Spectroscopic evidence for intermolecular M-H center dot center dot center dot H-OR hydrogen bonding: Interaction of WH(CO)(2)(NO)L(2) hydrides with acidic alcohols

Intermolecular hydrogen bonding of acidic alcohols (PhOH, (CF3)(2)CHOH (HFIP), (CF3)(3)CHOH (PFTB)) to the hydride ligand of WH(CO)(2)(NO)L(2) (L = PMe(3) (1), PEt(3) (2), P((OPr)-Pr-i)(3) (3), PPh(3) (4)) has been observed and characterized by IR and NMR spectroscopy in hexane, toluene-d(8), and CD2Cl2 solutions. The H-bonding is an equilibrium process with medium -Delta H degrees of 4.1-6.9 kcal/mol; the enthalpy increases on going from 4 to 1, i.e., the strongest bonding is found for the smallest and the most basic L = PMe(3). The value of -Delta H degrees depends on the pK(a) of the proton donors, increasing as the acidity does (PhOH < HFIP < PFTB). The IR and NMR data suggest C-2v symmetry around tungsten in the ROH ... HW(CO)(2)(NO)L(2) adduct, with the H ... H distance of 1.77 Angstrom (L = PMe(3)) estimated from the hydride T-1 min relaxation time. The relevance of the hydrogen bonding to the mechanism of protonation of metal hydrides is suggested.