Much of the recent research in the Barron group has involved the tert -butyl derivatives of the Group 13 metals. The rationale for employing the tert -butyl group is multiple. First, the steric bulk as measured by the Tolman cone angle (q = 126û) is sufficiently large to allow isolation, yet not large enough to preclude formation of analogs of sterically less demanding alkyl groups. Second, the lack of stable tert-butyl bridges between two Group 13 metals allows for isolation of species that would ordinarily be fluxional. Third, the majority of tert -butyl compounds of the Group 13 metals are solids and thus amenable to X-ray crystallographic characterization without significant disorder of the substituents. All these factors have enabled us to prepare and structurally charactize a wide range of new compound types, including the elusive alkylalumoxanes. As part of our studies into the chemistry of aluminum, we have structurally characterized a number of tert -butyl derivatives of aluminum. These results will be discussed.
Deposition onto ZnS particles was carried out using AlH3(NMe2Et) and water vapor as co-precursors within a fluidized-bed reactor. Silver nitrate and lead acetate tests, as well as SEM/microprobe analyses confirmed the presence of a conformal coating of alumina. Deposition onto silicon wafers, quartz and carbon fibers were also carried out utilizing the same precursors within a horizontal hot-wall APCVD apparatus. Growth rates were on the order of 40-80 Å/min. Films were characterized by SEM, microprobe, X-ray diffraction and by conductivity measurements. The conformality of the films were illustrated using silicon wafers that were gas-etched prior to deposition. In all cases, the films consisted of pure amorphous alumina, possessing less than 3 wt% carbon.
The interest in single-source precursor routes to thin semiconducting films of Group 13 chalcogenides led to the synthesis of a wide variety of volatile solids with the generic form [(Me3C)Ga(m3-E)]4 (E = S, Se, Te). The MOCVD of [(Me3C)Ga(m3-S)]4 produces a GaS cubic phase with a structure reminiscent of the cubane core. However, application of UV light during the MOCVD of GaS causes an apparent core cleavage and hexagonal GaS grows instead of cubic GaS. Since previous work with UV light suggested that photolysis enhances decomposition/degradation of molecular cubanes, we undertook the present study to assess the structural stability of various cubanes in solution upon exposure to high intensity UV light. Details of the photolytic decompostion of the [(Me3C)Ga(m3-E)]4 clusters and their potential application towards solution nanoparticle growth will be discussed.
The reaction of TMA with the [(tBu)Al(O)]6 has been yields two isomers of the hybrid tert-butyl-methylalumoxane, [Al7(O)6(tBu)6Me3]; the structures of which have been determined by NMR spectroscopy. The activity of [Al7(O)6(tBu)6Me3], for the [iPr(Cp)(Flu)]ZrBz2 catalyzed polymerization of 1,5-hexadiene, is significantly increased in comparison to [(tBu)Al(O)]6. The effect of additional equivalents of TMA to the co-catalytic activity of [Al7(O)6(tBu)6Me3] suggests that a maximum activity is obtained at a [(tBu)Al(O)]6 to AlMe3 ratio of 1:6. Under conditions of equal Al:Zr ratio the [(tBu)Al(O)]6(AlMe3)6 system has a higher activity than commercial MAO. The reaction of Cp2Zr(CD3)2 with [Al7(O)6(tBu)6Me3] demonstrates that methyl exchange does not occur between a metallocene and the alkyls of the alumoxane cage, but does occur with the complexed (tBu)AlMe2.
Increasing awareness of the significance of molecular asymmetry in the pharmacological activity and pharmacokinetics of chiral compounds has lead to an intensive search for new and improved methods for the separation of enantiomers. We will report on a novel material, carboxylate-alumoxanes, as a chiral stationary phase (CSP) for enantiomer separations using HPLC. Specifically, a CSP in which a (R,R)-N,N'-dialkyltartamide derivative is linked to the surface of boehmite (Al(O)(OH)) via a carboxylate group will be investigated. Carboxylate-alumoxanes are organic substituted alumina nano-particles synthesized from boehmite in aqueous solution and are an inexpensive and environmentally-benign material. This presentation will include synthesis of the CSP, optical resolution for selected chiral compounds, and comparison to the common method of silica gel based CSPs.
