Plumbylenes dimerization

In order to elucidate the relationship between the structure of plumbylene dimers and the bulkiness of substituents, Weidenbruch and coworkers synthesized and characterized... 

Figure 5 Plumbylene dimer stabilized by two molecules of MgBr2...

Although Lappert et al. reported on the isolation of the purple dialkylplumbylene 65 as early as 1973, its structure has only recently been determined. In the solid state, compound 65 exists as a loosely bonded dimer with a second plumbylene molecule and has a Pb- -Pb separation of 412.9 pm, with trans-bent angles between the PbC2 plane and the Pb- -Pb vector of 34.2°.82... 

The first bis(organyl)plumbylene isolated and structurally characterized (at first in the gas phase only) was the dark blue bis[bis(trimethylsilyl)methyl]plumbylene Bsi2Pb 3046. Recently we were able to determine the crystal structure of the low-melting solid and, to our surprise, found (very) weakly associated dimers (Figure 6)49. Despite a Pb Pb distance not much shorter than twice the van der Waals radius of Pb, the dimer adopts a trans-bent conformation typical for the heavier congeners of the alkenes. In spite of the same conformation (syn, syn) in the gas phase and in the solid state, the structure displays markedly differing C—Pb—C angles of 103.6° (30) and 93.6°((30)2), respectively. 

FIGURE 6. Molecular structure of plumbylene 30 in the crystal formation of loose dimers... 

For E = Pb, neither base-stabilized nor base-free derivatives have been isolated so far. The plumbanethione 123 could be indeed prepared by desulfurization of the respective tetrathiaplumbolane at —78 °C and trapped with several reagents, but has not been isolated to date. Instead, at ambient conditions the plumbylene 124 and the head-to-tail dimer, the 1,3,2,4-dithiadiplumbetane 125, is obtained (equation 16)165. 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Experimental aspects of the dimerization of germylenes, stannylenes and plumbylenes were discussed in a review363. Relations between the characteristics of ER2 (E = C, Si, Ge, Sn, Pb R = H, F) and the structure of their dimers E2R4 were studied theoretically by Trinquier and coworkers364-367. 

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