The N?H Functional Group in Organometallic Catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst–substrate, ligand–ligand, ligand–catalyst, and catalyst–catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.     

Ligand‐Directed Divergent Synthesis of Carbo‐ and Heterocyclic Ring Systems

Chemical tools that enable a catalytic reaction to selectively and efficiently yield different products will allow charting of wider chemical space. In ligand‐directed divergent synthesis, a common mode of catalysis is modulated by employing different ligands for catalytic organometallic complexes to transform either common substrates or common reactive intermediates into distinct molecular scaffolds. The strategy has the potential to create important and diverse scaffolds and to unveil novel modes of catalytic transformations for wider synthetic applications. This strategy is described and recent efforts in this emerging field of catalysis, focusing on transition‐metal catalysis for the synthesis of carbo‐ and heterocyclic ring systems, are reviewed.     

A Quantitative Approach of the Interaction between Metal Triflates and Organic Ligands Using Electrospray Mass Spectrometry

The interaction between two Lewis ??superacid?? catalysts Zn(OTf)₂ and In(OTf)₃ and series of amide and phosphate ligands is quantitatively characterized by electrospray ionization mass spectrometry (ESI-MS). A specific feature of the ESI-MS spectra of the mixture of metal triflates and Lewis bases is the formation of ionic adducts resulting from the displacement of one triflate anion by two neutral ligands. A ligand competition model is developed, which describes the relative intensities of the ionic adducts as a function of relative ligand concentrations. The relative affinities deduced from the ligand competition method are combined in an affinity scale for the metal triflate.     

Biometallorganische Chemie: Ein faszinierendes Forschungsgebiet

Bioorganometallic Chemistry is a new research area in which organometallic compounds are coupled with biomolecules (sugars, peptides, DNA and its constituents, steroides, vitamines, enzymes). In a narrow sense such organometallic complexes occur in nature (vitamin B₁₂), have a biological function (e.g. nickel enzymes in bacterias) or are of potential medical use (as novel drug or as marker for biomolecules). In a wider sense bioorganometallic chemistry includes simply metal complexes which besides organometallic ligands (e.g. CO, π‐hydrocarbon) have a biomolecule as ligand (e.g. with interesting structures, for catalysis).     

The Measure of All Rings—N‐Heterocyclic Carbenes

Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N‐Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition‐metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.     

Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts.By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).     

Kinetics and mechanisms of substitution at paramagnetic metal centers in organometallic complexes

The mechanism of ligand substitution in 17- and 19-electron organometallic radicals is discussed. These species substitute ligands by an associative process some 10⁶ to 10¹⁰ faster than analogous 18-electron complexes. When 17-electron species can be generated by bond homolysis or electron transfer reactions of 18-electron complexes, they can act as intermediates in radical chain reactions of 18-electron complexes. A 17–19 electron rule is proposed to explain transformations of organometallic radicals just as the 16–18 electron rule finds use for closed shell organometallic complexes. The origin of this rule is the favorable two-center three-electron bond that can form when an odd electron in a sterically accessible metal d-orbital interacts with an electron pair on an entering nucleophile. Besides simple substitution, these radicals can disproportionate, dimerize, and undergo insertion or atom abstraction reactions.     

Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.     

Recent advances in oxidation catalysis using ionic liquids as solvents

Ionic liquids are an interesting alternative to classical solvents presenting several advantages. A variety of catalysts show good or even enhanced activities when applied in ionic liquids. Oxidation catalysis represents a large segment of industrial chemistry, providing both bulk chemicals and intermediates for high value added special products and pharmaceuticals. Particularly for the latter products organometallic and inorganic catalysts are being developed that can be applied in systems consisting of or containing ionic liquids. This work provides an overview on recent developments in this field.     

N‐Heterocyclic Carbenes: A New Concept in Organometallic Catalysis

N‐Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N‐heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C−H activation, C−C, C−H, C−O, and C−N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N‐heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.     

Steuerliganden für Katalysatoren

Ever since the isolation of the first free N‐heterocyclic carbene more than 20 years ago, this ligand class has become essential in modern chemistry. This development is decisively owed to the stability of the metal‐carbon bond, the smooth fine‐tuning of ligand properties, and the facile synthetic access to such complexes. Acting as steering ligands, they are integral parts of many organometallic compounds that currently push the frontiers of chemistry in many areas, for example the synthesis and application of immobilized, water soluble, or asymmetric systems that act as efficient catalysts in organic transformations. Additionally, metal NHC complexes find promising applications in fields beyond catalysis, such as medicine, optics, and material science.     

Deconstructing Covalent Organocatalysis

Modern organocatalysis has rapidly evolved into an essential component of contemporary organic synthesis. One of the most distinctive aspects of organocatalytic processes is the biomimetic nature in which the catalyst engages the substrate, often forming covalently bound intermediates in a manner reminiscent of enzyme catalysis. Indeed, the process of intramolecularization is often accompanied by a conformational change of the catalyst scaffold, further accentuating this analogy with biological systems. The isolation and study of these catalytic intermediates facilitate the rapid generation of conformation and reactivity profiles to assist in organocatalytic reaction development and/or clarify reaction outcomes. Emulating the formative advances that have derived from studying reaction intermediates in mechanistic organometallic and enzymatic catalysis, the deconstruction of covalently bound organocatalysis intermediates is gaining momentum as a design strategy.     

