Distibine thermochromic

Unlike many distibines and dibismuthines, diarsines do not appear to exhibit thermochromic effects (105). 

An intermolecular analog of this inter-ring pnictogen bonding is found in the thermochromic distibines and dibismuthines.45 For example, 2,2, 5,5 -tetramethylbistibole (87) crystallizes so that the Sb atoms are aligned in chains with close contacts between the Sb atoms of adjacent molecules. It seems more than coincidental that the intermolecular contacts in 87 occur at virtually the same distance as the intramolecular contacts in 29. See Figure 3. 

Preceding reviews on distibines, R2Sb-SbR25,7,66-68 feature mainly the structural aspects in relation to the thermochromic properties of some of these compounds, but also overviews on the earlier work of the coordination chemistry of distibine ligands have been reported.7,68 All the distibines that... 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

As a part of his classic study of reactive free radicals in the early 1930s, Paneth observed that methyl radicals react with a heated antimony mirror to produce tetramethyldistibine (22) (27). This distibine has quite remarkable properties. It forms intensely colored red crystals which melt reversibly to a yellow oil. Similarly, solutions of tetramethyldistibine are pale yellow. Although these color changes are clearly due to changes in phase rather than strictly temperature, they have been termed thermochromic... 

Nine other thermochromic distibines have been reported. All show a yellow melt, but the solid colors range from deep yellow to violet-blue. Seven thermochromic dibismuthines are known. The liquid colors are all red while solid colors are variable. The thermochromic distibines and dibismuthines are listed in Table II (7,10,17a,21,23-41) along with the melting points and colors of the crystals. Nonthermochromic dibismuthines are collected in Table III (20,29,32,42). 

On crystallization, nonthermochromic distibines and dibismuthines show little visual change. The solid colors of two nonthermochromic distibines (8 and 9) and two nonthermochromic dibismuthines (45 and 46) have been characterized by diffuse reflectance. In each case, only very modest changes in the absorption maxima were observed and between solid and solution. On the other hand, the intense colors shown by the solid phases of the thermochromic distibines are red shifted by 200-250 nm from their solution phase maxima (see Table IV) (7,25,29,33,34,37, 38b,40). The dibismuthines are red shifted even further. There is complete correspondence between dibismuthines and the analogous distibines, with the dibismuthines being red shifted by around 100 nm (see Fig. 1). 

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. 

While tetraphenyldibismuthine (46) is not isostructural with tetraphenyl-distibine (1), it does not show any close intramolecular Bi—Bi contacts. Although no other structural data are available, it might be noted that there is a complete match of the thermochromic properties for the dibis-muthines with the corresponding distibines. Thus, in all cases if a distibine is thermochromic the analogous dibismuthine is as well. Conversely, nonthermochromic distibines correspond to nonthermochromic dibis-muthines. A structural correspondence also seems likely. 

The thermochromic effect of distibines has been treated in three papers by Hoffmann and colleagues using a tight bonding model based on extended Hiickel calculations (33,47,48). These calculations treated only unsaturated distibines, and major attention was focused on bistibole (47). The important orbitals of the stacked bistibole are derived from molecular orbitals of the SbC4H4 unit (see Fig. 6). The HOMO results from the in-phase mixing of the Sb(pz) and Sb(n ) orbitals and is largely localized on the Sb atoms. At the zone center (k = 0) this band has primarily lone pair... 

No theoretical treatment is available for dibismuthines. However, the qualitative features of the band structures of the thermochromic dibismuthines can be anticipated from the Hoffmann model for the corresponding distibines. Thus, the substitution of Bi for Sb is unlikely to affect greatly the ligand v orbitals. But the trg orbital will likely be raised to reflect the weaker metal-metal bond (28). A smaller band gap is predicted, as is observed. 

Raman spectroscopy is extremely useful for characterization of distibines and dibismuthines, since the metal-metal stretching vibrations give rise to intense, easily identifiable peaks. Several thermochromic and nonthermochromic distibines have been examined as both solids and liquids (see Table VI) (7,40,49-51). With the exception of tetramethyldisti-bine, there are only small shifts in ysbsb between solid and liquid. Nor are there systematic differences between thermochromic and nonthermochromic distibines. The observed range of ySbSb appears to be predominantly... 

The Raman spectra of the thermochromic tetramethyldistibine (22) and tetrakis(trimethylsilyl)distibine (36) show a great enhancement of the intensity of the r sbsb band in the solid over the liquid. In addition, new low-frequency bands near 50 cm-1 are found for the solid which are not observed for the liquid (50). These bands have been assigned to the inter-molecular Sb2---Sb2 stretch. Neither effect is observed for nonthermo-chromic distibines. 

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. 

---