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Selective Anion Binding Drives the Formation of AgI8L6 and AgI12L6 Six-Stranded Helicates

Cite this: J. Am. Chem. Soc. 2021, 143, 2, 664–670
Publication Date (Web):December 31, 2020
https://doi.org/10.1021/jacs.0c11905
Copyright © 2020 American Chemical Society
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Abstract

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Here we describe the formation of an unexpected and unique family of hollow six-stranded helicates. The formation of these structures depends on the coordinative flexibility of silver and the 2-formyl-1,8-napthyridine subcomponent. Crystal structures show that these assemblies are held together by Ag4I, Ag4Br, or Ag6(SO4)2 clusters, where the templating anion plays an integral structure-defining role. Prior to the addition of the anionic template, no six-stranded helicate was observed to form, with the system instead consisting of a dynamic mixture of triple helicate and tetrahedron. Six-stranded helicate formation was highly sensitive to the structure of the ligand, with minor modifications inhibiting its formation. This work provides an unusual example of mutual stabilization between metal clusters and a self-assembled metal–organic cage. The selective preparation of this anisotropic host demonstrates new modes of guiding selective self-assembly using silver(I), whose many stable coordination geometries render design difficult.

Self-assembly can produce complex metal–organic architectures from simple starting materials. (1−5) Such structures have been the subject of intense recent exploration, with applications spanning guest binding, stabilization of reactive species, biomolecular interactions, and chemical purification. (6−9) These applications often depend on binding a target in the pseudospherical cavity of a metal–organic cage. These isotropic cavities can bind roughly spherical guests or guest agglomerates (10−13) but are ill-adapted to bind asymmetric and anisotropic guests. The introduction of flexible organic ligands (14−16) or metal coordination spheres (17−20) has led to the formation of new metal–organic cages, with nonspherical internal cavities, partially alleviating these limitations. (21−25) Silver(I), in combination with dipyridyl peptidic linkers, has recently been shown to generate a wealth of complex knotted architectures via self-assembly. (26−28) The strategy of incorporating a guest of interest into the architecture formed, as a template (29−31) or other structural element, (32,33) can enhance selectivity and sensitivity in guest binding. (34−36) Furthermore, if the guest is anionic, (37−39) the diverse coordination chemistry of anions can be used to effect the selective recognition (40−42) of targeted anions. (43)

We hypothesized that the flexible coordination sphere of silver(I) ions, (44−49) in combination with organic ligands that assemble in situ around these metal-ion templates, would provide access to new structure types that bind anions as structural elements. Zhao and co-workers have previously shown that nitrogen containing macrocycles can stabilize atomically precise silver clusters with defined geometries, supporting this hypothesis. (50,51)

Here we describe the formation of a family of complex six-stranded silver helicates upon the addition of three anions: iodide, bromide, and sulfate. This family comprises two novel structure types, with sulfate generating a structure distinct from those templated by halides. Key structural elements within these architectures are unique silver(I)-anion clusters, (50,51) whose geometries are molded by the central anions, which in turn are held in an unusual, polarized, environment.

Building on the discovery that silver(I) assembles with 2-formyl-1,8-naphthyridine (1), (52) a tritopic subcomponent, and anionic templates to form a trigonal prism with disilver vertices, (17) we investigated the use of linear ditopic anilines in place of triangular ones. Initial experiments, involving the mixture of benzidine (2) together with 1, various silver salts, and prospective guests in acetonitrile (Figure 1a), gave in all cases an intractable gel (SI Section 8).

Figure 1

Figure 1. Self-assembly of Ag4L3 and Ag8L6 architectures. Conditions: (a) AgNTf2 (2 equiv), 2 (1 equiv), 1 (2 equiv), d3-MeCN, 5 min; (b) AgNTf2 (2 equiv), 3 or 4 (1 equiv), 1 (2 equiv), d3-MeCN, 5 min. Structures of 5 and 6 are MM3-optimized models. (c) DOSY NMR of 5 and 6.

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Reasoning that increasing steric hindrance and widening the torsion angle between the phenylene groups of the dianiline could lead to a different outcome, (53) we explored the self-assembly of 2,2′-dimethyl-[1,1′-biphenyl]-4,4′-diamine (3) with 1 in acetonitrile, and observed the formation of discrete species with various silver(I) salts (Figure 1b). With silver perchlorate, we observed a 1:1 ratio of integrals between two species (Figure 1c). Diffusion ordered spectroscopy (DOSY) NMR revealed that one had a significantly larger diffusion coefficient (Figure 1c). Mass spectrometry indicated that the smaller species had Ag4L3 composition, with the larger species corresponding to Ag8L6 (Figures S72 and S75). Approximately 400 attempts to grow crystals of these species failed.

The observation of well-defined bands of peaks in the DOSY spectrum is consistent with the formation of discrete species, as opposed to poorly defined oligomers in solution. (54,55) We modeled potential structures for the Ag8L6 architecture and found that a tetrahedral geometry was preferred by 300–400 kcal mol–1 (SI Section 9). (56) Although we cannot definitively assign the product structures without crystallographic data, we infer that the two species are likely to be Ag4L3 helicate 5 and Ag8L6 tetrahedron 6, consistent with previously reported systems, (57) our modeling studies, and the solution data (SI Section 4.4). Investigations of host–guest behavior showed binding to a range of anionic and organic guests, with some altering the 5:6 equilibrium (SI Section 7). (58,59) When dianiline 4 was used in place of 3 we observed similar results (Figure 1b and SI Section 10).

Having extensively screened potential guest species, we next turned to the addition of halides to these silver(I) based assemblies. We had initially avoided the use of halides, anticipating precipitation of silver halide species (the solubility product of AgI is 10–14.5 in acetonitrile). (60) However, upon addition of TBA iodide, a new species, 9, immediately formed and, to our surprise, no precipitate was observed.

Characteristic 1H NMR signals were observed for 9 at 6 ppm, ca. 1 ppm upfield of any signals of 5 or 6 (Figure 1c). Furthermore, a twofold desymmetrization was observed, with two 1H NMR signals observed for each proton environment in free ligand (Figure S1). DOSY spectroscopy gave results consistent with the formation of a single species (Figure S8). Mass spectrometry confirmed that a Ag8L6I2 architecture had been formed (Figures S69 and S77). (61)

The X-ray crystal structure of 9 revealed its highly unusual six-stranded helicate structure (Figure 3a,b), which is capped at each end by a Ag4I cluster consisting of a Ag3 triangle capped by an apical Ag on the outside and iodide on the inside (Figure 3e). The six ligand strands bridge two such Ag4I clusters, grouped into three pairs of ligands that show aromatic stacking interactions between naphthyridine moieties, with distances of 3.1–3.7 Å between stacked rings.

Atypical coordination environments for the Ag centers were observed in 9. One arm of each ligand coordinates via all three available nitrogen donors, and the other via only a single inner naphythridine nitrogen. This differentiation leads to the twofold desymmetrization seen in the 1H NMR spectrum

The presence of 12 uncoordinated nitrogen donors within 9 violates the principle of maximal coordinative saturation, which has often, and successfully, been used to predict the product of metal–organic self-assembly processes. (62) The absence of coordinative stabilization may be a consequence of the nonchelating coordination vectors of 1, which precluded the formation of simple structures. The lack of coordinative saturation is compensated for by the extensive aromatic stacking seen in the crystal structure of 9. (63)

Silver–silver separations were 2.96–3.00 Å between silver atoms bridged by a single naphthyridine moiety, greater than those observed in simpler mononuclear naphthyridine-bridged silver complexes. (63) The iodide ion coordinated to all four Ag ions in the cluster, with Ag–I separations of 2.79–2.88 Å, consistent with previous reports of Ag4I clusters. (50,51)

Having determined the structure of 9, we investigated whether alternative anions might lead to the generation of further examples of this new structure type. Addition of tetramethylammonium sulfate to a mixture of 1, 3, and silver triflimide brought about conversion to an alternate species, 10, as the uniquely observed product (Figure 2). This product again showed twofold desymmetrization in the 1H NMR (Figure S9) and a single species by DOSY NMR (Figure S14). We initially anticipated that a structure analogous to 9 would be formed, with Ag8L6(SO4)2 stoichiometry, based upon similarities between 1H NMR spectra (Figure S9). However, mass spectrometry indicated that instead a Ag12L6(SO4)4 species formed (Figures S70 and S78). Six-stranded helicate formation was confirmed by single-crystal X-ray diffraction (Figure 3c,d). The organic portion of the structure was similar to 9, yet the silver clusters at the ends of both assemblies are dramatically different. Instead of the Ag4I clusters of 9, the vertices of 10 consist of Ag6(SO4)2 clusters composed of inner and outer Ag3 triangles. The externally facing sulfate coordinates to the outer triangle of silver ions via a single, triply coordinated, oxygen atom. (64) The coordination of this sulfate is reinforced by nonclassical hydrogen bonding from three naphthyridine CH groups (CH···O distances 2.40–2.43 Å), stabilizing the assembly (Figure 3f). (65) Each silver ion of this outer triangle is also coordinated by the internal sulfate via a single, triply coordinated oxygen. The interior sulfate additionally coordinates to the internal, more widely spaced, triangle of silver ions. The two Ag triangles form pairs of silver ions in close proximity, with each bridged by two naphthyridine moieties. The sulfur atoms of the internal anions are 11.58 Å apart, farther than the iodide anions in 9 (10.47 Å), and show nonclassical hydrogen bonds (CH···O distances 2.58–2.69 Å) to internally facing CH groups (Figure 3g). Ligand coordination again shows pairwise alternation, here between three and two coordinating nitrogen atoms per ligand arm. The uncoordinated donor atoms were again imine nitrogens

Figure 2

Figure 2. (a) Synthesis of six-stranded helicates 9 and 10, formed only during self-assembly from dianiline 3. (i) Tetrabutylammonium iodide (0.34 equiv), 5 min; (ii) tetramethylammonium sulfate (1.0 equiv), 6 h. Structures of 5 and 6 are MM3 optimized models, and those of 9 and 10 are based on crystallographic data (vide infra). Simplified representation of six-stranded helicate (b) 9 and (c) 10.

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Figure 3

Figure 3. (a) X-ray crystal structure of 9; (b) schematic view of 9. (c) X-ray crystal structure of 10; (d) schematic view of 10. (e) End-on view of crystal structure of 9 showing cluster geometry. (f) End-on view of crystal structure of 10 showing the silver cluster and nonclassical hydrogen bonds to the exterior sulfate. (g) View from within the crystal structure of 10, showing nonclassical hydrogen bonds to the internal sulfate.

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We next investigated whether other anions could template structures similar to 9 and 10. Among the 38 anions tested (SI Sections 6.7 and 6.8), only bromide proved able to efficiently template a six-stranded helicate (11). The 1H NMR spectrum of 11 again exhibited a twofold desymmetrization, and a single species was observed by DOSY spectroscopy, with a hydrodynamic radius of 11.9 Å, similar to the cases of 9 and 10 (Figures S8, S15, and S22). Attempts to grow crystals suitable for X-ray diffraction proved unsuccessful. However, we inferred the Ag8L6Br2 structure of 11 to be an analogue of 9 by comparing the 1H NMR, COSY, and HSQC spectra of 9–11. The spectra of 9 and 11 were clearly similar, whereas that of 10 was notably different (Figure 4a and SI Section 5).

Figure 4

Figure 4. (a) Comparison of 1H NMR spectra of 10 (top), 11 (middle), and 9 (bottom), showing the similarity between the spectra of 9 and 11. Simplified (b) schematic and (c) cartoon views of six-stranded helicate 11.

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We then probed further the selectivity of the assembly process. Silver tetrafluoroborate, hexafluorophosphate, perchlorate, and triflate all furnished six-stranded helicates adopting the framework of 9 when combined with 1, 3, and potassium iodide (Figures S38 and S39). Titration of TBA bromide into a mixture of 5 and 6 revealed no intermediate species (i.e., from binding a single bromide). Instead, formation of 11 (containing two bromide anions) was seen immediately, in the continued presence of 5 and 6 (Figures S42 and S46), suggesting that the six-stranded helicate assembled cooperatively (SI Sections 6.3 and 6.6). Using 2 or 4 in place of 3 led to immediate gelation (for 2) or shifts in the equilibrium of 7 and 8 (for 4, Figures S55 and S68).

These results highlight the extent to which the subcomponent self-assembly of metal–organic architectures may depend critically upon subtle variations in subcomponent structure. The lack of methyl groups on 2 favored polymerization over the assembly of discrete structures. The subtle steric and electronic differences between the methyl groups of 3 and the trifluoromethyl groups of 4 disfavored, in the latter case, the formation of six-stranded helicates analogous to 9–11. We hypothesize this sensitivity to be due to the slightly weaker ligand field in the case of ligands incorporating 4, which disfavors structures that incorporate the more highly cationic silver clusters incorporated into the new structure types 9–11.

This work describes the development of a system of novel six-stranded helicates, which assemble around atomically precise silver clusters. Specific anionic templates, in turn, serve to shape these clusters, such that the identity of the anion dictates the architecture observed. The ability of 2-formyl-1,8-napthyridine to bridge silver ions enables these complex structures to form from simple subcomponents. These new assemblies are sensitive to the precise nature of the ligand chosen and are selective for the templates employed, with potential applications in sensing specific analytes.

The ability to use atomically precise clusters in place of mono- or dimetallic vertices in metal–organic cages has the potential to generate a vastly increased diversity of architectures, as we continue to uncover the principles underpinning silver–naphthyridine self-assembly. Future work will focus on exploring the photophysical properties of these novel clusters (66) and on expanding the range of architectures formed by the interplay of anion templation, ligand design, and coordinational flexibility to generate increased structural diversity.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.0c11905.

  • Experimental procedure and details; MM3 models and calculated energies; mass spectrometry data; X-ray crystallography data (PDF)

  • X-ray data for 9 (CCDC 2024152) (CIF)

  • X-ray data for 10 (CCDC 2024153) (CIF)

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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.

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Acknowledgments

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This work was supported by the Engineering and Physical Sciences Research Council (EPSRC, EP/P027067/1) and the European Research Council (695009). We thank the University of Cambridge Mass Spectrometry Service Centre for high-resolution mass spectrometry and Diamond Light Source (UK) for synchrotron beamtime on I19 (CY21497). C.T.M. thanks the Leverhulme Trust and the Isaac Newton Trust, and Sidney Sussex College, Cambridge, for Fellowship support.

