Aluminum alkyls dimers

The association of aluminum alkyls provides a good opportunity to illustrate the principles discussed in Chapter 6 in regard to properties of liquids. Measurements of the molecular weights in benzene solutions indicate that the methyl, ethyl, and n-propyl compounds are completely dimerized. However, the heat of dissociation of (A1R3)2 dimers varies with nature of the alkyl group as shown by the following heats of dissociation ... 

Other organometallic compounds of aluminum include the alkyl hydrides, R2A1H. Molecular association of these compounds leads to cyclic tetramers. When the dimeric and trimeric compounds are dissolved in a basic aprotic solvent, the aggregates separate as a result of formation of bonds between A1 and the unshared pair of electrons on the solvent molecule. Toward Lewis bases such as trimeth-ylamine, aluminum alkyls are strong Lewis acids (as are aluminum halides). 

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

The early developments in the chemistry of boron hydrides and aluminum alkyls quickly led to the realization that these species did not conform to the accepted theories of bonding, but required the development of new concepts that could account for the dimeric or polymeric nature of these derivatives. These concepts, were initially provided in a number of papers by Mulliken (88), Longuet-Higgins (70), Pitzer (93), Pitzer and Gutowsky (94), and Rundle (97, 98). Much of this work has been summarized recently in books by Wade (118), by Lipscomb (69),... 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

Smith, M. B., The monomer-dimer equilibria of liquid aluminum alkyls. I. Triethyl-aluminum, J. Phys. Chem. 71, 364 (1967). 

It has been suggested that alkene or alkyne complexes are mechanistically important intermediates in hydroalumination and carboalumination reactions (124-126). Clear spectroscopic evidence for n interactions stems from investigations of alkenylaluminum compounds having a suitable intramolecular separation between the aluminum center and the double bond (127). IR and NMR data of these compounds show comparably lower alkene stretching frequencies and deshielded vinylic protons. Furthermore, these molecules are monomeric in solution this indicates that the tendency toward 7i-complex formation is stronger than that toward the dimeric bonding usual in aluminum alkyls. 

One of the interesting aspects of the chemistry of aluminum alkyls is the fact that they show a strong tendency to dimerize. In contrast, GaR3, InR3, and T1R3 do not dimerize, but they are also less stable than their A1R3 analogs. 

Compounds of the type Cp2LnR THF have distorted tetrahedral geometry of the ligands around the metal. By coordination to a solvent molecule, the monomeric species is obtained. A heterometallic entity like aluminum alkyl or a bidentate ligand may stabilize the species. Stabilization may also occur through dimerization [69] as in [Cp2Y(/i-Me)]2. Other compounds that have been prepared and characterized are given in Table 6.3. 

The higher alkyls are progressively less highly polymerized diethyl- and diiso-propylberyllium are dimeric in benzene, but the terf-butyl is monomeric the same trend is found in aluminum alkyls. 

Aluminum alkyl halides of the types R2A1X and RA1X2 are halide-bridged dimers in which, unlike Al2Me6, Al is surrounded by an electron octet. They are stronger Lewis acids than the trialkyls. The sterically very crowded compounds mes MX2 are monomeric they fail to give complexes with Et20 (M = Al, Ga, In X = Cl, Br).27... 

The sttuctures of organoaluminum halides R AlXs are dominated by dimeric structures with AI2X2 four-membered rings for X = Cl, Br, and I. Unlike the aluminum alkyls, these hahde bridges are electron-precise and result from intermolecular Lewis acid-base complexation. These dimers do dissociate and readily react with Lewis bases, but the enthalpies of dissociation are higher than those for the aluminum alkyls. In many cases, the strength of the association is sufficient that dimers are observed in the gas phase. 

Aluminum alkyls are used as Lewis acids and activators for several related transition metal-catalyzed reactions. In the Dimersol process, nickel complexes activated by an aluminum alkyl catalyze alkene dimerization, as in the conversion of butene to isooctene for use in preparing... 

Catalyst Ratio CAl/Til. The molar ratio of aluminum alkyl to titanium alkoxide is recognized as an important parameter in the dimerization of ethylene to butene-1. A molar ratio less than 10 favors dimerization while a ratio higher than 10 favors polymerization [1]. An optimal catalyst activity was found to exist at Al/Ti range of (2- 4) (subject to the reaction temperature), where it was also noticed that there was no remarkable increase in the polymer formation. 

In order for it to function as a Lewis acid catalyst, the dimer AbCU can undergo the less energetic opening of only one of the two chlorine bridges to form the half-bridged structure 3 [Eq. (2.2)] in which the three-coordinate metal atom can function as a strong Lewis acid. " Similar half-bridged structures in dimeric aluminum alkyls and other associated metal alkyls can be expected to play roles in their reactions with bases. 

While a limited amount of experimental evidence does lend support to the bimetallic concept, major objections were voiced by Ziegler, who was of the opinion that like dimeric aluminum alkyls the Ti-Al complex is not likely to be the effective catalyst agent. Other more recent work also favors the second and simpler alternative, the monometallic mechanism. 

It should be noted that the aluminum alkyls are known to be mainly dimeric in solution, existing in equilibrium with the monomeric species ... 

Reductive dimerization of alkenes and alkynes can be achieved with various catalysts in the presence of dihydrogen or other hydrogen donors, such as tetrahydroborates or / -H-eliminat-ing aluminum alkyls. 

H1 nuclear magnetic resonance measurements have established that, at least for the dialkyl alkynyl alanes, bridge bonds are formed exclusively by the a-carbon atom of the alkynyl group (30). This bond is stronger for the alkynyl than for the alkenyl compounds. In accordance with this, the dialkylalkynyl alanes may be distilled at reduced pressure as dimers without decomposition, whereas the corresponding alkenyl compounds decompose when heated and then undergo further reaction, in which addition of the A1—C bond to the C=C double bond occurs. The resulting aluminum alkyls disproportionate subsequently to trialkylalane and polymeric compounds (253). 

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