Synthesis and Reactivity of a Benzo-Fused 1,2-Diborete Biradicaloid

The synthesis of 1,2-(BCl₂)₂C₆H₄ (1) was optimised by performing a SiMe₃-BCl₂ metathesis reaction between 1-SiMe₃-2-(BCl₂)C₆H₄ with BCl₃ in the dilute gas phase at 95 °C, with 5-9% of its meta-isomer 1,3-(BCl₂)₂C₆H₄ (2) formed as an inseparable byproduct. The addition of two equiv. of IiPr (1,3-diisopropylimidazol-2-ylidene) or CAAC (1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) to 1 yielded the twofold carbene adducts 3-IiPr and 3-CAAC, alongside their minor 1,3-isomers 4-IiPr and 4-CAAC. In CH₂Cl₂3-CAAC converted into the chloride-bridged boronium cation 5, with an unusual linear HCl₂^‒ counteranion. The twofold reduction of 3-IiPr yielded the doubly fused C₉B₂N₂ heterocycle 6, presumably via an unstable 4π-antiaromatic 1,2-diborete intermediate (7) undergoing intramolecular ring expansion of one IiPr ligand and B=B bond splitting. In contrast, 3-CAAC (or 5) underwent stepwise reduction to the monoboryl radical 8 (1e^‒), the bis(boryl) biradical 9 (2e^‒), the benzo-fused 1,2-diborete biradical 10 (4e^‒), and the closed-shell diborete dianions 11-M (6e^‒, M = Li, Na, K). Furthermore, the reduction of 5 under CO atmosphere yielded a mixture of the benzo-bridged diborylketone 12 and the bis((CAAC,CO)borylene) 12, the latter being accessible selectively by adduct formation of diborete 10 with CO. EPR spectroscopy and computational analyses confirmed the biradical nature of 10, with an open-shell singlet ground state and a thermally accessible triplet state, while nucleus-independent chemical shift (NICS) calculations indicate a small but non-negligible 2π-aromatic character for 10.