References

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    A water-sol. self-assembled supramol. FeII4L4 tetrahedron binds to single stranded DNA, mismatched DNA base pairs, and three-way DNA junctions. Binding of the coordination cage quenches fluorescent labels on the DNA strand, which provides an optical means to detect the interaction and allows the position of the binding site to be gauged with respect to the fluorescent label. Using the quenching and binding properties of the coordination cage, the authors developed a simple and rapid detection method based on fluorescence quenching to detect unpaired bases in double-stranded DNA.
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  9. 9
    Hotze, A. C. G.; Kariuki, B. M.; Hannon, M. J. Dinuclear Double-Stranded Metallosupramolecular Ruthenium Complexes: Potential Anticancer Drugs. Angew. Chem., Int. Ed. 2006, 45, 4839– 4842,  DOI: 10.1002/anie.200601351
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    9
    Dinuclear double-stranded metallosupramolecular ruthenium complexes: potential anticancer drugs
    Hotze Anna C G; Kariuki Benson M; Hannon Michael J
    Angewandte Chemie (International ed. in English) (2006), 45 (29), 4839-42 ISSN:1433-7851.
    There is no expanded citation for this reference.
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  10. 10
    Chen, B.; Holstein, J. J.; Horiuchi, S.; Hiller, W. G.; Clever, G. H. Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes. J. Am. Chem. Soc. 2019, 141, 8907– 8913,  DOI: 10.1021/jacs.9b02207
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    10
    Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes
    Chen, Bin; Holstein, Julian J.; Horiuchi, Shinnosuke; Hiller, Wolf G.; Clever, Guido H.
    Journal of the American Chemical Society (2019), 141 (22), 8907-8913CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Fullerenes and their derivs. are of tremendous technol. relevance. Synthetic access and application are still hampered by tedious purifn. protocols, peculiar soly., and limited control over regioselective derivatization. Authors present a modular self-assembly system based on a new low-mol.-wt. binding motif, appended by two palladium(II)-coordinating units of different steric demands, to either form a [Pd2L14]4+ cage or an unprecedented [Pd2L23(MeCN)2]4+ bowl (with L1 = pyridyl, L2 = quinolinyl donors). The former was used as a selective induced-fit receptor for C60. The latter, owing to its more open structure, also allows binding of C70 and fullerene derivs. By exposing only a fraction of the bound guests' surface, the bowl acts as fullerene protecting group to control functionalization, as demonstrated by exclusive monoaddn. of anthracene. In a hierarchical manner, sterically low-demanding dicarboxylates were found to bridge pairs of bowls into pill-shaped dimers, able to host two fullerenes. The hosts allow transferring bound fullerenes into a variety of org. solvents, extending the scope of possible derivatization and processing methodologies.
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  11. 11
    Rizzuto, F. J.; von Krbek, L. K. S.; Nitschke, J. R. Strategies for Binding Multiple Guests in Metal-Organic Cages. Nat. Chem. Rev. 2019, 3, 204– 222,  DOI: 10.1038/s41570-019-0085-3
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    Takezawa, H.; Shitozawa, K.; Fujita, M. Enhanced Reactivity of Twisted Amides Inside a Molecular Cage. Nat. Chem. 2020, 12, 574– 578,  DOI: 10.1038/s41557-020-0455-y
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    12
    Enhanced reactivity of twisted amides inside a molecular cage
    Takezawa, Hiroki; Shitozawa, Kosuke; Fujita, Makoto
    Nature Chemistry (2020), 12 (6), 574-578CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
    Inclusion of N-aryl amides in octahedral (Td-sym.) metal cages generates inclusion complexes in which the amides have significantly twisted geometries. The structures of a variety of inclusion complexes, including ternary inclusion complexes, were detd. by X-ray crystallog.; the cis-twisted conformers were favored in some cases over the trans-planar amide conformers with twist angles of up to 34°. The kinetics of the basic hydrolysis of amide inclusion complexes was detd.; the rates of hydrolysis for included amides were significantly increased over the rate of hydrolysis of the free (unincluded) amide.
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  13. 13
    Chepelin, O. Luminescent, Enantiopure, Phenylatopyridine Iridium-Based Coordination Capsules. J. Am. Chem. Soc. 2012, 134, 19334– 19337,  DOI: 10.1021/ja309031h
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    13
    Luminescent, Enantiopure, Phenylatopyridine Iridium-Based Coordination Capsules
    Chepelin, Oleg; Ujma, Jakub; Wu, Xiaohua; Slawin, Alexandra M. Z.; Pitak, Mateusz B.; Coles, Simon J.; Michel, Julien; Jones, Anita C.; Barran, Perdita E.; Lusby, Paul J.
    Journal of the American Chemical Society (2012), 134 (47), 19334-19337CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The 1st mol. capsule based on an [Ir(ppy)2]+ unit (ppy = 2-phenylatopyridine) was prepd. Following the development of a method to resolve rac-[(Ir(ppy)2Cl)2] into its enantiopure forms, homochiral Ir6L4 octahedra where obtained with the tritopic 1,3,5-tricyanobenzene. Soln. studies and x-ray diffraction show that these capsules encapsulate four of the six assocd. counteranions and that these can be exchanged for other anionic guests. Initial photophys. studies showed that an ensemble of weakly coordinating ligands can lead to luminescence not present in comparable mononuclear systems.
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    Mosquera, J.; Ronson, T. K.; Nitschke, J. R. Subcomponent Flexibility Enables Conversion between D4-Symmetric CdII4L4 Assemblies. J. Am. Chem. Soc. 2016, 138, 1812– 1815,  DOI: 10.1021/jacs.5b12955
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    14
    Subcomponent Flexibility Enables Conversion between D4-Symmetric CdII8L8 and T-Symmetric CdII4L4 Assemblies
    Mosquera, Jesus; Ronson, Tanya K.; Nitschke, Jonathan R.
    Journal of the American Chemical Society (2016), 138 (6), 1812-1815CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A flexible tris-formylpyridine subcomponent A (I) was obsd. to produce three distinct products following CdII-templated self-assembly with different anilines. Two of the products were CdII4L4 tetrahedra (C192H172Cd4F48N32O46S16), one with ligands puckered inward, and the other outward. The third product was a CdII8L8 structure (C399H382.5Cd8F48N63.5O72S16) having all mer stereochem., contrasting with the fac stereochem. of the tetrahedra. These three complexes coexist in soln. The equil. between them could be influenced through guest binding and specific interactions between aniline subcomponents, allowing a selected one of the three to predominate under defined conditions.
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  15. 15
    Cullen, W.; Misuraca, M. C.; Hunter, C. A.; Williams, N. H.; Ward, M. D. Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage. Nat. Chem. 2016, 8, 231– 236,  DOI: 10.1038/nchem.2452
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    15
    Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage
    Cullen, William; Misuraca, M. Cristina; Hunter, Christopher A.; Williams, Nicholas H.; Ward, Michael D.
    Nature Chemistry (2016), 8 (3), 231-236CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    The hollow cavities of coordination cages can provide an environment for enzyme-like catalytic reactions of small-mol. guests. Here, we report a new example (catalysis of the Kemp elimination reaction of benzisoxazole with hydroxide to form 2-cyanophenolate) in the cavity of a water-sol. M8L12 coordination cage, with two features of particular interest. First, the rate enhancement is among the largest obsd. to date: at pD 8.5, the value of kcat/kuncat is 2 × 105, due to the accumulation of a high concn. of partially desolvated hydroxide ions around the bound guest arising from ion-pairing with the 16+ cage. Second, the catalysis is based on two orthogonal interactions: (1) hydrophobic binding of benzisoxazole in the cavity and (2) polar binding of hydroxide ions to sites on the cage surface, both of which were established by competition expts.
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  16. 16
    Liu, Y.; Zhang, R.; He, C.; Dang, D. B.; Duan, C. Y. A Palladium(II) Triangle as Building Blocks of Microporous Molecular Materials: Structures and Catalytic Performance. Chem. Commun. 2010, 46, 746– 748,  DOI: 10.1039/B916916D
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    16
    A palladium(II) triangle as building blocks of microporous molecular materials: structures and catalytic performance
    Liu, Yang; Zhang, Rong; He, Cheng; Dang, Dongbin; Duan, Chunying
    Chemical Communications (Cambridge, United Kingdom) (2010), 46 (5), 746-748CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A porous mol. cryst. solid based on amide-contg. Pd(II) triangles, [Pd3L3](NO3)6·34CH3OH·8H2O (L = 3,3',5,5'-tetrakis(pyridin-4-ylmethylaminocarbonyl)diphenylmethane) was prepd., characterized by x-ray crystallog., and evaluated for size-selective heterogeneous catalysis of the Knoevenagel condensation reaction.
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  17. 17
    Carpenter, J. P.; McTernan, C. T.; Ronson, T. K.; Nitschke, J. R. Anion Pairs Template a Trigonal Prism with Disilver Vertices. J. Am. Chem. Soc. 2019, 141, 11409– 11413,  DOI: 10.1021/jacs.9b05432
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    17
    Anion Pairs Template a Trigonal Prism with Disilver Vertices
    Carpenter, John P.; McTernan, Charlie T.; Ronson, Tanya K.; Nitschke, Jonathan R.
    Journal of the American Chemical Society (2019), 141 (29), 11409-11413CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Here authors describe the formation of a trigonal prismatic cage, utilizing 2-formyl-1,8-naphthyridine subcomponents to bind pairs of silver(I) ions in close proximity. This cage is the first example of a new class of subcomponent self-assembled polyhedral structures having bimetallic vertices, as opposed to the single metal centers that typically serve as structural elements within such cages. The new cage self-assembles around a pair of anionic templates, which are shown by crystallog. and soln.-phase data to bind within the central cavity of the structure. Many different anions serve as competent templates and guests. Elongated dianions, such as the strong oxidizing agent peroxysulfate, also serve to template and bind within the cavity of the prism. The principle of using subcomponents that have more than one spatially close, but nonchelating, binding site may thus allow access to other higher-order structures with multimetallic vertices.
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  18. 18
    Huang, S.; Lin, Y.; Hor, T. S. A.; Jin, J. Cp*Rh-based Heterometallic Metallarectangles: Size-Dependent Borromean Link Structures and Catalytic Acyl Transfer. J. Am. Chem. Soc. 2013, 135, 8125– 8128,  DOI: 10.1021/ja402630g
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    18
    Cp*Rh-Based Heterometallic Metallarectangles: Size-Dependent Borromean Link Structures and Catalytic Acyl Transfer
    Huang, Sheng-Li; Lin, Yue-Jian; Hor, T. S. Andy; Jin, Guo-Xin
    Journal of the American Chemical Society (2013), 135 (22), 8125-8128CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Cp*Rh-based functional metallarectangles were synthesized from metallaligands, e.g., [Cu(opba)]2- [opba = o-phenylenebis(oxamato)]. Enlargement of one linker gives two novel Borromean link architectures. All these complexes are intact in soln., as evident from ESI-MS spectroscopic anal. Arising from the combination of open Cu centers and favorable cavity space, {(Cp*Rh)4(bpe)2[Cu(opba)·2MeOH]2}4(OTf)·6MeOH shows extraordinary catalytic abilities with high efficiency and wide substrate selectivity in the acyl-transfer reaction.
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  19. 19
    Yue, N. L. S.; Jennings, M. C.; Puddephatt, R. J. Disilver(I) Macrocycles: Variation of Cavity Size with Anion Binding. Inorg. Chem. 2005, 44, 1125– 1131,  DOI: 10.1021/ic048549c
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    19
    Disilver(I) Macrocycles: Variation of Cavity Size with Anion Binding
    Yue, Nancy L. S.; Jennings, Michael C.; Puddephatt, Richard J.
    Inorganic Chemistry (2005), 44 (4), 1125-1131CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Reaction of the N-methylated bis(amidopyridine) ligand, LL = C6H4(1,3-CONMe-4-C5H4N)2, with AgNO3, AgO2CCF3, AgO3SCF3, AgBF4, and AgPF6 gave the corresponding cationic disilver(I) macrocycles [Ag2(μ-LL)2]X2, 2a-e, resp. The transannular Ag···silver distance in the macrocycles varies greatly from 2.99 to 7.03 Å, and these differences arise through a combination of different modes of anion binding and from the presence or absence of Ag···silver secondary bonding. In all complexes, the ligand adopts a conformation in which the Me group and O atom of the MeNCO units are mutually cis, but the overall macrocycle can exist in either boat (X = PF6 only) or chair conformation. Short transannular Ag···silver distances are found in complexes 2b,c, in which the anions CF3CO2- and CF3SO3- bind above and below the macrocycle, but longer Ag···silver distances are found for 2a,d,e, in which the anions are present, at least in part, inside the disilver macrocycle. Easy anion exchange occurs in soln., and studies using ESI-MS indicate that the anion binding to form [Ag2X(μ-LL)2]+ follows the sequence X = CF3CO2- > NO3- > CF3SO3-.
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  20. 20
    Dong, Y.-B.; Geng, Y.; Ma, J.-P.; Huang, R.-Q. Organometallic Silver(I) Supramolecular Complexes Generated from Multidentate Furan-Containing Symmetric and Unsymmetric Fulvene Ligands and Silver(I) Salts. Inorg. Chem. 2005, 44, 1693– 1703,  DOI: 10.1021/ic048518h
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    20
    Organometallic Silver(I) Supramolecular Complexes Generated from Multidentate Furan-Containing Symmetric and Unsymmetric Fulvene Ligands and Silver(I) Salts
    Dong, Yu-Bin; Geng, Yan; Ma, Jian-Ping; Huang, Ru-Qi
    Inorganic Chemistry (2005), 44 (6), 1693-1703CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    One new conjugated sym. fulvene ligand L1 (I, R = 2-furyl) and two new unsym. fulvene ligands L2 and L3 (I, R = 4- and 3-cyanophenyl, resp.) were synthesized. Five new supramol. complexes, Ag2(L1)3(SO3CF3)3 (1, monoclinic, space group P21/c; a 12.702(3), b 26.118(7), c 13.998(4) Å, β 96.063(4)°, Z = 4), [Ag(L1)]ClO4 (2, monoclinic, space group C2/c; a 17.363(2), b 13.2794(18), c 13.4884(18) Å, β 100.292(2)°, Z = 8), [Ag(L1)(C6H6)SbF6]·0.5C6H6·H2O (3, monoclinic, P21/c; a 6.8839(11), b 20.242(3), c 18.934(3) Å, β 91.994(3)°, Z = 4), Ag(L2)(SO3CF3) (4, triclinic, P‾1; a 8.629(3), b 10.915(3), c 11.178(3) Å, α 100.978(4), β 91.994(3), γ 105.652(4)°, Z = 2), and Ag(L3)(H2O)(SO3CF3) (5, triclinic, P‾1; a 8.914(5), b 10.809(6), c 11.283(6) Å, α 69.255(8), β 87.163(9), γ 84.993(8)°, Z = 2) were obtained through self-assembly based on these three new fulvene ligands in a benzene/toluene mixed-solvent system. Compds. 1-5 were fully characterized by IR spectroscopy, elemental anal., and single-crystal x-ray diffraction. The coordination chem. of new fulvene ligands is versatile. They can adopt either cis- or trans-conformation to bind soft acid Ag(I) ion through not only the terminal -CN and furan functional groups but also the fulvene carbon atoms into organometallic coordination polymers or discrete complexes. The luminescent properties of L1-L3 and their Ag(I) complexes were studied preliminarily in EtOH and solid state.
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  21. 21
    Wang, Q.; Gonell, S.; Leenders, S. H. A. M.; Dürr, M.; Ivanović-Burmazović, I.; Reek, J. N. H. Self-Assembled Nanospheres with Multiple Endohedral Binding Sites Pre-Organize Catalysts and Substrates for Highly Efficient Reactions. Nat. Chem. 2016, 8, 225– 230,  DOI: 10.1038/nchem.2425
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    21
    Self-assembled nanospheres with multiple endohedral binding sites pre-organize catalysts and substrates for highly efficient reactions
    Wang, Qi-Qiang; Gonell, Sergio; Leenders, Stefan H. A. M.; Duerr, Maximilian; Ivanovic-Burmazovic, Ivana; Reek, Joost N. H.
    Nature Chemistry (2016), 8 (3), 225-230CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Tuning reagent and catalyst concns. is crucial in the development of efficient catalytic transformations. In enzyme-catalyzed reactions the substrate is bound-often by multiple noncovalent interactions-in a well-defined pocket close to the active site of the enzyme; this pre-organization facilitates highly efficient transformations. Here the authors report an artificial system that coencapsulates multiple catalysts and substrates within the confined space defined by an M12L24 nanosphere that contains 24 endohedral guanidinium-binding sites. Cooperative binding means that sulfonate guests are bound much more strongly than carboxylates. This difference was used to fix Au-based catalysts firmly, with the remaining binding sites left to pre-organize substrates. This strategy was applied to a Au(I)-catalyzed cyclization of acetylenic acid to enol lactone in which the pre-organization resulted in much higher reaction rates. Also the encapsulated sulfonate-contg. Au(|) catalysts did not convert neutral (acid) substrates, and so could have potential in the development of substrate-selective catalysis and base-triggered on/off switching of catalysis.
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    Lisboa, L. S.; Findlay, J. A.; Wright, L. J.; Hartinger, C. G.; Crowley, J. D. A Reduced Symmetry Heterobimetallic [PdPtL4]4+ Cage: Assembly, Guest Binding and Stimulus-Induced Switching. Angew. Chem., Int. Ed. 2020, 59, 11101– 11107,  DOI: 10.1002/anie.202003220
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    22
    A Reduced-Symmetry Heterobimetallic [PdPtL4]4+ Cage: Assembly, Guest Binding, and Stimulus-Induced Switching
    Lisboa, Lynn S.; Findlay, James A.; Wright, L. James; Hartinger, Christian G.; Crowley, James D.
    Angewandte Chemie, International Edition (2020), 59 (27), 11101-11107CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A strategy is presented that enables the quant. assembly of a heterobimetallic [PdPtL4]4+ cage. The presence of two different metal ions (PdII and PtII) with differing labilities enables the cage to be opened and closed selectively at one end upon treatment with suitable stimuli. Combining an inert PtII tetrapyridylaldehyde complex with a suitably substituted pyridylamine and PdII ions led to the assembly of the cage. 1H and DOSY NMR spectroscopy and ESI mass spectrometry data were consistent with the quant. formation of the cage, and the heterobimetallic structure was confirmed using single-crystal x-ray crystallog. The structure of the host-guest adduct with a 2,6-diaminoanthraquinone guest mol. was detd. Addn. of N,N'-dimethylaminopyridine (DMAP) resulted in the formation of the open-cage [PtL4]2+ compd. and [Pd(DMAP)4]2+ complex. This process could then be reversed, with the reformation of the cage, upon addn. of p-toluenesulfonic acid (TsOH).
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    Ueda, Y.; Ito, H.; Fujita, D.; Fujita, M. Permeable Self-Assembled Molecular Containers for Catalyst Isolation Enabling Two-Step Cascade Reactions. J. Am. Chem. Soc. 2017, 139, 6090– 6093,  DOI: 10.1021/jacs.7b02745
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    23
    Permeable Self-Assembled Molecular Containers for Catalyst Isolation Enabling Two-Step Cascade Reactions
    Ueda, Yoshihiro; Ito, Hiroaki; Fujita, Daishi; Fujita, Makoto
    Journal of the American Chemical Society (2017), 139 (17), 6090-6093CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Establishment of a general one-pot cascade reaction protocol would dramatically reduce the effort of multistep org. synthesis. We demonstrate that the unique structure of M12L24 self-assembled complexes gives them the potential to serve as catalyst carriers for enabling continuous chem. transformations. A stereoselective cascade reaction (allylic oxidn. followed by Diels-Alder cyclization) with two intrinsically incompatible catalysts was demonstrated. Our system is advantageous in terms of availability, scalability, and predictability.
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    Holloway, L. R.; Bogie, P. M.; Lyon, Y.; Ngai, C.; Miller, T. F.; Julian, R. R.; Hooley, R. J. Tandem Reactivity of a Self-Assembled Cage Catalyst with Endohedral Acid Groups. J. Am. Chem. Soc. 2018, 140, 8078– 8081,  DOI: 10.1021/jacs.8b03984
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    24
    Tandem Reactivity of a Self-Assembled Cage Catalyst with Endohedral Acid Groups
    Holloway, Lauren R.; Bogie, Paul M.; Lyon, Yana; Ngai, Courtney; Miller, Tabitha F.; Julian, Ryan R.; Hooley, Richard J.
    Journal of the American Chemical Society (2018), 140 (26), 8078-8081CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Self-assembly of a carboxylic acid-contg. ligand into an Fe4L6 iminopyridine cage allows endohedral positioning of the acid groups while maintaining a robust cage structure. The cage is an effective supramol. catalyst, providing up to 1000-fold rate enhancement of acetal solvolysis. This enhanced reactivity allows a tandem deprotection/cage-to-cage interconversion that cannot be achieved with other acid catalysts. The combination of rate enhancements and sequestration of the reactive function confers both activity and selectivity on the process, mimicking enzymic behavior.
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    Hua, B.; Shao, L.; Zhang, Z.; Liu, J.; Huang, F. Cooperative Silver Ion-Pair Recognition by Peralkylated Pillar[5]arenes. J. Am. Chem. Soc. 2019, 141, 15008– 15012,  DOI: 10.1021/jacs.9b08257
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    25
    Cooperative Silver Ion-Pair Recognition by Peralkylated Pillar[5]arenes
    Hua, Bin; Shao, Li; Zhang, Zhihua; Liu, Jiyong; Huang, Feihe
    Journal of the American Chemical Society (2019), 141 (38), 15008-15012CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Ion-pair recognition of metal salts by pillararenes has never been reported. This limits the further development and applications of pillararene supramol. chem. Herein we report the first examples of pillararene-based metal ion-pair recognition. They are host-guest complexes between three peralkylated pillar[5]arenes and the silver salt CF3COOAg. Single crystal X-ray anal. shows that CF3COOAg forms a unique dinuclear silver structure that penetrates into the pillar[5]arene cavity in these complexes. Besides the silver-π interactions, multiple C-H···O and C-H···F hydrogen bonds between the counterion and the pillar[5]arene host also contribute to stabilizing and maintaining the host-guest structures. Moreover, benefiting from the silver ion-pair recognition, perethylated pillar[5]arene EtP5 can be used as a solid absorbent to capture CF3COOAg from soln. efficiently, showing a potential application in precious metal extn. and recycling.
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  26. 26
    Sawada, T.; Fujita, M. Folding and Assembly of Metal-Linked Peptidic Nanostructures. Chem. 2020, 6, 1861– 1876,  DOI: 10.1016/j.chempr.2020.07.002
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    26
    Folding and Assembly of Metal-Linked Peptidic Nanostructures
    Sawada, Tomohisa; Fujita, Makoto
    Chem (2020), 6 (8), 1861-1876CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    A review. Folding and assembly are intra- and inter-mol. processes, resp., for spontaneously generating predetd., well-defined mol. structures. Though the cooperative processes of these mechanisms have been studied in coiled coils and β-sheets, they have been seldom utilized simultaneously in the synthetic fields. We have been developing a new folding-and-assembly (F&A) strategy, in which the folding and metal-directed self-assembly of a short peptide fragment occur simultaneously by helping and inducing the processes of each other. Depending on the sequence design, each short peptide ligand exhibits a specific conformation during metal coordination and simultaneously assembles into an advanced nanostructure, which has been hardly achieved by known synthetic strategies. The concepts, examples of the F&A strategy, the function of the obtained structures as well as future directions are disclosed in this perspective.
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  27. 27
    Sawada, T.; Inomata, Y.; Shimokawa, K.; Fujita, M. A Metal-Peptide Capsule by Multiple Ring Threading. Nat. Commun. 2019, 10, 5687,  DOI: 10.1038/s41467-019-13594-4
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    27
    A metal-peptide capsule by multiple ring threading
    Sawada, Tomohisa; Inomata, Yuuki; Shimokawa, Koya; Fujita, Makoto
    Nature Communications (2019), 10 (1), 5687CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Cavity creation is a key to the origin of biol. functions. Small cavities such as enzyme pockets are created simply through liner peptide folding. Nature can create much larger cavities by threading and entangling large peptide rings, as learned from gigantic virus capsids, where not only chem. structures but the topol. of threaded rings must be controlled. Although interlocked mols. are a topic of current interest, they have for decades been explored merely as elements of mol. machines, or as a synthetic challenge. No research has specifically targeted them for, and succesfully achieved, cavity creation. Here we report the emergence of a huge capsular framework via multiple threading of metal-peptide rings. Six equivalent C4-propeller-shaped rings, each consisting of four oligopeptides and Ag+, are threaded by each other a total of twelve times (crossing no.: 24) to assemble into a well-defined 4 nm-sized sphere, which acts as a huge mol. capsule.
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  28. 28
    Inomata, Y.; Sawada, T.; Fujita, M. Metal-Peptide Torus Knots from Flexible Short Peptides. Chem 2020, 6, 294,  DOI: 10.1016/j.chempr.2019.12.009
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    28
    Metal-Peptide Torus Knots from Flexible Short Peptides
    Inomata, Yuuki; Sawada, Tomohisa; Fujita, Makoto
    Chem (2020), 6 (1), 294-303CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Entanglements of strings are regularly encountered on the macroscale yet rarely utilized in nanoscale construction. Here, we report highly entangled peptide-mimic structures created through folding and assembly of silver(I) ions and a triglycine ligand (L). The highly flexible triglycine strand assembled into a septafoil knot ([Ag·L]7) and its eight-crossing link analog ([Ag·L]8), both of which were formed for the first time by chem. synthesis. The two structures result from circular oligomerization of the same 1-crossing Ag·L motif. We also obtained poly[n]catenane [Ag·L]n from a topol. isomeric 2-crossing motif. Our observations reveal that dynamic linkages of short peptides enable easy access to knotted structures, which are restricted in protein structures because of the entropic (and/or kinetic) disadvantage of self-threading processes in long peptide chains, with remarkable stereoselectivity in the self-assembly process.
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  29. 29
    Barendt, T. A.; Docker, A.; Marques, I.; Félix, V.; Beer, P. D. Selective Nitrate Recognition by a Halogen-Bonding Four-Station [3]Rotaxane Molecular Shuttle. Angew. Chem., Int. Ed. 2016, 55, 11069– 11076,  DOI: 10.1002/anie.201604327
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    29
    Selective Nitrate Recognition by a Halogen Bonding Four Station [3]Rotaxane Molecular Shuttle
    Barendt, Timothy A.; Docker, Andrew; Marques, Igor; Felix, Vitor; Beer, Paul D.
    Angewandte Chemie, International Edition (2016), 55 (37), 11069-11076CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis of the first halogen bonding [3]rotaxane host system contg. a bis-iodo triazolium-bis-naphthalene diimide four station axle component is reported. Proton NMR anion binding titrn. expts. revealed the halogen bonding rotaxane is selective for nitrate over the more basic acetate, hydrogen carbonate and dihydrogen phosphate oxoanions and chloride, and exhibits enhanced recognition of anions relative to a hydrogen bonding analog. This elaborate interlocked anion receptor functions via a novel dynamic pincer mechanism where upon nitrate anion binding, both macrocycles shuttle from the naphthalene diimide stations at the periphery of the axle to the central halogen bonding iodo-triazolium station anion recognition sites to form a unique 1:1 stoichiometric nitrate anion-rotaxane sandwich complex. Mol. dynamics simulations carried out on the nitrate and chloride halogen bonding [3]rotaxane complexes corroborate the 1H NMR anion binding results.
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  30. 30
    Langton, M. J.; Beer, P. D. Rotaxane and Catenane Host Structures for Sensing Charged Guest Species. Acc. Chem. Res. 2014, 47, 1935– 1949,  DOI: 10.1021/ar500012a
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    30
    Rotaxane and Catenane Host Structures for Sensing Charged Guest Species
    Langton, Matthew J.; Beer, Paul D.
    Accounts of Chemical Research (2014), 47 (7), 1935-1949CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The promise of mech. interlocked architectures, such as rotaxanes and catenanes, as prototypical mol. switches and shuttles for nanotechnol. applications, has stimulated an ever increasing interest in their synthesis and function. The elaborate host cavities of interlocked structures, however, can also offer a novel approach toward mol. recognition: this Account describes the use of rotaxane and catenane host systems for binding charged guest species, and for providing sensing capability through an integrated optical or electrochem. reporter group. Particular attention is drawn to the exploitation of the unusual dynamic properties of interlocked mols., such as guest-induced shuttling or conformational switching, as a sophisticated means of achieving a selective and functional sensor response. The authors initially survey interlocked host systems capable of sensing cationic guests, before focusing on the accomplishments in synthesizing rotaxanes and catenanes designed for the more challenging task of selective anion sensing. In the group, the authors have developed the use of discrete anionic templation to prep. mech. interlocked structures for anion recognition applications. Removal of the anion template reveals an interlocked host system, possessing a unique three-dimensional geometrically restrained binding cavity formed between the interlocked components, which exhibits impressive selectivity toward complementary anionic guest species. By incorporating reporter groups within such systems, the authors have developed both electrochem. and optical anion sensors which can achieve highly selective sensing of anionic guests. Transition metals, lanthanides, and org. fluorophores integrated within the mech. bonded structural framework of the receptor are perturbed by the binding of the guest, with a concomitant change in the emission profile. The authors have also exploited the unique dynamics of interlocked hosts by demonstrating that an anion-induced conformational change can be used as a means of signal transduction. Electrochem. sensing was realized by integration of the redox-active ferrocene functionality within a range of rotaxane and catenanes; binding of an anion perturbs the metallocene, leading to a cathodic shift in the ferrocene/ferrocenium redox couple. To obtain practical sensors for target charged guest species, confinement of receptors at a surface is necessary to develop robust, reuseable devices. Surface confinement also offers advantages over soln. based receptors, including amplification of signal, enhanced guest binding thermodn. and the negation of soly. problems. The authors have fabricated anion-templated rotaxanes and catenanes on gold electrode surfaces and demonstrated that the resulting mech. bonded self-assembled monolayers are electrochem. responsive to the binding of anions, a crucial 1st step toward the advancement of sophisticated, highly selective, anion sensory devices. Rotaxane and catenane host mols. may be engineered to offer a superior level of mol. recognition, and the incorporation of optical or electrochem. reporter groups within these interlocked frameworks can allow for guest sensing. Advances in synthetic templation strategies has facilitated the synthesis of interlocked architectures and widened their interest as prototype mol. machines. However, their unique host-guest properties are only now beginning to be exploited as a sophisticated approach to chem. sensing. The development of functional host-guest sensory systems such as these is of great interest to the interdisciplinary field of supramol. chem.
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  31. 31
    Kishi, N.; Akita, M.; Kamiya, M.; Hayashi, S.; Hsu, H.-F.; Yoshizawa, M. Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube. J. Am. Chem. Soc. 2013, 135, 12976– 12979,  DOI: 10.1021/ja406893y
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    31
    Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube
    Kishi, Norifumi; Akita, Munetaka; Kamiya, Motoshi; Hayashi, Shigehiko; Hsu, Hsiu-Fu; Yoshizawa, Michito
    Journal of the American Chemical Society (2013), 135 (35), 12976-12979CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A novel M2L2 mol. tube capable of binding fullerene C60 was synthesized from bispyridine ligands (I; R = OC2H4OMe) with embedded anthracene panels and Ag(I) hinges. Unlike previous mol. cages and capsules, this open-ended tubular host can accommodate a single mol. of various C60 derivs. with large substituents. The fullerene guest can then be released by using the ideal, noninvasive external stimulus, light.
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  32. 32
    Riddell, I. A.; Smulders, M. M. J.; Clegg, J. K.; Hristova, Y. R.; Breiner, B.; Thoburn, J. D.; Nitschke, J. R. Anion-induced Reconstitution of a Self-Assembly System to Express a Chloride-Binding Co10L15 Pentagonal Prism. Nat. Chem. 2012, 4, 751– 756,  DOI: 10.1038/nchem.1407
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    32
    Anion-induced reconstitution of a self-assembling system to express a chloride-binding Co10L15 pentagonal prism
    Riddell, Imogen A.; Smulders, Maarten M. J.; Clegg, Jack K.; Hristova, Yana R.; Breiner, Boris; Thoburn, John D.; Nitschke, Jonathan R.
    Nature Chemistry (2012), 4 (9), 751-756CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
    Biochem. systems are adaptable, capable of reconstitution at all levels to achieve the functions assocd. with life. Synthetic chem. systems are more limited in their ability to reorganize to achieve new functions; they can reconfigure to bind an added substrate (template effect) or one binding event may modulate a receptor's affinity for a second substrate (allosteric effect). Here the authors describe a synthetic chem. system that is capable of structural reconstitution on receipt of one anionic signal (perchlorate) to create a tight binding pocket for another anion (chloride). A decanuclear cobalt(II) complex [Cl- ⊂ Co10L15](ClO4)19 (L = 6,6'-bis[(4-methylphenyl)iminomethyl]-3,3'-bipyridine) was prepd. by the reaction of p-toluidine and 6,6'-diformyl-3,3'-bipyridine and Co(ClO4)2, whereas a tetranuclear complex [Co4L6](OTf)8 results when Co(OTf)2 is used as the cobalt(II) source. The tetranuclear complex is converted to the decanuclear complex upon addn. of LiClO4. The decanuclear complex acts as a receptor for halides, N3-, OCN- and SCN-. The complex, barrel-like structure of the chloride receptor is templated by five perchlorate anions. This second-order templation phenomenon allows chem. networks to be envisaged that express more complex responses to chem. signals than is currently feasible.
