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Sensing and Liquid–Liquid Extraction of Dicarboxylates Using Dicopper Cryptates

  • Sonia La Cognata
    Sonia La Cognata
    Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
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  • , 
  • Riccardo Mobili
    Riccardo Mobili
    Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
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  • , 
  • Francesca Merlo
    Francesca Merlo
    Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
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  • , 
  • Andrea Speltini
    Andrea Speltini
    Department of Drug Sciences, Università degli Studi di Pavia,via Taramelli 12, Pavia 27100, Italy
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    Orcidhttp://orcid.org/0000-0002-6924-7170
  • , 
  • Massimo Boiocchi
    Massimo Boiocchi
    Centro Grandi Strumenti, Università degli Studi di Pavia, via A. Bassi 21, Pavia 27100, Italy
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  • , 
  • Teresa Recca
    Teresa Recca
    Centro Grandi Strumenti, Università degli Studi di Pavia, via A. Bassi 21, Pavia 27100, Italy
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  • Louis J. Maher III
    Louis J. Maher, III
    Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota 55905, United States
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    Orcidhttp://orcid.org/0000-0002-5043-6422
  • , and 
  • Valeria Amendola*
    Valeria Amendola
    Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0001-5219-6074
Cite this: ACS Omega 2020, 5, 41, 26573–26582
Publication Date (Web):October 7, 2020
https://doi.org/10.1021/acsomega.0c03337

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Abstract

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We report the investigation of dicopper(II) bistren cryptate, containing naphthyl spacers between the tren subunits, as a receptor for polycarboxylates in neutral aqueous solution. An indicator displacement assay for dicarboxylates was also developed by mixing the azacryptate with the fluorescent indicator 5-carboxyfluorescein in a 50:1 molar ratio. Fluorimetric studies showed a significant restoration of fluorophore emission upon addition of fumarate anions followed by succinate and isophthalate. The introduction of hexyl chains on the naphthalene groups created a novel hydrophobic cage; the corresponding dicopper complex was investigated as an extractant for dicarboxylates from neutral water into dichloromethane. The liquid–liquid extraction of succinate—as a model anion—was successfully achieved by exploiting the high affinity of this anionic guest for the azacryptate cavity. Extraction was monitored through the changes in the UV–visible spectrum of the dicopper complex in dichloromethane and by measuring the residual concentration of succinate in the aqueous phase by HPLC-UV. The successful extraction was also confirmed by 1H-NMR spectroscopy. Considering the relevance of polycarboxylates in biochemistry and in the environmental field, e.g., as waste products of industrial processes, our results open new perspectives for research in all contexts where recognition, sensing, or extraction of polycarboxylates is required.

Introduction

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Polycarboxylates are relevant in the industrial and environmental fields. In addition, these anionic species are natural cellular metabolites with roles as intermediates of the tricarboxylic acid (TCA) cycle. For these reasons, molecular systems capable of selectively recognizing target polycarboxylates have attracted attention in the supramolecular chemistry field over many years. (1) Interest will undoubtedly grow because of the recent medical recognition of the pathological roles of some dicarboxylates, such as succinate, beyond normal metabolism. (2,3)
Molecular receptors are appreciated as powerful tools for the selective recognition of either neutral or ionic guests in competing media. (1,3−6) In the context of anion recognition, research has led to the development of a plethora of anion sensors, anion-responsive materials, and organocatalytic processes involving anion complexation. (7−9) Compared to the macrocyclic parents, the preorganization and the well-defined cavity in macrobicycles give an important contribution to the selectivity of these systems for anionic guests. (10)
Bistren-like azacryptands, in particular, have emerged among cage-like anion receptors. (1,3,4,6) In these systems, when the recognition of bidentate anionic guests is concerned, selectivity can be maximized by properly choosing the spacers between the tren subunits on the azacryptand skeleton. (1) Anion binding properties can be further improved by including two transition metal ions in the receptor cavity. The obtained dinuclear azacryptates form stable complexes with those bidentate anions—including polycarboxylates—for which the separation between the binding units better fits the separation between the metal ions in the cavity. Besides anion recognition, molecular receptors were successfully applied in the selective liquid–liquid or solid–liquid extraction of target anions and as anion transporters across cell membranes. (11−14) The latter applications have generally involved the extraction and/or transport of halides—chloride in particular—or oxoanions. (15−19) Despite the promising features of cage-like systems, their application in these fields is difficult to achieve.
In particular, in the extraction of polycarboxylates from a neutral aqueous solution into an aprotic solvent, the process must be both electroneutral and selective; i.e., charge neutrality must be maintained and the extractant should discriminate among similar guests. Some years ago, Lu and co-workers (20) demonstrated that the dizinc(II) complex of the cryptand L1 (reported in Figure 1) successfully discriminates between fumarate (fum2–) and its geometric isomer, maleate (male2–): the affinity constants were 4.6 and 3.4 log units for fum2– and male2–, respectively. A tendency to form 1:1 inclusion complexes with fum2– and succinate was also found for the dicopper(II) analogue complex, as shown by the crystal structures of the adducts. (20)

Figure 1

Figure 1. Azacryptands L1 and L2 and the series of polycarboxylates studied in this work.

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In this work, by applying the indicator displacement approach (21) to the [Cu2(L1)]4+ complex, we obtained an assay for the detection of fum2– in neutral water. This approach, which is well known in the literature, has previously been applied by our group in the determination of polycarboxylates in competing media. (22)
The skeleton of L1 was also modified by appending n-hexyl chains on the naphthyl spacers, thus obtaining a novel hydrophobic cryptand, L2, suitable for applications in the liquid–liquid extraction of dicarboxylates from water to immiscible solvents. (23) Our tests with the succinate anion showed that [Cu2(L2)]4+ extracts suc2– from neutral aqueous solution to dichloromethane; extraction is accompanied by the inclusion of the anion into the cage cavity.

Results and Discussion

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L1 and L2 were synthesized by Schiff-base condensation of tren with the proper dialdehyde, mixed in a 2:3 molar ratio, followed by reduction. In the case of L1, we followed the procedure described by Lu et al. (20) The synthesis of L2 (see Figure S1) and other experimental details are reported in the Experimental Section of this manuscript.

Potentiometric and pH-Spectrophotometric Investigations

Protonation and complexation equilibria involving L1 and L2 were first investigated by potentiometric titrations.
In the case of L1, titrations were performed on both the ligand alone and in the presence of two equivalents of Cu(CF3SO3)2 in a 30% v/v water/MeOH mixture ([NaNO3] = 0.05 M; T = 25 °C). This medium allowed us to solubilize both the free cage and the dicopper complex up to pH 11. The calculated constants (obtained through a non-linear least squares procedure, using the HyperQuad package) (24) allowed us to build the distribution diagrams of the species, reported as % abundance relative to L1 versus pH. The corresponding distribution diagrams, with and without Cu(II), are shown in Figure 2 and Figure S2, respectively (see the equilibrium constants in Table S1).

Figure 2

Figure 2. Distribution diagram showing species present at the equilibrium over the course of the potentiometric titration of L1 (0.4 mM) in the presence of 2 equiv of Cu(CF3SO3)2; the pH-spectrophotometric profiles of Mol Abs at 877 and 667 nm (red and green triangles, respectively) vs pH are superimposed (MeOH/water 30% v/v, 0.05 M NaNO3; T = 25 °C). The lines in the diagram correspond to the species: H6L16+, gray; H5L15+, purple; H4L14+, orange; [Cu(L1H3)]5+, cyan; [Cu2(L1)]4+, red; [Cu2(L1)(OH)]3+, green; [Cu2(L1)(OH)2]2+, blue.

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Around pH 2, L1 is in the hexaprotonated form. The mononuclear species [Cu(H3L1)]5+ (i.e., with the copper(II) ion occupying one of the tren subunits) prevails between pH 4 and 5.5, while the dinuclear complex [Cu2(L1)]4+, with one metal center for each tren subunit, becomes the major species in solution over pH 5.5. In [Cu2(L1)]4+, each copper(II) ion is penta-coordinated, with a water molecule occupying the apical position left free by the polyamino ligand. At pH > 8, the coordinated water molecules undergo deprotonation, leading to the species [Cu2(L1)(OH)]3+ and [Cu2(L1)(OH)2]2+ (>80% around pH 10).
A pH-spectrophotometric titration was performed in the same conditions and the obtained experimental profiles of molar absorbance versus pH were superimposed to the distribution diagram of the species (as shown in Figure 2). The family of spectra is available in the Supporting Information (see Figure S4). Around pH 7, the spectrum shows a band around 880 nm and a shoulder at 667 nm, attributable to the [Cu2(L1)]4+ complex with Cu(II) ions in a trigonal bipyramidal geometry. (25) The development of Nsec → Cu(II) (LMCT) bands is also observed near 330 nm.
Above pH 8, the deprotonation of the water molecules coordinated to Cu(II) ions leads to development of the hydroxide-containing species [Cu2(L1)(OH)]3+ and [Cu2(L1)(OH)2]2+. Deprotonation is accompanied by a change in the intensity ratio between the d–d bands at 880 and 667 nm. A color change is also observed in solution from green to light blue (see the blue line in Figure S4).
Potentiometric and pH-spectrophotometric titrations have been also performed on the lipophilic cage L2. For solubility reasons, in this case, the dioxane/water 20% v/v mixture, [TBA]NO3 0.05 M, was employed (T = 25 °C). The distribution diagram of the species, reported in Figures S3 and S6, shows a trend similar to that found for L1. Working in the presence of 2 equiv of Cu(II), the monometallic complex [Cu(L2H3)]5+ prevails in solution between pH 4 and 5.2, while the dimetallic cryptate [Cu2(L2)]4+ is the dominant species (>99%) in the pH range 6–10. The pH-spectrophotometric experiment confirmed the trend found for the non-functionalized ligand. In particular, around neutral pH, the UV–vis spectrum of [Cu2(L2)]4+ (see the green line in Figure S5) is characterized by a d–d band around 900 nm and a shoulder at 730 nm. These bands are peculiar of the coordination of Cu(II) ions to the tren subunits, with water molecules completing the penta-coordination sphere of the two metal centers. The binding of Cu(II) ions to the tren compartments is also accompanied by the development of Nsec → Cu(II) (LMCT) bands near 330 nm. Over pH 11, the deprotonation of the coordinated water molecules causes spectral variations similar to those observed for L1, with the change in the intensity ratio between the d–d bands at 845 and 680 nm. The profiles of the molar absorbances at 650 and 885 nm (green and red triangles, respectively) are superimposed to the distribution diagram of the species versus pH in Figure S6.

Crystal Structures Analysis

Through the slow evaporation of an aqueous solution containing the dicarboxylate sodium salt and the in situ-prepared complex [Cu2(L1)](CF3SO3)4 in a 1:1 molar ratio, we obtained single crystals suitable for X-ray diffraction analysis of the inclusion complexes with glutarate (glut2–), alpha-ketoglutarate (α-keto2–), and acetylendicarboxylate (ace2–) anions. Simplified sketches of the molecular structures are reported in Figure 3 (additional information is available in the Supporting Information).

Figure 3

Figure 3. From the left, pairs of simplified sketches of [Cu2(L1)(glut)]2+, [Cu2(L1)(α-keto)]2+, and [Cu2(L1)(ace)]2+ species. The bridging portion of each dicarboxylate anion is drawn as large spheres, in order to emphasize similitudes and differences among the guests. Hydrogen atoms are omitted for clarity, and atom names are reported only for Cu, N, and O.

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The hydrated crystals of [Cu2(L1)(glut)](CF3SO3)2·5(H2O) and [Cu2(L1)(α-keto)](CF3SO3)2·4(H2O) are isostructural and originate from two monoclinic crystal structures.
As described in the experimental section, the X-ray diffraction quality of the synthetic crystals containing the glut2– and α-keto2– adducts was poor. However, the results of the crystallographic study have been considered suitable to the aims of this work because the chemically significant parts of the two structures (i.e., the [Cu2(L1)]4+ cations and the dicarboxylate ions) are unequivocally determined, albeit with larger than typical esd’s values for interatomic distances and angles. Furthermore, the solid-state analysis reveals very similar molecular geometries of the [Cu2(L1)]4+ cryptate in the presence of the two dicarboxylate anions having a common backbone of five C atoms. As already observed by Lu et al. (20) for the inclusion complexes with fum2– and suc2–, all the anionic guests reside in the cage cavity, which exhibits a cradle-like arrangement (see Figure 3 and the Supporting Information): the dicarboxylate anion bridges the two Cu(II) ions, both presenting a penta-coordination geometry, with four N atoms of the tren subunits and an O atom of the anion. The longer carbon chains of glut2– and α-keto2– anions as well as the linear backbone of ace2– disfavor an optimal match within the cage’s cavity and generally cause an increase of the Cu(II)···Cu(II) separation in the cryptate. The Cu(II)···Cu(II) separation is 8.77(1) Å in both [Cu2(L1)(fum)]2+ and [Cu2(L1)(suc)]2+4, whereas it is 8.95(1) Å in [Cu2(L1)(glut)]2+ and 8.98(1) Å in [Cu2(L1)(α-keto)]2+. For [Cu2(L1)(ace)]2+, the intermediate value of 8.88(1) Å is found. Complete geometrical features of the metal-center coordinations are reported in the Supporting Information.
The coordination geometry of the metal centers was distinguished using the structural parameter τ, (26) calculated by eq 1.
(1)
where β and α are the two greatest bond angles of the metal center, with β > α. The τ value is close to 0 in the presence of a square pyramidal (SP) geometry, while a value close to 1 reveals a trigonal bipyramidal (TB) geometry. (26)
In the inclusion complexes with fum2– and suc2–, (20) the couples of τ values for the two metal centers are τ1 and τ2 = 0.64 and 0.89 and τ1 and τ2 = 0.60 and 0.89, respectively. Therefore, in both adducts, the geometry around one metal center is close to TB, while for the other one, it is intermediate between TB and SP. In the complex [Cu2(L1)(ace)]2+, an intermediate TB/SP geometry is found for both metals (τ1 and τ2 = 0.51 and 0.69, respectively). In the structures with glut2– and α-keto2–, the TB → SP distortion is more significant and it involves both Cu(II) ions: τ1 and τ2 = 0.30 and 0.34 in [Cu2(L1)(glut)]2+; τ1 and τ2 = 0.33 and 0.39 in [Cu2(L1)(α-keto)]2+.
The TB → SP distortion is emphasized by the bond angle Nter–Cu–Ocar, involving the two axial atom positions of the trigonal bipyramid: Nter–Cu–Ocar is almost flat in the presence of fum2– and suc2– ions [being in the range 175.4(2)–178.8(2)°] and remains close to these values in the presence of the linear ace2– ion, being the observed values 173.57(14) and 178.26(15)° for the two metal centers, respectively. On the other hand, Nter–Cu–Ocar decreases to values in the range 167.3(2)–169.1(2)° in the presence of glut2– and α-keto2–. All these latter dicarboxylates extend their carbon chain outside the cavity of the cradle-like dimetallic cage while keeping the best plane of the dicarboxylate groups almost parallel to the best plane of the naphthalene rings of the two lateral arms (the dihedral angles for the studied molecular compounds are in the range 2.2–8.8°). However, for longer dicarboxylates, the sliding off of the cradle increases the separations between the centroids of the naphthalenes and the centroids of the dicarboxylate moieties. In particular, the two centroid–centroid separations are 3.42 and 3.45 Å in [Cu2(L1)(glut)]2+ and 3.57 and 3.60 Å in [Cu2(L1)(α-keto)]2+.
These values are longer than the centroid–centroid separations found in [Cu2(L1)(fum)]2+ (3.33 and 3.37 Å) and somewhat longer than the values observed in [Cu2(L1)(ace)]2+ (3.37 and 3.44 Å). This suggests minor face-to-face π-stacking interactions in the case of dicarboxylates with a backbone of five C atoms.

Spectrophotometric Studies on [Cu2(L1)]4+ with Dicarboxylates

In this work, we aimed to deepen our understanding of the binding tendencies of [Cu2(L1)]4+ through spectrophotometric titrations with a series of carboxylates. In particular, we tested the affinity of [Cu2(L1)]4+ for polycarboxylates (i) of different lengths, (ii) with either an aliphatic or aromatic skeleton, or (iii) with the same number of carbon atoms but presenting different substituents or unsaturation degrees (see Table 1 and Table S7).
Table 1. Conditional Binding Constants (as log K11 Values) Obtained by UV–Vis Titrations on [Cu2(L1)]4+ and [Cu2(L2)]4+a
anionic guestlog K11 [Cu2(L1)]4+log K11 [Cu2(L2)]4+
fum2–>65.76(2)
suc2–>65.60(3)
isopht2–>64.98(4)
α-keto2–6.00(1)n.a.
ace2–5.90(2)n.a.
glut2–5.39(1)4.14(3)
adi2–2.58(1)3.47(1)
male2–2.78(1)3.65(5)
a

Conditions: [Cu2(L1)]4+ water, 0.05 M HEPES, pH 7; [Cu2(L2)]4+ in dioxane/water solution 20% v/v, 0.025 M HEPES, pH 7 (T = 25 °C). Standard deviations are shown in parentheses. Titration data were processed with the HyperQuad package. See the Supporting Information for details. n.a.: not available.

Our UV–vis titrations, performed in aqueous buffer at pH 7 (0.05 M HEPES buffer), pointed out the formation of 1:1 adducts with all tested anions with the exception of malonate and oxalate.
For both these anions, a significant decrease in the intensity of the d–d bands was observed under titration, attributable to an anion-induced decomposition of the dicopper cryptate.
The log K11 values, reported in Table 1 and Table S7, point out a stronger affinity for dicarboxylates with a rigid structure and with two or three carbon atoms between the −COO– groups (named C2 and C3 anions, respectively). With longer dicarboxylates (e.g., adipate and pimelate, C4 and C5, respectively), a significant drop of the affinity is observed.
Among the investigated anionic species, stronger interactions are found with fum2–, suc2–, and isopht2–: for all these guests, the titration profiles are too steep to allow accurate determination of the binding constants by UV–vis titrations. (31) Interestingly, the affinity for these dicarboxylates is even greater than that found for the tricarboxylate citrate anion (see Table S7).
The position of the d–d bands in the initial UV–vis spectrum of [Cu2(L1)]4+ is consistent with a TB coordination geometry around the copper(II) ions, with water molecules occupying the positions left free by the tren subunits. The band at 877 nm and the shoulder around 660 nm are typical of complexes of the [Cu(tren)(X)]2+ type. (25) Under titrations with dicarboxylates, a significant blue shift of the band at 877 nm is observed, accompanied by an increase in intensity of the shoulder (see, e.g., Figure 4 and Figure S9).

Figure 4

Figure 4. Spectra taken over the course of the titration of [Cu2(L1)]4+ (0.5 mM) with an aqueous solution of fumaric acid in 0.05 M HEPES at pH 7. Inset: profiles of ε (i.e., Mol Abs) at 691 (blue triangles) and 903 nm (red triangles) vs equivalents of the added guest.

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For the investigated C3 anions, affinity increases along the series: glut2– < α-keto2– < isopht2–. The crystal structures of the inclusion complexes with glut2– and α-keto2– reveal that these anions bridge the Cu(II) centers of the cage while standing a bit outside the cavity. This binding mode induces a relevant TB → SP distortion of the coordination geometry around the Cu(II) ions. A similar distortion can be hypothesized in solution, as indicated by the exchange of the intensity ratio between the d–d bands at 660 and 877 nm (see Figures S10 and S11). (25)
Among aromatic dicarboxylates, a peak of selectivity is found for isophthalate, whose bite length corresponds to those in fum2– and suc2–. A relevant drop of the affinity is instead observed with phthalate and terephthalate (log K11 ≈ 2.5 and 3.5, respectively, see the Supporting Information for details), which are too small and too large, respectively, for the cavity.

Spectrofluorimetric Studies on [Cu2(L1)]4+ with 5-FAM and Dicarboxylates

The fluorescent indicator displacement approach was applied to investigate [Cu2(L1)]4+ with those anions—e.g., fum2–, suc2–, and isopht2–—for which the constants could not be determined by UV–vis titrations. 5-Carboxyfluorescein (5-FAM) was chosen as the indicator because it possesses an isophthalate unit apt to bridge the Cu(II) ions in the azacryptate cavity (see Scheme 1).

Scheme 1

Scheme 1. Displacement of 5-FAM from the [Cu2(L1)]4+ Cavity Promoted by the Fumarate Anion
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First, the [Cu2(L1)]4+/5-FAM association constant was determined by spectrofluorimetric titration of a solution of 5-FAM (1 μM) with a solution of the dicopper complex in buffered aqueous solution (0.05 M HEPES, pH 7). The family of spectra and the corresponding titration profile are shown in Figure 5 (λexc = 473 nm; λem = 520 nm).

Figure 5

Figure 5. Spectra taken over the course of the fluorimetric titration of 5-FAM (1 μM) with a solution of [Cu2(L1)]4+ in 0.05 M HEPES at pH 7. Inset: profile of I/I0 at 520 nm (triangles) vs equivalents of the dicopper complex superimposed to the distribution diagram of the species (as % abundance vs equivalents of dicopper complex): red line = % free 5-FAM; gray line = % [Cu2(L1)(5-FAM)]+ .

