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

The mono and bis N-heterocyclic carbene (NHC) complexes Ru(ICy)(PPh<INF>3</INF>)<INF>2</INF>(CO)H<INF>2</INF> (<BO>1</BO>) and Ru(ICy)<INF>2</INF>(PPh<INF>3</INF>)(CO)H<INF>2</INF> (<BO>2</BO>) (ICy = 1,2-dicyclohexylimidazol-2-ylidene) were isolated from the reaction of Ru(PPh<INF>3</INF>)<INF>3</INF>(CO)H<INF>2</INF> with ICy at elevated temperature. X-ray crystallography revealed that <BO>1</BO> contains a trans arrangement of the PPh<INF>3</INF> ligands with the ICy trans to hydride; in complex <BO>2</BO>, the two ICy ligands are cis to one another. Both <BO>1</BO> and <BO>2</BO> undergo H/D exchange with D<INF>2</INF>, as well as reacting rapidly with CO to give Ru(ICy)(PPh<INF>3</INF>)(CO)<INF>3</INF> (<BO>3</BO>) and Ru(ICy)<INF>2</INF>(CO)<INF>3</INF> (<BO>4</BO>). Addition of CO<INF>2</INF> to solutions of <BO>2</BO> resulted in insertion into one of the Ru−H bonds to afford both the κ¹- and κ²-formate complexes Ru(ICy)<INF>2</INF>(PPh<INF>3</INF>)(CO)(κ¹-OCHO)H (<BO>5</BO>) and Ru(ICy)<INF>2</INF>(CO)(κ²-OCHO)H (<BO>6</BO>). Addition of CO<INF>2</INF> to <BO>1</BO> gave solely Ru(ICy)(PPh<INF>3</INF>)(CO)(κ²-OCHO)H (<BO>7</BO>). The product from reaction of ICy with Ru(PPh<INF>3</INF>)<INF>3</INF>HCl proves to be solvent dependent, generating two isomers of the mono-NHC complex Ru(ICy)(PPh<INF>3</INF>)<INF>2</INF>HCl (<BO>8a</BO>,<BO>b</BO>) in dichloromethane but the bis-NHC species Ru(ICy)<INF>2</INF>(PPh<INF>3</INF>)HCl (<BO>10</BO>) in THF. An agostic interaction between Ru and a β-CH<INF>2</INF> hydrogen of the ICy ligand is apparent both in the X-ray crystal structure of <BO>8a</BO> and also in solution. This interaction is broken upon addition of CO, which yields Ru(ICy)(PPh<INF>3</INF>)<INF>2</INF>(CO)HCl (<BO>9</BO>). The molecular structure of the coordinatively unsaturated species <BO>10</BO> is devoid of any agostic bonding.