Models for Copper-Dioxygen Complexes: The chemistry of copper(II) with some planar tridentate nitrogen ligands

The solution chemistry of Cu(II) with a series of five planar tridentate nitrogen ligands, 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy, 1 ), 2,6-bis(l-methylbenzimidazol-2-yl)pyridine (mbzimpy, 2 ) 2,6-bis(benzothiazol-2-yl) pyridine (bzthpy, 3 ), 2,6-bis(benzoxazol-2-yl)pyridine (bzoxpy, 4 ), and 2,2′, 6′, 2″-terpyridyl (terpy, 5 ) is reported. Electronic and EPR spectra are consistent with the complexes CuL]²⁺ having essentially tetragonal structure in solution, with the fourth coordination site in the plane of the ligand occupied by solvent. bzthpy and bzoxpy show smaller ligand-field splittings than bzimpy, mbzimpy, and terpy, and are easily decomplexed from the copper. Substitution of the coordinated solvent molecule in the plane of the ligand is observed with Cl^? and OH^? (provided that the ligand has no acidic protons) for all ligands except terpy. The reaction between Cu(mbzimpy)]²⁺ and imidazole has been studied by potentiometric titration in MeCN/H₂O 1:1 and shows strong binding of the imidazole in the plane (log K = 4.5 at 25°), and also the formation of an imidazolate-bridged dinuclear species.