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As expected, the binding of 5-FAM to the azacryptate cavity is accompanied by the quenching of the fluorophore emission. (22) The experimental profiles could be fitted for a single equilibrium, leading to the formation of the 1:1 complex [Cu2(L1)(5-FAM)]+ with an association constant of 7.20(2) log units.
For the experiments with dicarboxylates, stock solutions of 5-FAM (0.1 μM) with 50 equiv of the dicopper cryptate were prepared and used in titrations with chosen anions. As shown by Figure 6, a significant displacement of the indicator from the receptor cavity, accompanied by a significant restoration of the emission, was only obtained with the strongly binding anions fum2– and suc2–. In contrast, e.g., with adi2–, male2–, and citrate, a slight increase of the emission was only observed in the presence of a large excess of the guest. From the fitting of the titration profiles, we obtained the binding constants, which are reported in Table 2. Addition material is available in the Supporting Information.

Figure 6

Figure 6. Profiles of the spectrofluorimetric titrations of the indicator displacement assay (0.1 μM 5-FAM, 5 μM [Cu2(L1)]4+) with solutions of the anionic guests in 0.05 M HEPES at pH 7 (fum2– = red triangles, suc2– = blue triangles; α-keto2– = dark blue triangles; glut2– = green triangles; adi2– = dark green triangles; diamonds = citrate). I = emission intensity; Imax = emission intensity of a solution of 5-FAM in the absence of the dicopper complex (λexc = 473 nm; λem = 520 nm).

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Table 2. Conditional Binding Constants (as log K11 Values) Obtained
guestlog K11a,bbite (Å)cτ1, τ2c
fum2–>6a, 8.21(3)b5.000.64, 0.89
suc2–>6a, 7.38(3)b5.020.60, 0.89
isopht2–>6a, 7.20(4)bn.a.n.a.
α-keto2–6.00(1)a, 5.95(3)b5.280.33, 0.39
ace2–5.90(2)a, 5.71(3)b5.100.51, 0.69
glut2–5.39(1)a, 5.40(4)b5.270.30, 0.34
adi2–2.58(1)an.a.n.a.
male2–2.78(1)an.a.n.a.
a

Obtained by UV–vis titrations on [Cu2(L1)]4+ with the anionic guest.

b

Obtained by fluorimetric indicator displacement titrations with selected anions; 5-FAM was used as the fluorescent indicator.

c

The anion bite and the τ1 and τ2 values were obtained from the crystal structures reported in this work or taken from the literature. (20)

Spectrophotometric Studies on [Cu2(L2)]4+

In order to evaluate the effect of spacers’ functionalization on the anion-binding properties of the azacryptate, the new receptor [Cu2(L2)]4+ was investigated with a series of dicarboxylates by UV–vis titrations (see Table 1 and Figures S24–S30). These studies were performed in dioxane/water solution (20% v/v), buffered at pH 7 by HEPES buffer 0.025 M (T = 25 °C), because of the low solubility of the dicopper complex in only aqueous media. Appreciating that organic solvents in the aqueous mixture may alter dissociation equilibria, we determined pKa values for two relevant acids, H2suc and H2glut, through potentiometric titrations in dioxane/water (20% v/v), 0.05 M [TBA]NO3 (T = 25 °C; see Supporting Information). As expected, complete deprotonation of H2suc and H2glut was obtained only at pH > 8 (see the Supporting Information), while the abundance of the corresponding dicarboxylates in pure water was already greater than 99% at pH 7.

Figure 7

Figure 7. Family of spectra taken over the course of the UV–vis titration of [Cu2(L2)]4+ (50 μM) with a solution of H2suc in dioxane/water (20% v/v) at pH 7 (0.025 M HEPES, path length: 10 cm). Initial spectrum and final spectrum: red and blue lines, respectively. Inset figure: plot of ε (i.e., Mol Abs) at 687 nm vs equiv of the added guest (triangles), superimposed to the distribution diagram of the species calculated for a log K11 = 5.60; % [Cu2(L2)]4+ (red line) and [Cu2(L2)(suc)]2+ (blue line) vs equiv of succinate. (23) See the Supporting Information for more details.

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Even if the deprotonation of dicarboxylic acids is disfavored in the chosen aqueous mixture, the coordination of the corresponding dianion to the metal centers within the cavity of the cage may provide sufficient driving force to promote complete deprotonation. This was confirmed by our studies on [Cu2(L2)]4+ with the guests reported in Table 1. In particular, [Cu2(L2)]4+ was found to form 1:1 adducts with all of the investigated anions, with the greatest selectivity for fum2– and suc2–. Reduced affinity was found for anions with longer chains, e.g., glut2– and adi2–. The spectral changes observed over the course of the titration experiments of [Cu2(L2)]4+ are quite similar to those observed for the non-functionalized ligand L1 (Figure 7). In fact, the [Cu2(L2)]4+ spectrum shows a d–d band at 900 nm, with the shoulder at 700 nm, typical of a TB geometry for the copper(II) ions, with water molecules in the positions left free by the tren subunits. The variations of these bands, observed during titrations with anions, are very similar to those found with [Cu2(L1)]4+ (see the Supporting Information for details).
Despite several efforts, no single crystals suitable for X-ray analysis of the inclusion complexes with anions for [Cu2(L2)]4+ were obtained. Therefore, the binding of suc2– to the lipophilic azacryptate [Cu2(L2)]4+, with the formation of the [Cu2(L2)(suc)]2+ adduct, was confirmed by HRMS-ESI spectroscopy in an acetonitrile solution of [Cu2(L2)]4+ treated with 1 equiv aqueous Na2suc. The HRMS-ESI spectrum (see the Supporting Information) displays a peak at 796.4592 m/z, attributable to the doubly charged ion [Cu2(L2)(suc)]2+.
As shown in Table 1, the selectivity displayed by [Cu2(L2)]4+ is lower than that found for [Cu2(L1)]4+, but the affinity trend is rather similar. Discrepancies in the log K11 values for the binding of dicarboxylates by the two azacryptates can be attributed to the different structures of the cages and to the different media employed for measurements.

Extraction Experiments

Given its high affinity for polycarboxylate guests and its enhanced lipophilicity, we tested the cage-like complex [Cu2(L2)]4+ as a potential extractant of dicarboxylates from neutral water into an immiscible organic solvent, i.e., dichloromethane. (23) It should be noticed that the complex [Cu2(L1)]4+ cannot be used as an extracting agent because of its insolubility in dichloromethane.
The stability of the [Cu2(L2)(suc)]2+ adduct in the chosen solvent was first verified by UV–vis titration of [Cu2(L2)]4+ (30 μM) with [TBA]2suc in dichloromethane (path length 10 cm; T = 25 °C). This study confirmed the high affinity of suc2– for the azacryptate and the formation of a receptor–anion adduct, with a 1:1 stoichiometry (see Supporting Information, Figure S31).
We then performed liquid–liquid extraction experiments. A 0.20 mM solution of [Cu2(L2)]4+ in dichloromethane was stirred for 10 min in contact with an aqueous solution of 1 mM H2suc in HEPES buffer 0.050 M at pH 7. The successful extraction of H2suc from water was demonstrated by recording UV–vis spectra of the organic phase before and after mixing (see Figure S33). Notably, the final spectrum of the dichloromethane phase displayed the same changes in the d–d bands of the [Cu2(L2)]4+ complex as those found upon titration of the azacryptate with [TBA]2suc in this medium. This result indicates that suc2– is successfully transferred from neutral water to dichloromethane, where it is stabilized by complexation with [Cu2(L2)]4+.
These initial tests were then followed by quantitative extraction experiments in which the suc2– concentration in the aqueous layer was monitored by HPLC-UV. These experiments (see the Experimental Section for details) again employed 0.20 mM solutions of [Cu2(L2)]4+ in dichloromethane and 1.0 mM solutions (i.e., 5-fold excess vs [Cu2(L2)]4+) of H2suc in 0.050 M HEPES buffer at pH 7.
Seven extraction experiments were performed independently over non-consecutive days. HPLC-UV analyses indicated that suc2– was effectively extracted into the organic phase only in the presence of the azacryptate. The final concentration of succinate in the organic phase actually matched the concentration of dicopper cryptate in the organic phase; in fact, the measured average extraction yield—calculated with respect to the initial concentration of suc2– in the aqueous phase—was 20 ± 4% (n = 7, α = 0.05) (see the Supporting Information for details). Notably, this result also corresponds to the maximum amount of suc2– that would result in the organic phase if the entire amount of [Cu2(L2)]4+ forms a 1:1 complex with the anion in DCM. This is in full agreement with the data obtained by UV–vis spectroscopy (i.e., by measuring the spectrum of the [Cu2(L2)]4+ in the organic phase before and after extraction).
These findings were corroborated by additional trials showing that succinate does not measurably enter the organic phase without complexation by [Cu2(L2)]4+: the organic phase was evaporated to dryness after extraction in the absence of the cryptand. Chromatographic analysis of the residue, reconstituted with 1 mL of 0.050 M HEPES buffer, showed an undetectable concentration of suc2–.
A further confirmation of suc2– extraction was obtained through an 1H-NMR experiment, consisting of the quantification of suc2– in buffered D2O solution (1.5 mL, 0.05 M PBS, pH 7), before and after extraction with a solution of the dicopper complex in DCM (see the Experimental Section for details). Very interestingly, starting from a 1.044 mM solution of suc2– (1.57 mmol) in buffered D2O, after 10 min. Stirring with a 0.42 mM solution of [Cu2(L2)]4+ in DCM (1.5 mL, 0.63 mmol [Cu2(L2)]4+), a 0.638 mM residual concentration of suc2– in the D2O phase was found. This indicates that 0.61 mmol of the anion are extracted into the organic phase, corresponding to a 97% extraction yield relative to the cage. The 1H-NMR spectra are shown in the Supporting Information. A control experiment in which a buffered solution of suc2– was stirred with a pure DCM layer confirmed that suc2– is not extracted, from the PBS buffer into DCM, in the absence of the cage.

Conclusions

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In this work, we show that an indicator displacement assay, consisting of the dicopper azacryptate [Cu2(L1)]4+ and the fluorescent indicator 5-FAM, can be successfully applied in the sensing of the fumarate anion in neutral water.
In addition, we demonstrate that the lipophilic cage-like complex [Cu2(L2)]4+can be successfully applied in the extraction of highly hydrated polyanionic species from water.
Extraction studies, using succinate as a model anion, showed that the quantitative extraction of the dicaboxylate guest can be achieved by employing the lipophilic azacryptate [Cu2(L2)]4+ as the extractant. This complex is actually capable of stabilizing the polyanion in dichloromethane through encapsulation in the cavity. Given the relevance of polycarboxylates as normal metabolites and pathological oncometabolites as well as industrial waste products, our results open new possibilities for research and application in selective recognition, trapping, and extraction of target polycarboxylates.

Experimental Section

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Materials and Methods

All reagents for syntheses were purchased from Sigma-Aldrich and used without further purification. All reactions were performed under dinitrogen. High-resolution mass spectra were recorded on a Thermo Scientific Q Exactive Plus instrument. 1H- and 13C-NMR spectra were recorded on a Bruker AVANCEIII 400 MHz (operating at 9.37 T, 400 MHz), equipped with a 5 mm BBO probe head with Z-gradient (Bruker BioSpin). For the extraction study, 1H-NMR analysis was performed using the AVANCEIII 400 MHz. Spectra were acquired using a Bruker pulse sequence noesygppr1d for water suppression, after optimization of 90°pulse, shims, and O1 parameter. Phase correction and baseline correction were applied using an automatic procedure. Succinate concentration was determined using the Topspin Eretic2 package. UV–vis spectra were collected using a Varian Cary 50 SCAN spectrophotometer, with quartz cuvettes of the appropriate path length (1 or 10 cm) at 25.0 ± 0.1 °C. Emission spectra were collected using a Perkin Elmer LS50B fluorimeter, with a quartz cuvette of 1 cm path length. HPLC gradient-grade acetonitrile and ultrapure water were purchased from VWR (Milan, Italy). The HPLC-UV apparatus consisted of a Shimadzu (Milan, Italy) LC-20AT solvent delivery module equipped with a DGU-20A3 degasser and interfaced with a SPD-20A UV detector. A Sepachrom Adamas C18-AQ (250 × 4.6 mm, 5 μm) column coupled with a Supelco Supelguard Ascentis C18 (20 × 2.1 mm, 5 μm) guard column was used. 1,5-Di(hexyloxy)naphthalene-2,6-dicarboxaldehyde, [TBA]2suc, and L1 were synthesized following known procedures. (27−29)

Synthesis of L2

A solution of 1,5-di(hexyloxy)naphthalene-2,6-dicarboxaldehyde (0.12 g; 0.31 mmol; 3.0 equiv) in dicloromethane (DCM; 10 mL) and MeOH (70 mL) was added to a solution of AgNO3 (0.05 g; 0.31 mmol; 3.0 equiv) in MeOH (70 mL). The reaction mixture was then refluxed under an Ar atmosphere. A solution of tris(2-aminoethyl)amine (0.03 mL; 0.21 mmol; 2.0 equiv) in MeOH (50 mL) was then slowly added dropwise, and the reaction was refluxed for 24 h. The solution containing the polyimine cage compound was then transferred in a round-bottom flask, and DCM was evaporated under vacuum. The methanol solution was refluxed again, and NaBH4 (3.0 g) was added in small portions in order to reduce both imine bonds and Ag+. After 12 h, the mixture was cooled to room temperature and filtered to eliminate Ag0 particles. The solvent was evaporated under vacuum to give a white solid that was dissolved with brine (200 mL) and extracted with DCM (4 × 50 mL). The collected organic phase was then dried with Na2SO4. The solvent was evaporated under vacuum to obtain 0.13 g of a yellowish dense oil. Yield: 91%; 1H-NMR (400 MHz, DMSO-d6 + HNO3, T = 353 K) δ: 7.84–7.61 (6H, naph), 4.44 (m, 12H, N-CH2-naph), 3.91 (m, 12H, O-CH2-C), 3.20–2.80 (br m, 24H, N-CH2CH2-N), 1.97 (m, 12H, OCH2-CH2-C), 1.51–1.27 (m, 36H, O(CH2)2-(CH2)3-C), 0.91 (t, 18H, -CH3); 13C-NMR (100 MHz, DMSO-d6, T = 298 K) δ: 154.02, 129.16, 126.89, 122.22, 118.91, 76.32, 51.07, 45.68, 45.26, 31.78, 30.29, 25.53, 22.61, 14.40; HRMS-ESI (MeOH) m/z: [M + H+]+ calculated for C84H132N8O6, 1350.0348, found 1350.0529.

Potentiometric and pH-Spectrophotometric Titrations

Protonation constants of ligand L1 were determined in a water/methanol (30% v/v) mixture, 0.05 M in NaNO3. Protonation constants of ligand L2 were determined in a dioxane/water (20% v/v) mixture, 0.05 M in [TBA]NO3. In a typical experiment, 15 mL of a 5 × 10–4 M (L1) or 4 × 10–4 M (L2) ligand solution was treated with an excess of 1.0 M HNO3. Titrations were performed by addition of 10 μL aliquots of carbonate-free standard 0.1 M NaOH, recording 80–100 points for each titration. Complexation constants of L1 and L2 cages were determined by performing a potentiometric titration experiment, working in the presence of 2 equiv of Cu(CF3SO3)2 or Cu(NO3)2, respectively. Prior to each potentiometric titration, the standard electrochemical potential (E°) of the glass electrode was determined in the proper solvent mixture by a titration experiment according to the Gran method. (29) Protonation and complexation titration data (emf vs mL of NaOH) were processed with the HyperQuad package (24) to estimate the equilibrium constants.
pH-spectrophotometric titrations were performed on solutions of L1 and L2 (4 × 10–4 M) in the presence of Cu(II) (8 × 10–4 M) in the same solvent mixture utilized in potentiometric titrations in water/methanol (30% v/v) with 0.05 M NaNO3 and dioxane/water (20% v/v) with [TBA]NO3 0.05 M, 25 °C, respectively). In a typical experiment, aliquots of carbonate-free standard 0.1 M NaOH were added to the solution of the azacryptate and copper, both the electrochemical potential and the UV–vis spectrum of the solution were recorded after each addition.

Spectrophotometric Titrations

In a typical experiment, a concentrated solution of the dicopper(II) azacryptate in DMSO was diluted with buffer at pH 7, i.e., 0.05 M HEPES buffer in pure water for [Cu2(L1)]4+ or 0.025 M HEPES buffer in dioxane/water (20% v/v) for [Cu2(L2)]4+: the final concentration of the azacryptate ranged between 50 and 500 μM. The buffered solution of the azacryptate was then titrated with a standard solution of either the chosen dicarboxylic acid or the corresponding disodium salt, in the case of phthalates. After each addition of the titrant solution, the UV–vis spectrum was recorded. It should be noted that with terephthalate and phthalate anions, the formation of a precipitate over the course of the titration prevented accurate determination of association constants. The concentration of the azacryptate solution was chosen on the basis of the p parameter (p = [concentration of the azacryptate–anion adduct]/[maximum possible concentration of the azacryptate–anion adduct]), which should range between 0.2 and 0.8. (30) Titration data were processed with the HyperQuad package (24) to estimate equilibrium constants. In some cases, even working with very dilute solutions of the azacryptate, the curvature of the titration profile was too sharp to allow accurate calculation of binding constants. In these cases, distribution diagrams were traced assuming an estimated value of the constant.

Spectrofluorimetric Titrations

A solution of 5-FAM (1 μM) was first titrated with a solution of the in situ-prepared [Cu2(L2)]4+ complex in 0.05 M HEPES buffer at pH 7. The indicator was excited at 478 nm, i.e., an isosbestic point of the corresponding UV–vis titration. Titration data were processed with the HyperQuad package (24) to estimate receptor–indicator binding constants.
Titrations with anions were then performed on solutions of 5-FAM (0.1 μM) and the in situ-prepared [Cu2(L2)]4+ complex (5 μM) in 0.05 M HEPES buffer at pH 7. Titration data were processed with the HyperQuad package (24) to estimate equilibrium constants.

X-ray Diffraction Studies

Diffraction data for [Cu2(L1)(glut)](CF3SO3)2·4H2O crystal (greenish azure, 0.30 × 0.26 × 0.04 mm3), [Cu2(L1)(α-keto)](CF3SO3)2·5H2O crystal (greenish azure, 0.30 × 0.18 × 0.02 mm3), and [Cu2(L1)(ace)](CF3SO3)2·4H2O crystal (bluish azure, 0.26 × 0.22 × 0.13 mm3) were collected by means of a Bruker-AXS diffractometer equipped with a SMART-APEX CCD detector. Data collection were performed at room temperature with Mo Kα X-ray radiation (λ = 0.7107 Å), and crystal data are reported on Table S8. CCD frames were processed with the SAINT software, (31) and intensities were corrected for Lorentz and polarization effects; absorption effects were empirically evaluated by the SADABS software, (32) and absorption correction was applied to the data. All crystal structures were solved by direct methods (SIR 97) (33) and refined by full-matrix least-squares procedures on F2 using all reflections (SHELXL-2018). (34) Anisotropic displacement parameters were used for all non-hydrogen atoms. Hydrogens were placed at calculated positions, and their positions were refined according to a riding model. Positions for H atoms belonging to water solvent molecules were not determined.
The X ray diffraction quality of the synthetic crystals of the glut2–and α-keto2– was poor. The poor diffraction quality produced a low accuracy for the atom positions of the triflate counterions and of the water solvent molecules, which exhibited large and elongated atom displacement parameters, probably related to some unresolved positional disorder. In general, soft geometrical restraints and restraints on the atom displacement parameters were applied for the refinement of these atom positions. More details on the positional disorder identified in these crystal structures and on the restraints applied in the final least-squares cycles of structure refinements are reported in the Supporting Information.
CCDC 1971576, 1996979, and 2006355 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

Extraction Experiments

After an equilibration period of 5 min, 20 μL of each sample was injected into the HPLC system. The mobile phases were (A) 25 mM aqueous H3PO4 (pH 2.5) and (B) acetonitrile, working with an isocratic elution 10% B for 7 min, followed by a washing step (linear gradient to 70% B until 9 min, hold for 3 min). The flow rate was 0.8 mL min–1, and the detection wavelength was 214 nm.
Extraction experiments were performed with solutions of succinic acid (1.0 mM), prepared in aqueous buffer (0.05 M HEPES buffer) at pH 7, and an organic phase obtained by dilution with DCM of a stock solution of [Cu2(L2)](NO3)4 (9 mM) in DMSO. The final concentration of [Cu2(L2)]4+ in DCM was 0.20 mM [the % DMSO into the final solution being about 2% v/v]. The extraction of succinate from water into DCM was quantitatively evaluated by comparing the dicarboxylate concentration in the buffered aqueous solution, before and after mixing with the solution of [Cu2(L2)]4+ in DCM. The solutions were vigorously stirred for 20 min and then allowed to separate for 10 min. A blank experiment was run by mixing the aqueous solution of succinate with pure DCM under the same conditions as in the cryptate-mediated extraction. For each repeat, the stock solution of succinic acid was freshly prepared in order to reduce experimental errors. Succinate extraction was also monitored by 1H-NMR spectroscopy. For the experiment, a solution of succinic acid (1.076 mM) was prepared in D2O (0.05 M PBS at pH 7). The organic phase was obtained by dilution with DCM of a stock solution of [Cu2(L2)](CF3SO3)4 (5.1 mM) in DMSO. The final concentration of [Cu2(L2)]4+ in DCM was 0.42 mM [the % DMSO into the final solution being about 6% v/v]. The extraction of succinate from the D2O solution into DCM was quantitatively evaluated by comparing the dicarboxylate concentration in D2O, before and after mixing with the solution of [Cu2(L2)]4+ in DCM. The solutions were vigorously stirred for 20 min and then allowed to separate for 10 min. A blank experiment was run by mixing the aqueous solution of succinate with DCM/6% DMSO (v/v) under the same conditions as in the cryptate-mediated extraction. Succinate concentration was determined using the Topspin Eretic2 package, using glycine (1.00 mM) as an internal reference for the D2O phase.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03337.

