Reaction of hydrated CuCl2 with equimolar amounts of Na[L1]·H2O (HL1 = 2-hydroxy-5-methyl-1,4-benzoquinone) and K[TpPh2] ([TpPh2]− = tris-[3,5-diphenylpyrazol-1-yl]hydridoborate) in CH2Cl2 at room temperature afforded [Cu(L1)(TpPh2)] 1 in moderate yields. A similar complexation employing hydrated Zn(BF4)2, Na[L1]·H2O and K[TpPh2] in refluxing CH2Cl2 affords [Zn(L1)(TpPh2)] 2. The single crystal structure of 1·0.8CH2Cl2 reveals a near-regular square pyramidal copper(II) centre, with a chelating [L1]− ligand. In contrast, the structure of 2 shows a distorted trigonal bipyramidal geometry, with a long interaction to the chelating carbonyl O donor. IR, UV/vis, NMR and/or EPR data demonstrate that 1 and 2 adopt the same molecular structures in CH2Cl2 solution as in the solid state. The cyclic voltammogram of 2 in CH2Cl2–0.5 M NBun4BF4 at 293 K exhibits chemically reversible 1-electron [L1]−–[L2]2− and [L2]2−–[L3]3− (H2L2 = 2-hydroxy-5-methyl-1,4-semiquinone; H3L3 = 2,4,5-trihydroxytoluene) couples. The CV of 1 under these conditions is more complex, showing an irreversible CuII–CuI couple, with daughter waves that suggest that reduction of the Cu in 1 results in decomplexation of [L1]−. These results imply that a previously proposed stepwise mechanism for the oxidative half-reaction of copper-containing amine oxidase may only take place if the enzyme’s hydroquinone cofactor is coordinated to the active site copper ion.
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