Bistiboles

The reduction of octaethyl-l,r-biphosphole, -biarsole, and -bistibole with strontium and barium in THF yields the l,l -dipentelaoctaethylstrontocenes and -barocenes (ECsEt4H)2M (E = P 157, As, Sb). The strontocene crystallizes as a THF adduct whereas the barocene 157 precipitates co-ligand-free but as a coordination polymer (Figure 83).260... 

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... 

Fig. 6. Derivation of the HOMO and LUMO of stacked bistibole from C4H4Sb orbitals.

Fig. 7. Band structure of bistibole. The higher and lower bands have been omitted for clarity. [Reprinted with permission from A. J. Ashe III, C. M. Kausch, and O. Eisenstein (33), Organometallics 6, 1185 (1987). Copyright 1987 American Chemical Society.)...

Distibanes and dibismuthanes in which the antimony or bismuth atoms are incorporated in five-membered heterocycles are named bistiboles and bibismoles. This important class of thermochromic compounds has been studied intensively by the group of Ashe. The synthesis and X-ray structure determination of tetramethylbistibole. 

TABLE 5. Structural data of tetramethyl biarsole and bistibole... 

The thermochromic effect of heterocyclic bistiboles has been the subject of several theoretical publications " " with focus on the unsaturated compounds. Band structures have been calculated for the antimony chain of... 

The bistiboles are orange solids that melt to orange liquids. In contrast, the bibismoles are lustrous deep-green crystals that melt to give, reversibly, orange (R = Me) and red (R = MejSi) oils. 

Red crystals of 10 have been reported to be separable manually from a mixture of biarsoie 8 and bistibole 9 (equation 147 ). 

Arsoles, stiboles, and bismoles are fairly well-characterized by C NMR, and this technique is used routinely. Select data for these compounds, as well as for l,T-bistiboles and -bibismoles, is included in Table 3. Occasionally, the bismuth a-carbon is broad or is not observed <920M3492, 93JOM(447)l97>. Table 4 contains C NMR data for tricoordinate arsolenes data for other derivatives, their salts, and organometallic complexes may be found elsewhere <90JOM(396)C8, 94J0M(467)57>. 

The l,l -bistibole (92) is converted into either the l-chloro-(93) or the thermally unstable 1,1,1-trichloro-(94) derivative by controlling reaction stoichiometry (Scheme 17) <77JOM(l36)l73>. 

Preparation of 1,1 -biheterole systems containing arsenic, antimony, and bismuth has been important in understanding thermochromic behavior in terms of structure and bonding. Their interesting solid state structures help explain why some show color transitions, while others do not (see Section 2.16.11.2). The coupled products are sometimes very air-sensitive, but can be isolated by low temperature crystallization for nonpolar solvents or by sublimation. The biarsoles, bistiboles, and bibismoles tend to be thermally stable, while the bibismolanes usually decompose at room temperature. In this section, the different methods by which these compounds may be prepared and isolated are examined. 

There is a strong propensity to form these dimers. For example, during the reaction of 2,5-dimethylstibolyl anion with FeCU to make distibaferrocene, the l,r bistibole (150) is produced in about 11 % yield. Since the ferrocene is stable to Fe " under the reaction conditions, it is likely that the bistibole is formed by oxidation of the metal <80JOM(202)C95>. It was determined in 1991, that the ferrocene can be prepared free of the bistibole if the reaction is run at — 78°C rather than at 25 °C <91OM2068>. 

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