Methylation of PNP pincer arms has emerged as a new versatile tool to control steric hindrance and disable metal-ligand cooperative or ligand-centered reactivity. This Concept describes applications of the methylation approach in the control of catalytic activity and selectivity in hydrogenation, hydroboration and semihydrogenation, with Ru and Fe complexes.

Abstract

The widespread use of pyridine-based PNP pincer ligands has inspired the concept of metal-ligand cooperation (MLC), in which the reactivity at the deprotonated CH₂ (or NH) arm of the ligand is proposed to play an important role. Several groups developed a family of PNP-type pincer ligands with methylated arms which were initially introduced to test the effect of blocking MLC in catalysis, but eventually led to unexpected consequences such as stabilization of unusual oxidation states, beneficial catalytic activity, or selectivity. Analysis of the sterics imposed by introducing Me groups revealed that arm protection can be an efficient tool to control sterics around the metal as an alternative to phosphine substitution, leading to much greater steric hinderance above and below pincer's coordination plane. This Concept will describe several illustrative examples which contrast the reactivity of classical CH₂/NH-arm PNP pincers with their CMe₂/NMe-armed counterparts, in particular related to Ru-catalyzed alcohol dehydrogenative coupling, Fe-catalyzed hydrogenation, hydroboration, and alkyne semihydrogenation.