Beyond the Limits of Perbromo-Substituted Octahedral Pnictogenaboranes: A Spectroscopic and Computational Study

Abstract

Octahedral closo-1,2-Pn2B4Br4 (Pn=P, As) molecules react with tetrahydrothiophene (THT), regarded as a soft base, much slower than with tetrahydrofuran (THF), considered a hard base. Namely, the reaction begins with only Pn=P at temperatures above 140 degrees C; 4-Br(CH2)4S-closo-1,2-P2B4Br3 has been detected as the first product resulting from the cleavage and insertion of a thiobutylate group into the B-Br bond. Unlike in the reaction with THF, the addition of a second THT moiety has not been observed. Conversely, also starting from 140 degrees C, conjuncto-3,3'-(1,2-Pn2B4Br3)2 has been found as the final product of the reaction, which indicates the presence of 3-Br(CH2)4S-closo-1,2-P2B4Br3 as another intermediate during the conversion. A computational examination has revealed that it occurs through a pentagonal pyramidal stationary point as an additional intermediate, the latter of which serves as the crucial structural assembly for the thermal conversion that produces the conjuncto motif. Computations of the energy balance and 11B and 31P NMR chemical shifts are in agreement with experimental observations. This reaction is made possible by the presence of sigma-holes on the bromine atoms. In addition, the presence of the sigma-holes on the pnictogens has enabled ESI-MS to detect various anions appearing during earlier and current syntheses. Octahedral closo-1,2-Pn2B4Br4 (Pn=P, As) react with tetrahydrotoluene (THT) to get conjuncto-3,3'-(1,2-P2B4Br3)2. 4-THT-closo-1,2-P2B4Br3 has also been detected which is isomerized through a pentagonal pyramidal intermediate to the corresponding 3-THT isomer to yield the conjuncto motif. This reaction is due to the presence of sigma-holes on Br's. The sigma-holes on Pn has enabled ESI-MS to detect various anions. image