Zhang's group at the College of Chemistry and Molecular Engineering, Peking University has recently published a paper titled "Nonplanar Aromaticity of Dinuclear Rare-Earth Metallacycles" in Journal of the American Chemical Society (Dajiang Huang, Wei Liu, Yu Zheng, Rui Feng, Zhengqi Chai, Junnian Wei, and Wen-Xiong Zhang,* J. Am. Chem. Soc. 2024, doi.org/10.1021/jacs.4c04683).
Since Hoffmann proposed the concept of metal aromaticity in 1979 and Roper reported the first aromatic metallabenzene in 1982, the synthesis of various aromatic metallacycles has been a key focus for scientists. Over the years, many novel aromatic metallacycles have been synthesized, primarily featuring planar or near-planar geometries formed by π-type or mixed (π+σ) p-d orbital overlaps. However, until now nonplanar aromatic rare-earth metallacycles adopting σ-type orbital overlaps had not been reported (Fig. 1a). This is mainly due to the following reasons:
a. The unique (n−1)d0 electron configuration of rare-earth ions, which lack additional d electrons to form a conjugated system.
b. The high-lying valence d orbitals of rare-earth ions, which make it difficult to effectively overlap with ligand p orbitals.
c. The traditional π-type overlap mode cannot maximize orbital overlap in nonplanar conjugated systems.
Therefore, synthesizing nonplanar aromatic rare-earth metallacycles poses a significant challenge (Fig. 1b). To address these issues, inspired by their previous work, they further reduced the butadiene dianion ligand and injected "two electrons" into its anti-bonding orbitals to design an electron-rich 2-butene tetraanion (BTA) ligand. Reacting this ligand with various rare-earth ions, they successfully synthesized the first non-planar aromatic rare-earth metallacycle (Fig. 1c).
Fig. 1 p-d orbital overlap mode in metallacycles, nonplanar aromatic transition metallacycles, and synthesis of nonplanar aromatic rare-earth metallacycles.
Zhang's group has been dedicated to the research of rare-earth metallacyclic chemistryfor many years (Angew. Chem. Int. Ed. 2017, 56, 15886; J. Am. Chem. Soc. 2019, 141,6843; J. Am. Chem. Soc. 2019, 141, 8773; J. Am. Chem. Soc. 2019, 141, 20547; J. Am. Chem. Soc. 2020, 142,10705; J. Am. Chem. Soc. 2021, 143, 9151; Cell Rep. Phys. Sci. 2022, 3, 100831; J. Am. Chem. Soc. 2023, 145,6633; Cell Rep. Phys. Sci. 2023, 4, 101479). In 2017, they synthesized and characterized the first bridged bis-alkylidene scandium(III) complexes (Chem. Sci. 2017, 8, 6852). This scandium metallacycle exhibited bond length equalization but had a significantly nonplanar structure (with angles exceeding 60°), which deviates greatly from the structures of traditional aromatic metal metallacycles. Consequently, at that time, the aromaticity characteristics of this complex were not deeply investigated. In 2020, they reported structurally similar BTA bridged dinuclear samarium(III) complexes (J. Am. Chem. Soc. 2020, 142, 10705). This compound also displayed bond length equalization. This similarity raised the question: Do these two non-planar rare-earth metallacycles, both showing bond length equalization, possess aromaticity? The synthesis and characterization of aromatic rare-earth metallacycles represent a new research frontier. Zhang's group reported the first aromatic rare-earth metallacycle, cyclopropenyl (J. Am. Chem. Soc. 2019, 141, 20547), followed by a collaboration with Hu Hanshi from Tsinghua University and Wang Laisheng from Brown University, reporting the aromatic cyclo-PrB2− compound (Chem. Sci. 2022, 13, 10082). Both of these examples are planar aromatic rare-earth metallacycles. However, to date, nonplanar cases have not been reported in the literature.
