Solvent-Dependent Studies of Intervalence Transfer of Mixed-Valence Complexes Containing Ferrocenylpyridine and Rutheniumammines

Abstract

Two heterobimetallic complexes of [Fc(4-Py)Ru(NH 3 ) 5 ](PF 6 ) 2 ( 1 ) and [Fc(3-Py)Ru(NH 3 ) 5 ](PF 6 ) 2 ( 2 ) (Fc(4-Py) = 4-ferrocenylpyridine, Fc(3-Py) = 3-ferrocenylpyridine) have been synthesized and characterized for the purpose of investigating optical and thermal electron transfer. The mixed-valence species generated in situ using ferrocenium hexafluorophosphate as the oxidant show Robin and Day class II behavior, and the oxidized sites are ruthenium centered. Δ E 1/2 °, E 1/2 °(Fe III /Fe II ) − E 1/2 °(Ru III /Ru II ), an estimate of Δ E o that is an energetic difference between the donor and acceptor sites, changes sharply with variation of solvents. Good linear relationship exists between Δ E 1/2 ° and Gutmann solvent donor number (DN) and Δ E 1/2 ° versus DN plots yield a slope of 20.4 ± 1.6 mV/DN for [Fc(4-Py)Ru(NH 3 ) 5 ] 2+/3+/4+ and a slope of 21.1 ± 2.2 mV/DN for [Fc(3-Py)Ru(NH 3 ) 5 ] 2+/3+/4+ . The solvent-dependent IT bands were found to vary almost exclusively with Δ E 1/2 °. The continuum dielectric approximation is found to be adequate, and the ( E op − Δ E o ) versus (1/ε op − 1/ε s ) plot yields a straight line with a slope of 3848 ± 1444 and an intercept of 4265 ± 227 cm - 1 for [Fc(4-Py)Ru(NH 3 ) 5 ] 3+ . The corresponding values for [Fc(3-Py)Ru(NH 3 ) 5 ] 3+ are 2328 ± 1560 and 6712 ± 100 cm - 1 . The thermal electron transfer (the reverse of the optical process, viz., Ru II → Fe III electron transfer) is adiabatic for [Fc(4-Py)Ru(NH 3 ) 5 ] 3+ but somewhat nonadiabatic (κ ≈ 0.6) for [Fc(3-Py)Ru(NH 3 ) 5 ] 3+ . The thermal transfer rate constants of both complexes decrease exponentially with increasing solvent donor number and show close magnitude in all solvents despite [Fc(3-Py)Ru(NH 3 ) 5 ] 3+ having apparently a shorter through-space distance and lower activation energies. However, these calculated k th values should be used with caution because no experimental data as measured by flash photolysis techniques are available.