Thermal and Light-Induced Electron Transfer between Iron(II) and Cobalt(III) Mediated by Bridging Pyrazines

The first-order, specific rates of reduction of Co(III) by Fe(II) in complexes of the type [(NH3)(4 or 5>CoLFe(CN)5] with L as chelating 2-carboxylatopyrazine, pyrazine, and 2-methylpyrazine are 1.3 X 10-2, 5.5 X 10-2, and 30 X 10-2 s-1, respectively, at 25 °C, pH 6-7, µ = 0.15 M. For the first two reaction systems ΔH⧧ equals 22.7 ± 1.0 and 24.8 ± 1.4 kcal/mol, respectively, while ΔS⧧ is 9.5 ± 3.0 and 18.5 ± 5.0 cal mol-1 K-1, respectively. Photoinduced electron transfer occurs upon irradiation of the iron(II) to heterocycle charge transfer bands near 620 nm. Within experimental error, the quantum yield is unity for the first two species. The complexes are formed by aqueous reaction of pentacyanoaquaferrate(II) with the appropriate Co(III) ammine incorporating the bridging ligand. Kinetics of formation (µ = 0.15 M) are first order in each of the reactants with specific rates of 3.1 X 103, 7.0 X 103, and 3.7 X 103 M-1 s-1, respectively, for the three complexes at 25 °C. ΔH⧧ and ΔS⧧ for formation of the first two are approximately 17 kcal/mol and 16 cal mol-1 K-1. pH and ionic strength dependences of the formation and electron transfer steps are discussed.