Based on these observations and considerations, they conducted aromaticity calculations on these two nonplanar dinuclear rare-earth metallacycles, revealing significantly negative NICS values. Additionally, calculations of ICSS, AICD, and ISE all indicated prominent aromaticity features (Figs. 2c-f). To explore the origin of this unique aromaticity, they conducted electronic structure studies on these two nonplanar aromatic rare-earth metallacycles. DFT calculations showed that they share similar electronic structures. Their aromaticity originates from a unique π-HOMO orbital in the system, primarily formed by the overlap between the ligand's occupied antibonding π* orbitals and the empty d orbitals of the rare-earth ions (Fig. 2g). By further reducing the butadiene dianion ligand and "injecting" two electrons into the ligand's antibonding π* orbitals, the formation of this occupied π* orbital not only provided additional electrons to build the conjugated system but also reduced the energy gap between the ligand orbitals and the metal orbitals. Notably, the σ-type bonding mode between it and the metal d orbitals maximized orbital overlap in this nonplanar structure, forming a π-HOMO with significant covalency, along with the other two π orbitals, ultimately forming a five-center-six-electron (5c-6e) conjugated system (Fig. 2h). Other chemical bond analysis methods (AdNDP, MCBI, and PIO, etc.) also reached consistent conclusions from different perspectives.
Fig. 2 Crystalstructure, DFT optimized structure, aromaticity evaluation, and electronic structure calculation of nonplanar aromatic dinuclear scandium and samarium metallacycles.
The aforementioned studies have shed light on the origin of nonplanar aromaticity, which in turn contributes tothe stability of the electron-rich BTA ligand. Building on this understanding, the researchers hypothesized the potential existence of other nonplanar aromatic rare-earth metallacycles. To explore this, they conducted DFT calculations and optimized the structures of dinuclear rare-earth metallacycles of lutetium and gadolinium (Fig. 3a). These two molecules exhibited similar characteristics, including equalized bond lengths and significant aromaticity. Their electronic structures are essentially identical to those of scandium and samarium metallacycles, featuring a 5c-6e conjugated system (Figs. 3b-e).
Fig. 3 DFT predicted structures, aromaticity evaluation, and electronic structure calculation of dinuclear lutetium and gadolinium metallacycles.
Based on the results of DFT calculations, they persistently pursued and successfully synthesized the predicted dinuclear lutetium and gadolinium metallacycles. X-ray crystal diffraction analysis corroborated that both metallacycles displayed bond length equalization, consistent with the bond lengths calculated by DFT (Fig. 4a). Given the presence of unpaired 4f electrons in the dinuclear gadolinium aromatic metallacycle, temperature-dependent magnetic susceptibility analysis was performed to determine its preferred ground state. The results revealed the presence of antiferromagnetic coupling between the two trivalent gadolinium ions (J=−0.67 cm−1), resulting in anopen-shell singlet ground state (Fig. 4b). These experiments validated the results of unrestricted DFT calculations (Fig. 3d). Nevertheless, owing to the extremely weak antiferromagnetic coupling, the high-spin state and singlet state were nearly degenerate (<100 cm−1), with DFT calculations indicating that these two electronic states had minimal impact onthe structure and bonding of the system.
Fig. 4 Crystal structures of dinuclear lutetium and gadolinium metallacycles and temperature-dependent magnetization analysis of gadolinium metallacycles.
In summary, this study identifies the first example of nonplanar aromatic rare-earth metallacycles, predicts the existence of other nonplanar aromatic rare-earth metallacycles using DFT calculations, and successfully synthesizes the predicted nonplanar aromatic rare-earth metallacycles. This research expands our understanding of aromaticity, by revealing the nonplanar aromatic characteristics of dinuclear rare-earth metallacycles. It also provides a new theoretical basis and experimental guidance for designing and synthesizing various nonplanar aromatic rare-earth metallacycles. Dajiang Huang and Wei Liu are co-first authors of the paper, with Prof. Wen-Xiong Zhang as the corresponding author. This work was supported by the National Key R&D Programof China and the National Natural Science Foundation of China. Special thanks to Prof. Dr. Zhenfeng Xi of Peking University for useful discussions and comments on the manuscript, and to Prof. Congqing Zhu of Nanjing University and Dr. Qiong Yuan of Peking University for their assistance in the analysis of magnetization.
Link: https://doi.org/10.1021/jacs.4c04683
