Kinetics and thermodynamics of proton transfer to Cp∗Ru(dppe)H: Via dihydrogen bonding and (η2-H2)-complex to the dihydride

The interaction between Cp∗RuH(dppe) and a series of proton donors (HA) of increasing strength: CFH2CH2OH (MFE), CF3CH2OH (TFE), (CF3)2CHOH (HFIP), p-nitrophenol, CF3COOH and HBF4 has been investigated spectroscopically by variable-temperature IR, UV–Vis, and NMR spectroscopy in solvents of differing polarity (n-hexane, dichloromethane and their mixture). The low-temperature IR study shows the establishment of a hydrogen-bond which involves the hydride ligand as the proton accepting site. The basicity factor Ej for the hydride was found to be 1.39. All techniques indicate that an equilibrium exists between the dihydrogen-bonded complex and the cationic dihydrogen complex, [Cp∗Ru(η2-H2)(dppe)]+, the formation of which is shown here for the first time. The proton transfer from HFIP is characterized by ΔH∘ = −8.1 ± 0.6 kcal mol−1 and ΔS∘ = −17 ± 3 eu. The activation parameters for the subsequent irreversible isomerization leading to the classical dihydride complex, [Cp∗Ru(H)2(dppe)]+, are ΔH‡ = 20.9 ± 0.8 kcal mol−1 and ΔS‡ = 9 ± 3 eu as determined from 1H NMR spectroscopy for protonation by HBF4. Computational results at the DFT/B3PW91 level confirm the experimentally observed hydride basicity increase on descending the Group from iron to ruthenium and also the formation of the non-classical complex as an intermediate, prior to the thermodynamically favored dihydride.