Intervalence electron transfer through a thiolate bridge ligand
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Date
2000
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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)]PF6, [Cp(dppe)Fe-SPy]PF6 or [{Cp(dppe)Fe}(2)-mu -SPy](PF6)(2) depending on the reaction condition. Reaction of Cp(dppe)FeI with dithiobispyridines in presence of TIPF6 as halide abstractor and using CH2Cl2 as a solvent gives the complexes [Cp(dppe)Fe-4.4'-S-2(Py)(2))(2)]PF6 (1) and [CpFe(dppe)-2,2'-S-2(Py)(2)]PF6 (2) whereas the same reaction using CH3OH as a solvent and NH4PF6 as the halide abstractor leads to the formation of the Fe-III-thiolate complex [Cp(dppe)Fe-2,2'-SPy]PF6 (3) and the mixed-valence complex [Cp(dppe)Fe-III-mu SPy-Fe-II(dppe)Cp](PF6)(2) (4). Magnetic and ESR measurements are in agreement with one unpaired electron delocalized between them. Mossbauer 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 CH2Cl2). By applying the Hush theory the intervalence parameters were obtained; alpha = 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 x 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. (C) 2000 Elsevier Science B.V. All rights reserved.
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Keywords
thiolate-bridged complexes, mixed-valence, electron-transfer, iron