Effect of the Nature of the Metal Atom on Hydrogen Bonding and Proton Transfer to [Cp*MH3(dppe)]: Tungsten versus Molybdenum

The hydrogen-bonding and proton-transfer pathway to complex [Cp*W(dppe)H-3] (CP* = eta(5)-C5Me5; dppe=Ph2PCH2CH2PPh2) was investigated experimentally by IR, NMR, UV/Vis spectroscopy in the presence of fluorinated alcohols, p-nitrophenol, and HBF4, and by using DFT calculations for the [CpW(dhpe)H-3] model (Cp= eta(5)-C5H5; dhpe=H2PCH2CH2PH2) and for the real system. A study of the interaction with weak acids (CH2FCH2OH, CF3CH2OH, (CF3)(2)CHOH) allowed the determination of the basicity factor, E-j=1.73 +/- 0.01, making this compound the most basic hydride complex reported to date. A computational investigation revealed several minima for the [CpW-(dhpe)H-3] adducts with CF3CH2OH, (CF3)(2)CHOH, and 2(CF3)(2)CHOH and confirms that these interactions are stronger than those established by the Mo analogue. Their geometries and relative energies are closely related to those of the homologous Mo systems, with the most stable adducts corresponding to H bonding with M-H sites, however, the geometric and electronic parameters reveal that the metal center plays a greater role in the tungsten systems. Proton-transfer equilibria are observed with the weaker proton donors, the proton-transfer step for the system [Cp*W(dppe)H3]/HOCH(CF3)2 in toluene having Delta H = (-3.9 +/- 0.3) kcal mol(-1) and Delta S=(-17 +/- 2)cal mol(-1)K(-1). The thermodynamic stability of the proton-transfer product is greater for W than for Mo. Contrary to the Mo system, the protonation of the [Cp*W(dppe)H-3] appears to involve a direct proton transfer to the metal center without a nonclassical intermediate, although assistance is provided by a hydride ligand in the transition state.