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.

Hydrogenation of Arenes under Mild Conditions Using Rhodium Pyridylphosphine and Bipyridyl Complexes Tethered to a Silica-Supported Palladium Heterogeneous Catalyst

Hydrodefluorination of Fluorobenzene and 1,2-Difluorobenzene under Mild Conditions over Rhodium Pyridylphosphine and Bipyridyl Complexes Tethered on a Silica-Supported Palladium Catalyst

Transition metal complex catalysts tethered to organic or inorganic supports1 have received much attention in the past few decades because they can, in principle, combine the advantages of homogeneous and heterogeneous catalysts. Such complexes can be easily tethered on silica surfaces through a ligand in the complex which has alkoxyor chlorosilane functional groups that react with surface hydroxyl groups on the SiO2. Silica-supported heterogeneous metal catalysts such as Pd-SiO2, Rh-SiO2, and Pt-SiO2 also have surface hydroxyl groups that could be used to tether transition metal complex homogeneous catalysts. These combination catalysts consisting of a tethered complex on a supported metal (TCSM) catalyst (Figure 1) could function by synergistic action of both catalyst components. For hydrogenation reactions of unsaturated organic substrates, one might imagine that these TCSM catalysts could function in a way that H2 is activated on the supported metal (e.g., Pd, Rh, or Pt) with the resulting hydrogen atoms spilling over onto the silica where they could react with the unsaturated organic substrate that is simultaneously coordinated and activated by the tethered complex. This mechanism for the functioning of a TCSM catalyst depends on the well-known phenomenon of hydrogen spillover on supported metal catalysts.3 In other mechanisms, the tethered complex may interact more directly with molecules that are activated on the supported metal. In this paper, we report an example, the first to our knowledge, of a tethered complex on a supported metal (TCSM) catalyst, whose activity for the hydrogenation of arenes is substantially higher than that of the tethered complex or the supported metal separately. In fact, its activity is higher than that of any reported homogeneous or immobilized metal complex catalyst under the mild conditions of 1 atm of H2 and 40 °C. Two TCSM catalysts were prepared by tethering either of the rhodium isocyanide complexes, RhCl[CN(CH2)3Si(OC2H5)3]3 or RhCl(CO)[CN(CH2)3Si(OC2H5)3]2, to a silica-supported palladium metal catalyst (Pd-SiO2). The rhodium isocyanide complex RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 (Rh-CNR2) was prepared by the reaction of [Rh(CO)2Cl]2 with 4 equiv of CN(CH2)3Si(OC2H5)3 in toluene, in a reaction similar to that described for the synthesis of RhCl(CO)[CNBu]2. The complex RhCl[CN(CH2)3Si(OC2H5)3]3 (Rh-CNR3) was prepared in the reaction of [Rh(COD)Cl]2 (COD ) cyclooctadiene) with 6 equiv of CN(CH2)3Si(OC2H5)3 according to a procedure used for the preparation of RhCl[CN(2,6-xylyl)]3. The toluene solution containing RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 or RhCl[CN(CH2)3Si(OC2H5)3]3 was refluxed with the silica-supported palladium catalyst Pd-SiO2 (Pd, 10 wt %) for 4 h. After filtration, the solid was washed with toluene and then dried in vacuum at room temperature. The resulting tethered catalysts, Rh-CNR2/Pd-SiO2 (Rh content, 1.10 wt %) and Rh-CNR3/PdSiO2 (Rh content, 1.35 wt %), gave IR spectra (DRIFTS) with ν(CN-) and ν(CO) bands (2197 (s) and 2017 (s) cm-1 for RhCNR2/Pd-SiO2; 2176 (s) and 2124 (w) cm-1 for Rh-CNR3/PdSiO2) that are very similar in position and relative intensity to those of the untethered Rh-CNR2 and Rh-CNR3 complexes,4,8 which indicates that the complexes retain their structures after being tethered to the Pd-SiO2 surface. The rates of hydrogenation (Table 1) of toluene to methylcyclohexane at 40 °C while being stirred under 1 atm of H2 in the presence of the TCSM catalysts or the separate homogeneous and heterogeneous catalysts were determined by following the rate of H2 uptake. The catalysts are active from the outset but the TOF (turnover frequency) values increase to a maximum value of 4.8 for Rh-CNR2/Pd-SiO2 after 1 h and to 5.5 for RhCNR3/Pd-SiO2 after 6.5 h. After several hours at the maximum TOF levels, the activities decrease slightly. From the data in Table 1, it can be seen that the Rh-CNR2/Pd-SiO2 catalyst activity (as measured by the maximum TOF, turnover number (TO), or H2 uptake) is at least 7 times greater than that of the simple heterogeneous SiO2-supported Pd (Pd-SiO2), the RhCNR2 complex tethered to just SiO2(Rh-CNR2/SiO2), just the ligand (CN(CH2)3Si(OC2H5)3) tethered to Pd-SiO2(CNR/PdSiO2), or the homogeneous catalyst (Rh-CNR2) even with relatively large amounts of Rh (20 μmol) as compared with 6.3 μmol in Rh-CNR2/Pd-SiO2. Similarly, Rh-CNR3/Pd-SiO2 is at least 9 times more active than Pd-SiO2, homogeneous Rh-CNR3, tethered Rh-CNR3/SiO2, or CNR/Pd-SiO2. The most active TCSM catalyst, Rh-CNR3/Pd-SiO2, has a maximum turnover frequency of 5.5 mol H2/(mol of Rh min) and a turnover number (1) (a) Hartley, F. R. Supported Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1985. (b) Iwasaka, Y. Tailored Metal Catalysts; Reidel: Tokyo, 1986. (c) Cornils, B.; Hermann, W. A. Applied Homogeneous Catalysis with Organometallic Compounds; VCH: Weinheim, 1996; p 351. (2) (a) Blumel, J. Inorg. Chem. 1994, 33, 5050. (b) Capka, M.; Czakova, M.; Wlodzimierz, U.; Schubert, U. J. Mol. Catal. 1992, 74, 335. (c) Allum, K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organomet. Chem. 1975, 87, 203. (d) Capka, M.; Hetflejs, J. Collect. Czech. Chem. Commun. 1974, 39, 154. (e) Pugin, B. J. Mol. Catal. A: Chem. 1996, 107, 273. (f) Czakova, M.; Capka, M. J. Mol. Catal. 1981, 11, 313. (3) (a) Pajonk, G. M.; Teichner, S. J.; Germain, J. E. SpilloVer of Adsorbed Species; Elsevier: Amsterdam, 1983. (b) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. AdV. Catal. 1986, 34, 1. (c) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (d) Inui, T.; Fujimoto, K.; Uchijima, T.; Masai, M. New Aspects of SpilloVer Effects in Catalysis; Elsevier: Amsterdam, 1993. (4) Selected data for RhCl(CO)[CN(CH2)3Si(OC2H5)3]2: 1H NMR (CDCl3) δ 3.82 (q, 12H, OCH2CH3), 3.67 (t, 4H, CNCH2), 1.90 (m, 4H, CH2CH2CH2), 1.21 (t, 18H, OCH2CH3), 0.75 (t, 4H, SiCH2); IR (in toluene) ν(CN-) 2192 (s) cm-1, ν(CO) 1996 (s) cm-1. (5) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84. (6) (CH3CH2O)3SiCH2CH2CH2NC was prepared from (CH3CH2O)3SiCH2CH2CH2NHCHO and Cl3COC(dO)Cl following a procedure developed for the synthesis of other alkyl isocyanides (Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977, 16, 259); IR (in CH2Cl2), ν(CN-) 2150 cm-1; 1H NMR (CDCl3) δ 3.81 (q, 6H, OCH2CH3), 3.38 (m, 2H, CNCH2), 1.78 (m, 2H, CH2CH2CH2), 1.20 (t, 9H, OCH2CH3), 0.72 (t, 2H, SiCH2). (7) Deeming, A. J. J. Organomet. Chem. 1979, 175, 105. (8) Selected data for RhCl[CN(CH2)3Si(OC2H5)3]3: 1H NMR (CDCl3) δ 3.82 (q, 18H, OCH2CH3), 3.58 (t, 4H, CNCH2), 3.46 (t, 2H, CNCH2), 1.85 (m, 6H, CH2CH2CH2), 1.23 (t, 27H, OCH2CH3), 0.73 (t, 6H, SiCH2); IR (in toluene) ν(CN-) 2158 (s), 2119 (m) cm-1. Anal. Calcd for C30H63O9N3Si3ClRh: C, 43.28; H, 7.63; N, 5.05. Found: C, 42.70; H, 7.37; N, 4.57. (9) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88. (10) Yamamoto, Y.; Yamazaki, H. J. Organomet. Chem. 1977, 140, C33. (11) Pd-SiO2 was prepared by the incipient wetness method by impregnation of SiO2 using an aqueous solution of H2PdCl4, calcining at 500 °C for 4 h and reducing with H2 at 380 °C for 4 h. Figure 1. Conceptual illustration of a TCSM catalyst consisting of a tethered homogeneous complex catalyst on a supported metal heterogeneous catalyst. 6937 J. Am. Chem. Soc. 1997, 119, 6937-6938