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  33. 33
    Zhang, W.; Yang, D.; Zhao, J.; Hou, L.; Sessler, J. L.; Yang, Z.-J.; Wu, B. Controlling the Recognition and Reactivity of Alkyl Ammonium Guests Using an Anion Coordination-Based Tetrahedral Cage. J. Am. Chem. Soc. 2018, 140, 5248– 5256,  DOI: 10.1021/jacs.8b01488
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    33
    Controlling the Recognition and Reactivity of Alkyl Ammonium Guests Using an Anion Coordination-Based Tetrahedral Cage
    Zhang, Wenyao; Yang, Dong; Zhao, Jie; Hou, Lekai; Sessler, Jonathan L.; Yang, Xiao-Juan; Wu, Biao
    Journal of the American Chemical Society (2018), 140 (15), 5248-5256CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Caged structures have found wide application in a variety of areas, including guest encapsulation and catalysis. Although metal-based cages have dominated the field, anion-coordination-based cages are emerging as a new type of supramol. ensemble with interesting host-guest properties. In the current work, we report a C3-sym. tris-bis(urea) ligand based on the 2,4,6-triphenyl-1,3,5-triazine spacer, which assembles with phosphate anions to form an A4L4-type (A = anion, L = ligand) tetrahedral cage, 3, with unusually high packing coeffs. (up to 99.5% for the best substrate). Cage 3 is able to adjust its size and shape (from 136 to 216 Å3) by bending of the triphenyltriazine plane. This allows it to accommodate relatively large guests. In the case of DABCO, inclusion within the cage allows the degree of methylation to be controlled and the monomethylated product to be isolated cleanly under conditions where mixts. of the mono- and dimethylated adduct are obtained in the absence of cage 3.
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    Custelcean, R. Anion Encapsulation and Dynamics in Self-Assembled Coordination Cages. Chem. Soc. Rev. 2014, 43, 1813– 1824,  DOI: 10.1039/C3CS60371G
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    34
    Anion encapsulation and dynamics in self-assembled coordination cages
    Custelcean, Radu
    Chemical Society Reviews (2014), 43 (6), 1813-1824CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. The first part of the review describes various examples of anion-encapsulating coordination cages, categorized on the basis of their MxLy stoichiometry (M = metal cation; L = org. ligand). The second part deals with the dynamic aspects of anion encapsulation, including the kinetics and mechanism of anion binding, release, and exchange, as well as the structural evolution of the coordination complexes involved.
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  35. 35
    Bowman-James, K. Alfred Werner Revisited: The Coordination Chemistry of Anions. Acc. Chem. Res. 2005, 38, 671– 678,  DOI: 10.1021/ar040071t
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    35
    Alfred Werner Revisited: The Coordination Chemistry of Anions
    Bowman-James, Kristin
    Accounts of Chemical Research (2005), 38 (8), 671-678CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review is presented on the complexation of anions within macrocyclic receptors. Macrocyclic receptors were designed to probe the influence of four factors, hydrogen bonding, charge, dimensionality, and topol., on anion binding. Monocyclic and bicyclic polyammonium and polyamide receptors were synthesized from either 2,2'-diaminodiethylamine derivs. (dien) or 2,2',2''-aminoethylamine (tren) building blocks, plus arom. or heterocyclic spacers. Supramol. complexes of these hosts with three simple anion topologies were probed: spherical (halides), trigonal planar (nitrate), and tetrahedral (sulfate). Results indicate a no. of corollaries with transition-metal coordination chem. in terms of binding concepts such as the chelate effect and dual valencies, as well as geometries for anion complexes that are strikingly similar to those obsd. in transition-metal coordination chem.
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  36. 36
    Custelcean, R.; Bonnesen, P. V.; Duncan, N. C.; Zhang, X.; Watson, L. A.; Van Berkel, G.; Parson, W. B.; Hay, B. P. Urea-Functionalized M4L6 Cage Receptors: Anion-Templated Self-Assembly and Selective Guest Exchange in Aqueous Solutions. J. Am. Chem. Soc. 2012, 134, 8525– 8534,  DOI: 10.1021/ja300677w
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    36
    Urea-Functionalized M4L6 Cage Receptors: Anion-Templated Self-Assembly and Selective Guest Exchange in Aqueous Solutions
    Custelcean, Radu; Bonnesen, Peter V.; Duncan, Nathan C.; Zhang, Xiaohua; Watson, Lori A.; Van Berkel, Gary; Parson, Whitney B.; Hay, Benjamin P.
    Journal of the American Chemical Society (2012), 134 (20), 8525-8534CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The authors present an extensive study of a novel class of de novo designed tetrahedral M4L6 (M = Ni, Zn) cage receptors, wherein internal decoration of the cage cavities with urea anion-binding groups, via functionalization of the org. components L, led to selective encapsulation of tetrahedral oxoanions EO4n- (E = S, Se, Cr, Mo, W, n = 2; E = P, n = 3) from aq. solns., based on shape, size, and charge recognition. External functionalization with tBu groups led to enhanced soly. of the cages in aq. MeOH solns., thereby allowing for their thorough characterization by multinuclear (1H, 13C, 77Se) and diffusion NMR spectroscopies. Addnl. exptl. characterization by electrospray ionization mass spectrometry, UV-visible spectroscopy, and single-crystal x-ray diffraction, as well as theor. calcns., led to a detailed understanding of the cage structures, self-assembly, and anion encapsulation. The cage self-assembly is templated by EO4n- oxoanions (n ≥ 2), and upon removal of the templating anion the tetrahedral M4L6 cages rearrange into different coordination assemblies. The exchange selectivity among EO4n- oxoanions was studied with 77Se NMR spectroscopy using 77SeO42- as an anionic probe, which found the following selectivity trend: PO43- » CrO42- > SO42- > SeO42- > MoO42- > WO42-. In addn. to the complementarity and flexibility of the cage receptor, a combination of factors contribute to the obsd. anion selectivity, including the anions' charge, size, hydration, basicity, and H-bond acceptor abilities.
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    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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    37
    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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    Liu, Y.; Sengupta, A.; Raghavachari, K.; Flood, A. M. Anion Binding in Solution: Beyond the Electrostatic Regime. Chem 2017, 3, 411– 417,  DOI: 10.1016/j.chempr.2017.08.003
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    Anion Binding in Solution: Beyond the Electrostatic Regime
    Liu, Yun; Sengupta, Arkajyoti; Raghavachari, Krishnan; Flood, Amar H.
    Chem (2017), 3 (3), 411-427CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    A fundamental understanding of anion binding by receptors is essential for managing salts during energy, water, and food prodn. However, the limited understanding of solvent effects in ion recognition leads to a persistent blind spot that prevents effective receptor design. We exptl. discovered an underlying 1/εr dependence of anion affinity on solvent dielec. const. (εr). We found this relationship by measuring how chloride binds to macrocyclic triazolophane receptors across a wide range of solvents: εr = 4.7-56.2. Solvent weakens affinity by screening electrostatics; electrostatics dominates when εr < 4.7 (chloroform) and then transits a cross-over region (4.7 < εr < 20.5; acetone), after which it no longer governs affinity (acetonitrile and DMSO). D. functional theory helped us understand this dependence. Our theory-backed model accurately predicts Cl- affinity in solvents used in liq.-liq. extns. in the nuclear fuel cycle. This model offers a general foundation for anion recognition and electrostatically driven complexation.
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  39. 39
    Zhao, W.; Qiao, B.; Tropp, J.; Pink, M.; Azoulay, J. D.; Flood, A. H. Linear Supramolecular Polymers Driven by Anion-Anion Dimerization of Difunctional Phosphonate Monomers Inside Cyanostar Macrocycles. J. Am. Chem. Soc. 2019, 141, 4980– 4989,  DOI: 10.1021/jacs.9b00248
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    39
    Linear Supramolecular Polymers Driven by Anion-Anion Dimerization of Difunctional Phosphonate Monomers Inside Cyanostar Macrocycles
    Zhao, Wei; Qiao, Bo; Tropp, Joshua; Pink, Maren; Azoulay, Jason D.; Flood, Amar H.
    Journal of the American Chemical Society (2019), 141 (12), 4980-4989CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Supramol. polymers have enabled far-reaching fundamental science and the development of diverse macromol. technologies owing to the reversible and noncovalent chem. connectivities that define their properties. Despite the unabated development of these materials using highly tailorable recognition elements, anion-based polymers remain rare as a result of the weak interactions they mediate. Here, we use design rules inspired by cation-driven polymers to demonstrate a new noncovalent link based on receptor-stabilized anion-anion interactions that enables the efficient linear polymn. of simple difunctional phosphonates. The linear main chain connectivity and mol. topol. were confirmed by single crystal X-ray diffraction, which demonstrates the rare 2:2 stoichiometry between the anionic phosphonate end groups and a pair of π-stacked cyanostar macrocycles. The stability of these links enables rapid polymn. of difunctional phosphonates employing different aliph. linkers (C6H12, C8H16, C10H20, C12H24). Diphosphonates with greater chain flexibility (C12H24) enable greater polymn. with an av. d.p. of nine emerging at 10 mM. Viscosity measurements show a transition from oligomers to polymers at the crit. polymn. concn. of 5 mM. In a rare correlation, NMR spectroscopy shows a coincident mol. signature of the polymn. at 5 mM. These polymers are highly concn. dependent, reversibly polymerize with acid and base, and respond to competitive anions. They display the design simplicity of metallo-supramol. polymers with transfer of the strong 2:2 recognition chem. to macromols. The simplicity and understanding of this new class of supramol. polymer is anticipated to open opportunities in tailoring anion-based functional materials.
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  40. 40
    Wu, X.; Wang, P.; Turner, P.; Lewis, W.; Catal, O.; Thomas, D. S.; Gale, P. A. Tetraurea Macrocycles: Aggregation-Driven Binding of Chloride in Aqueous Solutions. Chem. 2019, 5, 1210– 1222,  DOI: 10.1016/j.chempr.2019.02.023
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    40
    Tetraurea Macrocycles: Aggregation-Driven Binding of Chloride in Aqueous Solutions
    Wu, Xin; Wang, Patrick; Turner, Peter; Lewis, William; Catal, Osman; Thomas, Donald S.; Gale, Philip A.
    Chem (2019), 5 (5), 1210-1222CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    Artificial receptors that recognize anionic species via non-covalent interactions have a wide range of biomedical, industrial, and environmental applications. A major challenge in this area of research is to achieve high-affinity and selective anion binding in aq. media. So far, only a few examples of receptors capable of strong (>105 M-1) anion binding in solns. contg. >50% water are available, and none show selectivity for chloride. We report here the discovery of a D4h-sym. fluorinated tetraurea macrocycle that fulfills this function owing to its unique self-assembly properties. The macrocycle has a strong tendency to self-assoc. into columnar aggregates via intermol. hydrogen bonds and arom. stacking. In aq. solns., macrocycle aggregation generates solvent-shielding and size-selective binding pockets favorable for hydrogen bonding with chloride. As a result, micromolar affinity and highly selective chloride binding have been achieved with this simple small mol. (MW < 700) in 60 vol % water/acetonitrile.
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    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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    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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    Chen, L.; Berry, S. N.; Wu, X.; Howe, E. N. W.; Gale, P. A. Advances in Anion Receptor Chemistry. Chem. 2020, 6, 61– 141,  DOI: 10.1016/j.chempr.2019.12.002
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    Advances in Anion Receptor Chemistry
    Chen, Lijun; Berry, Stuart N.; Wu, Xin; Howe, Ethan N. W.; Gale, Philip A.
    Chem (2020), 6 (1), 61-141CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    This review covers advances in anion complexation in the years 2017 and 2018. The review focuses on the applications of anion receptor chem., including sensing, self-assembly, extn., transport, catalysis, and fundamental advances in the area.
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    Custelcean, R. Urea-Functionalized Crystalline Capsules for Recognition and Separation of Tetrahedral Oxoanions. Chem. Commun. 2013, 49, 2173– 2182,  DOI: 10.1039/c2cc38252k
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    Urea-functionalized crystalline capsules for recognition and separation of tetrahedral oxoanions
    Custelcean, Radu
    Chemical Communications (Cambridge, United Kingdom) (2013), 49 (22), 2173-2182CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. The persistent ability of tripodal TREN-based tris-urea receptors (TREN = tris(2-aminoethyl)amine) to self-assemble with a variety of oxoanions into dimeric capsules upon crystn. is reviewed. The capsule crystn. allows for charge-, shape-, and size-selective encapsulation of tetrahedral XO4n- anions (n = 2,3), and provides an effective way to sep. these anions from competitive aq. environments.
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    Schäfer, S.; Gamer, M. T.; Lebedkin, S.; Weigend, F.; Kappes, M. M.; Roesky, P. W. Bis(6-methylene-2,2’-bipyridine)phenylphosphine – A Flexible Ligand for the Construction of Trinuclear Coinage-Metal Complexes. Chem. - Eur. J. 2017, 23, 12198– 12209,  DOI: 10.1002/chem.201701091
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    Luo, G.-G.; Guo, Q.-L.; Wang, Z.; Sun, C.-F.; Lin, J.-Q.; Sun, D. New Protective Ligands for Atomically Precise Silver Nanoclusters. Dalton Trans. 2020, 49, 5406– 5415,  DOI: 10.1039/D0DT00477D
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    New protective ligands for atomically precise silver nanoclusters
    Luo, Geng-Geng; Guo, Qi-Lin; Wang, Zhi; Sun, Cun-Fa; Lin, Jin-Qing; Sun, Di
    Dalton Transactions (2020), 49 (17), 5406-5415CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    A review. Atomically precise silver nanoclusters (NCs) have emerged as a hot topic attracting immense research interest. Protecting ligands are needed for direct capping on cluster surfaces to prevent aggregation and to stabilize NCs. Protective ligands are crit. to detg. the sizes, structures and properties of silver NCs. The past decades have witnessed conventionally used org. ligands (thiolates/selenols, phosphines and alkynyls) and inorg. ligands (chalcogens and halogens) being extensively used to passivate NC surfaces. However, only in the most recent years have new-type protecting ligands beyond the conventional ones begun to be introduced in the protecting sphere of new functional silver NCs. The present Frontier article covers the most recent examples of some new protective agents for well-defined silver NCs. The authors describe four classes of novel silver NCs stabilized by newly-developed surface ligands, namely, nitrogen-donor org. ligands, oxygen-donor inorg. ligands, metalloligands and macrocyclic hosts, paying attention to the synthesis, structures and properties of these silver NCs. This Frontier article will hopefully attract more cluster scientists to explore more freshly ligated atomically precise silver NCs with novel structures and properties in the years ahead. The literature survey in this review is based on publications up to Feb. 2020. Some suggestions for future directions in this field are also given.
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    Wang, X. Discrete Ag6L6 Coordination Nanotubular Structures Based on a T-Shaped Pyridyl Diphosphine. Chem. Commun. 2011, 47, 3849– 3851,  DOI: 10.1039/c0cc05235c
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    Discrete Ag6L6 coordination nanotubular structures based on a T-shaped pyridyl diphosphine
    Wang, Xiaobing; Huang, Jing; Xiang, Shenglin; Liu, Yu; Zhang, Jianyong; Eichhofer, Andreas; Fenske, Dieter; Bai, Shi; Su, Cheng-Yong
    Chemical Communications (Cambridge, United Kingdom) (2011), 47 (13), 3849-3851CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Ag6L6-type coordination nanotubular structures were assembled from 6 Ag(I) ions and 6 T-shaped ligands, 4-(3,5-bis(diphenylphosphino)phenyl)pyridine. The nanotubes represent a discrete mol. architecture of a no. of polymeric structures assembled from dimeric building blocks.
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    Zhang, Y.-W.; Bai, S.; Wang, Y.-Y.; Han, Y.-F. A Strategy for the Construction of Triply Interlocked Organometallic Cages by Rational Design of Poly-NHC Precursors. J. Am. Chem. Soc. 2020, 142, 13614– 13621,  DOI: 10.1021/jacs.0c06470
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    A Strategy for the Construction of Triply Interlocked Organometallic Cages by Rational Design of Poly-NHC Precursors
    Zhang, Ya-Wen; Bai, Sha; Wang, Yao-Yu; Han, Ying-Feng
    Journal of the American Chemical Society (2020), 142 (31), 13614-13621CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Three-dimensional (3D) triply interlocked catenanes are a family of chem. topologies that consist of two identical, mech. interlocked coordination cage components with intriguingly complex structures. Although only a few successful constructions of 3-dimensional interlocked catenanes were achieved to date via metal-mediated assembly, these complex structures have thus far only been targeted by metal-N/O coordination techniques. Here, taking advantage of rational ligand design, the authors report the efficient construction of 3-dimensional triply interlocked [2]catenanes [Ag3L2]2, wherein the metal ions exclusively form bonds to N-heterocyclic carbene (NHC) units, and their subsequent transmetalation to the corresponding [Au3L2]2 Au analogs. The formation and transmetalation reactions proceed under mild conditions and are generally applicable. Characterization techniques were applied to confirm the formation and structure of the desired 3-dimensional triply interlocked architectures: multinuclear NMR spectroscopy, ESI-MS, and single-crystal x-ray diffraction anal. The solid-state structure of [Ag3(1a)2]2(PF6)6 unambiguously confirms the existence of a 3-dimensional catenane that consists of two identical, mech. interlocked trinuclear hexacarbene cage components. The interlocking of two 3-dimensional cages into a [2]catenane is driven by the efficient π···π stacking of triazine-triazine stacks with cooperative interactions between imidazo[1,5-a]pyridine subunits. Notably, the triply interlocked organometallic cages exhibit good stability toward various org. solvents, concns., and temps., and no disassembly occurred in the presence of coronene or pyrene. The future construction of mech. interlocked architectures using metal-carbene bonds rather than metal-N bonds may provide assemblies with interesting properties for as-yet-unimagined applications in fields such as sensors and mol. elec. conductors.
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    Jin, G.-X.; Zhu, G.-Y.; Sun, Y.-Y.; Shi, Q.-X.; Liang, L.-P.; Wang, H.-Y.; Wu, Z.-W.; Ma, J.-P. [Ag-Ag]2+ Unit-Encapsulated Trimetallic Cages: One-Pot Syntheses and Modulation of Argentophilic Interactions by the Uncoordinated Substituents. Inorg. Chem. 2019, 58, 2916– 2920,  DOI: 10.1021/acs.inorgchem.8b03388
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    [Ag-Ag]2+ Unit-Encapsulated Trimetallic Cages: One-Pot Syntheses and Modulation of Argentophilic Interactions by the Uncoordinated Substituents
    Jin, Guo-Xia; Zhu, Gui-Ying; Sun, Yan-Yan; Shi, Qing-Xiu; Liang, Li-Ping; Wang, Hai-Ying; Wu, Xiang-Wen; Ma, Jian-Ping
    Inorganic Chemistry (2019), 58 (5), 2916-2920CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Four [Ag-Ag]2+ unit-encapsulated trimetallic cages were synthesized from one new tripodal ligand L and Ag salts in different solvent systems by a 1-pot method. The formation of coordination cages occurred simultaneously with the condensation of amino groups and ketone. The remarkable structural feature of the four cages is their spontaneous incorporation of [Ag-Ag]2+ cationic units. Moreover, the argentophilic interactions are modulated by the uncoordinated amino substituents. The study herein shows that modification and subtle changes of the cage structures could be realized by a 1-pot synthetic method.
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    Schmidbaur, H.; Schier, A. Argentophilic Interactions. Angew. Chem., Int. Ed. 2015, 54, 746– 784,  DOI: 10.1002/anie.201405936
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    Argentophilic Interactions
    Schmidbaur, Hubert; Schier, Annette
    Angewandte Chemie, International Edition (2015), 54 (3), 746-784CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The decade 1990-2000 saw a growing interest in aurophilic interactions in gold chem. These interactions were found to influence significantly a variety of structural and other phys. characteristics of gold(I) compds. The attention paid to this unusual and counterintuitive type of intra- and intermol. bonding between seemingly closed-shell metal centers has rapidly been extended to also include silver chem. Hundreds of exptl. and computational studies have since been dedicated to the argentophilicity phenomenon. The results of this development are reviewed herein focusing on mol. systems where two or more silver(I) centers are in close contact leading to specific structural characteristics and a variety of novel phys. properties. These include strongly modified ligand-to-metal charge-transfer processes obsd. in absorption and emission spectroscopy, but also colossal pos. and neg. thermal expansion on the one hand and unprecedented neg. linear compressibility of crystal parameters on the other.
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    Zhang, Q.-Y.; He, X.; Zhao, L. Macrocycle-Assisted Synthesis of Non-Stoichimetric Silver(I) Halide Electrocatalysts for Efficient Chlorine Evolution Reaction. Chem. Sci. 2017, 8, 5662– 5668,  DOI: 10.1039/C7SC00575J
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    50
    Macrocycle-assisted synthesis of non-stoichiometric silver(I) halide electrocatalysts for efficient chlorine evolution reaction
    Zhang, Qiong-You; He, Xin; Zhao, Liang
    Chemical Science (2017), 8 (8), 5662-5668CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The electrocatalytic oxidn. of chloride to chlorine is a fundamental and important electrochem. reaction in industry. Herein we report the synthesis of non-stoichiometric silver halide nanoparticles through a novel macrocycle-assisted bulk-to-cluster-to-nano transformation. The acquired pos. charged nanoparticles expedite chloride transportation by electrostatic attraction and facilitate the formation of silver polychloride catalytic species on the surface, thus functioning as efficient and selective electrocatalysts for the chlorine evolution reaction (CER) at a very low overpotential and within a wide concn. range of chloride. The formation of uncommon non-stoichiometric nanoparticles prevents the formation of a AgCl ppt. and exposes more coordination unsatd. silver atoms to catalyze CER, finally causing a large enhancement of the at. catalytic efficiency of silver. This study showcases a promising approach to achieve efficient catalysts from a bottom-up design.
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    Zhang, S.; Zhao, L. Macrocycle-Encircled Polynuclear Metal Clusters: Controllable Synthesis, Reactivity Studies, and Applications. Acc. Chem. Res. 2018, 51, 2535– 2545,  DOI: 10.1021/acs.accounts.8b00283
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    51
    Macrocycle-encircled polynuclear metal clusters: Controllable synthesis, reactivity studies, and applications
    Zhang, Siqi; Zhao, Liang
    Accounts of Chemical Research (2018), 51 (10), 2535-2545CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Macrocyclic ligands have been extensively applied to recognize single metal ions with high selectivity and good affinity based on the size-match principle. The resulting metal-macrocycle complexes play a significant role in mimicking the function of natural metal ion carriers and understanding and reproducing the catalytic activity of metalloenzymes. Because of the known macrocyclic effect, those single metal-macrocycle adducts often show an enhanced kinetic and thermodn. stability in comparison with their open-chain analogs. By virtue of such extraordinary coordination properties of macrocyclic ligands, it is expected that larger macrocycles with multiple coordination sites could properly act as an outer scaffold to direct the formation of multiatom species inside, such as polynuclear metal cluster aggregates, whose assembly may largely depend on the template positioning of coordinative atoms in the macrocyclic ring. Thus, the employment of polydentate macrocyclic ligands may provide a convenient tool to access polynuclear metal clusters in a controllable way. In this Account, we review our studies of the metal ion binding process of a class of polydentate macrocyclic ligands, azacalixpyridines (Py[n]s), and the application of Py[n]s as an outer template to direct the controllable synthesis of polynuclear metal clusters. Our investigations revealed that Py[n]s show a significant cooperative coordination effect in the metal ion binding process that facilitated the easy formation of a polymetallic assembled structure. Taking advantage of the cooperative coordination effect and the tunable and highly fluxional conformation of Py[n]s, we laid our focus on control of the nuclearity no. by tuning the size of Py[n]s and the adoption of Py[n]s with different anionic centers in metal cluster synthesis. As an important example for application, this new established macrocycle-directed method has been employed to achieve a variety of metal-cluster-centered capsule, rotaxane, catenane, polygon, and other supramol. assemblies. Furthermore, a cluster-to-cluster transformation inside the cavity of Py[n]s is presented to showcase the use of the acquired metal cluster-macrocycle complexes to achieve unconventional metal cluster entities.With regard to the application of the newly synthesized macrocycle-encircled metal clusters, examples of the fabrication of functional materials and catalysts are presented. With the assistance of Py[n]s, a bulk-to-cluster-to-nanoparticle transformation of silver sulfide (Ag2S) and silver halides (AgX) has been conducted to produce a series of nonstoichiometric silver sulfide and halide nanoparticles. The resulting Ag-S nanoparticle material with a high Ag/S ratio, which is inherited from the Py[n]-protected polysilver sulfide clusters, has a large energy gap relative to conventional Ag2S nanoparticles. Moreover, the nonstoichiometric silver halide nanoparticles can act as a new kind of electrocatalyst for the chlorine evolution reaction, showing excellent selectivity and high catalytic efficiency. Overall, in this account we try to highlight the application of polydentate macrocycles as an outer template to guide the synthesis of polynuclear metal clusters in a controllable manner. This unique synthesis will provide a new avenue to access unconventional metal clusters of different metal kinds and diverse anionic centers, which are expected to have promising and significant applications in many interdisciplinary areas of chem.
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    Desnoyer, A. N.; Nicolay, A.; Rios, P.; Ziegler, M. S.; Tilley, T. D. Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands. Acc. Chem. Res. 2020, 53, 1944– 1956,  DOI: 10.1021/acs.accounts.0c00382
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    52
    Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands
    Desnoyer, Addison N.; Nicolay, Amelie; Rios, Pablo; Ziegler, Micah S.; Tilley, T. Don
    Accounts of Chemical Research (2020), 53 (9), 1944-1956CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    Bimetallic motifs are a structural feature common to some of the most effective and synthetically useful catalysts known, including in the active sites of many metalloenzymes and on the surfaces of industrially relevant heterogeneous materials. However, the complexity of these systems often hampers detailed studies of their fundamental properties. To glean valuable mechanistic insight into how these catalysts function, this research group has prepd. a family of dinucleating 1,8-naphthyridine ligands that bind two first-row transition metals in close proximity, originally designed to help mimic the proposed active site of metal oxide surfaces. Of the various bimetallic combinations examd., dicopper(I) is particularly versatile, as neutral bridging ligands adopt a variety of different binding modes depending on the configuration of frontier orbitals available to interact with the Cu centers. Organodicopper complexes are readily accessible, either through the traditional route of salt metathesis or via the activation of tetraarylborate anions through aryl group abstraction by a dicopper(I) unit. The resulting bridging aryl complexes engage in C-H bond activations, notably with terminal alkynes to afford bridging alkynyl species. The μ-hydrocarbyl complexes are surprisingly tolerant of water and elevated temps. This stability was leveraged to isolate a species that typically represents a fleeting intermediate in Cu-catalyzed azide-alkyne coupling (CuAAC); reaction of a bridging alkynyl complex with an org. azide afforded the first example of a well-defined, sym. bridged dicopper triazolide. This complex was an intermediate during CuAAC, providing support for a proposed bimetallic mechanism. These platforms are not limited to formally low oxidn. states; chem. oxidn. of the hydrocarbyl complexes cleanly results in formation of mixed valence CuICuII complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidn. states, e.g., dicopper(II), are easily accessed via oxidn. of a dicopper(I) compd. with air to give a CuII2(μ-OH)2 complex. Redn. of this compd. with silanes resulted in the unexpected formation of pentametallic copper(I) dihydride clusters or trimetallic monohydride complexes, depending on the nature of the silane. Finally, development of an unsym. naphthyridine ligand with mixed donor side-arms enables selective synthesis of an isostructural series of six heterobimetallic complexes, demonstrating the power of ligand design in the prepn. of heterometallic assemblies.
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    Jansze, S. M. Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages. J. Am. Chem. Soc. 2016, 138, 2046– 2054,  DOI: 10.1021/jacs.5b13190
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    53
    Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages
    Jansze, Suzanne M.; Cecot, Giacomo; Wise, Matthew D.; Zhurov, Konstantin O.; Ronson, Tanya K.; Castilla, Ana M.; Finelli, Alba; Pattison, Philip; Solari, Euro; Scopelliti, Rosario; Zelinskii, Genrikh E.; Vologzhanina, Anna V.; Voloshin, Yan Z.; Nitschke, Jonathan R.; Severin, Kay
    Journal of the American Chemical Society (2016), 138 (6), 2046-2054CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    It is possible to control the geometry and the compn. of metallasupramol. assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic FeII8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral FeII4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of PdII-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral PdII4L8 assembly, whereas an octahedral PdII6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the compn. of dynamic mixts. of PdII cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of PdII4L8 and PdII6L12 cages, whereas the other did not.
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    Giuseppone, N.; Schmitt, J.-L.; Allouche, L.; Lehn, J.-M. DOSY NMR Experiments as a Tool for the Analysis of Constitutional and Motional Dynamic Processes: Implementation for the Driven Evolution of Dynamic Combinatorial Libraries of Helical Strands. Angew. Chem., Int. Ed. 2008, 47, 2235– 2239,  DOI: 10.1002/anie.200703168
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    DOSY NMR experiments as a tool for the analysis of constitutional and motional dynamic processes: implementation for the driven evolution of dynamic combinatorial libraries of helical strands
    Giuseppone, Nicolas; Schmitt, Jean-Louis; Allouche, Lionel; Lehn, Jean-Marie
    Angewandte Chemie, International Edition (2008), 47 (12), 2235-2239CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Diffusion ordered spectroscopy (DOSY) NMR expts. have been used to characterize a dynamic combinatorial library of helical strands and grid-type metallosupramol. architectures. The technique allows the deconvolution of very similar chem. structures differing only by their hydrodynamic radius. Moreover, the occurrence of springlike, extension-contraction conformational motions in helical strands can be revealed as a function of the temp.
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    Zhang, Z. Intra- and Intermolecular Self-Assembly of a 20-nm-Wide Supramolecular Hexagonal Grid. Nat. Chem. 2020, 12, 468– 474,  DOI: 10.1038/s41557-020-0454-z
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    55
    Intra- and intermolecular self-assembly of a 20-nm-wide supramolecular hexagonal grid
    Zhang, Zhe; Li, Yiming; Song, Bo; Zhang, Yuan; Jiang, Xin; Wang, Ming; Trumbleson, Ryan; Liu, Changlin; Wang, Pingshan; Hao, Xin-Qi; Rojas, Tomas; Ngo, Anh T.; Sessler, Jonathan L.; Newkome, George R.; Hla, Saw Wai; Li, Xiaopeng
    Nature Chemistry (2020), 12 (5), 468-474CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
    For the past three decades, the coordination-driven self-assembly of three-dimensional structures has undergone rapid progress; however, parallel efforts to create large discrete two-dimensional architectures-as opposed to polymers-have met with limited success. The synthesis of metallo-supramol. systems with well-defined shapes and sizes at 10-100 nm remains challenging. Here the authors report the construction of a series of giant supramol. hexagonal grids, with diams. ∼20 nm and mol. wts. >65 kDa, through a combination of intra- and intermol. metal-mediated self-assembly steps. The hexagonal intermediates and the resulting self-assembled grid architectures were imaged at submol. resoln. by scanning tunneling microscopy. Characterization (including by scanning tunneling spectroscopy) enabled the unambiguous at.-scale detn. of fourteen hexagonal grid isomers.
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    Although the coordinative flexibility shown by the napthyridine–silver system limits the degree of certainty of these modeling results, the large difference in energy between the tetrahedral architecture and alternate structures lends credence to the assignment of the Ag8L6 structure as a tetrahedron.

