Sorting Phenomena and Chirality Transfer in Fluoride-Bridged Macrocyclic Rare Earth Complexes

The reaction of fluoride anions with mononuclear lanthanide(III) and yttrium(III) hexaaza-macrocyclic complexes results in the formation of dinuclear fluoride-bridged complexes. As indicated by X-ray crystal structures, in these complexes two metal ions bound by the macrocycles are linked by two or three bridging fluoride anions, depending on the type of the macrocycle. In the case of the chiral hexaaza-macrocycle L1 derived from trans-1,2-diaminocyclohexane, the formation of these μ2-fluorido dinuclear complexes is accompanied by enantiomeric self-recognition of macrocyclic units. In contrast, this kind of recognition is not observed in the case of complexes of the chiral macrocycle L2 derived from 1,2-diphenylethylenediamine. The reaction of fluoride with a mixture of mononuclear complexes of L1 and L2, containing two different Ln(III) ions, results in narcissistic sorting of macrocyclic units. Conversely, a similar reaction involving mononuclear complexes of L1 and complexes of achiral macrocycle L3 based on ethylenediamine results in sociable sorting of macrocyclic units and preferable formation of heterodinuclear complexes. In addition, formation of these heterodinuclear complexes is accompanied by chirality transfer from the chiral macrocycle L1 to the achiral macrocycle L3 as indicated by CPL and CD spectra.


■ INTRODUCTION
Chiral structures and chiral recognition phenomena are fundamental features of molecular biological systems, and chirality is a central issue in various areas of organic and inorganic chemistry. For instance, chiral metal complexes and chiral supramolecular assemblies are studied as enantioselective catalysts, chiroptical probes, and nonlinear optical materials. Similarly, the recognition and self-organization phenomena characteristic for complex biological systems have triggered research in many areas of chemistry. Both social self-sorting (self-discrimination) and narcissistic selfsorting (self-recognition) are examples of such phenomena that attract increasing attention. 1−33 Chiral sorting corresponds to enantiomeric self-recognition or enantiomeric self-discrimination, and these processes have been documented for supramolecular systems, 1−6 metal complexes, 7−21 and organic systems, including macrocyclic compounds. 22−31 Chiral sorting phenomena are most often demonstrated for solid state, while examples of enantiomeric self-recognition well documented for solutions of metal complexes are less common. Another important issue in the synthesis of elaborate enantiopure metal complexes or supramolecular assemblies is chirality transfer, 32−43 e.g., the transmission of chiral information from enantiopure ligands to metal centers. While there are many chiral transition metal complexes with well-defined stable configurations, the control over chirality of lanthanide complexes, in particular poly-nuclear complexes, 34−36,44−58 is more difficult due to the lack of spatial preferences, lability, and high-coordination numbers of these ions. For similar reasons recognition and self-sorting phenomena 4,[6][7][8][9][10][11][12]20,21 in lanthanide systems are not so well explored in comparison with the systems based on stable organic compounds or more rigid and inert transition metal complexes.
Here we describe fluoride derivatives of lanthanide(III) and Y(III) (denoted as Ln(III)) complexes of hexaaza-macrocycles L1−L3 derived from 1,2-diformylpyridine and various diamines ( Figure 1). We show that dinuclear complexes of this type may contain two different macrocyclic units and that their formation is governed by self-sorting phenomena. By using circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy, we also demonstrate chirality transfer from chiral to achiral macrocycle in these mixed dinuclear complexes.
