Hydrogen bonding and proton transfer to the trihydride complex [Cp*MoH3(dppe)]:: IR, NMR, and theoretical investigations

The interaction between [Cp*MoH3(dppe)] (dppe = Ph(2)pCH(2)CH(2)pPh(2)) and a variety of proton donors has been investigated by a combination of experiments and DFT calculations. Weak proton donors [2-monofluoroethanol (MFE) and trifluoroethanol (TFE)] allow the determination of basicity factor (E-j = 1.42 +/- 0.02) and thermodynamic parameters for the hydrogen bond formation (Delta H-HB = -4.9 +/- 0.2 and -6.1 +/- 0.3 kcal mol(-1); Delta S-HB = -15.7 +/- 0.7 and -20.4 +/- 1 cal mol(-1)K(-1) for MFE and TFE, respectively). For TFE, a stable low-temperature proton-transfer equilibrium (220-240 K) with the cationic classical tetrahydrido derivative [Cp*MoH4 (dppe) could be investigated independently by UV/Vis (Delta H degrees(PT) -2.8 +/- 0.4 kcalmol(-1) and Delta S degrees(PT) = -15 +/- 2calmol(-1)K(-1)) and H-1 NMR (Delta H degrees(PT) = -2.7 +/- 0.5 kcal mol(-1) and Delta S degrees(PT) = -11 +/- 2 cal mol(-1)K(-1)) spectroscopy. Upon warming, however, the tetrahydride evolves by dihydrogen loss and formation of a hydride-free diamagnetic product. Stronger proton donors [hexafluoroisopropanol (HFIP), p-nitrophenol (PNP), perfluoro-tert-butyl alcohol (PFTB), and HBF4 center dot OEt2] lead to more extensive proton transfer at lower donor/Mo ratios. A 1: 1 proton-transfer stoichiometry is indicated independently by a titration experiment with UV/Vis monitoring for the [Cp*MoH3 (dppe)]-PNP reaction, and by a stopped-flow kinetics investigation for the [Cp*MoH3(dppe)]-HFIP reaction. For all proton-transfer processes investigated, the classical tetrahydrido cation forms directly, without the observation of a nonclassical intermediate. DFT calculations have been carried out on the interaction between TFE and HFIP and the model compound [CpMoH3(dpe)] (dpe = H2PCH2CH2PH2) both in the gas phase and in CH2Cl2 solvent with the polarizable continuum model and, to a more limited extent, on the full [Cp*MoH3(dppe)] system. A detailed comparison of the observed and calculated frequency shifts for the M-H vibrations is presented. The calculations have explored the relative energy and geometry of various configurations involving either a hydride ligand or the metal as the principal proton-accepting site. They have also probed two principal proton-transfer pathways, leading to the unobserved nonclassical intermediate and to the observed classical product. From these studies, it appears that a nonclassical intermediate may be obtained by a kinetically controlled proton transfer to a hydride site, followed by an intramolecular rearrangement through a very low energy barrier. However, a competitive low-energy pathway for direct proton transfer at the metal site is also revealed by the calculations.