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    Mass spectrometry of these silver complexes is challenging, presumably due to the dynamic nature of the naphthyridine–silver interactions. We see extensive fragmentation of all complexes under even mild conditions. By tuning ionization conditions and through choice of counterion, we were able to gather data on these architectures in both LRMS and HRMS. We found that using hexafluorophosphate as the counterion was particularly effective for obtaining good quality mass spectra.

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    A minor isomer was also resolved in the crystallographic data, whereby an exterior sulfate coordinates via three oxygen atoms instead. Please see SI Section 12 for further details.

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    These silver–naphthyridine systems proved to be extremely stable to light, which was unexpected. Samples could be left exposed to ambient light for 2–3 months with no sign of decomposition by NMR, or precipitation.

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  • Abstract

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    Figure 1

    Figure 1. Self-assembly of Ag4L3 and Ag8L6 architectures. Conditions: (a) AgNTf2 (2 equiv), 2 (1 equiv), 1 (2 equiv), d3-MeCN, 5 min; (b) AgNTf2 (2 equiv), 3 or 4 (1 equiv), 1 (2 equiv), d3-MeCN, 5 min. Structures of 5 and 6 are MM3-optimized models. (c) DOSY NMR of 5 and 6.

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    Figure 2

    Figure 2. (a) Synthesis of six-stranded helicates 9 and 10, formed only during self-assembly from dianiline 3. (i) Tetrabutylammonium iodide (0.34 equiv), 5 min; (ii) tetramethylammonium sulfate (1.0 equiv), 6 h. Structures of 5 and 6 are MM3 optimized models, and those of 9 and 10 are based on crystallographic data (vide infra). Simplified representation of six-stranded helicate (b) 9 and (c) 10.

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    Figure 3

    Figure 3. (a) X-ray crystal structure of 9; (b) schematic view of 9. (c) X-ray crystal structure of 10; (d) schematic view of 10. (e) End-on view of crystal structure of 9 showing cluster geometry. (f) End-on view of crystal structure of 10 showing the silver cluster and nonclassical hydrogen bonds to the exterior sulfate. (g) View from within the crystal structure of 10, showing nonclassical hydrogen bonds to the internal sulfate.

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    Figure 4

    Figure 4. (a) Comparison of 1H NMR spectra of 10 (top), 11 (middle), and 9 (bottom), showing the similarity between the spectra of 9 and 11. Simplified (b) schematic and (c) cartoon views of six-stranded helicate 11.

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  • References

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    This article references 66 other publications.

