Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

Reduced transition metal colloids: a novel family of reusable catalysts

Fluorine has received great attention in all fields of science. “Small atom with a big ego” was the title of the Symposium at the ACS meeting in San Francisco in 2000, where a number of the current scientific and industrial aspects of fluorine chemistry made possible by the small size and high electronegativity of the atom were discussed. This small atom has provided mankind with significant benefits in special products such as poly(tetrafluroethylene) (PTFE), freon, fluoro-liquid crystals, optical fiber, pharmaceutical and agrochemical compounds, and so on, all of which have their own unique properties that are otherwise difficult to obtain.1 For instance, at present, up to 30% of agrochemicals and 10% of pharmaceuticals currently used contain fluorine atoms. Therefore, organic fluorine compounds have received a great deal of interest and attention from the scientists involved in diverse fields of science and technology. Now, not only C-F bond formation but also selective C-F bond activation have become current subjects of active investigation from the viewpoint of effective synthesis of fluoroorganic compounds. The former is highlighted by designing a sophisticated fluorinating reagent for regioand stereocontrolled fluorination and developing versatile multifunctional and easily prepared building blocks. C-F bond formation has been treated extensively in several reviews2 and books.3 The latter is a subject that has been less explored but would be promising for selective defluorination of aliphatic fluorides, cross-coupling with aryl fluorides, and * To whom correspondence should be addressed. Phone: 81-78-803-5799. Fax: 81-78-803-5799. E-mail: amii@kobe-u.ac.jp and uneyamak@cc.okayamau.ac.jp. † Kobe University. ‡ Okayama University. Chem. Rev. 2009, 109, 2119–2183 2119

C-F bond activation in organic synthesis.

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.

A review of the problem of distinguishing true homogeneous catalysis from soluble or other metal-particle heterogeneous catalysis under reducing conditions

Metal-mediated reductive hydrodehalogenation of organic halides.

The rhodium carbonyl thiolate complex, Rh2[μ-S(CH2)3Si(OCH3)3]2(CO)4 (Rh–S) was tethered to phosphine-modified Pd–SiO2 (P–(Pd–SiO2)), which was prepared by tethering the phosphine ligand Ph2P(CH2)3Si(OC2H5)3 to Pd–SiO2, to give the tethered complex catalyst Rh–S/P–(Pd–SiO2). Also, the phosphine-substituted Rh2[μ-S(CH2)3Si(OCH3)3]2[Ph2P(CH2)3Si(OC2H5)3]2(CO)2 (Rh–S–P) was tethered to several silica-supported metal heterogeneous catalysts M–SiO2 (M=Pd, Pt, Ru, Ir) to give Rh–S–P/M–SiO2. These complex catalysts, consisting of a tethered complex on a supported-metal (TCSM), were used to catalyze the hydroformylation of olefins and the hydrogenation of toluene under atmospheric pressure. In the presence of phosphorus donor-ligands, all the TCSM catalysts are active for the hydroformylation of olefins under the mild conditions of 60°C and 1 atm of H2 and CO (1:1). The most active catalysts, Rh–S/P–(Pd–SiO2) and Rh–S–P/Pd–SiO2, give maximum TOF values of 1.04 and 0.88 (mol aldehyde/mol Rh min), respectively, and TO values of 905 and 703 (mol aldehyde/mol Rh) during 22.5 h and 23.5 h in the hydroformylation of 1-octene. The Rh–S–P/Pd–SiO2∼PPh3 catalyst system maintains its activity through three cycles and 68 h to give a total turnover of 2118 mol aldehyde/mol Rh. These activities are higher than those of the homogeneous rhodium thiolate complex catalyst (Rh–S–P) and the rhodium thiolate complex catalysts tethered on just SiO2(Rh–S/P–SiO2 and Rh–S–P/SiO2). The effects of the phosphorus ligand and PR3/Rh mole ratio on the hydroformylation rates, conversions, and chemo- and regioselectivities for aldehyde were also investigated. The Rh–S/P–(Pd–SiO2) and Rh–S–P/Pd–SiO2 catalysts are also active for the hydrogenation of toluene under the mild conditions of 40°C and 1 atm of H2; they are much more active than the homogeneous rhodium thiolate complex catalyst Rh–S–P, the simple silica-supported heterogeneous palladium catalyst (Pd–SiO2) and the rhodium thiolate complex catalysts tethered on just SiO2(Rh–S/P–SiO2 and Rh–S–P/SiO2). The synergistic advantages of the two components (tethered complex and supported metal) of the TCSM catalysts are greater for the hydrogenation of toluene than for the hydroformylation of 1-octene.

