Proton-transfer reactions to half-sandwich ruthenium trihydride complexes bearing hemilabile P,N ligands: Experimental and density functional theory studies

The trihydride complexes [Cp*RuH3(κ1-P- iPr2PCH2X)] [X = pyridine (Py), 2a; quinoline (Quin), 2b] have been prepared by reaction of the corresponding chloro derivatives [Cp*RuCl(κ2-P,N-iPr 2PCH2X)] [X = Py (1a), Quin (1b)] with NaBH4 in methanol. Both 2a and 2b exhibit quantum-mechanical exchange coupling. The proton-transfer reactions to 2a and 2b using strong as well as weak proton donors have been experimentally and computationally studied. Density functional theory studies have been performed to analyze the stability of the proposed species, the hydrogen exchange, and the protonation pathway. The reactions with weak donors such as PhCOOH, indole, or salicylic acid in benzene or toluene result in the formation of hydrogen-bonded adducts between the proton donor and the pendant pyridine or quinoline group. However, in a more polar solvent such as dichloromethane, there is spectral evidence for the proton transfer to the hydride to yield a dihydrogen complex. The protonation with CF 3SO3H in CD2Cl2 occurs in a stepwise manner. In afirst step, the pendant pyridine or quinoline group is protonated to yield [Cp*RuH3(κ1P-iPr 2PCH2XH)]+ [X = Py (4a) or Quin (4b)]. The NH proton is then transferred to the hydride and one molecule of dihydrogen is released, furnishing the cationic mono(dihydrogen) complexes [Cp*Ru(H 2)(κ2-P,N-iPr2PCH 2X)]+J+ [X = Py (5a) or Quin (5b)]. These species are thermally stable and do not undergo irreversible rearrangement to their dihydride isomers. In the presence of an excess of acid, a second protonation occurs at the hydride site and the dicationic complexes [Cp*RuH 4(κ1-P,N-iPr2PCH 2XH)]2+ [X=Py (6a) or Quin (6b)] are generated. These species are stable up to 273 K and consist of equilibrium mixtures between bis(dihydrogen) and dihydrido(dihydrogen) tautomeric forms. Above this temperature, 6a and 6b are converted into the corresponding cationic mono(dihydrogen) complexes 5a/5b. The crystal structures of [Cp* RuCl(κ2-P,N-iPr2PCH2Quin)] (1b), [Cp*RuH3(κ1-P-iPr 2PCH2Quin)] (2b), [Cp*RuH3(κ 1-P-iPr2PCH2Py⋯H⋯ OOCC6H4OH)] (3a), [Cp*Ru(H2) (κ2-P,N-iPr2PCH2Quin) [BAr′] (5b), [Cp*Ru(N2)(κ2-P, N- iPr2PCH2Quin)][BAr′4] (8b), and [Cp*Ru(O2)(κ2-P,N-iPr 2PCH2Quin)][BAr′4] (9b) are reported. © 2010 American Chemical Society.