A combined experimental and theoretical study on the isomers of 2,3,4,5-tetracarba-nido-hexaborane(6) derivatives and their photophysical properties

Abstract

Building upon our ongoing studies on the high-yield synthetic route to 2,3,4,5-tetracarba-1,6-nido hexaborane derivative 5, we continue to explore the chemistry of this system to isolate a range of perphenylated C₄B₂-type carborane isomers. As a result, the photolysis of 5 for an elongated period yielded 6, which exhibits an intense luminescence upon excitation with a UV lamp (λ_ex=366 nm). In contrast, the conversion of 5 under thermal conditions (microwave heating) generated the isomer 7, which does not show any luminescence. The structural disparity among the three isomers 5–7 is the position of the butadiene moiety with respect to the C₄B₂-carborane cage as deduced from single-crystal X-ray diffraction analyses. In an attempt to generate the classical structural motif of 5, an equimolar amount of KOtBu was added to a THF solution of 5, which immediately provided the boratacyclopentadiene derivative 8 in good yield. The observed rearrangement reactions were further investigated by quantum chemical calculations to suggest a plausible reaction pathway. According to the DFT calculations, the energetically favored reaction mechanism most likely involves tricyclic transition states and classical intermediate structures. In addition, the fluorescence emission properties of 6 were investigated in solution as well as in the solid state.