Frank J. Feher

Silasesquioxanes as ligands in inorganic and organometallic chemistry

Abstract Silasesquioxanes are an interesting class of ligands for both main group and transition-metal elements. A variety of coordination environments can be supported by silasesquioxane ligands, and it is now possible to prepare metallasilsesquioxanes containing practically any stable element from Groups 1015. This article surveys the synthesis, characterization, and reactivity of silasesquioxane ligands. Structure, bonding and structural dynamics of both silasesquioxanes and metallasilasesquioxanes are also discussed. The chemistry of silasesquioxanes and metallasilasesquioxanes can provide important insights into the chemistry of silica and silica-supported transition-metal catalysts. The uniqueness of silasesquioxanes as models for silica is discussed and several systems which provide molecular-level insights into the surface chemistry of silica and silica-supported catalysts are discussed.

Syntheses of highly-functionalized polyhedral oligosilsesquioxanes

Abstract The hydrolytic condensation of p-ClCH2C6H4SiCl3 in aqueous acetone affords [p-ClCH2C6H4SiO3/2]8 (1), a synthetically useful precursor for the syntheses of octafunctional polyhedral oligosilsesquioxanes [p-XCH2C6H4SiO1.5]8, including 2 (X = I), 3 (X = OH), 4 (X = ONO2), 5 (X = OAc), 6 (X = p-nitrobenzoyl), and 7 (X = methylterephthaloyl).

Amine and ester-substituted silsesquioxanes: synthesis, characterization and use as a core for starburst dendrimers

Practical procedures are reported for the syntheses of amine and ester-substituted silsesquioxane frameworks with the formula R8Si8O12 with R = CH2CH2CH2NH2 (2), [CH2CH2CH2NH3]Cl (3) and CH2CH2CMe2CH2CO2Me (4); the use of 2 as a core for starburst dendrimers with R = CH2CH2CH2N(CH2CH2CO2Me)2 (5) and CH2CH2CH2N(CH2CH2C(O)NHCH2CH2NH2)2 (6) is also described.

New polyhedral oligosilsesquioxanes via the catalytic hydrogenation of aryl-containing silsesquioxanes

The hydrolytic condensation of RSiCl3 (R = benzyl, m-tolyl, 3,5-dimethylphenyl) gives good yields of the corresponding octameric aryl silsesquioxanes (1). A single-crystal X-ray diffraction study of highly soluble 1 (R = benzyl) reveals that highly efficient crystal packing can be accomplished without the inclusion of solvent or the strong intermolecular π-stacking arrangements that normally lead to poor solubility properties. The catalytic hydrogenation of aryl polyhedral oligosilsesquioxanes (POSS) affords high yields of the corresponding aliphatic silsesquioxanes. These new silsesquioxanes display thermal and physical properties comparable to the corresponding aryl-containing POSS but generally have much greater solubilities in common organic solvents. The catalytic hydrogenation of [Ph12Si12O20] affords iso-[Cy12Si12O20], which possesses local C2v rather than D6h symmetry.

Synthesis, Characterization and Reactivity of cis-cis-cis [(C6H5)4Si4O4(OH)4]

The hydrolytic condensation of PhSiCl3 (H2O/acetone, 0–5 C, 18 h) precipitates cis-cis-cis-[(C6H5)4Si4O4(OH)4] (5a) as the sole crystalline product in approximately 40% yield. The all-cis structure, which was originally assigned by J. F. Brown on the basis of IR spectroscopy and derivatization studies, was confirmed by a single-crystal X-ray diffraction study. The ether solvate of 5a crystallizes from Et2O/C6H6 as a hydrogen-bonded, dimeric structure, which kinetically stabilizes the molecule toward self-condensation reactions. In solution, particularly in the presence of weak amine bases, 5a rapidly self-condenses to form “T-resins”; self-condensation also occurs in the solid-state (25° C) over several months. Tetrasilanol 5a can be exhaustively silylated with Me3SiCl to afford excellent yields of cis-cis-cis-[(C6H5)4Si4O4(OSiMe3)4] (10), but all attempts to effect clean partial silylation produced complex mixtures of partially silylated T-resins. Attempts to effect catalytic hydrogenation of 5a to cis-c...

