Complexes of 2-hydroxy-5-methyl-1,4-benzoquinone as models for the 'TPQ-on' form of copper amine oxidases

Reaction of hydrated CuCl2 with equimolar amounts of Na[L-1].H2O (HL1=2-hyd roxy-5-methyl-1,4-benzoquinone) and K[Tp(Ph2)] ([Tp(Ph2)](-)=tris-[3,5-diph enylpyrazol-1-yl]hydridoborate) in CH2Cl2 at room temperature afforded [Cu( L-1)(Tp(Ph2))] 1 in moderate yields. A similar complexation employing hydra ted Zn(BF4)(2), Na[L-1].H2O and K[Tp(Ph2)] in refluxing CH2Cl2 affords [Zn( L-1)(Tp(Ph2))] 2. The single crystal structure of 1.0.8CH(2)Cl(2) reveals a near-regular square pyramidal copper(II) centre, with a chelating [L-1](-) ligand. In contrast, the structure of 2 shows a distorted trigonal bipyram idal geometry, with a long interaction to the chelating carbonyl O donor. I R, UV/vis, NMR and/or EPR data demonstrate that 1 and 2 adopt the same mole cular structures in CH2Cl2 solution as in the solid state. The cyclic volta mmogram of 2 in CH2Cl2-0.5 M (NBu4BF4)-B-n at 293 K exhibits chemically rev ersible 1-electron [L-1](-)-[L-2](2-) and [L-2](2-)-[L-3](3-) (H2L2=2-hydro xy-5-methyl-1,4-semiquinone; H3L3=2,4,5-trihydroxytoluene) couples. The CV of 1 under these conditions is more complex, showing an irreversible Cu-II- Cu-I couple, with daughter waves that suggest that reduction of the Cu in 1 results in decomplexation of [L-1](-). These results imply that a previous ly proposed stepwise mechanism for the oxidative half-reaction of copper-co ntaining amine oxidase may only take place if the enzyme's hydroquinone cof actor is coordinated to the active site copper ion.