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

Octahedral closo-1,2-Pn ₂B₄Br₄ (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 σ-holes on Br's. The σ-holes on Pn has enabled ESI-MS to detect various anions.

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 °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 °C, conjuncto-3,3’-(1,2-Pn ₂B₄Br₃)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 σ-holes on the bromine atoms. In addition, the presence of the σ-holes on the pnictogens has enabled ESI-MS to detect various anions appearing during earlier and current syntheses.

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