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

• Published in: Inorganic chemistry Vol. 38; no. 4; pp. 674 - 679
• Main Authors: Liu, Tsun-Yang, Chen, Yuan Jang, Tai, Chih-Cheng, Kwan, Keh Shin
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22.02.1999
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic9713341

Two heterobimetallic complexes of [Fc(4-Py)Ru(NH3)5](PF6)2 (1) and [Fc(3-Py)Ru(NH3)5](PF6)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. ΔE 1/2°, E 1/2°(FeIII/FeII) − E 1/2°(RuIII/RuII), an estimate of ΔE o that is an energetic difference between the donor and acceptor sites, changes sharply with variation of solvents. Good linear relationship exists between ΔE 1/2° and Gutmann solvent donor number (DN) and ΔE 1/2° versus DN plots yield a slope of 20.4 ± 1.6 mV/DN for [Fc(4-Py)Ru(NH3)5]2+/3+/4+ and a slope of 21.1 ± 2.2 mV/DN for [Fc(3-Py)Ru(NH3)5]2+/3+/4+. The solvent-dependent IT bands were found to vary almost exclusively with ΔE 1/2°. The continuum dielectric approximation is found to be adequate, and the (E op − ΔE o) versus (1/εop − 1/εs) plot yields a straight line with a slope of 3848 ± 1444 and an intercept of 4265 ± 227 cm-1 for [Fc(4-Py)Ru(NH3)5]3+. The corresponding values for [Fc(3-Py)Ru(NH3)5]3+are 2328 ± 1560 and 6712 ± 100 cm-1. The thermal electron transfer (the reverse of the optical process, viz., RuII → FeIII electron transfer) is adiabatic for [Fc(4-Py)Ru(NH3)5]3+ but somewhat nonadiabatic (κ ≈ 0.6) for [Fc(3-Py)Ru(NH3)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(NH3)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.