Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of N‑Centered Triphosphine Ligands

The reactivity of the novel tridentate phosphine ligand N­(CH₂PCyp₂)₃ (N-triphos^Cyp, 2; Cyp = cyclopentyl) with various ruthenium complexes was investigated and compared that of to the less sterically bulky and less electron donating phenyl derivative N­(CH₂PPh₂)₃ (N-triphos^Ph, 1). One of these complexes was subsequently investigated for reactivity toward levulinic acid, a potentially important biorenewable feedstock. Reaction of ligands 1 and 2 with the precursors [Ru­(COD)­(methylallyl)₂] (COD = 1,5-cycloocatadiene) and [RuH₂(PPh₃)₄] gave the tridentate coordination complexes [Ru­(tmm)­{N­(CH₂PR₂)₃-κ³P}] (R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH₂(PPh₃)­{N­(CH₂PR₂)₃-κ³P}] (R = Ph (5), Cyp (6)), respectively. Ligands 1 and 2 displayed different reactivities with [Ru₃(CO)₁₂]. Ligand 1 gave the tridentate dicarbonyl complex [Ru­(CO)₂{N­(CH₂PPh₂)₃-κ³P}] (7), while 2 gave the bidentate, tricarbonyl [Ru­(CO)₃{N­(CH₂PCyp₂)₃-κ²P}] (8). This was attributed to the greater electron-donating characteristics of 2, requiring further stabilization on coordination to the electron-rich Ru(0) center by more CO ligands. Complex 7 was activated via oxidation using AgOTf and O₂, giving the Ru­(II) complexes [Ru­(CO)₂(OTf)­{N­(CH₂PPh₂)₃-κ³P}]­(OTf) (9) and [Ru­(CO₃)­(CO)­{N­(CH₂PPh₂)₃-κ³P}] (11), respectively. Hydrogenation of these complexes under hydrogen pressures of 3–15 bar gave the monohydride and dihydride complexes [RuH­(CO)₂{N­(CH₂PPh₂)₃-κ³P}] (10) and [RuH₂(CO)­{N­(CH₂PPh₂)₃-κ³P}] (12), respectively. Complex 12 was found to be unreactive toward levulinic acid (LA) unless activated by reaction with NH₄PF₆ in acetonitrile, forming [RuH­(CO)­(MeCN)­{N­(CH₂PPh₂)₃-κ³P}]­(PF₆) (13), which reacted cleanly with LA to form [Ru­(CO)­{N­(CH₂PPh₂)₃-κ³P}­{CH₃CO­(CH₂)₂CO₂H-κ²O}]­(PF₆) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.