Tetrathiafulvalene–phosphine-based iron and ruthenium carbonyl complexes: Electrochemical and EPR studies

The radical cation of the redox active ligand 3,4-dimethyl-3′,4′-bis-(diphenylphosphino)-tetrathiafulvalene (P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S2CCS2 part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2˙+, one-electron oxidation of [Fe(P2)(CO)3] gives rise to the metal-centered oxidation species [Fe(I)(P2)(CO)3], characterized by a coupling with two 31P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe(I)(P2)(CO)3] reacts to give the complex [Fe(I)(P2)2(CO)] containing two TTF fragments. The two-electron oxidation of [Fe(P2)(CO)3] leads to decomplexation and to the P2˙+ spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe(P2)(CO)3] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments.