Syntheses, structures and electrochemistry of copper(II) salicylaldehyde/tris(3-phenylpyrazolyl)borate complexes as models for the radical copper oxidases

Abstract

2-Hydroxy-5-methyl-3-methylsulfanylbenzaldehyde (HL²) and 2-hydroxy-5-methyl-3-methylselanylbenzaldehyde (HL³) have been synthesized from 2-hydroxy-5-methylbenzaldehyde (HL¹), as have Schiff bases HL⁴R and HL⁵R (R = Me or Ph) derived from RNH₂ and HL¹ or HL² respectively. The complexes [Cu(L)(Tp^Ph)] ([L]^– = [L¹]^–, 1; [L²]^–, 2; [L³]^–, 3; [L⁴Me]^–, 4; or [L⁴Ph]^–, 5) have been prepared. Single crystal structure determinations of 1, 2, 4 and 5 show copper(II) centres with square pyramidal [CuN₃O₂] (1, 2) or [CuN₄O] (4, 5) co-ordination spheres; for 4 and 5 the basal plane of the complex is twisted by 20–25° because of the steric properties of the Schiff base Me or Ph substituent. The UV/vis and EPR spectra of 1–5 in CH₂Cl₂ show the presence of tetragonal copper(II) centres. Cyclic voltammograms of 1–5 and the uncomplexed phenols in CH₂Cl₂–0.5 M Buⁿ₄NPF₆ exhibit an irreversible or (for 2) reversible 1-electron oxidation to a phenoxyl radical. The oxidation potentials of HL² and HL³, and of 2 and 3, are barely distinguishable. However, the irreversibility of this process for 3 compared to that of 2 suggests that a selenoether substituent kinetically stabilises the phenoxyl unpaired spin less efficiently than a thioether one; this is borne out by EHMO calculations on L¹˙–L³˙. Spectroelectrochemical characterisation of [2]⁺, whose UV/vis/NIR spectrum is very similar to that of galactose oxidase, confirms its formulation as the antiferromagnetically coupled species [Cu^II(L²˙)(Tp^Ph)]⁺.

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