Structure and reactivity studies on dinuclear copper complexes of the ligand alpha,alpha '-Bis{bis[1-(1 '-methyl-2 '-benzimidazolyl)methyl]amino}-m-xylene

The ligand alpha,alpha'-bis{bis[1-(1'-methyl-2'-benzimidazolyl)-methyl] amino}-m-xylene (L-55) forms the complexes [Cu-2(I)(L-55)(MeCN)(2)](PF6)(2) (1) and [Cu-2(II)(L-55)(OMe)(2)][ClO4](2) (2), in which an extremely weak axial amine interaction is imposed by the small five-membered benzimidazole chelating ring units. This structural feature has been characterized by Xray crystal structure determination. In 1, the Cu-I centers are three-coordinate with ligation from two benzimidazolyl and one acetonitrile N-donors, in a slightly distorted trigonal arrangement. In 2, each Cull center has a distorted square-planar geometry and coordinates to the benzimidazolyl N atoms and the bridging methoxy groups in the basal positions, with a much weaker interaction with the tertiary amine N in an apical position. These structural features affect the reactivities of the copper(I)- and copper(II)-L-55 complexes toward dioxygen and hydrogen peroxide, respectively. [Cu-2(I)(L-55)(NleCN)(2)](2+) does not produce a stable peroxo complex, but undergoes an irreversible oxidation to copper(II) species, while [Cu-2(II)(L-55)(H2O)(2)](4+) reacts slowly with hydrogen peroxide to undergo regiospecific hydroxylation of the ligand at one benzylic carbon atom, which causes decomposition of the complex. [Cu-2(L-55)(H2O)(2)](4+) is easily converted into its bis(mu-hydroxo) adduct [Cu-2(L-55)(OH)(2)](2+). This pH-driven equilibrium was monitored by paramagnetic H-1 NMR spectroscopy, and the solution magnetic properties of the complexes were determined by the Evans susceptibility method. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).