Monomeric aluminum species

There is much discussion on the nature of the aluminum species present in slightly acidic and basic solutions. There is general agreement that in solutions below pH 4, the mononuclear Al " exists coordinated by six water molecules, ie, [ ( 20) ". The strong positive charge of the Al " ion polarizes each water molecule and as the pH is increased, a proton is eventually released, forming the monomeric complex ion [A1(0H)(H20) ]. At about pH 5, this complex ion and the hexahydrated Al " are in equal abundance. The pentahydrate complex ion may dimerize by losing two water molecules... 

In aquatic systems dissolved aluminum species are toxic to fish. There exists a vast number of aluminum species, ranging from inorganic monomeric to complex colloidal. polymeric and organic complexes. A major problem, when studying aluminum species in water is that the species quickly convert one into the other (Fairman and Sanz-Medel 1995). 

The simple lithium tetraorganoaluminates including LiAlEt4 are known to polymerize methyl methacrylate monomers [222]. This is, however, only possible at the very low temperature of -78°C. In contrast, preformation of the i-Bu2Al(BHT)-t-BuLi complex, then treatment of methyl methacrylate at 0°C afforded PMMA with a molecular weight of 28400 (Scheme 6.174) [223]. NMR analysis revealed that the alkyl group bonded to the end of the polymer chain is the t-Bu moiety derived from t-BuLi and not alkyl groups from aluminum. The contribution of two enolate intermediates, monomeric and dimeric aluminum species, was invoked to account for the structure of the initiation species. 

Table III. Gibbs Free Energies of Formation of Monomeric Aluminum Species...

Activities of monomeric aluminum species calculated in this manner can then be plotted on a graph of log activity as a function of pH and aging time. An example (for solution H) of such a plot is illustrated in... 

McLachlan et al. 1991b Meiri et al. 1993 Yokel 2000). Recent investigations suggest that AD is associated with enhanced bioavailability of certain aluminum species. AD patients have greater gastrointestinal absorption than controls, and specific exposure to organic monomeric aluminum increases risk of AD compared with exposure to other forms of aluminum (Gauthier et al. 2000 Moore et al. 2000). 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

The reaction of metal species with added colorimetric ligand has also been utilized to gain speciation information. A classic example is the use of pyro-catechol violet as a spectrophotometric reagent to determine labile monomeric (fast reacting) and organic aluminum species. 

Another example is the aluminum species Al(OH)3(a(7). Again, this is a monomeric uncharged species of A1 in solution. There is really no need to assume that it has three oxygens and three hydrogens attached to it. Whatever is attached to it, we can call it AlOj 5(ag), whose properties will differ from those of Al(OH)3(ag) by those of 1.5H20(/), because... 

Recently, Katsukiand coworkers reported that chiral Al(salen) complex (74) served as the catalyst for catalytic asymmetric hydrophosphonation of aldehydes with dimethylphosphite (Scheme 6.56) [71]. It was determined by X-ray crystallographic analysis that AI[(R,R)-saIene]CI (74) is a monomeric cis-P aluminum species. On the basis of the X-ray structure of complex (74), intramolecular phosphonylation mechanism was proposed. 

Only a limited number of structural studies have been reported on beryllium compounds. The simple alkyls appear to be polymeric with chain structures as shown in XVI (109). For comparison, the structure of di-(t-butyl)beryllium (XVII) is shown as determined from electron diffraction studies (6). In this case, the compound is a linear monomeric species with a Be—C bond length of 1.699 A. Similarly, dimethylberyl-lium has a Be—C bond distance of 1.70 A in the gas phase (5). Comparison of these beryllium structures with the polymer shows that the Be—C distance in the bridge is considerably greater than that in a normal Be—C single bond, a result similar to that observed for the aluminum derivatives. 

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