Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO)H(η2-BH4)] (L = NHC, PPh3)

Abstract

Thermolysis of [Ru(AsPh₃)₃(CO)H₂] 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)-4,5-dihydroimidazol-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(η²-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 η¹-borohydride species [Ru(IMes)(L)(CO)₂H(η¹-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)₂H₂] (L = IMes, L = PPh₃14) and, ultimately, [Ru(IMes)(L)(CO)₃] (L = IMes 13, L = PPh₃15). Both 8 and 11 lose BH₃ on addition of PMe₂Ph to give [Ru(IMes)(L)(L′)(CO)H₂](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 H₂. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr > IMes >> SIMes.