Supported metal hydrides

Supported metal hydrides of early transition metals catalyze the hydrogenolysis of alkanes at relatively low temperatures (50-150 °C) [29,90-92]. Noteworthy are their differences in product selectivities. For example, the hydrogenolysis of propane in the presence of a large excess of H2 gives a 1 1 mixture of ethane and methane in the case of zirconium hydride, a group 4... 

Reductions of organic compounds with polymer-supported reagents have included reports of the reduction of disulfides to thiols, of the use of alumina-supported materials for reduction of alkenes and ketones, and of the various reductions brought about by polymer-supported metal hydrides. The reductions of disulfides to thiols, mainly concerned with biologically related materials, will be discussed later (Chapter 15). 

Vidal V, Theolier A, Thivolle-Cazat and Basset J M 1997 Metathesis of alkanes catalyzed by silica-supported transition metal hydrides Soienoe 276 99-102... 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

The commonly investigated ligands on the supported metal clusters mentioned before include CO, hydride, and hydrocarbons. The evidence of hydrides and hydrocarbons is not as strong as one would wish. 

NMR spectroscopy has been used to detect hydrides on various oxide-supported metals in the presence of H2 and on La203-supported Ir4, in the absence of H2 [37]. The kinetics of chemisorption of H2 supports the inference of hydride formation by dissociative adsorption of H2 [38]. 

Tributyltin hydride reduction of carbonyl compounds. The reduction of carbonyl compounds with metal hydrides can also proceed via an electron-transfer activation in analogy to the metal hydride insertion into TCNE.188 Such a notion is further supported by the following observations (a) the reaction rates are enhanced by light as well as heat 189 (b) the rate of the reduction depends strongly on the reduction potentials of ketones. For example, trifluoroacetophenone ( re<1 = —1.38 V versus SCE) is quantitatively reduced by Bu3SnH in propionitrile within 5 min, whereas the reduction of cyclohexanone (Erea — 2.4 V versus SCE) to cyclohexanol (under identical... 

Hodoshima, S., H. Nakamura, A. Shono, K. Satoh, and Y. Saito, Hydrogen supply from organic chemical hydride with carbon-supported metallic catalyst under superheated liquid-film conditions.. Hydrogen Energy Syst. Soc. Jpn., 30(1), 36-41 (2005). 

Capture of Active Catalyst Using Solid Acidic Support with H2 Elution. The limit on the practical life of a catalyst solution may be determined by several factors including the presence of poisons or inhibitors, the buildup of less soluble materials such as the oxidation products of organophosphorus ligands, or an increase in the concentration of heavy aldehyde condensation products in the catalyst solution. In the latter case, there may be substantial amounts of active catalyst, but it is in a solvent that is unsuitable. Alternately, active rhodium catalyst may have been carried over with product. Technology has been disclosed [39] that permits the isolation of an active metal hydride catalyst. Steps include ... 

The formation of a metal-hydride entity in supported heterogeneous catalysts has been proposed by Rooney (43). Surface hydroxylic groups on the support provide the needed hydrogen. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

The proposed mechanism shown in Scheme 7.11 is supported by stoichiometric proton- and hydride-transfer reactions of metal hydrides that were dis-... 

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

The EU "STORMY" program aims to develop the next generation of compressed and liquefied hydrogen storage and metal hydrides. The EU consortium includes about 40 partners including car industry players. The program is support with a 10 million budget... 

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