Highly Fluorinated Aryl-Substituted
Tris(indazolyl)borate
Thallium Complexes: Diverse Regiochemistry at the B–N Bond
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Abstract
The synthesis and characterization (mainly by <sup>19</sup>F NMR
and X-ray diffraction) of highly fluorinated aryl-4,5,6,7-tetrafluoroindazoles
and their corresponding thallium hydrotris(indazolyl)borate complexes
are reported [aryl = phenyl, pentafluorophenyl, 3,5-dimethylphenyl,
3,5-bis(trifluoromethyl)phenyl]. Thanks to N–H···N
hydrogen bonds, the indazoles crystallize as dimers that pack differently
depending on the nature of the aryl group. The thallium hydrotris(indazolyl)borate
complexes Tl[Fn-Tp<sup>4Bo,3aryl</sup>] resulting from the reaction
of aryl-4,5,6,7-tetrafluoroindazoles [aryl = phenyl, 3,5-dimethylphenyl,
3,5-bis(trifluoromethyl)phenyl] with thallium borohydride adopt overall <i>C</i><sub>3<i>v</i></sub> symmetry with the indazolyl
groups bound to boron via their N-1 nitrogen in a conventional manner.
When the perfluorinated pentaphenyl-4,5,6,7-tetrafluoroindazole is
reacted with thallium borohydride, a single regioisomer of <i>C</i><sub><i>s</i></sub> symmetry having one indazolyl
ring bound to boron via its N-2 nitrogen, TlHB(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)<sub>2</sub>(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl[F27-Tp<sup>(4Bo,3C6F5)*</sup>], is obtained for the first time. Surprisingly,
the perfluorinated dihydrobis(indazolyl)borate complex Tl[F<sub>18</sub>-Bp<sup>3Bo,3C6F5</sup>], an intermediate on the way to the hydrotris(indazolyl)borate
complex, has <i>C</i><sub><i>s</i></sub> symmetry
with two indazolyl rings bound to boron via N-2. The distortion of
the coordination sphere around Tl and the arrangement of the complexes
in the crystal are discussed