Doubly N-heterocyclic-carbene-stabilized diborenes undergo facile reactions with CO 2 , initially providing dibora-βlactones. These lactones convert over time to their 2,4-diboraoxetan-3-one isomers through a presumed dissociative pathway and hypovalent boron species borylene carbonyls (LHBCO) and base-stabilized oxoboranes (LHBO). Repeating these reactions with doubly cyclic(alkyl)(amino)carbene-stabilized diborenes allowed the isolation of a borylene carbonyl intermediate, whereas a base-stabilized oxoborane could be inferred by the isolation of a boroxine from the reaction mixture. These results, supported by calculations, confirm the presumed mechanism of the diboralactone-to-diboraoxetanone isomerization while also establishing a surprising level of stability for three unknown or very rare hypovalent boron species: base-stabilized derivatives of the parent borylene carbonyl (LHBCO) and parent oxoborane (LHBO) as well as base-free oxoboranes (RBO).
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 11B NMR resonances of which are strongly upfield-shifted upon descending the halide group. The diborenes crystallize as the trans isomers, with relatively short BB 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 (E 1/2 = −1.16 to −1.50 V); the reducing power of B2X2(NHC)2 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 B2X2(NHC)2 is mostly localized on the BB 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–CNHC 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.
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