Stabilization of the Elusive 9‐Carbene‐9‐Borafluorene Monoanion

Abstract

Two-electron reduction of carbene-supported 9bromo-9-borafluorenes with excess KC 8 , Na, or Li-naphthalenide affords six N-heterocyclic carbene (NHC)-or cyclic-(alkyl)(amino) carbene (CAAC)-stabilized borafluorene anions (3-8)-the first isolated and structurally authenticated examples of the elusive 9-carbene-9-borafluorene monoanion. The electronic structure, bonding, and aromaticity of the boracyclic anions were comprehensively investigated via computational studies. Compounds 5 and 8 react with metal halides via salt elimination to give new B-E (E = Au, Se, Ge)containing materials (9-12). Upon reaction with diketones, the carbene ligand cleanly dissociates from 5 or 8 to yield new B-O containing spirocycles (13-14) that cannot be easily obtained using "normal" valent borafluorene compounds. Collectively, these results support the notion that carbene-stabilized monoanionic borafluorenes may serve as a new platform for the onestep construction of higher-value boracyclic materials.

σ,π‐Conjugated Bis(germylene) Adducts with NHC and CAACs

“…[3][4][5][6][7][8][9] The coordination of NHCs vs. CAACs often leads to drastic differences in (electronic) structure due to the stronger π-acceptor and σ-donor abilities of the latter. This has been evaluated experimentally and theoretically for group 13 and 15 with anionic borafluorenes by the Gilliard group, [10] the diborene carbene adducts by Engels et al [11] or the carbene phosphinidene adducts by Bertrand et al [12] In group 14, the considerably shorter SiÀ C CAAC bond distances in the (CAAC • SiR) 2 adducts (R=Cl, H, Me) [13] compared to the corresponding NHC adduct (NHC • SiCl) 2 [14] underpin these differences despite the pyramidal coordination environments of both adjacent tetrel(II) centers. In case of dihalosilylenes, predominantly NHC adducts have been reported.…”

“…The coordination of NHCs vs. CAACs often leads to drastic differences in (electronic) structure due to the stronger π‐acceptor and σ‐donor abilities of the latter. This has been evaluated experimentally and theoretically for group 13 and 15 with anionic borafluorenes by the Gilliard group, [10] the diborene carbene adducts by Engels et al [11] . or the carbene phosphinidene adducts by Bertrand et al [12] .…”