Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

A series of ruthenium(II) N-heterocyclic carbene chelate complexes were prepd. by using bidentate chelating alkene-carbene, thioether-carbene, carboxylate-carbene and bis-carbene ligands. Metalation of 1-R-3-X-imidazolium salts (1-3, R = Me, X = allyl, MeSCH2CH2; R = mesityl, X = O2CCH2) by either [RuCpCl(PPh2)2] or [RuCl2(cymene)]2 gave the corresponding n-donor-carbene chelates [CpRu(PPh3)[MeSCH2CH2NCN(Me)CH:CH-κS,κC2]][BF4] (6), [CpRu(PPh3)[η2-CH2:CHCH2NCN(Me)CH:CH-κC2]]Y (5a,b, Y = Cl, BF4), [CpRu(PPh3)[O2CCH2NCN(Ar)CH:CH-κO,κC2]] (7, Ar = mesityl), [(η6-cymene)RuCl[η2-CH2:CHCH2NCN(Me)CH:CH-κC2]][BF4] (8), [(η6-cymene)RuCl[MeSCH2CH2NCN(Me)CH:CH-κS,κC2]][BF4] (9), [(η6-cymene)RuCl[O2CCH2NCN(Ar)CH:CH-κO,κC2]] (10). Reaction of 3,3'-diisopropyl-1,1'-methylenebis(imidazolium) diiodide with [RuCl2(arene)]2 gave [(arene)RuCl[iPrNCN(CH:CH)CH2NCN(CH:CH)iPr-κC2,κC2']][PF6] (11, 12, arene = benzene, p-cymene). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic effects of the donor groups were evaluated by electrochem. analyses. The chelating donor group had a strong influence on the activity of the metal center in styrene hydrogenation reaction. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.