Kinetics and thermodynamics of proton transfer to Cp ∗Ru(dppe)H: Via dihydrogen bonding and (η 2-H 2)-complex to the dihydride

Abstract

The interaction between Cp ∗RuH(dppe) and a series of proton donors (HA) of increasing strength: CFH 2CH 2OH (MFE), CF 3CH 2OH (TFE), (CF 3) 2CHOH (HFIP), p-nitrophenol, CF 3COOH and HBF 4 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 E j 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(η 2-H 2)(dppe)] +, the formation of which is shown here for the first time. The proton transfer from HFIP is characterized by Δ H ∘ = −8.1 ± 0.6 kcal mol −1 and Δ S ∘ = −17 ± 3 eu. The activation parameters for the subsequent irreversible isomerization leading to the classical dihydride complex, [Cp ∗Ru(H) 2(dppe)] +, are Δ H ‡ = 20.9 ± 0.8 kcal mol −1 and ΔS ‡ = 9 ± 3 eu as determined from 1H NMR spectroscopy for protonation by HBF 4. 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.