Lu Xuefeng, Gopalakrishna Tullimilli Y, Han Yi, Ni Yong, Zou Ya, Wu Jishan
Department of Chemistry , National University of Singapore , 3 Science Drive 3 , Singapore 117543.
J Am Chem Soc. 2019 Apr 10;141(14):5934-5941. doi: 10.1021/jacs.9b00683. Epub 2019 Mar 29.
The synthesis of carbon nanobelts (CNBs) with well-defined size and structure remains a challenging topic in nanocarbon chemistry and materials science. Herein, we report the synthesis, physical characterization, and supramolecular properties of two fully conjugated bowl-shaped CNBs (CNB1/CNB2), in which six/eight cyclopenta-rings are fused onto a macrocycle containing three/four alternately linked 2,7-pyrenyl and 2,7-phenanthryl units. The existence of five-membered rings results in a bowl-shaped geometry, as revealed by X-ray crystallographic analysis and density functional theory calculations. Both molecules contain an alternate aromatic phenanthrene- co-quinoidal pyrene structure to satisfy Clar's aromatic sextet rule. The smaller size CNB1 has a deeper bowl depth ( d = 4.997 Å) than CNB2 ( d = 3.682 Å) and cannot undergo bowl-to-bowl (BTB) inversion below 373 K in toluene. However, the larger size CNB2 shows a smaller BTB inversion barrier (∼12 kcal/mol) at the coalescent temperature (248 K), which was estimated by variable-temperature NMR measurements. Both compounds exhibit a small energy gap and amphoteric redox behavior with multiple redox waves. The dications of CBN1 and CBN2, and the tetracation of CBN2, are experimentally accessible by chemical oxidation with NO·SbF, all displaying unusual open-shell singlet diradical character with a small singlet-triplet energy gap (Δ E = -2.71 kcal/mol for CBN1, -2.50 kcal/mol for CBN2, and -2.00 kcal/mol for CBN2). The dications are globally aromatic while the tetracations are globally antiaromatic according to NMR measurements and theoretical calculations (anisotropy of the induced current density, nucleus independent chemical shift, and 2D isochemical shielding surface). The small bowl-shaped CNB1 demonstrates selective encapsulation of fullerene C over C, with a large association constant ( K = 8.066 × 10 M in toluene). However, the larger size CNB2 does not exhibit any encapsulation with both C and C.
在纳米碳化学和材料科学领域，合成尺寸和结构明确的碳纳米带（CNBs）仍然是一个具有挑战性的课题。在此，我们报告了两种完全共轭的碗状碳纳米带（CNB1/CNB2）的合成、物理表征和超分子性质，其中六个/八个环戊环稠合在一个包含三个/四个交替连接的2,7-芘基和2,7-菲基单元的大环上。X射线晶体学分析和密度泛函理论计算表明，五元环的存在导致了碗状几何结构。两个分子都含有交替的芳香菲-共醌型芘结构，以满足克拉尔芳香六隅体规则。尺寸较小的CNB1碗深（d = 4.997 Å）比CNB2（d = 3.682 Å）更深，在甲苯中低于373 K时不能发生碗对碗（BTB）反转。然而，尺寸较大的CNB2在聚结温度（248 K）下显示出较小的BTB反转势垒（约12 kcal/mol），这是通过变温核磁共振测量估算得出的。两种化合物都表现出较小的能隙和具有多个氧化还原波的两性氧化还原行为。通过用NO·SbF进行化学氧化，实验上可得到CBN1和CBN2的双阳离子以及CBN2的四阳离子，它们都表现出不寻常的开壳单重态双自由基特征，单重态-三重态能隙较小（CBN1的ΔE = -2.71 kcal/mol，CBN2的ΔE = -2.50 kcal/mol，CBN2的ΔE = -2.00 kcal/mol）。根据核磁共振测量和理论计算（感应电流密度的各向异性、核独立化学位移和二维等化学屏蔽表面），双阳离子是全局芳香性的，而四阳离子是全局反芳香性的。小碗状的CNB1表现出对富勒烯C的选择性包封，而不是C，在甲苯中的缔合常数很大（K = 8.066 × 10 M）。然而，尺寸较大的CNB2对C和C都没有表现出任何包封作用。