  • Synthesis of L2; potentiometric and pH-spectrophotometric titrations of L1 and L2; spectrophotometric titrations of [Cu2(L1)] and [Cu2(L2)]; X-ray diffraction studies; extraction experiments; and characterization of L2 (PDF)

  • Crystallographic information file of [Cu2(L1)(glut)](CF3SO3)2·4(H2O) (CIF)

  • Crystallographic information file of [Cu2(L1)(α-keto)](CF3SO3)2·5(H2O) (CIF)

  • Crystallographic information file of [Cu2(L1)(ace)](CF3SO3)2·4(H2O) (CIF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Sonia La Cognata - Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
    • Riccardo Mobili - Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
    • Francesca Merlo - Department of Chemistry, Università degli Studi di Pavia, v.le T. Taramelli 12, Pavia 27100, Italy
    • Andrea Speltini - Department of Drug Sciences, Università degli Studi di Pavia,via Taramelli 12, Pavia 27100, Italy;  Orcidhttp://orcid.org/0000-0002-6924-7170
    • Massimo Boiocchi - Centro Grandi Strumenti, Università degli Studi di Pavia, via A. Bassi 21, Pavia 27100, Italy
    • Teresa Recca - Centro Grandi Strumenti, Università degli Studi di Pavia, via A. Bassi 21, Pavia 27100, Italy
    • Louis J. Maher - Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota 55905, United States;  Orcidhttp://orcid.org/0000-0002-5043-6422
  • Funding

    V.A. received funding for reagents and solvents from the Cariplo Foundation (grant 2019-2090). The National Inter-University Consortium of Materials Science and Technology (INSTM) is also acknowledged. R.M. and S.L.C. acknowledge the Italian Ministry of Education, University and Research and the University of Pavia for fellowships.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Clelia Cazzola is acknowledged for her contribution in the UV–vis titrations of [Cu2L2]4+ with anions. The Laboratorio Biochimica, Biotecnologie e Diagnostica Avanzata, Fondazione IRCCS Policlinico San Matteo (Pavia, Italy) is gratefully thanked for HRMS measurements.

ABBREVIATIONS

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α-keto2–

alpha-ketoglutarate

ace2–

acetylendicarboxylate

adi2–

adipate

5-FAM

5-carboxyfluorescein

cit3–

citrate

DCM

dichloromethane

fum2–

fumarate

glut2–

glutarate

HEPES

4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid

isopht2–

isophthalate

male2–

maleate

PBS

phosphate-buffered solution

pht2–

phthalate

suc2–

succinate

terepht2–

terephthalate

TBA

tetrabutylammonium

References

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    Three macrocyclic hexaamines I (n = 3, 7, 10) and the acyclic tetraamine RNH(CH2)3NH(CH2)nNH(CH2)3NHR (II; n = 10, R = H) and hexaamine II [n = 10, R = (CH2)3NH2] were prepd. The hexaamines I (n = 3, 7, 10) are ditopic coreceptor mols. contg. two triamine subunits which may bind anionic substrates when protonated. The stability consts. of the complexes between the protonated forms of the macrocyclic polyamines and terminal dicarboxylates -O2C-(CH2)m-CO2- as well as amino-acid and dipeptide dicarboxylates were detd. by pH-metric measurements. Around neutral pH, I (n = 7, 10) gave mainly complexes of the fully protonated species I·6H+, whereas I (n = 3) gave predominantly complexes of I·5H+ and I·4H+. The stability sequences of the complexes formed indicate preferential binding of the dianionic substrates whose length is compatible with the sepn. of the triammonium binding subunits in the protonated receptor mols. I (n = 3, 7, 10). This selectivity pattern corresponds to a process of linear mol. recognition based on ditopic binding between the two ammonium subunits of the coreceptor and the terminal carboxylates of the substrate of complementary length. The complexes of the acyclic ligands II are much weaker and much less selective, indicating a marked macrocyclic effect on both stability and selectivity of binding, i.e. on recognition.
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    (b) Lehn, J.-M.; Méric, R.; Vigneron, J.-P.; Bkouche-Waksman, I.; Pascard, C. Molecular Recognition of Anionic Substrates. Binding of Carboxylates by a Macrobicyclic Coreceptor and Crystal Structure of Its Supramolecular Cryptate with the Terephthalate Dianion. J. Chem. Soc., Chem. Commun. 1991, 2, 62– 64,  DOI: 10.1039/C39910000062
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    (c) Wang, Q.-Q.; Day, V. W.; Bowman-James, K. Tunable, Shape-Shifting Capsule for Dicarboxylates. Chem. Sci. 2011, 2, 1735– 1738,  DOI: 10.1039/c1sc00292a
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    3c
    Tunable, shape-shifting capsule for dicarboxylates
    Wang, Qi-Qiang; Day, Victor W.; Bowman-James, Kristin
    Chemical Science (2011), 2 (9), 1735-1738CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A cylindrical amide-based tricycle provides a ditopic framework for encapsulating both aliph. and arom. dicarboxylate anions. Due to the flexible spacer between the 2 macrocyclic receptor sites, it can modulate its shape to conform to dicarboxylates of varying lengths. In the uncomplexed form, the host is elongated along its cylindrical axis, but when encapsulating the guest, it compresses to ensure the best structural fit.
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    (d) Santos, M. M.; Marques, I.; Carvalho, S.; Moiteiro, C.; Félix, V. Recognition of Bio-Relevant Dicarboxylate Anions by an Azacalix[2]Arene[2]Triazine Derivative Decorated with Urea Moieties†. Org. Biomol. Chem. 2015, 13, 3070– 3085,  DOI: 10.1039/C4OB02283A
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    3d
    Recognition of bio-relevant dicarboxylate anions by an azacalix[2]arene[2]triazine derivative decorated with urea moieties
    Santos, Miguel M.; Marques, Igor; Carvalho, Silvia; Moiteiro, Cristina; Felix, Vitor
    Organic & Biomolecular Chemistry (2015), 13 (10), 3070-3085CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)
    A new dichloroazacalix[2]arene[2]triazine receptor with two chiral urea binding groups is reported. The binding affinity of this macrocycle was evaluated by 1H NMR titrns. in CDCl3 for the dicarboxylate anions oxalate (ox2-), malonate (mal2-), succinate (suc2-), glutarate (glu2-), diglycolate (dg2-), fumarate (fum2-), maleate (male2-), and (R,R)- and (S,S)-tartrate (tart2-) enantiomers. Among the first five linear anions, the higher assocn. consts. were calcd. for the larger anions glu2- and dg2- and for the smallest anion ox2-, with Kass values following the sequence dg2- > glu2- > ox2- > suc2- > mal2-. Despite the high binding affinity for both tart2- enantiomers, no enantioselectivity was obsd. By contrast, Kass for fum2- is ca. 8.9 times greater than that for male2-, showing the selectivity of 1 for the trans isomer. These binding preferences were further elucidated by theor. methods. Mol. dynamics simulations showed that the linear anions are lodged between both pendant arms and that each anion can assume two distinct binding poses, with one or two carboxylate groups establishing intermittent hydrogen bonds with both urea binding units. On the other hand, the recognition of male2- by this azacalix[2]arene[2]triazine deriv. ensues in an alternative scenario, characterized by the interaction between a carboxylate group and a single urea binding unit, mirroring the lower exptl. binding affinity relatively to fum2-. A linear increase of the receptor's Nurea···Nurea and the anions' -O2C···CO2- distances vs. exptl. Kass was established for mal2-, suc2-, glu2- and dg2- assocns., indicating that the match between these two distances dets. the anion binding strength. The affinity for ox2- was assocd. with the most neg. values of electrostatic potential positioned near carboxylate groups. Dicarboxylic acid salts included ethanedioic acid ammonium salt (1:2) (diammonium oxalate), propanedioic acid ammonium salt (1:2) (diammonium malonate), butanedioic acid ammonium salt (1:2) (diammonium succinate), pentanedioic acid ammonium salt (1:2) (diammonium glutarate), (oxy)bis[acetic acid diammonium salt, (2E)-2-butenedioic acid ammonium salt (1:2) (diammonium fumaric acid), (2Z)-2-butenedioic acid ammonium salt (1:2) (diammonium maleic acid), (2R,3R)-2,3-dihydroxybutanedioic acid ammonium salt (1:2) [diammonium L-(+)-tartrate], (2R,3R)-2,3-dihydroxybutanedioic acid ammonium salt (1:2) [diammonium L-(+)-tartrate], (2S,3S)-2,3-dihydroxybutanedioic acid ammonium salt (1:2) [diammonium D-tartrate]. The synthesis of the target compd. was achieved by a reaction of [(1S)-1-(isocyanato)ethyl]benzene with ethanolamine, formation of a urea deriv., esterification with dinitrobenzoyl chloride, redn. of nitro groups, formation of a diamine deriv. and reaction with 2,4,6-trichloro-1,3,5-triazine (cyanuric chloride) and subsequent dimerization. The title compd. thus formed was a 2,4,6,8,14,16,18,20,26,28-decaazapentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3,5,7(28),9,11,13(27),15,17,19(26),21,23-dodecaene deriv. (urea-triazine calixarene analog).
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  4. 4
    (a) Mateus, P.; Delgado, R.; André, V.; Duarte, M. T. Dicarboxylate Recognition Properties of a Dinuclear Copper(II) Cryptate. Inorg. Chem. 2015, 54, 229– 240,  DOI: 10.1021/ic502230q
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    4a
    Dicarboxylate Recognition Properties of a Dinuclear Copper(II) Cryptate
    Mateus, Pedro; Delgado, Rita; Andre, Vania; Duarte, M. Teresa
    Inorganic Chemistry (2015), 54 (1), 229-240CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A ditopic polyamine macrobicyclic compd. with biphenylmethane spacers was prepd., and its dinuclear copper(II) complex was studied as a receptor for the recognition of dicarboxylate anions of varying chain length in H2O/MeOH (50:50 (vol./vol.)) soln. The acid-base behavior of the compd., the stability consts. of its complexes with Cu2+ ion, and the assocn. consts. of the copper(II) cryptate with succinate (suc2-), glutarate (glu2-), adipate (adi2-), and pimelate (pim2-) were detd. by potentiometry at 298.2 ± 0.1 K in H2O/MeOH (50:50 (vol./vol.)) and at ionic strength 0.10 ± 0.01 M in KNO3. The assocn. consts. of the same cryptate as receptor for arom. dicarboxylate substrates, such as phthalate (ph2-), isophthalate (iph2-), and terephthalate (tph2-), were detd. through competition expts. by spectrophotometry in the UV region. Remarkably high values of assocn. consts. at 7.34-10.01 log units were found that are, to the best of the authors' knowledge, the highest values of assocn. consts. reported for the binding of dicarboxylate anions in aq. soln. A very well defined peak of selectivity was obsd. with the binding const. values increasing with the chain length and reaching the max. for substrates with four carbon atoms between the carboxylate groups. Single-crystal x-ray diffraction detns. of the cascade complexes with adi2- and tph2- assisted in the understanding of the selectivity of the cryptate toward these substrates. The Hirshfeld surface analyses of both cascade complexes suggest that the establishment of several van der Waals interactions between the substrates and the walls of the receptor also contributes to the stability of the assocns.
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    (b) Esteves, C. V.; Mateus, P.; André, V.; Bandeira, N. A. G.; Calhorda, M. J.; Ferreira, L. P.; Delgado, R. Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions. Inorg. Chem. 2016, 55, 7051– 7060,  DOI: 10.1021/acs.inorgchem.6b00945
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    4b
    Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions
    Esteves, Catarina V.; Mateus, Pedro; Andre, Vania; Bandeira, Nuno A. G.; Calhorda, Maria Jose; Ferreira, Liliana P.; Delgado, Rita
    Inorganic Chemistry (2016), 55 (14), 7051-7060CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Searching for receptors selective for the binding of dicarboxylate anions, the copper(II) complexes of the known ditopic octaazacryptand (t2pN8), derived from bistren [tren = tris(2-aminoethyl)amine] linked by p-xylyl spacers, were reexamd., with the expectation of observing a selective binding of oxalate or malonate by bridging the two copper centers of the [Cu2(t2pN8)(H2O)2]4+ receptor. Soln. studies involving the supramol. species formed by the receptor and oxalate (oxa2-), malonate (mal2-), and succinate (suc2-) anions are reported. The detd. assocn. consts. revealed the unexpected formation of a 3:1:1 Cu/t2pN8/anion stoichiometry for the cascade species with oxa2- and mal2-, and the single crystal x-ray structural characterization confirmed the presence of tricopper(II) complexes, with an unusual binding mode for the dicarboxylate anions. Each of the two copper atoms binds four nitrogen donor atoms of the t2pN8 cryptand and one addnl. hydroxide group, which bridges to the 3rd copper. The square planar environment of this one is complete with two oxygen atoms from the oxalate (or the malonate). The two copper centers bound to the tren heads are ∼6.5 Å apart, each at ∼3.5 Å from the 3rd Cu center. These studies were complemented by SQUID magnetization measurements and DFT calcns. The magnetic susceptibility measurements of the oxalate cascade complex showed a strong magnetic coupling (J = - 210 cm-1) between the Cu centers at a short distance (3.5 Å), while the coupling between the two equiv. Cu atoms (∼6.5 Å) was only -70 cm-1. This result was well reproduced by DFT calcns.
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    (c) Chakraborty, S.; Saha, S.; Lima, L. M. P.; Warzok, U.; Sarkar, S.; Akhuli, B.; Nandi, M.; Bej, S.; Adarsh, N. N.; Schalley, C. A.; Delgado, R.; Ghosh, P. Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase. J. Org. Chem. 2017, 82, 10007– 10014,  DOI: 10.1021/acs.joc.7b01431
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    4c
    Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase
    Chakraborty, Sourav; Saha, Subrata; Lima, Luis M. P.; Warzok, Ulrike; Sarkar, Sayan; Akhuli, Bidyut; Nandi, Mandira; Bej, Somnath; Adarsh, Nayarassery N.; Schalley, Christoph A.; Delgado, Rita; Ghosh, Pradyut
    Journal of Organic Chemistry (2017), 82 (19), 10007-10014CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc2-) through glutarate (glu2-), α-ketoglutarate (kglu2-), adipate (adi2-), pimelate (pim2-), suberate (sub2-), to azelate (aze2-) have shown adipate preference with assocn. const. value of K = 4900 M-1 in a H2O/DMSO (50:50 vol./vol.) binary solvent mixt. The binding const. increases from glu2- to adi2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the assocns. along with electrostatic interactions. Neg.-mode electrospray ionization of aq. solns. of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the obsd. binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the soln. state binding studies.
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  5. 5
    Curiel, D.; Más-Montoya, M.; Sánchez, G. Complexation and Sensing of Dicarboxylate Anions and Dicarboxylic Acids. Coord. Chem. Rev. 2015, 284, 19– 66,  DOI: 10.1016/j.ccr.2014.09.010
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    5
    Complexation and sensing of dicarboxylate anions and dicarboxylic acids
    Curiel, David; Mas-Montoya, Miriam; Sanchez, Guzman
    Coordination Chemistry Reviews (2015), 284 (), 19-66CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. The relevance of dicarboxylate anions and dicarboxylic acids in biol. and industrial processes requires that such species are easily detected and quantified. In this regard, the research about synthetic receptors, which can offer a straightforward anal. signal, produced significant advances in the last decades. This review describes the different structural approaches followed to achieve an efficient interaction between the receptor and the dicarboxylic species. A comprehensive revision of the literature was made which covers the results published up to early 2014.
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    and references therein

  6. 6
    (a) Fabbrizzi, L. Cryptands and Cryptates; World Scientific Publishing Europe Ltd., London.
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    There is no corresponding record for this reference.
    (b) Amendola, V.; Bergamaschi, G.; Miljkovic, A. Azacryptands as Molecular Cages for Anions and Metal Ions. Supramol. Chem. 2018, 30, 236– 242,  DOI: 10.1080/10610278.2017.1339885
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    6b
    Azacryptands as molecular cages for anions and metal ions
    Amendola, Valeria; Bergamaschi, Greta; Miljkovic, Ana
    Supramolecular Chemistry (2018), 30 (4), 236-242CODEN: SCHEER; ISSN:1029-0478. (Taylor & Francis Ltd.)
    This is a short overview on azacryptands, as mol. receptors for cations and anions. A particular attention was devoted to the results obtained by women researchers working in the field. The terms 'cryptand' and 'cryptate' were coined by Lehn. Since then, much work has been done to improve the knowledge on this class of receptors. Small azacryptands, as free bases, were found to bind a single metal ion within their cavities. When fully protonated, the same systems could also behave as selective hosts for anions, through the cooperation of H-bonding and electrostatic interactions. Proceeding to systems with larger cavities, the inclusion of two metal ions and a bridging anion was possible, forming the so-called 'cascade' complexes. Azacryptates carrying fluorescent spacers or exploiting the indicator displacement paradigm allowed the sensing of anionic species in water at micromolar concns. Moreover, immobilization on solid matrixes and surfaces yielded new materials for the solid-phase extn. of anionic pollutants and the construction of selective electrodes for analytes in water.
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    (c) Thevenet, A.; Marie, C.; Tamain, C.; Amendola, V.; Miljkovic, A.; Guillaumont, D.; Boubals, N.; Guilbaud, P. Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium. Dalton Trans. 2020, 49, 1446– 1455,  DOI: 10.1039/C9DT04314D
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    6c
    Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium
    Thevenet, Alexiane; Marie, Cecile; Tamain, Christelle; Amendola, Valeria; Miljkovic, Ana; Guillaumont, Dominique; Boubals, Nathalie; Guilbaud, Philippe
    Dalton Transactions (2020), 49 (5), 1446-1455CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Technetium is present as the pertechnetate anion in spent nuclear fuel solns., and its extn. by several extractant systems is a major problem for the liq.-liq. extn. processes used to sep. uranium and plutonium. To prevent technetium extn. into the org. phase, a complexing agent may be added to the aq. nitric acid phase to selectively bind the pertechnetate anion. Liq.-liq. extn. expts. reveal that technetium distribution ratios are considerably lowered with addn. of an azacryptand, which is a good receptor for pertechnetate anion recognition. This ligand is able to overcome the Hofmeister bias and selectively bind techetium in nitric acid soln. Coordination studies using IR and Raman spectoscopies and DFT calcns. show the formation of an inclusion complex with hydrogen bonds stabilizing the oxo-anion within the cavity. For the first time, the cage mols. were studied for an extn. process.
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  7. 7
    (a) Busschaert, N.; Caltagirone, C.; Van Rossom, W.; Gale, P. A. Applications of Supramolecular Anion Recognition. Chem. Rev. 2015, 115, 8038– 8155,  DOI: 10.1021/acs.chemrev.5b00099
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    7a
    Applications of Supramolecular Anion Recognition
    Busschaert, Nathalie; Caltagirone, Claudia; Van Rossom, Wim; Gale, Philip A.
    Chemical Reviews (Washington, DC, United States) (2015), 115 (15), 8038-8155CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. This review focuses on the applications of anion complexation in research over the past decade. This spans a wide range of areas, but for the purposes of this review are divided into sensing, extn., transport through lipid bilayers, the roles anions can play in the formation of mol. assemblies, and, finally, organocatalysis.
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    (b) Gale, P. A.; Caltagirone, C. Fluorescent and Colorimetric Sensors for Anionic Species. Coord. Chem. Rev. 2018, 354, 2– 27,  DOI: 10.1016/j.ccr.2017.05.003
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    7b
    Fluorescent and colorimetric sensors for anionic species
    Gale, Philip A.; Caltagirone, Claudia
    Coordination Chemistry Reviews (2018), 354 (), 2-27CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    The development of new fluorescent and colorimetric anion sensors is surveyed in this review including hydrogen and halogen bond donating chemosensors, charged systems, boron-based chemosensors, systems that employ anion-pi interactions and excimer formation, mol. logic gates and arrays of sensors.
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  8. 8
    (a) Kubik, S. Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors. Acc. Chem. Res. 2017, 50, 2870– 2878,  DOI: 10.1021/acs.accounts.7b00458
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    8a
    Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors
    Kubik, Stefan
    Accounts of Chemical Research (2017), 50 (11), 2870-2878CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Anion receptors often rely on coordinative or multiple ionic interactions to be active in water. In the absence of such strong interactions, anion binding in water can also be efficient, however, as demonstrated by a no. of anion receptors developed in recent years. The cyclopeptide-derived receptors comprising an alternating sequence of L-proline and 6-aminopicolinic acid subunits are an example. These cyclopeptides are neutral and, at 1st sight, can only engage in hydrogen-bond formation with an anionic substrate. Nevertheless, they even interact with strongly solvated sulfate anions in water. The intrinsic anion affinity of these cyclopeptides can be related to structural aspects of their highly preorganized concave binding site, which comprises a wall of hydrophobic proline units arranged around the peptide NH groups at the cavity base. When anions are incorporated into this cavity they can engage in hydrogen-bonding interactions to the NH groups, and complex formation also benefits from cavity dehydration. Formation of 1:1 complexes, in which an anion binds to a single cyclopeptide ring, is assocd. with only small stability consts., however, whereas significantly more stable complexes are formed if the anion is buried between two cyclopeptide mols. A major contribution to the formation of these sandwich complexes derives from the addn. of the 2nd ring to the initially formed 1:1 cyclopeptide-anion complex. This step brings the apolar proline residues of both cyclopeptides in close proximity, which causes the resulting structure to be stabilized to a large extent by hydrophobic effects. Solvent dependent binding studies provided an est. to which degree these solvent effects contribute to the overall complex stability. In these studies, bis(cyclopeptides) were used, featuring two cyclopeptide rings covalently connected via linkers that enable both rings to simultaneously interact with the anion. Bis(cyclopeptides) with addnl. solubilizing groups allowed binding studies in a wide range of solvents, including in water. The systematic anal. of the solvent dependence of anion affinity yielded a quant. correlation between complex stability and parameters relating to the solvation of the anions and solvent properties, confirming that solvent effects contribute to anion binding. The thermodn. signature of complex formation in water mirrors that of sulfate binding to a protein complex but is opposite to that of other recently described anion receptors, which also do not engage in ionic or coordinative interactions with the substrate. These receptors not only differ in terms of the thermodn. of binding from the cyclopeptides but also possess a characteristically different anion selectivity in that they prefer to bind weakly coordinating anions but fail to bind sulfate. Solvent effects likely control the anion binding of both receptors types but their impact on complex formation and anion selectivity seems to be profoundly different. Future work in the area of anion coordination chem. will benefit from the deeper understanding of these effects and how they can be controlled.
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    (b) Langton, M. J.; Serpell, C. J.; Beer, P. D. Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective. Angew. Chem. Int. Ed. 2016, 55, 1974– 1987,  DOI: 10.1002/anie.201506589
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    Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective
    Langton, Matthew J.; Serpell, Christopher J.; Beer, Paul D.
    Angewandte Chemie, International Edition (2016), 55 (6), 1974-1987CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The recognition of anions in water remains a key challenge in modern supramol. chem., and is essential if proposed applications in biol., medical, and environmental arenas that typically require aq. conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water was achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C-H hydrogen bonding and halogen bonding. The authors also look beyond the field of small-mol. recognition into the macromol. domain, covering recent advances in anion recognition based on biomols., polymers, and nanoparticles.
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    and references therein