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      Self-assembly is invaluable in the construction of giant mol. structures via a bottom-up approach. Although living organisms naturally make the most use of self-assembly and freely handle the mechanism at will, scientists are still far behind the level of nature. Inspired by the elegant structures of virus capsids, the authors have previously constructed roughly spherical giant metal complexes with the symmetry of an octahedron, cuboctahedron, and rhombicuboctahedron M6L12, M12L24, and M24L48, resp. Here, the authors report the 1st successful synthesis of an M30L60 mol. icosidodecahedron that consists of ∼100 components: 30 Pd(II) ions and 60 (2,5-bis(4-pyridyl-1,4-tetramethylphenylene)thiophene) ligands that assemble into the largest well-defined spherical macromol. to date (diam. of ∼8.2 nm). Tuning the flexibility of the ligand was the key for successful self-assembly. A highly complex but sym. organized structure was identified through x-ray crystallog. anal. The interior space of the mol. complex is large enough (157,000 Å3) to enclose proteins.
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    4. 4
      Niki, K.; Tsutsui, T.; Yamashina, M.; Akita, M.; Yoshizawa, M. Recognition and Stabilization of Unsaturated Fatty Acids by a Polyaromatic Receptor. Angew. Chem., Int. Ed. 2020, 59, 10489– 10492,  DOI: 10.1002/anie.202003253
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      4
      Recognition and Stabilization of Unsaturated Fatty Acids by a Polyaromatic Receptor
      Niki, Keita; Tsutsui, Takahiro; Yamashina, Masahiro; Akita, Munetaka; Yoshizawa, Michito
      Angewandte Chemie, International Edition (2020), 59 (26), 10489-10492CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Selective recognition of natural fatty acids is intrinsically difficult owing to the long, flexible, and poorly interactive hydrocarbon chains. Inspired by biol. recognition systems, we herein demonstrate the exclusive binding of a monounsatd. fatty acid by an artificial polyarom. receptor from a mixt. of the unsatd. and corresponding satd. substrates (i.e., oleic and stearic acids) in water. The selectivity stems from multiple CH-π/π-π interactions between the host framework and the guest in its roughly coiled conformation. Moreover, competitive binding expts. elucidate higher binding affinities of the receptor for oligo- and polyunsatd. fatty acids (e.g., α-linolenic acid and EPA). Within the receptor, the biosubstrates are remarkably stabilized against air, light, and heat owing to the polyarom. shielding effect.
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    5. 5
      Howlader, P.; Mukherjee, P. S. Face and Edge Directed Self-Assembly of Pd12 Tetrahedral Nano-Cages and their Self-Sorting. Chem. Sci. 2016, 7, 5893– 5899,  DOI: 10.1039/C6SC02012G
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      5
      Face and edge directed self-assembly of Pd12 tetrahedral nano-cages and their self-sorting
      Howlader, Prodip; Mukherjee, Partha Sarathi
      Chemical Science (2016), 7 (9), 5893-5899CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Reactions of a cis-blocked Pd(II) 90° acceptor [cis-(tmeda)Pd(NO3)2] (M) with 1,4-di(1H-tetrazol-5-yl)benzene (H2L1) and [1,3,5-tri(1H-tetrazol-5-yl)benzene] (H3L2) in 1 : 1 and 3 : 2 molar ratios resp., yielded soft metallogels G1 and G2 [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. Post-metalation of the gels G1 and G2 with M yielded highly water-sol. edge and face directed self-assembled Pd12 tetrahedral nano-cages T1 and T2, resp. Such facile conversion of Pd(II) gels to discrete tetrahedral metallocages is unprecedented. Moreover, distinct self-sorting of these two tetrahedral cages of similar sizes was obsd. in the self-assembly of M with a mixt. of H2L1 and H3L2 in aq. medium. The edge directed tetrahedral cage (T1) was successfully used to perform Michael reactions of a series of water insol. nitro-olefins assisted by encapsulation into the cage in aq. medium.
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    6. 6
      Kaphan, D. M.; Levin, M. D.; Bergman, R. G.; Raymond, K. N.; Toste, F. D. A Supramolecular Microenvironment Strategy for Transition Metal Catalysis. Science 2015, 350, 1235– 1238,  DOI: 10.1126/science.aad3087
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      6
      A supramolecular microenvironment strategy for transition metal catalysis
      Kaphan, David M.; Levin, Mark D.; Bergman, Robert G.; Raymond, Kenneth N.; Toste, F. Dean
      Science (Washington, DC, United States) (2015), 350 (6265), 1235-1238CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      A self-assembled supramol. complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent transition metal complexes [such as gold(III) and platinum(IV)], the central bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramol. assembly acts as a functional enzyme mimic, applying the concepts of enzymic catalysis to a reactivity manifold not represented in biol. Kinetic expts. delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (kcat/kuncat) up to 1.9 × 107 (here kcat and kuncat are the Michaelis-Menten enzymic rate const. and obsd. uncatalyzed rate const., resp.). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramol. microenvironment catalyst and the transition metal catalyst operating in concert to achieve efficient turnover.
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    7. 7
      Zhang, D.; Ronson, T. K.; Lavendomme, R.; Nitschke, J. R. Selective Separation of Polyaromatic Hydrocarbons by Phase Transfer of Coordination Cages. J. Am. Chem. Soc. 2019, 141, 18949– 18953,  DOI: 10.1021/jacs.9b10741
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      7
      Selective Separation of Polyaromatic Hydrocarbons by Phase Transfer of Coordination Cages
      Zhang, Dawei; Ronson, Tanya K.; Lavendomme, Roy; Nitschke, Jonathan R.
      Journal of the American Chemical Society (2019), 141 (48), 18949-18953CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Here the authors report a new supramol. strategy for the selective sepn. of specific polycyclic arom. hydrocarbons (PAHs) from mixts. The use of a triethylene glycol-functionalized formylpyridine subcomponent allowed the construction of an FeII4L4 tetrahedron 1 that was capable of transferring between H2O and nitromethane layers, driven by anion metathesis. Cage 1 selectively encapsulated coronene from among a mixt. of 8 different types of PAHs in nitromethane, bringing it into a new nitromethane phase by transiting through an intermediate H2O phase. The bound coronene was released from 1 upon addn. of benzene, and both the cage and the purified coronene could be sepd. via further phase sepn.
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    8. 8
      Zhu, J.; Haynes, C. J. E.; Kieffer, M.; Greenfield, J. L.; Greenhalgh, R. D.; Nitschke, J. R.; Keyser, U. F. FeII4L4 Tetrahedron Binds to Nonpaired DNA Bases. J. Am. Chem. Soc. 2019, 141, 11358– 11362,  DOI: 10.1021/jacs.9b03566
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      8
      FeII4L4 Tetrahedron Binds to Nonpaired DNA Bases
      Zhu, Jinbo; Haynes, Cally J. E.; Kieffer, Marion; Greenfield, Jake L.; Greenhalgh, Ryan D.; Nitschke, Jonathan R.; Keyser, Ulrich F.
      Journal of the American Chemical Society (2019), 141 (29), 11358-11362CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A water-sol. self-assembled supramol. FeII4L4 tetrahedron binds to single stranded DNA, mismatched DNA base pairs, and three-way DNA junctions. Binding of the coordination cage quenches fluorescent labels on the DNA strand, which provides an optical means to detect the interaction and allows the position of the binding site to be gauged with respect to the fluorescent label. Using the quenching and binding properties of the coordination cage, the authors developed a simple and rapid detection method based on fluorescence quenching to detect unpaired bases in double-stranded DNA.
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    9. 9
      Hotze, A. C. G.; Kariuki, B. M.; Hannon, M. J. Dinuclear Double-Stranded Metallosupramolecular Ruthenium Complexes: Potential Anticancer Drugs. Angew. Chem., Int. Ed. 2006, 45, 4839– 4842,  DOI: 10.1002/anie.200601351
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      9
      Dinuclear double-stranded metallosupramolecular ruthenium complexes: potential anticancer drugs
      Hotze Anna C G; Kariuki Benson M; Hannon Michael J
      Angewandte Chemie (International ed. in English) (2006), 45 (29), 4839-42 ISSN:1433-7851.
      There is no expanded citation for this reference.
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    10. 10
      Chen, B.; Holstein, J. J.; Horiuchi, S.; Hiller, W. G.; Clever, G. H. Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes. J. Am. Chem. Soc. 2019, 141, 8907– 8913,  DOI: 10.1021/jacs.9b02207
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      10
      Pd(II) Coordination Sphere Engineering: Pyridine Cages, Quinoline Bowls, and Heteroleptic Pills Binding One or Two Fullerenes
      Chen, Bin; Holstein, Julian J.; Horiuchi, Shinnosuke; Hiller, Wolf G.; Clever, Guido H.
      Journal of the American Chemical Society (2019), 141 (22), 8907-8913CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Fullerenes and their derivs. are of tremendous technol. relevance. Synthetic access and application are still hampered by tedious purifn. protocols, peculiar soly., and limited control over regioselective derivatization. Authors present a modular self-assembly system based on a new low-mol.-wt. binding motif, appended by two palladium(II)-coordinating units of different steric demands, to either form a [Pd2L14]4+ cage or an unprecedented [Pd2L23(MeCN)2]4+ bowl (with L1 = pyridyl, L2 = quinolinyl donors). The former was used as a selective induced-fit receptor for C60. The latter, owing to its more open structure, also allows binding of C70 and fullerene derivs. By exposing only a fraction of the bound guests' surface, the bowl acts as fullerene protecting group to control functionalization, as demonstrated by exclusive monoaddn. of anthracene. In a hierarchical manner, sterically low-demanding dicarboxylates were found to bridge pairs of bowls into pill-shaped dimers, able to host two fullerenes. The hosts allow transferring bound fullerenes into a variety of org. solvents, extending the scope of possible derivatization and processing methodologies.
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    11. 11
      Rizzuto, F. J.; von Krbek, L. K. S.; Nitschke, J. R. Strategies for Binding Multiple Guests in Metal-Organic Cages. Nat. Chem. Rev. 2019, 3, 204– 222,  DOI: 10.1038/s41570-019-0085-3
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      There is no corresponding record for this reference.
    12. 12
      Takezawa, H.; Shitozawa, K.; Fujita, M. Enhanced Reactivity of Twisted Amides Inside a Molecular Cage. Nat. Chem. 2020, 12, 574– 578,  DOI: 10.1038/s41557-020-0455-y
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      12
      Enhanced reactivity of twisted amides inside a molecular cage
      Takezawa, Hiroki; Shitozawa, Kosuke; Fujita, Makoto
      Nature Chemistry (2020), 12 (6), 574-578CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
      Inclusion of N-aryl amides in octahedral (Td-sym.) metal cages generates inclusion complexes in which the amides have significantly twisted geometries. The structures of a variety of inclusion complexes, including ternary inclusion complexes, were detd. by X-ray crystallog.; the cis-twisted conformers were favored in some cases over the trans-planar amide conformers with twist angles of up to 34°. The kinetics of the basic hydrolysis of amide inclusion complexes was detd.; the rates of hydrolysis for included amides were significantly increased over the rate of hydrolysis of the free (unincluded) amide.
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    13. 13
      Chepelin, O. Luminescent, Enantiopure, Phenylatopyridine Iridium-Based Coordination Capsules. J. Am. Chem. Soc. 2012, 134, 19334– 19337,  DOI: 10.1021/ja309031h
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      13
      Luminescent, Enantiopure, Phenylatopyridine Iridium-Based Coordination Capsules
      Chepelin, Oleg; Ujma, Jakub; Wu, Xiaohua; Slawin, Alexandra M. Z.; Pitak, Mateusz B.; Coles, Simon J.; Michel, Julien; Jones, Anita C.; Barran, Perdita E.; Lusby, Paul J.
      Journal of the American Chemical Society (2012), 134 (47), 19334-19337CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The 1st mol. capsule based on an [Ir(ppy)2]+ unit (ppy = 2-phenylatopyridine) was prepd. Following the development of a method to resolve rac-[(Ir(ppy)2Cl)2] into its enantiopure forms, homochiral Ir6L4 octahedra where obtained with the tritopic 1,3,5-tricyanobenzene. Soln. studies and x-ray diffraction show that these capsules encapsulate four of the six assocd. counteranions and that these can be exchanged for other anionic guests. Initial photophys. studies showed that an ensemble of weakly coordinating ligands can lead to luminescence not present in comparable mononuclear systems.
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    14. 14
      Mosquera, J.; Ronson, T. K.; Nitschke, J. R. Subcomponent Flexibility Enables Conversion between D4-Symmetric CdII4L4 Assemblies. J. Am. Chem. Soc. 2016, 138, 1812– 1815,  DOI: 10.1021/jacs.5b12955
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      14
      Subcomponent Flexibility Enables Conversion between D4-Symmetric CdII8L8 and T-Symmetric CdII4L4 Assemblies
      Mosquera, Jesus; Ronson, Tanya K.; Nitschke, Jonathan R.
      Journal of the American Chemical Society (2016), 138 (6), 1812-1815CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A flexible tris-formylpyridine subcomponent A (I) was obsd. to produce three distinct products following CdII-templated self-assembly with different anilines. Two of the products were CdII4L4 tetrahedra (C192H172Cd4F48N32O46S16), one with ligands puckered inward, and the other outward. The third product was a CdII8L8 structure (C399H382.5Cd8F48N63.5O72S16) having all mer stereochem., contrasting with the fac stereochem. of the tetrahedra. These three complexes coexist in soln. The equil. between them could be influenced through guest binding and specific interactions between aniline subcomponents, allowing a selected one of the three to predominate under defined conditions.
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    15. 15
      Cullen, W.; Misuraca, M. C.; Hunter, C. A.; Williams, N. H.; Ward, M. D. Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage. Nat. Chem. 2016, 8, 231– 236,  DOI: 10.1038/nchem.2452
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      15
      Highly efficient catalysis of the Kemp elimination in the cavity of a cubic coordination cage
      Cullen, William; Misuraca, M. Cristina; Hunter, Christopher A.; Williams, Nicholas H.; Ward, Michael D.
      Nature Chemistry (2016), 8 (3), 231-236CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      The hollow cavities of coordination cages can provide an environment for enzyme-like catalytic reactions of small-mol. guests. Here, we report a new example (catalysis of the Kemp elimination reaction of benzisoxazole with hydroxide to form 2-cyanophenolate) in the cavity of a water-sol. M8L12 coordination cage, with two features of particular interest. First, the rate enhancement is among the largest obsd. to date: at pD 8.5, the value of kcat/kuncat is 2 × 105, due to the accumulation of a high concn. of partially desolvated hydroxide ions around the bound guest arising from ion-pairing with the 16+ cage. Second, the catalysis is based on two orthogonal interactions: (1) hydrophobic binding of benzisoxazole in the cavity and (2) polar binding of hydroxide ions to sites on the cage surface, both of which were established by competition expts.
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    16. 16
      Liu, Y.; Zhang, R.; He, C.; Dang, D. B.; Duan, C. Y. A Palladium(II) Triangle as Building Blocks of Microporous Molecular Materials: Structures and Catalytic Performance. Chem. Commun. 2010, 46, 746– 748,  DOI: 10.1039/B916916D
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      16
      A palladium(II) triangle as building blocks of microporous molecular materials: structures and catalytic performance
      Liu, Yang; Zhang, Rong; He, Cheng; Dang, Dongbin; Duan, Chunying
      Chemical Communications (Cambridge, United Kingdom) (2010), 46 (5), 746-748CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A porous mol. cryst. solid based on amide-contg. Pd(II) triangles, [Pd3L3](NO3)6·34CH3OH·8H2O (L = 3,3',5,5'-tetrakis(pyridin-4-ylmethylaminocarbonyl)diphenylmethane) was prepd., characterized by x-ray crystallog., and evaluated for size-selective heterogeneous catalysis of the Knoevenagel condensation reaction.
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    17. 17
      Carpenter, J. P.; McTernan, C. T.; Ronson, T. K.; Nitschke, J. R. Anion Pairs Template a Trigonal Prism with Disilver Vertices. J. Am. Chem. Soc. 2019, 141, 11409– 11413,  DOI: 10.1021/jacs.9b05432
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      17
      Anion Pairs Template a Trigonal Prism with Disilver Vertices
      Carpenter, John P.; McTernan, Charlie T.; Ronson, Tanya K.; Nitschke, Jonathan R.
      Journal of the American Chemical Society (2019), 141 (29), 11409-11413CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Here authors describe the formation of a trigonal prismatic cage, utilizing 2-formyl-1,8-naphthyridine subcomponents to bind pairs of silver(I) ions in close proximity. This cage is the first example of a new class of subcomponent self-assembled polyhedral structures having bimetallic vertices, as opposed to the single metal centers that typically serve as structural elements within such cages. The new cage self-assembles around a pair of anionic templates, which are shown by crystallog. and soln.-phase data to bind within the central cavity of the structure. Many different anions serve as competent templates and guests. Elongated dianions, such as the strong oxidizing agent peroxysulfate, also serve to template and bind within the cavity of the prism. The principle of using subcomponents that have more than one spatially close, but nonchelating, binding site may thus allow access to other higher-order structures with multimetallic vertices.
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    18. 18
      Huang, S.; Lin, Y.; Hor, T. S. A.; Jin, J. Cp*Rh-based Heterometallic Metallarectangles: Size-Dependent Borromean Link Structures and Catalytic Acyl Transfer. J. Am. Chem. Soc. 2013, 135, 8125– 8128,  DOI: 10.1021/ja402630g
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      18
      Cp*Rh-Based Heterometallic Metallarectangles: Size-Dependent Borromean Link Structures and Catalytic Acyl Transfer
      Huang, Sheng-Li; Lin, Yue-Jian; Hor, T. S. Andy; Jin, Guo-Xin
      Journal of the American Chemical Society (2013), 135 (22), 8125-8128CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Cp*Rh-based functional metallarectangles were synthesized from metallaligands, e.g., [Cu(opba)]2- [opba = o-phenylenebis(oxamato)]. Enlargement of one linker gives two novel Borromean link architectures. All these complexes are intact in soln., as evident from ESI-MS spectroscopic anal. Arising from the combination of open Cu centers and favorable cavity space, {(Cp*Rh)4(bpe)2[Cu(opba)·2MeOH]2}4(OTf)·6MeOH shows extraordinary catalytic abilities with high efficiency and wide substrate selectivity in the acyl-transfer reaction.
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    19. 19
      Yue, N. L. S.; Jennings, M. C.; Puddephatt, R. J. Disilver(I) Macrocycles: Variation of Cavity Size with Anion Binding. Inorg. Chem. 2005, 44, 1125– 1131,  DOI: 10.1021/ic048549c
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      19
      Disilver(I) Macrocycles: Variation of Cavity Size with Anion Binding
      Yue, Nancy L. S.; Jennings, Michael C.; Puddephatt, Richard J.
      Inorganic Chemistry (2005), 44 (4), 1125-1131CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Reaction of the N-methylated bis(amidopyridine) ligand, LL = C6H4(1,3-CONMe-4-C5H4N)2, with AgNO3, AgO2CCF3, AgO3SCF3, AgBF4, and AgPF6 gave the corresponding cationic disilver(I) macrocycles [Ag2(μ-LL)2]X2, 2a-e, resp. The transannular Ag···silver distance in the macrocycles varies greatly from 2.99 to 7.03 Å, and these differences arise through a combination of different modes of anion binding and from the presence or absence of Ag···silver secondary bonding. In all complexes, the ligand adopts a conformation in which the Me group and O atom of the MeNCO units are mutually cis, but the overall macrocycle can exist in either boat (X = PF6 only) or chair conformation. Short transannular Ag···silver distances are found in complexes 2b,c, in which the anions CF3CO2- and CF3SO3- bind above and below the macrocycle, but longer Ag···silver distances are found for 2a,d,e, in which the anions are present, at least in part, inside the disilver macrocycle. Easy anion exchange occurs in soln., and studies using ESI-MS indicate that the anion binding to form [Ag2X(μ-LL)2]+ follows the sequence X = CF3CO2- > NO3- > CF3SO3-.
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    20. 20
      Dong, Y.-B.; Geng, Y.; Ma, J.-P.; Huang, R.-Q. Organometallic Silver(I) Supramolecular Complexes Generated from Multidentate Furan-Containing Symmetric and Unsymmetric Fulvene Ligands and Silver(I) Salts. Inorg. Chem. 2005, 44, 1693– 1703,  DOI: 10.1021/ic048518h
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      20
      Organometallic Silver(I) Supramolecular Complexes Generated from Multidentate Furan-Containing Symmetric and Unsymmetric Fulvene Ligands and Silver(I) Salts
      Dong, Yu-Bin; Geng, Yan; Ma, Jian-Ping; Huang, Ru-Qi
      Inorganic Chemistry (2005), 44 (6), 1693-1703CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      One new conjugated sym. fulvene ligand L1 (I, R = 2-furyl) and two new unsym. fulvene ligands L2 and L3 (I, R = 4- and 3-cyanophenyl, resp.) were synthesized. Five new supramol. complexes, Ag2(L1)3(SO3CF3)3 (1, monoclinic, space group P21/c; a 12.702(3), b 26.118(7), c 13.998(4) Å, β 96.063(4)°, Z = 4), [Ag(L1)]ClO4 (2, monoclinic, space group C2/c; a 17.363(2), b 13.2794(18), c 13.4884(18) Å, β 100.292(2)°, Z = 8), [Ag(L1)(C6H6)SbF6]·0.5C6H6·H2O (3, monoclinic, P21/c; a 6.8839(11), b 20.242(3), c 18.934(3) Å, β 91.994(3)°, Z = 4), Ag(L2)(SO3CF3) (4, triclinic, P‾1; a 8.629(3), b 10.915(3), c 11.178(3) Å, α 100.978(4), β 91.994(3), γ 105.652(4)°, Z = 2), and Ag(L3)(H2O)(SO3CF3) (5, triclinic, P‾1; a 8.914(5), b 10.809(6), c 11.283(6) Å, α 69.255(8), β 87.163(9), γ 84.993(8)°, Z = 2) were obtained through self-assembly based on these three new fulvene ligands in a benzene/toluene mixed-solvent system. Compds. 1-5 were fully characterized by IR spectroscopy, elemental anal., and single-crystal x-ray diffraction. The coordination chem. of new fulvene ligands is versatile. They can adopt either cis- or trans-conformation to bind soft acid Ag(I) ion through not only the terminal -CN and furan functional groups but also the fulvene carbon atoms into organometallic coordination polymers or discrete complexes. The luminescent properties of L1-L3 and their Ag(I) complexes were studied preliminarily in EtOH and solid state.
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    21. 21
      Wang, Q.; Gonell, S.; Leenders, S. H. A. M.; Dürr, M.; Ivanović-Burmazović, I.; Reek, J. N. H. Self-Assembled Nanospheres with Multiple Endohedral Binding Sites Pre-Organize Catalysts and Substrates for Highly Efficient Reactions. Nat. Chem. 2016, 8, 225– 230,  DOI: 10.1038/nchem.2425
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      21
      Self-assembled nanospheres with multiple endohedral binding sites pre-organize catalysts and substrates for highly efficient reactions
      Wang, Qi-Qiang; Gonell, Sergio; Leenders, Stefan H. A. M.; Duerr, Maximilian; Ivanovic-Burmazovic, Ivana; Reek, Joost N. H.
      Nature Chemistry (2016), 8 (3), 225-230CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Tuning reagent and catalyst concns. is crucial in the development of efficient catalytic transformations. In enzyme-catalyzed reactions the substrate is bound-often by multiple noncovalent interactions-in a well-defined pocket close to the active site of the enzyme; this pre-organization facilitates highly efficient transformations. Here the authors report an artificial system that coencapsulates multiple catalysts and substrates within the confined space defined by an M12L24 nanosphere that contains 24 endohedral guanidinium-binding sites. Cooperative binding means that sulfonate guests are bound much more strongly than carboxylates. This difference was used to fix Au-based catalysts firmly, with the remaining binding sites left to pre-organize substrates. This strategy was applied to a Au(I)-catalyzed cyclization of acetylenic acid to enol lactone in which the pre-organization resulted in much higher reaction rates. Also the encapsulated sulfonate-contg. Au(|) catalysts did not convert neutral (acid) substrates, and so could have potential in the development of substrate-selective catalysis and base-triggered on/off switching of catalysis.
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    22. 22
      Lisboa, L. S.; Findlay, J. A.; Wright, L. J.; Hartinger, C. G.; Crowley, J. D. A Reduced Symmetry Heterobimetallic [PdPtL4]4+ Cage: Assembly, Guest Binding and Stimulus-Induced Switching. Angew. Chem., Int. Ed. 2020, 59, 11101– 11107,  DOI: 10.1002/anie.202003220
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      22
      A Reduced-Symmetry Heterobimetallic [PdPtL4]4+ Cage: Assembly, Guest Binding, and Stimulus-Induced Switching
      Lisboa, Lynn S.; Findlay, James A.; Wright, L. James; Hartinger, Christian G.; Crowley, James D.
      Angewandte Chemie, International Edition (2020), 59 (27), 11101-11107CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A strategy is presented that enables the quant. assembly of a heterobimetallic [PdPtL4]4+ cage. The presence of two different metal ions (PdII and PtII) with differing labilities enables the cage to be opened and closed selectively at one end upon treatment with suitable stimuli. Combining an inert PtII tetrapyridylaldehyde complex with a suitably substituted pyridylamine and PdII ions led to the assembly of the cage. 1H and DOSY NMR spectroscopy and ESI mass spectrometry data were consistent with the quant. formation of the cage, and the heterobimetallic structure was confirmed using single-crystal x-ray crystallog. The structure of the host-guest adduct with a 2,6-diaminoanthraquinone guest mol. was detd. Addn. of N,N'-dimethylaminopyridine (DMAP) resulted in the formation of the open-cage [PtL4]2+ compd. and [Pd(DMAP)4]2+ complex. This process could then be reversed, with the reformation of the cage, upon addn. of p-toluenesulfonic acid (TsOH).
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    23. 23
      Ueda, Y.; Ito, H.; Fujita, D.; Fujita, M. Permeable Self-Assembled Molecular Containers for Catalyst Isolation Enabling Two-Step Cascade Reactions. J. Am. Chem. Soc. 2017, 139, 6090– 6093,  DOI: 10.1021/jacs.7b02745
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      23
      Permeable Self-Assembled Molecular Containers for Catalyst Isolation Enabling Two-Step Cascade Reactions
      Ueda, Yoshihiro; Ito, Hiroaki; Fujita, Daishi; Fujita, Makoto
      Journal of the American Chemical Society (2017), 139 (17), 6090-6093CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Establishment of a general one-pot cascade reaction protocol would dramatically reduce the effort of multistep org. synthesis. We demonstrate that the unique structure of M12L24 self-assembled complexes gives them the potential to serve as catalyst carriers for enabling continuous chem. transformations. A stereoselective cascade reaction (allylic oxidn. followed by Diels-Alder cyclization) with two intrinsically incompatible catalysts was demonstrated. Our system is advantageous in terms of availability, scalability, and predictability.
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    24. 24
      Holloway, L. R.; Bogie, P. M.; Lyon, Y.; Ngai, C.; Miller, T. F.; Julian, R. R.; Hooley, R. J. Tandem Reactivity of a Self-Assembled Cage Catalyst with Endohedral Acid Groups. J. Am. Chem. Soc. 2018, 140, 8078– 8081,  DOI: 10.1021/jacs.8b03984
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      24
      Tandem Reactivity of a Self-Assembled Cage Catalyst with Endohedral Acid Groups
      Holloway, Lauren R.; Bogie, Paul M.; Lyon, Yana; Ngai, Courtney; Miller, Tabitha F.; Julian, Ryan R.; Hooley, Richard J.
      Journal of the American Chemical Society (2018), 140 (26), 8078-8081CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Self-assembly of a carboxylic acid-contg. ligand into an Fe4L6 iminopyridine cage allows endohedral positioning of the acid groups while maintaining a robust cage structure. The cage is an effective supramol. catalyst, providing up to 1000-fold rate enhancement of acetal solvolysis. This enhanced reactivity allows a tandem deprotection/cage-to-cage interconversion that cannot be achieved with other acid catalysts. The combination of rate enhancements and sequestration of the reactive function confers both activity and selectivity on the process, mimicking enzymic behavior.
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    25. 25
      Hua, B.; Shao, L.; Zhang, Z.; Liu, J.; Huang, F. Cooperative Silver Ion-Pair Recognition by Peralkylated Pillar[5]arenes. J. Am. Chem. Soc. 2019, 141, 15008– 15012,  DOI: 10.1021/jacs.9b08257
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      25
      Cooperative Silver Ion-Pair Recognition by Peralkylated Pillar[5]arenes
      Hua, Bin; Shao, Li; Zhang, Zhihua; Liu, Jiyong; Huang, Feihe
      Journal of the American Chemical Society (2019), 141 (38), 15008-15012CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Ion-pair recognition of metal salts by pillararenes has never been reported. This limits the further development and applications of pillararene supramol. chem. Herein we report the first examples of pillararene-based metal ion-pair recognition. They are host-guest complexes between three peralkylated pillar[5]arenes and the silver salt CF3COOAg. Single crystal X-ray anal. shows that CF3COOAg forms a unique dinuclear silver structure that penetrates into the pillar[5]arene cavity in these complexes. Besides the silver-π interactions, multiple C-H···O and C-H···F hydrogen bonds between the counterion and the pillar[5]arene host also contribute to stabilizing and maintaining the host-guest structures. Moreover, benefiting from the silver ion-pair recognition, perethylated pillar[5]arene EtP5 can be used as a solid absorbent to capture CF3COOAg from soln. efficiently, showing a potential application in precious metal extn. and recycling.
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    26. 26
      Sawada, T.; Fujita, M. Folding and Assembly of Metal-Linked Peptidic Nanostructures. Chem. 2020, 6, 1861– 1876,  DOI: 10.1016/j.chempr.2020.07.002
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      26
      Folding and Assembly of Metal-Linked Peptidic Nanostructures
      Sawada, Tomohisa; Fujita, Makoto
      Chem (2020), 6 (8), 1861-1876CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      A review. Folding and assembly are intra- and inter-mol. processes, resp., for spontaneously generating predetd., well-defined mol. structures. Though the cooperative processes of these mechanisms have been studied in coiled coils and β-sheets, they have been seldom utilized simultaneously in the synthetic fields. We have been developing a new folding-and-assembly (F&A) strategy, in which the folding and metal-directed self-assembly of a short peptide fragment occur simultaneously by helping and inducing the processes of each other. Depending on the sequence design, each short peptide ligand exhibits a specific conformation during metal coordination and simultaneously assembles into an advanced nanostructure, which has been hardly achieved by known synthetic strategies. The concepts, examples of the F&A strategy, the function of the obtained structures as well as future directions are disclosed in this perspective.
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    27. 27
      Sawada, T.; Inomata, Y.; Shimokawa, K.; Fujita, M. A Metal-Peptide Capsule by Multiple Ring Threading. Nat. Commun. 2019, 10, 5687,  DOI: 10.1038/s41467-019-13594-4
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      27
      A metal-peptide capsule by multiple ring threading
      Sawada, Tomohisa; Inomata, Yuuki; Shimokawa, Koya; Fujita, Makoto
      Nature Communications (2019), 10 (1), 5687CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Cavity creation is a key to the origin of biol. functions. Small cavities such as enzyme pockets are created simply through liner peptide folding. Nature can create much larger cavities by threading and entangling large peptide rings, as learned from gigantic virus capsids, where not only chem. structures but the topol. of threaded rings must be controlled. Although interlocked mols. are a topic of current interest, they have for decades been explored merely as elements of mol. machines, or as a synthetic challenge. No research has specifically targeted them for, and succesfully achieved, cavity creation. Here we report the emergence of a huge capsular framework via multiple threading of metal-peptide rings. Six equivalent C4-propeller-shaped rings, each consisting of four oligopeptides and Ag+, are threaded by each other a total of twelve times (crossing no.: 24) to assemble into a well-defined 4 nm-sized sphere, which acts as a huge mol. capsule.
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    28. 28
      Inomata, Y.; Sawada, T.; Fujita, M. Metal-Peptide Torus Knots from Flexible Short Peptides. Chem 2020, 6, 294,  DOI: 10.1016/j.chempr.2019.12.009
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      28
      Metal-Peptide Torus Knots from Flexible Short Peptides
      Inomata, Yuuki; Sawada, Tomohisa; Fujita, Makoto
      Chem (2020), 6 (1), 294-303CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Entanglements of strings are regularly encountered on the macroscale yet rarely utilized in nanoscale construction. Here, we report highly entangled peptide-mimic structures created through folding and assembly of silver(I) ions and a triglycine ligand (L). The highly flexible triglycine strand assembled into a septafoil knot ([Ag·L]7) and its eight-crossing link analog ([Ag·L]8), both of which were formed for the first time by chem. synthesis. The two structures result from circular oligomerization of the same 1-crossing Ag·L motif. We also obtained poly[n]catenane [Ag·L]n from a topol. isomeric 2-crossing motif. Our observations reveal that dynamic linkages of short peptides enable easy access to knotted structures, which are restricted in protein structures because of the entropic (and/or kinetic) disadvantage of self-threading processes in long peptide chains, with remarkable stereoselectivity in the self-assembly process.
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    29. 29
      Barendt, T. A.; Docker, A.; Marques, I.; Félix, V.; Beer, P. D. Selective Nitrate Recognition by a Halogen-Bonding Four-Station [3]Rotaxane Molecular Shuttle. Angew. Chem., Int. Ed. 2016, 55, 11069– 11076,  DOI: 10.1002/anie.201604327
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      29
      Selective Nitrate Recognition by a Halogen Bonding Four Station [3]Rotaxane Molecular Shuttle
      Barendt, Timothy A.; Docker, Andrew; Marques, Igor; Felix, Vitor; Beer, Paul D.
      Angewandte Chemie, International Edition (2016), 55 (37), 11069-11076CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The synthesis of the first halogen bonding [3]rotaxane host system contg. a bis-iodo triazolium-bis-naphthalene diimide four station axle component is reported. Proton NMR anion binding titrn. expts. revealed the halogen bonding rotaxane is selective for nitrate over the more basic acetate, hydrogen carbonate and dihydrogen phosphate oxoanions and chloride, and exhibits enhanced recognition of anions relative to a hydrogen bonding analog. This elaborate interlocked anion receptor functions via a novel dynamic pincer mechanism where upon nitrate anion binding, both macrocycles shuttle from the naphthalene diimide stations at the periphery of the axle to the central halogen bonding iodo-triazolium station anion recognition sites to form a unique 1:1 stoichiometric nitrate anion-rotaxane sandwich complex. Mol. dynamics simulations carried out on the nitrate and chloride halogen bonding [3]rotaxane complexes corroborate the 1H NMR anion binding results.
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    30. 30
      Langton, M. J.; Beer, P. D. Rotaxane and Catenane Host Structures for Sensing Charged Guest Species. Acc. Chem. Res. 2014, 47, 1935– 1949,  DOI: 10.1021/ar500012a
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      30
      Rotaxane and Catenane Host Structures for Sensing Charged Guest Species
      Langton, Matthew J.; Beer, Paul D.
      Accounts of Chemical Research (2014), 47 (7), 1935-1949CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. The promise of mech. interlocked architectures, such as rotaxanes and catenanes, as prototypical mol. switches and shuttles for nanotechnol. applications, has stimulated an ever increasing interest in their synthesis and function. The elaborate host cavities of interlocked structures, however, can also offer a novel approach toward mol. recognition: this Account describes the use of rotaxane and catenane host systems for binding charged guest species, and for providing sensing capability through an integrated optical or electrochem. reporter group. Particular attention is drawn to the exploitation of the unusual dynamic properties of interlocked mols., such as guest-induced shuttling or conformational switching, as a sophisticated means of achieving a selective and functional sensor response. The authors initially survey interlocked host systems capable of sensing cationic guests, before focusing on the accomplishments in synthesizing rotaxanes and catenanes designed for the more challenging task of selective anion sensing. In the group, the authors have developed the use of discrete anionic templation to prep. mech. interlocked structures for anion recognition applications. Removal of the anion template reveals an interlocked host system, possessing a unique three-dimensional geometrically restrained binding cavity formed between the interlocked components, which exhibits impressive selectivity toward complementary anionic guest species. By incorporating reporter groups within such systems, the authors have developed both electrochem. and optical anion sensors which can achieve highly selective sensing of anionic guests. Transition metals, lanthanides, and org. fluorophores integrated within the mech. bonded structural framework of the receptor are perturbed by the binding of the guest, with a concomitant change in the emission profile. The authors have also exploited the unique dynamics of interlocked hosts by demonstrating that an anion-induced conformational change can be used as a means of signal transduction. Electrochem. sensing was realized by integration of the redox-active ferrocene functionality within a range of rotaxane and catenanes; binding of an anion perturbs the metallocene, leading to a cathodic shift in the ferrocene/ferrocenium redox couple. To obtain practical sensors for target charged guest species, confinement of receptors at a surface is necessary to develop robust, reuseable devices. Surface confinement also offers advantages over soln. based receptors, including amplification of signal, enhanced guest binding thermodn. and the negation of soly. problems. The authors have fabricated anion-templated rotaxanes and catenanes on gold electrode surfaces and demonstrated that the resulting mech. bonded self-assembled monolayers are electrochem. responsive to the binding of anions, a crucial 1st step toward the advancement of sophisticated, highly selective, anion sensory devices. Rotaxane and catenane host mols. may be engineered to offer a superior level of mol. recognition, and the incorporation of optical or electrochem. reporter groups within these interlocked frameworks can allow for guest sensing. Advances in synthetic templation strategies has facilitated the synthesis of interlocked architectures and widened their interest as prototype mol. machines. However, their unique host-guest properties are only now beginning to be exploited as a sophisticated approach to chem. sensing. The development of functional host-guest sensory systems such as these is of great interest to the interdisciplinary field of supramol. chem.
