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(PPh3)3(CO)HF to afford Ru(6-Mes)(PPh3)(CO)HF (1), which is converted to the five-coordinate C−H activated carbene complex Ru(6-Mes)′(PPh3)(CO)H (2) upon treatment with Et3SiH. The hydride chloride precursor Ru(PPh3)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-iPr) to give the six-coordinate activated complex Ru(6-iPr)′(PPh3)2(CO)H (3a), in which the hydride is trans to the methylene arm of the activated NHC. This complex isomerizes in solution with ΔH and ΔS 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-iPr with Ru(PPh3)3(CO)H2.