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[L¹]·H₂O (HL¹ = 2-hydroxy-5-methyl-1,4-benzoquinone) and K[Tp^Ph2] ([Tp^Ph2]⁻ = tris-[3,5-diphenylpyrazol-1-yl]hydridoborate) in CH₂Cl₂ at room temperature afforded [Cu(L¹)(Tp^Ph2)] 1 in moderate yields. A similar complexation employing hydrated Zn(BF₄)₂, Na[L¹]·H₂O and K[Tp^Ph2] in refluxing CH₂Cl₂ affords [Zn(L¹)(Tp^Ph2)] 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 Cu^II–Cu^I 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.