Synthesis and structural characterization of homochiral coordination polymers with imidazole-based monocarboxylate ligands.

Chiral Na[(S)-L] (R = Me, 1a; Pr, 1b; CHPr, 1c, and (S)-Bu, 1d) and Na[(R)-L] (R = Me, 1a') compounds were synthesised following standard procedures. New compounds 1d and 1a' were analytically and spectroscopically characterised. 1a and 1c were structurally identified by single-crystal X-ray diffraction methods as homochiral 2D coordination polymers, {Na(HO)[(S)-L]} and {Na[(S)-L]}, respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: μκOκO' for 1a and μκOκO' for 1c. Enantiomeric species 1a', {Na(HO)[(R)-L]}, showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal structure. DFT calculations on the [L] anions confirmed their coordination capabilities as ditopic linkers. In fact, the reaction of Na[L] with several metal salts yielded the following coordination polymers: {Ag[(S)-L]}, 2a, {Ag[(R)-L]}, 2a', {Cu[(S)-L]} (R = Me, 3a; Pr, 3b), {Cu[(R)-L]}, 3a', {Zn[(S)-L]} (R = Me, 4a; Pr, 4b; (S)-Bu, 4d) and {Zn[(R)-L]}, 4a'. For the known compounds 3a and 4a, this procedure is a new synthetic route that avoided high temperature reaction conditions. New complexes 2, 3a', b, and 4a', b, d were characterised by elemental analysis, infrared and XRPD methods and complex 2a by single-crystal X-ray diffraction. This complex is also a two-dimensional coordination polymer in which the [(S)-L] anion acts as a μκN,κO,κO' bridging ligand. Compounds 1-4a' are the first examples of homochiral coordination polymers with imidazole-monocarboxylate ligands based on non-natural amino acids. Preliminary studies on the metal-catalysed preparation of chiral α-aminophosphonates were carried out but, unfortunately, no enantioselectivity was observed.