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    31. 31
      Kishi, N.; Akita, M.; Kamiya, M.; Hayashi, S.; Hsu, H.-F.; Yoshizawa, M. Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube. J. Am. Chem. Soc. 2013, 135, 12976– 12979,  DOI: 10.1021/ja406893y
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      31
      Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube
      Kishi, Norifumi; Akita, Munetaka; Kamiya, Motoshi; Hayashi, Shigehiko; Hsu, Hsiu-Fu; Yoshizawa, Michito
      Journal of the American Chemical Society (2013), 135 (35), 12976-12979CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A novel M2L2 mol. tube capable of binding fullerene C60 was synthesized from bispyridine ligands (I; R = OC2H4OMe) with embedded anthracene panels and Ag(I) hinges. Unlike previous mol. cages and capsules, this open-ended tubular host can accommodate a single mol. of various C60 derivs. with large substituents. The fullerene guest can then be released by using the ideal, noninvasive external stimulus, light.
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    32. 32
      Riddell, I. A.; Smulders, M. M. J.; Clegg, J. K.; Hristova, Y. R.; Breiner, B.; Thoburn, J. D.; Nitschke, J. R. Anion-induced Reconstitution of a Self-Assembly System to Express a Chloride-Binding Co10L15 Pentagonal Prism. Nat. Chem. 2012, 4, 751– 756,  DOI: 10.1038/nchem.1407
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      32
      Anion-induced reconstitution of a self-assembling system to express a chloride-binding Co10L15 pentagonal prism
      Riddell, Imogen A.; Smulders, Maarten M. J.; Clegg, Jack K.; Hristova, Yana R.; Breiner, Boris; Thoburn, John D.; Nitschke, Jonathan R.
      Nature Chemistry (2012), 4 (9), 751-756CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
      Biochem. systems are adaptable, capable of reconstitution at all levels to achieve the functions assocd. with life. Synthetic chem. systems are more limited in their ability to reorganize to achieve new functions; they can reconfigure to bind an added substrate (template effect) or one binding event may modulate a receptor's affinity for a second substrate (allosteric effect). Here the authors describe a synthetic chem. system that is capable of structural reconstitution on receipt of one anionic signal (perchlorate) to create a tight binding pocket for another anion (chloride). A decanuclear cobalt(II) complex [Cl- ⊂ Co10L15](ClO4)19 (L = 6,6'-bis[(4-methylphenyl)iminomethyl]-3,3'-bipyridine) was prepd. by the reaction of p-toluidine and 6,6'-diformyl-3,3'-bipyridine and Co(ClO4)2, whereas a tetranuclear complex [Co4L6](OTf)8 results when Co(OTf)2 is used as the cobalt(II) source. The tetranuclear complex is converted to the decanuclear complex upon addn. of LiClO4. The decanuclear complex acts as a receptor for halides, N3-, OCN- and SCN-. The complex, barrel-like structure of the chloride receptor is templated by five perchlorate anions. This second-order templation phenomenon allows chem. networks to be envisaged that express more complex responses to chem. signals than is currently feasible.
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    33. 33
      Zhang, W.; Yang, D.; Zhao, J.; Hou, L.; Sessler, J. L.; Yang, Z.-J.; Wu, B. Controlling the Recognition and Reactivity of Alkyl Ammonium Guests Using an Anion Coordination-Based Tetrahedral Cage. J. Am. Chem. Soc. 2018, 140, 5248– 5256,  DOI: 10.1021/jacs.8b01488
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      33
      Controlling the Recognition and Reactivity of Alkyl Ammonium Guests Using an Anion Coordination-Based Tetrahedral Cage
      Zhang, Wenyao; Yang, Dong; Zhao, Jie; Hou, Lekai; Sessler, Jonathan L.; Yang, Xiao-Juan; Wu, Biao
      Journal of the American Chemical Society (2018), 140 (15), 5248-5256CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Caged structures have found wide application in a variety of areas, including guest encapsulation and catalysis. Although metal-based cages have dominated the field, anion-coordination-based cages are emerging as a new type of supramol. ensemble with interesting host-guest properties. In the current work, we report a C3-sym. tris-bis(urea) ligand based on the 2,4,6-triphenyl-1,3,5-triazine spacer, which assembles with phosphate anions to form an A4L4-type (A = anion, L = ligand) tetrahedral cage, 3, with unusually high packing coeffs. (up to 99.5% for the best substrate). Cage 3 is able to adjust its size and shape (from 136 to 216 Å3) by bending of the triphenyltriazine plane. This allows it to accommodate relatively large guests. In the case of DABCO, inclusion within the cage allows the degree of methylation to be controlled and the monomethylated product to be isolated cleanly under conditions where mixts. of the mono- and dimethylated adduct are obtained in the absence of cage 3.
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      Custelcean, R. Anion Encapsulation and Dynamics in Self-Assembled Coordination Cages. Chem. Soc. Rev. 2014, 43, 1813– 1824,  DOI: 10.1039/C3CS60371G
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      34
      Anion encapsulation and dynamics in self-assembled coordination cages
      Custelcean, Radu
      Chemical Society Reviews (2014), 43 (6), 1813-1824CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. The first part of the review describes various examples of anion-encapsulating coordination cages, categorized on the basis of their MxLy stoichiometry (M = metal cation; L = org. ligand). The second part deals with the dynamic aspects of anion encapsulation, including the kinetics and mechanism of anion binding, release, and exchange, as well as the structural evolution of the coordination complexes involved.
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      Bowman-James, K. Alfred Werner Revisited: The Coordination Chemistry of Anions. Acc. Chem. Res. 2005, 38, 671– 678,  DOI: 10.1021/ar040071t
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      35
      Alfred Werner Revisited: The Coordination Chemistry of Anions
      Bowman-James, Kristin
      Accounts of Chemical Research (2005), 38 (8), 671-678CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review is presented on the complexation of anions within macrocyclic receptors. Macrocyclic receptors were designed to probe the influence of four factors, hydrogen bonding, charge, dimensionality, and topol., on anion binding. Monocyclic and bicyclic polyammonium and polyamide receptors were synthesized from either 2,2'-diaminodiethylamine derivs. (dien) or 2,2',2''-aminoethylamine (tren) building blocks, plus arom. or heterocyclic spacers. Supramol. complexes of these hosts with three simple anion topologies were probed: spherical (halides), trigonal planar (nitrate), and tetrahedral (sulfate). Results indicate a no. of corollaries with transition-metal coordination chem. in terms of binding concepts such as the chelate effect and dual valencies, as well as geometries for anion complexes that are strikingly similar to those obsd. in transition-metal coordination chem.
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    36. 36
      Custelcean, R.; Bonnesen, P. V.; Duncan, N. C.; Zhang, X.; Watson, L. A.; Van Berkel, G.; Parson, W. B.; Hay, B. P. Urea-Functionalized M4L6 Cage Receptors: Anion-Templated Self-Assembly and Selective Guest Exchange in Aqueous Solutions. J. Am. Chem. Soc. 2012, 134, 8525– 8534,  DOI: 10.1021/ja300677w
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      36
      Urea-Functionalized M4L6 Cage Receptors: Anion-Templated Self-Assembly and Selective Guest Exchange in Aqueous Solutions
      Custelcean, Radu; Bonnesen, Peter V.; Duncan, Nathan C.; Zhang, Xiaohua; Watson, Lori A.; Van Berkel, Gary; Parson, Whitney B.; Hay, Benjamin P.
      Journal of the American Chemical Society (2012), 134 (20), 8525-8534CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The authors present an extensive study of a novel class of de novo designed tetrahedral M4L6 (M = Ni, Zn) cage receptors, wherein internal decoration of the cage cavities with urea anion-binding groups, via functionalization of the org. components L, led to selective encapsulation of tetrahedral oxoanions EO4n- (E = S, Se, Cr, Mo, W, n = 2; E = P, n = 3) from aq. solns., based on shape, size, and charge recognition. External functionalization with tBu groups led to enhanced soly. of the cages in aq. MeOH solns., thereby allowing for their thorough characterization by multinuclear (1H, 13C, 77Se) and diffusion NMR spectroscopies. Addnl. exptl. characterization by electrospray ionization mass spectrometry, UV-visible spectroscopy, and single-crystal x-ray diffraction, as well as theor. calcns., led to a detailed understanding of the cage structures, self-assembly, and anion encapsulation. The cage self-assembly is templated by EO4n- oxoanions (n ≥ 2), and upon removal of the templating anion the tetrahedral M4L6 cages rearrange into different coordination assemblies. The exchange selectivity among EO4n- oxoanions was studied with 77Se NMR spectroscopy using 77SeO42- as an anionic probe, which found the following selectivity trend: PO43- » CrO42- > SO42- > SeO42- > MoO42- > WO42-. In addn. to the complementarity and flexibility of the cage receptor, a combination of factors contribute to the obsd. anion selectivity, including the anions' charge, size, hydration, basicity, and H-bond acceptor abilities.
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    37. 37
      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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      37
      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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    38. 38
      Liu, Y.; Sengupta, A.; Raghavachari, K.; Flood, A. M. Anion Binding in Solution: Beyond the Electrostatic Regime. Chem 2017, 3, 411– 417,  DOI: 10.1016/j.chempr.2017.08.003
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      38
      Anion Binding in Solution: Beyond the Electrostatic Regime
      Liu, Yun; Sengupta, Arkajyoti; Raghavachari, Krishnan; Flood, Amar H.
      Chem (2017), 3 (3), 411-427CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      A fundamental understanding of anion binding by receptors is essential for managing salts during energy, water, and food prodn. However, the limited understanding of solvent effects in ion recognition leads to a persistent blind spot that prevents effective receptor design. We exptl. discovered an underlying 1/εr dependence of anion affinity on solvent dielec. const. (εr). We found this relationship by measuring how chloride binds to macrocyclic triazolophane receptors across a wide range of solvents: εr = 4.7-56.2. Solvent weakens affinity by screening electrostatics; electrostatics dominates when εr < 4.7 (chloroform) and then transits a cross-over region (4.7 < εr < 20.5; acetone), after which it no longer governs affinity (acetonitrile and DMSO). D. functional theory helped us understand this dependence. Our theory-backed model accurately predicts Cl- affinity in solvents used in liq.-liq. extns. in the nuclear fuel cycle. This model offers a general foundation for anion recognition and electrostatically driven complexation.
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    39. 39
      Zhao, W.; Qiao, B.; Tropp, J.; Pink, M.; Azoulay, J. D.; Flood, A. H. Linear Supramolecular Polymers Driven by Anion-Anion Dimerization of Difunctional Phosphonate Monomers Inside Cyanostar Macrocycles. J. Am. Chem. Soc. 2019, 141, 4980– 4989,  DOI: 10.1021/jacs.9b00248
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      39
      Linear Supramolecular Polymers Driven by Anion-Anion Dimerization of Difunctional Phosphonate Monomers Inside Cyanostar Macrocycles
      Zhao, Wei; Qiao, Bo; Tropp, Joshua; Pink, Maren; Azoulay, Jason D.; Flood, Amar H.
      Journal of the American Chemical Society (2019), 141 (12), 4980-4989CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Supramol. polymers have enabled far-reaching fundamental science and the development of diverse macromol. technologies owing to the reversible and noncovalent chem. connectivities that define their properties. Despite the unabated development of these materials using highly tailorable recognition elements, anion-based polymers remain rare as a result of the weak interactions they mediate. Here, we use design rules inspired by cation-driven polymers to demonstrate a new noncovalent link based on receptor-stabilized anion-anion interactions that enables the efficient linear polymn. of simple difunctional phosphonates. The linear main chain connectivity and mol. topol. were confirmed by single crystal X-ray diffraction, which demonstrates the rare 2:2 stoichiometry between the anionic phosphonate end groups and a pair of π-stacked cyanostar macrocycles. The stability of these links enables rapid polymn. of difunctional phosphonates employing different aliph. linkers (C6H12, C8H16, C10H20, C12H24). Diphosphonates with greater chain flexibility (C12H24) enable greater polymn. with an av. d.p. of nine emerging at 10 mM. Viscosity measurements show a transition from oligomers to polymers at the crit. polymn. concn. of 5 mM. In a rare correlation, NMR spectroscopy shows a coincident mol. signature of the polymn. at 5 mM. These polymers are highly concn. dependent, reversibly polymerize with acid and base, and respond to competitive anions. They display the design simplicity of metallo-supramol. polymers with transfer of the strong 2:2 recognition chem. to macromols. The simplicity and understanding of this new class of supramol. polymer is anticipated to open opportunities in tailoring anion-based functional materials.
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    40. 40
      Wu, X.; Wang, P.; Turner, P.; Lewis, W.; Catal, O.; Thomas, D. S.; Gale, P. A. Tetraurea Macrocycles: Aggregation-Driven Binding of Chloride in Aqueous Solutions. Chem. 2019, 5, 1210– 1222,  DOI: 10.1016/j.chempr.2019.02.023
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      40
      Tetraurea Macrocycles: Aggregation-Driven Binding of Chloride in Aqueous Solutions
      Wu, Xin; Wang, Patrick; Turner, Peter; Lewis, William; Catal, Osman; Thomas, Donald S.; Gale, Philip A.
      Chem (2019), 5 (5), 1210-1222CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      Artificial receptors that recognize anionic species via non-covalent interactions have a wide range of biomedical, industrial, and environmental applications. A major challenge in this area of research is to achieve high-affinity and selective anion binding in aq. media. So far, only a few examples of receptors capable of strong (>105 M-1) anion binding in solns. contg. >50% water are available, and none show selectivity for chloride. We report here the discovery of a D4h-sym. fluorinated tetraurea macrocycle that fulfills this function owing to its unique self-assembly properties. The macrocycle has a strong tendency to self-assoc. into columnar aggregates via intermol. hydrogen bonds and arom. stacking. In aq. solns., macrocycle aggregation generates solvent-shielding and size-selective binding pockets favorable for hydrogen bonding with chloride. As a result, micromolar affinity and highly selective chloride binding have been achieved with this simple small mol. (MW < 700) in 60 vol % water/acetonitrile.
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      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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      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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    42. 42
      Chen, L.; Berry, S. N.; Wu, X.; Howe, E. N. W.; Gale, P. A. Advances in Anion Receptor Chemistry. Chem. 2020, 6, 61– 141,  DOI: 10.1016/j.chempr.2019.12.002
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      Advances in Anion Receptor Chemistry
      Chen, Lijun; Berry, Stuart N.; Wu, Xin; Howe, Ethan N. W.; Gale, Philip A.
      Chem (2020), 6 (1), 61-141CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      This review covers advances in anion complexation in the years 2017 and 2018. The review focuses on the applications of anion receptor chem., including sensing, self-assembly, extn., transport, catalysis, and fundamental advances in the area.
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    43. 43
      Custelcean, R. Urea-Functionalized Crystalline Capsules for Recognition and Separation of Tetrahedral Oxoanions. Chem. Commun. 2013, 49, 2173– 2182,  DOI: 10.1039/c2cc38252k
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      Urea-functionalized crystalline capsules for recognition and separation of tetrahedral oxoanions
      Custelcean, Radu
      Chemical Communications (Cambridge, United Kingdom) (2013), 49 (22), 2173-2182CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. The persistent ability of tripodal TREN-based tris-urea receptors (TREN = tris(2-aminoethyl)amine) to self-assemble with a variety of oxoanions into dimeric capsules upon crystn. is reviewed. The capsule crystn. allows for charge-, shape-, and size-selective encapsulation of tetrahedral XO4n- anions (n = 2,3), and provides an effective way to sep. these anions from competitive aq. environments.
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    44. 44
      Schäfer, S.; Gamer, M. T.; Lebedkin, S.; Weigend, F.; Kappes, M. M.; Roesky, P. W. Bis(6-methylene-2,2’-bipyridine)phenylphosphine – A Flexible Ligand for the Construction of Trinuclear Coinage-Metal Complexes. Chem. - Eur. J. 2017, 23, 12198– 12209,  DOI: 10.1002/chem.201701091
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      Luo, G.-G.; Guo, Q.-L.; Wang, Z.; Sun, C.-F.; Lin, J.-Q.; Sun, D. New Protective Ligands for Atomically Precise Silver Nanoclusters. Dalton Trans. 2020, 49, 5406– 5415,  DOI: 10.1039/D0DT00477D
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      New protective ligands for atomically precise silver nanoclusters
      Luo, Geng-Geng; Guo, Qi-Lin; Wang, Zhi; Sun, Cun-Fa; Lin, Jin-Qing; Sun, Di
      Dalton Transactions (2020), 49 (17), 5406-5415CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      A review. Atomically precise silver nanoclusters (NCs) have emerged as a hot topic attracting immense research interest. Protecting ligands are needed for direct capping on cluster surfaces to prevent aggregation and to stabilize NCs. Protective ligands are crit. to detg. the sizes, structures and properties of silver NCs. The past decades have witnessed conventionally used org. ligands (thiolates/selenols, phosphines and alkynyls) and inorg. ligands (chalcogens and halogens) being extensively used to passivate NC surfaces. However, only in the most recent years have new-type protecting ligands beyond the conventional ones begun to be introduced in the protecting sphere of new functional silver NCs. The present Frontier article covers the most recent examples of some new protective agents for well-defined silver NCs. The authors describe four classes of novel silver NCs stabilized by newly-developed surface ligands, namely, nitrogen-donor org. ligands, oxygen-donor inorg. ligands, metalloligands and macrocyclic hosts, paying attention to the synthesis, structures and properties of these silver NCs. This Frontier article will hopefully attract more cluster scientists to explore more freshly ligated atomically precise silver NCs with novel structures and properties in the years ahead. The literature survey in this review is based on publications up to Feb. 2020. Some suggestions for future directions in this field are also given.
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    46. 46
      Wang, X. Discrete Ag6L6 Coordination Nanotubular Structures Based on a T-Shaped Pyridyl Diphosphine. Chem. Commun. 2011, 47, 3849– 3851,  DOI: 10.1039/c0cc05235c
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      46
      Discrete Ag6L6 coordination nanotubular structures based on a T-shaped pyridyl diphosphine
      Wang, Xiaobing; Huang, Jing; Xiang, Shenglin; Liu, Yu; Zhang, Jianyong; Eichhofer, Andreas; Fenske, Dieter; Bai, Shi; Su, Cheng-Yong
      Chemical Communications (Cambridge, United Kingdom) (2011), 47 (13), 3849-3851CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Ag6L6-type coordination nanotubular structures were assembled from 6 Ag(I) ions and 6 T-shaped ligands, 4-(3,5-bis(diphenylphosphino)phenyl)pyridine. The nanotubes represent a discrete mol. architecture of a no. of polymeric structures assembled from dimeric building blocks.
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    47. 47
      Zhang, Y.-W.; Bai, S.; Wang, Y.-Y.; Han, Y.-F. A Strategy for the Construction of Triply Interlocked Organometallic Cages by Rational Design of Poly-NHC Precursors. J. Am. Chem. Soc. 2020, 142, 13614– 13621,  DOI: 10.1021/jacs.0c06470
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      47
      A Strategy for the Construction of Triply Interlocked Organometallic Cages by Rational Design of Poly-NHC Precursors
      Zhang, Ya-Wen; Bai, Sha; Wang, Yao-Yu; Han, Ying-Feng
      Journal of the American Chemical Society (2020), 142 (31), 13614-13621CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Three-dimensional (3D) triply interlocked catenanes are a family of chem. topologies that consist of two identical, mech. interlocked coordination cage components with intriguingly complex structures. Although only a few successful constructions of 3-dimensional interlocked catenanes were achieved to date via metal-mediated assembly, these complex structures have thus far only been targeted by metal-N/O coordination techniques. Here, taking advantage of rational ligand design, the authors report the efficient construction of 3-dimensional triply interlocked [2]catenanes [Ag3L2]2, wherein the metal ions exclusively form bonds to N-heterocyclic carbene (NHC) units, and their subsequent transmetalation to the corresponding [Au3L2]2 Au analogs. The formation and transmetalation reactions proceed under mild conditions and are generally applicable. Characterization techniques were applied to confirm the formation and structure of the desired 3-dimensional triply interlocked architectures: multinuclear NMR spectroscopy, ESI-MS, and single-crystal x-ray diffraction anal. The solid-state structure of [Ag3(1a)2]2(PF6)6 unambiguously confirms the existence of a 3-dimensional catenane that consists of two identical, mech. interlocked trinuclear hexacarbene cage components. The interlocking of two 3-dimensional cages into a [2]catenane is driven by the efficient π···π stacking of triazine-triazine stacks with cooperative interactions between imidazo[1,5-a]pyridine subunits. Notably, the triply interlocked organometallic cages exhibit good stability toward various org. solvents, concns., and temps., and no disassembly occurred in the presence of coronene or pyrene. The future construction of mech. interlocked architectures using metal-carbene bonds rather than metal-N bonds may provide assemblies with interesting properties for as-yet-unimagined applications in fields such as sensors and mol. elec. conductors.
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    48. 48
      Jin, G.-X.; Zhu, G.-Y.; Sun, Y.-Y.; Shi, Q.-X.; Liang, L.-P.; Wang, H.-Y.; Wu, Z.-W.; Ma, J.-P. [Ag-Ag]2+ Unit-Encapsulated Trimetallic Cages: One-Pot Syntheses and Modulation of Argentophilic Interactions by the Uncoordinated Substituents. Inorg. Chem. 2019, 58, 2916– 2920,  DOI: 10.1021/acs.inorgchem.8b03388
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      48
      [Ag-Ag]2+ Unit-Encapsulated Trimetallic Cages: One-Pot Syntheses and Modulation of Argentophilic Interactions by the Uncoordinated Substituents
      Jin, Guo-Xia; Zhu, Gui-Ying; Sun, Yan-Yan; Shi, Qing-Xiu; Liang, Li-Ping; Wang, Hai-Ying; Wu, Xiang-Wen; Ma, Jian-Ping
      Inorganic Chemistry (2019), 58 (5), 2916-2920CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Four [Ag-Ag]2+ unit-encapsulated trimetallic cages were synthesized from one new tripodal ligand L and Ag salts in different solvent systems by a 1-pot method. The formation of coordination cages occurred simultaneously with the condensation of amino groups and ketone. The remarkable structural feature of the four cages is their spontaneous incorporation of [Ag-Ag]2+ cationic units. Moreover, the argentophilic interactions are modulated by the uncoordinated amino substituents. The study herein shows that modification and subtle changes of the cage structures could be realized by a 1-pot synthetic method.
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      Schmidbaur, H.; Schier, A. Argentophilic Interactions. Angew. Chem., Int. Ed. 2015, 54, 746– 784,  DOI: 10.1002/anie.201405936
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      Argentophilic Interactions
      Schmidbaur, Hubert; Schier, Annette
      Angewandte Chemie, International Edition (2015), 54 (3), 746-784CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The decade 1990-2000 saw a growing interest in aurophilic interactions in gold chem. These interactions were found to influence significantly a variety of structural and other phys. characteristics of gold(I) compds. The attention paid to this unusual and counterintuitive type of intra- and intermol. bonding between seemingly closed-shell metal centers has rapidly been extended to also include silver chem. Hundreds of exptl. and computational studies have since been dedicated to the argentophilicity phenomenon. The results of this development are reviewed herein focusing on mol. systems where two or more silver(I) centers are in close contact leading to specific structural characteristics and a variety of novel phys. properties. These include strongly modified ligand-to-metal charge-transfer processes obsd. in absorption and emission spectroscopy, but also colossal pos. and neg. thermal expansion on the one hand and unprecedented neg. linear compressibility of crystal parameters on the other.
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    50. 50
      Zhang, Q.-Y.; He, X.; Zhao, L. Macrocycle-Assisted Synthesis of Non-Stoichimetric Silver(I) Halide Electrocatalysts for Efficient Chlorine Evolution Reaction. Chem. Sci. 2017, 8, 5662– 5668,  DOI: 10.1039/C7SC00575J
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      50
      Macrocycle-assisted synthesis of non-stoichiometric silver(I) halide electrocatalysts for efficient chlorine evolution reaction
      Zhang, Qiong-You; He, Xin; Zhao, Liang
      Chemical Science (2017), 8 (8), 5662-5668CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The electrocatalytic oxidn. of chloride to chlorine is a fundamental and important electrochem. reaction in industry. Herein we report the synthesis of non-stoichiometric silver halide nanoparticles through a novel macrocycle-assisted bulk-to-cluster-to-nano transformation. The acquired pos. charged nanoparticles expedite chloride transportation by electrostatic attraction and facilitate the formation of silver polychloride catalytic species on the surface, thus functioning as efficient and selective electrocatalysts for the chlorine evolution reaction (CER) at a very low overpotential and within a wide concn. range of chloride. The formation of uncommon non-stoichiometric nanoparticles prevents the formation of a AgCl ppt. and exposes more coordination unsatd. silver atoms to catalyze CER, finally causing a large enhancement of the at. catalytic efficiency of silver. This study showcases a promising approach to achieve efficient catalysts from a bottom-up design.
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      Zhang, S.; Zhao, L. Macrocycle-Encircled Polynuclear Metal Clusters: Controllable Synthesis, Reactivity Studies, and Applications. Acc. Chem. Res. 2018, 51, 2535– 2545,  DOI: 10.1021/acs.accounts.8b00283
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      51
      Macrocycle-encircled polynuclear metal clusters: Controllable synthesis, reactivity studies, and applications
      Zhang, Siqi; Zhao, Liang
      Accounts of Chemical Research (2018), 51 (10), 2535-2545CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      Macrocyclic ligands have been extensively applied to recognize single metal ions with high selectivity and good affinity based on the size-match principle. The resulting metal-macrocycle complexes play a significant role in mimicking the function of natural metal ion carriers and understanding and reproducing the catalytic activity of metalloenzymes. Because of the known macrocyclic effect, those single metal-macrocycle adducts often show an enhanced kinetic and thermodn. stability in comparison with their open-chain analogs. By virtue of such extraordinary coordination properties of macrocyclic ligands, it is expected that larger macrocycles with multiple coordination sites could properly act as an outer scaffold to direct the formation of multiatom species inside, such as polynuclear metal cluster aggregates, whose assembly may largely depend on the template positioning of coordinative atoms in the macrocyclic ring. Thus, the employment of polydentate macrocyclic ligands may provide a convenient tool to access polynuclear metal clusters in a controllable way. In this Account, we review our studies of the metal ion binding process of a class of polydentate macrocyclic ligands, azacalixpyridines (Py[n]s), and the application of Py[n]s as an outer template to direct the controllable synthesis of polynuclear metal clusters. Our investigations revealed that Py[n]s show a significant cooperative coordination effect in the metal ion binding process that facilitated the easy formation of a polymetallic assembled structure. Taking advantage of the cooperative coordination effect and the tunable and highly fluxional conformation of Py[n]s, we laid our focus on control of the nuclearity no. by tuning the size of Py[n]s and the adoption of Py[n]s with different anionic centers in metal cluster synthesis. As an important example for application, this new established macrocycle-directed method has been employed to achieve a variety of metal-cluster-centered capsule, rotaxane, catenane, polygon, and other supramol. assemblies. Furthermore, a cluster-to-cluster transformation inside the cavity of Py[n]s is presented to showcase the use of the acquired metal cluster-macrocycle complexes to achieve unconventional metal cluster entities.With regard to the application of the newly synthesized macrocycle-encircled metal clusters, examples of the fabrication of functional materials and catalysts are presented. With the assistance of Py[n]s, a bulk-to-cluster-to-nanoparticle transformation of silver sulfide (Ag2S) and silver halides (AgX) has been conducted to produce a series of nonstoichiometric silver sulfide and halide nanoparticles. The resulting Ag-S nanoparticle material with a high Ag/S ratio, which is inherited from the Py[n]-protected polysilver sulfide clusters, has a large energy gap relative to conventional Ag2S nanoparticles. Moreover, the nonstoichiometric silver halide nanoparticles can act as a new kind of electrocatalyst for the chlorine evolution reaction, showing excellent selectivity and high catalytic efficiency. Overall, in this account we try to highlight the application of polydentate macrocycles as an outer template to guide the synthesis of polynuclear metal clusters in a controllable manner. This unique synthesis will provide a new avenue to access unconventional metal clusters of different metal kinds and diverse anionic centers, which are expected to have promising and significant applications in many interdisciplinary areas of chem.
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    52. 52
      Desnoyer, A. N.; Nicolay, A.; Rios, P.; Ziegler, M. S.; Tilley, T. D. Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands. Acc. Chem. Res. 2020, 53, 1944– 1956,  DOI: 10.1021/acs.accounts.0c00382
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      52
      Bimetallics in a Nutshell: Complexes Supported by Chelating Naphthyridine-Based Ligands
      Desnoyer, Addison N.; Nicolay, Amelie; Rios, Pablo; Ziegler, Micah S.; Tilley, T. Don
      Accounts of Chemical Research (2020), 53 (9), 1944-1956CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      Bimetallic motifs are a structural feature common to some of the most effective and synthetically useful catalysts known, including in the active sites of many metalloenzymes and on the surfaces of industrially relevant heterogeneous materials. However, the complexity of these systems often hampers detailed studies of their fundamental properties. To glean valuable mechanistic insight into how these catalysts function, this research group has prepd. a family of dinucleating 1,8-naphthyridine ligands that bind two first-row transition metals in close proximity, originally designed to help mimic the proposed active site of metal oxide surfaces. Of the various bimetallic combinations examd., dicopper(I) is particularly versatile, as neutral bridging ligands adopt a variety of different binding modes depending on the configuration of frontier orbitals available to interact with the Cu centers. Organodicopper complexes are readily accessible, either through the traditional route of salt metathesis or via the activation of tetraarylborate anions through aryl group abstraction by a dicopper(I) unit. The resulting bridging aryl complexes engage in C-H bond activations, notably with terminal alkynes to afford bridging alkynyl species. The μ-hydrocarbyl complexes are surprisingly tolerant of water and elevated temps. This stability was leveraged to isolate a species that typically represents a fleeting intermediate in Cu-catalyzed azide-alkyne coupling (CuAAC); reaction of a bridging alkynyl complex with an org. azide afforded the first example of a well-defined, sym. bridged dicopper triazolide. This complex was an intermediate during CuAAC, providing support for a proposed bimetallic mechanism. These platforms are not limited to formally low oxidn. states; chem. oxidn. of the hydrocarbyl complexes cleanly results in formation of mixed valence CuICuII complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidn. states, e.g., dicopper(II), are easily accessed via oxidn. of a dicopper(I) compd. with air to give a CuII2(μ-OH)2 complex. Redn. of this compd. with silanes resulted in the unexpected formation of pentametallic copper(I) dihydride clusters or trimetallic monohydride complexes, depending on the nature of the silane. Finally, development of an unsym. naphthyridine ligand with mixed donor side-arms enables selective synthesis of an isostructural series of six heterobimetallic complexes, demonstrating the power of ligand design in the prepn. of heterometallic assemblies.
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    53. 53
      Jansze, S. M. Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages. J. Am. Chem. Soc. 2016, 138, 2046– 2054,  DOI: 10.1021/jacs.5b13190
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      53
      Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages
      Jansze, Suzanne M.; Cecot, Giacomo; Wise, Matthew D.; Zhurov, Konstantin O.; Ronson, Tanya K.; Castilla, Ana M.; Finelli, Alba; Pattison, Philip; Solari, Euro; Scopelliti, Rosario; Zelinskii, Genrikh E.; Vologzhanina, Anna V.; Voloshin, Yan Z.; Nitschke, Jonathan R.; Severin, Kay
      Journal of the American Chemical Society (2016), 138 (6), 2046-2054CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      It is possible to control the geometry and the compn. of metallasupramol. assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic FeII8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral FeII4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of PdII-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral PdII4L8 assembly, whereas an octahedral PdII6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the compn. of dynamic mixts. of PdII cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of PdII4L8 and PdII6L12 cages, whereas the other did not.
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    54. 54
      Giuseppone, N.; Schmitt, J.-L.; Allouche, L.; Lehn, J.-M. DOSY NMR Experiments as a Tool for the Analysis of Constitutional and Motional Dynamic Processes: Implementation for the Driven Evolution of Dynamic Combinatorial Libraries of Helical Strands. Angew. Chem., Int. Ed. 2008, 47, 2235– 2239,  DOI: 10.1002/anie.200703168
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      DOSY NMR experiments as a tool for the analysis of constitutional and motional dynamic processes: implementation for the driven evolution of dynamic combinatorial libraries of helical strands
      Giuseppone, Nicolas; Schmitt, Jean-Louis; Allouche, Lionel; Lehn, Jean-Marie
      Angewandte Chemie, International Edition (2008), 47 (12), 2235-2239CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Diffusion ordered spectroscopy (DOSY) NMR expts. have been used to characterize a dynamic combinatorial library of helical strands and grid-type metallosupramol. architectures. The technique allows the deconvolution of very similar chem. structures differing only by their hydrodynamic radius. Moreover, the occurrence of springlike, extension-contraction conformational motions in helical strands can be revealed as a function of the temp.
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      Zhang, Z. Intra- and Intermolecular Self-Assembly of a 20-nm-Wide Supramolecular Hexagonal Grid. Nat. Chem. 2020, 12, 468– 474,  DOI: 10.1038/s41557-020-0454-z
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      55
      Intra- and intermolecular self-assembly of a 20-nm-wide supramolecular hexagonal grid
      Zhang, Zhe; Li, Yiming; Song, Bo; Zhang, Yuan; Jiang, Xin; Wang, Ming; Trumbleson, Ryan; Liu, Changlin; Wang, Pingshan; Hao, Xin-Qi; Rojas, Tomas; Ngo, Anh T.; Sessler, Jonathan L.; Newkome, George R.; Hla, Saw Wai; Li, Xiaopeng
      Nature Chemistry (2020), 12 (5), 468-474CODEN: NCAHBB; ISSN:1755-4330. (Nature Research)
      For the past three decades, the coordination-driven self-assembly of three-dimensional structures has undergone rapid progress; however, parallel efforts to create large discrete two-dimensional architectures-as opposed to polymers-have met with limited success. The synthesis of metallo-supramol. systems with well-defined shapes and sizes at 10-100 nm remains challenging. Here the authors report the construction of a series of giant supramol. hexagonal grids, with diams. ∼20 nm and mol. wts. >65 kDa, through a combination of intra- and intermol. metal-mediated self-assembly steps. The hexagonal intermediates and the resulting self-assembled grid architectures were imaged at submol. resoln. by scanning tunneling microscopy. Characterization (including by scanning tunneling spectroscopy) enabled the unambiguous at.-scale detn. of fourteen hexagonal grid isomers.
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    56. 56

