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.