Dibismuthine 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. 

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... 

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). 

Fewer structural data are available for dibismuthines. Published structures include only the thermochromic tetrakis(trimethylsilyl)dibismu-thine (37) (57) and the nonthermochromic tetraphenyldibismuthine (46) (42) although some data from an unpublished structure of tetramethyl-... 

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. 

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... 

Raman spectra have also been reported for several dibismuthines. The PBiBi stretch is close to 110 cm-1 in all cases for both thermochromic and nonthermochromic dibismuthines in either solid or liquid (29). This insensitivity to substitution suggests that these bands are largely due to localized Bi-Bi vibrations. 

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. 

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