Ruthenium Hydride Complexes of 1,3-Dicyclohexylimidazol-2-ylidene

The mono and bis N-heterocyclic carbene (NHC) complexes Ru(ICy)(PPh3)2(CO)H2 (1) and Ru(ICy)2(PPh3)(CO)H2 (2) (ICy = 1,3-dicyclohexyl-1,3-dihydro-2H-imidazol-2-ylidene) were isolated from the reaction of Ru(PPh3)3(CO)H2 with ICy at elevated temp. X-ray crystallog. revealed that 1 contains a trans arrangement of the PPh3 ligands with the ICy trans to hydride; in complex 2, the two ICy ligands are cis to one another. Both 1 and 2 undergo H/D exchange with D2, as well as reacting rapidly with CO to give Ru(ICy)(PPh3)(CO)3 (3) and Ru(ICy)2(CO)3 (4). Addn. of CO2 to solns. of 2 resulted in insertion into one of the Ru-H bonds to afford both the k1- and k2-formate complexes Ru(ICy)2(PPh3)(CO)(k1-OCHO)H (5) and Ru(ICy)2(CO)(k2-OCHO)H (6). Addn. of CO2 to 1 gave solely Ru(ICy)(PPh3)(CO)(k2-OCHO)H (7). The product from reaction of ICy with Ru(PPh3)3HCl proves to be solvent dependent, generating two isomers of the mono-NHC complex Ru(ICy)(PPh3)2HCl (8a,b) in CH2Cl2 but the bis-NHC species Ru(ICy)2(PPh3)HCl (10) in THF. An agostic interaction between Ru and a beta -CH2 hydrogen of the ICy ligand is apparent both in the x-ray crystal structure of 8a and also in soln. This interaction is broken upon addn. of CO, which yields Ru(ICy)(PPh3)2(CO)HCl (9). The mol. structure of the coordinatively unsatd. species 10 is devoid of any agostic bonding.