Hydrogen reduction hydride formation

Further mechanistic insights into hydrogenations catalyzed by HRuCl(PPh3)3 (7, p. 83) have been obtained indirectly, from studies on hydrogenation of some ruthenium(III) phosphine complexes (83). A frequently considered mechanism for hydrogen reduction of metal salts involves slow formation of an intermediate monohydride, followed by a faster reaction between the hydride and starting complex (/, p. 72), Eqs. (2) and (3) ... 

Figure 12.3. Hydrogen as the source for hydride formation (4) and oxidative addition /reductive elimination related to hydride formation (5)...

The formation of formate esters in the hydroformylation reaction (90, 64) may be explained by a CO-alkoxide insertion reaction as well as by the CO-hydride insertion mechanism mentioned above. Aldehydes formed in the hydroformylation reaction can be reduced by cobalt hydrocarbonyl (27) presumably by way of an addition of the hydride to the carbonyl group (90, 2). If the intermediate in the reduction is an alkoxycobalt carbonyl, carbon monoxide insertion followed by hydrogenation would give formate esters (90, 64). 

In type 2, the homogeneous redox reaction of the electrogenerated and regenerated redox catalyst consists of a chemical reaction. For oxidations, these reactions may be hydride ion or hydrogen atom abstraction, oxygen transfer, or an intermediate complex or bond formation. For reductions, hydride or car-banion transfer from a metal complex is often observed. In all these cases, very large potential differences between the standard potential of the substrate and the redox catalyst may be overcome. The selectivity can be very high and may... 

Another possible effect of PdAu deposits on PdAu/SnOx sensors is through the formation of a Schottky barrier between PdAu and SnOx, as in the case of the Pd/CdS hydrogen sensor. If such a barrier is formed, then a depletion layer is created inside the semiconductor tin oxide. Since the Pd work function can be reduced by hydrogen absorption through dipole or hydride formation (14,15), the width of the depletion layer in tin oxide may be reduced. The reduction of the depletion layer width causes the sample resistance to decrease. Such a possibility was checked and was ruled out, because a good ohmic contact was obtained between Pd (-50 nm thick) and SnOx- It is also commonly known that gold forms an ohmic contact with tin oxide. 

In principle, the same methods used for the preparation of platinum catalysts may be applied for palladium catalysts. When palladium chloride is used as a starting material, it is usually dissolved into an aqueous solution as chloropalladic acid by adding hydrochloric acid prior to reduction or formation of precipitates. Unsupported and supported palladium catalysts have been prepared by reduction of palladium salts with alkaline formaldehyde,139 168 sodium formate,169 hydrazine,150 hydrogen,170,171 sodium borohydride,146,172 or sodium hydride-t-AmOH,173 or by reduction of palladium hydroxide174,175 or palladium oxide176 with hydrogen. 

An early review by Koelle on transition metal catalyzed proton reduction nicely developed the various chemical steps involved in hydrogen evolution including metal hydride formation, hydride acidity (basicity) and protonation and requisite redox potentials.284 The complexes review here have little structural relevance to the hy-drogenase active sites but many show promising catalytic activity. More recently... 

A complex reducing agent was prepared from NaH, RONa and nickel(II) acetate This catalyst (referred to as Nic), similarly to the P-1 and P-2 nickel catalysts, is a selective catalyst in diene reductions. The reactive parts of Nic are metal hydrides and the key step in the hydrogenation is the formation of M—H bonds. The sodium salt of the alcohol added plays an important role as an activating agent in reductions using Nic. Whereas P-1 and P-2 nickels are selective and sensitive to the double-bond structure and show a rather low propensity toward isomerization, Nic has no propensity toward disproportionation. 

Kinetic data have been reported for cyclohexene reduction with a 1 6 Cr(acac)3- Bu3Al catalyst in heptane at 30 C, which showed a first-order dependence on catalyst and H2. Hydrogenation rates generally decrease with increasing substitution of the alkene substrate. Similar kinetic results were independently obtained for the Cr(acac)3- Bu3Al catalyst. A proposed mechanism involves alkylation of the metal-halide [equation (a)], hydride formation [equation (b)], followed by reversible insertion of the olefin substrate into the metal-hydride bond [equation (c)], and hydrogenolysis of the resulting metal-alkyl bond [equation (d)]. ... 

A transition metal catalyst has also been used to effect the reductive alkylation of amino groups on proteins [41], This reaction uses [Cp Ir(4-4 -dimethoxybipy)(H20)]S04 31 as a mild transfer hydrogenation catalyst and formate ion as the stoichiometric hydride source, in Fig. 10.3-11 (a). Presumably, this reaction occurs via the reversible formation of imine 33 with free amino groups on the protein surface, followed by reduction of iridium hydride 32. For most proteins, multiple modifications are observed (Fig. 10.3-ll(b)), although the overall level of conversion can be altered through variation of either the reaction temperature or the concentrations of the aldehyde and catalyst. In general, the reaction has shown excellent reliability for protein alkylation between pH 5 and 7.4. 

The reactions of a number of such compounds have been studied and a wide variety of rates of reaction are evident. It is of interest to note that molecular hydrogen reacts with sodium vapour in the gas phase under conditions where it is used as a carrier gas in sodium flame studies. A slow steady decrease in total pressure occurs as a run proceeds which can be attributed to sodium hydride formation. This can have an effect on the saturation of the carrier gas with sodium vapour when very long runs are employed. Thomas [108] has shown that after about six hours there is a considerable reduction in the amount of sodium vapour emerging from the nozzle, an effect that is absent for nitrogen as a carrier gas. The literature on sodium flame reactions makes no mention of this effect with the exception of the paper by Hodgins et al. [109]. 

Two types of sodium hydride are available commercially a dry, granular material about 8 to 200 mesh in size, and a semidispersion of micronsized crystals in mineral oil. The oil-dispersed sodium hydride is the safer and easier to handle, as the high reactivity of the hydride is protected by the oil. The principal use of sodium hydride is to carry out condensation and alkylation reactions which proceed through the formation of a car-banion (base-catalyzed). The sodium hydride dispersion has been evaluated in comparison with dry sodium hydride, sodium metal, soda-mide, and sodium methylate. Yields and reaction rates in the self-condensation of esters, ester-keto condensations, and the Dieckmann condensation have been outstandingly superior. Amines can be successfully alkylated by a new technique employing polar solvents. Dehalogenations do not occur, nor does reduction unless there is no a-hydrogen present. Acyloin formation and reduction side reactions do not interfere when sodium hydride is used. 

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