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

Abstract

The hydrogen-bonding and proton-transfer pathway to complex [Cp*W(dppe)H(3)] (Cp*=eta(5)-C(5)Me(5); dppe=Ph(2)PCH(2)CH(2)PPh(2)) was investigated experimentally by IR, NMR, UV/Vis spectroscopy in the presence of fluorinated alcohols, p-nitrophenol, and HBF(4), and by using DFT calculations for the [CpW(dhpe)H(3)] model (Cp=eta(5)-C(5)H(5); dhpe=H(2)PCH(2)CH(2)PH(2)) and for the real system. A study of the interaction with weak acids (CH(2)FCH(2)OH, CF(3)CH(2)OH, (CF(3))(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 CF(3)CH(2)OH, (CF(3))(2)CHOH, and 2(CF(3))(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)H(3)]/HOCH(CF(3))(2) in toluene having DeltaH=(-3.9+/-0.3) kcal mol(-1) and DeltaS=(-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.

Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

“…Also the crucial H1−H2 distance is slightly shorter in Cp i Pr 4 molecule than in Cp t Bu 3 (Table 1). Theoretical investigations of similar complexes 26,27 produced distances in agreement with our structures, and a molecular dynamic simulation study confirmed a high probability of trihydride structures for related Mo complexes. 28 Full sets of geometrical data for all compounds in GP, THF, and MeCN as well as superposition of the most stable calculated structures with the experimental ones are given in section S.II of the SI.…”

Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms

“…E j Cp 2 Ru (0.67) < Cp 2 Os (0.81) and Cp* 2 Ru (0.85) < Cp* 2 Os (1.05)]. [21,70,71] The orbital component of a classical hydrogen bond is of donor-acceptor nature; the lone pair orbital of the base donates electrons to an empty s à AH orbital that has its main coeffi- www.chemphyschem.org cient on the protonic hydrogen atom (Figure 9 a). Such increase of the M···H interaction strength for 5d transition metals seems to be a general trend.…”

“…[2] Hydrogen bonds play a key role as initial stage of the proton-transfer reaction to organic bases. Participation of the metal atom in hydrogen bonding at a hydride site was shown to assist direct proton transfer to W and Os in the case of [Cp*WH 3 (dppe)] [21] (Cp* = h 5 -C 5 Me 5 ) and [Cp*OsH(dppe)] [22] (dppe = k 2 -Ph 2 PCH 2 CH 2 PPh 2 ). [7,17] Basic knowledge accumulated over the last two decades suggests that a direct proton transfer to a metal lone pair is an unlikely event in the presence of hydride ligands, which are considered to be the kinetic site of proton attack even if classical di(poly)hydride were the thermodynamic protonation product.…”

“…Participation of the metal atom in hydrogen bonding at a hydride site was shown to assist direct proton transfer to W and Os in the case of [Cp*WH 3 (dppe)] [21] (Cp* = h 5 -C 5 Me 5 ) and [Cp*OsH(dppe)] [22] (dppe = k 2 -Ph 2 PCH 2 CH 2 PPh 2 ). [21,26] Interestingly short metal-proton distance and significant Mo···HF overlap population were found in one of the first computational works on DHB [27] but were barely discussed. [24] For the [Cp*MH(dppe)] series, passing from ruthenium to osmium changes the reaction mechanism: proton transfer yields dihydrogen complex for Ru but cis-dihydride for Os.…”

Pentabenzylcyclopentadienyl molybdenum and tungsten hydrides: Syntheses, structures and electrochemistry of [MHCpBz(CO)2(L)] (L=CO, PMe3, PPh3)

“…The electron removal is accompanied by a drastic pKa decrease [24] and therefore the 17-electron species suffer deprotonation. The proton may be captured by the medium, namely a base, the solvent or traces of H 2 O, or by the neutral hydride that may itself be susceptible to protonation, leading to cationic dihydrogen or dihydride compounds [25][26][27][28]. Electrochemical studies of halfsandwich molybdenum and tungsten complexes have received considerable attention and it was reported that subsequently to deprotonation, the 17-electron radicals may be trapped by the solvent and further oxidised or dimerize through radical coupling [24,[29][30][31][32][33][34][35][36][37][38][39][40][41][42].…”