A major problem with traditional "green body" ceramic processing is that significant shrinkage and therefore porosity exists in the finished ceramic body. A second problem with ceramic composites processed through traditional routes is the formation of a rough surface. This occurs from both the sintering step, as well as physical handling and post-sintering machining. We have investigated the application of carboxylate-alumoxane nano-particles through post-process infiltration and re-sintering as a solution to these problems.
Infiltration of porous YAG (Y3Al5O12) ceramic bodies with aqueous solutions of acetate-alumoxane (A-A) has been investigated. The effects of solution concentration (2 - 12 wt%), infiltration method, infiltration time, and number of infiltration/dry/fire steps has been determined. SEM, XRD, and microprobe results will be presented along with an outline of the optimum processing conditions.
We report that p-hydroxybenzoate or lysine substituted alumoxanes are readily prepared from the reaction of boehmite, [AlO(OH)]n, with the parent acid. The surface hydroxides and amines of these alumoxanes reacts with epoxides such as the diglycidylether of bisphenol-A to give a new class of inorganic-organic hybrid material. Details of the process will be reported along with the physical properties of a variety of alumoxane based composites. Solid state and conventional liquid NMR data will be presented for the cross-linked materials, along with X-Ray crystallographic data for several model systems.
A series of compounds, X-OCH2CH(OH)CH2NH-Y, where X and Y are variously sized substituents ranging from small aliphatics to bulky phenyl groups, have been examined crystallographically. These systems were investigated as models for functionalized carboxylate-alumoxanes cross-linked with conventional epoxy resin materials. The crystal packing motifs in these compounds are established by hydrogen bonding, however, the aggregation of the molecules may be controlled by the direct selection of X and Y groups. In most cases, the molecules are arranged as dimeric units that consist of the two enantiomeric forms related by a center of symmetry. However, careful choice of X and Y combinations will yield a non-centrosymmetric structure consisting of four molecules (two of each enantiomer). We shall discuss the factors which give rise to this controllability including the relative positions of the hydrogen-bonded components, and the effect of the near overall linearity of the molecule. Finally, a general approach for conferring desirable properties upon routine racemic mixtures will be presented.
Reaction of K[CpFe(CO)2] with a large excess of GaCl3 yields [{CpFe(CO)2}Ga(Cl.GaCl3)(m-Cl)]2 (1), while reactions with 1 and 0.5 equivalents yields [{CpFe(CO)2}GaCl2]n (2), and [{CpFe(CO)2}2Ga(m-Cl)]¥ (3), respectively. The molecular structure of compound 1 can be considered to be a GaCl3 complex of dimeric 2, in which the two GaCl3 moieties coordinate via a near linear chloride bridge. Compound 3 is polymeric in the solid state; involving an infinite Ga-Cl...Ga-Cl backbone with pendent [CpFe(CO)2] units, however, the solubility of [{CpFe(CO)2}2Ga(m-Cl)]¥ in CH2Cl2 and toluene suggests that its polymeric structure is cleaved in solution. Compound 2 reacts with MeCN to yield [CpFe(CO)2]GaCl2(MeCN). Reduction of compound 3 with potassium in Et2O yields the previously reported [CpFe(CO)2]3Ga and gallium metal. Reaction of K[CpFe(CO)2] with GaI3 yields [CpFe(CO)2]GaI2, which upon hydrolysis gives the unusual galloxane, [CpFe(CO)2]6Ga6(m3-O)4(m-OH)2I2 (4). Reaction of CpMo(CO)3H with Ga(tBu)3 yields [CpMo(CO)3]Ga(tBu)2 (5). The structure of compound 5 shows evidence of unusual intra- and inter-molecular carbonyl...gallium interactions. The structures of compounds 1, 3, 4.Et2O, and 5 have been determined by X-ray crystallography.
Return to Barron Group Home Page