Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO) HCl] and [Ru(NHC)(L)(CO) H(eta(2)-BH4)] (L = NHC, PPh3)

Thermolysis of [Ru(AsPh3)(3)(CO)H-2] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr.(C6F5) H (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5dihydroimidazol-2-ylidene), followed by addition of CH2Cl2, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)(2)(CO)HCl] (NHC = IMes 1, IPr 2, SIPr 3). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes 4-6. Reduction of 1-3 and [Ru(IMes)(PPh3)(CO)HCl] (7) with NaBH4 yields the isolable borohydride complexes [Ru(NHC)(L)(CO) H(eta(2)-BH4)] (8-11, L = NHC, PPh3). Both the bis-IMes complex 8 and the IMes-PPh3 species 11 react with CO at low temperature to give the eta(1)-borohydride species [Ru(IMes)(L)(CO)(2)H(eta(1)-BH4)] (L = IMes 12, PPh3), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)(2)H-2] (L = IMes, L = PPh3 14) and, ultimately, [Ru(IMes)(L)(CO)(3)] (L = IMes 13, L = PPh3 15). Both 8 and 11 lose BH3 on addition of PMe2Ph to give [Ru(IMes)(L)(L')(CO)H-2](L = L' = PMe2Ph; L = PPh3, L' = PMe2Ph). Compounds 1-4 and 8-11 have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of iPrOH and H-2. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr > IMes >> SIMes.