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

Chiral Na[(S)-L (R)] (R = Me, 1a; Pr-i, 1b; (CH2Pr)-Pr-i, 1c, and (S)-Bu-sec, 1d) and Na[(R)-L (R)] (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(H2O)[(S)-L-Me]}(n) and {Na[(S)-(L2Pr)-Pr-CH-Pr-i]}(n), respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: mu(3)kappa O-2 kappa O ' for 1a and mu(4)kappa O-2 kappa O-2 ' for 1c. Enantiomeric species 1a ', {Na(H2O)[(R)-L-Me]}(n), showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal structure. DFT calculations on the [L (R)](-) anions confirmed their coordination capabilities as ditopic linkers. In fact, the reaction of Na[L (R)] with several metal salts yielded the following coordination polymers: {Ag[(S)-L-Me]}(n), 2a, {Ag[(R)-L-Me]}(n), 2a ', {Cu[(S)-L (R)](2)}(n) (R = Me, 3a; Pr-i, 3b), {Cu[(R)-L-Me]}(n), 3a ', {Zn[(S)-L (R)](2)}(n) (R = Me, 4a; Pr-i, 4b; (S)-Bu-sec, 4d) and {Zn[(R)-L-Me](2)}(n), 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-Me](-) anion acts as a mu(4)kappa N,kappa O-2,kappa O-2 ' 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 alpha-aminophosphonates were carried out but, unfortunately, no enantioselectivity was observed.