Substrate-assisted product release in hydrogenation of LOHCs facilitated by hemilabile pyridine in Ru(ii)–NNC pincer complexes and enhanced activity of proton-responsive protic-NHC ligands

Abstract

An interesting mechanism of product release similar to substrate-assisted product release found in enzyme catalysis is observed in the case of transition-metal catalysis. A set of Ru(II)–pincer complexes Ru1–4 with benzimidazolylidene-based protic- and classical-NHCs, recently reported by our group, and new complexes Ru5–8 with triazolylidene-based protic- and classical-NHCs have been used for the selective hydrogenation of nitriles and N-heteroarenes as potential LOHC candidates. The benzimidazolylidene-based protic-NHC complex Ru1 emerges as an optimal precatalyst, which was found to be converted to the anionic-NHC Ru–hydride complex Ru2′ during the catalysis. Additionally, Ru(II)–nitrile complex Ru9′ was synthesized and characterized by spectroscopic techniques. The isolated nitrile complex Ru9′ was found to exhibit better catalytic activity compared to the precatalyst Ru1. The classical-NHC analogue Ru3 with a methyl substituent on the β-nitrogen wingtip showed lower conversion (18%), indicating anionic-NHC's role in H₂-activation. Several control experiments elucidating the effect of other gasses, like CO, CH₄, O₂, etc., and ligands, like excess PPh₃ and dppe, are carried out to obtain more insights into the catalyst behaviour. A combination of in situ monitoring of the catalytic reaction, stoichiometric experiments, and DFT studies enabled us to understand the complete catalytic cycle. Computational analysis indicates that the initial cooperative heterolytic splitting of hydrogen molecules by anionic-NHC complex Ru1′ is feasible but the non-MLC pathway is favoured for subsequent steps. Interestingly, product release by direct dissociation of the metal-bound amine product was found to be unfavourable. An alternative path for the product release, where another nitrile substrate molecule first binds in place of a hemilabile pyridine unit and subsequent rearrangement pushes out the amine product, was found to be energetically more favourable during DFT studies. This is supported by the analysis of the catalyst deactivation mechanism by CO, which was found to occupy the same coordination site.