  9. 9
    Carnegie, R. S.; Gibb, C. L. D.; Gibb, B. C. Anion Complexation and the Hofmeister Effect. Angew. Chem. Int. Ed. 2014, 53, 11498– 11500,  DOI: 10.1002/anie.201405796
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    9
    Anion Complexation and The Hofmeister Effect
    Carnegie, Ryan S.; Gibb, Corinne L. D.; Gibb, Bruce C.
    Angewandte Chemie, International Edition (2014), 53 (43), 11498-11500CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The 1H NMR spectroscopic anal. of the binding of the ClO4- anion to the hydrophobic, concave binding site of a deep-cavity cavitand is presented. The strength of assocn. between the host and the ClO4- anion is controlled by both the nature and concn. of co-salts in a manner that follows the Hofmeister series. A model that partitions this trend into the competitive binding of the co-salt anion to the hydrophobic pocket of the host and counterion binding to its external carboxylate groups successfully accounts for the obsd. changes in ClO4- affinity.
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  10. 10
    Jagleniec, D.; Dobrzycki, Ł.; Karbarz, M.; Romański, J. Ion-Pair Induced Supramolecular Assembly Formation for Selective Extraction and Sensing of Potassium Sulfate. Chem. Sci. 2019, 10, 9542– 9547,  DOI: 10.1039/C9SC02923K
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    10
    Ion-pair induced supramolecular assembly formation for selective extraction and sensing of potassium sulfate
    Jagleniec, Damian; Dobrzycki, Lukasz; Karbarz, Marcin; Romanski, Jan
    Chemical Science (2019), 10 (41), 9542-9547CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Selective extn. of sulfates in the form of alkali metal salts using charge-neutral mol. receptors is one of the holy grails of supramol. chem. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped with a crown ether site that is able to ext. potassium sulfate from the aq. to the org. phase (an analogus monotopic anion receptor lacking the crown ether unit lacks this ability). A 1H NMR, UV-vis, DOSY-NMR, DLS, and MS expts. and the solid-state single crystal structure provided evidence of the formation of a supramol. core-shell like assembly upon interaction of the receptor with potassium sulfate. The presence of monovalent potassium salts, in contrast, promoted the formation of simple 1 : 1 complexes. Unlike the 4 : 1 assembly, the 1 : 1 complexes are poorly sol. in org. media. This feature was utilized to overcome the Hofmeister bias and allow for selective extn. of extremely hydrophilic sulfates over lipophilic nitrate anions, which was unambiguously proved by quant. AES and ion chromatog. measurements. A simple modification of the receptor structure led to a "naked eye" optical sensor able to selectively detect sulfates under both SLE and LLE conditions.
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  11. 11
    Wichmann, K.; Antonioli, B.; Söhnel, T.; Wenzel, M.; Gloe, K.; Gloe, K.; Price, J. R.; Lindoy, L. F.; Blake, A. J.; Schröder, M. Polyamine-based anion receptors: Extraction and structural studies. Coord. Chem. Rev. 2006, 250, 2987– 3003,  DOI: 10.1016/j.ccr.2006.07.010
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    11
    Polyamine-based anion receptors: Extraction and structural studies
    Wichmann, Kathrin; Antonioli, Bianca; Soehnel, Tilo; Wenzel, Marco; Gloe, Kerstin; Gloe, Karsten; Price, Jason R.; Lindoy, Leonard F.; Blake, Alexander J.; Schroeder, Martin
    Coordination Chemistry Reviews (2006), 250 (23+24), 2987-3003CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. In the discussion that follows some of the more recent progress in the area of anion binding by synthetic polyamine receptors is presented, with emphasis given to work undertaken by the authors' groups. A continuing theme in these studies has been the relationship between receptor structure and its anion extn. properties. Systematic solvent extn. and structural studies for halide and perrhenate complexes with polyamines of tripodal, macrocyclic and macrobicyclic architecture that contain both arom. moieties and four to eight amine functions have been performed in order to derive relevant structure-binding/extractability relationships. The results demonstrate that the binding and extn. behavior of the polyamines towards halides and perrhenate is a complex function of their structural features, degree of protonation and lipophilic properties. The extn. is characterized by the preferred formation of mono- and diprotonated amine species in the org. phase. X-ray structure studies of iodide and perrhenate complexes with open-chain tetraamino derivs. and octaamino cryptands in different protonation states lead to the conclusion that in the first case only limited chelation of the anion occurs and in the second only highly protonated species are able to encapsulate the anion. The structural patterns obsd. are strongly influenced by the presence of water mols. in the crystals.
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  12. 12
    (a) Wu, X.; Howe, E. N. W.; Gale, P. A. Supramolecular Transmembrane Anion Transport: New Assays and Insights. Acc. Chem. Res. 2018, 51, 1870– 1879,  DOI: 10.1021/acs.accounts.8b00264
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    12a
    Supramolecular Transmembrane Anion Transport: New Assays and Insights
    Wu, Xin; Howe, Ethan N. W.; Gale, Philip A.
    Accounts of Chemical Research (2018), 51 (8), 1870-1879CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Transmembrane anion transport has been the focus of a no. of supramol. chem. research groups for a no. of years. Much of this research is driven by the biol. relevance of anion transport and the search to find new treatments for diseases such as cystic fibrosis, which is caused by genetic problems leading to faulty cystic fibrosis transmembrane conductance regulator (CFTR) channels, which in turn lead to reduced chloride and bicarbonate transport through epithelial cell membranes. Considerable effort has been devoted to the development of new transporters, and the group along with others have been searching for combinations of org. scaffolds and anion binding groups that produce highly effective transporters that work at low concn. These compds. may be used in the future as "channel replacement therapies", restoring the flux of anions through epithelial cell membranes and ameliorating the symptoms of cystic fibrosis. Less effort has been put into gaining a fundamental understanding of anion transport processes. Over the last 3 years, the group has developed a no. of new transport assays that allow anion transport mechanisms to be detd. This Account covers the latest developments in this area, providing a concise review of the new techniques the authors can use to study anion transport processes individually without resorting to measurement of exchange processes and the new insights that these assays provide. The Account provides an overview of the effects of anion transporters on cells and an explanation of why many systems perturb pH gradients within cells in addn. to transporting chloride. The authors discuss assays to det. whether anionophores facilitate chloride or HCl transport and how this latter assay can be modified to det. chloride vs. proton selectivity in small-mol. anion receptors. The authors show how mol. design can be used to produce receptors that are capable of transporting chloride without perturbing pH gradients. The authors cover the role that anion transporters in the presence of fatty acids play in dissipating pH gradients across lipid bilayer membranes and the effect that this process has on chloride-selective transport. The authors also discuss how coupling of anion transport to cation transport by natural cationophores can be used to det. whether anion transport is electrogenic or electroneutral. In addn., the authors compare these new assays to the previously used chloride/nitrate exchange assay and show how this exchange assay can underestimate the chloride transport ability of certain receptors that are rate-limited by nitrate transport.
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    and references therein

    (b) Grauwels, G.; Valkenier, H.; Davis, A. P.; Jabin, I.; Bartik, K. Repositioning Chloride Transmembrane Transporters: Transport of Organic Ion Pairs. Angew. Chem. Int Ed. 2019, 58, 6921– 6925,  DOI: 10.1002/anie.201900818
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    12b
    Repositioning chloride transmembrane transporters: Transport of organic ion pairs
    Grauwels, Glenn; Valkenier, Hennie; Davis, Anthony P.; Jabin, Ivan; Bartik, Kristin
    Angewandte Chemie, International Edition (2019), 58 (21), 6921-6925CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Given the biol. importance of org. cations, the facilitated transport of org. ion pairs could find many applications. Calix[6]arene tris(thio)ureas, which possess a cavity that can accommodate primary ammonium ions, can not only act as carriers for Cl-/NO3- antiport but can also perform the cotransport of PrNH3Cl. Transport was monitored by fluorescence spectroscopy and the presence of the different species inside the vesicles was characterized by 1H and 35Cl NMR expts. involving shift reagents. The cotransport of PrNH3Cl was also obsd. by receptors deprived of a cavity, but the presence of the cavity conveys an advantage, as the cotransport by calix[6]arenes was obsd. to be more efficient than the Cl-/NO3- antiport, which is not the case with receptors without a cavity. The role played by the cavity was further highlighted by the disappearance of this advantage when using a bulky ammonium ion, which cannot be complexed within the cavity.
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  13. 13
    Bowman-James, K. Supramolecular Cages Trap Pesky Anions. Science 2019, 365, 124– 125,  DOI: 10.1126/science.aax9369
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    13
    Supramolecular cages trap pesky anions
    Bowman-James, Kristin
    Science (Washington, DC, United States) (2019), 365 (6449), 124-125CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The design of cryptands-org. mols. that can capture simple inorg. and org. ions from soln. through multiple weak interactions-can be quite challenging, esp. when the task is to capture a specific ion with high affinity. In many instances, targeted recognition is necessary to remove an unwanted species from some site because of its overabundance, to remove contamination, or even to capture it for its economic value (e.g., gold from seawater) (). Neg. charged ions, esp. smaller anions such as Cl-, are esp. troublesome. Greater energies are generally needed to peel away their more tightly held hydration shells relative to larger anions of the same charge, such as I-, which has a more diffuse charge cloud. Likewise, anions possess higher free energies of hydration than cations of similar size and charge. On page 159 of this issue, Liu et al. () report the design of a bicyclic cryptand that specifically recognizes Cl- ions with high affinity. Their results build on a long history of increasingly complex cryptand cages specifically designed to target anions (see the figure).
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  14. 14
    Liu, Y.; Zhao, W.; Chen, C.-H.; Flood, A. H. Chloride Capture Using a C–H Hydrogen Bonding Cage. Science 2019, 365, 159– 161,  DOI: 10.1126/science.aaw5145
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    14
    Chloride capture using a C-H hydrogen-bonding cage
    Liu, Yun; Zhao, Wei; Chen, Chun-Hsing; Flood, Amar H.
    Science (Washington, DC, United States) (2019), 365 (6449), 159-161CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Tight binding and high selectivity are hallmarks of biomol. recognition. These behaviors with synthetic receptors has usually been assocd. with OH and NH hydrogen bonding. Contrary to this conventional wisdom, we designed a chlorideselective receptor in the form of a cryptand-like cage using only CH hydrogen bonding. Crystallog. showed chloride stabilized by six short 2.7-angstrom hydrogen bonds originating from the cage's six 1,2,3-triazoles. Attomolaraffinity (1017 M-1) was detd. using liq.-liq. extns. of chloride from water into nonpolar dichloromethane solvents. Controls verified the addnl. role of triazoles in rigidifying the three-dimensional structure to effect recognition affinity and selectivity: Cl- > Br- > NO3- > I-. This cage shows anti-Hofmeister salt extn. and corrosion inhibition.
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  15. 15
    (a) He, Q.; Williams, N. J.; Oh, J. H.; Lynch, V. M.; Kim, S. K.; Moyer, B. A.; Sessler, J. L. Selective Solid–Liquid and Liquid–Liquid Extraction of Lithium Chloride Using Strapped Calix[4]Pyrroles. Angew. Chem. Int Ed. 2018, 57, 11924– 11928,  DOI: 10.1002/anie.201805127
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    15a
    Selective Solid-Liquid and Liquid-Liquid Extraction of Lithium Chloride Using Strapped Calix[4]pyrroles
    He, Qing; Williams, Neil J.; Oh, Ju Hyun; Lynch, Vincent M.; Kim, Sung Kuk; Moyer, Bruce A.; Sessler, Jonathan L.
    Angewandte Chemie, International Edition (2018), 57 (37), 11924-11928CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    LiCl is a classic "hard" ion salt that is present in lithium-rich brines and a key component in end-of-life materials (i.e., used lithium-ion batteries). Its isolation and purifn. from like salts is a recognized challenge with potential strategic and economic implications. Herein, we describe two ditopic calix[4]pyrrole-based ion-pair receptors (2 and 3, see Fig. 1 in text of paper), that are capable of selectively capturing LiCl. Under solid-liq. extn. conditions, using 2 as the extractant, LiCl could be sepd. from a NaCl/KCl salt mixt. contg. as little as 1 % LiCl with circa 100 % selectivity, while receptor 3 achieved similar sepns. when the LiCl level was as low as 200 ppm. Under liq.-liq. extn. conditions using nitrobenzene as the non-aq. phase, the extn. preference displayed by 2 is KCl>NaCl>LiCl. In contrast, 3 exhibits high selectivity towards LiCl over NaCl and KCl, with no appreciable extn. being obsd. for the latter two salts.
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    (b) Ji, X.; Wu, R.-T.; Long, L.; Guo, C.; Khashab, N. M.; Huang, F.; Sessler, J. L. Physical Removal of Anions from Aqueous Media by Means of a Macrocycle-Containing Polymeric Network. J. Am. Chem. Soc. 2018, 140, 2777– 2780,  DOI: 10.1021/jacs.7b13656
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    15b
    Physical Removal of Anions from Aqueous Media by Means of a Macrocycle-Containing Polymeric Network
    Ji, Xiaofan; Wu, Ren-Tsung; Long, Lingliang; Guo, Chenxing; Khashab, Niveen M.; Huang, Feihe; Sessler, Jonathan L.
    Journal of the American Chemical Society (2018), 140 (8), 2777-2780CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Reported here is a hydrogel-forming polymer network that contains a water-sol. tetracationic macrocycle. Upon immersion of this polymer network in aq. solns. contg. various inorg. and org. salts, changes in the phys. properties are obsd. that are consistent with absorption of the constituent anions into the polymer network. This absorption is ascribed to host-guest interactions involving the tetracationic macrocyclic receptor. Removal of the anions may then be achieved by lifting the resulting hydrogels out of the aq. phase. Treating the anion-contg. hydrogels with dil. HCl leads to the protonation-induced release of the bound anions. This allows the hydrogels to be recycled for reuse. The present polymer network thus provides a potentially attractive approach to removing undesired anions from aq. environments.
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    (c) He, Q.; Vargas-Zúñiga, G. I.; Kim, S. H.; Kim, S. K.; Sessler, J. L. Macrocycles as Ion Pair Receptors. Chem. Rev. 2019, 119, 9753– 9835,  DOI: 10.1021/acs.chemrev.8b00734
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    15c
    Macrocycles as Ion Pair Receptors
    He, Qing; Vargas-Zuniga, Gabriela I.; Kim, Seung Hyun; Kim, Sung Kuk; Sessler, Jonathan L.
    Chemical Reviews (Washington, DC, United States) (2019), 119 (17), 9753-9835CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Cation and anion recognition have both played central roles in the development of supramol. chem. Much of the assocd. research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chem., emerging from cation and anion coordination chem., is a specific research field where cocomplexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chem. of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extn., ion transport, and logic gate construction.
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    and references therein

