System with Potential Dual Modes of Metal–Ligand Cooperation: Highly Catalytically Active Pyridine‐Based PNNH–Ru Pincer Complexes

Abstract

Metal-ligand cooperation (MLC) plays an important role in catalysis. Systems reported so far are generally based on a single mode of MLC. We report here a system with potential for MLC by both amine-amide and aromatization-dearomatization ligand transformations, based on a new class of phosphino-pyridyl ruthenium pincer complexes, bearing sec-amine coordination. These pincer complexes are effective catalysts under unprecedented mild conditions for acceptorless dehydrogenative coupling of alcohols to esters at 35 °C and hydrogenation of esters at room temperature and 5 atm H2. The likely actual catalyst, a novel, crystallographically characterized monoanionic de-aromatized enamido-Ru(II) complex, was obtained by deprotonation of both the N-H and the methylene proton of the N-arm of the pincer ligand.

A Multifunctional Pincer Ligand Supports Unsaturated Cobalt: Five Functionalities in One Pincer

“…In contrast, an example of a singly deprotonated benzylic arm on Re I augments this analogous C−C bond formation with an Re−O bond . Other examples of benzylic pincer deprotonation have shown that Li + associates closely with the π system of the dearomatized ring of the conjugate base –. Several cobalt pincer complexes with monoanionic pincer ligands were reported in the last decade (Figure ) –.…”

Pincer‐Type Complexes for Catalytic (De)Hydrogenation and Transfer (De)Hydrogenation Reactions: Recent Progress

“…In general, the yield of the corresponding cross‐ester is excellent, whereas in only a few cases were small amounts of homocoupling products of the primary alcohol observed. Recently, Milstein’s group reported the modified Ru PNNH pincer complex 80 , exhibiting secondary amine coordination (Scheme ) 100c. In contrast to the previously described examples 19 and 21 , this new type of Ru pincer complex combines both prevailing MLC modes by amine/amide, as well as aromatization/dearomatization ligand transformation (see section 1.3).…”

Manganese‐Catalyzed Hydrogenation of Esters to Alcohols

“…Recently our group reported the synthesis and reactivity of a series of analogous [Ru(PNNH)(CO)(H)(Cl)] pincer complexes, which showed exceptional reactivity in hydrogenation and dehydrogenation processes . Exploring the catalytic activity of the PNNH complex 1 , attempted hydrogenation of hexyl hexanoate under 20 bar of H 2 and catalyst loading of 1 mol % in toluene was not successful (Table , entry 1).…”