Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non‐Innocent Scaffolds for Homogeneous Catalysis

Abstract

Ligands in coordination chemistry and homogeneous catalysis are traditionally "static" spectators that do not actively participate in the catalytic cycle. However, such classic systems do not provide additional "handles" that could facilitate or trigger alternative productive reaction pathways. Recent advances in the use of novel nitrogen-centered pincer systems have unveiled interesting opportunities for cooperative catalysis. The chemistry of pyridine-derived, neutral ligands is discussed, with a specific focus on their non-innocent behavior and potential as facilitators for metal-mediated organic transformations. This overview should provide inspiration and an incentive to incorporate non-innocent ligands and their metal complexes within old and new homogeneously catalyzed reactions.

Comparative Study of Novel Phosphordiamidite and Phosphite Ligands Used in Alkene Hydroformylation; Synthesis, Characterization, Metalation, and Catalytic Evaluation

“…In addition to the coordination possibilities already mentioned for L1 and L3 , L2 has the potential for κ 3 coordination similar to that which has been observed with other pyridine based PNP and PNN ligands , . However the pyridine nitrogen typically does not participate in metal bonding when the arms to the central pyridine unit are longer than one atom , , .…”

Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia

“…This means that a mononuclear unit of the dinuclear molybdenum–dinitrogen complex bearing the PNP-type pincer ligands works as a mobile ligand to the other unit as an active site. This result is in sharp contrast to the common role of the dinitrogen-bridged dinuclear metal complexes bearing PNP-type and PCP-type pincer ligands, where dinitrogen-bridged dinuclear metal complexes are known to be used as precursors of mononuclear reactive metal species545556. We consider that our new findings described in this paper provide a new opportunity to design and develop novel and more effective catalytic systems including not only the catalytic formation of ammonia from molecular dinitrogen (nitrogen fixation) but also other catalytic transformations of organic molecules by using dinitrogen-bridged dinuclear metal complexes as catalysts.…”

Metal–ligand cooperation by aromatization–dearomatization as a tool in single bond activation

“…We have reported a new mode of metal-ligand cooperation involving aromatizationdearomatization processes of pyridine-based pincer complexes derived from PNP and PNN ligands (scheme 2), as summarized in recent reviews [11][12][13][14][15], leading to unusual bond activation processes and to novel, environmentally benign catalysis, including hydrogenation reactions of polar double and triple bonds, and several acceptorless dehydrogenation reactions. Applications of acceptorless dehydrogenation reactions in synthesis were recently reviewed [16].…”

“…Pincer-type complexes of several transition metals exhibit MLC based on aromatizationdearomatization, including complexes of Ru, Fe, Co, Rh, Ir, Ni, Pd, Pt, Mn and Re, leading to facile activation of various chemical bonds. Upon deprotonation of pyridine-based pincer complexes at the methylenic group, dearomatization of the pyridine core takes place [11][12][13][14][15][17][18][19][20], as judged from crystal structures, which exhibit an exo-cyclic double bond. Activation of chemical bonds (Y-H = O-H, N-H, C-H, B-H, Si-H) by cooperation between the metal and the dearomatized ligand results in ligand aromatization, with no formal change in the oxidation state of the metal (scheme 2).…”