Investigations of synthetic magnesium silicate hydrate (M-S-H) samples have shown that M-S-H aged for 1 year can exhibit variable compositions with molar Mg/Si ratios in the range 0.7 ≤ Mg/Si ≤ 1.5. At lower Mg/Si ratio, additional silica is present whereas brucite is observed for Mg/Si ≥1.3. FT-IR and 29 Si NMR data reveal a high degree of silicate polymerisation, indicating the formation of silicate sheets. TGA shows the presence of bound water and of hydroxyl groups bound to Mg and as silanol groups in the M-S-H, in accord with 29 Si{ 1 H}CP/MAS and high-speed 1 H NMR measurements. Raman and XRD data suggest that the M-S-H structure is related to a disordered talc precursor at low Mg/Si and to a serpentine precursor at high Mg/Si ratio. Solubility products for M-S-H phases were calculated on basis of the compositions of the aqueous solutions and a solid solution model was suggested.

We report on the successful synthesis of Si(5)Mes(6) (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal [1.1.1] cluster core featuring two ligand-free, "inverted tetrahedral" bridgehead silicon atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si-Si single bond. A detailed analysis of the electronic characteristics of Si(5)Mes(6) reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon-silicon bond in this "nonclassical" molecule.

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Two heteronuclear [1.1.1]propellanes of group 14, Ge2Si3Mes6 (1) and Sn2Si3Mes6 (2) (Mes = 2,4,6-Me3C6H2), were prepared by reductive coupling of Mes2SiCl2 and GeCl2·dioxane or SnCl2. Both compounds were characterized in detail, including X-ray structure analyses on single crystals. In each case it was found that the E2Si3 cluster core consists of three bridging {SiMes2} units and two ligand-free bridgehead atoms (Eb). As a result of the different size of the bridging units, the distances between the bridgehead atoms are considerably shorter (0.10 Å for 1 and 0.27 Å for 2) than in the homonuclear counterparts Ge5Mes6 and Sn5Dep6 (Dep = 2,6-Et2C6H3) known from the literature. The stronger Eb···Eb interactions in 1 and 2 were confirmed by electrochemical studies using cyclic voltammetry. UV/vis studies, together with density functional theory (DFT) calculations, further supported these findings. A correlation of the Eb···Eb distances and the singlet and triplet A2 transitions for a series of homo- and heteronuclear [1.1.1]propellanes revealed that higher 3A2 excitation wavelengths, and thus lower ΔE S→T energies, are obtained either by increasing the distances between the bridgehead atoms or by arranging the involved orbitals in close spatial proximity. Reactivity studies on 1 and 2 using selected reagents showed that Me3SnH or the disulfide FcS−SFc (Fc = ferrocenyl), which are prone to radical-type reactivity, can be readily added across the bridge (the tin hydride reacts only with 1). The resulting 1,3-disubstituted bicyclo[1.1.1]pentane derivatives Me3Sn−Ge(SiMes2)3Ge−H (3) and FcS−E(SiMes2)3E−SFc (4 (E = Ge) and 5 (E = Sn)) were characterized in detail, including X-ray structures of 4 and 5. Interestingly, the homolytic S−S bond addition reactions were found to be susceptible to light. Even though the tin-containing propellane 2 turned out to be more reactive than 1, both conversions can be drastically enhanced simply by using daylight in the lab.

Ge, whiz! A detailed study of the synthesis, structure, redox chemistry, and bonding properties of pentagerma[1.1.1]propellane (1, see picture) examines fundamental aspects of the interactions between the bridgehead germanium atoms. DFT and CASSCF calculations unravel the biradicaloid characteristics of 1, and preliminary reactivity studies indicate that 1 features some radical-type behavior.

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Time to B radical: One‐electron reduction of 1‐ferrocenyl‐2,3,4,5‐tetraphenylborole results in a radical anion with 5 π electrons in the borole ring. Both EPR spectroscopic investigations and spin density calculations confirm the formation of a borol radical (see picture). Further reduction stimulates an intramolecular [(C5H5)Fe] migration from the cyclopentadienyl anion to the borole dianion.