Hydrogen addition reactions with clusters

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

Over zinc oxide it is clear that only a limited number of sites are capable of type I hydrogen adsorption. This adsorption on a Zn—O pair site is rapid with a half-time of less than 1 min hence, it is fast enough so that H2-D2 equilibration (half-time 8 min) can readily occur via type I adsorption. If the active sites were clustered, one might expect the reaction of ethylene with H2-D2 mixtures to yield results similar to those obtained for the corresponding reaction with butyne-2 over palladium That is, despite the clean dideutero addition of deuterium to ethylene, the eth-... 

Consistent with the two-electron donor nature of H2, the reaction behaved as an n=2 Nernst redox reaction. It showed a pH dependence of 66mV per pH unit, so again one proton was taken up for each electron. It is not known where all incoming protons are localized in the enzyme. The reaction shows that in addition to the light-sensitive hydrogen species bound to the active site in the Nia-C " state, a second hydrogen can react at the active site and deliver its two electrons to the enzyme. We hence proposed that the active site of the A. vinosum enzyme has two sites where hydrogen can bind. If H2 is completely removed, the Nia-C state persists for hours this is unlike the situation in redox titrations in the presence of redox mediators. As the active site in the Nig-SR state has one electron more than that in the Nia-C state, an Fe-S cluster has to be involved in this reaction with H2. 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

Chemisorption of cyclohexane, n-hexane, and 2,3-dimethylbutane on small platinum clusters has been reported to be quite facile. These species also readily exhibit sequential reactions. Even the platinum atom forms a di-addition product with cyclohexane. The first report of this reaction utilized high-fluence 6.42-eV photons to ionize the reaction products. The resultant mass spectra showed extensive hydrogen loss, and a C H ratio near 1 was found for most product ions (Fig. 2 in Trevor et al. ). As discussed above, extensive hydrogen loss may be due to laser cooking during ionization. More recent experiments, which utilized low-fluence 7.87-eV ionizing pho-... 

The oxidation state of Cu ions in zeolite Y depends on the pretreatment procedure. After activation for more than 8 hours at 675 K phases similar to CuO occur. Admitting hydrogen at 575 K, a reduction to Cu(I) but not to Cu(0) metallic clusters is observed. Probably a reaction with extraframework oxygen takes place. However, a reduction to rather small metallic copper clusters is observed after co-addition of water vapour and hydrogen. This demonstrates the important role of strong ligands like water in the reduction mechanism, enabling a controlled formation of copper clusters. 

The sequential addition of a proton and a hydride to a metal complex may be considered equivalent to an oxidative addition of hydrogen. With this in mind, the cationic hydrido derivative [Ru3(//-H)(/t3-ampy)(/<-PhC=CHPh)(CO)8][BF4] (42) (Fig. 11) was made and subsequently treated with [PPN] BH4]. This latter reaction was found to lead to Z- and -stilbene and to a mixture of cluster decomposition products. However, the cluster 40 is reformed when it is sequentially treated with HBF4 and [PPN][BH4] in the presence of diphenylacetylene. Z- and -stilbene are also produced in this reaction.It should be noted that complex 42 is coor-dinatively saturated and that a vacant site is needed before its reaction with [BH4]. ... 

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