New Insight into the Reactivity of Pyridine-Functionalized Phosphine Complexes of Ruthenium(II) with Respect to Olefin Metathesis and Transfer Hydrogenation

The present paper deals with the synthesis and full characterization of a series of pyridine-functionalized phosphine complexes of Ru(II), namely, RuCl 2 ( L nx )(PPh 3 ) ( L nx = R 2 PCH 2 (C 5 H 2 R′R′′N)), differing in the nature of the substituents on the phosphorus (superscript label n in L nx defined as n = 1 for R = Ph, n = 2 for R = Cy) and/or on the pyridyl group (superscript label x in L nx defined as x = a for picolyl, noted pic, and x = b for quinolyl, noted quin) and discloses new aspects of their reactivity with respect to catalysis. The ligands 2-[(diphenylphosphino)methyl]-6-methylpyridine, L 1a, 2-[(diphenylphosphino)methyl]quinoline, L 1b, 2-[(dicyclohexylphosphino)methyl]-6-methylpyridine, L 2a, and 2-[(dicyclohexylphosphino)methyl]quinoline, L 2b, were prepared and respectively reacted with RuCl 2 (PPh 3 ) 3 under optimized experimental conditions. In a preliminary test, the reaction of RuCl 2 (PPh 3 ) 3 with L 1a using a stoichiometric 1/1 metal/ligand ratio gave three complexes, namely, [RuCl 2 (PPh 3 ) 2 ] 2 ( 1 ), [(PPh 3 ) 2 ClRu(μ-Cl) 3 Ru( L 1a )(PPh 3 )] ( 2 1a ), and RuCl 2 ( L 1a ) 2 ( 3 1a ). These were isolated by fractional crystallization and, at that stage, identified only by single-crystal X-ray diffraction. The formation of 1 and 2 1a reflects the existence of the elusive 14 e − fragment “RuCl 2 (PPh 3 ) 2 ”, which tends to relieve its unsaturation by intermolecular association. By contrast, controlled addition of 2-(phosphinomethyl)pyridine type ligands L nx to RuCl 2 (PPh 3 ) 2 leads selectively to the desired 16 e − species RuCl 2 ( L nx )(PPh 3 ) ( 4 nx ). For example, with L 1b, the green complex RuCl 2 ( L 1b )(PPh 3 ) ( 4 1b -trans-Cl ) was identified as the kinetic product of ligand addition. It slowly and irreversibly converts into the more stable isomer RuCl 2 ( L 1b )(PPh 3 ) ( 4 1b -cis-Cl ), representing the thermodynamic product. Both isomers were fully characterized by NMR spectroscopy and X-ray diffraction. Similar transformations, taking place at different rates, were observed within the ligand series examined here. All isomeric forms of type 4 na complexes react cleanly with a terminal alkyne-like phenylacetylene to give a new complex identified by NMR spectroscopy as the vinylidene species RuCl 2 ( L )(CCHPh)(PPh 3 ) ( 5 na ). The reaction of 4 nb -cis-Cl with an excess of ethyl diazoacetate at –60 °C gives the novel complex RuCl 2 ( L na ){ cis -EtO(O)C(H)C═C(H)C(O)OEt} ( 6 na ) with concomitant elimination of the phosphonium ylide, Ph 3 P═C(H)C(O)OEt. Whereas 1 equiv of diazoalkane thus serves as phosphine scavenger, the uptake of two more carbene units by the remaining 14 e − fragment “RuCl 2 ( L 1a )” results in their coupling, providing diethyl maleate, intercepted in 6 na as a coordinated ligand. Preliminary catalytic tests indicate that the complexes 4 nx act as catalyst precursors for the ROMP of norbornene in the presence of trimethylsilyldiazomethane as the carbene source. The same compounds 4 nx are also used as catalyst precursors in the transfer hydrogenation of a series of ketone substrates using alcohol as the hydrogen source. For example, the hydrogenation of cyclohexanone is achieved in 99% yield within 45 s with only 0.01 mol (0.1 mol %) of the precatalyst RuCl 2 (Ph 2 PCH 2 pic)(PPh 3 ) -trans-Cl ( 4 1a ), representing a turnover frequency of 272 571 h −1 . The X-ray structure analyses of 1, 2 1a, 3 1a, 4 1b (both trans -Cl and cis -Cl isomers), and 6 1a are reported.