Pincer Chemistry Research Articles

Current Frontiers in Pincer Chemistry

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.

Pincer Chemistry and Catalysis

This themed collection highlights the different and versatile applications of pincer metal compounds in modern chemistry, ranging from chemical biology to catalysis, including the activation of small molecules, diverse organic synthesis applications and electrochemistry. These works serve as an illustration of the richness and state of the art of current research in pincer metal chemistry and catalysis, amply demonstrating the abundant opportunities and future prospects of the field.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands.

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of MI(CO) and MIIICl2(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Rhodapentalenes: Pincer Complexes with Internal Aromaticity.

SummaryPincer complexes are a remarkably versatile family benefited from their stability, diversity, and tunability. Many of them contain aromatic organic rings at the periphery, and aromaticity plays an important role in their stability and properties, whereas their metallacyclic cores are not aromatic. Herein, we report rhodapentalenes, which can be viewed as pincer complexes in which the metallacyclic cores exhibit considerable aromatic character. Rhodapentalenes show good thermal stability, although the rhodium-carbon bonds in such compounds are fragile. Experimental and computational studies suggest that the stabilization of rigid CCC pincer architectures together with an intrinsic aromaticity is vital for these metallacyclic rhodium species. Dearomatization-aromatization reactions, corresponding to metal-ligand cooperation of classical aromatic pincer complexes, were observed in this system. These findings suggest a new concept for pincer chemistry, the internal aromaticity involving metal d-orbitals, which would be useful for exploiting the nature of construction motif and inspire further applications.

Extending the Application Scope of Organophosphorus(V) Compounds in Palladium(II) Pincer Chemistry

1-Dimethylthiocarbamoyloxy-3-diphenylphosphinobenzene was used as a key precursor for the synthesis of a whole series of organophosphorus(V) pincer ligands combining thiocarbamate donor group with ...

ChemInform Abstract: Ruthenium Pincer Complexes: Ligand Design and Complex Synthesis

AbstractReview: 270 refs.

Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (CF3PCP)Os(cod)(H)

Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (^CF₃PCP)Os(cod)(H)

ChemInform Abstract: Cleavage of Unreactive Bonds with Pincer Metal Complexes

AbstractReview: 91 refs.

SYNTHESIS AND CATALYTIC APPLICATIONS OF IRON PINCER COMPLEXES

Transition metal complexes bearing pincer ligands have been extensively studied during the past decade. One of the less-explored or perhaps even neglected areas of pincer chemistry is on the reactivity of iron complexes that incorporate these enormously popular ligand sets. Meanwhile, recent emphasis on homogeneous catalysis has been placed on the developments of catalytic reactions promoted by inexpensive and environmentally benign metals, for which iron is particularly attractive. In that regard, investigating the reactivity patterns of well-defined iron pincer complexes can potentially provide valuable guidelines for the rational design of iron catalysis. This review analyzes the challenges and successes of synthesizing iron complexes with different pincer-type ligands, and discusses the established or potential utility of these complexes for catalytic reactions.

Cleavage of unreactive bonds with pincer metal complexes

Since the first reports some three decades ago, the chemistry of pincer metal complexes has seen a tremendous development with impact on materials chemistry, supramolecular chemistry, bioorganometallics, and, presumably most significantly, on (catalytic) bond making and breaking processes. The remarkable progress is due to a large extent to the well-defined nature and tunability of the pincer ligand which allows the reactivity of the metal center to be modified and eventually tailored to specific needs. This Perspective summarizes the achievements in employing pincer complexes for mediating and catalyzing the cleavage of typically unreactive bonds such as C-H, C-C, C-E, and E-H bonds, arguably one of the most spectacular applications of pincer chemistry.

Pincer oxazolines: emerging tools in coordination chemistry and catalysis – where to next?

