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 ¹H NMR, FT-IR and electrospray mass spectrometry. The ligand forms a dimeric neutral M₂L₂ 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 ≈ Ni > Pb > Cu and solution speciation has been defined by ESMS showing non-selective formation of, for example, ML, MLH, M₂L₂X species with all ions but zinc, where the M₂L₂ species was predominant.