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.

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

“…In compound 5, the potassium cations Thermal ellipsoids set at 50 % probability and H atoms were omitted for clarity. [16]…”

“…The solid-state structures of the borafluorene anions varied drastically based on the ligand and cation (Figure 2). [16] Compounds 3 and 6 display solvated lithium cations with no contact to the central borafluorene ring. Generally, the solidstate structures of boryl anions have boron-and/or cation-p interactions and remarkably 3 and 6 represent examples of free boryl anions (Figure S37), with B-Li distances of 6.33 and 6.58 (> 4.21 longer than the sum of the covalent radii (R(B-Li) = 2.12 ) [17] and significantly longer than boryllithium (2.291 ) (Figure 1 a).…”

“…Thermal ellipsoids set at 50 % probability and H atoms were omitted for clarity. [16] Selected bond lengths []: 13: B1-O1 1.5103( 15), B1-O2 1.5156( 15), O1-C13 1.3796( 14), O2-C20 1.3801( 14), C13-C20 1.3565 (16). 14: B1-O1 1.52( 3), B1-O2 1.57( 3), O1-C13 1.42( 2), O2-C14 1.35( 2), C13-C14 1.30(3).…”

“…Figure 2. Molecular structures of CAAC-BF-anions [3(a), 4(b), 5 (c)] and NHC-BF-anions [6(d), 7(e), 8(f)].Thermal ellipsoids set at 50 % probability and H atoms were omitted for clarity [16]. …”

“…Molecular structures of 13 (a) and 14 (b). Thermal ellipsoids set at 50 % probability and H atoms were omitted for clarity [16]. Selected bond lengths []: 13: B1-O1 1.5103(15), B1-O2 1.5156(15), O1-C13 1.3796(14), O2-C20 1.3801(14), C13-C20 1.3565(16).…”