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.