Molecular Hydrogen and Water Activation by Transition Metal Frustrated Lewis Pairs Containing Ruthenium or Osmium Components: Catalytic Hydrogenation Assays

The transition metal frustrated Lewis pair compounds [(Cym)M(κS,P,N-HL1)][SbF] (Cym = η--MeCHiPr; H2L1 = -(-tolyl)-'-(2-diphenylphosphanoethyl)thiourea; M = Ru (5), Os (6)) have been prepared from the corresponding dimer [{(Cym)MCl}(μ-Cl)] and H2L1 by successive chloride abstraction with NaSbF and AgSbF and NH deprotonation with NaHCO. Complexes 5 and 6 and the previously reported phosphano-guanidino compounds [(Cym)M(κP,N,N'-HL2)][SbF] [H2L2 = ,'-bis(-tolyl)-''-(2-diphenylphosphanoethyl) guanidine; M = Ru (7), Os (8)] and pyridinyl-guanidino compounds [(Cym)M(κN,N',N''-HL3)][SbF] [H2L3 = ,'-bis(-tolyl)-''-(2-pyridinylmethyl) guanidine; M = Ru (9), Os (10)] heterolytically activate H in a reversible manner affording the hydrido complexes [(Cym)MH(H2L)][SbF] (H2L = H2L1; M = Ru (11), Os (12); H2L = H2L2; M = Ru (13), Os (14); H2L = H2L3; M = Ru (15), Os (16)). DFT calculations carried out on the hydrogenation of complex 7 support an FLP mechanism for the process. Heating 9 and 10 in methanol yields the orthometalated complexes [(Cym)M(κN,N',C-H2L3-H)][SbF] (M = Ru (17), Os (18)). The phosphano-guanidino complex 7 activates deuterated water in a reversible fashion, resulting in the gradual deuteration of the three cymene methyl protons through sequential C(sp)-H bond activation. From DFT calculations, a metal-ligand cooperative reversible mechanism that involves the O-H bond activation and the formation of an intermediate methylene cyclohexenyl complex has been proposed. Complexes 5-10 catalyse the hydrogenation of the CC double bond of styrene and a range of acrylates, the CO bond of acetophenone and the CN bond of -benzylideneaniline and quinoline. The CC double bond of methyl acrylate adds to catalyst 9, affording complex 19 in which a new ligand exhibiting a κN,N',C coordination mode has been incorporated.