Phenylantimony hexamer

Attempts to extend the reductive preparative procedures for phenylarsenic to phenyl-antimony have met with little success. Reduction of phenylstibonic acid or phenylanti-mony dihalides only led to polymeric materials. The phenylantimony hexamer was, however, obtained as orange or yellow crystalline solvates (PhSb)6 solv (solv = benzene, toluene, THF, 1,4-dioxane) by reaction of PhSbH2 with styrene or phenylacetylene (equation 9) or by slow reaction of PhSb(SiMe3)2 in air - (equation 10). 

The packing of the arsenic or antimony rings in the crystals leads to stacks of rings. Structural data are given in Table 1. It is remarkable that the shortest intermolecular contacts between antimony atoms are considerably shorter than between arsenic atoms. With respect to intermolecular association in the solid state, however, (PhSb)g is a borderline case. The contact distances are close to the van der Waals distances between antimony atoms (440 pm). For the crystallization of the phenylantimony hexamer, the presence of cyclic organic molecules that are incorporated in the solid plays an important role. 

This observation sheds some light on the question as to why pentamers are formed with n-alkyl substituents but hexamers with slim aryl substituents. A possible explanation, at least for the organoantimony homocycles, is that in solution jjentamers dominate both with alkyl and aryl substituents. In the solid state, however, the six-membered form allows a closer packing of the phenylantimony rings than the pentameric form. 

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