New Polymer Precursors To SiNCB Materials

The first borazine/silazane backbone copolymers derived from the parent borazine, B3N3H6, have been obtained by the thermal condensation of borazine with either of two silazanes, tris(trimethylsilylamino)silane (TTS) or 1,1,3,3,5,5- hexamethylcyclotrisilazane (HCT), to yield copolymers of typical composition (B3N3H4)1.00(N)1.17(SiMe 3)1.16 (SiH)0.34 and (B3N3H4) 1.00(N) 1.67(SiMe 2)1.49(H) 1.5, respectively. Despite their similar compositions, upon pyrolysis the TTS copolymers yield B1.0N1.0Si<0.2 ceramics, while the ceramics derived from the HCT copolymers showed greater retention of silicon and carbon with typical compositions of B1.0N1.5Si0.4C0.2 The XRD spectra show the materials are amorphous to 1400 °C, but show crystalline phases of β-Si3N4, °C, with no diffraction from any boron-containing species. The DRIFT spectra of the ceramics, however, indicate the presence of boron nitride.

Formation of nanostructured, nanocrystalline boron nitride microparticles with diatom-derived 3-D shapes

The first use of diatom frustules as shape-dictating 3-D templates for the syntheses of nanostructured, nanocrystalline micro-particles of a non-oxide ceramic, boron nitride, is demonstrated.

Silicon-based ceramics from polymer precursors

A hyperbranched polycarbosilane of the type [R3SiCH2−]x[−SiR2CH2−]y[−SiR(CH2−)1.5]z[−Si(CH2−)2]l (R = H, –CH2CH=CH2, or OR) has been prepared, which was used as a source of inorganic/organic hybrid materials and, through pyrolysis, as a precursor to inorganic solids with unusual microstructures and properties. A partially allyl-substituted derivative “AHPCS”, nominally ['Si(allyl)0.1H0.9CH2']n, has been extensively studied as a precursor to silicon carbide (SiC) and is now used commercially as a SiC matrix source for C- and SiC-fiber-reinforced composites and binder for particulate ceramics. The alkoxy derivatives, ['Si(OR)2CH2'], (R = Me, Et) yield, after hydrolysis and condensation, carbosilane/siloxane gels with unusually high surface areas (700–900 m2/g) and microporosity that is retained in the resultant SiOxCy ceramics formed after pyrolysis to 1000 °C. The fully condensed ['Si(O)CH2'] gel in the latter case was obtained as thin, adherent films on Si surfaces by spin coating and was found to exhibit dielectric constants as low as 2.0 after heating to 400 °C. The SiC precursor, AHPCS, has also been used recently, along with other polymeric precursors, to make two-phase (SiC/C and SiC/BN) amorphous ceramics that exhibit unusual microstructures and thermal/mechanical properties. These microstructures are formed during the mixing and thermosetting of the constituent polymers, which undergo phase separation due to their immiscibility. Certain of the SiC/C composites, which have the C phase uniformly distributed as ca.1-µm droplets in a SiCx matrix, exhibit high oxidation resistance, and microindentation tests on the SiC/BN system suggest unusual toughness.

Transition-metal-promoted reactions of boron hydrides. 17. Titanium-catalyzed decaborane-olefin hydroborations

The titanium-catalyzed hydroboration reactions of decaborane with a variety of terminal olefins have been found to result in the exclusive, high-yield formation of monosubstituted decaborane 6-R-B(10)H(13) products, arising from anti-Markovnikov addition of the cage B6-H to the olefin. The titanium-catalyzed reactions are slow, often less than one turnover per hour; however, their high selectivities and yields coupled with the fact that they are simple, one-pot reactions give them significant advantages over the previously reported routes to 6-R-B(10)H(13) compounds. The catalyst also has extended activity with reactions carried out for as long as 13 days, showing little decrease in reactivity, thereby allowing for the production of large amounts of 6-R-B(10)H(13). The titanium-catalyzed reactions of decaborane with the nonconjugated diolefins, 1,5-hexadiene and diallylsilane, were found to give, depending upon reaction conditions and stoichiometries, high yields of either alkenyl-substituted 6-(CH(2)=CH(CH(2))(4))-B(10)H(13) (4) and 6-(CH(2)=CHCH(2)SiMe(2)(CH(2))(3))-B(10)H(13) (5) or linked-cage 6,6'-(CH(2))(6)-(B(10)H(13))(2) (6) and Me(2)Si(6-(CH(2))(3)-B(10)H(13))(2) (7) compounds, respectively. The unique tetra-cage product, Si(6-(CH(2))(3)-B(10)H(13))(4) (8), was obtained by the catalyzed reaction of 4 equiv of decaborane with tetraallylsilane. Sequential use of the titanium catalyst and previously reported platinum catalysts (PtBr(2) or H(2)PtCl(6).6H(2)O with an initiator) provides an efficient pathway to asymmetrically substituted 6-R-9-R'-B(10)H(12) species. The structures of compounds 5, 6, and 8, as well as a platinum derivative, (PSH(+))(2)-commo-Pt-[nido-7-Pt-8-(n-C(8)H(17))B(10)H(11)](2)(2-), of 6-(n-octyl)decaborane have been established by single-crystal crystallographic determinations.

