First Investigation of Non-Classical Dihydrogen Bonding between an Early Transition-Metal Hydride and Alcohols:IR,NMR,and DFT Approach

摘要

The interaction of[NbCp_2H_3]with fluorinated alcohols to give dihy-drogen-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_2H_3]/ HOR_f 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(+-)O.Skcal mor~(-1)for TFE(TFE-tri-fluoroethanol)and 5.7(+-)0.3 kcal mor~(-1) for HFIP.~1H NMR studies of solutions of[NbCp_2H_2~BH~A]and HFIP in[D_8]tolu-ene 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 centralhydride H~A,but are also consistent with a bifurcated character of the dihy-drogen 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[NbCp_2H_3]with a series of non-fluorinated(MeOH,CH_3COOH)and fluorinated(CF_3OH,TFE,HFIP,PFTB and CF_3COOH) 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[NbCp_2H_3],the former interaction being of bifurcated type and energetically slightly more favourable.In thecase of the strong acid H_3O~+ ,the proton transfer occurs without any barrier,and no dihydrogen-bonded intermediates are found.Proton transfer to[NbCp_2H_3]gives bis(dihydrogen)[NbCp_2(if-H_2)_2]~+ and dihydride(dihy-drogen)complexes[NbCp_2(H)_2(rp-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(if-H_2)_2]~+ and the ho-moconjugated anion pair(CF_3COO-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.