Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

Abstract

The combination of NHC (NHC = N-heterocyclic carbene) and pincer framework has received extensive attention. The relevant transition metal complexes have found manifold applications in various areas spanning from catalysis to optical material chemistry. This minireview focuses on the advances in the synthesis, reactivity study and applications of metal pincer NHCs.

Direct Access to Palladium(II) Complexes Based on AnionicC,C,C-Phosphonium Ylide Core Pincer Ligand

“…Following this pioneering report, a wide variety of pincer architectures differing in the nature of the backbone, the coordinating extremities, and the metallacycle ring size were designed . Thanks to the unique meridional geometry of these robust and constrained metallic systems, unusual bonding modes and peculiar reactivities were observed, resulting in a large number of applications, especially in the field of homogeneous catalysis . In line with our continued efforts to design ligands that push the limits of the electronic parameter scale, whether extremely electron-poor or electron-rich, we have recently focused on the preparation of pincer ligands exhibiting only carbon-based donor extremities.…”

Influence of the Flexibility of Nickel PCP‐Pincer Complexes on C−H and P−C Bond Activation and Ethylene Reactivity: A Combined Experimental and Theoretical Investigation

“…The general relevance of NHC chemistry was further highlighted in 2018 by a special issue of Chemical Reviews [19] . The incorporation of NHCs into multidentate, hybrid ligands provides increased chemical diversity and can lead to particularly robust catalysts [20–36] . Since their inception in the 1970s, pincer‐type ligands, defined as any tridentate ligand that binds to a metal centre in a meridional fashion, have become ubiquitous in modern molecular chemistry, largely because of the kinetic and thermodynamic stabilization they confer to their metal complexes.…”

“…[19] The incorporation of NHCs into multidentate, hybrid ligands provides increased chemical diversity and can lead to particularly robust catalysts. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] Since their inception in the 1970s, pincer-type ligands, defined as any tridentate ligand that binds to a metal centre in a meridional fashion, have become ubiquitous in modern molecular chemistry, largely because of the kinetic and thermodynamic stabilization they confer to their metal complexes. Incorporating NHC donor(s) in pincer-type architectures has rapidly attracted considerable interest and the huge diversity and relatively easy functionalization of pincer ligands facilitates the fine-tuning of their stereoelectronic features and, consequently, of the reactivity and catalytic applications of their metal complexes.…”

Ring-Opening Polymerization of ε-Caprolactone Utilizing Aluminum Alkyl Complexes Bearing Dianionic Scorpionate Ligands

“…Scorpionate ligands, on the other hand, were first developed in the late 1960s by Trofimenko, and include species like tris­(pyrazolyl)­borate , and tris­(pyrazolyl)­methane (Figure ). The defining features of these ligands are a fac coordination geometry and the ability to interchange between tridentate and bidentate binding modes. , This hemilability can lead to higher reactivity levels over other tridentate ligands, such as pincer ligands, which tend to form very stable complexes. , Moreover, scorpionate ligands are extremely versatile in that their bonding strengths, electronic parameters, and steric encumbrance can be readily tuned. Since their development, complexes incorporating these types of ligands have been widely studied for medical purposes, catalysis, as well as other applications. ,, Surprisingly, despite their versatility, to the best of our knowledge, only a handful of scorpionate ligands have been explored on aluminum centers. − Martínez et al investigated a family of aluminum scorpionate complexes for the catalytic synthesis of cyclic carbonates from epoxides and carbon dioxide. ,− Additionally, aluminum scorpionate complexes have been utilized by the Otero group, and others, to affect the ROP of lactide and ε-CL. ,− By increasing the steric bulk of the tridentate ligands, more control over polymerization was seen and products with narrower PDIs were generated .…”

“…58,59 This hemilability can lead to higher reactivity levels over other tridentate ligands, such as pincer ligands, which tend to form very stable complexes. 60,61 Moreover, scorpionate ligands are extremely versatile in that their bonding strengths, electronic parameters, and steric encumbrance can be readily tuned. Since their development, complexes incorporating these types of ligands have been widely studied for medical purposes, 62 catalysis, 63 as well as other applications.…”