title: 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 C4 B2 -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 (λex =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 C4 B2 -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.
authors.affiliation: Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg (Germany), Fax: (+49) 931-31-84623. h.braunschweig@uni-wuerzburg.de.
authors.forename: Holger
authors.initials: H
authors.lastname: Braunschweig
authors.forename: Sundargopal
authors.initials: S
authors.lastname: Ghosh
authors.forename: J Oscar C
authors.initials: JO
authors.lastname: Jimenez-Halla
authors.forename: Johannes H
authors.initials: JH
authors.lastname: Klein
authors.forename: Christoph
authors.initials: C
authors.lastname: Lambert
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Radacki
authors.forename: Andreas
authors.initials: A
authors.lastname: Steffen
authors.forename: Alfredo
authors.initials: A
authors.lastname: Vargas
authors.forename: Johannes
authors.initials: J
authors.lastname: Wahler
keywords: DFT calculations; boron clusters; nido-carborane; photoluminescence; sigmatropic rearrangement