Synthesis and solution complexation behaviour of dimeric zinc-selective bis(benzimidazole) derivatives

The synthesis of 1,1'-bis(4-tert-butylbenzyl)-2,2'-bis(benzimidazole)-4,4'- dicarboxylic acid and its N-substituted analogues is reported, Complexation of the ligand by divalent Zn, Cu, Ni, Pb and Cd ions has been studied in methanolic solution by H-1 NMR, FT-IR and electrospray mass spectrometry. The ligand forms a dimeric neutral M(2)L(2) complex selectively with zinc, favoured by the two rigid tetrahedral binding sites created by the cooperative ligation of two orthogonally disposed ligands. Overall binding affinities follow the sequence Zn > Cd approximate to Ni > Pb > Cu and solution speciation has been defined by ESMS showing non-selective formation of, for example, ML, MLH, M(2)L(2)X species with all ions but zinc, where the M(2)L(2) species was predominant.