Ruthenium Hydride Complexes of 1,2-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 = 1,2-dicyclohexylimidazol-2-ylidene) were isolated from the reaction of Ru(PPh₃)₃(CO)H₂ with ICy at elevated temperature. X-ray crystallography 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). Addition of CO₂ to solutions of 2 resulted in insertion into one of the Ru−H bonds to afford both the κ¹- and κ²-formate complexes Ru(ICy)₂(PPh₃)(CO)(κ¹-OCHO)H (5) and Ru(ICy)₂(CO)(κ²-OCHO)H (6). Addition of CO₂ to 1 gave solely Ru(ICy)(PPh₃)(CO)(κ²-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 dichloromethane but the bis-NHC species Ru(ICy)₂(PPh₃)HCl (10) in THF. An agostic interaction between Ru and a β-CH₂ hydrogen of the ICy ligand is apparent both in the X-ray crystal structure of 8a and also in solution. This interaction is broken upon addition of CO, which yields Ru(ICy)(PPh₃)₂(CO)HCl (9). The molecular structure of the coordinatively unsaturated species 10 is devoid of any agostic bonding.