Catenated derivatives

Derivatives with silicon-silicon bonds include cycles, chains and polymers. The relationships between derivatives that contain two or more contiguous silicon centers is shown in Scheme 5. In most cases the origin of the catenated derivatives may be traced back to a dihalosilane. 

The effect of substituents on the thermal stability of catenated derivatives is worth noting stability increases in the sequence halide < organo. Significantly, the substituents which stabilize catenated compounds are not those which form the strongest bonds to the Group IV elements, but those which release electrons most readily. It is likely that the M—M bond energy in a derivative pC(8-)]3M(8- -)-M(8-t-)pC(8-)]3 is sensitive to the size of the positive charge 8- - on M, and if, as seems likely in many cases, fission of the M—M bond is a key step in their decomposition, the stability sequence is readily understood. 

The ability of C to catenate (i.e. to form bonds to itself in compounds) is nowhere better illustrated than in the compounds it forms with H. Hydrocarbons occur in great variety in petroleum deposits and elsewhere, and form various homologous series in which the C atoms are linked into chains, branched chains and rings. The study of these compounds and their derivatives forms the subject of organic chemistry and is fully discussed in the many textbooks and treatises on that subject. The matter is further considered on p. 374 in relation to the much smaller ability of other Group 14 elements to form such catenated compounds. Methane, CH4, is the archetype of tetrahedral coordination in molecular compounds some of its properties are listed in Table 8.4 where they are compared with those of the... 

Catenation is well established in organotin chemistry and distannane derivatives can be prepared by standard methods (see Ge, p. 396). The compounds are more reactive than organodiger-manes e.g. Sn2Meg (mp 23°) inflames in air at its bp (182°) and absorbs oxygen slowly at room temperature to give (Me2Sn)20. Typical routes to higher polystannanes are ... 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

The synthesis of the key catenate [Cu(I)N4]+ PF6- = 2(4) + (Fig. 14.8a) (one should notice that, as usual, the subscripts 4, 5, and 6 indicate the coordination number of the copper center) derives from the usual three-dimensional template strategy.23,24... 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

The mechanism of the photochemical degradation of catenated silicon derivatives has received considerable attention (25). Substituted cyclic derivatives photochemically extrude a silylene fragment which can be intercepted by appropriate trapping reagents (e.g., trialkylsilanes or 2,3-dimethyl butadiene). This extrusion results in the formation of the corresponding ring contracted cyclopolysilane. The process continues upon additional irradiation until a cyclotetrasilane results which then undergoes... 

Shimada and coworkers recently reported the synthesis of the poly[2]catenand 56, which is structurally related to the poly[2]catenand 51b (Scheme 20) [60]. Similarly to what has been observed for the co-polycondensation of the dihy-droxy[2]catenand 50b with the terephthalic acid derivative 47, the reaction of the diamino[2]catenand 53 with adipoyl dichloride affords mainly the Pretzel -shaped compound 54 [3, 41, 60]. The poly[2]catenand 56 has been obtained via the polycondensation of the diamino[2]catenate 54 with adipoyl dichloride, yielding the poly[2]catenate 55. Subsequently, the demetalation of the latter affords the linear poly[2]catenand 56 with M = 8.1xl05, using polystyrene standards, which corre-... 

CATENATED NITROGEN LIGANDS PART II.1 TRANSITION METAL DERIVATIVES OF TRIAZOLES, TETRAZOLES, PENTAZOLES, AND HEXAZINE... 

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