Combined Homogeneous and Heterogeneous Catalysts. Rhodium and Platinum Isocyanide Complexes Tethered on Silica-Supported Metal Heterogeneous Catalysts: Arene and Cyclohexanone Hydrogenation †
TL;DR: In this paper, the TCSM (tethered complex on supported metal) catalysts were used to catalyze the hydrogenation of arenes (Rh−CNR2/Pd−SiO2 and rhCl[CN(CH2)3Si(OC2H5)3]2) under mild conditions of 40 °C and 1 atm.
About: This article is published in Organometallics.The article was published on 1999-02-26. It has received 59 citations till now. The article focuses on the topics: Rhodium & Catalysis.
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TL;DR: In this article, the authors consider cases in which a discrete transition-metal complex is used as a precatalyst for reductive catalysis and focus on the problem of determining if the true catalyst is a metal-complex homogeneous catalyst or if it is a soluble or other metal-particle heterogeneous catalyst.
Abstract: This review considers cases in which a discrete transition-metal complex is used as a precatalyst for reductive catalysis; it focuses on the problem of determining if the true catalyst is a metal-complex homogeneous catalyst or if it is a soluble or other metal-particle heterogeneous catalyst. The various experiments that have been used to distinguish homogeneous and heterogeneous catalysis are outlined and critiqued. A more general method for making this distinction is then discussed. Next, the circumstances that make heterogeneous catalysis probable, and the telltale signs that a heterogeneous catalyst has formed, are outlined. Finally, catalytic systems requiring further study to determine if they are homogeneous or heterogeneous are listed. The major findings of this review are: (i) the in situ reduction of transition-metal complexes to form soluble-metal-particle heterogeneous catalysts is common; (ii) the formation of such a catalyst is easy to miss because colloidal solutions often appear homogeneous to the naked eye; (iii) a variety of experiments have been used to distinguish homogeneous catalysis from heterogeneous catalysis, but there is no single definitive experiment for making this distinction; (iv) experiments that provide kinetic information are key to the correct identification of the true catalyst; and (v) a more general approach for distinguishing homogeneous catalysis from heterogeneous catalysis has been developed. Additionally, (vi) the conditions under which a heterogeneous catalyst is likely to form include: (a) when easily reduced transition-metal complexes are used as precatalysts; (b) when forcing reaction conditions are employed; (c) when nanocluster stabilizers are present; and (d) when monocyclic arene hydrogenation is observed. Finally, (vii) the telltale signs of heterogeneous catalysis include the formation of dark reaction solutions, metallic precipitates, and the observation of induction periods and sigmoidal kinetics.
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TL;DR: In this article, various approaches towards the immobilization of molecular homogeneous catalysts are introduced, focusing on catalysts where an organic molecule is attached to the surface of an inorganic support material via a covalent bond forming the so-called hybrid organic-inorganic catalysts.
Abstract: Various approaches towards the immobilization of molecular homogeneous catalysts are introduced, focusing on catalysts where an organic molecule is attached to the surface of an inorganic support material via a covalent bond forming the so-called hybrid organic–inorganic catalysts. The application of this new class of catalysts in a wide variety of organic reactions is reviewed.
213 citations
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...Conceptual illustration of TCSM catalysts (116)....
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TL;DR: In this article, the mechanism for arene hydrogenation is described and extended to include polynuclear (cluster, colloidal and nanoparticulate) catalysts, and various experiments used to determine whether catalysis is by homogeneous or heterogeneous species are assessed.
Abstract: A few years ago the concept of homogeneous arene hydrogenation catalysis was called into question, since many catalysts originally assigned as homogeneous have subsequently been shown to be precursors to heterogeneous (colloidal or nanoparticulate) catalysts. In this Perspective, the mechanism for arene hydrogenation is described and extended to include polynuclear (cluster, colloidal and nanoparticulate) catalysts. The various experiments used to determine whether catalysis is by homogeneous or heterogeneous species are then assessed. The current catalysts in the literature are categorised, including those able to bring about partial reduction of aromatic compounds. Finally, the role of the solvent, including ionic liquids, is described, as the solvent could prove to be crucial in stabilising one form of catalyst over the other and in providing regioselective hydrogenation.