https://www.chem.ccu.edu.tw/~joyce/referance/gump/Rh/TCSM/J.%20Am.%20Chem.%20Soc.%201997,%20119,%206937.pdf

Combination Catalysts Consisting of a Homogeneous Catalyst Tethered to a Silica-Supported Palladium Heterogeneous Catalyst: Arene Hydrogenation

https://www.chem.ccu.edu.tw/~joyce/referance/gump/Rh/TCSM/Organometallics%201999,%2018,%20989.pdf

Combined Homogeneous and Heterogeneous Catalysts. Rhodium and Platinum Isocyanide Complexes Tethered on Silica-Supported Metal Heterogeneous Catalysts: Arene and Cyclohexanone Hydrogenation †

Hanrong Gao

Papers

Hydrogenation of Arenes under Mild Conditions Using Rhodium Pyridylphosphine and Bipyridyl Complexes Tethered to a Silica-Supported Palladium Heterogeneous Catalyst

Hydrodefluorination of Fluorobenzene and 1,2-Difluorobenzene under Mild Conditions over Rhodium Pyridylphosphine and Bipyridyl Complexes Tethered on a Silica-Supported Palladium Catalyst

Combination Catalysts Consisting of a Homogeneous Catalyst Tethered to a Silica-Supported Palladium Heterogeneous Catalyst: Arene Hydrogenation

Combined Homogeneous and Heterogeneous Catalysts. Rhodium and Platinum Isocyanide Complexes Tethered on Silica-Supported Metal Heterogeneous Catalysts: Arene and Cyclohexanone Hydrogenation †

Hydroformylation of 1-Octene under Atmospheric Pressure Catalyzed by Rhodium Carbonyl Thiolate Complexes Tethered to Silica