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, RuCl2(Lnx)(PPh3) (Lnx = R2PCH2(C5H2R′R′′N)), differing in the nature of the substituents on the phosphorus (superscript label n in Lnx defined as n = 1 for R = Ph, n = 2 for R = Cy) and/or on the pyridyl group (superscript label x in Lnx 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, L1a, 2-[(diphenylphosphino)methyl]quinoline, L1b, 2-[(dicyclohexylphosphino)methyl]-6-methylpyridine, L2a, and 2-[(dicyclohexylphosphino)methyl]quinoline, L2b, were prepared and respectively reacted with RuCl2(PPh3)3 under optimized experimental conditions. In a preliminary test, the reaction of RuCl2(PPh3)3 with L1a using a stoichiometric 1/1 metal/ligand ratio gave three complexes, namely, [RuCl2(PPh3)2]2 (1), [(PPh3)2ClRu(μ-Cl)3Ru(L1a)(PPh3)] (21a), and RuCl2(L1a)2 (31a). These were isolated by fractional crystallization and, at that stage, identified only by single-crystal X-ray diffraction. The formation of 1 and 21a reflects the existence of the elusive 14 e fragment “RuCl2(PPh3)2”, which tends to relieve its unsaturation by intermolecular association. By contrast, controlled addition of 2-(phosphinomethyl)pyridine type ligands Lnx to RuCl2(PPh3)2 leads selectively to the desired 16 e species RuCl2(Lnx)(PPh3) (4nx). For example, with L1b, the green complex RuCl2(L1b)(PPh3) (41b-trans-Cl) was identified as the kinetic product of ligand addition. It slowly and irreversibly converts into the more stable isomer RuCl2(L1b)(PPh3) (41b-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 4na complexes react cleanly with a terminal alkyne-like phenylacetylene to give a new complex identified by NMR spectroscopy as the vinylidene species RuCl2(L)(CCHPh)(PPh3) (5na). The reaction of 4nb-cis-Cl with an excess of ethyl diazoacetate at –60 °C gives the novel complex RuCl2(Lna){cis-EtO(O)C(H)CC(H)C(O)OEt} (6na) with concomitant elimination of the phosphonium ylide, Ph3P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 “RuCl2(L1a)” results in their coupling, providing diethyl maleate, intercepted in 6na as a coordinated ligand. Preliminary catalytic tests indicate that the complexes 4nx act as catalyst precursors for the ROMP of norbornene in the presence of trimethylsilyldiazomethane as the carbene source. The same compounds 4nx 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 RuCl2(Ph2PCH2pic)(PPh3)-trans-Cl (41a), representing a turnover frequency of 272 571 h−1. The X-ray structure analyses of 1, 21a, 31a, 41b (both trans-Cl and cis-Cl isomers), and 61a are reported.