Construction of Homochiral Complexes Based on Imidazole-Decorated Salen-Type Schiff Bases: Photochromism and Enantiomer Recognition of Penicillamine

Abstract

Two imidazole-decorated salen-type Schiff base ligands (H2LR and H2LS) were synthesized and characterized. Five pairs of homochiral complexes, namely, {[Cd(H2LR)Cl2]·DMF}n (1a), {[Cd(H2LS)Cl2]·DMF}n (1b), {[Cd(H2LR)Br2]·DMF}n (2a), {[Cd(H2LS)Br2]·DMF}n (2b), {[Cd(H2LR)I2]·2DMF·H2O}n (3a), {[Cd(H2LS)I2]·2DMF·H2O}n (3b), {[Cu2(LR2−)Cl2]·DMF·MeCN}n (4a), {[Cu2(LS2−)Cl2]·DMF·MeCN}n (4b), [Ni(LR2−)]·DMF·2H2O (5a), and [Ni(LS2−)]·DMF·2H2O (5b), were synthesized by reacting the two ligands with Cd(II), Cu(II), and Ni(II) ions, respectively. Crystal structures of these complexes revealed three different coordination modes of the ligands, resulting in the formation of microparticles with different morphologies in methanol. The photochromic properties of the ligands and complexes were examined and the results revealed that the ligands and the three pairs of Cd(II) complexes, especially 2a/2b, exhibited photochromism originating from enol−keto tautomerism followed by cis−trans isomerization of the salicylaldehyde imine units. Circular dichroism spectra and density functional theory calculations revealed that 5a/5b exhibited obvious enantiomer recognition of D-/L-penicillamine (D-/L-Pen) in their host−guest binding interactions.