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

Abstract

Chiral Na[(S)-L^R] (R = Me, 1a; ⁱPr, 1b; CH₂ⁱPr, 1c, and (S)-^secBu, 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)-L^CH₂iPr]}_n, respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: μ₃κ²OκO′ for 1a and μ₄κ²Oκ²O′ 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; ⁱPr, 3b), {Cu[(R)-L^Me]}_n, 3a′, {Zn[(S)-L^R]₂}_n (R = Me, 4a; ⁱPr, 4b; (S)-^secBu, 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 μ₄κN,κ²O,κ²O′ 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.