Use of the electron-reservoir [Fe(I)Cp(arene)] sandwiches as efficient and selective electrocatalysts: Syntheses of homo- and heterodinuclear zwitterionic transition-metal fulvalene complexes

The electron-reservoir complexes [Fe(I)Cp(C6H6)], [Fe(I)Cp(C(6)Me(6))], and [Fe(I)Cp*(C(6)Me(6))] (Cp = eta(5)-C5H5; Cp* = eta(5)-C(5)Me(5)) have been used as initiators in THF for the electron-transfer-chain-catalyzed (electrocatalyzed) synthesis of the homobimetallic zwitterions [(Co)(3)M(-)FvM(+)(Co)(2)(PR(3))(2)] (M = Mo, W; Fv = mu(2)-eta(10)-fulvalene; R = Me, OMe) from [M(2)Fv-(CO)(6)] and PR(3) and of the heterobimetallic zwitterions [(CO)(3)M(I)FvM(2)(CO)(PR(3))(2)] (M(1) = Mo, W; M(2) = Fe, Ru) from [M(1)M(2)Fv(CO)(5)] and PR(3). Cyclic voltammetry (CV) experiments (DMF, 0.1 M n-Bu(4)NBF(4), Pt, 0.400 V s(-1)) show that the CV's of the homobimetallic starting materials are unchanged in the presence of PR(3) (R = Me, OMe) whereas those of the heterobimetallic complexes in the presence of PMe(3) show only the CV's of the zwitterions. This indicates that the electrocatalytic process of the homobimetallic complexes is slow on the electrochemical time scale whereas that of the heterobimetallic complexes with PMe(3) is fast on the same time scale. This dichotomy is taken into account in terms of the very low concentration of the primary radical anion responsible for the reactivity with PR(3) in the case of the homodinuclear systems due to an intrinsically high disproportionation constant (K-disp); with heterodinuclear complexes, the dissymmetry is responsible for a relatively good thermodynamic stability and, thus, a higher concentration of the primary radical anion [(CO)(3)M(1) (-)FvM(2)(CO)(2)(.)], which reacts with PR(3). The effect of the PMe(3) concentration is also important, consistent with second-order kinetics. Subsequently, the K-disp values are qualitatively found in the following order, which is opposite to that of the electrocatalytic reactivity: RuRu(unreactive) >> WW > MoMo > 1 > RuMo, RuW > FeW. In THF, initiation with [Fe(I)Cp(C(6)Me(6)s)] of the reaction of [(CO)(3)WFvRu(CO2)] with PR(3) yields the monophosphine zwitterionic adduct [(CO)(3)W-FVRu+(C0)2(PMe3)], whose formation is partially driven by its insolubility. On the other hand, with [FeCp*(C(6)Me(6))] as the initiator, the bis(phosphine) zwitterion [(CO)(3)W(-)FvRu(+)(CO)(PMe(3))(2)] is formed as a result of the stronger driving force in the initiation electron-transfer step. The synergistic roles of the insolubility of the monophosphine intermediate and of the driving force provided by the electron-reservoir initiator are confirmed for electrocatalytic experiments in solvents of high dielectric constants (synthesis in MeCN or electrochemistry in DMF) in which the monophosphine zwitterion is neither formed nor detected. In conclusion, initiation of electrocatalytic reactions by the electron-reservoir [Fe(I)Cp(arene)] complexes is very useful(cobaltocene is inefficient in many cases), highly efficient (no side reactions), and highly selective (as a function of the number of Me groups on the ligands providing a wide range of redox potentials).