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

The 2-fold reduction of BX(NHC) (X = Cl, Br, I; NHC = (un)saturated N-heterocyclic carbene) yields the corresponding green-colored 1,2-dihalodiborenes BX(NHC), the B NMR resonances of which are strongly upfield-shifted upon descending the halide group. The diborenes crystallize as the isomers, with relatively short B═B double bonds (1.513(9) to 1.568(4) Å). Cyclic voltammetric experiments with these diborenes reveal reversible one-electron oxidation processes to the corresponding diboron radical cation ( = -1.16 to -1.50 V); the reducing power of BX(NHC) increasing with the electronegativity of the halide and for the less π-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 BX(NHC) is mostly localized on the B═B π bond, with the contribution from halide p orbitals decreasing down the group, and the saturated NHCs affording some π-bonding delocalization over the B-C 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 BBr(NHC) 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 BSe radical cation. Computational analyses show that the localization of the HOMO and HOMO - 1 of the diboraseleniranes is inverted compared to the diborathiiranes.