The diarylplumbylene R 2 Pb: (3), R ) 2-tBu-4,5,6-Me 3 C 6 H, and the rearranged alkylarylplumbylene RR′Pb:, R ) 2,4,6-tBu 3 C 6 H 2 , R′ ) CH 2 C(CH 3 ) 2 -3,5-tBu 2 C 6 H 3 , were synthesized and characterized by NMR and UV/vis spectroscopy, as well as by X-ray crystallography. Treatment of 3 with the disilylplumbylene R′′ 2 Pb:, R′′ ) Si(SiMe 3 ) 3 , furnished the heteroleptic plumbylene RR′′Pb: (8), which, in the solid state, forms the plumbylene dimer RR′′PbPbRR′′ (9). The X-ray structure analysis of 9 reveals a trans-bent angle of 46.5°and a Pb ... Pb separation of 3.37 Å, the shortest observed so far between the lead atoms of two plumbylenes.

Treatment of the diarylplumbylene R2Pb: (R = 2,4,6‐iPr3C6H2) with the disilylplumbylene R′2Pb: [R′ = Si(SiMe3)3] furnishes the heteroleptic plumbylene RR′Pb: which exists as the diplumbene RR′Pb=PbRR′ (7) in the solid state. The X‐ray structure analysis of 7 reveals a trans‐bent angle of 42.7° and a Pb–Pb bond length of 298.99(5) pm, the shortest observed so far for diplumbenes with a lead–lead double bond. Reaction of mesitylmagnesium bromide with PbCl2 yielded black crystals of an MgBr2 stabilized dimesitylplumbylene dimer with a large trans‐bent angle of 71°, a lead–lead separation of 335.49(6) pm, and long Pb···Br contacts of 315.71(8) pm.

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Treatment of GeC1, . dioxane with the Grignard reagent RMgBr (R = 2-tBu-4,5,6-Me3C6H) furnishes the tetraaryldigermene R2Ge=GeR2 (8). X-ray structure-analysis of 8 reveals a short Ge=Ge double bond, a nearly planar environment of both germanium atoms and extremely widened C-Ge-C angles of 128". In solution, 8 dissociates into two germylene molecules R2Ge: which can be trapped with oxygen or by [4 + 11 cycloaddition reactions with l,4-dihetero-1,3-dienes. A similar cycloadduct of a stannylene was obtained by thermolysis of (R;SII)~ (R' = 2,4,6-iPr3C6H2), in the presence of a 1,4-diazabutadiene and characterized by an Xrdy structure-analysis.The double-bond system of alkenes is usually characterized by a planar arrangement of the substituents. Twisting of the C=C double bond can only occur in the presence of very bulky groups. However, homonuclear double bonds between the heavier homologues of carbon exhibit a different behaviour and, in particular, digermenes and distannenes show marked trans folding of the substituentsc21. Thus, for example, trans bent angles y~ of 35 to 47" have been calculated for H2Ge=GeH2, the parent compound of the digermene~ ['-~]. Such calculations are corroborated by experiment: X-ray-crystallographic studies of the tetraalkyland tetraarpldigermenes 1-3 have in each case revealed some degree of trans pyramidalization where, in addition, a slight twisting of the double bond by the angle z may also occur in order to reduce the steric overcrowding (Scheme 1y3.8-101. We have now found that a similar planar arrangement of the substituents about the Ge=Ge double bond is not only realized with electropositive groups, but can also be achieved in a tetraaryldigermene by judicious choice of the orrho-alkyl groups. Thus, the reaction of the germanium(T1) chloride . dioxane adduct with the Grignard reagent 7 resulted in the formation of the digermene 8, which was isolated in good yield in the form of orange-yellow, air-sensitive crystals. An X-ray crystal-structure analysis of 8 revealed some interesting features (Figure 1).The steric influence of the bulky ortho-ter/-butyl groups does not lead to a stretching of the Ge=Ge double bond as is the case in compounds 2 and 3. Instead, the system responds to possible interactions between these groups by a twisting of z = 20.4". The short germanium-germanium separation of 225.2 pm is also surprising when one considers that the analogously substituted distannene R2SnSnR2 exhibits a very large tin-tin separation, which IS appreciably larger than even a normal tin-tin single-bond Even more unusual are the completely planar environments of both germanium atoms, the two @so-carbon atoms and the respective other gerinanium atom being exactly coplanar (angular sums in each case 360").On the other hand, the largest displacement parameters UAGe) = 84 pm2 > 1/2( U, I + U33) = 38 pm' can lead toChevn. Beu./Recueil1997,130, [735][736][737][738]

Future stochastic biomimetic chemistry within toroidal supramolecules will depend on the availability of large, transmembrane, and functionalized internal space of high stability, four properties that characterize the nanopore formed by the hexameric, cationic rigid‐rod β‐barrel (see schematic representation).