Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

Abstract

Aminodiphosphines and their monoanionic amido derivatives are among the most widely used tridentate ligands. A good share of these ligands, in particular the ones that are based on a rigid N‐heterocyclic core, is predisposed to coordinate in a meridional fashion. Negatively charged derivatives of these meridionally coordinating N‐heterocyclic ligands satisfy all the criteria for PNP pincer scaffolds. Herein, these ligands and their coordination chemistry is reviewed from the perspective of coordination chemistry. For our discussion, ligands containing a protonated N‐heterocycle (e.g. pyrrole‐ or carbazole‐based PNP scaffolds) are to be distinguished from their counterparts that do not contain such an NH‐functionality (e.g. pyridine‐ or triazine‐based PNP scaffolds). Different synthetic methodologies for the preparation of PNP pincer complexes are presented and shown to often depend on the presence or absence of the aforementioned NH‐proton. The different reactivity patterns of the resulting complexes are sub‐divided into two categories, namely into metal‐centered reactions and reactions that result in a chemical transformation at the metal and the ligand. The latter represent ligand‐cooperative transformations, which often play a key role in catalysis, and are showcased by discussing selected applications in catalysis. It will become evident in this review that this relatively young research field is still emerging and far from reaching maturity. Finally, a brief overview on ligand/metal combinations that have not been explored to date is provided, to stimulate future research on PNP pincers featuring a central N‐heterocycle.

Pyrrole‐based pincer ligands containing iminophosphorane moieties and their coordination chemistry with group 1 metals and magnesium

“…Because alkali metal salts are traditionally employed for salt metathesis reaction of pincer ligands, [6,17] we were curious to explore the coordination chemistry of 1 towards various alkali metals. Like ( t Bu PNP)H, [17a,b] the iminophosphorane 1 can be deprotonated by alkali metal silylamides such as [Li(N(SiMe 3 ) 2 (OEt 2 )] and [M(N(SiMe 3 ) 2 ] (M=Na, K, Cs) in toluene solutions to yield the corresponding alkali metal compounds 1‐M (M=Li, Na, K, Cs) as colorless solids in excellent yields (Scheme 4).…”

“…However, as part of the pincer‐ligand evolution, other donor functionalities were introduced, such as those based on heteroatoms. Examples of traditional N‐donor‐based “PNP” systems based on disilylamido [4] or diphenylamido [5] backbones are shown in Scheme 1, but more recently pincer ligands containing N‐heterocycles, such as pyridine and pyrroles, have been introduced to extend the class of “PNP” ligands [6] …”

“…Examples of traditional N-donor-based "PNP" systems based on disilylamido [4] or diphenylamido [5] backbones are shown in Scheme 1, but more recently pincer ligands containing N-heterocycles, such as pyridine and pyrroles, have been introduced to extend the class of "PNP" ligands. [6] As well as changes in the pincer backbone, the tethers and side-arm donors have also undergone several modifications over the years. [3] An interesting development in this context involves the further functionalization of the P-donor functionality to yield iminophosphoranes.…”

Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

“…Despite the efficacy and selectivity of these catalysts across a variety of substrates, recent work, particularly in the area of homogeneous catalysis, has seen a shift in focus to more earth-abundant transition metal systems. Several first-row transition metals have shown great promise as homogeneous hydrogenation catalysts with activities rivaling their precious metal counterparts in several instances. − The ligand frameworks used to support these systems vary, but pincers have emerged as an especially effective class of ligands for the creation of well-defined catalysts. , Use of pincer ligands among the mid 3 d metals (Mn, Fe, and Co) has permitted detailed investigations into the mechanisms of catalytic hydrogenations, revealing pathways unique from those of precious metal systems. − …”

Catalytic Conversion of Nitriles by Metal Pincer Complexes

“…In 1976, Moulton and Shaw were the first to report organometallic complexes based on pincer ligands [1]. Since then, pincer complexes have received increasing attention due to their unique stability, selectivity and often unusual reactivity [2][3][4][5][6][7][8][9][10][11][12]. Taking advantage of these properties, efficient methodologies have been developed for the activation of nitriles that are often tolerant of many other functional groups.…”