Anstöter Cate S, Gibson Christopher M, Fowler Patrick W
Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK.
Phys Chem Chem Phys. 2020 Jul 22;22(28):15919-15925. doi: 10.1039/d0cp02284e.
Despite a long tradition of descriptions of borazine as an 'inorganic benzene', this molecule is a non-aromatic species according to the magnetic (ring-current) criterion. Borazine, borazocine, and the larger neutral (BN)nH2n azabora-annulene heterocycles in planar conformations, although π-isoelectronic with [2n]annulenes, support only localized induced currents in perpendicular magnetic fields. The π-current maps of these systems comprise superpositions of separate 'lone-pair' circulations on all nitrogen centres. For the systems with n > 4, planarity must be enforced by a constraint. Qualitative orbital analysis based on the ipsocentric approach to calculation of induced current density suggests that global induced currents could be produced through strategic changes to the π electron count. In ab initio calculations, azabora-annulenes with rings of size [8]- and larger were indeed found to support global diatropic ring currents in both anionic and cationic forms with (4N + 2) π electron counts. The planar conformation of the charged ring typically occupies a stationary point of higher order on the potential energy surface, rather than a minimum. However, the borazocine dianion, [B4N4H8]2-, occupies a planar minimum, supports a diatropic ring current of strength comparable to that in benzene, and is predicted to participate in sandwich compounds; it is therefore a good candidate for an aromatised azabora-annulene.
尽管长期以来一直将硼嗪描述为“无机苯”，但根据磁性（环电流）标准，该分子是一种非芳香族物种。硼嗪、硼氮嗪以及平面构象中更大的中性(BN)nH2n氮杂硼环轮烯杂环，尽管与[2n]环轮烯具有等π电子数，但在垂直磁场中仅支持局域感应电流。这些体系的π电流图由所有氮中心上单独的“孤对”环流叠加而成。对于n>4的体系，必须通过约束来强制其平面性。基于计算感应电流密度的等中心方法的定性轨道分析表明，通过对π电子数进行策略性改变可以产生全局感应电流。在从头算计算中，确实发现具有[8]及更大环的氮杂硼环轮烯在(4N + 2) π电子数的阴离子和阳离子形式中均支持全局抗磁环电流。带电环的平面构象通常占据势能面上的高阶驻点，而不是最小值。然而，硼氮嗪二价阴离子[B4N4H8]2-占据平面最小值，支持强度与苯中相当 的抗磁环电流，并且预计会参与夹心化合物的形成；因此，它是芳香化氮杂硼环轮烯的良好候选物。
