USE OF THE ELECTRON-RESERVOIR [FE(I)CP(ARENE)] SANDWICHES AS EFFICIENT AND SELECTIVE ELECTROCATALYSTS - SYNTHESES OF HOMODINUCLEAR 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-c atalyzed (electrocatalyzed) synthesis of the homobimetallic zwitterion s [(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 het erobimetallic 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 voltamm etry (CV) experiments (DMF, 0.1 M n-Bu(4)NBF(4), Pt, 0.400 V s(-1)) sh ow that the CV's of the homobimetallic starting materials are unchange d in the presence of PR(3) (R = Me, OMe) whereas those of the heterobi metallic 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 whe reas 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 t he reactivity with PR(3) in the case of the homodinuclear systems due to an intrinsically high disproportionation constant (K-disp); with he terodinuclear complexes, the dissymmetry is responsible for a relative ly good thermodynamic stability and, thus, a higher concentration of t he primary radical anion [(CO)(3)M(1) (-)FvM(2)(CO)(2)(.)], which reac ts with PR(3). The effect of the PMe(3) concentration is also importan t, consistent with second-order kinetics. Subsequently, the K-disp val ues are qualitatively found in the following order, which is opposite to that of the electrocatalytic reactivity: RuRu(unreactive) >> WW > M oMo > 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 monoph osphine zwitterionic adduct [(CO)(3)W-FVRu+(C0)2(PMe3)], whose formati on is partially driven by its insolubility. On the other hand, with [F eCp(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 d riving force in the initiation electron-transfer step. The synergistic roles of the insolubility of the monophosphine intermediate and of th e driving force provided by the electron-reservoir initiator are confi rmed for electrocatalytic experiments in solvents of high dielectric c onstants (synthesis in MeCN or electrochemistry in DMF) in which the m onophosphine zwitterion is neither formed nor detected. In conclusion, initiation of electrocatalytic reactions by the electron-reservoir [F e(I)Cp(arene)] complexes is very useful(cobaltocene is inefficient in many cases), highly efficient (no side reactions), and highly selectiv e (as a function of the number of Me groups on the ligands providing a wide range of redox potentials).