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

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 (HLR)-R-4 and (HLR)-R-5 (R = Me or Ph) derived from RNH2 and HL1 or HL2 respectively. The complexes [Cu(L)(Tp(Ph))] ([L](-) = [L-1](-), 1; [L-2](-), 2; [L-3](-), 3; [(LMe)-Me-4](-), 4; or [(LPh)-Ph-4](-), 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 degrees 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 (Bu4NPF6)-N-n exhibit an irreversible or (for 2) reversible l-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 L-1 .-L-3 .. 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(L-2 .)(Tp(Ph))](+).