First investigation of non-classical dihydrogen bonding between an early transition-metal hydride and alcohols:: IR, NMR, and DFT approach

The interaction of [NbCp2H3] with fluorinated alcohols to give dihydrogen-bonded complexes was studied by a combination of IR, NMR and DFT methods. IR spectra were examined in the range from 200-295 K, affording a clear picture of dihydrogen-bond formation when [NbCp2H3]/HORf mixtures (HORf = hexafluoro-isopropanol (HFIP) or perfluoro-tert-butanol (PFTB)) were quickly cooled to 200 K. Through examination of the OH region, the dihydrogen-bond energetics were determined to be 4.5 +/- 0.3 kcal mol(-1) for TFE (TFE = tri-fluoroethanol) and 5.7 +/- 0.3 kcal mol(-1) for HFIR H-1 NMR studies of solutions of [(NbCp2H2HA)-H-B] and HFIP in [D-8]toluene revealed high-field shifts of the hydrides H-A and H-B, characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T-1 relaxation time measurements show preferential coordination of the alcohol to the central hydride H-A, but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T-1 data are in good accord with the results of DFT calculations. DFT calculations for the interaction of [NbCp2H3] with a series of non-fluorinated (MeOH, CH3COOH) and fluorinated (CF3OH, TFE, HFIP, PFTB and CF3COOH) proton donors of different strengths showed dihydrogen-bond formation, with binding energies ranging from -5.7 to -12.3 kcal mol(-1), depending on the proton donor strength. Coordination of proton donors occurs both to the central and to the lateral hydrides of [NbCp2H3], the former interaction being of bifurcated type and energetically slightly more favourable. In the case of the strong acid H3O+ the proton transfer occurs without any barrier, and no dihydrogen-bonded intermediates are found. Proton transfer to [NbCp2H3] gives bis(dihydrogen) [NbCp2(eta(2)-H-2)(2)](+) and dihydride(dihydrogen) complexes [NbCp2(H)(2)(eta(2)-H-2](+) (with lateral hydrides and central dihydrogen), the former product being slightly more stable. When two molecules of TFA were included in the calculations, in addition to the dihydrogen-bonded adduct, an ionic pair formed by the cationic bis(dihydrogen) complex [NbCp2(eta(2)-H-2)(2)](+) and the homoconjugated anion pair (CF3COO...H...OOCCF3)(-) was found as a minimum. It is very likely that these ionic pairs may be intermediates in the H/D exchange between the hydride ligands. and the OD group observed with the more acidic alcohols in the NMR studies.