Influence of media and homoconjugate pairing on transition metal hydride protonation. An IR and DFT study on proton transfer to CpRuH(CO)(PCy₃)

The interaction of the ruthenium hydride complex CpRuH(CO)(PCy3) (1) with proton donors HOR of different strength was studied in hexane and compared with data in dichloromethane. The formation of dihydrogen-bonded complexes (2) and ion pairs stabilized by hydrogen bonds between the dihydrogen ligand and the anion (3) was observed. Kinetics of the interconversion from 2 to 3 was followed at different (CF3)3COH concentrations between 200 and 240 K. The activation enthalpy and entropy values for proton transfer from the dihydrogen-bonded complex 2 to the (η2-H2)-complex 3 (ΔH‡ = 11.0 ± 0.5 kcal/mol and ΔS‡ = -19 ± 3 eu) were obtained for the first time. The results of the DFT study of the proton transfer process, taking CF3COOH and (CF3)3COH as a proton donors and introducing solvent effects in the calculation with the PCM method, are presented. The role of homoconjugate pairs [ROHOR]- in the protonation is analyzed by means of the inclusion of an additional ROH molecule in the calculations. The formation of the free cationic complex [CpRu(CO)(PCy3)(η2-H2)]+ is driven by the formation of the homoconjugated anionic complex [ROHOR]-. Solvent polarity plays a significant role stabilizing the charged species formed in the process. The theoretical study also accounts for the dihydrogen release and production of CpRu(OR)(CO)(PCy3), observed at temperatures above 250 K.