  16. 16
    Fowler, C. J.; Haverlock, T. J.; Moyer, B. A.; Shriver, J. A.; Gross, D. E.; Marquez, M.; Sessler, J. L.; Hossain, M. A.; Bowman-James, K. Enhanced Anion Exchange for Selective Sulfate Extraction: Overcoming the Hofmeister Bias. J. Am. Chem. Soc. 2008, 130, 14386– 14387,  DOI: 10.1021/ja806511b
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    16
    Enhanced Anion Exchange for Selective Sulfate Extraction: Overcoming the Hofmeister Bias
    Fowler, Christopher J.; Haverlock, Tamara J.; Moyer, Bruce A.; Shriver, James A.; Gross, Dustin E.; Marquez, Manuel; Sessler, Jonathan L.; Hossain, Md. Alamgir; Bowman-James, Kristin
    Journal of the American Chemical Society (2008), 130 (44), 14386-14387CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In this communication, a new approach to enhancing the efficacy of liq.-liq. anion exchange is demonstrated. It involves the concurrent use of appropriately chosen hydrogen-bond-donating (HBD) anion receptors in combination with a traditional quaternary ammonium extractant. The fluorinated calixpyrroles and the tetraamide macrocycle were found to be particularly effective receptors. Specifically, their use allowed the extn. of sulfate by tricaprylmethylammonium nitrate to be effected in the presence of excess nitrate. As such, the present work provides a rare demonstration of overcoming the Hofmeister bias in a competitive environment and the first to the authors' knowledge wherein this difficult-to-achieve objective is attained using a neutral HBD-based anion binding agent under conditions of solvent extn.
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  17. 17
    Qin, L.; Vervuurt, S. J. N.; Elmes, R. B. P.; Berry, S. N.; Proschogo, N.; Jolliffe, K. A. Extraction and transport of sulfate using macrocyclic squaramide receptors. Chem. Sci. 2020, 11, 201– 207,  DOI: 10.1039/C9SC04786G
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    17
    Extraction and transport of sulfate using macrocyclic squaramide receptors
    Qin, Lei; Vervuurt, Sacha J. N.; Elmes, Robert B. P.; Berry, Stuart N.; Proschogo, Nicholas; Jolliffe, Katrina A.
    Chemical Science (2020), 11 (1), 201-207CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The selective extn. of the hydrophilic sulfate ion from water is highly challenging because the high free energy of hydration of this ion makes it more difficult to ext. than less hydrophilic ions such as chloride and nitrate. Lipophilic macrocyclic squaramide receptors 1 and 2 were synthesized. Receptor 2 efficiently extd. sulfate from aq. sodium sulfate solns. into a chloroform phase, via exchange with nitrate ions, overcoming the Hofmeister bias. The resulting 2·SO42- complex was readily recycled through pptn. of BaSO4. Transport of sulfate across a bulk chloroform membrane by 2 was demonstrated across a wide pH range (pH 3.2-9.4) and in the presence of high concns. of competing anions (chloride, nitrate and dihydrogenphosphate), opening the door to the use of 2 for the selective removal of sulfate from water across a range of applications.
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  18. 18
    Baragaña, B.; Blackburn, A. G.; Breccia, P.; Davis, A. P.; de Mendoza, J.; Padrón-Carrillo, J. M.; Prados, P.; Riedner, J.; de Vries, J. G. Enantioselective Transport by a Steroidal Guanidinium Receptor. Chem. - Eur. J. 2002, 8, 2931– 2936,  DOI: 10.1002/1521-3765(20020703)8:13<2931::AID-CHEM2931>3.0.CO;2-H
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    18
    Enantioselective transport by a steroidal guanidinium receptor
    Baragana, Beatriz; Blackburn, Adrian G.; Breccia, Perla; Davis, Anthony P.; De Mendoza, Javier; Padron-Carrillo, Jose M.; Prados, Pilar; Riedner, Jens; De Vries, Johannes G.
    Chemistry - A European Journal (2002), 8 (13), 2931-2936CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)
    The cationic steroidal receptors I (R = OMe, OC20H41) have been synthesized from cholic acid. Receptor I (R = MeO) exts. N-acetyl-α-amino acids from aq. media into chloroform with enantioselectivities (L:D) of 7-10:1. The lipophilic variant I (R = OC20H41) has been employed for the enantioselective transport of N-acetylphenylalanine, (a) through dichloromethane (DCM) and dichloroethane (DCE) bulk liq. membranes (U-tube app.), and (b) through 2.5% (vol./vol.) octanol/hexane via hollow fiber membrane contactors. Significant enantioselectivities and multiple turnovers were obsd. for both types of app.
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    Urban, C.; Schmuck, C. Active Transport of Amino Acids by a Guanidiniocarbonyl-Pyrrole Receptor. Chem. - Eur. J. 2010, 16, 9502– 9510,  DOI: 10.1002/chem.201000509
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    19
    Active Transport of Amino Acids by a Guanidiniocarbonyl-Pyrrole Receptor
    Urban, Christian; Schmuck, Carsten
    Chemistry - A European Journal (2010), 16 (31), 9502-9510CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein the authors report the synthesis and characterization of a guanidiniocarbonyl-pyrrole deriv. I as transporter for N-acetylamino acids. I is a conjugate of a guanidiniocarbonyl pyrrole cation, one of the most efficient carboxylate binding motifs reported so far, and a hydrophobic tris(dodecylbenzyl) group, which ensures soly. in org. solvents. In its protonated form, I binds N-acetylamino acid carboxylates in wet org. solvents with assocn. consts. in the range of 104 M-1 as estd. by extn. expts. Arom. amino acids are preferred due to addnl. cation-π-interactions of the amino acid side chain with the guanidiniocarbonyl pyrrole moiety. U-tube expts. established efficient transport across a bulk liq. chloroform phase with fluxes approaching 10-6 mol m-2 s-1. In expts. with single substrates, the release rate of the amino acid from the receptor-substrate complex at the interface with the receiving phase is rate detg. In contrast to this, in competition expts. with several substrates, the thermodn. affinity to I becomes decisive. As I can only transport anions in its protonated form and has pKa ≈ 7, pH-driven active transport of amino acids is also possible. Transport occurs as a symport of the amino acid carboxylate and a proton.
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    Xie, G.-Y.; Jiang, L.; Lu, T.-B. Discrimination of Cis-Trans Isomers by Dinuclear Metal Cryptates at Physiological PH: Selectivity for Fumarate vs Maleate. Dalton Trans. 2013, 42, 14092– 14099,  DOI: 10.1039/c3dt51501j
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    20
    Discrimination of cis-trans isomers by dinuclear metal cryptates at physiological pH: selectivity for fumarate vs. maleate
    Xie, Gao-Yi; Jiang, Long; Lu, Tong-Bu
    Dalton Transactions (2013), 42 (39), 14092-14099CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Cryptand L (L = N[(CH2)2NHCH2(2,6-C10H6)CH2NH(CH2)2]3N) and its dinuclear metal cryptates [Zn2L](NO3)4 (1) and [Cu2L](ClO4)4 (2) were prepd., and the binding properties of the cryptates with fumarate and its cis isomer maleate were studied using fluorescent spectra, 1H NMR titrns. and single crystal x-ray diffraction anal. for [(Cu2L)(fum)][ClO4]2 (3) (fum = fumarate). Thanks to the size and shape matching effect, the cryptates can selectively recognize fumarate at physiol. pH, with an assocn. const. almost 18-fold larger than that of maleate, forming a cradle-like cascade complex.
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    (a) Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn, E. V. Teaching Old Indicators New Tricks. Acc. Chem. Res. 2001, 34, 963– 972,  DOI: 10.1021/ar9600796
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    Teaching old indicators new tricks
    Wiskur, Sheryl L.; Ait-Haddou, Hassan; Lavigne, John J.; Anslyn, Eric V.
    Accounts of Chemical Research (2001), 34 (12), 963-972CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Most synthetic sensors are designed with covalent attachment between a receptor and a reporter moiety. The authors describe the current progress of the authors' use of noncovalently attached indicators to signal binding of analytes. With these systems, analyte binding leads to indicator displacement from the binding cavity, which in turn yields an optical signal modulation. The authors include previous examples, the strategies involved in the authors' development, and the advantages as well as disadvantages of this method. Finally, the authors' latest research in this field is briefly presented.
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    (b) Anzenbacher, P., Jr.; Lubal, P.; Buček, P.; Palacios, M. A.; Kozelkova, M. E. A practical approach to optical cross-reactive sensor arrays. Chem. Soc. Rev. 2010, 39, 3954– 3979,  DOI: 10.1039/b926220m
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    21b
    A practical approach to optical cross-reactive sensor arrays
    Anzenbacher, Pavel, Jr.; Lubal, Premysl; Bucek, Pavel; Palacios, Manuel A.; Kozelkova, Maria E.
    Chemical Society Reviews (2010), 39 (10), 3954-3979CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Supramol. anal. chem. has emerged as a new discipline at the interface of supramol. and anal. chem. It focuses on anal. applications of mol. recognition and self-assembly. One of the important outcomes of the supramol. anal. chem. is the understanding of mol. aspects of sensor design, synthesis and binding studies of sensors while using rigorous methods of anal. chem. as a touchstone to verify the viability of the supramol. aspects of the sensor design. This crit. review provides a simplified version of the chemometric procedures involved in realizing a successful anal. expt. that utilizes cross-reactive optical sensor arrays, and summarizes the current research in this field. This review also shows several examples of use of described chemometric methods for evaluation of chemosensors and sensor arrays. Thus, this review is aimed mostly at the readers who want to test their newly-developed chemosensors in cross-reactive arrays (169 refs.).
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    (c) Nguyen, B. T.; Anslyn, E. V. Coord. Chem. Rev. 2006, 250, 3118– 3127,  DOI: 10.1016/j.ccr.2006.04.009
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    Indicator-displacement assays
    Nguyen, Binh T.; Anslyn, Eric V.
    Coordination Chemistry Reviews (2006), 250 (23+24), 3118-3127CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Indicator displacement assays (IDAs) are now a popular method for converting most any synthetic receptor into an optical sensor. In this review many such assays are highlighted, along with biol. counterparts. The focus is upon colorimetric, fluorescent, and metal contg. IDAs. The power of the method can be readily appreciated by the large diversity of analytes that have been targeted with this technique. It is clear that the method is now well accepted and will continue to be one of many methods used to create optical detection methods from synthetic receptors.
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    (d) Mako, T. L.; Racicot, J. M.; Levine, M. Supramolecular Luminescent Sensors. Chem. Rev. 2019, 119, 322– 477,  DOI: 10.1021/acs.chemrev.8b00260
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    21d
    Supramolecular Luminescent Sensors
    Mako, Teresa L.; Racicot, Joan M.; Levine, Mindy
    Chemical Reviews (Washington, DC, United States) (2019), 119 (1), 322-477CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramol. luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramol. chem. as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examd., and some remaining unsolved challenges in the area of chemosensors are discussed.
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    (e) Aletti, A. B.; Gillen, D. M.; Gunnlaugsson, T. Luminescent/Colorimetric Probes and (Chemo-) Sensors for Detecting Anions Based on Transition and Lanthanide Ion Receptor/Binding Complexes. Coord. Chem. Rev. 2018, 354, 98– 120,  DOI: 10.1016/j.ccr.2017.06.020
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    21e
    Luminescent/colorimetric probes and (chemo-) sensors for detecting anions based on transition and lanthanide ion receptor/binding complexes
    Aletti, Anna B.; Gillen, Dermot M.; Gunnlaugsson, Thorfinnur
    Coordination Chemistry Reviews (2018), 354 (), 98-120CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Herein, some of the recent developments in the design and study of transition metal (d-metal) and lanthanide (f-metal) based sensors, probes and reagents for detecting anions in org. or competitive media will be reviewed. Some examples of main group and actinide-based sensors are also featured. In all cases, the anion recognition is probed by monitoring changes in the various photophys. properties of these complexes, with particular focus being paid to recent examples from the literature where the anion recognition event is communicated through colorimetric or luminescent (fluorescence or phosphorescence) changes. A select no. of examples reported within the last 5 years (since 2011) are featured; the focus of this review is on those developed from org. ligands that can, in a synergetic manner with the metal ions, directly aid or enhance the anion recognition and sensing processes. Examples where such synergy is provided by hydrogen bonding interactions are particularly discussed.
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    (f) Minami, T.; Liu, Y.; Akdeniz, A.; Koutnik, P.; Esipenko, N. A.; Nishiyabu, R.; Kubo, Y.; Anzenbacher, P., Jr. Intramolecular Indicator Displacement Assay for Anions: Supramolecular Sensor for Glyphosate. J. Am. Chem. Soc. 2014, 136, 11396– 11401,  DOI: 10.1021/ja504535q
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    21f
    Intramolecular Indicator Displacement Assay for Anions: Supramolecular Sensor for Glyphosate
    Minami, Tsuyoshi; Liu, Yuanli; Akdeniz, Ali; Koutnik, Petr; Esipenko, Nina A.; Nishiyabu, Ryuhei; Kubo, Yuji; Anzenbacher, Pavel
    Journal of the American Chemical Society (2014), 136 (32), 11396-11401CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    One of the well-known strategies for anion sensing is an indicator (dye) displacement assay. However, the disadvantage of the dye displacement assays is the low sensitivity due to the excess of the dye used. To overcome this setback, we have developed an "Intramol. Indicator Displacement Assay (IIDA)". The IIDAs comprise a receptor and a spacer with an attached anionic chromophore in a single-mol. assembly. In the resting state, the environment-sensitive anionic chromophore is bound by the receptor, while the anionic substrate competes for binding into the receptor. The photophys. properties of the dye exhibit change in fluorescence when displaced by anions, which results in cross-reactive response. To illustrate the concept, we have prepd. IID sensors (I) and (II). Here, the characterization of sensors and microtiter arrays comprising the IIDA are reported. The microtiter array including IID sensors I and II is capable of recognizing biol. phosphates in water. The utility of the IIDA approach is demonstrated on sensing of a phosphonate herbicide glyphosate and other biol. important anions such as pyrophosphate in the presence of interferent sodium chloride.
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  22. 22
    (a) Boiocchi, M.; Bonizzoni, M.; Fabbrizzi, L.; Piovani, G.; Taglietti, A. A Dimetallic Cage with a Long Ellipsoidal Cavity for the Fluorescent Detection of Dicarboxylate Anions in Water. Angew. Chem., Int. Ed. 2004, 43, 3847– 3852,  DOI: 10.1002/anie.200460036
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    22a
    A dimetallic cage with a long ellipsoidal cavity for the fluorescent detection of dicarboxylate anions in water
    Boiocchi, Massimo; Bonizzoni, Marco; Fabbrizzi, Luigi; Piovani, Giulio; Taglietti, Angelo
    Angewandte Chemie, International Edition (2004), 43 (29), 3847-3852CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The dicopper(II) complex of a bistren cage contg. ditolyl spacers (I) binds dicarboxylate anions in aq. solns. The inclusion is highly selective from the distance between the COO- groups. For example, terephthalate is discriminated from isophthalate and phthalate.
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    (b) Merli, D.; La Cognata, S.; Balduzzi, F.; Miljkovic, A.; Toma, L.; Amendola, V. A Smart Supramolecular Device for the Detection of t,t-Muconic Acid in Urine†. New J. Chem. 2018, 42, 15460– 15465,  DOI: 10.1039/C8NJ02156B
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    22b
    A smart supramolecular device for the detection of t,t-muconic acid in urine
    Merli, Daniele; La Cognata, Sonia; Balduzzi, Federica; Miljkovic, Ana; Toma, Lucio; Amendola, Valeria
    New Journal of Chemistry (2018), 42 (18), 15460-15465CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
    The authors combined smartphone sensing with the indicator displacement approach for the detn. of trans,trans-muconic acid (i.e.tt-MA), a benzene biomarker, in urine. Dicopper(II) azacryptate was employed as the receptor. The good match of tt-MA with the receptor cavity was suggested by DFT calcns., and confirmed exptl. by UV-visible titrns. in buffered aq. solns. The binding const. was detd. through fluorometric titrns. using the indicator displacement approach. This method also confirmed the high affinity of the azacryptate for tt-MA in artificial and spiked urine samples. A portable device was then obtained by adsorption of the chemosensing ensemble on silica gel using ELISA-like plastic wells as the support. Indicator displacement was detected and quantified by exposing the microplate to a UV-lamp (366 nm) and recording the RGB values with a smartphone. A good correlation was found between the R index values and urinary tt-MA in the occupational concn. range.
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    (c) Miljkovic, A.; La Cognata, S.; Bergamaschi, G.; Freccero, M.; Poggi, A.; Amendola, V. Towards Building Blocks for Supramolecular Architectures Based on Azacryptates. Molecules 2020, 25, 1733,  DOI: 10.3390/molecules25071733
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    22c
    Towards building blocks for supramolecular architectures based on azacryptates
    Miljkovic, Ana; Cognata, Sonia La; Bergamaschi, Greta; Freccero, Mauro; Poggi, Antonio; Amendola, Valeria
    Molecules (2020), 25 (7), 1733CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
    In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with tri-Ph spacers. The binding properties of its dicopper complex for arom. dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramol. structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu2L]4+ in water/acetonitrile mixt. (pH = 7) showed a high affinity for biphenyl-4,40 -dicarboxylate (dfc2-), with a binding const. of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage's cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu2L]4+ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc2-) and short anions (e.g., terephthalate). The good affinity of [Cu2L]4+ for these dicarboxylates, together with hydrophobic interactions within the cage's cavity, may promote the self-assembly of a stable 1:1 complex in water mixt. These results represent a good starting point for the application of these mol. systems as building units for the design of new supramol. architectures based on non-covalent interactions, which could be of interest in all fields related to supramol. devices.
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  23. 23
    Mobili, R.; La Cognata, S.; Merlo, F.; Speltini, A.; Boiocchi, M.; Recca, T. Liquid-liquid extraction of succinate using a dicopper cryptate. ChemRxiv 2020,  DOI: 10.26434/chemrxiv.11786784.v1
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    Gans, P.; Sabatini, A.; Vacca, A. Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs. Talanta 1996, 43, 1739– 1753,  DOI: 10.1016/0039-9140(96)01958-3
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    Investigation of equilibria in solution. Determination of equilibrium constants with HYPERQUAD suite of programs
    Gans, Peter; Sabatini, Antonio; Vacca, Alberto
    Talanta (1996), 43 (10), 1739-1753CODEN: TLNTA2; ISSN:0039-9140. (Elsevier)
    A new series of 10 programs for the study of equil. consts. and soln. equil. is described. The programs include data prepn., pretreatment, equil. const. refinement and post-run anal. Data prepn. is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point detn., absorbance error detn., spectral baseline corrections, factor anal. and detn. of molar absorbance spectra. Equil. consts. can be detd. from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on exptl. measurements are kept in sep. files.
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    Duggan, M.; Ray, N.; Hathaway, B.; Tomlinson, G.; Brint, P.; Pelin, K. Crystal Structure and Electronic Properties of Ammine[tris(2-aminoethyl)amine]copper(I) Diperchlorate and Potassium Penta-amminecopper(II) Tris( hexafluorophosphate). J. Chem. Soc., Dalton Trans. 1980, 1342– 1348,  DOI: 10.1039/dt9800001342
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    Crystal structure and electronic properties of ammine[tris(2-aminoethyl)amine]copper(II) diperchlorate and potassium pentaamminecopper(II) tris(hexafluorophosphate)
    Duggan, Mary; Ray, Noel; Hathaway, Brian; Tomlinson, Gustav; Brint, Paul; Pelin, Kevin
    Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1980), (8), 1342-8CODEN: JCDTBI; ISSN:0300-9246.
    The crystal and mol. structures of [CuL(NH3)][ClO4]2 [L = N[(CH2)2NH2]3] and K[Cu(NH3)5][PF6]3 were detd. by x-ray crystallog. using 3-dimensional diffraction data and were refined to R 0.0401 and 0.0888 for 649 and 619 unique reflections, resp. Crystals of [CuL(NH3)][ClO4]2 are cubic, space group P213, with a 11.626 Å and Z = 4 whereas crystals of K[Cu(NH3)5][PF6]3 are orthorhombic, space group Imma, with a 14.90, b 11.79, c 10.57 Å, and Z = 6. [CuL(NH3)]+ is trigonal bipyramidal with strict C3 symmetry whereas [Cu(NH3)5]+ is a square-based pyramid with strict C2v symmetry. The ESR of Cu-doped [ZnL(NH3)][ClO4]2 and the ESR and visible spectra of single crystals of [CuL(NH3)][ClO4]2 and K[Cu(NH3)5][PF6]3 were also detd. and related to the 1-electron energy levels calcd. using EHMO methods and other data on high-symmetry 5-coordinate CuN5 chromophores.
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    Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349– 1356,  DOI: 10.1039/DT9840001349
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    Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulfur donor ligands: the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2'-yl)-2,6-dithiaheptane]copper(II) perchlorate
    Addison, Anthony W.; Rao, T. Nageswara; Reedijk, Jan; Van Rijn, Jacobus; Verschoor, Gerrit C.
    Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1984), (7), 1349-56CODEN: JCDTBI; ISSN:0300-9246.
    The complexes [CuL(OH2)][ClO4]2 (I) and CuL(ClO4)2.2H2O (II) of the linear quadridentate N2S2 donor ligand III (L) were prepd.; they are significantly more stable towards autoredn. than the nonmethyl analogs. The structure of I was detd. by x-ray crystallog.; results were refined to an R of 0.047 for 3343 reflections. The Cu coordination is intermediate between trigonal bipyramidal and square pyramidal. In the solid state the coordination sphere in II may be a topoisomer of I. A new angular structural parameter, τ, is defined as an index of trigonality, a general descriptor of 5-coordinate centric mols. By this criterion the coordination of I in the solid state is described as 48% along the path of distortion from square pyramidal to trigonal bipyramidal. The S → Cu charge-transfer bands in the electronic spectrum of I are assigned. ESR and ligand field spectra show that the Cu compds. adopt a tetragonal structure in donor solvents.
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    Urban, M.; Durka, K.; Jankowski, P.; Serwatowski, J.; Luliński, S. Highly Fluorescent Red-Light Emitting Bis(boranils) Based on Naphthalene Backbone. J. Org. Chem. 2017, 82, 8234– 8241,  DOI: 10.1021/acs.joc.7b01001
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    Highly fluorescent red-light emitting bis(boranils) based on naphthalene backbone
    Urban, Mateusz; Durka, Krzysztof; Jankowski, Piotr; Serwatowski, Janusz; Lulinski, Sergiusz
    Journal of Organic Chemistry (2017), 82 (15), 8234-8241CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    Ten bis(boranils) I (1-10; R1 = F, Ph, 2,6-F2C6H3, C6F5, R1-R1 = C6H4OC6H4; R2, R3 = H, NO2, NEt2; R2 = NO2, R3 = NEt2), differently substituted at the boron atom and iminophenyl groups were synthesized from 1,5-dihydroxynaphthalene-2,6-dicarboxaldehyde using a simple one-pot protocol. Their photophys. properties can be easily tuned in a wide range by the variation of substituents. Their absorption and emission spectral bands are significantly red-shifted (λmax = 495-590 nm, λem = 533-683 nm) when compared with simple boranils, whereas fluorescence quantum yields are strongly improved to reach 83%. The attachment of pendant NO2 and NEt2 groups at the opposite positions of the π-conjugated bis(boranil) scaffold resulted in the formation of an unprecedented system featuring push-pull architecture.
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    Allen, C. R.; Richard, P. L.; Ward, A. J.; van de Water, L. G. A.; Masters, A. F.; Maschmeyer, T. Facile synthesis of ionic liquids possessing chiral carboxylates. Tetrahedron Lett. 2006, 47, 7367– 7370,  DOI: 10.1016/j.tetlet.2006.08.007
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    Facile synthesis of ionic liquids possessing chiral carboxylates
    Allen, Christine R.; Richard, Paulina L.; Ward, Antony J.; van de Water, Leon G. A.; Masters, Anthony F.; Maschmeyer, Thomas
    Tetrahedron Letters (2006), 47 (41), 7367-7370CODEN: TELEAY; ISSN:0040-4039. (Elsevier Ltd.)
    The synthesis of 23 new chiral ionic liqs. is achieved in high yields and with good purity by the reaction of an amino acid or a chiral carboxylic acid with tetrabutylammonium hydroxide in water.
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    Gran, G. Determination of the equivalence point in potentiometric titrations Part II. Analyst 1952, 77, 661– 671,  DOI: 10.1039/an9527700661
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    Determination of the equivalence point in potentiometric titrations. II
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    cf. C.A. 44, 8815c. When there is only a slight change in e.m.f. at the end point it has been customary to plot a ΔE/Δ V curve and take the peak of this curve as the equiv. point. In the previous paper, a method of transforming these curves by a numerical manipulation was shown and here another method is explained. Some rather complicated expressions are given in the math. treatment but practically all the necessary calcns. can be done easily with the slide rule.
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    Determining association constants from titration experiments in supramolecular chemistry
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    Chemical Society Reviews (2011), 40 (3), 1305-1323CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. The most common approach for quantifying interactions in supramol. chem. is a titrn. of the guest to soln. of the host, noting the changes in some phys. property through NMR, UV-Vis, fluorescence or other techniques. Despite the apparent simplicity of this approach, there are several issues that need to be carefully addressed to ensure that the final results are reliable. This includes the use of non-linear rather than linear regression methods, careful choice of stoichiometric binding model, the choice of method (e.g., NMR vs. UV-Vis) and concn. of host, the application of advanced data anal. methods such as global anal. and finally the estn. of uncertainties and confidence intervals for the results obtained. This tutorial review will give a systematic overview of all these issues-highlighting some of the key messages herein with simulated data anal. examples.
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    Krause, L.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination. J. Appl. Crystallogr. 2015, 48, 3– 10,  DOI: 10.1107/S1600576714022985
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    Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination
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  • Abstract

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    Figure 1

    Figure 1. Azacryptands L1 and L2 and the series of polycarboxylates studied in this work.

