Ring-Expanded N-Heterocyclic Carbene Complexes of Ruthenium

The six-membered N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene (6-Mes) reacts with Ru(PPh 3 ) 3 (CO)HF to afford Ru(6-Mes)(PPh 3 )(CO)HF ( 1 ), which is converted to the five-coordinate C−H activated carbene complex Ru(6-Mes)′(PPh 3 )(CO)H ( 2 ) upon treatment with Et 3 SiH. The hydride chloride precursor Ru(PPh 3 ) 3 (CO)HCl affords a mixture of products with 6-Mes, but reacts cleanly with 1,3-bis(isopropyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene (6- i Pr) to give the six-coordinate activated complex Ru(6- i Pr)′(PPh 3 ) 2 (CO)H ( 3a ), in which the hydride is trans to the methylene arm of the activated NHC. This complex isomerizes in solution with Δ H ⧧ and Δ S ⧧ values of 98.2 ± 4.6 kJ mol −1 and 15.5 ± 14.5 J mol −1 K −1 . The major product from the isomerization, 3b, in which the hydride ligand is trans to carbene, can be made directly by reaction of 6- i Pr with Ru(PPh 3 ) 3 (CO)H 2 .