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

The reaction of Cp(dppe)FeI with the ligands 2,2'- and 4,4'-dithiobispyridi ne (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](P F6)(2) depending on the reaction condition. Reaction of Cp(dppe)FeI with di thiobispyridines 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) a nd [CpFe(dppe)-2,2'-S-2(Py)(2)]PF6 (2) whereas the same reaction using CH3O H as a solvent and NH4PF6 as the halide abstractor leads to the formation o f the Fe-III-thiolate complex [Cp(dppe)Fe-2,2'-SPy]PF6 (3) and the mixed-va lence complex [Cp(dppe)Fe-III-mu SPy-Fe-II(dppe)Cp](PF6)(2) (4). Magnetic a nd ESR measurements are in agreement with one unpaired electron delocalized between them. Mossbauer data indicate clearly the presence of two differen t iron sites, each one of the N-bonded and S-bonded iron atoms, with interm ediate oxidation state Fe-II-Fe-III. An electron transfer intervalence abso rption 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 ele ctron 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-sphe re reorganizational parameters, which were analyzed and discussed. The elec tronic interaction parameters compare well with those found for electron tr ansfer in metalloproteins. (C) 2000 Elsevier Science B.V. All rights reserv ed.