Geminal (2)J((29)Si-O-(29)Si) couplings in oligosiloxanes and their relation to direct (1)J((29) Si-(13)C) couplings

Benzocyclobutene-functionalized hyperbranched polysiloxane for low-k materials with good thermostability

Although hyperbranched polysiloxanes have been extensively studied, they have limited practical applications because of their low glass transition temperatures. In this study, we synthesized benzocyclobutene-functionalized hyperbranched polysiloxane (HB-BCB) via the Piers-Rubinsztajn reaction. The synthesized material was cured and crosslinking occurred at temperatures greater than 200 °C, forming a low-k thermoset resin with high thermostability. The structure of the resin was characterized using nuclear magnetic resonance (NMR) spectroscopy, viz. ¹H NMR and ¹³C NMR spectroscopy. ²⁹Si NMR spectroscopy was used to calculate the degree of branching. Differential scanning calorimetry, dynamic mechanical analysis, and thermogravimetric analysis revealed that the cured resin possesses good high-temperature mechanical properties and exhibits a high thermal decomposition temperature (T_d5 = 512 °C). In addition, the cured resin has a low dielectric constant (k = 2.70 at 1 MHz) and low dissipation factor (2.13 × 10^-3 at 1 MHz). Thus, the prepared resin can function as a low-k material with excellent high-temperature performance. These findings indicate that the performance of crosslinked siloxane is significantly attributed to the introduction of BCB groups and the formation of the highly crosslinked structure.

Oxo Ligand Substitution in a Cationic Uranyl Complex: Synergistic Interaction of an Electrophile and a Reductant

Reaction of [U(VI)O2(dppmo)2(OTf)][OTf] (dppmo = Ph2P(O)CH2P(O)Ph2) with 4 equiv of Ph3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex U(IV)(OTf)4(dppmo)2 (1), as a yellow-green crystalline solid in 83% yield, along with Ph3SiOSiPh3 and [Cp2Co][OTf]. This reaction proceeds via a U(IV) silyloxide intermediate, [U(IV)(OSiPh3)(dppmo)2(OTf)2][OTf] (2), which we have isolated and structurally characterized. Similarly, reaction of [U(VI)O2(TPPO)4][OTf]2 (TPPO = Ph3PO) with 6 equiv of Me3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex, [Cp2Co][U(IV)(OTf)5(TPPO)2] (3), as a yellow-green crystalline solid in 76% yield, concomitant with formation of Me3SiOSiMe3, [Ph3POSiMe3][OTf], and [Cp2Co][OTf]. Complexes 1 and 3 have been fully characterized, including analysis by X-ray crystallography. The conversion of [U(VI)O2(dppmo)2(OTf)][OTf] and [U(VI)O2(TPPO)4][OTf]2 to complexes 1 and 3, respectively, represents rare examples of well-defined uranyl oxo ligand substitution.

Siloxane-Bond Formation Promoted by Lewis Acids: A Nonhydrolytic Sol-Gel Process and the Piers-Rubinsztajn Reaction

Siloxane formation reactions of both the nonhydrolytic sol-gel process and Piers-Rubinsztajn reaction can be integrated as Lewis acid promoted siloxane syntheses without involving silanol groups. The former was developed in the field of inorganic materials chemistry and the latter was initiated in polymer chemistry. We have realized both reactions are quite similar, in terms of 1) the nonhydrolytic reaction, 2) the use of alkoxysilanes, 3) the group-exchange reactions competing with the siloxane formation, and 4) the proposed reaction mechanisms. This Minireview focuses on the above two reactions. The evolution of both reactions should realize a more sophisticated molecular design of siloxane compounds, which surely contributes to the development of advanced functional materials.

Spectra edited by relative signs of homonuclear couplings of low abundance nuclei

The proposed homonuclear coupling sign edited (HCSE) experiment can detect signed homonuclear couplings between low abundant nuclei like (13)C, (29)Si and (15)N in linear spin systems, that is, in systems where two nuclei are coupled by the measured coupling, and one of them is coupled by a second coupling to a nucleus of different kind. The third nucleus is usually high abundant hydrogen. Two spectra are measured during the HCSE experiment. Their weighed sum and difference yield two other spectra, one containing peaks coupled only by positive measured couplings and the other having peaks coupled by negative measured couplings. The usual E.COSY-type experiment requires all three couplings in the three spin system (triangular spin system) and not only two couplings as the HCSE experiment. The experiment was successfully tested on known carbon-carbon and silicon-silicon two bond couplings. A set of six simple siloxanes with |(2) J(Si-O-Si)| couplings ranging from 0.5 to 9.0 Hz was measured for the first time, and all the couplings were found to be positive.