NHC-Stabilized 1,2-Dihalodiborenes: Synthesis, Characterization, and Reactivity Toward Elemental Chalcogens

The 2-fold reduction of B2X4(NHC)(2) (X = Cl, Br, I; NHC = (un)saturated N-heterocyclic carbene) yields the corresponding green-colored 1,2-dihalodiborenes B2X2(NHC)(2), the B-11 NMR resonances of which are strongly upfield-shifted upon descending the halide group. The diborenes crystallize as the trans isomers, with relatively short B=B double bonds (1.513(9) to 1.568(4) angstrom). Cyclic voltammetric experiments with these diborenes reveal reversible one-electron oxidation processes to the corresponding diboron radical cation (E-1/2 = -1.16 to -1.50 V); the reducing power of B2X2(NHC)(2) increasing with the electro-negativity of the halide and for the less p-accepting unsaturated NHCs. The main UV-vis absorption (393-463 nm), which corresponds mainly to a highest occupied molecular orbital (HOMO) -> lowest unoccupied molecular orbital (LUMO) transition, undergoes a blueshift upon descending the halide group and shows some dependence on the stereoelectronics of the NHC ligands. Computational analyses show that the HOMO of B2X2(NHC)(2) is mostly localized on the B=B pi bond, with the contribution from halide p orbitals decreasing down the group, and the saturated NHCs affording some p-bonding delocalization over the B-C-NHC bonds. The calculated HOMO and LUMO energies decrease upon descending the halide group, while the HOMO-LUMO gap also decreases, correlating well with the cyclovoltammetry and UV-vis data. The reactions of B2Br2(NHC)(2) with elemental sulfur and red selenium lead to the formation of the corresponding diborathiiranes and seleniranes, respectively, which were characterized by NMR and UV-vis spectroscopy, cyclic voltammetry, and X-ray diffraction analyses. In one case, an additional one-electron oxidation yields a unique cyclic B2Se radical cation. Computational analyses show that the localization of the HOMO and HOMO - 1 of the diboraseleniranes is inverted compared to the diborathiiranes.

Automated summary

Reduction of B2X4(NHC)(2) yields green 1,2-dihalodiborenes B2X2(NHC)(2) with short B=B double bonds and reversible one-electron oxidation to diboron radical cations. UV-vis, computational analysis, and X-ray diffraction confirm the correlations between properties such as HOMO localization, HOMO-LUMO energies, and chemical reactivity.