REACTIVITY OF THE DIBORON TETRAHALIDES - DIBORATION OF ETHYLENE WITH DIBORON TETRABROMIDE AND THERMAL-DECOMPOSITION AND LIGAND EXCHANGES OF DIBORON TETRABROMIDE AND DIBORON TETRACHLORIDE IN CARBON TETRABROMIDE AND CARBON-TETRACHLORIDE

In contrast to previous assertions, we find that B2Br4 reacts readily with ethylene to generate BBr2CH2CH2BBr2, which can be isolated in 95% yield. While it has been reported numerous times that the disproportionation of B2Cl4, which eventually yields BCl3 and the polyhedral boron halides BnXn (n = 8-12), is slowed by the addition of haloalkenes, we have found that the disproportionation rate of both B2Cl4 and B2Br4 is also significantly decreased by the addition of the haloalkanes CBr4 and CCl4. For example, in the presence of a 7-fold excess of CBr4, the amount of B2Br4 recovered after 198 h at 96-degrees-C corresponded to 65% of that originally present. Although less dramatic, significant retardation of the B2Cl4 and B2Br4 disproportionations has also been observed when CCl4 was added. These results are interpreted in terms of the transient formation of (dihaloboryl)alkanes, a proposal that is buttressed by the observation of C(BBr2)4 and partially chlorinated (dihaloboryl)methanes like C(BBrCl)4 which were identified by mass spectrometry. The reaction between B2Br4 and excess CCl4 generates B2Cl4 in 84% yield, but the interaction of B2Cl4 with excess CBr4 does not appear to be an effective route to B2Br4.