The Influence of N‐Heterocyclic Carbenes (NHC) on the Reactivity of [Ru(NHC) 4 H] + With H 2 , N 2 , CO and O 2

The five‐coordinate ruthenium N‐heterocyclic carbene (NHC) hydrido complexes [Ru(I i Pr 2 Me 2 ) 4 H][BAr F 4 ] ( 1 ; I i Pr 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 δ −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 (η 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 .