Siloxane cage structure

The following review is concerned with the synthetic and structural chemistry of molecular alumo-siloxanes, which combine in a molecular entity the elements aluminum and silicon connected by oxygen. They may be regarded as molecular counterparts of alumo-silicates, which have attracted considerable attention owing to their solid-state cage structures (see for example zeolites).1 3 Numerous applications have been found for these solid-state materials for instance the holes and pores can be used in different separation techniques.4,5 Recently the channel and pore structures of zeolites and other porous materials have been used as templates for nano-structured materials and for catalytical purposes.6 9... 

Cage structures are exemplified by the first reported polyhedral indium siloxanes synthesized when RSi(OH)3 (R = l,3-Pr2C6H3) is reacted with hiMe3 in a 1 2 ratio via partial elimination of the CH3 groups, leading to a drum-shaped indium siloxane. Complete elimination of all... 

A tantalum-siloxane cage complex (163) has been synthesized by two routes and characterized structurally (Scheme 33).3 1 A tetranuclear Ta2Si2 unit is the basis of a distorted bicapped cubane core [Ta2Si2(/i3-0)2(xi2-0)4]6+.331 Other cyclic Nb— and Ta— siloxanes based on But2Si(OH)2 have been prepared. 

There are two technically important methods by which extended Si/0 systems can be formed from molecular precursors. The first is by reaction of chlorosilanes with oxygen at high temperatures, while the second is by hydrolysis and condensation reactions of chloro- or alkoxysilanes. Chapters 32 and 33 deal with the structural evolution of siloxane structures in such reactions from an experimental and theoretical viewpoint. M. Binnewies et al. compare the stepwise formation of Si-0 networks from SiCU for both the combustion and hydrolysis reactions. The stability and reactivity of intermediate chlorosiloxanes is an important issue in this work. Both the initial process in the reaction of SiCfi with O2 and the growth of larger siloxane cages are investigated theoretically in the contribution of K. Jug. 

CLASS Cage structure polymers Dp-carborane siloxanes... 

The crystal structure of 1 is shown in Fig. 1 the atom labeling is omitted for clarity. The molecule contains a cubic SigOi2 framework with a 4-(trimethylsilylethynyl)phenyl substituent bound to each of the silicon atoms. The silicon atoms of the siloxane cage are approximately tetrahedrally coordinated. The whole molecule shows C symmetry. In contrast to the solid state, 1 shows perfect Of, symmetry in solution, which was verified by NMR spectroscopy. 

Other types of metallosiloxane guests for alkali metals are presumably possible, as suggested by the unusual structure of an indium-siloxane cage which incorporates sodium cations [48a]. 

The molecular weight of the condensed silane is also influenced by the same factors. Another factor that is important for the silane coupling effect is the molecular architecture of the siloxane networks. Caged structures and ladder structures are the extreme cases of the different siloxane structures (11,12). The solution pH, topochemical effects of the solid surface, the structure of the organofunctional group, and the concentration of the silane are all believed to influence siloxane structure, although no detailed study has yet been reported. 

A higher level of structure is illustrated by the preparation of micropo-rous hybrid polymers from functionalized cubic siloxane cages. ... 

Fig. 6.11. Different kinds of organosiloxanes in ring or cage structures. The cage siloxanes are referred to as silsesquioxanes...

The self-organization of polysilanols in the presence of other hydrogen bond acceptors has been studied by several groups.512-516 Several other publications have dealt with the stepwise synthesis of siloxane and siloxanol chains.450,517-522 Recent work on fully condensed siloxane rings and silsesquioxane cages involves the non-aqueous hydrolysis of chlorosilanes - as well as mechanistic and structural studies. 

Useful chemical reactions have been carried out in the nano-sized cavity, as illustrated by the in situ isolation of a labile cyclic siloxane trimer (Fig. 20.3.19). In the first step, three to four molecules of phenyltrimethoxysilane enter the cage and are hydrolyzed to siloxane molecules. Next, condensation takes place in the confined environment to generate the cyclic trimer SiPh(0H)0- 3, which is trapped and stabilized in a pure form. The overall reaction yields an inclusion complex [ SiPh(0H)0- 3 c Pt(bipy) 6L4](N03)i2-7H20, which can be crystallized from aqueous solution in 92% yield. The all-cis configuration of the cyclic siloxane trimer and the structure of the inclusion complex have been determined by NMR and ESI-MS. 

