Molecular hydrogen and water activation by transition metal frustrated Lewis pairs containing ruthenium or osmium components: catalytic hydrogenation assays

Abstract

The transition metal frustrated Lewis pair compounds [(Cym)M(κ³S,P,N-HL1)][SbF₆] (Cym = η⁶-p-MeC₆H₄iPr; H₂L1 = N-(p-tolyl)-N′-(2-diphenylphosphanoethyl)thiourea; M = Ru (5), Os (6)) have been prepared from the corresponding dimer [{(Cym)MCl}₂(μ-Cl)₂] and H₂L1 by successive chloride abstraction with NaSbF₆ and AgSbF₆ and NH deprotonation with NaHCO₃. Complexes 5 and 6 and the previously reported phosphano–guanidino compounds [(Cym)M(κ³P,N,N′-HL2)][SbF₆] [H₂L2 = N,N′-bis(p-tolyl)-N′′-(2-diphenylphosphanoethyl) guanidine; M = Ru (7), Os (8)] and pyridinyl–guanidino compounds [(Cym)M(κ³N,N′,N′′-HL3)][SbF₆] [H₂L3 = N,N′-bis(p-tolyl)-N′′-(2-pyridinylmethyl) guanidine; M = Ru (9), Os (10)] heterolytically activate H₂ in a reversible manner affording the hydrido complexes [(Cym)MH(H₂L)][SbF₆] (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(κ³N,N′,C-H₂L3_-H)][SbF₆] (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(sp³)–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 CC double bond of styrene and a range of acrylates, the CO bond of acetophenone and the CN bond of N-benzylideneaniline and quinoline. The CC double bond of methyl acrylate adds to catalyst 9, affording complex 19 in which a new ligand exhibiting a fac κ³N,N′,C coordination mode has been incorporated.