Preparation of gold thin films by epitaxial growth on mica and the effect of flame annealing

Highly crystalline thin films of gold were grown on scratch-free mica. Epitaxial growth was accomplished by heating a fleshly cleaved 8 mm×8 mm piece of mica to 380 °C for 12 h at 1.9×10−7 Torr, followed by gold deposition at 1 A/s and 380 °C until 100 nm was deposited. After flame annealing, these gold films are observed to have grains that are on average 6300 A in width with a peak to peak topography of a few atomic layers. Adhesion of gold to mica was accomplished by controlling the temperature and length of bakeout in addition to the temperature of deposition. Films prepared in this manner exhibited excellent adhesion when immersed in all solvents except water. The nearly universally [111] terminated gold films were characterized by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). STM imaging showed that the gold surfaces exhibit the 22×√3 reconstruction. Flame annealing of films removes contaminants and increases the flat surface area by a factor of 25 relative to the u...

A new route to incompletely-condensed silsesquioxanes: base-mediated cleavage of polyhedral oligosilsesquioxanes

Readily available R8Si8O12 frameworks react selectively with aqueous Et4NOH to afford discrete incompletely-condensed silsesquioxanes: reaction of R8Si8O12 (2a, R = c-C6H11; 2b, R = Bui) with Et4NOH first produces endo-D2-R8Si8O11(OH)2 (3a, R = c-C6H11; 3b, R = Bui), which reacts further with Et4NOH to produce endo-C2-R8Si8O10(OH)4 (5a and 5b) and endo-C3-R7Si7O9(OH)3 (4a, R = c-C6H11; 4b, R = Bui); the reactions of Et4NOH with Me8Si8O12 (2c), (c-C6H11)7(H)Si8O12 (2d) and (c-C6H11)7(OH)Si8O12 (2d) also produce the corresponding trisilanols (i.e. endo-C3-R7Si7O9(OH)3 (4a, R = c-C6H11; 4c, R = Me).

Controlled cleavage of R8Si8O12 frameworks: a revolutionary new method for manufacturing precursors to hybrid inorganic–organic materials

Cube-octameric polyhedral silsesquioxanes (R8Si8O12) react with strong acids (HX) to produce R8Si8O11X2 frameworks resulting from selective cleavage of one Si–O–Si linkage; subsequent hydrolysis affords R8Si8O11(OH)2 frameworks derived from the net hydrolysis of one Si–O–Si linkage in R8Si8O12; these results demonstrate for the first time that readily available R8Si8O12 frameworks can be used as precursors to incompletely condensed Si/O frameworks and have important implications for the manufacture of hybrid inorganic–organic materials based on discrete polyhedral clusters of silicon and oxygen.

The chemistry of methyl and ethyl radicals on Pt(111) from the decomposition of tri-alkyl bismuth compounds

Abstract The chemistry of trimethyl and triethyl bismuth compounds adsorbed on Pt(111) have been studied using thermal desorption spectroscopy, Auger electron spectroscopy, and high resolution electron energy loss spectroscopy. The tri-alkyl bismuth compounds adsorb molecularly on Pt(111) at 110 K. Upon heating, the R3Bi compound decomposes and alkyl radicals are introduced to the surface. Alkyl radicals and bismuth atoms are delivered to the Pt(111) surface in a 3 : 1 (alkyl : bismuth) ratio. In the presence of Bi, it is expected that the alkyl radical chemistry is not perturbed substantially. Below multilayer coverage, the methyl and ethyl radicals, delivered to the surface by thermal decomposition of the parent, show similar trends in their chemistry. There is some fraction of parent (BiR3) desorption at ~ 190 K The remaining methyl or ethyl radicals follow two reaction pathways. The first reaction channel is dehydrogenation of the methyl or ethyl radical leading to CH or ethylidyne species, respectively, remaining on the surface. The second channel is hydrogenation involving surface hydrogen produced by the dehydrogenation channel which leads to CH4 or CH3CH3 formation, respectively. This pathway leads to reaction rate limited desorption of CH4 with a peak temperature at 280 K and CH3CH3 with a peak temperature at 295 K.

Syntheses of highly functionalized cube-octameric polyhedral oligosilsesquioxanes (R8Si8O12)

Reactions of [H2N(CH2)3]8Si8O12 or its octahydrochloride salt with a variety of electrophiles, including anhydrides, lactones, acid chlorides, α,β-unsaturated esters and isocyanates, afforded functionalized R8Si8O12 frameworks in good to excellent yields. Practical methods for the synthesis of [HO(CH2)3]8Si8O12, [OCN(CH2)3]8Si8O12 and [(Ph2PCH2)2N(CH2)3]8Si8O12 are also reported.