title: The influence of N-heterocyclic carbenes (NHC) on the reactivity of [Ru(NHC)(4)H](+) with H(2), N(2), CO and O(2).
abstract: The five-coordinate ruthenium N-heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr(2)Me(2))(4)H][BAr(F) (4)] (1; IiPr(2)Me(2)=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; Ar(F)=3,5-(CF(3))(2)C(6)H(3)), [Ru(IEt(2)Me(2))(4)H][BAr(F) (4)] (2; IEt(2)Me(2)=1,3-diethyl-4,5-dimethylimidazol-2-ylidene) and [Ru(IMe(4))(4)H][BAr(F) (4)] (3; IMe(4)=1,3,4,5-tetramethylimidazol-2-ylidene) have been synthesised following reaction of [Ru(PPh(3))(3)HCl] 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 delta -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(IMe(4))(4)(MeCN)H][BAr(F) (4)], 4. The reactivity of 1-3 towards H(2) and N(2) depends on the size of the N-substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H(2) and N(2) only at low temperature and incompletely, while 3 affords [Ru(IMe(4))(4)(eta(2)-H(2))H][BAr(F) (4)] (7) and [Ru(IMe(4))(4)(N(2))H][BAr(F) (4)] (8) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1-3 to give [Ru(NHC)(4)(CO)H][BAr(F) (4)] (9-11). Addition of O(2) to solutions of 2 and 3 leads to rapid oxidation, from which the Ru(III) species [Ru(NHC)(4)(OH)(2)][BAr(F) (4)] and the Ru(IV) oxo chlorido complex [Ru(IEt(2)Me(2))(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 >> O(2) > N(2) > H(2).
authors.affiliation: Department of Chemistry, University of Bath, Claverton Down, UK.
authors.forename: Suzanne
authors.initials: S
authors.lastname: Burling
authors.forename: L Jonas L
authors.initials: LJ
authors.lastname: Häller
authors.forename: Elena
authors.initials: E
authors.lastname: Mas-Marzá
authors.forename: Aitor
authors.initials: A
authors.lastname: Moreno
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Macgregor
authors.forename: Mary F
authors.initials: MF
authors.lastname: Mahon
authors.forename: Paul S
authors.initials: PS
authors.lastname: Pregosin
authors.forename: Michael K
authors.initials: MK
authors.lastname: Whittlesey