Synthesis, properties and structure of borafluorene-based conjugated polymers with kinetically and thermodynamically stabilized tetracoordinated boron atoms

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

Two-electron reduction of carbene-supported 9-bromo-9-borafluorenes with excess KC₈ , 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 one-step construction of higher-value boracyclic materials.

Ring-Forming Polymerization toward Perfluorocyclobutyl and Ortho-Diynylarene-Derived Materials: From Synthesis to Practical Applications

Many desirable characteristics of polymers arise from the method of polymerization and structural features of their repeat units, which typically are responsible for the polymer’s performance at the cost of processability. While linear alternatives are popular, polymers composed of cyclic repeat units across their backbones have generally been shown to exhibit higher optical transparency, lower water absorption, and higher glass transition temperatures. These specifically include polymers built with either substituted alicyclic structures or aromatic rings, or both. In this review article, we highlight two useful ring-forming polymer groups, perfluorocyclobutyl (PFCB) aryl ether polymers and ortho-diynylarene- (ODA) based thermosets, both demonstrating outstanding thermal stability, chemical resistance, mechanical integrity, and improved processability. Different synthetic routes (with emphasis on ring-forming polymerization) and properties for these polymers are discussed, followed by their relevant applications in a wide range of aspects.

(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant p_z orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.

(Hetero)arene-fused boroles, ‘antiaromatic’ 2n-electron π-systems, more stable and more functionalizable than boroles, offer greater potential for a variety of applications.

Persistent Borafluorene Radicals

N‐Heterocyclic carbene (NHC)‐ and cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borafluorene radicals have been isolated and characterized by elemental analysis, single‐crystal X‐ray diffraction, UV/Vis absorption, cyclic voltammetry (CV), electron paramagnetic resonance (EPR) spectroscopy, and theoretical studies. Both the CAAC–borafluorene radical (2) and the NHC–borafluorene radical (4) have a considerable amount of spin density localized on the boron atoms (0.322 for 2 and 0.369 for 4). In compound 2, the unpaired electron is also partly delocalized over the CAAC ligand ^carbeneC and N atoms. However, the unpaired electron in compound 4 mainly resides throughout the borafluorene π‐system, with significantly less delocalization over the NHC ligand. These results highlight the Lewis base dependent electrostructural tuning of materials‐relevant radicals. Notably, this is the first report of crystalline borafluorene radicals, and these species exhibit remarkable solid‐state and solution stability.