Stannylenes spectroscopy

The low-temperature photolysis of hexakis(2,4,6-triisopropylphenyl) cyclotristannane leads quantitatively to the corresponding distannene (Eq. 30).90 The distannene was found to be in equilibrium with the cyclotristannane over a wide range of temperatures. The simplest explanation for this is that the stannylene is formed as an intermediate from either the cyclotristannane or the distannene, but quickly adds to the distannene or dimerizes. This equilibrium can be followed by ll9Sn NMR spectroscopy at elevated temperatures.96... 

The synthesis of doubly bonded tin compounds by the coupling of stannylenes, however useful, is limited by the need for a stable stannylene and often a second divalent species (for example, a carbene or isonitrile). The simplest example of this reaction is the formation of tetrakis[bis(tri-methylsilyl)methyl]distannene from two molecules of the corresponding stannylene,83 with which it is in equilibrium in solution as evidenced by NMR spectroscopy.91... 

Apart from the determination of the structures of stannylenes by diffraction methods (X-ray or electron diffraction) many other physico-chemical techniques can be exployed to characterize these compounds more completely. Besides the classical methods such as IR-, Raman-, PE-, UV- and NMR-spectroscopy, MoBbauer-119 m-tin spectroscopy is widely used for the determination of the oxidation states of tin atoms and of their coordination 1n8-12°-123>. jt is not in the scope of this report to study the dependence of MoBbauer constants such as isomer shift and quadrupole splitting on structural parameters. Instead, we want to concentrate on one question Which information can we deduce from the structure of stannylenes to evaluate their reactivity ... 

This section is devoted to the electronic spectra of germylenes, stannylenes and plumby-lenes carrying polyatomic substituents. The available data on the absorption maxima of these species are collected in Table 4, from which it can be seen that only stable polyatomic stannylenes and plumbylenes have been characterized by UV spectroscopy. 

The number of polyatomic germylenes, stannylenes and plumbylenes characterized by their vibrational spectra is still very limited. Only IR spectroscopy was used for this purpose. Unstable molecules were studied in low-temperature inert matrices. The stable germylenes, stannylenes and plumbylenes were treated by standard means. 

When the two oxygen atoms involved in the stannylene acetal are diaste-reotopic, three dimers, two with C2 symmetry, can be formed, as shown in Fig. 7 for those obtained from 1,2-propanediol. Dimers are named by means of the numbers of the tricoordinate oxygen atoms.44 Steric effects appear to be the most important factor in determining the relative populations of the three dimers. In particular, stannylene acetals derived from fram-diols with one adjacent axial substituent exist in solution, to the level of detection of 119Sn NMR spectroscopy, as the symmetric dimer in which the tricoordinate oxygen atom is not adjacent to the axial substituent.44 Similarly, dialkylstannylene acetals from carbohydrate-derived terminal 1,2-diols exist predominantly as symmetric dimers with the primary oxygen atoms tricoordinate (Fig. 8).19,37... 

Reactivity of the highly crowded silicon-substituted cyclic stannylene 70 was investigated in detail <20020M2430>. Upon treatment with iodoalkanes, enones, and diones, the derivatives 71, 72, and 73 were obtained (Figure 3) and their structure was confirmed by H, 13C, z9Si, and 119Sn NMR spectroscopy as well as X-ray crystallographic studies. 

Stannyl radicals R3Sn can be identified by ESR spectroscopy, but there is no equivalent sensitive and selective technique for observing the transient simple (singlet) stannylenes, R2Sn . Their formation usually has to be inferred from their reactions, and it is not always unambiguous as to whether monomeric R2Sn has existed as a kinetically free entity the same problem of course occurs with the carbenes R2C . 

These compounds are less aggregated in solution. For instance, 2,2-dibutyl-l,3,2-dioxastannolane, a polymer in the solid state, has been shown, by variable temperature Sn NMR spectroscopy, to be a mixture of dimers, trimers, and tetramers in solution, with dimers predominating at room temperature and above. This technique has also indicated that most carbohydrate-derived stannylene acetals are present predominantly as dimers in solution." " " Supporting evidence has been obtained from mass spectral studies and by comparison of solid-state NMR spectra with those of solutions." Some dibutylstannylene acetals derived from cw-diols contain an observable proportion of higher oligomers benzyl 4,6-0-benzylidene-2,3-0-dibutylstannylene-o -D-mannopyranoside is present as a mixture of a dimer and a trimer in chloroform-d at -60 °C but, in the less polar solvent toluene-dg, is mainly present as a tetramer at that temperature. ... 

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