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

Abstract

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.