Highly Fluorinated Aryl-Substituted Tris(indazolyl)borate Thallium Complexes: Diverse Regiochemistry at the B–N Bond

The synthesis and characterization (mainly by 19F 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-Tp4Bo,3aryl] 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 C3v 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 Cs symmetry having one indazolyl
ring bound to boron via its N-2 nitrogen, TlHB­(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)2(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl­[F27-Tp(4Bo,3C6F5)*], is obtained for the first time. Surprisingly,
the perfluorinated dihydrobis­(indazolyl)­borate complex Tl­[F18-Bp3Bo,3C6F5], an intermediate on the way to the hydrotris­(indazolyl)­borate
complex, has Cs 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.