From Corannulene to Indacenopicene: Effect of Carbon Framework Topology on Aromaticity and Reduction Limits

The
electronic structure, reduction limits, and coordination abilities
of a bowl-shaped polycyclic aromatic hydrocarbon, indacenopicene (C26H12, 1), have been investigated for
the first time using a combination of theoretical and experimental
tools. A direct comparison with the prototypical corannulene bowl
(C20H10, 2) revealed the effects
of carbon framework topology and symmetry change on the electronic
properties and aromaticity of indacenopicene. The accessibility of
two reduction steps for 1 was predicted theoretically
and then confirmed experimentally. Two reversible one-electron reduction
processes with the formal reduction potentials at −1.92 and
−2.29 V vs Fc+/0 were detected by cyclic voltammetry
measurements, demonstrating the stability of the corresponding mono-
and dianionic states of 1. The products of the doubly
reduced indacenopicene have been isolated as rubidium and cesium salts
and fully characterized. Their X-ray diffraction study revealed the
formation of tetranuclear organometallic building blocks with the
[M2(18-crown-6)]2+ (M = Rb (3)
and Cs (4)) cations occupying the concave cavities of
two C26H122– anions. The coordination
of two outside exo-bound rubidium ions is terminated
by crown ether molecules in 3 to form the discrete [Rb+4(18-crown-6)3­(C26H122–)2] tetramer. In contrast,
the larger cesium ions allow the 1D polymeric chain propagation in 4 to afford [Cs+2(18-crown-6)2­(THF)­(C26H122–)]∞.

首次采用理论分析与实验手段相结合的方法，对碗状多环芳烃——茚满吡咯烯（C26H12，1）的电子结构、还原极限以及配位能力进行了深入研究。通过与原型杯芳烃碗（C20H10，2）的直接对比，揭示了碳框架拓扑结构及对称性变化对茚满吡咯烯电子性质和芳香性的影响。理论预测了1的两个还原步骤，并通过实验得到了证实。循环伏安法测量检测到两个具有标准还原电位分别为−1.92 V和−2.29 V vs Fc+/0的可逆单电子还原过程，证明了1相应的单负离子和双负离子状态的稳定性。经鉴定，双还原茚满吡咯烯产物以铷和铯盐的形式被分离出来，并进行了全面表征。X射线衍射研究表明，形成了包含[M2(18-crown-6)]2+（M = Rb（3）和Cs（4））阳离子占据两个C26H122–阴离子凹槽的四核有机金属构建模块。在3中，通过冠醚分子终止两个外向外配位的铷离子配位，形成了离散的[Rb+4(18-crown-6)3(C26H122–)2]四聚体。相比之下，较大的铯离子使得4中可以形成一维聚合物链，从而得到[Cs+2(18-crown-6)2(THF)(C26H122–)]∞。