Distibine similar compound

The five distibines (1, 22, 30, 36, and 40) that have been investigated by X-ray crystallography show very similar molecular structures (see Fig. 2 for the molecular structure of 2,2, 5,5 -tetramethylbistibole, 30). In each case the distibine adopts a staggered trans conformation so that there is an inversion center through the Sb—Sb bond. The Sb—Sb bond lengths vary between 2.84 and 2.88 A, values somewhat shorter than that of elemental antimony (2.90 A) (43). The bond angles about antimony are close to 90° and similar to those reported for other trivalent antimony compounds (44). In fact, none of the molecular parameters seems extraordinary. However, the four thermochromic distibines show unusual crystal packing (see Fig. 3, Table V) (22,25,36,40,41,45). In each case, the antimony atoms are... 

Although the Sb chains are qualitatively similar, there is a variation in the linearity of the chain and in the intermolecular distances. Thus, the chains are nearly linear (4 Sb—Sb—Sb 179.2°) and the Sb—Sb distances are short (3.68 A) for the sterically undemanding tetramethyldistibine (22), while the chains are distinctly zig-zag (4 Sb—Sb---Sb 165.8°) and the distances (3.99 A) are longer for the more bulky tetrakis(trimethylsilyl)dis-tibine (36). While short intermolecular contacts seem necessary for the thermochromic effect, the UV absorption maxima of the solid distibines do not correlate with the intermolecular Sb---Sb distances. Thus, the red compounds 22,36, and 40 have nearly identical absorption maxima despite a variation of more than 0.3 A in the intermolecular Sb contacts. On the other hand, the intermolecular Sb—Sb separations of 22 and 30 are nearly the same although the absorption maxima differ by 75 nm. 

No diarsines or diphosphines show thermochromic behavior similar to that of the distibines and dibismuthines. For example, in the series of dipnictogen compounds A, B, and C illustrated in Scheme 6, the thermochromic distibines and dibismuthines correspond to nonthermochromic diarsines and diphosphines. Structural data are available to compare the three diarsines 51 (46), 52 (57), and 53 (5S) with the corresponding distibines 22 (25,26), 36 (37), and 30 (22). Diarsines 52 and 53 crystallize in gauche conformations as opposed to the trans-staggered conformation of the distibines. In neither case are there intermolecular As---As contacts shorter than 4 A. However, the lack of conformational correspondence makes any comparison tenuous. 

A number of simple diarsines have been structurally investigated and, although individual molecules are similar to those of the corresponding distibines, the diarsines do not show the extended chain structures and consequent thermochromism often found for the latter. The geometry at the two arsenic atoms is pyramidal, though the angles at arsenic can be asymmetric. The substituents occupy anti (gauche) conformations and in some cases the molecules have imposed Cj symmetry. Parameters for four of the compounds are summarized in Table 2. 

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