DD-Na is easily converted into the sila-functional silsesquioxane (DD-OH). The structure of DD-OH was confirmed by Si-NMR and other spectroscopic analyses (Scheme 2) [10,11], In quite recent time, Lee reported that the double-decker shaped silsesquioxane seems to be produced with the consumption of the partial cage and the cyclic siloxanes in the same condition [12],... 

Fig. 14. A side view along the >> axis of bonding of cyclic Sis-siloxane molecules cage type and ladder type siloxane molecules at maximum possible occupancy on an a-quartz (001) surface (3x4 unit cell) after manual matching on the X-ray structure, followed by energy minimization with DISCOVER/COMPASS at periodic-boundary conditions, with cut-off radius 9.5 A (white vertical lines mark the computational box), a, c, e methyl derivatives, b, d, f iso-octyl derivatives, a, b cyclic Sis-siloxanes, c, d cage molecules (type VII according to Ref. [75]), e, f ladder molecules. Specifications of occupancy (per 3x4 unit cell), where A= number of bound siloxane molecules, R = number of aliphatic side-chains, F == number of remaining free OH groups on the quartz surface, E = bond enthalpy (kcal moP unit cell), a iV = 4, / = 12, F = 0, A = -319.5, b V = 4, F = 12, F= 0, AF = -1 50.9, c 4,F = 36, F= 8, A = -1950, d V= 1, A = 9, F= 11,...

Among the siloxanes, the silsesquioxanes Si 03 /2R are of special interest. Si80i2R8 has a cube-like structure with the eight silicon atoms at the comers and the twelve oxygen atoms bridging the silicon atoms. Many cages of this kind have been synthesized and their properties analyzed. " Hydrido-silsesquioxanes with R = H and n = 8-18 have also been prepared and their sfructures investigated.Most recently, the size of experimentally detected silsesquioxanes was extended to n = 38. ... 

For further improvement of thermal properties of the polymer, some functional groups, such as vinyl, hydride, hydroxy, and epoxy groups, instead of a methyl group on the silylene unit were introduced into the polymer structure. Tris(penta-fluorophenyl)borane [B(CgF5)3] was also found very effective for the formation of siloxane bond by cross-condensation between silane and silanol or alkoxysUane. We used this catalyst to synthesize polysiloxanes with cage silsesquioxane in place of... 

In the formation of cage or ladder structures of silsesquioxanes, the stereochemical structure around the sihcon atom also seems importanL When two all-cis-R-T4-tetrols condense by forming a siloxane linkage, there are two arrangements, facing and apart for the two rings as illustrated in Scheme 4.10. 

REACTIVITY OF SILANE AND SILANOL FUNCTIONS IN CAGE POSS STRUCTURE SILOXANE BOND FORMATION IN THE PRESENCE OF TRIS(PENTAFLUOROPHENYL)BORANE... 

Since 1990 the ab initio calculation of NMR chemical shift parameters has been established as a reliable tool for structure elucidation of various types of compounds [1]. Based on a set of model disiloxanes, the predictive powers of various computational levels (HF, DFT, MP2) [2] for Si NMR chemical shift calculations for linear, cyclic and cage-like polysiloxanes are compared. The dependence of the calculated NMR parameters on structural changes is discussed. Finally, the performance of our theoretical approach is tested using a series of 25 siloxanes and silanes. 

Pan, G. Mark, J. E. Schaefer, D. W., Synthesis and Characterization of Fillers of Controlled Structure Based on Polyhedral Oligomeric Silsesquioxane Cages, and Their Use in Reinforcing Siloxane Elastomers. J. Polym. Sei., Polym. Phys. Ed. 2003,41,3314-3323. 

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