Spectroscopic Evidence for Intermolecular M−H···H−OR Hydrogen Bonding:  Interaction of WH(CO)2(NO)L2 Hydrides with Acidic Alcohols

Intermolecular hydrogen bonding of acidic alcohols (PhOH, (CF3)2CHOH (HFIP), (CF3)3CHOH (PFTB)) to the hydride ligand of WH(CO)2(NO)L2 (L = PMe3 (1), PEt3 (2), P(OiPr)3 (3), PPh3 (4)) has been observed and characterized by IR and NMR spectroscopy in hexane, toluene-d8, and CD2Cl2 solutions. The H-bonding is an equilibrium process with medium −ΔH° 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 = PMe3. The value of −ΔH° depends on the pKa of the proton donors, increasing as the acidity does (PhOH < HFIP < PFTB). The IR and NMR data suggest C2v symmetry around tungsten in the ROH···HW(CO)2(NO)L2 adduct, with the H···H distance of 1.77 A (L = PMe3) estimated from the hydride T1min relaxation time. The relevance of the hydrogen bonding to the mechanism of protonation of metal hydrides is suggested.