Mechanistic insights into catalytic carboxylic ester hydrogenation with cooperative Ru(II)-bis1,2,3-triazolylidene pyridine pincer complexes

Transmetallation of newly designed lutidine-based CNC or CNN ligands L, featuring flanking 1,2,3-triazolylidene (tzNHCs) moieties, from Ag(I) to Ru(II) provided access to well-defined cationic [Ru^II(CO)(H)(L)(PPh₃)]⁺ complexes 2 and 5. Spectroscopic investigations confirm that, in both complexes, the tridentate ligand binds in a rare facial mode to the metal center. The complexes, that exhibit ligand-based reversible deprotonation/dearomatization reactivity, are active in catalytic ester hydrogenation in the presence of KOtBu (≥20 mol%) as an exogenous base. The beneficial effect of the base on catalytic activity relates to transesterification of substrates to the corresponding tert-butyl ester derivatives, which are hydrogenated considerably faster than methyl esters. The mechanistic findings in this work confirm that this transformation is very complex, with this transesterification, metal-ligand cooperative reactivity, base strength and possibly product inhibition all playing a role. Furthermore, relevant Ru(CNC)(hydride) species have been observed by NMR spectroscopy under near-catalytic conditions.