Intervalence electron transfer through a thiolate bridge ligand: a Fe III–S–R–Fe II mixed valence complex

Abstract

The reaction of Cp(dppe)FeI with the ligands 2,2′- and 4,4′-dithiobispyridine (S 2(Py) 2) give the mononuclear or binuclear complexes of the type [Cp(dppe)Fe-S 2(Py) 2]PF 6, [Cp(dppe)FeSPy]PF 6 or [{Cp(dppe)Fe} 2-μ-SPy](PF 6) 2 depending on the reaction condition. Reaction of Cp(dppe)FeI with dithiobispyridines in presence of TlPF 6 as halide abstractor and using CH 2Cl 2 as a solvent gives the complexes [Cp(dppe)Fe-4,4′-S 2(Py) 2) 2]PF 6 ( 1) and [CpFe(dppe)-2,2′-S 2(Py) 2]PF 6 ( 2) whereas the same reaction using CH 3OH as a solvent and NH 4PF 6 as the halide abstractor leads to the formation of the Fe III–thiolate complex [Cp(dppe)Fe-2,2′-SPy]PF 6 ( 3) and the mixed-valence complex [Cp(dppe)Fe III-μSPy-Fe II(dppe)Cp](PF 6) 2 ( 4). Magnetic and ESR measurements are in agreement with one unpaired electron delocalized between them. Mössbauer data indicate clearly the presence of two different iron sites, each one of the N-bonded and S-bonded iron atoms, with intermediate oxidation state Fe IIFe III. An electron transfer intervalence absorption was observed for this complex at 780 nm (in CH 2Cl 2). By applying the Hush theory the intervalence parameters were obtained; α=0.028, H ab=361 cm −1 which indicate Class II Robin–Day. Estimation of the rate electron transfer affords a value k th=6.5×10 6 s −1. Solvent effect on the intervalence transition follow the Hush prediction for high dielectric constants solvents which permit the evaluation of the outer and inner-sphere reorganizational parameters, which were analyzed and discussed. The electronic interaction parameters compare well with those found for electron transfer in metalloproteins.