The influence of N-heterocyclic carbenes (NHC) on the reactivity of [Ru(NHC)₄H]₊ with H₂, N₂, CO and O₂

The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr₂Me₂)₄H][BAr^F ₄] (1; IiPr₂Me₂ = l,3-diisopropyl4,5- dimethylimidazol-2-ylidene ; Ar^F = 3,5-(CF₃) ₂C₆H₃), [Ru(IEt₂Me₂) ₄H]-[BAr^F₄] (2; IEt₂Me₂ = l,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe₄) ₄H][BAr^F₄] (3; IMe₄=l,3,4,5- tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh₃)₃HCl] with 4-8 equivalents of the free carbenes at ambient temperature. Complexes 1-3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about (3 -41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe₄)₄(MeCN)H] [BAr^F₄], 4. The reactivity of 1-3 towards H₂ and N₂ depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H₂ and N₂ only at low temperature and incom-pletely, while 3 affords [Ru(IMe₄)₄(η²H₂)H] [BAr ^F₄] (7) and [Ru(IMe₄)₄(N ₂)H][BAr^F₄] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC) ₄(CO)H][BAr^F₄] (911). Addition of O₂ to solutions of 2 and 3 leads to rapid oxidation, from which the Ru ^III species [Ru(NHC)₄(OH)₂][BAr ^F₄] and the Ru^IV oxo chlorido complex [Ru(IEt₂Me₂)₄(O)Cl][BAr^F ₄] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO ≫ O₂ > N₂ > H₂.