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