Combined homogeneous and heterogeneous catalysts.: Rhodium carbonyl thiolate complexes tethered on silica-supported metal heterogeneous catalysts: olefin hydroformylation and arene hydrogenation

The rhodium–phosphine complex, RhCl(CO)[Ph2P(CH2)3Si(OC2H5)3]2 (Rh–P), was tethered to silica-supported metal heterogeneous catalysts, Pd–SiO2, Ni–SiO2 and Au–SiO2, to give the tethered complex catalysts Rh–P/Pd–SiO2, Rh–P/Ni–SiO2 and Rh–P/Au–SiO2. An IR (DRIFTS) study shows that the structure of the tethered rhodium–phosphine complex (Rh–P) in all of these combination catalysts is the same as that of the free Rh–P complex. These heterogenized complex catalysts consisting of a tethered complex on a supported metal (TCSM) were used to catalyze the hydrogenation of arenes under the mild conditions of 40°C and 1 atm of H2. The most active TCSM catalyst Rh–P/Pd–SiO2 is not only much more active than the homogeneous Rh–P complex and Pd–SiO2 but also much more active than the rhodium–phosphine complex tethered on just SiO2 (Rh–P/SiO2). The Rh–P/Pd–SiO2 catalyst gives a maximum turnover frequency (TOF, mol H2/mol Rh min) of 2.9 and a turnover (TO, mol H2/mol Rh) of 1940 during a 23 h period in the hydrogenation of toluene. When the tethered rhodium–phosphine complex catalyst stands in air for 2 months or longer, the phosphine ligand is oxidized to the phosphine oxide. This air-aged Rh–P/Pd–SiO2 is much more active than the fresh catalyst for the hydrogenation of toluene, but the air-aged Rh–P/SiO2 becomes inactive for this reaction. These two air-aged catalysts were characterized by solid state 31 P NMR and DRIFTS using CO as a probe.

Rhodium–phosphine complex catalysts tethered on silica-supported heterogeneous metal catalysts: arene hydrogenation under atmospheric pressure

Rhodium amine complexes, RhCl(CO)2[Et2N(CH2)3Si(OCH3)3] (Rh–NEt2), RhCl(CO)2[H2N(CH2)3Si(OC2H5)3] (Rh–NH2) and RhCl(COD)[H2NCH2CH2NH(CH2)3Si(OCH3)3] (Rh(COD)(N–N)), were tethered to the silica-supported metal heterogeneous catalysts, M–SiO2 (M=Pd, Ni, Au), to give the TCSM (tethered complex on supported metal) catalysts, Rh–NEt2/M–SiO2 (M=Pd, Ni, Au), Rh–NH2/Pd–SiO2 and Rh(COD)(N–N)/Pd–SiO2. These TCSM catalysts exhibit activities, at 40°C and 1 atm of H2 pressure, for the hydrogenation of arenes that are higher than those of the separate homogeneous rhodium amine complexes, the separate silica-supported metal heterogeneous catalysts or the rhodium complex catalysts tethered on just SiO2. The activities of the TCSM catalysts are strongly affected by both the tethered rhodium amine complex and the SiO2-supported metal. Among these TCSM catalysts, Rh–NEt2/Pd–SiO2 exhibits the highest activity for the hydrogenation of toluene; its maximum TOF is 7.2 mol H2 (mol Rh min)-1 while its TO is 1919 mol H2 (mol Rh)-1 during a 5 h period. IR(DRIFT) spectral studies of the TCSM catalysts before and after being used for the hydrogenation of toluene show that during the hydrogenation, the two CO ligands of Rh–NEt2/M–SiO2 (M=Pd, Ni, Au) are lost from the rhodium center. After standing in air for one month, Rh–NEt2/Pd–SiO2 becomes more active for the hydrogenation of toluene, but the activity of the air-aged Rh(COD)(N–N)/Pd–SiO2 is lower than that of the fresh catalyst.

Rhodium amine complexes tethered on silica-supported metal catalysts. highly active catalysts for the hydrogenation of arenes

The present invention provides new catalyst formats which comprise a supported catalyst tethered to a second and different catalyst by a suitable tethering ligand. A preferred system comprises a heterogeneous supported metal catalyst tethered to a homogeneous catalyst. This combination of homogeneous and heterogeneous catalysts has a sufficient lifetime and unusually high catalytic activity in arene hydrogenations, and potentially many other reactions as well, including, but not limited to hydroformylation, hydrosilation, olefin oxidation, isomerization, hydrocyanation, olefin metathesis, olefin polymerization, carbonylation, enantioselective catalysis and photoduplication. These catalysts are easily separated from the products, and can be reused repeatedly, making these systems very economical.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1090&context=patents

Hanrong Gao

Papers

Combined homogeneous and heterogeneous catalysts.: Rhodium carbonyl thiolate complexes tethered on silica-supported metal heterogeneous catalysts: olefin hydroformylation and arene hydrogenation

Rhodium–phosphine complex catalysts tethered on silica-supported heterogeneous metal catalysts: arene hydrogenation under atmospheric pressure

Rhodium amine complexes tethered on silica-supported metal catalysts. highly active catalysts for the hydrogenation of arenes

Catalyst system comprising a first catalyst system tethered to a supported catalyst

Epoxidation of Olefins by Molecular Oxygen Over Supported Metal Heterogeneous Catalysts