The number of well-defined molecular lanthanide(III) complexes containing fluoride ligands is limited due to the tendency to precipitate insoluble Ln(III) fluoride salts as well as due to the difficulty in controlling the coordination sphere of these labile ions. 59−72 In particular, macrocyclic ligands based on tetraaza-cyclen framework strongly bind lanthanide-(III) ions and form stable fluoride derivatives. Some of these cyclen-based complexes are mononuclear and contain terminal fluoride anions, 66−68 while other are dinuclear where two macrocyclic units are linked by a single linear μ 2 -fluorido bridge. 69−72 The binding of fluoride by these cyclen-based Eu(III) and Tb(III) complexes has been studied in the context of sensing of fluoride anions by using luminescence spectroscopy. In addition, a terminal fluoride anion bound in the axial position in cyclen-based Dy(III) complexes and in polychelate Dy(III) complexes generates high magnetic anisotropy of the Dy(III) ions and enhances single-ion magnet (SIM) properties. It has been also suggested that the Dy(III) complex with a hexaaza-macrocycle derived from 1,2-diacetylpyridine and ethylenediamine should exhibit exceptional magnetic anisotropy and SIM behavior. 73 CPL, the emission analogue to CD, involves the emission of circularly polarized luminescence from a chiral compound. 74−83 Unlike CD spectroscopy, CPL is only dependent on the active CPL species and free of potentially interfering background signals. It must be noted that a combination of positive and negative CPL signs ensures the splitting of narrow emission lines of the Ln 3+ ions, which provide unique chiroptical properties that can be used to probe for chiral phenomena. Thus, the CPL activity typically acts as a "fingerprint" to indicate any structural changes within the Ln(III)-containing system and/or around the local environment of the Ln(III) metal.

■ RESULTS AND DISCUSSION
Mononuclear Lanthanide(III) Complexes of Macrocycle L2. The new enantiopure rare earth(III) complexes of macrocycle L2 have been obtained in a template synthesis from the lanthanide(III) (Ln = Pr, Nd, Tb) or yttrium(III) chlorides, 2,6-diformylpyridine, and (1R,2R)-1,2-diphenylethylenediamine or (1S,2S)-1,2-diphenylethylenediamine in the same manner as it was reported for La, Eu, and Dy complexes. 84,85 The crystal structure of the [La(L2 R )Cl 3 ]· 2.5MeOH·0.5H 2 O complex, isomorphic to the previously reported Ce(III) derivative, 84 shows 9-coordinate La(III) ion bound by the six nitrogen atoms of the macrocycle and three axial chloride anions ( Figure 2). The macrocycle L2 is relatively flat in this complex with moderate helical twist of the pyridine fragments and very small folding of the macrocycle reflected by almost linear arrangement of the two pyridine nitrogen atoms and the central metal ion. In contrast, in the related Tb(III) complex the macrocycle is not only helically twisted but also sizably folded ( Figure 3) ions. The overall structures of these two cations are similar, but they differ in the set of axial ligandsone of them contains two axial chloride anions and coordinated water molecule, while the other contains two axial chloride anions and coordinated methanol molecule. Unlike the La(III) case, the macrocycle L2 is considerably folded in its Tb(III) complex similarly as it was observed for the Eu(III) complex 84     The NMR spectra of the Pr(III), Nd(III), Tb(III), and Dy(III) complexes of L2 cover a wide range of chemical shifts and show very broad lines (in particular in the case of Tb(III) and Dy(III) derivatives) in accord with the binding of the paramagnetic metal ion in the center of the macrocycle. The 1 H NMR spectra of the [Ln(L2)Cl 3 ] complexes consist of seven signals of the ligand L2. This number of lines indicates an effective D 2 symmetry of the complexes reflecting dynamic averaging of the structures observed in the crystalline state. This process most likely results from fast axial ligand exchange on the NMR time scale.
