Current Frontiers in Pincer Chemistry

Abstract

Ever since the introduction of the archetypal (bisphosphinomethyl)phenyl PCP pincer ligand by Shaw in 1976, the use of this class of tridentate ligands with a central phenyl moiety has found widespread utility in coordination chemistry and homogeneous catalysis. Initially, van Koten's definition of pincer ligands, which was inspired by the claws of the crab, only included rigid, meridionally binding, monoanionic ligands. The rapid growth in popularity of pincer chemistry in the 1990s can be associated with the modular tunability of the steric and electronic properties of these ligands, which can give pincer supported complexes enhanced chemical stability and well-defined reactivity. Consequently, the application of pincer supported complexes has had a significant impact on the advancement of key areas in small molecule activation and catalysis, such as C–H activation, olefin polymerization and cross-coupling. As of today, the term pincer ligand is used more broadly and now comprises a wide range of more or less flexible, neutral and charged, tridentate ligands with a large variety of central and flanking donor groups. While this development means that there is no longer a clear definition of a pincer ligand, the broadening of the field has allowed for the combination of new, chemically active functional groups with the beneficial structural properties of pincer ligands. Accordingly, the utility of pincer ligands was enhanced by the introduction of functional pincer ligands, which adopt active roles in chemical transformations, such as reversible electron and/or proton transfer steps, and extend pincer ligands beyond traditional inert auxiliary ligands. This conceptual advance has contributed to recent progress in many fields, like the renaissance of base metal catalysis, the use of non-innocent, redox active and cooperating ligands in catalysis, or the development of acceptorless de-/hydrogenation strategies for organic synthesis. This themed issue of the Zeitschrift für Anorganische und Allgemeine Chemie dedicated to Pincer Chemistry addresses three current focal areas in the field with contributions from leading groups in Asia, America, and Europe: (a) The synthesis of new pincer-type ligands, (b) the electronic structures of pincer supported complexes, and (c) the application of pincer ligated complexes in chemical transformations and catalysis. The work from Caulton et al., Huang et al., Langer and Vogt et al., Nishibayashi et al., Ozerov et al. and Walter et al. focuses on the development of synthetic methodologies towards new pincer platforms with respect to the central donor site, the connecting backbone, and the flanking “wings” of the ligands. These studies offer insight into the tunability of donor strength, rigidity and steric properties in pincer ligands. Contributions from Anderson et al., Kirchner et al., Milsmann et al. and Ray and Swart et al. disclose detailed spectroscopic and computational studies of iron and copper pincer complexes. Particular focus lies on their electronic structures, redox state assignments and control of accessible spin states and reactivity. Guan et al., Kuwata et al., Morris et al., Jones and Nachtigall et al. and Schneider et al. examined the proton/electron transfer model chemistry of complexes with proton responsive and redox active pincer ligands. These contributions are complemented by a systematic computational study from Pidko et al. that evaluates the effects of pincer substitution on metal-ligand cooperativity. Finally, Goswami et al. utilize such functional pincer ligands as hydrogen relays in transfer hydrogenation catalysis. These contributions reflect the current frontiers and the state-of-the-art in Pincer Chemistry and intend to stimulate the use pincer ligands in coordination chemistry and catalysis.