No abstract

Complexes of formula [RuCl 2 (arene)(κ 1 -dpim)] (dpim ) 2-(diphenylphosphino)-1-methylimidazole) (arene ) p-cymene, 1a; C 6 H 6 , 1b) were prepared by the reaction of [RuCl 2 (pcymene)] 2 or [RuCl 2 (C 6 H 6 )(CH 3 CN)] with dpim. Complexes 1a and 1b were structurally characterized by NMR spectroscopy and X-ray diffraction. The reaction of these precursors with BF 4salts led, in dichloromethane, to cationic complexes of formula [RuCl(arene)(κ 2dpim)]BF 4 (arene ) p-cymene, 2a; C 6 H 6 , 2b). However, in methanol the products were unexpected phosphinite derivatives of the type [RuCl(arene)(HImMe){κ 1 -PPh 2 (OMe)}]A (A ) BF 4 , arene ) p-cymene, 3a; C 6 H 6 , 3b; A ) BPh 4 , arene ) p-cymene, 3d) (ImMe ) methylimidazole). This transformation implies the existence of an easy P-C bond cleavage and phosphine functionalization with methanol at room temperature. The precursors 1a,b or the analogous derivative with 2-(diphenylphosphino)pyridine (PPh 2 py), [RuCl 2 (p-cymene)-(κ 1 -PPh 2 py)], 1c, reacted with HBF 4 to give cationic derivatives by protonation of the imidazole or the pyridine fragment, [RuCl 2 (arene)(κ 1 -PNH)]BF 4 (PNH ) dpimH, arene ) p-cymene, 4a; C 6 H 6 , 4b; PNH ) PPh 2 pyH, arene ) p-cymene, 4c). In these compounds the existence of an asymmetric and bifurcated hydrogen bond NH‚‚‚Cl 2 has been structurally determined (even by X-ray studies for 4a,b). Complexes 2a and 4a also yield the corresponding and analogous phosphinite derivatives in the presence of methanol-d 4 but at a markedly slower rate. NMR and spectrophotometric studies provided information concerning the formation of the phosphinite derivatives. It was concluded that the phosphine is not functionalized if it is not coordinated and that, very probably, a methanol solvatesintermediate between 1a and 2asparticipates in the P-C bond cleavage and allows the aforementioned transformation. Some preliminary catalytic tests involving the transfer hydrogenation of cyclohexanone and the hydrogenation of phenylacetylene have also been carried out.

The use of the phosphine PPh2py instead of PPh3 in complexes of the type [Cp*RuH(P)2] enormously alters the kinetic control of the proton-transfer reactions over this compound and its chemical behavior. The reaction at low temperature of [Cp*RuH(PPh2py)2], 2, with HBF4 gives as products the classical dihydride trans-[Cp*RuH2(PPh2py)2](BF4), 3 (1 equiv of HBF4) or the dihydrogen-bonded complex [Cp*RuHH(PPh2pyH)(PPh2py)](BF4)2, 4 (2 equiv of HBF4). These complexes exhibit very accessible intramolecular processes of proton transfer, and finally, a slow release of H2 takes place at room temperature. Derivatives 2 and 3 are active catalysts for the deuterium labeling of H2 using methanol-d4 as an isotopic source. This demonstrates that the release of hydrogen is reversible, that the heterolytic activation of H2 is an easy process, and that acid species participate in the intramolecular proton-transfer processes. These observations are supported by reaction-coordinate calculations at the DFT/B3LYP level that show the existence of a low-energy reaction path that easily transforms the classical trans dihydride complex into the nonclassical cis dihydrogen compound in a reversible way, through the involvement of hydrogen- and dihydrogen-bonded intermediates and the essential participation of the pyridine centers. The different energy minima of this reaction profile are very accessible through low-energy transition states, all of which have been located.