Because the [Ln(L2)Cl 3 ] complexes can be obtained in enantiopure form and Eu(III) and Tb(III) complexes may be luminescent, we were interested in CPL activity of the complexes of the L2 R and L2 S optical isomers of the macrocycle. The CPL measurements were performed in nondeuterated and deuterated 2:1 chloroform/methanol solutions at concentrations of 1 mM. The transitions that we studied are the magnetic dipole allowed transitions, 5 D 0 → 7 F 1 for Eu(III) and 5 D 4 → 7 F 5 for Tb(III), where one predicts the CPL would be large. We were able to measure a CPL signal for the set of enantiomeric pairs of the chiral Eu(III)-containing compounds upon UV excitation ( Figure 4). The luminescence dissymmetry ratio, g lum , is defined as follows: where I L and I R refer respectively to the intensity of left and right circularly polarized light. For the pair of the Eu(III) complexes [Eu(L2 R )Cl 3 ] and [Eu(L2 S )Cl 3 ], we were able to record opposite CPL spectra, which show the Eu(III)-centered polarized emission following excitation at about 333−329 nm (nondeuterated solutions) and 329−330 nm (deuterated solutions), respectively. The transition that we studied is the magnetic dipole allowed transitions, 5 D 0 → 7 F 1 for Eu(III). In addition, we also recorded the CPL activity for the (Eu) 5 D 0 → 7 F 2 . The CPL activity observed from the two enantiomeric forms of the Eu(III)-containing complexes is roughly similar, with a magnitude of the g lum values a little smaller for the samples measured in deuterated versus nondeuterated 2:1 chloroform/ methanol solutions. The g lum values are −0.05/+0.05, +0.05/− 0.05, −0.03/+0.03 versus −0.05/+0.05, +0.07/−0.06, −0.03/ +0.04 for the three components (∼591, 596, and 618 nm) of the CPL spectra for the Eu(III) complexes of the L2 S /L2 R enantiomers of the macrocycle, respectively ( Figure 4).
It was not possible to record the CPL activity of the two enantiomeric forms of the Tb(III)-containing complex L2 in nondeuterated and deuterated 2:1 chloroform/methanol solutions at concentrations of 1 and 10 mM. The intensity was too weak, suggesting that there is most likely an effective back-transfer from the Tb(III) to the ligand and/or an inefficient intersystem crossing between the singlet and triplet states of the ligand taking place for the Tb(III)-containing complexes. However, the observation of the CPL activity for the Eu(III)-containing compounds tends to favor an efficient Tb(III) back-transfer phenomenon. 86 Crystal In the [Lu 2 (L1 R ) 2 (μ 2 -F) 2 (NO 3 ) 2 ] 2+ dimers the ten-coordinate Lu(III) cation is bound in equatorial positions by six nitrogen atoms of the chiral macrocycle L1, while the axial positions are occupied by two bridging fluoride anions and a bidentate nitrate anion. The overall structure of this cationic complex is similar to the structure of the analogous hydroxo-bridged 21 The macrocyclic ligand L1 in these complexes is helically twisted, and the direction of the helical twist is determined by the configuration at the asymmetric carbon atoms of the diaminocyclohexane units. Thus, the twist of the two pyridine rings of L1 corresponding to the mutual Δ orientation is associated with the L1 S enantiomer of the ligand, while the opposite Λ twist of these units is associated with the L1 R enantiomer.
Dimeric fluorido-bridged complexes are also formed in the reactions of mononuclear nitrate-type complexes [Ln(L3)-(NO 3 ) 2 ](NO 3 ) of the macrocycle L3 derived from ethylenediamine. The molecular structure of the cationic complex [Y 2 (L3) 2 (μ 2 -F) 2 (NO 3 ) 2 ] 2+ , present in the crystal structure of the dimeric Y(III) complex of L3, is analogous to the abovediscussed complexes of L1 ( Figure 7 and Figure S6). The conformations of the chiral macrocycle L1 and the achiral macrocycle L3 in these fluoride-bridged complexes are similar. Importantly, the achiral ligand L3 is helically twisted in the complexed form, and thus, it assumes a chiral conformation. Within the fluorido-bridged complex [Y 2 (L3) 2 (μ 2 -F) 2 (NO 3 ) 2 ]-(NO 3 ) 2 ·CHCl 3 ·MeOH·H 2 O both macrocyclic units adopt the same direction of helical twist, either ΔΔ or ΛΛ, and the centrosymmetric crystals of this complex contain both forms as a racemic mixture.