A brief overview of the coordination chemistry aspects of pincer ligands and their complexes containing at least one oxazoline (i.e., 4,5-dihydro-2-oxazole) unit is presented. This historical perspective is placed into a context of the possible future direction(s) in this particular arena of pincer chemistry. These ideas are compared and contrasted to the overall direction of pincer chemistry since the first such complexes were reported in the 1970s.

Linear homobimetallic 4-thioacetyl-substituted NCN pincer palladium(II) and platinum(II) complexes with N-bidentate connecting units (NCN = [C 6H 2(CH 2NMe 2) 2-2,6-R-4] −)

The synthesis and characterization of homobimetallic palladium and platinum complexes of type [(Me(O)CS-4-NCN–M ← N ∩N → M–NCN-4-SC(O)Me](OTf) 2 (Me(O)CS-4-NCN = [C 6H 2(CH 2NMe 2) 2-2,6-SC(O)Me-4] −; N ∩N = 4,4′-bipyridine (bipy); M = Pd, 12; M = Pt, 13) is reported. The required bifunctional thio-acetyl NCN pincer starting compound NC(Br)N-4-SC(O)Me ( 2) has been synthesized by the consecutive reactions of NC(Br)N–I (I-1-C 6H 2(CH 2NMe 2) 2-3,5-Br-4) ( 1) with t BuLi, S 8 and Me(O)CCl, respectively. Chemoselective metallation at the C aryl–Br bond was achieved by the reaction of 2 with the palladium(0) source [Pd 2(dba) 3] ( 3) (dba = dibenzylidene acetone). Treatment of thus formed [Pd(NCN-4-SC(O)Me)(Br)] ( 4) with [AgOTf] ( 8) (OTf = triflate, OSO 2CF 3) gave [Pd(NCN-4-SC(O)Me)(H 2O)][OTf] ( 9) which was further reacted with 0.5 equiv. of 4,4′-bipyridine ( 11a) to afford rigid-rod structured 12. When [Pt(tol) 2(SEt 2)] 2 ( 5) (tol = 4-tolyl) was used instead of 3, then 13 was produced via the in situ formation of [PtBr(NCN-4-SC(O)Me)] ( 7) and [Pt(NCN-4-SC(O)Me)(H 2O)][OTf] ( 10). Another possibility to synthesize 7 relied upon the subsequent reaction of 1 with 0.5 equiv. of 5 to give [PtBr(NCN-4-I)] ( 6) which further reacted with t BuLi, 1/8 S 8 and Me(O)CCl to afford 7. The cyclic voltammograms of 2, 7, and 13 are discussed. Complex 7 was structurally characterized by single crystal X-ray crystallography. Organometallic 7 crystallizes with three independent molecules in the asymmetric unit and displays a monomeric structure as commonly encountered in d 8-metal pincer chemistry.

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts In memory of Prof. Dr. Luigi M. Venanzi and his pioneering work in organometallic chemistry, particularly in PCP pincer chemistry.

Since the first reports in the late 1970s on transition metal complexes containing pincer-type ligands-named after the particular coordination mode of these ligands-these systems have attracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typically, such a ligand comprises an anionic aryl ring which is ortho,ortho-disubstituted with heteroatom substituents, for example, CH(2)NR(2), CH(2)PR(2) or CH(2)SR, which generally coordinate to the metal center, and therefore support the M-C sigma bond. This commonly results in a terdentate and meridional coordination mode consisting of two metallacycles which share the M-C bond. Detailed studies of the formation and the properties of a large variety of pincers containing platinum group metal complexes have provided direct access to both a fundamental understanding of a variety of reactions in organometallic chemistry and to a range of new applications of these complexes. The discovery of alkane dehydrogenation catalysts, the mechanistic elucidation of fundamental transformations (for example, C-C bond activation), the construction of the first metallodendrimers for sustainable homogeneous catalysis, and the engineering of crystalline switches for materials processing represent only a few of the many highlights which have emanated from these numerous investigations. This review discusses the synthetic methodologies that are currently available for the preparation of platinum group metal complexes containing pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineering of sensors, switches, and catalysts.