ESR/UV–Vis–NIR cyclovoltammetry of macrocyclic complex [Cu I (bite)]BF 4 at different temperatures Peter Rapta a,c, * , Jozef Ko z  ı  sek a , Martin Breza a , Milan Gembick y b , Lothar Dunsch c a Department of Physical Chemistry, Slovak University of Technology, Radlinsk eho 9, SK-812 37 Bratislava, Slovak Republic b Chemistry Department, State University of New York at Buffalo, Buffalo, NY, USA c IFW Dresden, Abteilung Elektrochemie und leitf€ ahige Polymere, Helmholtzstr. 20, D-01069 Dresden, Germany Received 31 July 2003; received in revised form 20 October 2003; accepted 5 November 2003 Abstract Using in situ ESR/UV–Vis–NIR cyclovoltammetry at different temperatures, model systems for blue copper proteins like [Cu I (bite)]BF 4 and [Cu II (bite)](BF 4 ) 2 (bite ¼ biphenyldiimino dithioether) were studied with respect to their redox behavior. Chemically reversible processes in the electrode reaction were observed by ESR and UV–Vis–NIR spectroscopy during the repetitive redox cycling of [Cu I (bite)]BF 4 . The complex cyclovoltammetric feature of the compounds in acetonitrile can be explained by adsorption and a slow heterogeneous electron transfer with rate constant k s ¼ 2  10 4 cm s 1 . Spectroelectrochemical studies of the redox products at 77 K confirmed the formation of a metastable pseudo-tetrahedral [Cu II (bite)] 2þ intermediate. This is direct ex- perimental proof of processes in redox reactions of the [Cu I (bite)] þ within a dual-pathway square reaction scheme indicating preferable electron transfer from the pseudo-tetrahedral [Cu I (bite)] þ as the initial step. The DFT optimized geometries of quasi- tetrahedral and quasi-planar [Cu I (bite)] þ=2þ complexes are in reasonable agreement with experimental data. Ó 2003 Elsevier B.V. All rights reserved. Keywords: In situ ESR/UV–Vis–NIR spectroelectrochemistry; Macrocyclic copper complexes; Electron transfer; Adsorption; Molecular structure; Blue copper proteins 1. Introduction The relationship between the geometry of the copper site in redox-active enzymes and their electron transfer rate has long been a subject of special interest in copper chemistry. The electron transfer in copper(I/II) systems involves an unusually large change in coordination ge- ometry. Copper(II) complexes generally exist as Jahn- Teller distorted six- (octahedral) or five-coordinate (square pyramidal) species, while copper(I) complexes are predominantly four-coordinate tetrahedral species. In a series of studies [1] it has been shown that the change in conformation can become rate-limiting itself under specific circumstances so that the reactions be- come first order, independent of the counter reagents, a condition known as ‘‘gated’’ electron transfer. When this limiting condition is reached, further increase in the reaction driving force may result in a switch to an alternative mechanistic path so that the overall mechanism can be described as a dual-pathway square scheme (Scheme 1) [1–3]. In this scheme, Cu II L (O) and Cu I L (R) (L is a multidentate ligand) are the thermo- dynamically stable species while Cu II L (Q) and Cu I L (P) are metastable intermediates in which the coordina- tion geometry nearly resembles that of the thermo- dynamically stable species of the opposite oxidation state. The macrocyclic compound [Cu I (bite)]BF 4 (bite ¼ biphenyldiimino dithioether) is believed to represent a model compound of the type-I copper blue proteins [4] and has been recently intensively studied with respect to the electron transfer reactions of the [Cu II=I (bite)] 2þ=þ redox couple both using electrochemical and spectro- scopic techniques [2,5]. Special attention is focused on the changes in the coordination sphere of this complex under electron transfer [5] as the environment around * Corresponding author. Tel.: +421-2-59325-537; fax: +421-2-5249- 3198. E-mail address: rapta@cvt.stuba.sk (P. Rapta). 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.11.019 Journal of Electroanalytical Chemistry 566 (2004) 123–129 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry