Protic- or anionic-NHCs with a classical-NHC in a single [Ru(CNC)(PPh 3 ) 2 Cl]Cl pincer complex: direct comparison of structure & electronic properties and heterolytic H 2 splitting.

Herein, we report the first set of pincer complexes 1 and 2 with the general formula [Ru(CNC)(PPh 3 ) 2 Cl]Cl having a protic- and classical-NHC in the same molecule and nearly identical environments. Deprotonation of the protic-NHC complex 2 with one equivalent of base leads to the formation of anionic-NHC complex 2'. These complexes allow a direct comparison of protic- and anionic-NHCs with the classical-NHC ligand. A comparison of the molecular structure indicated that the metal carbene bond length trend is anionic-NHC > protic-NHC > classical-NHC. The electrochemical investigation revealed the electron donation tendency is classical-NHC > protic-NHC and anionic-NHC > protic-NHC. Cooperation between the metal and the ligand is indicated by the reaction of 2' with H 2 gas at 1 atm pressure and 110 °C to give the Ru-hydride complex 3.