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    Figure 2

    Figure 2. Distribution diagram showing species present at the equilibrium over the course of the potentiometric titration of L1 (0.4 mM) in the presence of 2 equiv of Cu(CF3SO3)2; the pH-spectrophotometric profiles of Mol Abs at 877 and 667 nm (red and green triangles, respectively) vs pH are superimposed (MeOH/water 30% v/v, 0.05 M NaNO3; T = 25 °C). The lines in the diagram correspond to the species: H6L16+, gray; H5L15+, purple; H4L14+, orange; [Cu(L1H3)]5+, cyan; [Cu2(L1)]4+, red; [Cu2(L1)(OH)]3+, green; [Cu2(L1)(OH)2]2+, blue.

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    Figure 3

    Figure 3. From the left, pairs of simplified sketches of [Cu2(L1)(glut)]2+, [Cu2(L1)(α-keto)]2+, and [Cu2(L1)(ace)]2+ species. The bridging portion of each dicarboxylate anion is drawn as large spheres, in order to emphasize similitudes and differences among the guests. Hydrogen atoms are omitted for clarity, and atom names are reported only for Cu, N, and O.

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    Figure 4

    Figure 4. Spectra taken over the course of the titration of [Cu2(L1)]4+ (0.5 mM) with an aqueous solution of fumaric acid in 0.05 M HEPES at pH 7. Inset: profiles of ε (i.e., Mol Abs) at 691 (blue triangles) and 903 nm (red triangles) vs equivalents of the added guest.

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    Scheme 1

    Scheme 1. Displacement of 5-FAM from the [Cu2(L1)]4+ Cavity Promoted by the Fumarate Anion
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    Figure 5

    Figure 5. Spectra taken over the course of the fluorimetric titration of 5-FAM (1 μM) with a solution of [Cu2(L1)]4+ in 0.05 M HEPES at pH 7. Inset: profile of I/I0 at 520 nm (triangles) vs equivalents of the dicopper complex superimposed to the distribution diagram of the species (as % abundance vs equivalents of dicopper complex): red line = % free 5-FAM; gray line = % [Cu2(L1)(5-FAM)]+ .

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    Figure 6

    Figure 6. Profiles of the spectrofluorimetric titrations of the indicator displacement assay (0.1 μM 5-FAM, 5 μM [Cu2(L1)]4+) with solutions of the anionic guests in 0.05 M HEPES at pH 7 (fum2– = red triangles, suc2– = blue triangles; α-keto2– = dark blue triangles; glut2– = green triangles; adi2– = dark green triangles; diamonds = citrate). I = emission intensity; Imax = emission intensity of a solution of 5-FAM in the absence of the dicopper complex (λexc = 473 nm; λem = 520 nm).

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    Figure 7

    Figure 7. Family of spectra taken over the course of the UV–vis titration of [Cu2(L2)]4+ (50 μM) with a solution of H2suc in dioxane/water (20% v/v) at pH 7 (0.025 M HEPES, path length: 10 cm). Initial spectrum and final spectrum: red and blue lines, respectively. Inset figure: plot of ε (i.e., Mol Abs) at 687 nm vs equiv of the added guest (triangles), superimposed to the distribution diagram of the species calculated for a log K11 = 5.60; % [Cu2(L2)]4+ (red line) and [Cu2(L2)(suc)]2+ (blue line) vs equiv of succinate. (23) See the Supporting Information for more details.

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  • References

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      Mateus, Pedro; Delgado, Rita; Andre, Vania; Duarte, M. Teresa
      Inorganic Chemistry (2015), 54 (1), 229-240CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A ditopic polyamine macrobicyclic compd. with biphenylmethane spacers was prepd., and its dinuclear copper(II) complex was studied as a receptor for the recognition of dicarboxylate anions of varying chain length in H2O/MeOH (50:50 (vol./vol.)) soln. The acid-base behavior of the compd., the stability consts. of its complexes with Cu2+ ion, and the assocn. consts. of the copper(II) cryptate with succinate (suc2-), glutarate (glu2-), adipate (adi2-), and pimelate (pim2-) were detd. by potentiometry at 298.2 ± 0.1 K in H2O/MeOH (50:50 (vol./vol.)) and at ionic strength 0.10 ± 0.01 M in KNO3. The assocn. consts. of the same cryptate as receptor for arom. dicarboxylate substrates, such as phthalate (ph2-), isophthalate (iph2-), and terephthalate (tph2-), were detd. through competition expts. by spectrophotometry in the UV region. Remarkably high values of assocn. consts. at 7.34-10.01 log units were found that are, to the best of the authors' knowledge, the highest values of assocn. consts. reported for the binding of dicarboxylate anions in aq. soln. A very well defined peak of selectivity was obsd. with the binding const. values increasing with the chain length and reaching the max. for substrates with four carbon atoms between the carboxylate groups. Single-crystal x-ray diffraction detns. of the cascade complexes with adi2- and tph2- assisted in the understanding of the selectivity of the cryptate toward these substrates. The Hirshfeld surface analyses of both cascade complexes suggest that the establishment of several van der Waals interactions between the substrates and the walls of the receptor also contributes to the stability of the assocns.
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      (b) Esteves, C. V.; Mateus, P.; André, V.; Bandeira, N. A. G.; Calhorda, M. J.; Ferreira, L. P.; Delgado, R. Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions. Inorg. Chem. 2016, 55, 7051– 7060,  DOI: 10.1021/acs.inorgchem.6b00945
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      4b
      Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions
      Esteves, Catarina V.; Mateus, Pedro; Andre, Vania; Bandeira, Nuno A. G.; Calhorda, Maria Jose; Ferreira, Liliana P.; Delgado, Rita
      Inorganic Chemistry (2016), 55 (14), 7051-7060CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Searching for receptors selective for the binding of dicarboxylate anions, the copper(II) complexes of the known ditopic octaazacryptand (t2pN8), derived from bistren [tren = tris(2-aminoethyl)amine] linked by p-xylyl spacers, were reexamd., with the expectation of observing a selective binding of oxalate or malonate by bridging the two copper centers of the [Cu2(t2pN8)(H2O)2]4+ receptor. Soln. studies involving the supramol. species formed by the receptor and oxalate (oxa2-), malonate (mal2-), and succinate (suc2-) anions are reported. The detd. assocn. consts. revealed the unexpected formation of a 3:1:1 Cu/t2pN8/anion stoichiometry for the cascade species with oxa2- and mal2-, and the single crystal x-ray structural characterization confirmed the presence of tricopper(II) complexes, with an unusual binding mode for the dicarboxylate anions. Each of the two copper atoms binds four nitrogen donor atoms of the t2pN8 cryptand and one addnl. hydroxide group, which bridges to the 3rd copper. The square planar environment of this one is complete with two oxygen atoms from the oxalate (or the malonate). The two copper centers bound to the tren heads are ∼6.5 Å apart, each at ∼3.5 Å from the 3rd Cu center. These studies were complemented by SQUID magnetization measurements and DFT calcns. The magnetic susceptibility measurements of the oxalate cascade complex showed a strong magnetic coupling (J = - 210 cm-1) between the Cu centers at a short distance (3.5 Å), while the coupling between the two equiv. Cu atoms (∼6.5 Å) was only -70 cm-1. This result was well reproduced by DFT calcns.
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      (c) Chakraborty, S.; Saha, S.; Lima, L. M. P.; Warzok, U.; Sarkar, S.; Akhuli, B.; Nandi, M.; Bej, S.; Adarsh, N. N.; Schalley, C. A.; Delgado, R.; Ghosh, P. Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase. J. Org. Chem. 2017, 82, 10007– 10014,  DOI: 10.1021/acs.joc.7b01431
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      4c
      Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase
      Chakraborty, Sourav; Saha, Subrata; Lima, Luis M. P.; Warzok, Ulrike; Sarkar, Sayan; Akhuli, Bidyut; Nandi, Mandira; Bej, Somnath; Adarsh, Nayarassery N.; Schalley, Christoph A.; Delgado, Rita; Ghosh, Pradyut
      Journal of Organic Chemistry (2017), 82 (19), 10007-10014CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc2-) through glutarate (glu2-), α-ketoglutarate (kglu2-), adipate (adi2-), pimelate (pim2-), suberate (sub2-), to azelate (aze2-) have shown adipate preference with assocn. const. value of K = 4900 M-1 in a H2O/DMSO (50:50 vol./vol.) binary solvent mixt. The binding const. increases from glu2- to adi2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the assocns. along with electrostatic interactions. Neg.-mode electrospray ionization of aq. solns. of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the obsd. binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the soln. state binding studies.
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    5. 5
      Curiel, D.; Más-Montoya, M.; Sánchez, G. Complexation and Sensing of Dicarboxylate Anions and Dicarboxylic Acids. Coord. Chem. Rev. 2015, 284, 19– 66,  DOI: 10.1016/j.ccr.2014.09.010
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      5
      Complexation and sensing of dicarboxylate anions and dicarboxylic acids
      Curiel, David; Mas-Montoya, Miriam; Sanchez, Guzman
      Coordination Chemistry Reviews (2015), 284 (), 19-66CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. The relevance of dicarboxylate anions and dicarboxylic acids in biol. and industrial processes requires that such species are easily detected and quantified. In this regard, the research about synthetic receptors, which can offer a straightforward anal. signal, produced significant advances in the last decades. This review describes the different structural approaches followed to achieve an efficient interaction between the receptor and the dicarboxylic species. A comprehensive revision of the literature was made which covers the results published up to early 2014.
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      and references therein

    6. 6
      (a) Fabbrizzi, L. Cryptands and Cryptates; World Scientific Publishing Europe Ltd., London.
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      (b) Amendola, V.; Bergamaschi, G.; Miljkovic, A. Azacryptands as Molecular Cages for Anions and Metal Ions. Supramol. Chem. 2018, 30, 236– 242,  DOI: 10.1080/10610278.2017.1339885
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      6b
      Azacryptands as molecular cages for anions and metal ions
      Amendola, Valeria; Bergamaschi, Greta; Miljkovic, Ana
      Supramolecular Chemistry (2018), 30 (4), 236-242CODEN: SCHEER; ISSN:1029-0478. (Taylor & Francis Ltd.)
      This is a short overview on azacryptands, as mol. receptors for cations and anions. A particular attention was devoted to the results obtained by women researchers working in the field. The terms 'cryptand' and 'cryptate' were coined by Lehn. Since then, much work has been done to improve the knowledge on this class of receptors. Small azacryptands, as free bases, were found to bind a single metal ion within their cavities. When fully protonated, the same systems could also behave as selective hosts for anions, through the cooperation of H-bonding and electrostatic interactions. Proceeding to systems with larger cavities, the inclusion of two metal ions and a bridging anion was possible, forming the so-called 'cascade' complexes. Azacryptates carrying fluorescent spacers or exploiting the indicator displacement paradigm allowed the sensing of anionic species in water at micromolar concns. Moreover, immobilization on solid matrixes and surfaces yielded new materials for the solid-phase extn. of anionic pollutants and the construction of selective electrodes for analytes in water.
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      (c) Thevenet, A.; Marie, C.; Tamain, C.; Amendola, V.; Miljkovic, A.; Guillaumont, D.; Boubals, N.; Guilbaud, P. Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium. Dalton Trans. 2020, 49, 1446– 1455,  DOI: 10.1039/C9DT04314D
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      6c
      Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium
      Thevenet, Alexiane; Marie, Cecile; Tamain, Christelle; Amendola, Valeria; Miljkovic, Ana; Guillaumont, Dominique; Boubals, Nathalie; Guilbaud, Philippe
      Dalton Transactions (2020), 49 (5), 1446-1455CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Technetium is present as the pertechnetate anion in spent nuclear fuel solns., and its extn. by several extractant systems is a major problem for the liq.-liq. extn. processes used to sep. uranium and plutonium. To prevent technetium extn. into the org. phase, a complexing agent may be added to the aq. nitric acid phase to selectively bind the pertechnetate anion. Liq.-liq. extn. expts. reveal that technetium distribution ratios are considerably lowered with addn. of an azacryptand, which is a good receptor for pertechnetate anion recognition. This ligand is able to overcome the Hofmeister bias and selectively bind techetium in nitric acid soln. Coordination studies using IR and Raman spectoscopies and DFT calcns. show the formation of an inclusion complex with hydrogen bonds stabilizing the oxo-anion within the cavity. For the first time, the cage mols. were studied for an extn. process.
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    7. 7
      (a) Busschaert, N.; Caltagirone, C.; Van Rossom, W.; Gale, P. A. Applications of Supramolecular Anion Recognition. Chem. Rev. 2015, 115, 8038– 8155,  DOI: 10.1021/acs.chemrev.5b00099
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      7a
      Applications of Supramolecular Anion Recognition
      Busschaert, Nathalie; Caltagirone, Claudia; Van Rossom, Wim; Gale, Philip A.
      Chemical Reviews (Washington, DC, United States) (2015), 115 (15), 8038-8155CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. This review focuses on the applications of anion complexation in research over the past decade. This spans a wide range of areas, but for the purposes of this review are divided into sensing, extn., transport through lipid bilayers, the roles anions can play in the formation of mol. assemblies, and, finally, organocatalysis.
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      (b) Gale, P. A.; Caltagirone, C. Fluorescent and Colorimetric Sensors for Anionic Species. Coord. Chem. Rev. 2018, 354, 2– 27,  DOI: 10.1016/j.ccr.2017.05.003
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      7b
      Fluorescent and colorimetric sensors for anionic species
      Gale, Philip A.; Caltagirone, Claudia
      Coordination Chemistry Reviews (2018), 354 (), 2-27CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      The development of new fluorescent and colorimetric anion sensors is surveyed in this review including hydrogen and halogen bond donating chemosensors, charged systems, boron-based chemosensors, systems that employ anion-pi interactions and excimer formation, mol. logic gates and arrays of sensors.
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    8. 8
      (a) Kubik, S. Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors. Acc. Chem. Res. 2017, 50, 2870– 2878,  DOI: 10.1021/acs.accounts.7b00458
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      8a
      Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors
      Kubik, Stefan
      Accounts of Chemical Research (2017), 50 (11), 2870-2878CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      Anion receptors often rely on coordinative or multiple ionic interactions to be active in water. In the absence of such strong interactions, anion binding in water can also be efficient, however, as demonstrated by a no. of anion receptors developed in recent years. The cyclopeptide-derived receptors comprising an alternating sequence of L-proline and 6-aminopicolinic acid subunits are an example. These cyclopeptides are neutral and, at 1st sight, can only engage in hydrogen-bond formation with an anionic substrate. Nevertheless, they even interact with strongly solvated sulfate anions in water. The intrinsic anion affinity of these cyclopeptides can be related to structural aspects of their highly preorganized concave binding site, which comprises a wall of hydrophobic proline units arranged around the peptide NH groups at the cavity base. When anions are incorporated into this cavity they can engage in hydrogen-bonding interactions to the NH groups, and complex formation also benefits from cavity dehydration. Formation of 1:1 complexes, in which an anion binds to a single cyclopeptide ring, is assocd. with only small stability consts., however, whereas significantly more stable complexes are formed if the anion is buried between two cyclopeptide mols. A major contribution to the formation of these sandwich complexes derives from the addn. of the 2nd ring to the initially formed 1:1 cyclopeptide-anion complex. This step brings the apolar proline residues of both cyclopeptides in close proximity, which causes the resulting structure to be stabilized to a large extent by hydrophobic effects. Solvent dependent binding studies provided an est. to which degree these solvent effects contribute to the overall complex stability. In these studies, bis(cyclopeptides) were used, featuring two cyclopeptide rings covalently connected via linkers that enable both rings to simultaneously interact with the anion. Bis(cyclopeptides) with addnl. solubilizing groups allowed binding studies in a wide range of solvents, including in water. The systematic anal. of the solvent dependence of anion affinity yielded a quant. correlation between complex stability and parameters relating to the solvation of the anions and solvent properties, confirming that solvent effects contribute to anion binding. The thermodn. signature of complex formation in water mirrors that of sulfate binding to a protein complex but is opposite to that of other recently described anion receptors, which also do not engage in ionic or coordinative interactions with the substrate. These receptors not only differ in terms of the thermodn. of binding from the cyclopeptides but also possess a characteristically different anion selectivity in that they prefer to bind weakly coordinating anions but fail to bind sulfate. Solvent effects likely control the anion binding of both receptors types but their impact on complex formation and anion selectivity seems to be profoundly different. Future work in the area of anion coordination chem. will benefit from the deeper understanding of these effects and how they can be controlled.
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      (b) Langton, M. J.; Serpell, C. J.; Beer, P. D. Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective. Angew. Chem. Int. Ed. 2016, 55, 1974– 1987,  DOI: 10.1002/anie.201506589
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      8b
      Anion Recognition in Water: Recent Advances from a Supramolecular and Macromolecular Perspective
      Langton, Matthew J.; Serpell, Christopher J.; Beer, Paul D.
      Angewandte Chemie, International Edition (2016), 55 (6), 1974-1987CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The recognition of anions in water remains a key challenge in modern supramol. chem., and is essential if proposed applications in biol., medical, and environmental arenas that typically require aq. conditions are to be achieved. However, synthetic anion receptors that operate in water have, in general, been the exception rather than the norm to date. Nevertheless, a significant step change towards routinely conducting anion recognition in water was achieved in the past few years, and this Review highlights these approaches, with particular focus on controlling and using the hydrophobic effect, as well as more exotic interactions such as C-H hydrogen bonding and halogen bonding. The authors also look beyond the field of small-mol. recognition into the macromol. domain, covering recent advances in anion recognition based on biomols., polymers, and nanoparticles.
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      and references therein

