O,O-Monochelate complexes of silicon and germanium halides: The derivatives of l-mandelic N,N-dimethylamide

Reactions of O-trimethylsilyl-l-mandelic N,N-dimethylamide (1) with tetrachlorosilane and tetrachlorogermane lead to O,O-monochelate complexes, [1-(dimethylcarbamoyl)-1-phenylmethoxy]trichlorosilane (2) and [1-(dimethylcarbamoyl)-1-phenylmethoxy]trichlorogermane (3). Pentacoordination of silicon and germanium in these complexes was confirmed by X-ray studies.X-ray data show that the Si and Ge atoms in 2 and 3 have TBP environments where the ether oxygen and two halogens are equatorial while the third halogen and the amide oxygen occupy axial positions. The axial O–M and Cl–M (M = Si, Ge) distances are somewhat longer than those in similar compounds of tetracoordinate silicon and germanium.Intramolecular coordination in compounds 1–3 and relative stabilities of different conformations of their molecules were studied by quantum-chemical calculations.     

Influence of the initial chemical conditions on the rational design of silica particles

The influence of the water content in the initial composition on the size of silica particles produced using the Stöber process is well known. We have shown that there are three morphological regimes defined by compositional boundaries. At low water levels (below stoichiometric ratio of water:tetraethoxysilane), very high surface area and aggregated structures are formed; at high water content (>40?wt%) similar structures are also seen. Between these two boundary conditions, discrete particles are formed whose size are dictated by the water content. Within the compositional regime that enables the classical Stöber silica, the structural evolution shows a more rapid attainment of final particle size than the rate of formation of silica supporting the monomer addition hypothesis. The clearer understanding of the role of the initial composition on the output of this synthesis method will be of considerable use for the establishment of reliable reproducible silica production for future industrial adoption.

Open image in new window

     

Hydrogenation of

The course of the hydrogenation of [5]- and [6]metacyclophane (1b and 1c) and their thermochemistry is described. Both compounds are hydrogenated rapidly (within 10 s) to furnish the bridgehead olefins 13b and 12c. The accompanying hydrogenation enthalpies are -220 and -141 kJmol(-1), respectively. Strain energies (SE) and olefinic strains (OS) of a number of bridgehead olefins have been evaluated by DFT calculations; it was concluded that 13b belongs to the class of hyperstable olefins which correlates nicely with its reluctance to undergo hydrogenation. By combining experimental hydrogenation enthalpies and DFT calculations, SE of 187 and 121 kJmol(-1) were derived for 1b and 1c.