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 [NbCp 2H3]/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 = trifluoroethanol) and 5.7 ± 0.3 kcal mol-1 for HFIP. 1H NMR studies of solutions of [NbCp2H2BHA] and HFIP in [D8]toluene revealed high-field shifts of the hydrides HA and HB, characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T1 relaxation time measurements show preferential coordination of the alcohol to the central hydride HA, but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T1 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 [NbCp 2H3] gives bis(dihydrogen) [NbCp2(η 2-H2)2]+ and dihydride(dihydrogen) complexes [NbCp2(H)2(η2-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 [NbCp 2(η2-H2)2]+ and the homoconjugated anion pair (CF3COO⋯ H⋯OOCCF 3)- 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.
|Journal||Chemistry - A European Journal|
|Publication status||Published - 6 Feb 2004|
- Ab initio calculations
- Hydrogen bonds
- IR spectroscopy
- NMR spectroscopy