    9. 9
      Carnegie, R. S.; Gibb, C. L. D.; Gibb, B. C. Anion Complexation and the Hofmeister Effect. Angew. Chem. Int. Ed. 2014, 53, 11498– 11500,  DOI: 10.1002/anie.201405796
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      9
      Anion Complexation and The Hofmeister Effect
      Carnegie, Ryan S.; Gibb, Corinne L. D.; Gibb, Bruce C.
      Angewandte Chemie, International Edition (2014), 53 (43), 11498-11500CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The 1H NMR spectroscopic anal. of the binding of the ClO4- anion to the hydrophobic, concave binding site of a deep-cavity cavitand is presented. The strength of assocn. between the host and the ClO4- anion is controlled by both the nature and concn. of co-salts in a manner that follows the Hofmeister series. A model that partitions this trend into the competitive binding of the co-salt anion to the hydrophobic pocket of the host and counterion binding to its external carboxylate groups successfully accounts for the obsd. changes in ClO4- affinity.
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    10. 10
      Jagleniec, D.; Dobrzycki, Ł.; Karbarz, M.; Romański, J. Ion-Pair Induced Supramolecular Assembly Formation for Selective Extraction and Sensing of Potassium Sulfate. Chem. Sci. 2019, 10, 9542– 9547,  DOI: 10.1039/C9SC02923K
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      10
      Ion-pair induced supramolecular assembly formation for selective extraction and sensing of potassium sulfate
      Jagleniec, Damian; Dobrzycki, Lukasz; Karbarz, Marcin; Romanski, Jan
      Chemical Science (2019), 10 (41), 9542-9547CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Selective extn. of sulfates in the form of alkali metal salts using charge-neutral mol. receptors is one of the holy grails of supramol. chem. Herein we describe, for the first time, a squaramide-based ion pair receptor equipped with a crown ether site that is able to ext. potassium sulfate from the aq. to the org. phase (an analogus monotopic anion receptor lacking the crown ether unit lacks this ability). A 1H NMR, UV-vis, DOSY-NMR, DLS, and MS expts. and the solid-state single crystal structure provided evidence of the formation of a supramol. core-shell like assembly upon interaction of the receptor with potassium sulfate. The presence of monovalent potassium salts, in contrast, promoted the formation of simple 1 : 1 complexes. Unlike the 4 : 1 assembly, the 1 : 1 complexes are poorly sol. in org. media. This feature was utilized to overcome the Hofmeister bias and allow for selective extn. of extremely hydrophilic sulfates over lipophilic nitrate anions, which was unambiguously proved by quant. AES and ion chromatog. measurements. A simple modification of the receptor structure led to a "naked eye" optical sensor able to selectively detect sulfates under both SLE and LLE conditions.
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    11. 11
      Wichmann, K.; Antonioli, B.; Söhnel, T.; Wenzel, M.; Gloe, K.; Gloe, K.; Price, J. R.; Lindoy, L. F.; Blake, A. J.; Schröder, M. Polyamine-based anion receptors: Extraction and structural studies. Coord. Chem. Rev. 2006, 250, 2987– 3003,  DOI: 10.1016/j.ccr.2006.07.010
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      11
      Polyamine-based anion receptors: Extraction and structural studies
      Wichmann, Kathrin; Antonioli, Bianca; Soehnel, Tilo; Wenzel, Marco; Gloe, Kerstin; Gloe, Karsten; Price, Jason R.; Lindoy, Leonard F.; Blake, Alexander J.; Schroeder, Martin
      Coordination Chemistry Reviews (2006), 250 (23+24), 2987-3003CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. In the discussion that follows some of the more recent progress in the area of anion binding by synthetic polyamine receptors is presented, with emphasis given to work undertaken by the authors' groups. A continuing theme in these studies has been the relationship between receptor structure and its anion extn. properties. Systematic solvent extn. and structural studies for halide and perrhenate complexes with polyamines of tripodal, macrocyclic and macrobicyclic architecture that contain both arom. moieties and four to eight amine functions have been performed in order to derive relevant structure-binding/extractability relationships. The results demonstrate that the binding and extn. behavior of the polyamines towards halides and perrhenate is a complex function of their structural features, degree of protonation and lipophilic properties. The extn. is characterized by the preferred formation of mono- and diprotonated amine species in the org. phase. X-ray structure studies of iodide and perrhenate complexes with open-chain tetraamino derivs. and octaamino cryptands in different protonation states lead to the conclusion that in the first case only limited chelation of the anion occurs and in the second only highly protonated species are able to encapsulate the anion. The structural patterns obsd. are strongly influenced by the presence of water mols. in the crystals.
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    12. 12
      (a) Wu, X.; Howe, E. N. W.; Gale, P. A. Supramolecular Transmembrane Anion Transport: New Assays and Insights. Acc. Chem. Res. 2018, 51, 1870– 1879,  DOI: 10.1021/acs.accounts.8b00264
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      12a
      Supramolecular Transmembrane Anion Transport: New Assays and Insights
      Wu, Xin; Howe, Ethan N. W.; Gale, Philip A.
      Accounts of Chemical Research (2018), 51 (8), 1870-1879CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Transmembrane anion transport has been the focus of a no. of supramol. chem. research groups for a no. of years. Much of this research is driven by the biol. relevance of anion transport and the search to find new treatments for diseases such as cystic fibrosis, which is caused by genetic problems leading to faulty cystic fibrosis transmembrane conductance regulator (CFTR) channels, which in turn lead to reduced chloride and bicarbonate transport through epithelial cell membranes. Considerable effort has been devoted to the development of new transporters, and the group along with others have been searching for combinations of org. scaffolds and anion binding groups that produce highly effective transporters that work at low concn. These compds. may be used in the future as "channel replacement therapies", restoring the flux of anions through epithelial cell membranes and ameliorating the symptoms of cystic fibrosis. Less effort has been put into gaining a fundamental understanding of anion transport processes. Over the last 3 years, the group has developed a no. of new transport assays that allow anion transport mechanisms to be detd. This Account covers the latest developments in this area, providing a concise review of the new techniques the authors can use to study anion transport processes individually without resorting to measurement of exchange processes and the new insights that these assays provide. The Account provides an overview of the effects of anion transporters on cells and an explanation of why many systems perturb pH gradients within cells in addn. to transporting chloride. The authors discuss assays to det. whether anionophores facilitate chloride or HCl transport and how this latter assay can be modified to det. chloride vs. proton selectivity in small-mol. anion receptors. The authors show how mol. design can be used to produce receptors that are capable of transporting chloride without perturbing pH gradients. The authors cover the role that anion transporters in the presence of fatty acids play in dissipating pH gradients across lipid bilayer membranes and the effect that this process has on chloride-selective transport. The authors also discuss how coupling of anion transport to cation transport by natural cationophores can be used to det. whether anion transport is electrogenic or electroneutral. In addn., the authors compare these new assays to the previously used chloride/nitrate exchange assay and show how this exchange assay can underestimate the chloride transport ability of certain receptors that are rate-limited by nitrate transport.
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      and references therein

      (b) Grauwels, G.; Valkenier, H.; Davis, A. P.; Jabin, I.; Bartik, K. Repositioning Chloride Transmembrane Transporters: Transport of Organic Ion Pairs. Angew. Chem. Int Ed. 2019, 58, 6921– 6925,  DOI: 10.1002/anie.201900818
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      12b
      Repositioning chloride transmembrane transporters: Transport of organic ion pairs
      Grauwels, Glenn; Valkenier, Hennie; Davis, Anthony P.; Jabin, Ivan; Bartik, Kristin
      Angewandte Chemie, International Edition (2019), 58 (21), 6921-6925CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Given the biol. importance of org. cations, the facilitated transport of org. ion pairs could find many applications. Calix[6]arene tris(thio)ureas, which possess a cavity that can accommodate primary ammonium ions, can not only act as carriers for Cl-/NO3- antiport but can also perform the cotransport of PrNH3Cl. Transport was monitored by fluorescence spectroscopy and the presence of the different species inside the vesicles was characterized by 1H and 35Cl NMR expts. involving shift reagents. The cotransport of PrNH3Cl was also obsd. by receptors deprived of a cavity, but the presence of the cavity conveys an advantage, as the cotransport by calix[6]arenes was obsd. to be more efficient than the Cl-/NO3- antiport, which is not the case with receptors without a cavity. The role played by the cavity was further highlighted by the disappearance of this advantage when using a bulky ammonium ion, which cannot be complexed within the cavity.
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    13. 13
      Bowman-James, K. Supramolecular Cages Trap Pesky Anions. Science 2019, 365, 124– 125,  DOI: 10.1126/science.aax9369
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      13
      Supramolecular cages trap pesky anions
      Bowman-James, Kristin
      Science (Washington, DC, United States) (2019), 365 (6449), 124-125CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The design of cryptands-org. mols. that can capture simple inorg. and org. ions from soln. through multiple weak interactions-can be quite challenging, esp. when the task is to capture a specific ion with high affinity. In many instances, targeted recognition is necessary to remove an unwanted species from some site because of its overabundance, to remove contamination, or even to capture it for its economic value (e.g., gold from seawater) (). Neg. charged ions, esp. smaller anions such as Cl-, are esp. troublesome. Greater energies are generally needed to peel away their more tightly held hydration shells relative to larger anions of the same charge, such as I-, which has a more diffuse charge cloud. Likewise, anions possess higher free energies of hydration than cations of similar size and charge. On page 159 of this issue, Liu et al. () report the design of a bicyclic cryptand that specifically recognizes Cl- ions with high affinity. Their results build on a long history of increasingly complex cryptand cages specifically designed to target anions (see the figure).
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    14. 14
      Liu, Y.; Zhao, W.; Chen, C.-H.; Flood, A. H. Chloride Capture Using a C–H Hydrogen Bonding Cage. Science 2019, 365, 159– 161,  DOI: 10.1126/science.aaw5145
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      14
      Chloride capture using a C-H hydrogen-bonding cage
      Liu, Yun; Zhao, Wei; Chen, Chun-Hsing; Flood, Amar H.
      Science (Washington, DC, United States) (2019), 365 (6449), 159-161CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Tight binding and high selectivity are hallmarks of biomol. recognition. These behaviors with synthetic receptors has usually been assocd. with OH and NH hydrogen bonding. Contrary to this conventional wisdom, we designed a chlorideselective receptor in the form of a cryptand-like cage using only CH hydrogen bonding. Crystallog. showed chloride stabilized by six short 2.7-angstrom hydrogen bonds originating from the cage's six 1,2,3-triazoles. Attomolaraffinity (1017 M-1) was detd. using liq.-liq. extns. of chloride from water into nonpolar dichloromethane solvents. Controls verified the addnl. role of triazoles in rigidifying the three-dimensional structure to effect recognition affinity and selectivity: Cl- > Br- > NO3- > I-. This cage shows anti-Hofmeister salt extn. and corrosion inhibition.
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    15. 15
      (a) He, Q.; Williams, N. J.; Oh, J. H.; Lynch, V. M.; Kim, S. K.; Moyer, B. A.; Sessler, J. L. Selective Solid–Liquid and Liquid–Liquid Extraction of Lithium Chloride Using Strapped Calix[4]Pyrroles. Angew. Chem. Int Ed. 2018, 57, 11924– 11928,  DOI: 10.1002/anie.201805127
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      15a
      Selective Solid-Liquid and Liquid-Liquid Extraction of Lithium Chloride Using Strapped Calix[4]pyrroles
      He, Qing; Williams, Neil J.; Oh, Ju Hyun; Lynch, Vincent M.; Kim, Sung Kuk; Moyer, Bruce A.; Sessler, Jonathan L.
      Angewandte Chemie, International Edition (2018), 57 (37), 11924-11928CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      LiCl is a classic "hard" ion salt that is present in lithium-rich brines and a key component in end-of-life materials (i.e., used lithium-ion batteries). Its isolation and purifn. from like salts is a recognized challenge with potential strategic and economic implications. Herein, we describe two ditopic calix[4]pyrrole-based ion-pair receptors (2 and 3, see Fig. 1 in text of paper), that are capable of selectively capturing LiCl. Under solid-liq. extn. conditions, using 2 as the extractant, LiCl could be sepd. from a NaCl/KCl salt mixt. contg. as little as 1 % LiCl with circa 100 % selectivity, while receptor 3 achieved similar sepns. when the LiCl level was as low as 200 ppm. Under liq.-liq. extn. conditions using nitrobenzene as the non-aq. phase, the extn. preference displayed by 2 is KCl>NaCl>LiCl. In contrast, 3 exhibits high selectivity towards LiCl over NaCl and KCl, with no appreciable extn. being obsd. for the latter two salts.
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      (b) Ji, X.; Wu, R.-T.; Long, L.; Guo, C.; Khashab, N. M.; Huang, F.; Sessler, J. L. Physical Removal of Anions from Aqueous Media by Means of a Macrocycle-Containing Polymeric Network. J. Am. Chem. Soc. 2018, 140, 2777– 2780,  DOI: 10.1021/jacs.7b13656
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      15b
      Physical Removal of Anions from Aqueous Media by Means of a Macrocycle-Containing Polymeric Network
      Ji, Xiaofan; Wu, Ren-Tsung; Long, Lingliang; Guo, Chenxing; Khashab, Niveen M.; Huang, Feihe; Sessler, Jonathan L.
      Journal of the American Chemical Society (2018), 140 (8), 2777-2780CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Reported here is a hydrogel-forming polymer network that contains a water-sol. tetracationic macrocycle. Upon immersion of this polymer network in aq. solns. contg. various inorg. and org. salts, changes in the phys. properties are obsd. that are consistent with absorption of the constituent anions into the polymer network. This absorption is ascribed to host-guest interactions involving the tetracationic macrocyclic receptor. Removal of the anions may then be achieved by lifting the resulting hydrogels out of the aq. phase. Treating the anion-contg. hydrogels with dil. HCl leads to the protonation-induced release of the bound anions. This allows the hydrogels to be recycled for reuse. The present polymer network thus provides a potentially attractive approach to removing undesired anions from aq. environments.
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      (c) He, Q.; Vargas-Zúñiga, G. I.; Kim, S. H.; Kim, S. K.; Sessler, J. L. Macrocycles as Ion Pair Receptors. Chem. Rev. 2019, 119, 9753– 9835,  DOI: 10.1021/acs.chemrev.8b00734
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      15c
      Macrocycles as Ion Pair Receptors
      He, Qing; Vargas-Zuniga, Gabriela I.; Kim, Seung Hyun; Kim, Sung Kuk; Sessler, Jonathan L.
      Chemical Reviews (Washington, DC, United States) (2019), 119 (17), 9753-9835CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Cation and anion recognition have both played central roles in the development of supramol. chem. Much of the assocd. research has focused on the development of receptors for individual cations or anions, as well as their applications in different areas. Rarely is complexation of the counterions considered. In contrast, ion pair recognition chem., emerging from cation and anion coordination chem., is a specific research field where cocomplexation of both anions and cations, so-called ion pairs, is the center of focus. Systems used for the purpose, known as ion pair receptors, are typically di- or polytopic hosts that contain recognition sites for both cations and anions and which permit the concurrent binding of multiple ions. The field of ion pair recognition has blossomed during the past decades. Several smaller reviews on the topic were published roughly 5 years ago. They provided a summary of synthetic progress and detailed the various limiting ion recognition modes displayed by both acyclic and macrocyclic ion pair receptors known at the time. The present review is designed to provide a comprehensive and up-to-date overview of the chem. of macrocycle-based ion pair receptors. We specifically focus on the relationship between structure and ion pair recognition, as well as applications of ion pair receptors in sensor development, cation and anion extn., ion transport, and logic gate construction.
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      and references therein

