B–N–B Bond Embedded Phenalenyl and Its Anions

• Published in: Journal of the American Chemical Society 2017-11, Vol.139 (44), p.15760-15767
• Main Authors: Wei, Haipeng, Liu, Yulan, Gopalakrishna, Tullimilli Y, Phan, Hoa, Huang, Xiaobo, Bao, Lipiao, Guo, Jing, Zhou, Jun, Luo, Shenglian, Wu, Jishan, Zeng, Zebing
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.7b07375

Despite the great potential of heteroatom-containing polycyclic aromatic hydrocarbons in organic optoelectronics, there are very limited reports on heteroaromatics containing a B–N–B bond in the π-scaffold. Herein, stable 1,9-dibora-9a-azaphenalenyl (DBAP) derivatives, named BNB-embedded phenalenyls, are presented. The DBAP skeleton contains a three-center two-π-electron B–N–B moiety with 12 π-electrons and can be regarded as the isoelectronic structure of the phenalenyl cation. Chemical reduction of the phenyl derivative of DBAP by potassium generated the dianion containing 14 delocalized π-electrons, which can be regarded as the isoelectronic structure of the phenalenyl anion. The dianion is sandwiched and stabilized by two bulky [K([18]­crown-6)]+ counterions according to its X-ray structure. However, its monoanion (an isoelectronic structure of the henalenyl radical) generated by mixing equal moles of neutral compound and dianion gave an unusual B–N–B-embedded benzo­[cd]­fluoranthene dianion, which again was confirmed by X-ray crystallographic analysis. The new dianion containing 20 π-electrons is highly aromatic and is further stabilized by [K([18]­crown-6)]+ counterions. An aromaticity driven rearrangement mechanism was proposed for this unusual transformation. Based on X-ray structures and theoretical calculations, the B–N–B moiety in the neutral and anionic DBAP participates in the π-electron delocalization along the whole DBAP skeleton like their phenalenyl cation/radical/anion counterparts, but with more localized character. Therefore, our studies report the first synthesis and characterization of a B–N–B-embedded phenalenyl and its anionic species, which show unique electronic structure and unusual reactivity different from that of their all-carbon phenalenyl analogues.