Since the first reports in the late 1970s on transition metal complexes contain- ing pincer-type ligands—named after the particular coordination mode of these ligands—these systems have at- tracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typical- ly, such a ligand comprises an anionic aryl ring which is ortho,ortho-disubsti- tuted with heteroatom substituents, for example, CH2NR2 ,C H 2PR2 or CH2SR, which generally coordinate to the met- al center, and therefore support the MC s bond. This commonly results in a terdentate and meridional coordina- tion mode consisting of two metalla- cycles which share the MC 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 under- standing 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 mecha- nistic elucidation of fundamental transformations (for example, CC bond activation), the construction of the first metallodendrimers for sustain- able 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 inves- tigations. This review discusses the synthetic methodologies that are cur- rently available for the preparation of platinum group metal complexes con- taining pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineer- ing of sensors, switches, and catalysts.

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors, Switches, and Catalysts

Catalysis by Palladium Pincer Complexes

Advances in the chemistry of high-oxidation-state copper and palladium demonstrate their great potential as catalysts. Homogeneous copper and palladium catalysts are used to synthesize a broad range of organic molecules, including pharmaceuticals, commodity chemicals and polymers. Improvements in the activity, selectivity and scope of these catalysts have the potential to increase their usefulness and reduce the environmental impact of the chemical reactions. In this Review, Amanda Hickman and Melanie Sanford summarize recent advances by organometallic chemists working with a subset of catalysts that operate through an intermediate containing palladium in the +3 or +4 oxidation state or copper in the +3 oxidation state. Copper and palladium catalysts are critically important in numerous commercial chemical processes. Improvements in the activity, selectivity and scope of these catalysts could drastically reduce the environmental impact, and increase the sustainability, of chemical reactions. One rapidly developing strategy for achieving these goals is to use ‘high-valent’ organometallic copper and palladium intermediates in catalysis. Here we describe recent advances involving both the fundamental chemistry and the applications of these high-valent metal complexes in numerous synthetically useful catalytic transformations.

High-valent organometallic copper and palladium in catalysis

ConspectusDirect C–H bond activation is an important reaction in synthetic organic chemistry. This methodology has the potential to simplify reactions by avoiding the use of prefunctionalized reagents. However, selectivity, especially site selectivity, remains challenging. Sequential reactions, in which different molecules or groups are combined in an ordered sequence, represent a powerful tool for the construction of complex molecules in a single operation.We have discovered and developed a synthetic methodology that combines selective C–H bond activation with sequential reactions. This procedure, which is now known as the “Catellani reaction”, enables the selective functionalization of both the ortho and ipso positions of aryl halides. The desired molecules are obtained with high selectivity from a pool of simple precursors. These molecules are assembled under the control of a palladacycle, which is formed through the joint action of a metal (Pd) and an olefin such as norbornene. These two species act c...

Pd/Norbornene: A Winning Combination for Selective Aromatic Functionalization via C-H Bond Activation.

Metal-mediated and metal-catalyzed reactions of isocyanides.

and the two isomers of complex 2.[*] Prof. Dr. J. Vicente, Prof. Dr. A. Arcas, M. Sc. F. Juli-HernndezGrupo de Qumica OrganometlicaDepartamento de Qumica Inorgnica, Facultad de QumicaUniversidad de Murcia, Aptdo. 4021, 30071 Murcia (Spain)E-mail: jvs1@um.esHomepage: http://www.um.es/gqo/Dr. D. BautistaSAI, Universidad de Murcia, Aptdo. 4021, 30071 Murcia (Spain)Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.201102214.

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Synthesis of a Palladium(IV) Complex by Oxidative Addition of an Aryl Halide to Palladium(II) and Its Use as Precatalyst in a CC Coupling Reaction

Aqua palladium complexes: synthesis, properties and applications

Mono- and di-nuclear complexes of ortho-palladated and -platinated 4,4′-dimethylazobenzene with bis(diphenylphosphino)methane. More data on transphobia

[Pd{CH2C(O)Me}Cl]n, a Key Product for the Synthesis of Acetonylpalladium(II) Complexes

The complex [Pd(O,N,C-L)(OAc)], in which L is a monoanionic pincer ligand derived from 2,6-diacetylpyridine, reacts with 2-iodobenzoic acid at room temperature to afford the very stable pair of Pd(IV) complexes (OC-6-54)- and (OC-6-26)-[Pd(O,N,C-L)(O,C-C(6)H(4)CO(2)-2)I] (1.5:1 molar ratio, at -55 °C). These complexes and the Pd(II) species [Pd(O,N,C-L)(OX)] and [Pd(O,N,C-L')(NCMe)]ClO(4), (X = MeC(O) or ClO(3), L' = another monoanionic pincer ligand derived from 2,6-diacetylpyridine), are precatalysts for the arylation of CH(2)=CHR (R = CO(2)Me, CO(2)Et, Ph) using IC(6)H(4)CO(2)H-2 and AgClO(4). These catalytic reactions have been studied and a tentative mechanism is proposed. The presence of two Pd(IV) complexes was detected by ESI(+)-MS during the catalytic process. All the data obtained strongly support a Pd(II)/Pd(IV) catalytic cycle.

Aurelia Arcas

Papers

Synthesis of a Palladium(IV) Complex by Oxidative Addition of an Aryl Halide to Palladium(II) and Its Use as Precatalyst in a CC Coupling Reaction

Aqua palladium complexes: synthesis, properties and applications

Mono- and di-nuclear complexes of ortho-palladated and -platinated 4,4′-dimethylazobenzene with bis(diphenylphosphino)methane. More data on transphobia

[Pd{CH2C(O)Me}Cl]n, a Key Product for the Synthesis of Acetonylpalladium(II) Complexes

Providing Support in Favor of the Existence of a PdII/PdIV Catalytic Cycle in a Heck-Type Reaction