Metal alkyl clusters

Although it is clear that alkane activation is possible, the work done so far using soft metals as catalyst has not produced a reaction that converts alkanes into useful products. Nor does it point very clearly in the direction where progress will be made. It may be that studies on dimeric and cluster compounds, on polyhydrides, or on transition metal alkyls will indicate the paths to be explored. The work using hard catalysts is at present very limited and can be expected to be developed much further in future years. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

Alkenes may also be used as reagents in side-chain alkylation. Zeolites with occluded alkali metal and alkali metal oxide clusters were tested in alkylation of toluene with ethylene439 440 Alkali metal clusters created by decomposing Na or... 

Cs azide were active catalysts in the alkylation reaction, whereas alkali metal oxide clusters showed no activity at all. 

The only apparent exception to this rule is the hydrido-bridged osmium cluster (/u-CHjX/u-H lOsalCOhol (75) (Table IV). The alkanediyl derivatives (type G) may even be distinguished from the closely related metal alkyls. The chemical shifts of the latter compounds typically cover the... 

Both alkyl and aryl tellurosilanes act as a good source of Rib and Te2 ligands, forming transition metal-tellurium clusters with cobalt(II) and copper(I) salts. The nuclearity of the cluster is determined primarily by the phosphine present, but also by... 

In the area of technological application there are two major topics the (real or postulated) involvement of metal atom clusters in catalysis and the remarkable superconducting properties of the Chevrel phases. The former topic has been abundantly covered in current review literature (54), although there are cautions to be recognized (55), and will not be further discussed here. Even the Rh2 + complexes have been reported to be catalytically active. Thus, Rh2(02CCF3)i, exhibits catalytic activity, toward cyclopropanation of alkenes with alkyl... 

The first example of insertion of NO into a cluster metal-alkyl bond has recently been reported in compound (44) (224). 

In the presence of the radical substitution promoter [Fe2(CO)4()u,-SR)i(PPh3)2], tertiary amines react with transition metal carbonyl clusters under exceptionally mild conditions. Modified allenyl and allylic clusters similar to those described earlier have been isolated from such reactions. Two distinct types of products have been isolated (i) those involving the elimination of an alkyl group and (ii) those involving C-C coupling reactions. The product formation described in Scheme 31 was preceded by amine coordination, C-H activation, C-N cleavage, carbene-amine complex formation, transamination, and C-C coupling (6Ia.h)- Such processes are of interest in the area of hydrodenitrification (6/c). 

The catalysts derived from supported iron clusters exhibit in Fischer-Tropsch synthesis a high selectivity for propylene. Those catalysts are also selective for the stoechiometric homologation of ethylene to propylene and of propylene to n and iso butenes. The results are explained on the basis of a new mode of C-C bond formation which implies < - olefin coordination to surface methylene fragments or methylene insertion into a metal alkyl bond. 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

Reactions of alkenes with metal carbonyl clusters may give simple substitution products such as Os3(CO)n(ri -C2H4) or may involve oxidative addition of one or more C—H bonds. Reaction 23.76 illustrates the reaction of Ru3(CO)i2 with RHC=CH2 (R = alkyl) to give isomers of H2Ru3(CO)9(RCCH) in which the organic ligand acts as a 4-electron donor (one tt- and two tr-interactions). 

Some metal carbonyl clusters have vertices of post-transition elements such as phosphorus, sulfur, and their heavier congeners. Let us consider an alkyl-phosphinidene vertex R-P. The phosphorus atom of a normal alkylphosphinidene vertex uses only its 5" and p orbitals for chemical bonding leading to four tetrahedrally hybridized orbitals, one of which is an external orbital for bonding to the alkyl group and the remaining three of which are internal orbitals for the cluster skeletal bonding. However, in addition, hypervalent alkylphosphinidene vertices are... 

Of the 33 papers presented at the symposium 32 are included in this volume. They are of two general types. One type discusses inorganic compounds exhibiting a specific property, most frequently electrical or photochemical. The second type reviews recent developments in the synthetic chemistry of a class of inorganic compounds— most often transition metal or lanthanide complexes— which are of potential interest to persons looking for new systems with unusual properties. Classes of compounds covered by this second type include metal alkyls, metal alk-oxides, metal alkylamides, metal chelates, and metal clusters as well as metal complexes with polyboranes, polyphosphines, macrocyclic derivatives, di- and triketones, and polypyrazolylborates as ligands. 

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