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)(2)CHOH (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 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(eta(2)-H-2)(dppe)](+), the formation of which is shown here for the first time. The proton transfer from HFIP is characterized by Delta H degrees = -8.1 +/- 0.6 kcal mol(-1) and Delta S degrees = -17 +/- 3 eu. The activation parameters for the subsequent irreversible isomerization leading to the classical dihydride complex, [Cp*Ru(H)(2)(dppe)](+), are Delta H-double dagger = 20.9 +/- 0.8 kcal mol(-1) and Delta S-double dagger = 9 +/- 3 eu as determined from H-1 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. (c) 2006 Elsevier B.V. All rights reserved.