Copper(II) complexes of hydroquinone-containing Schiff bases. Towards a structural model for copper amine oxidases

Complexation of the preformed ligand 2,5-dihydroxy-N-{pyridin-2-ylmethyl}-benzylideneamine (HL1) with hydrated Cu(BF4)2 afforded [{Cu(μ-L1)}2][BF4]21. The crystal structure of 1·MeNO2 shows a dimer of near-planar copper(II) ions, with a bridging apical BF4− anion. Variable temperature susceptibility measurements showed the copper(II) ions in 1 to be moderately antiferromagnetically coupled. The complexes [CuL2]X (X− = ClO4−2, NO3−3, Cl−4 or NCS−5) and [CuL3]ClO4 (6; HL2 = N-{pyridin-2-ylmethyl}-N′-{2,5-dihydroxybenzylidene}-1,2-diaminoethane, HL3 = N-{pyridin-2-ylmethyl}-N′-{2,4,5-trihydroxybenzylidene}-1,2-diaminoethane) have been prepared by template condensation of N-(pyridin-2-ylmethyl)-1,2-diaminoethane with the appropriate benzaldehyde derivative and copper salt. The single crystal structure of 2 shows a near-planar four-co-ordinate copper(II) centre, with a non-co-ordinated ClO4− anion. The chelate ligand backbone is disordered over two orientations, which correspond to different patterns of intermolecular hydrogen bonding in the lattice. UV/vis and EPR data in dmf solution suggest that 2–6 all undergo solvolysis to form an identical [CuL(dmf)x]+ (x = 0–2) species in solution. Cyclic voltammograms of HL1 and 1–6 are complex, and demonstrate rapid acid-catalysed decomposition of the benzoquinonecarbaldimine ligand oxidation products.