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

Abstract

Abstract The interaction between [Cp*MoH 3 (dppe)] (dppe = Ph 2 PCH 2 CH 2 PPh 2 ) 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 ( E j = 1.42 ± 0.02) and thermodynamicparameters for the hydrogen bond formation (Δ H HB =–4.9 ± 0.2 and –6.1 ± 0.3 kcal mol –1 ; Δ 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*MoH 4 (dppe)] + could be investigated independently by UV/Vis (Δ H ° PT = –2.8 ± 0.4 kcal mol –1 and Δ S ° PT =–15 ± 2 cal mol –1 K –1 ) and 1 H NMR (Δ H ° PT = –2.7 ± 0.5kcal mol –1 and Δ S ° 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 HBF 4 · OEt 2 ] 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*MoH 3 (dppe)]–PNP reaction, and by a stopped‐flow kinetics investigation for the [Cp*MoH 3 (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 [CpMoH 3 (dpe)] (dpe = H 2 PCH 2 CH 2 PH 2 ) both in the gas phase and in CH 2 Cl 2 solvent with the polarizable continuum model and, to a more limited extent, on the full [Cp*MoH 3 (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)