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

The mono and bis N-heterocyclic carbene (NHC) complexes Ru(ICy)(PPh 3 ) 2 (CO)H 2 ( 1 ) and Ru(ICy) 2 (PPh 3 )(CO)H 2 ( 2 ) (ICy = 1,2-dicyclohexylimidazol-2-ylidene) were isolated from the reaction of Ru(PPh 3 ) 3 (CO)H 2 with ICy at elevated temperature. X-ray crystallography revealed that 1 contains a trans arrangement of the PPh 3 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 D 2, as well as reacting rapidly with CO to give Ru(ICy)(PPh 3 )(CO) 3 ( 3 ) and Ru(ICy) 2 (CO) 3 ( 4 ). Addition of CO 2 to solutions of 2 resulted in insertion into one of the Ru−H bonds to afford both the κ 1 - and κ 2 -formate complexes Ru(ICy) 2 (PPh 3 )(CO)(κ 1 -OCHO)H ( 5 ) and Ru(ICy) 2 (CO)(κ 2 -OCHO)H ( 6 ). Addition of CO 2 to 1 gave solely Ru(ICy)(PPh 3 )(CO)(κ 2 -OCHO)H ( 7 ). The product from reaction of ICy with Ru(PPh 3 ) 3 HCl proves to be solvent dependent, generating two isomers of the mono-NHC complex Ru(ICy)(PPh 3 ) 2 HCl ( 8a, b ) in dichloromethane but the bis-NHC species Ru(ICy) 2 (PPh 3 )HCl ( 10 ) in THF. An agostic interaction between Ru and a β-CH 2 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)(PPh 3 ) 2 (CO)HCl ( 9 ). The molecular structure of the coordinatively unsaturated species 10 is devoid of any agostic bonding.