Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH3(dppe)]: IR, NMR, and Theoretical Investigations

The interaction between [Cp*MoH3(dppe)] (dppe = Ph2PCH2CH2PPh2) and a variety of proton donors hasbeen investigated by a combination of experiments andDFT calculations. Weak proton donors [2-monofluoroethanol (MFE) and trifluoroethanol (TFE)] allow the determination of basicity factor (Ej = 1.42 ± 0.02) and thermodynamicparameters for the hydrogen bond formation (ΔHHB =–4.9 ± 0.2 and –6.1 ± 0.3 kcal mol–1; ΔSHB = –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 (ΔH°PT = –2.8 ± 0.4 kcal mol–1 and ΔS°PT =–15 ± 2 cal mol–1 K–1) and 1H NMR (ΔH°PT = –2.7 ± 0.5kcal mol–1 and ΔS°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·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 therelative 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. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)