Crystal structures and magnetic properties of binuclear five-co-ordinate copper(II) complexes with a phenolate bridge and their catalytic functions in multielectron redox reactions

Abstract

Crystal structures of the binuclear five-co-ordinate copper(II) complexes [Cu₂(L¹)(CH₃CO₂)][PF₆]₂(1), [Cu₂(L¹)Cl₂]ClO₄(2), and [Cu₂(L²)(OH)][ClO₄]₂(3)(HL¹= 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol and HL²= 2,6-bis{bis[2-(methylthio)ethyl]aminomethyl}-4-methylphenol) have been determined by X-ray diffraction methods. The structures were solved by direct methods and refined by block-diagonal least squares to R= 0.071, 0.077, and 0.059 for (1), (2), and (3), respectively. All the complexes are binuclear; the two copper atoms are bridged by acetate and phenolate, phenolate, and hydroxide and phenolate groups with Cu–Cu separations of 3.549, 4.128, and 3.020 Å, for (1), (2), and (3), respectively. The magnitude of the antiferromagnetic interaction varies remarkably on going from (1) to (3), i.e., –2J= 80, 0, and 675 cm^–1 for (1), (2), and (3), respectively. The variation is explained in terms of overlapping between the copper 3d and intervening oxygen 2p orbitals based on the molecular structures determined. The catalytic activities of these complexes for the O₂ oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine were measured, together with those of mononuclear complexes of similar co-ordination environments. In spite of large Cu–Cu separations (> 3.5 Å), complexes (1) and (2) showed remarkably high catalytic activity compared with those of the mononuclear complexes. The result is discussed in terms of the co-ordination structures.