Aluminum clusters, reactions with

Leuchner, R. E. Harms, A. C. Castleman, A. W. Thermal metal cluster anion reactions Behavior of aluminum clusters with oxygen. J. Chem. Phys. 1989,91, 2753 Aluminum cluster reactions. 1991, 94, 1093. 

Figure 22. Series of mass spectra showing progression of the etching reaction of aluminum anions with oxygen. Note that magic number clusters corresponding to electron shell closings for 40 and 70 electrons (AI13 and AI23) appear as the terminal product species of reactions with oxygen at flow rates of (a) 0.0, (b) 7.5, and (c) 100.0 standard cubic centimeters per minute (seem). Taken with permission from ref. 92.

Clusters of the elements aluminum to thallium containing only one or two carbon atoms and strong direct element-element interactions, similar to boron rich car-baboranes, have not yet been synthesized, and also the corresponding silicon derivatives are relatively rare. To the best of our knowledge only one aluminum-silicon and one gallium-silicon cluster (1 and 2) has been reported in the literature. The reaction of metastable aluminum(I) chloride with decamethylsilicocene or with a mixture of SiCl4 and (AlCp )4, respectively, afforded black crystals of... 

The linear CO stretching frequency for the carbonylated platinum colloid while lower than that found for surface bound CO, is in the range reported for the platinum carbonyl clusters [Pt 3 (CO) 6 ] n / sind we find that the carbonylated colloid is easily transformed into the molecular cluster [Pt 12 (CO) 24 ] (10) reaction with water. The cluster was isolated in 50 yield based on platinum content of the precipitate by extraction with tetraethylammonium bromide in methanol from the aluminum hydroxide precipitated when water is added to the aluminoxane solution. The isolation of the platinum carbonyl cluster reveals nothing about the size or structure of the colloidal platinum particles, but merely emphasizes the high reactivity of metals in this highly dispersed state. The cluster isolated is presumably more a reflection of the stability of the [Pt3(CO)6]n family of clusters than a clue to the nuclearity of the colloidal metal particles - in a similar series of experiments with colloidal cobalt with a mean particle size of 20A carbonylation results in the direct formation of Co2(CO)8. 

The initial observation of remarkable size-dependent reactions of deuterium with clusters of cobalt and niobium was rapidly followed by other reports of dihydrogen addition reactions with clusters of many metals, in some cases containing more than 100 atoms. Further work was reported for nio-bium " and cobalt as well as for iron, vanadium, " nickel, platinum, rhodium, tantalum, and aluminum. ... 

The most size-specific reaction is that of aluminum clusters with D2, as shown in Fig. 2. The 6-atom cluster of aluminum is the most reactive, and no reaction is observed for clusters containing fewer atoms. Reactivity rapidly falls off for AI7 and Alg. Under the highest Dj pressures employed reaction can be observed up to about Al j. Similar results are obtained for H2, but with relative rate constants about 1.7 0.4 higher than for D2. The primary products of the reaction are A1 D2, and unlike most other systems discussed below, once one molecule has been added, the cluster adduct does not appear to react further. Saturation coverage appears to be one molecule of hydrogen. 

Aluminum clusters react more rapidly with methanoP " than either Dj or DjO. Alj, A1 4, and AI22 are the least reactive clusters. As a result of sequential addition reactions the observed product peaks are A1 (CH30H) with m = 1, 2, and 3. At low extent of reaction, products with m = 1 are obtained in the highest yield. The peak reactivity, occuring at Ali -i, is about 500 times greater than that of the most reactive cluster (Al ) toward D2 - As the methanol partial pressure is increased, the mass spectra become very complicated. 

Aluminum clusters selectively chemisorb D2O, with peak reactivity occurring at AI q. The rate constants for AI q is 50 times greater than that of the most reactive cluster (AI5) with The reaction is facile, not only for the... 

In the inhibition period aluminum cations react with the silica oligomers and subcolloidal particles to form isolated patches of alumino-silicates on the surface of the small silica particles. From the review work of Iler (Her 1979), it is known that the reaction of aluminum cations with silicate oligomers is fast compared to the clustering of particles. After the inhibition period, the size of the aggregates slowly increases. Increasing the concentration of aluminum cations in the solution involves the formation of more homogeneous alumino-silicate particles. At a concentration of 0.10 mol% aluminum in the silicate solution, the fractal dimensionality... 

Khanna et al. [136] proposed a mechanism of the reactions of aluminum based clusters with O, which lends a physical interpretation as to why the HOMO-LUMO gap of the clusters successfully predicts the oxygen etching behaviors. The importance of the HOMO-LUMO gap strongly suggests that the reactions of the metal clusters belong to the pseudoexcitation band. 

Lanthanide(III) isopropoxides show higher activities in MPV reductions than Al(OiPr)3, enabling their use in truly catalytic quantities (see Table 20.7 compare entry 2 with entries 3 to 6). Aluminum-catalyzed MPVO reactions can be enhanced by the use of TFA as additive (Table 20.7, entry 11) [87, 88], by utilizing bidentate ligands (Table 20.7, entry 14) [89] or by using binuclear catalysts (Table 20.7, entries 15 and 16) [8, 9]. With bidentate ligands, the aluminum catalyst does not form large clusters as it does in aluminum(III) isopropoxide. This increase in availability per aluminum ion increases the catalytic activity. Lanthanide-catalyzed reactions have been improved by the in-situ preparation of the catalyst the metal is treated with iodide in 2-propanol as the solvent (Table 20.7, entries 17-20) [90]. Lanthanide triflates have also been reported to possess excellent catalytic properties [91]. 

---