Studies on blue copper protein models: syntheses, properties, and electron-transfer reactions of bis[4-(alkylmercaptomethyl)imidazole]copper(II) diperchlorate and related complexes and X-ray crystallographic analysis of bis[4-(n-propylmercaptomethyl)imidazole-N3S]copper(II) diperchlorate

Abstract

Bis[4-(alkyl- and bis[4-(phenyl-mercaptomethyl)imidazole]copper(II) diperchlorates (alkyl = n-propyl, t-butyl, or benzyl) and 1,6-bis(imidazol-4′-yl)-2,5-dithiahexanecopper(II) diperchlorate were prepared. The cationic moieties of these complexes undergo electrochemically reversible redox reactions at the potentials E_½=+0.23 to +0.36 V vs. s.c.e. in methanol containing [NBuⁿ₄][BF₄](0.1 mol dm^–3) as a supporting electrolyte. Kinetic studies on the reductions of these complexes with ferrocene in methanol reveal that precursor complexes are formed prior to electron transfer between the reactants; the formation constants of the precursor complexes are in the range 9.7–32.2 dm³ mol^–1. The activation entropies for the electron-transfer reaction were obtained as positive values, +33 to +71 J K^–1 mol^–1, which suggests that the precursor complex undergoes desolvation in a transition state. Electronic absorption and e.s.r. spectra of the copper(II) complexes as well as the X-ray crystal structure of bis[4-(n-propylmercaptomethyl)imidazole]copper(II) diperchlorate are described. The X-ray crystal structure shows the copper to be centrosymmetrically co-ordinated by two thioether sulphur atoms [Cu–S 2.397(2)Å], two imidazole nitrogen atoms [Cu–N 1.940(6)Å], and two oxygen atoms from unidentate perchlorate ions [Cu–O 2.594(6)Å].