Synthesis of the Soft Lewis Superacid Tris(4-bromo-2,3,5,6-tetrafluorophenyl)borane B(C6F4Br)3 via C‒Ag to C‒B Transmetalation.

Abstract

The Lewis acid tris(4-bromo-2,3,5,6-tetrafluorophenyl)borane B(C₆F₄Br)₃ was synthesized via transmetalation from BBr₃ and AgC₆F₄Br. The latter was prepared from 1,4-C₆F₄BrSiMe₃ with AgF and crystallized as a toluene solvate [2(AgC₆F₄Br)₄•7.46(toluene)]. The solid-state structure of free B(C₆F₄Br)₃ and its Lewis base adducts with EtCN, H₂O, and POEt₃ was determined by single-crystal X-ray diffraction. DFT calculations show that B(C₆F₄Br)₃ has approximately 1% higher fluoride and hydride ion affinities (FIA and HIA) than tris(pentafluorophenyl)borane B(C₆F₅)₃. Similarly, the Gutmann-Beckett method indicates an increase in Lewis acidity of 2%, whereas IR spectroscopy of the CD₃CN adducts reveals a slightly lower ṽ(CN) vibration for B(C₆F₄Br)₃. The presence of bromine atoms is expected to facilitate structural analysis of Lewis acid-base adducts of B(C₆F₄Br)₃ by single-crystal X-ray diffraction in the future.