Gerard van Koten

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

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, CH2 NR2 , CH2 PR2 or CH2 SR, which generally coordinate to the metal center, and therefore support the M-C σ 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.

Fluorous phase separation techniques in catalysis

Fluorous solvents have limited miscibility with conventional organic solvents. Combined with the fact that compounds functionalized with perfluorinated groups often dissolve preferentially in fluorous solvents this can be used to extract fluorous components from reaction mixtures. This review discusses the application and potential of fluorous phase separation techniques for the recovery of soluble metal catalysts.

Ligand-Free Copper-Catalyzed C−S Coupling of Aryl Iodides and Thiols

A protocol for the copper-catalyzed aryl-sulfur bond formation between aryl iodides and thiophenols is reported. The reaction is catalyzed by a low amount (1-2.5 mol %) of readily available and ligand-free copper iodide salt. A variety of diaryl thioethers are synthesized under relatively mild reaction conditions with good chemoselectivity and functional group tolerance.

Homogeneous vanadium-based catalysts for the Ziegler–Natta polymerization of α-olefins

Although their activity is often inferior to that of other systems, the use of vanadium-based catalysts in homogeneous Ziegler–Natta polymerizations allows the preparation of high-molecular-weight polymers with narrow molecular-weight distributions, ethene/α-olefin copolymers with high α-olefin incorporation, and syndiotactic polypropene. The main reason for the low activity of these catalysts is their deactivation during catalysis by reduction of active vanadium species to low-valent, less active or inactive species. We here present an up-to-date review of this area with particular emphasis on the attempts to improve catalyst performance and stability by the use of additives or ancillary ligands.

Metallasupramolecular architectures, an overview of functional properties and applications

The synthesis of metallasupramolecular architectures, such as two-dimensional squares, triangles and polygons, and three-dimensional cages and polyhedra, has attracted much interest in the past decade. These structures are designed to have novel specific shapes and dimensions with interesting functional properties. In this overview the functional properties of metallasupramolecular architectures are highlighted with emphasis on potential applications such as catalysis, cavity-directed synthesis and sensing, that can be performed with these materials.

Organometallic Pincer Chemistry

Gerard van Koten: The Mono-anionic ECE-Pincer Ligand - a Versatile Privileged Ligand Platform: General Considerations.- Elena Poverenov, David Milstein: Non-Innocent Behavior of PCP and PCN Pincer Ligands of Late Metal Complexes.- Dean M. Roddick: Tuning of PCP Pincer Ligand Electronic and Steric Properties.- Gemma R. Freeman, J. A. Gareth Williams: Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence.- Davit Zargarian, Annie Castonguay, Denis M. Spasyuk: ECE-Type Pincer Complexes of Nickel.- Roman Jambor and Libor Dostal: The Chemistry of Pincer Complexes of 13 - 15 Main Group Elements.- Kalman J. Szabo: Pincer Complexes as Catalysts in Organic Chemistry.- Jun-ichi Ito and Hisao Nishiyama: Optically Active Bis(oxazolinyl)phenyl Metal Complexes as Multi-potent Catalysts.- Anthony St. John, Karen I. Goldberg, and D. Michael Heinekey: Pincer Complexes as Catalysts for Amine Borane Dehydrogenation.- Dmitri Gelman and Ronit Romm: PC(sp3)P Transition Metal Pincer Complexes: Properties and Catalytic Applications.- Jennifer Hawk and Steve Craig: Physical Applications of Pincer Complexes.

Hexacationic Dendriphos ligands in the Pd-catalyzed Suzuki-Miyaura cross-coupling reaction: scope and mechanistic studies.

The combination of Pd(2)dba(3) x CHCl(3) and hexacationic triarylphosphine-based Dendriphos ligands (1-3) leads to a highly active catalytic system in the Suzuki-Miyaura cross-coupling reaction. Under relatively mild reaction conditions, nonactivated aryl bromides and activated aryl chlorides can be coupled at a low Pd loading (0.1 mol %). The observed activity of this catalytic system, in particular in coupling reactions of aryl chlorides, is dramatically higher than that of conventional Pd catalysts employing triarylphosphine ligands. Through control and poisoning experiments, it is concluded that a homogeneous Pd(0)-Dendriphos complex is the active species in this catalytic system. Despite their triarylphosphine-based structure, Dendriphos ligands behave as very bulky phosphine ligands and lead to a preferential formation of coordinatively unsaturated and catalytically active Pd(0) species, which explains the observed high catalytic activity for these systems. The presence of six permanent cationic charges in the backbone of this class of ligands is proposed to result in a significant interligand Coulombic repulsion and plays a crucial role in their bulky behavior. In the coupling reactions of activated aryl chlorides, a positive dendritic kinetic effect was observed among the different Dendriphos generations, indicating an increased ability of the higher ligand generations to stabilize the active species due to steric effects. For aryl bromides, no dendritic effect was observed due to a shift in the rate-determining step in the catalytic cycle, from oxidative addition for aryl chlorides to transmetalation for aryl bromides.

Selective hydrovinylation of styrene in a membrane reactor : use of carbosilane dendrimers with hemilabile P,O ligands

A codimerization of styrene and ethene can be carried out continuously in a nanofiltration membrane reactor with dendritic Pd complexes such as 1. The selectivity of the reaction is increased considerably under continuous conditions. The activity and selectivity of monomeric model complexes and the dendritic catalysts were compared in batch reactions.

Recent Findings in Cyclometallation of meta-Substituted Aryl Ligands by Platinum Group Metal Complexes by Caryl–R Bond Activation (R=H, CR3, SiR3)

Cyclometallation of aryl ligands with two donor substituents in the meta-positions occurs by Caryl - R bond activation and is facilitated by intramolec- ular coordination of these donor substituents. This procedure has been used for the preparation of a variety of platinum group metal organometallic complexes with interesting catalytic and electronic properties. Some recent findings are detailed.

CHIRAL ARENETHIOLATOCOPPER(I) CATALYZED SUBSTITUTION REACTIONS OF ACYCLIC ALLYLIC SUBSTRATES WITH GRIGNARD REAGENTS

Abstract Asymmetric induction can be achieved in the γ-selective substitution reaction of allylic substrates (R′CHCHCH 2 Y) with n -BuMgI catalyzed by the chiral arenethiolatocopper(I) complex 1b . It was found that the enantiomeric excess of the γ-substituted product (R′CH( n -Bu)CHCH 2 ) is influenced by the coordinating ability of the leaving group Y, and e.e.s of up to 42% (R′ = Cy, Y = OAc) have been obtained.