    16. 16
      Fowler, C. J.; Haverlock, T. J.; Moyer, B. A.; Shriver, J. A.; Gross, D. E.; Marquez, M.; Sessler, J. L.; Hossain, M. A.; Bowman-James, K. Enhanced Anion Exchange for Selective Sulfate Extraction: Overcoming the Hofmeister Bias. J. Am. Chem. Soc. 2008, 130, 14386– 14387,  DOI: 10.1021/ja806511b
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      Enhanced Anion Exchange for Selective Sulfate Extraction: Overcoming the Hofmeister Bias
      Fowler, Christopher J.; Haverlock, Tamara J.; Moyer, Bruce A.; Shriver, James A.; Gross, Dustin E.; Marquez, Manuel; Sessler, Jonathan L.; Hossain, Md. Alamgir; Bowman-James, Kristin
      Journal of the American Chemical Society (2008), 130 (44), 14386-14387CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In this communication, a new approach to enhancing the efficacy of liq.-liq. anion exchange is demonstrated. It involves the concurrent use of appropriately chosen hydrogen-bond-donating (HBD) anion receptors in combination with a traditional quaternary ammonium extractant. The fluorinated calixpyrroles and the tetraamide macrocycle were found to be particularly effective receptors. Specifically, their use allowed the extn. of sulfate by tricaprylmethylammonium nitrate to be effected in the presence of excess nitrate. As such, the present work provides a rare demonstration of overcoming the Hofmeister bias in a competitive environment and the first to the authors' knowledge wherein this difficult-to-achieve objective is attained using a neutral HBD-based anion binding agent under conditions of solvent extn.
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    17. 17
      Qin, L.; Vervuurt, S. J. N.; Elmes, R. B. P.; Berry, S. N.; Proschogo, N.; Jolliffe, K. A. Extraction and transport of sulfate using macrocyclic squaramide receptors. Chem. Sci. 2020, 11, 201– 207,  DOI: 10.1039/C9SC04786G
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      Extraction and transport of sulfate using macrocyclic squaramide receptors
      Qin, Lei; Vervuurt, Sacha J. N.; Elmes, Robert B. P.; Berry, Stuart N.; Proschogo, Nicholas; Jolliffe, Katrina A.
      Chemical Science (2020), 11 (1), 201-207CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The selective extn. of the hydrophilic sulfate ion from water is highly challenging because the high free energy of hydration of this ion makes it more difficult to ext. than less hydrophilic ions such as chloride and nitrate. Lipophilic macrocyclic squaramide receptors 1 and 2 were synthesized. Receptor 2 efficiently extd. sulfate from aq. sodium sulfate solns. into a chloroform phase, via exchange with nitrate ions, overcoming the Hofmeister bias. The resulting 2·SO42- complex was readily recycled through pptn. of BaSO4. Transport of sulfate across a bulk chloroform membrane by 2 was demonstrated across a wide pH range (pH 3.2-9.4) and in the presence of high concns. of competing anions (chloride, nitrate and dihydrogenphosphate), opening the door to the use of 2 for the selective removal of sulfate from water across a range of applications.
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    18. 18
      Baragaña, B.; Blackburn, A. G.; Breccia, P.; Davis, A. P.; de Mendoza, J.; Padrón-Carrillo, J. M.; Prados, P.; Riedner, J.; de Vries, J. G. Enantioselective Transport by a Steroidal Guanidinium Receptor. Chem. - Eur. J. 2002, 8, 2931– 2936,  DOI: 10.1002/1521-3765(20020703)8:13<2931::AID-CHEM2931>3.0.CO;2-H
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      Enantioselective transport by a steroidal guanidinium receptor
      Baragana, Beatriz; Blackburn, Adrian G.; Breccia, Perla; Davis, Anthony P.; De Mendoza, Javier; Padron-Carrillo, Jose M.; Prados, Pilar; Riedner, Jens; De Vries, Johannes G.
      Chemistry - A European Journal (2002), 8 (13), 2931-2936CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH)
      The cationic steroidal receptors I (R = OMe, OC20H41) have been synthesized from cholic acid. Receptor I (R = MeO) exts. N-acetyl-α-amino acids from aq. media into chloroform with enantioselectivities (L:D) of 7-10:1. The lipophilic variant I (R = OC20H41) has been employed for the enantioselective transport of N-acetylphenylalanine, (a) through dichloromethane (DCM) and dichloroethane (DCE) bulk liq. membranes (U-tube app.), and (b) through 2.5% (vol./vol.) octanol/hexane via hollow fiber membrane contactors. Significant enantioselectivities and multiple turnovers were obsd. for both types of app.
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      Urban, C.; Schmuck, C. Active Transport of Amino Acids by a Guanidiniocarbonyl-Pyrrole Receptor. Chem. - Eur. J. 2010, 16, 9502– 9510,  DOI: 10.1002/chem.201000509
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      Active Transport of Amino Acids by a Guanidiniocarbonyl-Pyrrole Receptor
      Urban, Christian; Schmuck, Carsten
      Chemistry - A European Journal (2010), 16 (31), 9502-9510CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein the authors report the synthesis and characterization of a guanidiniocarbonyl-pyrrole deriv. I as transporter for N-acetylamino acids. I is a conjugate of a guanidiniocarbonyl pyrrole cation, one of the most efficient carboxylate binding motifs reported so far, and a hydrophobic tris(dodecylbenzyl) group, which ensures soly. in org. solvents. In its protonated form, I binds N-acetylamino acid carboxylates in wet org. solvents with assocn. consts. in the range of 104 M-1 as estd. by extn. expts. Arom. amino acids are preferred due to addnl. cation-π-interactions of the amino acid side chain with the guanidiniocarbonyl pyrrole moiety. U-tube expts. established efficient transport across a bulk liq. chloroform phase with fluxes approaching 10-6 mol m-2 s-1. In expts. with single substrates, the release rate of the amino acid from the receptor-substrate complex at the interface with the receiving phase is rate detg. In contrast to this, in competition expts. with several substrates, the thermodn. affinity to I becomes decisive. As I can only transport anions in its protonated form and has pKa ≈ 7, pH-driven active transport of amino acids is also possible. Transport occurs as a symport of the amino acid carboxylate and a proton.
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      Xie, G.-Y.; Jiang, L.; Lu, T.-B. Discrimination of Cis-Trans Isomers by Dinuclear Metal Cryptates at Physiological PH: Selectivity for Fumarate vs Maleate. Dalton Trans. 2013, 42, 14092– 14099,  DOI: 10.1039/c3dt51501j
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      Discrimination of cis-trans isomers by dinuclear metal cryptates at physiological pH: selectivity for fumarate vs. maleate
      Xie, Gao-Yi; Jiang, Long; Lu, Tong-Bu
      Dalton Transactions (2013), 42 (39), 14092-14099CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Cryptand L (L = N[(CH2)2NHCH2(2,6-C10H6)CH2NH(CH2)2]3N) and its dinuclear metal cryptates [Zn2L](NO3)4 (1) and [Cu2L](ClO4)4 (2) were prepd., and the binding properties of the cryptates with fumarate and its cis isomer maleate were studied using fluorescent spectra, 1H NMR titrns. and single crystal x-ray diffraction anal. for [(Cu2L)(fum)][ClO4]2 (3) (fum = fumarate). Thanks to the size and shape matching effect, the cryptates can selectively recognize fumarate at physiol. pH, with an assocn. const. almost 18-fold larger than that of maleate, forming a cradle-like cascade complex.
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      (a) Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn, E. V. Teaching Old Indicators New Tricks. Acc. Chem. Res. 2001, 34, 963– 972,  DOI: 10.1021/ar9600796
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      Teaching old indicators new tricks
      Wiskur, Sheryl L.; Ait-Haddou, Hassan; Lavigne, John J.; Anslyn, Eric V.
      Accounts of Chemical Research (2001), 34 (12), 963-972CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Most synthetic sensors are designed with covalent attachment between a receptor and a reporter moiety. The authors describe the current progress of the authors' use of noncovalently attached indicators to signal binding of analytes. With these systems, analyte binding leads to indicator displacement from the binding cavity, which in turn yields an optical signal modulation. The authors include previous examples, the strategies involved in the authors' development, and the advantages as well as disadvantages of this method. Finally, the authors' latest research in this field is briefly presented.
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      (b) Anzenbacher, P., Jr.; Lubal, P.; Buček, P.; Palacios, M. A.; Kozelkova, M. E. A practical approach to optical cross-reactive sensor arrays. Chem. Soc. Rev. 2010, 39, 3954– 3979,  DOI: 10.1039/b926220m
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      A practical approach to optical cross-reactive sensor arrays
      Anzenbacher, Pavel, Jr.; Lubal, Premysl; Bucek, Pavel; Palacios, Manuel A.; Kozelkova, Maria E.
      Chemical Society Reviews (2010), 39 (10), 3954-3979CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Supramol. anal. chem. has emerged as a new discipline at the interface of supramol. and anal. chem. It focuses on anal. applications of mol. recognition and self-assembly. One of the important outcomes of the supramol. anal. chem. is the understanding of mol. aspects of sensor design, synthesis and binding studies of sensors while using rigorous methods of anal. chem. as a touchstone to verify the viability of the supramol. aspects of the sensor design. This crit. review provides a simplified version of the chemometric procedures involved in realizing a successful anal. expt. that utilizes cross-reactive optical sensor arrays, and summarizes the current research in this field. This review also shows several examples of use of described chemometric methods for evaluation of chemosensors and sensor arrays. Thus, this review is aimed mostly at the readers who want to test their newly-developed chemosensors in cross-reactive arrays (169 refs.).
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      Indicator-displacement assays
      Nguyen, Binh T.; Anslyn, Eric V.
      Coordination Chemistry Reviews (2006), 250 (23+24), 3118-3127CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Indicator displacement assays (IDAs) are now a popular method for converting most any synthetic receptor into an optical sensor. In this review many such assays are highlighted, along with biol. counterparts. The focus is upon colorimetric, fluorescent, and metal contg. IDAs. The power of the method can be readily appreciated by the large diversity of analytes that have been targeted with this technique. It is clear that the method is now well accepted and will continue to be one of many methods used to create optical detection methods from synthetic receptors.
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      (d) Mako, T. L.; Racicot, J. M.; Levine, M. Supramolecular Luminescent Sensors. Chem. Rev. 2019, 119, 322– 477,  DOI: 10.1021/acs.chemrev.8b00260
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      Supramolecular Luminescent Sensors
      Mako, Teresa L.; Racicot, Joan M.; Levine, Mindy
      Chemical Reviews (Washington, DC, United States) (2019), 119 (1), 322-477CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      There is great need for stand-alone luminescence-based chemosensors that exemplify selectivity, sensitivity, and applicability and that overcome the challenges that arise from complex, real-world media. Discussed herein are recent developments toward these goals in the field of supramol. luminescent chemosensors, including macrocycles, polymers, and nanomaterials. Specific focus is placed on the development of new macrocycle hosts since 2010, coupled with considerations of the underlying principles of supramol. chem. as well as analytes of interest and common luminophores. State-of-the-art developments in the fields of polymer and nanomaterial sensors are also examd., and some remaining unsolved challenges in the area of chemosensors are discussed.
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      (e) Aletti, A. B.; Gillen, D. M.; Gunnlaugsson, T. Luminescent/Colorimetric Probes and (Chemo-) Sensors for Detecting Anions Based on Transition and Lanthanide Ion Receptor/Binding Complexes. Coord. Chem. Rev. 2018, 354, 98– 120,  DOI: 10.1016/j.ccr.2017.06.020
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      21e
      Luminescent/colorimetric probes and (chemo-) sensors for detecting anions based on transition and lanthanide ion receptor/binding complexes
      Aletti, Anna B.; Gillen, Dermot M.; Gunnlaugsson, Thorfinnur
      Coordination Chemistry Reviews (2018), 354 (), 98-120CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Herein, some of the recent developments in the design and study of transition metal (d-metal) and lanthanide (f-metal) based sensors, probes and reagents for detecting anions in org. or competitive media will be reviewed. Some examples of main group and actinide-based sensors are also featured. In all cases, the anion recognition is probed by monitoring changes in the various photophys. properties of these complexes, with particular focus being paid to recent examples from the literature where the anion recognition event is communicated through colorimetric or luminescent (fluorescence or phosphorescence) changes. A select no. of examples reported within the last 5 years (since 2011) are featured; the focus of this review is on those developed from org. ligands that can, in a synergetic manner with the metal ions, directly aid or enhance the anion recognition and sensing processes. Examples where such synergy is provided by hydrogen bonding interactions are particularly discussed.
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      (f) Minami, T.; Liu, Y.; Akdeniz, A.; Koutnik, P.; Esipenko, N. A.; Nishiyabu, R.; Kubo, Y.; Anzenbacher, P., Jr. Intramolecular Indicator Displacement Assay for Anions: Supramolecular Sensor for Glyphosate. J. Am. Chem. Soc. 2014, 136, 11396– 11401,  DOI: 10.1021/ja504535q
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      Intramolecular Indicator Displacement Assay for Anions: Supramolecular Sensor for Glyphosate
      Minami, Tsuyoshi; Liu, Yuanli; Akdeniz, Ali; Koutnik, Petr; Esipenko, Nina A.; Nishiyabu, Ryuhei; Kubo, Yuji; Anzenbacher, Pavel
      Journal of the American Chemical Society (2014), 136 (32), 11396-11401CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      One of the well-known strategies for anion sensing is an indicator (dye) displacement assay. However, the disadvantage of the dye displacement assays is the low sensitivity due to the excess of the dye used. To overcome this setback, we have developed an "Intramol. Indicator Displacement Assay (IIDA)". The IIDAs comprise a receptor and a spacer with an attached anionic chromophore in a single-mol. assembly. In the resting state, the environment-sensitive anionic chromophore is bound by the receptor, while the anionic substrate competes for binding into the receptor. The photophys. properties of the dye exhibit change in fluorescence when displaced by anions, which results in cross-reactive response. To illustrate the concept, we have prepd. IID sensors (I) and (II). Here, the characterization of sensors and microtiter arrays comprising the IIDA are reported. The microtiter array including IID sensors I and II is capable of recognizing biol. phosphates in water. The utility of the IIDA approach is demonstrated on sensing of a phosphonate herbicide glyphosate and other biol. important anions such as pyrophosphate in the presence of interferent sodium chloride.
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    22. 22
      (a) Boiocchi, M.; Bonizzoni, M.; Fabbrizzi, L.; Piovani, G.; Taglietti, A. A Dimetallic Cage with a Long Ellipsoidal Cavity for the Fluorescent Detection of Dicarboxylate Anions in Water. Angew. Chem., Int. Ed. 2004, 43, 3847– 3852,  DOI: 10.1002/anie.200460036
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      22a
      A dimetallic cage with a long ellipsoidal cavity for the fluorescent detection of dicarboxylate anions in water
      Boiocchi, Massimo; Bonizzoni, Marco; Fabbrizzi, Luigi; Piovani, Giulio; Taglietti, Angelo
      Angewandte Chemie, International Edition (2004), 43 (29), 3847-3852CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The dicopper(II) complex of a bistren cage contg. ditolyl spacers (I) binds dicarboxylate anions in aq. solns. The inclusion is highly selective from the distance between the COO- groups. For example, terephthalate is discriminated from isophthalate and phthalate.
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      (b) Merli, D.; La Cognata, S.; Balduzzi, F.; Miljkovic, A.; Toma, L.; Amendola, V. A Smart Supramolecular Device for the Detection of t,t-Muconic Acid in Urine†. New J. Chem. 2018, 42, 15460– 15465,  DOI: 10.1039/C8NJ02156B
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      22b
      A smart supramolecular device for the detection of t,t-muconic acid in urine
      Merli, Daniele; La Cognata, Sonia; Balduzzi, Federica; Miljkovic, Ana; Toma, Lucio; Amendola, Valeria
      New Journal of Chemistry (2018), 42 (18), 15460-15465CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
      The authors combined smartphone sensing with the indicator displacement approach for the detn. of trans,trans-muconic acid (i.e.tt-MA), a benzene biomarker, in urine. Dicopper(II) azacryptate was employed as the receptor. The good match of tt-MA with the receptor cavity was suggested by DFT calcns., and confirmed exptl. by UV-visible titrns. in buffered aq. solns. The binding const. was detd. through fluorometric titrns. using the indicator displacement approach. This method also confirmed the high affinity of the azacryptate for tt-MA in artificial and spiked urine samples. A portable device was then obtained by adsorption of the chemosensing ensemble on silica gel using ELISA-like plastic wells as the support. Indicator displacement was detected and quantified by exposing the microplate to a UV-lamp (366 nm) and recording the RGB values with a smartphone. A good correlation was found between the R index values and urinary tt-MA in the occupational concn. range.
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      (c) Miljkovic, A.; La Cognata, S.; Bergamaschi, G.; Freccero, M.; Poggi, A.; Amendola, V. Towards Building Blocks for Supramolecular Architectures Based on Azacryptates. Molecules 2020, 25, 1733,  DOI: 10.3390/molecules25071733
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      Towards building blocks for supramolecular architectures based on azacryptates
      Miljkovic, Ana; Cognata, Sonia La; Bergamaschi, Greta; Freccero, Mauro; Poggi, Antonio; Amendola, Valeria
      Molecules (2020), 25 (7), 1733CODEN: MOLEFW; ISSN:1420-3049. (MDPI AG)
      In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with tri-Ph spacers. The binding properties of its dicopper complex for arom. dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramol. structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu2L]4+ in water/acetonitrile mixt. (pH = 7) showed a high affinity for biphenyl-4,40 -dicarboxylate (dfc2-), with a binding const. of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage's cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu2L]4+ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc2-) and short anions (e.g., terephthalate). The good affinity of [Cu2L]4+ for these dicarboxylates, together with hydrophobic interactions within the cage's cavity, may promote the self-assembly of a stable 1:1 complex in water mixt. These results represent a good starting point for the application of these mol. systems as building units for the design of new supramol. architectures based on non-covalent interactions, which could be of interest in all fields related to supramol. devices.
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    23. 23
      Mobili, R.; La Cognata, S.; Merlo, F.; Speltini, A.; Boiocchi, M.; Recca, T. Liquid-liquid extraction of succinate using a dicopper cryptate. ChemRxiv 2020,  DOI: 10.26434/chemrxiv.11786784.v1
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      Gans, P.; Sabatini, A.; Vacca, A. Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs. Talanta 1996, 43, 1739– 1753,  DOI: 10.1016/0039-9140(96)01958-3
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      Investigation of equilibria in solution. Determination of equilibrium constants with HYPERQUAD suite of programs
      Gans, Peter; Sabatini, Antonio; Vacca, Alberto
      Talanta (1996), 43 (10), 1739-1753CODEN: TLNTA2; ISSN:0039-9140. (Elsevier)
      A new series of 10 programs for the study of equil. consts. and soln. equil. is described. The programs include data prepn., pretreatment, equil. const. refinement and post-run anal. Data prepn. is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point detn., absorbance error detn., spectral baseline corrections, factor anal. and detn. of molar absorbance spectra. Equil. consts. can be detd. from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on exptl. measurements are kept in sep. files.
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    25. 25
      Duggan, M.; Ray, N.; Hathaway, B.; Tomlinson, G.; Brint, P.; Pelin, K. Crystal Structure and Electronic Properties of Ammine[tris(2-aminoethyl)amine]copper(I) Diperchlorate and Potassium Penta-amminecopper(II) Tris( hexafluorophosphate). J. Chem. Soc., Dalton Trans. 1980, 1342– 1348,  DOI: 10.1039/dt9800001342
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      Crystal structure and electronic properties of ammine[tris(2-aminoethyl)amine]copper(II) diperchlorate and potassium pentaamminecopper(II) tris(hexafluorophosphate)
      Duggan, Mary; Ray, Noel; Hathaway, Brian; Tomlinson, Gustav; Brint, Paul; Pelin, Kevin
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1980), (8), 1342-8CODEN: JCDTBI; ISSN:0300-9246.
      The crystal and mol. structures of [CuL(NH3)][ClO4]2 [L = N[(CH2)2NH2]3] and K[Cu(NH3)5][PF6]3 were detd. by x-ray crystallog. using 3-dimensional diffraction data and were refined to R 0.0401 and 0.0888 for 649 and 619 unique reflections, resp. Crystals of [CuL(NH3)][ClO4]2 are cubic, space group P213, with a 11.626 Å and Z = 4 whereas crystals of K[Cu(NH3)5][PF6]3 are orthorhombic, space group Imma, with a 14.90, b 11.79, c 10.57 Å, and Z = 6. [CuL(NH3)]+ is trigonal bipyramidal with strict C3 symmetry whereas [Cu(NH3)5]+ is a square-based pyramid with strict C2v symmetry. The ESR of Cu-doped [ZnL(NH3)][ClO4]2 and the ESR and visible spectra of single crystals of [CuL(NH3)][ClO4]2 and K[Cu(NH3)5][PF6]3 were also detd. and related to the 1-electron energy levels calcd. using EHMO methods and other data on high-symmetry 5-coordinate CuN5 chromophores.
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    26. 26
      Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349– 1356,  DOI: 10.1039/DT9840001349
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      Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulfur donor ligands: the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2'-yl)-2,6-dithiaheptane]copper(II) perchlorate
      Addison, Anthony W.; Rao, T. Nageswara; Reedijk, Jan; Van Rijn, Jacobus; Verschoor, Gerrit C.
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1984), (7), 1349-56CODEN: JCDTBI; ISSN:0300-9246.
      The complexes [CuL(OH2)][ClO4]2 (I) and CuL(ClO4)2.2H2O (II) of the linear quadridentate N2S2 donor ligand III (L) were prepd.; they are significantly more stable towards autoredn. than the nonmethyl analogs. The structure of I was detd. by x-ray crystallog.; results were refined to an R of 0.047 for 3343 reflections. The Cu coordination is intermediate between trigonal bipyramidal and square pyramidal. In the solid state the coordination sphere in II may be a topoisomer of I. A new angular structural parameter, τ, is defined as an index of trigonality, a general descriptor of 5-coordinate centric mols. By this criterion the coordination of I in the solid state is described as 48% along the path of distortion from square pyramidal to trigonal bipyramidal. The S → Cu charge-transfer bands in the electronic spectrum of I are assigned. ESR and ligand field spectra show that the Cu compds. adopt a tetragonal structure in donor solvents.
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      Urban, M.; Durka, K.; Jankowski, P.; Serwatowski, J.; Luliński, S. Highly Fluorescent Red-Light Emitting Bis(boranils) Based on Naphthalene Backbone. J. Org. Chem. 2017, 82, 8234– 8241,  DOI: 10.1021/acs.joc.7b01001
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      Highly fluorescent red-light emitting bis(boranils) based on naphthalene backbone
      Urban, Mateusz; Durka, Krzysztof; Jankowski, Piotr; Serwatowski, Janusz; Lulinski, Sergiusz
      Journal of Organic Chemistry (2017), 82 (15), 8234-8241CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Ten bis(boranils) I (1-10; R1 = F, Ph, 2,6-F2C6H3, C6F5, R1-R1 = C6H4OC6H4; R2, R3 = H, NO2, NEt2; R2 = NO2, R3 = NEt2), differently substituted at the boron atom and iminophenyl groups were synthesized from 1,5-dihydroxynaphthalene-2,6-dicarboxaldehyde using a simple one-pot protocol. Their photophys. properties can be easily tuned in a wide range by the variation of substituents. Their absorption and emission spectral bands are significantly red-shifted (λmax = 495-590 nm, λem = 533-683 nm) when compared with simple boranils, whereas fluorescence quantum yields are strongly improved to reach 83%. The attachment of pendant NO2 and NEt2 groups at the opposite positions of the π-conjugated bis(boranil) scaffold resulted in the formation of an unprecedented system featuring push-pull architecture.
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    28. 28
      Allen, C. R.; Richard, P. L.; Ward, A. J.; van de Water, L. G. A.; Masters, A. F.; Maschmeyer, T. Facile synthesis of ionic liquids possessing chiral carboxylates. Tetrahedron Lett. 2006, 47, 7367– 7370,  DOI: 10.1016/j.tetlet.2006.08.007
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      Facile synthesis of ionic liquids possessing chiral carboxylates
      Allen, Christine R.; Richard, Paulina L.; Ward, Antony J.; van de Water, Leon G. A.; Masters, Anthony F.; Maschmeyer, Thomas
      Tetrahedron Letters (2006), 47 (41), 7367-7370CODEN: TELEAY; ISSN:0040-4039. (Elsevier Ltd.)
      The synthesis of 23 new chiral ionic liqs. is achieved in high yields and with good purity by the reaction of an amino acid or a chiral carboxylic acid with tetrabutylammonium hydroxide in water.
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      Gran, G. Determination of the equivalence point in potentiometric titrations Part II. Analyst 1952, 77, 661– 671,  DOI: 10.1039/an9527700661
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      Determination of the equivalence point in potentiometric titrations. II
      Gran, Gunnar
      (1952), 77 (), 661-71 ISSN:.
      cf. C.A. 44, 8815c. When there is only a slight change in e.m.f. at the end point it has been customary to plot a ΔE/Δ V curve and take the peak of this curve as the equiv. point. In the previous paper, a method of transforming these curves by a numerical manipulation was shown and here another method is explained. Some rather complicated expressions are given in the math. treatment but practically all the necessary calcns. can be done easily with the slide rule.
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      (a) Thordarson, P. Determining association constants from titration experiments in supramolecular chemistry. Chem. Soc. Rev. 2011, 40, 1305– 1323,  DOI: 10.1039/C0CS00062K
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      Determining association constants from titration experiments in supramolecular chemistry
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      A review. The most common approach for quantifying interactions in supramol. chem. is a titrn. of the guest to soln. of the host, noting the changes in some phys. property through NMR, UV-Vis, fluorescence or other techniques. Despite the apparent simplicity of this approach, there are several issues that need to be carefully addressed to ensure that the final results are reliable. This includes the use of non-linear rather than linear regression methods, careful choice of stoichiometric binding model, the choice of method (e.g., NMR vs. UV-Vis) and concn. of host, the application of advanced data anal. methods such as global anal. and finally the estn. of uncertainties and confidence intervals for the results obtained. This tutorial review will give a systematic overview of all these issues-highlighting some of the key messages herein with simulated data anal. examples.
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      Krause, L.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination. J. Appl. Crystallogr. 2015, 48, 3– 10,  DOI: 10.1107/S1600576714022985
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      Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination
      Krause, Lennard; Herbst-Irmer, Regine; Sheldrick, George M.; Stalke, Dietmar
      Journal of Applied Crystallography (2015), 48 (1), 3-10CODEN: JACGAR; ISSN:1600-5767. (International Union of Crystallography)
      The quality of diffraction data obtained using silver and molybdenum microsources has been compared for six model compds. with a wide range of absorption factors. The expts. were performed on two 30 W air-cooled Incoatec IμS microfocus sources with multilayer optics mounted on a Bruker D8 goniometer with a SMART APEX II CCD detector. All data were analyzed, processed and refined using std. Bruker software. The results show that Ag Kα radiation can be beneficial when heavy elements are involved. A numerical absorption correction based on the positions and indexes of the crystal faces is shown to be of limited use for the highly focused microsource beams, presumably because the assumption that the crystal is completely bathed in a (top-hat profile) beam of uniform intensity is no longer valid. Fortunately the empirical corrections implemented in SADABS, although originally intended as a correction for absorption, also correct rather well for the variations in the effective vol. of the crystal irradiated. In three of the cases studied (two Ag and one Mo) the final SHELXL R1 against all data after application of empirical corrections implemented in SADABS was below 1%. Since such corrections are designed to optimize the agreement of the intensities of equiv. reflections with different paths through the crystal but the same Bragg 2θ angles, a further correction is required for the 2θ dependence of the absorption. For this, SADABS uses the transmission factor of a spherical crystal with a user-defined value of μr (where μ is the linear absorption coeff. and r is the effective radius of the crystal); the best results are obtained when r is biased towards the smallest crystal dimension. The results presented here suggest that the IUCr publication requirement that a numerical absorption correction must be applied for strongly absorbing crystals is in need of revision.
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      Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97: a new tool for crystal structure determination and refinement. J. Appl. Crystallogr. 1999, 32, 115– 119,  DOI: 10.1107/S0021889898007717
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      SIR97: a new tool for crystal structure determination and refinement
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      Journal of Applied Crystallography (1999), 32 (1), 115-119CODEN: JACGAR; ISSN:0021-8898. (Munksgaard International Publishers Ltd.)
      SIR97 is the integration of two programs, SIR92 and CAOS, the 1st devoted to the soln. of crystal structures by direct methods, the 2nd to refinement via least-squares-Fourier procedures. Several new features were introduced in SIR97 with respect to the previous version, SIR92: greater automatization, increased efficiency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF files.
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      Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect. C 2015, 71, 3– 8,  DOI: 10.1107/S2053229614024218
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      Crystal structure refinement with SHELXL
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      Acta Crystallographica, Section C: Structural Chemistry (2015), 71 (1), 3-8CODEN: ACSCGG; ISSN:2053-2296. (International Union of Crystallography)
      The improvements in the crystal structure refinement program SHELXL have been closely coupled with the development and increasing importance of the CIF (Crystallog. Information Framework) format for validating and archiving crystal structures. An important simplification is that now only one file in CIF format (for convenience, referred to simply as 'a CIF') contg. embedded reflection data and SHELXL instructions is needed for a complete structure archive; the program SHREDCIF can be used to ext. the and files required for further refinement with SHELXL. Recent developments in SHELXL facilitate refinement against neutron diffraction data, the treatment of H atoms, the detn. of abs. structure, the input of partial structure factors and the refinement of twinned and disordered structures. SHELXL is available free to academics for the Windows, Linux and Mac OS X operating systems, and is particularly suitable for multiple-core processors.
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  • Supporting Information

    Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03337.

    • Synthesis of L2; potentiometric and pH-spectrophotometric titrations of L1 and L2; spectrophotometric titrations of [Cu2(L1)] and [Cu2(L2)]; X-ray diffraction studies; extraction experiments; and characterization of L2 (PDF)

    • Crystallographic information file of [Cu2(L1)(glut)](CF3SO3)2·4(H2O) (CIF)

    • Crystallographic information file of [Cu2(L1)(α-keto)](CF3SO3)2·5(H2O) (CIF)

    • Crystallographic information file of [Cu2(L1)(ace)](CF3SO3)2·4(H2O) (CIF)


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