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TL;DR: In this paper, the structure of rhodium in an ultradispersed 0.57 wt % Rh/y-Al,O, catalyst before and after CO adsorption was studied with extended X-ray absorption fine structure (EXAFS), XPS, electron spin resonance (ESR), temperature programmed reduction (TPR), CO infrared spectroscopy, and H2 and CO chemisorption.
Abstract: The structure of rhodium in an ultradispersed 0.57 wt % Rh/y-Al,O, catalyst before and after CO adsorption was studied with extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), temperature programmed reduction (TPR), CO infrared spectroscopy, and H2 and CO chemisorption. With the aid of these complementary techniques, it could be established that the structure of the rhodium catalyst was completely different before and after CO adsorption. Before CO adsorption and after reduction of the catalyst at 593 K, all the rhodium was reduced and in the form of three-dimensional metallic crystallites. CO adsorption disrupted the metal-metal bonds in the crystallites, leading to isolated rhodium geminal dicarbonyl species in which the rhodium was present as Rh'. Each rhodium ion was surrounded by two carbon monoxide molecules and three oxygen anions of the support.
291 citations
TL;DR: A number of reproducible synthetic routes to six-and seven-coordinate isocyanide complexes of molybdenum and tungsten have been developed over the years as mentioned in this paper.
Abstract: Publisher Summary This chapter discusses metal–isocyanide chemistry. In the interim, reviews have appeared on specific aspects of isocyanide chemistry. The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel's review, and has been used to rationalize variations in infrared (IR) stretching frequencies between bonded and nonbonded isocyanides and the better π-acceptor qualities of aryl versus alkyl isocyanide groups. A number of new synthetic routes to isocyanide complexes of chromium, molybdenum, and tungsten have been investigated. A number of attempts have been made over the years to develop reproducible synthetic routes to six- and seven-coordinate isocyanide complexes of molybdenum and tungsten. A number of routes have been employed for the synthesis of metal–isocyanide complexes by generating the isocyanide ligand on the metal atom. A number of monomeric complexes have been prepared, which may be considered as substitution products of the [Co(CNR) 5 ] + and [M(CNR) 4 ] + cations. Electrochemical investigations have been reported on a range of homoleptic and mixed carbonyl–isocyanide complexes, in attempts to rationalize substituent effects on the isocyanide with the electronic structure of the metal. Insertion reactions of isonitriles into metal–alkyl or metal–aryl bonds are now well established, occurring with metal–alkyl or –aryl groups from group IVA to IB and, recently, with uranium and thorium carbon bonds.
254 citations
05 May 1986
TL;DR: Polymeric Reagents and Catalysts: An Overview Soluble Polymer-Bound Reagents as mentioned in this paper and Catalysis with a Perfluorinated Ion-Exchange Polymer The Role of Substrate Transport in Catalyst Activity Stability of polymer-supported transition metal catalysts Polymeric Photosensitizers Polymer Bound Oxidizing Agents Wittig Reactions on Polymer Support Molecular Recognition in Polymers Prepared by Imprinting with Templates Polymeric Transfer Reagents for Organic Synthesis with Self-Control: Towards Automation in Organic synthesis with self-control
Abstract: Polymeric Reagents and Catalysts: An Overview Soluble Polymer-Bound Reagents and Catalysts Catalysis with a Perfluorinated Ion-Exchange Polymer The Role of Substrate Transport in Catalyst Activity Stability of Polymer-Supported Transition Metal Catalysts Polymeric Photosensitizers Polymer-Bound Oxidizing Agents Wittig Reactions on Polymer Supports Molecular Recognition in Polymers Prepared by Imprinting with Templates Polymeric Transfer Reagents for Organic Synthesis with Self-Control: Towards Automation in Organic Synthesis Site Isolation Organic Synthesis in Polystyrene Networks
188 citations