Polystannanes

Much of the interest in the polysilanes, polygermanes, and polystannanes involves their s-delocalization and their a-tr delocahzation when coupled with arenes or acetylenes. This is not imexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. Conductivity is increased by doping, illumination, and application of an electric field. 

There are numerous ways of forming bonds between two group 14 atoms. An important one, discussed above, involves the nucleophilic attack of a silyl anion on a silyl halide. Another method, called hydrostannolysis, involves condensation of a Sn-H bond and a Sn-NMe2 linkage, with the elimination of HNMe2. Th following reaction leads to a branched Sn4 

By analogy with silyl anion chemistry, it s not unreasonable to postulate stannyl anion intermediates, the first of which could be produced as depicted below  

The stannyl anion is an effective nucleophile that may kick out the first HNMe2 molecule. A two-step Siv[2-Si mechanism is as follows  

The distannane so produced may be deprotonated again, and the resulting anion may carry out a second nucleophilic displacement to produce a tristannane. The process continues until all the Sn-H bonds have been consumed and replaced by Sn-Sn bonds. 

An alternative to the above mechanism (involving stannyl anions) is to consider a concerted cyclic mechanism, as shown below  

The synthesis of tristannanes and linear tetrastannanes can be achieved in a straightforward manner by employing the hydrostannolysis reaction and diorganotin dihydrides (26) and 1,1,2,2-tetraorganodistannanes (9) according to Eq. (8) 49). As shown, this methodology can be used to 

Relatively few linear polystannanes have been the subject of crystallographic analysis Table IV and Fig. 4 provide some selected structural 

Selected Bond Lengths and Bond Angles for Linear Polystannanes 

The series of linear polystannanes represented by compounds 31, 33, 35, 48, and 49 have also been the subject of spectroscopic analysis. In this case, a similar trend is observed, although the magnitude of the red-shift per tin atom is less than that observed for the previous series (Fig. 6). This discrepency between the two series of linear polystannanes is likely to be a result of the strong nonbonded interactions in the former (i.e., 39, 40, 50, and 51), which severely restrict conformational mobility 

The Sn NMR parameters [S and J( Sn- Sn) values] for several selected linear tri- and tetrastannanes and for one pentastannane are provided in Table V. As with distannanes, some general trends can be observed from this compilation. To begin with, an increase in the a-branching of alkyl substituents has a pronounced effect on the chemical shifts of tristannanes for both the terminal and the bridging tin atoms, as seen in 

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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 ... 

Sita, Lawrence R., Structure/Property Relationships of Polystannanes. 38 189... 

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. 

The diorganotin dihydrides react with catalysts based on the metallocenes of titanium, zirconium, or hafnium459 162 to give oligo- and polystannanes. 

This review will focus on the NMR properties of Zintl ion complexes, namely the solution properties of the ions where E = Si, Ge, Sn, Pb, and the products derived from those clusters. Closely related clusters prepared by other means, such as the recent, elegant organo polystannane work of Schnepf, Power, Huttner, and Fischer, are briefly mentioned but are not the focus of this review. Related overviews of dynamic organometallic complexes [1,2] and the stmcture and bonding of Zintl ions [3-5] can be found in previous reviews and in other chapters of this book. 

Sita LR (1995) Stracture/property Relationships of Polystannanes. In Stone EGA, West R (eds) Advances in organometallic chemistry, vol 38. Academic, London, pp 189-243... 

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 would be expected to possess even more o-delocalised structures, as suggested by studies of linear oligostannanes with up to six tin... 

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. 

The polysilanes can be regarded as one-dimensional analogs to elemental silicon, on which, of course, nearly all of modem electronics is based. The photophysical behavior of polysilanes is not approached by any other materials, save for the less stable and more costly polygermanes and polystannanes. The remarkable properties of polysilanes have led to intense interest, and to numerous proposed high-tech applications. But the great promise of polysilanes as materials has yet to be realized. Their only commercial use at present is as precursors to silicon carbide ceramics, an application which takes no advantage of their optical or electronic properties. 

Highly coordinated tin enolates, reactivity, 9, 362 High-molecular weight polystannanes, preparation and properties, 12, 381... 

D. Redistributive Coupling of Hydrostannanes to Highly Branched Polystannanes. 161... 

As described in Section II.E, polystannanes, which can be prepared by the Wurtz coupling of chlorostannanes with alkali metals and by the dehydrocoupling of hydrostannanes, exhibit intriguing physical properties. The Sita group reported a peculiar polymerization of -Bu2SnH2 with a RhH(CO)(PPh3)3 catalyst [Eqs. (21a) and (21b)].63... 

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