On the mechanism of the palladium(II)-catalyzed decarboxylative olefination of arene carboxylic acids. Crystallographic characterization of non-phosphine palladium(II) intermediates and observation of their stepwise transformation in Heck-like processes

Mechanistic studies of a palladium-mediated decarboxylative olefination of arene carboxylic acids are presented, providing spectroscopic and, in two instances, crystallographic evidence for intermediates in a proposed stepwise process. Sequentially, the proposed pathway involves carboxyl exchange between palladium(II) bis(trifluoroacetate) and an arene carboxylic acid substrate, rate-determining decarboxylation to form an arylpalladium(II) trifluoroacetate intermediate (containing two trans-disposed S-bound dimethyl sulfoxide ligands in a crystallographically characterized form), then olefin insertion and beta-hydride elimination. Because of the unique mode of generation of the arylpalladium(II) trifluoroacetate intermediate, a species believed to be substantially electron-deficient relative to phosphine-containing arylpalladium(II) complexes previously studied, it has been possible to gain new insights into those steps that are common to the Heck reaction, namely, olefin insertion and beta-hydride elimination. The present results show that there are notable differences in reactivity between arylpalladium(II) intermediates generated by decarboxylative palladation and those produced in conventional Heck reactions. Specifically, we have found that more electron-rich alkenes react preferentially with an arylpalladium(II) trifluoroacetate intermediate formed by decarboxylative palladation, whereas an opposite trend is found in conventional Heck reactions. In addition, we have found that the aralkylpalladium(II) trifluoroacetate intermediates that are formed upon olefin insertion in the present study are stabilized with respect to beta-hydride elimination as compared to the corresponding phosphine-ligated aralkylpalladium(II) complexes. We have also crystallographically characterized an aralkylpalladium(II) trifluoroacetate intermediate derived from arylpalladium(II) insertion into norbornene, and this structure, too, contains an S-bound dimethyl sulfoxide ligand; the ipso-carbon of the transferred aryl group and trifluoroacetate function as the third and fourth ligands in the observed distorted square-planar palladium(II) complex.     

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki–Heck Reaction

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M–NHC binding in this metal–ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M−NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal–NHC binding and [PdX₃]⁻[NHC-R]⁺ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]⁺ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph–NHC coupling or reversible H–NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.     

Characterization of reaction intermediates by ion spectroscopy

In the last decade, we have experienced massive progress in spectroscopic methods for mass-selected ions. The aim of this tutorial review is to present action spectroscopy as a powerful tool for the investigation of ionic reaction intermediates. Examples span from ultraviolet and infrared photodissociation spectroscopy of model reaction intermediates to applications of infrared multiphoton dissociation spectroscopy (IRMPD) to intermediates directly sampled from reaction mixtures. The first example of double resonance IR-UV spectroscopy of model intermediates in an organometallic reaction is also mentioned.     

Functional ligands and complexes for new structures, homogeneous catalysts and nanomaterials

In this account, we focus on results from our laboratory to illustrate recent developments in various fields of organometallic chemistry. Studies on hemilabile P,N donor ligands and on the ion-pair behaviour of cationic Pd(II) complexes have led to the full characterization of complexes with η¹-allyl ligands. This still rare bonding mode for the allyl ligand in palladium chemistry allows facile insertion of CO into the Pd-C σ-bond, in contrast to the situation in related η³-allyl Pd(II) complexes. In order to develop new homogeneous catalysts for the selective dimerization and oligomerization of ethylene, a range of Ni(II) complexes have been prepared with new chelating P,N ligands where P represents a phosphine, phosphinite or phosphonite donor group and N a pyridine or oxazoline moiety. Finally, we shall examine bottom-up approaches to the formation of new nanomaterials of magnetic or catalytic interest by covalent anchoring of metal complexes and clusters into mesoporous materials using functional phosphine or alkyne ligands containing an alkoxysilyl group.     

Amide‐Substituted Titanocenes in Hydrogen‐Atom Transfer Catalysis

Two new catalytic systems for hydrogen‐atom transfer (HAT) catalysis involving the N?H bonds of titanocene(III) complexes with pendant amide ligands are reported. In a monometallic system, a bifunctional catalyst for radical generation and reduction through HAT catalysis depending on the coordination of the amide ligand is employed. The pendant amide ligand is used to activate Crabtree's catalyst to yield an efficient bimetallic system for radical generation and HAT catalysis.     

Ruthenium catalysts bearing N-heterocyclic carbene ligands in atom transfer radical reactions

The catalytic activity of ruthenium-p-cymene complexes bearing N-heterocyclic carbene ligands in atom transfer radical addition (ATRA) or polymerisation (ATRP) strongly depends on the substituents of the carbene ligand, thereby providing a nice illustration of the importance of organometallic engineering and ligand fine tuning in homogeneous catalysis.     

Chiral monophosphines as ligands for asymmetric organometallic catalysis

Chelating chiral diphosphines are often used as ligands of organometallic complexes. However monophosphines, or more generally ligands with one phosphorus linked to one or several heteroatom, may also be useful. This review gives the main results obtained in that area, by considering the classes of monodentate chiral ligands bearing one P(III) atom and involved in asymmetric catalysis with organometallic complexes.     

金属有机配合物的非线性光学特性

从过渡金属有机配合物的中心金属、配体和几何结构的多样性、多变的金属氧化态和金属与配体的电子供－受作用评述了金属有机配合物的二阶、三阶非线性光学效应的分子结构特征和最新进展。参考文献２６篇。