Guo Jin-Chang, Feng Lin-Yan, Zhai Hua-Jin
Department of Chemistry, Xinzhou Teachers University, Xinzhou 034000, Shanxi, China.
Phys Chem Chem Phys. 2018 Feb 28;20(9):6299-6306. doi: 10.1039/c7cp08420j.
Planar hypercoordinate carbons as exotic chemical species are dominated by 18-electron counting. We report herein a 16-electron planar tetracoordinate carbon (ptC) cluster, CBeAu, which is quasi-planar to be exact, being composed of a C center, a square-planar Be ring, and four outer Au bridges. The quasi-ptC cluster is established as a global minimum via computer structural searches, located 14.6 kcal mol below the nearest competitor at the CCSD(T) level. It shows thermodynamic and electronic robustness, with a low electron affinity (1.54 eV at B3LYP) and a large HOMO-LUMO gap (2.21 eV for excitation energy). Bonding analyses reveal 2π and 6σ double aromaticity, in addition to four three-center two-electron (3c-2e) Be-Au-Be σ bonds, confirming that 16-electron counting is perfect for the system. We believe that double (π and σ) aromaticity is a general concept that governs planar or quasi-planar carbons, which overrides the 18-electron rule. Competition between quasi-ptC and tetrahedral carbon (thC) isomers in the CBeM (M = K, Au, H, Cl) series is also examined, which sheds crucial light on factors that govern the ptC clusters. The present findings offer opportunities for further planar and unconventional molecules.
平面超配位碳作为奇特的化学物种，以18电子计数为主导。我们在此报告一种16电子平面四配位碳（ptC）簇合物CBeAu，确切地说是准平面的，它由一个C中心、一个平面正方形Be环和四个外部Au桥组成。通过计算机结构搜索确定该准ptC簇合物为全局最小值，在CCSD(T)水平下比最接近的竞争者低14.6 kcal/mol。它表现出热力学和电子稳定性，具有低电子亲和能（B3LYP水平下为1.54 eV）和大的HOMO-LUMO能隙（激发能为2.21 eV）。键合分析揭示了2π和6σ双芳香性，此外还有四个三中心两电子（3c-2e）Be-Au-Be σ键，证实16电子计数对该体系是完美的。我们认为双（π和σ）芳香性是支配平面或准平面碳的一个普遍概念，它超越了18电子规则。还研究了CBeM（M = K、Au、H、Cl）系列中准ptC和四面体碳（thC）异构体之间的竞争，这为支配ptC簇合物的因素提供了关键线索。目前的发现为进一步研究平面和非常规分子提供了机会。
