Chelation-Assisted Reactions of Phosphine- and Olefin-Tethered Imidazolium Derivatives and Their Affiliated N-Heterocyclic Carbenes with Roper’s Complex Ru(CO) 2 (PPh3)3

International audienceComplex Ru(CO)2(PPh3)3, 1, is a suitable starting compound for the generation of N-heterocyclic carbene complexes of Ru(0). Although monodentate NHCs are totally unreactive toward 1, phosphine- or olefin-functionalized N-heterocyclic carbenes, as well as their imidazolium precursors, react with 1 under chelation assistance where the phosphine or the olefin is acting as a directing group. Reactions of 1-mesityl-3-(2-diphenylphosphinoeth-1-yl)imidazolium bromide, [HL1a]+Br−, 1-mesityl-3-(2-diphenylphosphinoeth-1-yl)imidazolium tetrafluoroborate, [HL1a]+BF4−, and 1-(2,6-diisopropylphenyl)-3-(2-diphenylphosphinoeth-1-yl)imidazolium bromide, [HL1b]+Br−, with 1 give cationic hydrido species formulated as [RuH{L*1a,b}(CO)2(PPh3)]+X−, [2a,b]+X− (a, Ar = mesityl; b, Ar = 2,6 diisopropylphenyl), in which abnormal activation (symbolized by the asterisk) at the C(4) position of the heterocycle has taken place to yield the bidentate ligands L*1a,b. Deprotonation of [HL1b]+ with KOtBu gives the corresponding NHC/phosphine bidentate ligand, which reacts with 1 to give the chelated NHC/phosphine complex Ru{L1b}(CO)2(PPh3) (3b), the first analogue of Roper’s complex incorporating an NHC moiety. The olefin-functionalized imidazolium ligand 3-(but-3-enyl)-1-mesitylimidazolium bromide, [HL2a]+Br−, reacts with 1 via chelation-assisted C−H activation and H transfer to the olefin, giving Ru{Ar(N2C3H2)CH2C(H)(CH2CH3)}(CO)2(PPh3)Br (4a). Deprotonation of [HL2a]+Br− gives L2a, which reacts with 1 to give Ru{L2a}(CO)2(PPh3) (5a). Its protonation with HBF4 at −80 °C gives a cationic NHC/olefin-hydrido complex, [RuH{L2a}(CO)2(PPh3)]+BF4−, [6a]+BF4−. NMR data indicate the occurrence of a dynamic process involving a fast exchange between the hydride and the two terminal hydrogen atoms of the coordinated olefin, which can be rationalized in terms of the transient generation of an elusive higher energy NHC/alkyl intermediate, [Ru{Ar(N2C3H2)CH2CH2C(H)CH3)}(CO)2(PPh3)]+BF4−, [7a]+BF4−. At temperatures above −20 °C, [6a]+BF4− is irreversibly converted into the isomerized NHC/olefin-hydrido complex [RuH{Ar(N2C3H2)CH2CH═C(H)CH3}(CO)2(PPh3)]+BF4−, [8a]+BF4−. Here again, NMR data reveal a dynamic process involving fast exchange between the hydride and the terminal hydrogen atom of the coordinated olefin, now through the intermediacy of the elusive cationic NHC/alkyl species [Ru{Ar(N2C3H2)CH2C(H)CH2CH3}(CO)2(PPh3)]+BF4−, [9a]+BF4−. Although neither of the above unsaturated cationic alkyl intermediates [7a]+ or [9a]+ was observed, their occurrence could be inferred from trapping experiments. Indeed, the addition of [PPN]Cl to the above mixture after equilibration at 25 °C leads to the formation of the chloride analogue of 4a. Protonation with HCl instead of HBF4 allows capture of the first elusive intermediate [7a]+ by the halide, which quenches the isomerization process and promotes a migratory CO insertion yielding the NHC/alkyl derivative Ru{Ar(N2C3H2)CH2CH2C(H)(CH3)C═O}(CO)(PPh3), 10a. The X-ray structure analyses for 4 and 5 are included