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–)]∞.

本研究首次结合理论与实验联用手段，对碗状多环芳烃茚并苝烯（indacenopicene，C₂₆H₁₂，1）的电子结构、还原极限及配位能力开展了系统性探究。通过与典型碗烯（corannulene，C₂₀H₁₀，2）的直接对比，明确了碳骨架拓扑结构与对称性变化对茚并苝烯电子性质及芳香性的调控作用。本研究从理论上预测化合物1可经历两步还原反应，随后通过实验验证了该预测结果。通过循环伏安法（cyclic voltammetry）检测到两个可逆单电子还原过程，其形式还原电位分别为-1.92 V与-2.29 V（相对于Fc⁺/Fc电势标度），证实了化合物1对应的单阴离子与二阴离子态具备优异稳定性。双还原态茚并苝烯的产物以铷盐与铯盐形式被分离纯化，并完成了全面表征。X射线衍射（X-ray diffraction）分析表明，该类产物形成了四核有机金属结构基元：[M₂(18-冠-6（18-crown-6）)]²⁺（M=Rb（3）、Cs（4））阳离子占据两个C₂₆H₁₂²⁻阴离子的凹面空腔。在化合物3中，两个外环配位的铷离子的配位位点被冠醚分子封端，最终形成离散的四聚体[Rb₄⁺(18-冠-6)₃(C₂₆H₁₂²⁻)₂]。与之相反，尺寸更大的铯离子可促使化合物4发生一维聚合物链的延伸，最终得到无限长的聚合物[Cs₂⁺(18-冠-6)₂(四氢呋喃（THF）)(C₂₆H₁₂²⁻)]_∞。