Hydrogen Transfer Activation via Stabilization of Coordinatively Vacant Sites: Tuning Long-Range π-System Electronic Interaction between Ru(0) and NHC Pendants

The insertion of a pyridine substituent on the lateral chain of N-heterocyclic carbene ligands enhances the catalytic activity of [Ru(CO)2(cyclopentadienone)(NHC)] complexes, precursors of Shvo-type catalysts, toward the hydrogenation of 4-fluoroacetophenone in refluxing 2-propanol as hydrogen donor. DFT calculations evidence the role of pyridine in the donor/acceptor properties and complexes reactivity both in the case of imidazolylidene and triazolylidene ligands. Although the NHC-pyridine derived complexes perform somewhat worse than the forerunner Shvo dimer, the slower evolution of the reaction steps is compatible with the FT-ATR spectroscopy time scale and allows to identify in situ intermediates, invisible when using the original Shvo catalyst due to its higher reaction speed. Thus, mechanistic insight has been disclosed by synergic contribution of FT-ATR in situ experiments and DFT calculations performed on the slowest precatalyst (1N) chosen as a model. The proposed reaction mechanism has been based on both energy profile and experimentally identified intermediate.