      Although the coordinative flexibility shown by the napthyridine–silver system limits the degree of certainty of these modeling results, the large difference in energy between the tetrahedral architecture and alternate structures lends credence to the assignment of the Ag8L6 structure as a tetrahedron.

      There is no corresponding record for this reference.
    57. 57
      For example:von Krbek, L. K. S.; Roberts, D. A.; Pilgrim, B. S.; Schalley, C. A.; Nitschke, J. R. Multivalent Crown-ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe4L6 Tetrahedron. Angew. Chem., Int. Ed. 2018, 57, 14121– 14124,  DOI: 10.1002/anie.201808534
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      Multivalent Crown-Ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe4L6 Tetrahedron
      von Krbek, Larissa K. S.; Roberts, Derrick A.; Pilgrim, Ben S.; Schalley, Christoph A.; Nitschke, Jonathan R.
      Angewandte Chemie, International Edition (2018), 57 (43), 14121-14124CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors report a strategy for regulating the rate of internally bound anion exchange within an Fe4L6 metal-org. tetrahedron through external coordination of tripodal tris(alkylammonium) cations. The cage features three flexible 18-crown-6 receptors at each of its FeII vertexes, facilitating strong tritopic interactions with tris(ammonium) cations to "cap" the vertexes of the tetrahedron. This capping mechanism restricts the flexibility of the cage framework, thereby reducing the rate of anion exchange within its central cavity by 20-fold. Thus, the authors demonstrate the first use of an externally bound multivalent effector to allosterically control internal guest binding in a mol. cage.
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      Clegg, J. K.; Cremers, J.; Hogben, A. J.; Breiner, B.; Smulders, M. M. J.; Thoburn, J. D.; Nitschke, J. R. A Stimuli Responsive System of Self-Assembled Anion-Binding Fe4L68+ Cages. Chem. Sci. 2013, 4, 68– 76,  DOI: 10.1039/C2SC21486E
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      A stimuli responsive system of self-assembled anion-binding Fe4L68+ cages
      Clegg, Jack K.; Cremers, Jonathan; Hogben, Andrew J.; Breiner, Boris; Smulders, Maarten M. J.; Thoburn, John D.; Nitschke, Jonathan R.
      Chemical Science (2013), 4 (1), 68-76CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A new cationic Fe4L6 cage mol. was synthesized from 4,4'-diaminobiphenyl, 2-formylpyridine and Fe(II). The cage exists as a system of interconverting diastereomers in soln. The system adapts to the addn. of anionic guest mols., expressing a new combination of diastereomers that synergistically bind the guest mols. Not only do the cage diastereomers interconvert, the vol. of the individual cages adapts phys. through the rotation of bonds, providing a tailored binding pocket for the guest lined with H-bond donors. A model for the resulting complex network of species was developed that allowed the system to be fully described. The anion binding consts. and the kinetics of both diastereomer interconversion and guest exchange were measured.
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      Fernández-Galán, R.; Manzano, B. R.; Otero, A.; Lanfranchi, M.; Pellinghelli, M. A. 19F and 31P NMR Evidence for Silver Hexafluorophosphate Hydrolysis in Solution. New Palladium Difluorophosphate Complexes and X-ray Structure Determination of [Pd(η3-2-Me-C3H4)(PO2F2)(PCy3)]. Inorg. Chem. 1994, 33, 2309– 2312,  DOI: 10.1021/ic00088a039
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      19F and 31P NMR evidence for silver hexafluorophosphate hydrolysis in solution. New palladium difluorophosphate complexes and x-ray structure determination of [Pd(η3-2-Me-C3H4)(PO2F2)(PCy3)]
      Fernandez-Galan, Rafael; Manzano, Blanca R.; Otero, Antonio; Lanfranchi, Maurizio; Pellinghelli, Maria Angela
      Inorganic Chemistry (1994), 33 (10), 2309-12CODEN: INOCAJ; ISSN:0020-1669.
      The reaction of [Pd(η3-2-Me-C3H4)(μ-Cl)]2, 1, with AgPF6 in a noncoordinating solvent, CH2Cl2, yields unexpectedly [Pd(η3-2-Me-C3H4)(μ-PO2F2)]3, 2. The authors have demonstrated that the group PO2F2- must have been generated from the partial hydrolysis of the hexafluorophosphate ion. Studies of the hydrolysis process have shown that AgPF6 undergoes complete hydrolysis in CH2Cl2 in the absence of the palladium complex. However, when the palladium complex is present the hydrolysis continues only until the PO2F2- stage, probably due to the coordination of the difluorophosphate group. Several intermediates of the AgPF6 hydrolysis have been detected and a clear solvent dependence of this process demonstrated. The complexes [Pd(η3-2-Me-C3H4)(PO2F2)(PR3)] (R = Cy, 3; Ph, 4; p-tolyl, 5) are obtained from 2 and PR3. An x-ray structure detn. of 3 showed unequivocally the presence of the PO2F2- ion. The complex [Pd(η3-2-Me-C3H4)(CH3CN)2]PF6 is obtained from 2 and [Cu(CH3CN)4]PF6.
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    60. 60
      Salomon, M. Solubilities of the Silver Halides in Benzonitrile and Trichloroacetonitrile Mixtures with Propylene Carbonate. Can. J. Chem. 1976, 54, 1487– 1492,  DOI: 10.1139/v76-213
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      Solubilities of the silver halides in benzonitrile and trichloroacetonitrile mixtures with propylene carbonate
      Salomon, Mark
      Canadian Journal of Chemistry (1976), 54 (9), 1487-92CODEN: CJCHAG; ISSN:0008-4042.
      The complex solubilities of the silver halides, AgCl, AgBr, and AgI, have been detd. in benzonitrile and trichloroacetonitrile mixts. with propylene carbonate. The results are compared with previous work in acetonitrile and propionitrile and are discussed in relation to the solvent effect on ion solvation.
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    61. 61

