A Preorganized Ditopic Borane as Highly Efficient One- or Two-Electron Trap

Reduction of the bis­(9-borafluorenyl)­methane 1 with
excess lithium furnishes the red dianion salt Li2[1]. The corresponding dark green monoanion radical Li­[1] is accessible through the comproportionation reaction between 1 and Li2[1]. EPR spectroscopy on
Li­[1] reveals hyperfine coupling of the unpaired electron
to two magnetically equivalent boron nuclei (a(11B) = 5.1 ± 0.1 G, a(10B)
= 1.7 ± 0.2 G). Further coupling is observed to the unique B–CH–B bridgehead proton (a(1H) = 7.2 ± 0.2 G) and to eight aromatic protons (a(1H) = 1.4 ± 0.1 G). According to X-ray crystallography,
the B···B distances continuously decrease along the
sequence 1 → [1]•– → [1]2– with values of 2.534(2),
2.166(4), and 1.906(3) Å, respectively. Protonation of Li2[1] leads to the cyclic borohydride species Li­[1H] featuring a B–H–B two-electron-three-center
bond. This result strongly indicates a nucleophilic character of the
boron atoms; the reaction can also be viewed as rare example of the
protonation of an element–element σ bond. According to
NMR spectroscopy, EPR spectroscopy, and quantum-chemical calculations,
[1]2– represents a closed-shell singlet
without any spin contamination. Detailed wave function analyses of
[1]•– and [1]2– reveal strongly localized interactions of the two
boron pz-type orbitals, with small delocalized
contributions of the 9-borafluorenyl π systems. Overall, our
results provide evidence for a direct B–B one-electron and
two-electron bonding interaction in [1]•– and [1]2–, respectively.