Solvent Control in the Protonation of [Cp*Mo(dppe)H3] by CF3COOH

The outcome of the reaction between [Cp*Mo(dppe)H3] [dppe = 1,2-bis(diphenylphosphanyl)ethane] and trifluoroacetic acid (TFA) is highly dependent on the solvent and the TFA/Mo ratio. The dihydride compound [Cp*Mo(dppe)H2(O2CCF3)] is obtained selectively when the reaction is carried out in aromatic hydrocarbons (benzene, toluene) when using less than one equivalent of TFA. The dihydride is also the end product when THF or MeCN are used as solvent, independent of the TFA/Mo ratio. In benzene/toluene the use of excess acid has a profound effect, resulting in the formation of the tetrahydrido complex [Cp*Mo(dppe)H4]+, which did not further evolve into the dihydrido product. Monitoring of the reaction by NMR and IR spectroscopy under different conditions (solvent, temperature, TFA/Mo ratio) reveals the rapid establishment of an equilibrium between the dihydrogen-bonded adduct, [Cp*Mo(dppe)H3]···HO2CCF3, the ion-paired proton-transfer product, [Cp*Mo(dppe)H4]+···–O2CCF3, and the separated ions, followed by a slower irreversible transformation to the final dihydride product with H2 evolution. The activation parameters of the H2 evolution and M–OR product formation were determined. Excess TFA in low-polarity solvents stabilizes the separated charged species by forming the homoconjugate anion [CF3COO(CF3COOH)n]–. The effect of the solvent on the course of the reaction can be interpreted in terms of the different polarity, H-bonding ability, and coordinating power of the various solvent molecules.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)