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

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journal contribution
posted on 2014-04-07, 00:00 authored by Andreas Phanopoulos, Neil J. Brown, Andrew J. P. White, Nicholas J. Long, Philip W. Miller
The reactivity of the novel tridentate phosphine ligand N­(CH2PCyp2)3 (N-triphosCyp, 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-triphosPh, 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)33P}] (R = Ph (3), Cyp (4); tmm = trimethylenemethane) and [RuH2(PPh3)­{N­(CH2PR2)33P}] (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)33P}] (7), while 2 gave the bidentate, tricarbonyl [Ru­(CO)3{N­(CH2PCyp2)32P}] (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)33P}]­(OTf) (9) and [Ru­(CO3)­(CO)­{N­(CH2PPh2)33P}] (11), respectively. Hydrogenation of these complexes under hydrogen pressures of 3–15 bar gave the monohydride and dihydride complexes [RuH­(CO)2{N­(CH2PPh2)33P}] (10) and [RuH2(CO)­{N­(CH2PPh2)33P}] (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)33P}]­(PF6) (13), which reacted cleanly with LA to form [Ru­(CO)­{N­(CH2PPh2)33P}­{CH3CO­(CH2)2CO2H-κ2O}]­(PF6) (14). Complexes 3, 5, 7, 8, 11, and 12 were characterized by single-crystal X-ray crystallography.