Polystannanes, cyclic —

Most of the reactions that can be used to prepare distannanes can be extended to the preparation of oligo- and polystannanes, which have attracted interest for potential use in electronic and optical devices.455 The structures of the products are not necessarily those of completely linear catenanes, (R2Sn) , and different degrees of branching confer different properties. The formation of polymers is also frequently accompanied by the formation of cyclic pentamers or hexamers, and samples prepared by different methods may show substantially different properties. 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

The condensation of diorganotin dihydrides with diorganotin diamides is another route to cyclic polystannanes. This method is especially useful for the high-yield synthesis of the dimethyl derivative (Me2Sn)g [eqn (10.5)], which cannot be obtained by the dehydrocoupling process. 

Polystannanes have been shown to display very interesting properties. They are highly photosensitive and exhibit photobleaching behaviour and on UV irradiation depolymerise to yield cyclic oligomers. The materials are thermally stable to 200-270 °C in air and at more elevated temperatures function as precursors to Sn02. " By using rhodium catalysts, branched polystannanes have been prepared. 

Several cyclic and polycyclic oligostannanes with a rich structural diversity have been reported in the literature16,21 83 and it was concluded by Sita21 that the concepts and strategies established in organic chemistry can successfully be applied for the construction of polystannane frameworks as well. The polycyclic oligostannanes 16-19 are representative examples for this class of compounds. 

TABLE 6. n9Sn NMR chemical shifts (in ppm) of polystannanes and cyclic oligostannanes82 97 100 in benzene-d6... 

Studies in the held of tin-substituted cycloalkanes stem in part from a desire to understand the modifications, in terms of conformation and reactivity, brought to the carbacycle by the inclusion of increasing numbers of heteroatoms. Although not strictly heterocycles, cyclic polystannanes allow this line of investigation to be taken to its extreme conclusion, and as such they are included in this article for comparison. The synthetic routes to cyclostannanes are summarized in Eqs. (19)-(23), with details in Table I (91-103). 

Polystannanes are routinely characterized by and spectroscopy, and (GPC) against polystyrene standards of different molecular weights in THF. However, it is often " Sn NMR in particular that is a valuable tool to determine the structure of the linear oligomeric and polymeric chains and provides useful information regarding purity of polymer chains and ratios and types of cyclic oligomers. 

Even smaller band gaps are present in polystannanes with aryl substituents at tin" " For example, dehydropolymerization of R2SnH2 (R =/>- BuG6H4 or />- HexG6H4) afforded linear polymers (contaminated with some cyclics) with values up to 20,000. The materials, which are yellow in color, possess A ax values for the Sn a-a transition of 423-436 nm, that is red-shifted by 30 0 nm compared to those for poly(dialkylstannanes) and suggest that significant cr-7r-conjugation that involves the Sn backbone and the aryl substituents is present. 

---