- Facile Synthesis of Unsymmetrical Trialkoxysilanols: (RO)2(R′O)SiOH
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Trialkoxysilanols, (RO)3SiOH, are useful as ligands in transition-metal complexes because they provide models for silica-supported metal sites or precursors for the thermolytic precursor approach. However, their synthesis is mostly limited to symmetrical ones, where all RO ligands are the same. However, unsymmetrically substituted trialkoxysilanols could offer significant advantages over their symmetrical counterparts by facilitating crystallization of complexes, lowering crystallographic disorder, changing the thermal properties of the complexes made, and making the addition of pendant functional groups possible. Herein, a simple, general synthetic procedure yielding unsymmetrical trialkoxysilanols (RO)2(R′O)SiOH is presented using imidazole as a promoter.
- Docherty, Scott R.,Estes, Deven P.,Copéret, Christophe
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- Ga[OSi(OtBu)3]3·THF, a thermolytic molecular precursor for high surface area gallium-containing silica materials of controlled dispersion and stoichiometry
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The molecular precursor tris[(tri-tert-butoxy)siloxy]gallium, as the tetrahydrofuran adduct Ga[OSi(OtBu)3]3·THF (1), was synthesized via the salt metathesis reaction of gallium trichloride with NaOSi(OtBu)3. This complex serves as a model for isolated gallium in a silica framework. Complex 1 decomposes thermally in hydrocarbon solvent, eliminating isobutylene, water, and tert-butanol to generate high surface area gallium-containing silica at low temperatures. When thermal decomposition was performed in the presence of P-123 Pluronic as a templating agent the generated material displayed uniform vermicular pores. Textural mesoporosity was evident in untemplated material. Co-thermolysis of 1 with HOSi(OtBu)3 in the presence of P-123 Pluronic led to materials with Ga : Si ratios ranging from 1 : 3 to 1 : 50, denoted UCB1-GaSi3, UCB1-GaSi10, UCB1-GaSi20 and UCB1-GaSi50. After calcination at 500 °C these materials exhibited decreasing surface areas and broadening pore distributions with increasing silicon content, indicating a loss of template effects. The position and dispersion of the gallium in UCB1-GaSi materials was investigated using 71Ga MAS-NMR, powder XRD, and STEM/EDS elemental mapping. The results indicate a high degree of gallium dispersion in all samples, with gallium oxide clusters or oligomers present at higher gallium content.
- Dombrowski, James P.,Johnson, Gregory R.,Bell, Alexis T.,Tilley, T. Don
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p. 11025 - 11034
(2016/07/15)
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- Tris(tert-butoxy)siloxy derivatives of boron, including the boronous acid HOB[OSi(OtBu)3]2 and the metal (siloxy)boryloxide complex Cp2Zr(Me)OB[OSi(OtBu)3]2: A remarkable crystal structure with 18 independent molecules in its asymmetric unit
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Silanolysis of B(OtBu)3 with 2 and 3 equiv of HOSi(OtBu)3 led to the formation of BuOB[OSi(OtBu)3]2 (1) and B[OSi(OtBu)3]3 (2), respectively. Compounds 1 and 2 are efficient single-source molecular precursors to B/Si/O materials via thermolytic routes in nonpolar media, as demonstrated by the generation of BO1.5·2SiO2 (BOSi2xg and BO1.5·3SiO2 (BOSi3xg) xerogels, respectively. Use of a block copolymer template provided B/Si/O materials (BOSi2epe and BOSi3epe) with a broad distribution of mesopores (by N2 porosimetry) and smaller, more uniform particle sizes (by TEM) as compared to the nontemplated materials. Hydrolyses of 1 and 2 with excess H2O resulted in formation of the expected amounts of tBuOH and HOSi(OtBu)3; however, reaction of 1 with 1 equiv of H2O led to isolation of the new boronous acid HOB[OSi(OtBu)3]2 (3). This ligand precursor is well suited for the synthesis of new metal (siloxy)boryloxide complexes via proton-transfer reactions involving the BOH group. The reaction of 3 with Cp2ZrMe2 resulted in formation of Cp2Zr(Me)OB[OSi(OtBu)3]2 (4) in high yield. This rare example of a transition metal boryloxide complex crystallizes in the triclinic space group P1 and exhibits a crystal structure with an unprecedented number of independent molecules in its asymmetric unit (i.e., Z = 18 and Z = 36). This unusual crystal structure presented an opportunity to perform statistical analyses of the metric parameters for the 18 crystallographically independent molecules. Complex 4 readily converts to Cp2Zr[OSi(OtBu)3]2 (5) upon thermolysis or upon dissolution in Et2O at room temperature.
- Fujdala, Kyle L.,Oliver, Allen G.,Hollander, Frederick J.,Tilley, T. Don
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p. 1140 - 1150
(2008/10/08)
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