A report by Zhao Liang group of Tsinghua University and Zhu Jun group of Xiamen University on the superconjugated aromaticity of polymetallic aromatic heterocycles and its effect on the reactivity

Recently, Zhao Liang's research group of Tsinghua University and Zhu Jun's research group of Xiamen University published a research paper entitled "hyperconjunctive atomicity and protoauthorization reactivity of diversified indoliums" (DOI: 10.1038 / s41467-019-13663-8) in the journal Nature communications, A new type of hyperconjugated aromaticity and its effect on the reactivity of the compounds were reported Aromaticity is a concept used to evaluate the extra stability of cyclic organic compounds The common aromatic cyclic conjugated π systems (such as shockel and Mobius π systems) are all composed of multiple SP 2 hybrid carbon atoms or heteroatoms In 1999, theoretical chemists schleyer and nyul á szi found that two electron donating substituents on SP 3 hybrid carbon atom in cyclopentadiene can participate in π conjugation in the ring through hyperconjugation, forming hyperconjugative aromaticity (J am Chem SOC., 1999, 121, 6872) In 2016, the team of Zhao Liang, Department of chemistry, Tsinghua University, and Zhu Jun, School of chemistry and chemical engineering, Xiamen University, reported for the first time the existence of hyperconjugated aromaticity in indole tetranuclear gold clusters with transition metals as substituents, At the same time, they found that transition metal substituents can produce more significant hyperconjugated aromatic effect than traditional main group elements Recently, Zhao Liang's group of Tsinghua University and Zhu Jun's group of Xiamen University once again reported the difference of superconjugated aromaticity in indole tetranuclear and pentanuclear gold clusters and its effect on reactivity Compared with the indole tetranuclear gold cluster (2), the pentanuclear gold cluster (1) contains two adjacent gem cau 2 units, which has double hyperconjugation Generally speaking, the introduction of saturated carbon atoms into a ring will destroy the conjugation of the ring Interestingly, computational chemistry shows that both the indole rings of compounds 1 and 2 have closed-loop currents The nuclear independent chemical shifts (NICS (1) zz) of the six membered ring / five membered ring in compound 1 are - 24.5 / - 20.9 ppm, respectively Compared with - 26.4 / - 18.2 ppm of compound 2, the five membered ring of compound 1 may have more significant aromaticity At the same time, the bond length of indole five membered ring in the crystal structure of compound 1 is more average The calculation of the electron local function (ELF) and the electron density of delocalization (eddb) on the ring confirmed that the five membered ring of compound 1 indeed has a higher degree of delocalization In addition to the difference between 1 and 2 in computational chemistry, they also studied the effect of hyperconjugated aromaticity on the reactivity By monitoring the reaction process of the fluorescent probe molecule with 1 and 2 respectively, they confirmed that the probe molecule only reacts with compound 1 It was confirmed by UV-vis, 1H NMR, HRMS and fluorescence kinetic stop that the reaction rate was determined by the proton dissociation of n-au bond in compound 1 The difference in the reactivity of compounds 1 and 2 can be attributed to their hyperconjugated aromaticity Compound 1 has higher electron delocalization due to its more remarkable superconjugation aromaticity, so the n-indole nitrogen atom (NPA = - 0.604) of compound 1 has lower electron density (NPA = - 0.630) than that of N atom in compound 2, which makes the n-au bond energy of compound 1 lower than that of compound 2, and more prone to proton dissociation In this work, the difference between the superconjugated aromaticity of the polymetallic aromatic ring and the reaction activity are related, which fully shows that the superconjugated aromaticity is of great significance for understanding the structure and properties of some polymetallic organic intermediates The corresponding authors of this paper are Zhao Liangchang, associate professor and Zhu Jun, Professor of Xiamen University The first author provides theoretical calculation support for Xiao Kui, Ph.D student in the Department of chemistry of Tsinghua University and Zhao Yu, Ph.D student in the school of chemistry and chemical engineering of Xiamen University.