Ruthenium Arene Derivatives with PN Hemilabile Ligands. P−C Cleavage and Phosphine to Phosphinite Transformation

Complexes of formula [RuCl2(arene)(κ1-dpim)] (dpim = 2-(diphenylphosphino)-1-methylimidazole) (arene = p-cymene, 1a; C6H6, 1b) were prepared by the reaction of [RuCl2(p-cymene)]2 or [RuCl2(C6H6)(CH3CN)] with dpim. Complexes 1a and 1b were structurally
characterized by NMR spectroscopy and X-ray diffraction. The reaction of these precursors
with BF4- salts led, in dichloromethane, to cationic complexes of formula [RuCl(arene)(κ2-dpim)]BF4 (arene = p-cymene, 2a; C6H6, 2b). However, in methanol the products were
unexpected phosphinite derivatives of the type [RuCl(arene)(HImMe){κ1-PPh2(OMe)}]A (A
= BF4, arene = p-cymene, 3a; C6H6, 3b; A = BPh4, 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 (PPh2py), [RuCl2(p-cymene)(κ1-PPh2py)], 1c, reacted with HBF4 to give cationic derivatives by protonation of the imidazole
or the pyridine fragment, [RuCl2(arene)(κ1-PNH)]BF4 (PNH = dpimH, arene = p-cymene,
4a; C6H6, 4b; PNH = PPh2pyH, arene = p-cymene, 4c). In these compounds the existence
of an asymmetric and bifurcated hydrogen bond NH···Cl2 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-d4 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 solvateintermediate between 1a
and 2aparticipates 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.