Syntheses and structural and electrochemical characterizations of vanadatricarbadecaboranyl analogues of vanadocene and the structural characterization of the [Li(CH(3)CN)2+](6-CH(3)-nido-5,6,9-C(3)B(7)H(9-) tricarbadecaboranyl anion

A single-crystal X-ray determination of the [Li(CH(3)CN)(2)(+)](6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-)) salt has shown that the 6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-) tricarbadecaboranyl anion has a nido-cage geometry based on an octadecahedron missing the unique six-coordinate vertex. The resulting six-membered open face is puckered, with two of the cage carbons (C6 and C9) occupying the low-coordinate cage positions above the plane of the four remaining atoms (C5, B7, B8, and B10). The Li(+) ion is centered over the open face and is solvated by two acetonitrile molecules. The reactions of the 6-CH(3)-nido-5,6,9-C(3)B(7)H(9)(-) anion with various vanadium halide salts, including VCl(4), VCl(3), and VBr(2), each resulted in the isolation of the same five paramagnetic products (2-6) of composition V(CH(3)-C(3)B(7)H(9))(2). X-ray crystallographic determinations of 2-5 showed that the complexes consist of two octadecahedral VC(3)B(7) fragments sharing a common vanadium vertex and established their structures as commo-V-(1-V-4'-CH(3)-2',3',4'-C(3)B(7)H(9))(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9)) (2), commo-V-(1-V-5'-CH(3)-2',3',5'-C(3)B(7)H(9))(1-V-4-CH(3)-2,3,4-C(3)B(7)H(9)) (3), commo-V-(1-V-5'-CH(3)-2',3',5'-C(3)B(7)H(9))(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9)) (4), and commo-V-(1-V-2-CH(3)-2,3,4-C(3)B(7)H(9))(2) (5). These complexes can be considered as tricarbadecaboranyl analogues of vanadocene, (eta(5)-C(5)H(5))(2)V. However, unlike vanadocene, these complexes are air- and moisture-stable and have only one unpaired electron. The five complexes differ with respect to one another in that they either (1) contain different enantiomeric forms of the CH(3)-C(3)B(7)H(9) cages, (2) have a different twist orientation of the two cages, or (3) have the methyl group of the CH(3)-C(3)B(7)H(9) cage located in either the 2 or 4 position of the cage. Subsequent attempts to oxidize the compounds with reagents such as Br(2) and Ag(+) were unsuccessful, illustrating the ability of the tricarbadecaboranyl anion to stabilize metals in low oxidation states. Consistent with this, both the electrochemical oxidation and the reduction of 2 were much more positive than those of the same oxidation state changes in vanadocene. The one-electron reduction of 2 is a remarkable 2.9 V positive of that of Cp(2)V.

Cytotoxicity of ferratricarbadecaboranyl complexes in murine and human tissue cultured cell lines

The ferratricarbadecaboranyl salts [1-(eta 5-C5H5)Fe-2-CH3-2,3,4-C3B7H9] + X-(X- = AsF6-, 1 and SbF6-, 2), as well as the neutral complex, 1-(eta 5-C5H5)Fe-2-CH3-2,3,4-C3B7H9, 4, are effective cytotoxic agents, causing cell death in a number of tissue culture lines, e.g. L-1210, Tmolt3, HL-60, and HeLa-S3. In general, these agents were not active against the solid cell growth, i.e. KB nasopharynx, A431 skin, HCT-8 ileum, SW480 colon, osteosarcoma, and glioma. However, they were active against the growth of lung bronchogenic MB-9812. The mode of action of the derivatives involves inhibition of de novo purine synthesis of L-1210 cells, which reduces DNA and RNA syntheses. Purine synthesis was reduced by complexes [1-(eta 5-C5H5)Fe-2-CH3-2,3,4-C3B7H9]+ [SbF6]- 2 and ¿[1-(eta 5-C5H5)Fe-2-CH3-2,3,4-C3B7H9]+ [SbF6]- [1-(eta 5-C5H5)Fe-4-CH3-2,3,4-C3B7H9]+ [SbF6]-¿ [20:1] 3, at the regulatory enzyme, i.e. PRPP amido transferase. The agents lowered d[ATP] and d[CTP] pools, further reducing DNA synthesis. The salts 1, 2, and 3 were shown to be moderate inhibitors of isolated L-1210 DNA topoisomerase activity. DNA strand scission was evident after incubation with compounds 1-4 and [1-(eta 5-C5H5)Fe-2-CH3-2,3,4-C3B7H9 and 1-(eta 5-C5H5)Fe-4-CH3-2,3,4-C3B7H9] [20:1] 5, for 24 hours at 100 microM, lowering DNA synthesis, and causing cell death. The ferratricarbadecaboranes show activities and specificities different than those found for the analogous ferrocenium salts [(eta 5-C5H5)2Fe]+ [AsF6]- 6 and [(eta 5-C5H5)2Fe]+ [SbF6]- 7, and the neutral ferrocene compound (eta 5-C5H5)2Fe 8. The bioactivites of both the tricarbaboranyl and metallocenium salts are attributed to the cations, since Na+[AsF6]- 9, K+ [AsF6]- 10, Na+[SbF6]- 11, and K+[SbF6]- 12 were found to be inactive at equivalent ionic concentration.