      Mass spectrometry of these silver complexes is challenging, presumably due to the dynamic nature of the naphthyridine–silver interactions. We see extensive fragmentation of all complexes under even mild conditions. By tuning ionization conditions and through choice of counterion, we were able to gather data on these architectures in both LRMS and HRMS. We found that using hexafluorophosphate as the counterion was particularly effective for obtaining good quality mass spectra.

      There is no corresponding record for this reference.
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      Lehn, J.-M. Toward Complex matter: Supramolecular Chemistry and Self-Organization. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4763– 4768,  DOI: 10.1073/pnas.072065599
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      Toward complex matter: Supramolecular chemistry and self-organization
      Lehn, Jean-Marie
      Proceedings of the National Academy of Sciences of the United States of America (2002), 99 (8), 4763-4768CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A review outlining some of the conjectures for the future of supramol. chem. and self-organization.
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    63. 63
      Toyota, S.; Woods, C. R.; Benaglia, M.; Haldimann, R.; Wärnmark, K.; Hardcastle, K.; Siegel, J. S. Tetranuclear Copper(I)-Biphenanthroline Gridwork: Violation of the Principle of Maximal Donor Coordination Caused by Intercalation and CH-to-N Forces. Angew. Chem., Int. Ed. 2001, 40, 751– 754,  DOI: 10.1002/1521-3773(20010216)40:4<751::AID-ANIE7510>3.0.CO;2-4
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      Tetranuclear copper(I)-biphenanthroline gridwork: violation of the principle of maximal donor coordination caused by intercalation and CH-to-N forces
      Toyota, Shinji; Woods, Craig R.; Benaglia, Maurizio; Haldimann, Richard; Warnmark, Kenneth; Hardcastle, Kenneth; Siegel, Jay S.
      Angewandte Chemie, International Edition (2001), 40 (4), 751-754CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
      Rods of 3,8-linked biphenanthrolines provide scaffolds for the prepn. of mol. grids with copper(I) and silver (I). Thus, reaction of 2,2'-diaryl-8,8'-bis-1,10-phenanthrolines in MeCN with Cu(I) or Ag(I) provides 3 × 3 M4L6 mol. grids. The x-ray crystal structure of [Cu4L4](BF4)4 (aryl = p-anisyl in L) was detd. The complex is a 3 × 3 mol. grid with 4 ligands coordinatively bonded to Cu(I) ions to form a mol. square with two addnl. ligands filling the interstitial spaces. Grid spacing between host biphenanthroline rods is about 7.5 Å on av. and therefore well accommodates an addnl. rod as an arom. guest by intercalation. The uncoordinated N atoms of the guest biphenanthrolines are positioned within van der Waals contacts of one set of C9,C9' H atoms of the host scaffold. Effects that account for the stability of the complex include primarily dispersive and to a lesser extent electrostatic interactions (π stacking). Coincidentally, complimentary electrostatic interaction between the H atoms of C9 on the scaffold and the N atoms of the guest exist. Changing the aryl group of L to p-tolyl also affords a 3 × 3 grid, but mesityl groups do not produce a grid architecture. There must be a cooperativity between the host and guest components that gives Cu4L6 its unique 3 × 3 grid supramol. structure. The reaction of Cu4L6 (aryl = p-anisyl in L) with the free ligand L where aryl is p-tolyl results in a rapid exchange of guest followed by a slower incorporation into the host sites. Pure mixed host-guest complexes with either p-anisyl or p-tolyl aryl-contg. L intercalated in place of the other aryl-contg. L were prepd. via an alumina-supported system. The ligand exchange kinetics were studied by spin satn. transfer spectroscopy. Assuming a dissociative mechanism with the first step rate limiting, the entropy for the dissocn. is pos. and large (ca. 20 cal K-1 mol-1). This unusual entropy hints at a novel solvation effect akin to the hydrophobic effect in water.
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    64. 64

      A minor isomer was also resolved in the crystallographic data, whereby an exterior sulfate coordinates via three oxygen atoms instead. Please see SI Section 12 for further details.

      There is no corresponding record for this reference.
    65. 65
      Fatila, E. M.; Twum, E. B.; Sengupta, A.; Pink, M.; Karty, J. A.; Raghavachari, K.; Flood, A. H. Anion Stabilize Each Other Inside Macrocyclic Hosts. Angew. Chem., Int. Ed. 2016, 55, 14057– 14062,  DOI: 10.1002/anie.201608118
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      Anions Stabilize Each Other inside Macrocyclic Hosts
      Fatila, Elisabeth M.; Twum, Eric B.; Sengupta, Arkajyoti; Pink, Maren; Karty, Jonathan A.; Raghavachari, Krishnan; Flood, Amar H.
      Angewandte Chemie, International Edition (2016), 55 (45), 14057-14062CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Contrary to the simple expectations from Coulomb's law, Weinhold proposed that anions can stabilize each other as metastable dimers, yet exptl. evidence for these species and their mutual stabilization is missing. We show that two bisulfate anions can form such dimers, which stabilize each other with self-complementary hydrogen bonds, by encapsulation inside a pair of cyanostar macrocycles. The resulting 2:2 complex of the bisulfate homodimer persists across all states of matter, including in soln. The bisulfate dimer's OH···O hydrogen bonding is seen in a 1H NMR peak at 13.75 ppm, which is consistent with borderline-strong hydrogen bonds.
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    66. 66

      These silver–naphthyridine systems proved to be extremely stable to light, which was unexpected. Samples could be left exposed to ambient light for 2–3 months with no sign of decomposition by NMR, or precipitation.

      There is no corresponding record for this reference.

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