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

The trihydride complexes [Cp*RuH(3)(kappa(1)-P-(i)Pr(2)PCH(2)X)] [X = pyridine (Py), 2a; quinoline (Quin), 2b] have been prepared by reaction of the corresponding chloro derivatives [Cp*RuCl(kappa(2)-P,N-(i)Pr(2)PCH(2)X)] [X = Py (1a), Quin (1b)] with NaBH(4) 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(3)SO(3)H in CD(2)Cl(2) occurs in a stepwise manner. In a first step, the pendant pyridine or quinoline group is protonated to yield [Cp*RuH(3)(kappa(1)-P-(i)Pr(2)PCH(2)XH)](+) [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))(kappa(2)-P,N-(i)Pr(2)PCH(2)X)](+) [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)(kappa(1)-P,N-(i)Pr(2)PCH(2)XH)](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(kappa(2)-P,N-(i)Pr(2)PCH(2)Quin)] (1b), [Cp*RuH(3)(kappa(1)-P-(i)Pr(2)PCH(2)Quin)] (2b), [Cp*RuH(3)(kappa(1)-P-(i)Pr(2)PCH(2)Py...H...OOCC(6)H(4)OH)] (3a), [Cp*Ru(H(2))(kappa(2)-P,N-(i)Pr(2)PCH(2)Quin)][BAr'(4)] (5b), [Cp*Ru(N(2))(kappa(2)-P,N-(i)Pr(2)PCH(2)Quin)][BAr'(4)] (8b), and [Cp*Ru(O(2))(kappa(2)-P,N-(i)Pr(2)PCH(2)Quin)][BAr'(4)] (9b) are reported.