Stannylenes adducts with Lewis acids

Some adducts of stannylenes with Lewis acids, such as boron compounds" and AlClg are known. More common are adducts of stannylenes with Lewis bases, such as 1,4-dioxane or phosphines. Stannylenes form adducts with carbenes (Scheme 2.8.5) and with carbene analogs, such as gallium(I) derivatives or even other stannylenes. In the latter adducts between two tin(II) atoms, one stannylene acts as a Lewis base and the other one as a Lewis acid. 

A stannaketenimine has been synthesized by the low-temperature reaction of a diarylstannylene with mesityl isocyanide.87 The stannaketenimine is thermally stable, crystals being obtained by sublimation at 40°C/0.01 mmHg. In this case, the stannylene acts as the Lewis acid for formation of the adduct and, along with the stannenes described above, covers the full range of bonding modes available to the stannenes. 

In 1970 Harrison and Zuckerman 136) reported for the first time the existence of a stable adduct of a stannylene (dicyclopentadienyltin(II)) with a Lewis acid (tri-fluoroborane). In the meantime, P. G. Harrison was able to demonstrate that these Lewis-acid base interactions can be extended to a variety of main group acidsl37,138) (Eqs. (15) and (16)). 

Another indication for the Lewis acidic properties of NHSns is provided by the reaction of diben-zotetraazafulvalenes with stannylenes. Electron-rich enetetramines can be cleaved into N-heterocyclic carbenes. Pioneering work by Lappert has demonstrated that this cleavage, in the presence of an electrophilic metal center, leads to NHC complexes. Hahn and coworkers have used the electrophilic tin center in the NHSn 4 to cleave the dibenzotetraazafulvalene 5 with the formation of the zwitterionic stannylene-carbene adduct 6 (Scheme 2.8.5). Similar carbene adducts of di-aminogermylenes and diaminostannylenes have been described by Schumann et al. and Lappert et al. and even transient diorganogermylenes have been stabilized by formation of an adduct with an N-heterocyclic carbene. ... 

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