Fulvalenyl Mono- and Diiron Complexes: Photolytic and Electron-Transfer-Induced Substitution of the Benzene Ligands by Phosphines and CO in the Diiron Fulvalenyl Dibenzene Complexes [Fe2Fv(C6H6)2]2+/0. Generation of the Average-Valence Species [Fe2FvL6]3+(L = Phosphine)

Abstract

The bi-sandwich complex [Fe2Fv(C6H6)2]2+(PF6-)2 (12+; Fv = μ2-η5:η5-fulvalenyl, unless noted otherwise) synthesized from biferrocene, was photolyzed with visible light in acetonitrile in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) or bis(diphenylphosphino)methane (dppm) at −15 °C to give [Fe2Fv(dppe)2(NCMe)2]2+(PF6-)2 (2a2+) or [Fe2Fv(dppm)2(NCMe)2]2+(PF6-)2 (2b2+). The complexes 2a2+ and 2b2+ reacted in refluxing 1,2-dichloroethane with CO to give [Fe2Fv(dppe)2(CO)2]2+(PF6-)2 (3a2+) and [Fe2Fv(dppm)2(CO)2]2+(PF6-)2 (3b2+), and 2a2+ reacted similarly with PMe3 to give [Fe2Fv(dppe)2(PMe3)]2+(PF6-)2 (42+). The direduced 38-electron (38e) complex 1 reacted at −15 °C with 1 atm of CO to give [Fe2(μ2-η4:η4-Fv)(CO)6] (7) and with PMe3 to give [Fe2Fv(PMe3)4] (9). When Na+PF6- was present in stoichiometric amounts in THF, these reactions followed a different course and Na+PF6- induced electron transfer (disproportionation) by irreversibly dislocating ion pairs: the reaction of 1 with 1 atm of CO gave [Fe(η5-Fv)(η6-C6H6), Na+PF6-] (5), and that with PMe3 gave [Fe2Fv(PMe3)6]2+(PF6-)2 (82+) and the known complex [Fe(PMe3)4] (10). The cyclic voltammograms (CV) of 2a2+ and 2b2+ contain irreversible oxidation and reduction waves, but the CVs of 3a2+ and 3b2+ showed two close reversible monoelectronic reduction waves (no oxidation wave). The CVs of the hexaphosphine complexes indicated partially or fully irreversible reduction waves, respectively, but two reversible waves at +0.71 and +0.95 V for 42+ and +0.70 and +1.08 V for 82+ (vs SCE, Pt, DMF, 0.1 M n-Bu4NBF4 −30 °C). The bielectronic reduction of 2a2+ and the bielectronic oxidation of 42+ and 82+ using redox reagents led to decomposition, but the monoelectronic oxidation of 42+ and 82+ using (p-Br-C6H4)3N+SbCl6- in CH2Cl2 gave the stable mixed-valence trications 43+ and 83+, for which the Mössbauer spectra showed delocalized average valency on the Mössbauer time scale. These studies have opened the route to a variety of mono- and diiron fulvalenyl organometallic compounds, confirming the great importance of the presence of Na+PF6-. This salt can change reaction pathways and, in particular, induce electron-transfer reactions, underlining the extraordinary electronic flexibility of the fulvalenyl ligand and its ability to transfer the electron flow between two metal centers.