The reaction of the complex of the macrocycle L2, [La(L2 R )Cl 3 ], with NEt 4 F results in a different kind of dimer in comparison with the dimeric complexes of macrocycles L1 and L3 discussed above. Thus, the X-ray crystal structure of the obtained product [La 2 (L2 R ) 2 (μ 2 -F) 3 F(H 2 O)]Cl 2 ·5MeOH· H 2 O shows a dimeric structure where three fluoride anions bridge the La(III) ions ( Figure 8 and Figure S7). In contrast, in the dimeric Ln(III) complexes of macrocycles L1 and L3, metal ions are linked by only two μ 2 -F − bridges.
While dinuclear molecular Ln(III) complexes of the di-μ 2fluorido type are known, 87−90 to the best of our knowledge this is a first example of a molecular tri-μ 2 -fluorido dinuclear Ln(III) complex. The Ln 2 (μ 2 -F) 3 structural motif can be found, however, in the Ln(III)-containing cluster, oligomeric or polymeric compounds. The La−La distance in the complex cation [La 2 (L2 R ) 2 (μ 2 -F) 3 F(H 2 O)] 2+ is 3.71 Å, which is similar value to the values of the corresponding distances 3.66, 3.71, and 3.66 Å observed for the Lu(III), Y(III), and Yb(III) dinuclear complexes of L1 and L3, discussed above. The two macrocyclic units in this complex are close to each other, and      91,92 This effect, in turn, arises from the change of the parameters of the magnetic susceptibility tensor accompanying the change of    (Figure 11). In this case no heterodinuclear Yb−Lu complex was observed. This result is a proof of enantiomeric self-recognition, i.e., narcissistic self-sorting with respect to helicity of macrocyclic units (Scheme 1). This chiral recognition process was not observed in analogous experiments with complexes of macrocycle L2. In this case new heterodinuclear species were observed for the La−Nd couple irrespective of chirality of the macrocycle. With the mixed couples of complexes containing heavier Ln(III) ion or Y(III) the titration experiments suggest that mainly mononuclear fluoride species are generated because heterodinuclear species were not observed.
In another set of NMR titration experiments mixtures of two mononuclear complexes containing not only two different metal ions but also two different macrocycles were used. In the case of mixtures of complexes [Ln(L1)Cl 3 ] and [Ln′(L2)Cl 3 ] after addition of fluoride no dinuclear Ln−Ln′ species containing two different macrocycles L1 and L2 were observed in 1 H NMR spectra irrespective of the chirality of the macrocycles. This result points to narcissistic self-sorting of macrocyclic units L1 and L2 (Scheme 1). Similar results were obtained for the starting mononuclear complexes [Ln(L2)Cl 3 ] and [Ln′(L3)Cl 3 ] pointing to narcissistic self-sorting of macrocyclic units L2 and L3. In contrast, sociable self-sorting of macrocycles L1 and L3 was observed during formation of Similarly, our attempts to isolate the heterodinuclear complex containing two different macrocyclic units as pure solids were so far unsuccessful, and the crystallization process seems to shift the equilibrium between various dinuclear forms. It should be mentioned that the heteronuclear mixed-lanthanide complexes characterized in solution are rare. 99−116 Chirality  (Figure 12). The transition that was measured for this sample dissolved in deuterated 2:1 chloroform/methanol solution was the magnetic dipole allowed transition 5 D 0 → 7 F 1 for Eu(III). In addition, we also recorded the CPL activity for the (Eu) 5 D 0 → 7 F 2 . The g lum values are −0.05 and +0.04 for the (Eu) 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 transitions. The observation of an Eu(III)-centered CPL activity clearly indicates that the Eu(III) resides in a chiral nonracemic complex. The signals cannot arise from the enantiopure component [Y 2 (L1 S ) 2 (μ 2 -F) 2 (NO 3 ) 2 ](NO 3 ) 2 since it is nonluminescent and CPL silent. They also cannot arise from the Eu 2 (L3) 2 (μ 2 -F) 2 (NO 3 ) 2 ](NO 3 ) 2 component since, here, Eu(III) resides in a racemic mixture of Δ and Λ conformations of the achiral ligand L3. The conclusion is that CPL activity has to arise from the [Y(L1 S )Eu(L3)(μ 2 -F) 2 (NO 3 ) 2 ](NO 3 ) 2 component of the mixture and that the Eu(III) ion in the heterodinuclear complex cation [Y(L1 S )-Eu(L3)(μ 2 -F) 2 (NO 3 ) 2 ] 2+ is bound by achiral L3 which has assumed only one of the two possible directions of the helical twist (Scheme 3). It should be mentioned that the NMR spectra of the sample used for CPL measurements do not indicate characteristic paramagnetic signals of species where the Eu(III) ion is coordinated to the chiral macrocycle. Thus, the observed CPL signal of Eu(III) does not arise from the metal ion dissociation and scrambling between the L1 and L3 sites. ] 2+ complex generated from this mixture by addition of 1 equiv of NEt 4 F gives rise to weak and narrow CD signals in the 500−700 nm region ( Figure 13). This kind of signal cannot arise from the macrocyclic units, which do not absorb in this region. Similarly, it does not arise from the Y(III) ion which lacks f−f transitions. Thus, the observed CD signals have to arise from the f−f transitions of the Nd(III) ions, and they reflect the nonracemic, chiral environment of these ions bound by the L3 macrocycle. This chiral environment of Nd(III) is due to generation of preferred direction of helical twist of the achiral macrocycle L3 caused by the steric interactions with the macrocycle L1 within the heterodinuclear complex. NMR data were used to verify that the observed CD signals due to f−f transitions did not arise from metal ion exchange between the L1 and L2 macrocycles. Further addition of fluoride anions results in the change of CD signals. These changes reflect variations of crystal field parameters of Ln(III) ion and the formation of new species (most likely with additional terminal fluorides bound to Nd(III) centers), in accord with the results of NMR titrations. We have recently observed similar CD effects in the hydroxo-bridged dinuclear species. 21 In another CD experiment a starting mixture of monomeric [Nd(L1)(NO 3 ) 2 ](NO 3 ) and [Dy(L3)(NO 3 ) 2 ](NO 3 ) was used. This mixture generated CD signals of the Nd(III) ion residing within the chiral macrocycle L1, but no CD signals due to Dy(III) ions residing within achiral macrocycle L3 were observed. After addition of 1 equiv of NEt 4 F, the Nd(III) signal changed due to the formation of fluoride-bridged dinuclear species, and a new weak signal corresponding to the f−f transitions of Dy(III) ion appeared due to chirality transfer ( Figure 14). It is worth mentioning that the Nd(III) signals of the generated dinuclear species are very similar to those generated after addition of 1 equiv of fluoride in the previous experiment described above. Thus, the shape of the Nd(III) CD signal is analogous for the [Nd(L1 S )Dy(L3)(μ 2 -F) 2 (NO 3 ) 2 ] 2+ and [Y(L1 S )Nd(L3)(μ 2 -F) 2 (NO 3 ) 2 ] 2+ dinuclear

■ EXPERIMENTAL SECTION
Details of structure determination and CPL measurements are provided in the Supporting Information.
Synthesis of Mononuclear Complexes. The Ln(III) complexes of macrocycles L1 and L3 have been obtained as reported previously. 20,21,117 The [Ln(L2 R )Cl 3 ] (Ln = La, Ce, Eu, Dy) complexes were obtained in a similar manner as reported previously, 84,85 as were new complexes of this type (Ln = Pr, Nd, Tb, and Y) as well as enantiomeric complexes [Ln(L2) S Cl 3 ]. In a typical procedure 2,6-diformylpyridine (135 mg, 1 mmol), (1R,2R)-1,2-diphenylethylenediamine (212 mg, 1 mmol), and the appropriate lanthanide(III) chloride hexahydrate (0.5 mmol) were refluxed in 20 mL of methanol for 3 h. After cooling, the volume was reduced to ca. 3 mL by using a rotary evaporator. The white product that formed was filtered, washed with methanol, and dried in a vacuum. [