Self-Assembly and Interconversion of Tetranuclear Copper(II) Complexes

The ligand 1,2-bis(benzimidazol-2-yl)-1,2-ethanediol (H2bzimed, 1) and its N-methylated analogue (H2mbzimed, 2)
form a variety of polynuclear complexes with copper(II), all of which contain a planar Cu2O2 lozenge as a central
element and in which the bridging oxygen belongs to an alkoxo group of the ligand. Syntheses are reported for
dinuclear [Cu2(Hmbzimed)2](ClO4)2·1.5H2O, Cu222, and the tetranuclear species [Cu4(Hbzimed)4(ClO4)2](NO3)2·4H2O,
Cu414, [Cu4(Hmbzimed)2(mbzimed)Cl2](ClO4)2·2H2O·C2H5OH, Cu423, and rac-[Cu4(H2bzimed)4(bzimed)(ClO4)2](ClO4)4·1.5H2O·3.5C2H5OH, Cu415. Crystal structures are reported for the tetranuclear species. Cu414 shows a cubane
structure, Cu423 a stepped cubane structure, and rac-Cu415 a novel structure in which one doubly deprotonated
ligand lies between the two Cu2O2 units. Magnetic susceptibility measurements indicate that all complexes show
antiferromagnetic coupling in the solid state. Studies in solution (ESI-MS, CD, NMR) show that Cu222 and Cu423
persist in solution but that Cu414 dissociates partially and rac-Cu415 completely. The six coordination modes of the
ligands are discussed together with the effect of the N-methylation on the ligand conformation.