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

摘要

Reduction of the bis(9-borafluorenyl)methane 1 with excess lithium furnishes the red dianion salt Li_2[1]. The corresponding dark green monoanion radical Li[1] is accessible through the comproportionation reaction between 1 and Li_2[1]. EPR spectroscopy on Li[1] reveals hyperfine coupling of the unpaired electron to two magnetically equivalent boron nuclei (a(~(11)B) = 5.1 ± 0.1 G, a(~(10)B) = 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) A, respectively. Protonation of Li_2[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 p_z-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.