Electron transfer in non-oxovanadium(IV) and (V) complexes: Kinetic studies of an amavadin model

Lenhardt, Jeremy M.; Baruah, Bharat; Crans, Debbie C.; Johnson, Michael D.

doi:10.1351/PAC-CON-08-08-23

Pure Appl. Chem., 2009, Vol. 81, No. 7, pp. 1241-1249

http://dx.doi.org/10.1351/PAC-CON-08-08-23

Published online 2009-06-29

Electron transfer in non-oxovanadium(IV) and (V) complexes: Kinetic studies of an amavadin model

Jeremy M. Lenhardt¹, Bharat Baruah^2,3, Debbie C. Crans³* and Michael D. Johnson¹*

¹ Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88003, USA
² Department of Chemistry and Biochemistry, Kennasaw State University, Kennesaw, GA 30144, USA
³ Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA

Abstract: Electron-transfer reactions of the eight-coordinate vanadium complex, bis-(N-hydroxyiminodiacetate)vanadium(IV) [V(HIDA)₂]^2–, a synthetic analog of amavadin with ascorbic acid and hexachloroiridate(IV), have been studied. The self-exchange rate constant for this analog has been calculated from oxidation and reduction cross-reactions using Marcus theory and directly measured using ⁵¹V NMR paramagnetic line-broadening techniques. The average self-exchange rate constant for the bis-HIDA vanadium(IV/V) couple equals 1.5 × 10⁵ M^–1 s^–1. The observed rate enhancements are proposed to be due to the small structural differences between the oxidized and reduced forms of the HIDA complex and inner-sphere reorganizational energies. The electron-transfer reaction of this synthetic analog is experimentally indistinguishable from amavadin itself, although significant differences exist in the reduction potential of these compounds. This suggests that ligand modification effects the thermodynamic driving force and not the self-exchange process.