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)(TpPh)] ([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 [CuII(L2˙)(TpPh)]+.