The research group of Peking University Huang Wenliang and its collaborators have made new progress in the chemistry of low-cost rare earth metals.

SMM News: the oxidation state is one of the most basic properties of an element. According to the traditional view, most rare earth metals can only exist stably at positive trivalent in compounds. This is due to the fact that the valence bond orbitals of rare earth metals are near-nuclear 4f orbitals, which are less affected by ligands and are not easy to participate in bonding. In recent years, great breakthroughs have been made in the non-traditional oxidation chemistry of rare earth metals, and the synthesis of non-traditional high-valent and low-valent rare earth metal complexes has been reported one after another. In low-valent rare earth metal complexes, the electronic configuration of divalent rare earth metal ions can change between 4fn+1 and 4fn5d1, in which the former is classified as traditional low valence while the latter is considered to be unconventional low valence. Exploring the factors leading to the transformation of this electronic configuration is of great significance to understand the electronic structure and bonding properties of low-valent rare earth metal complexes.

Recently, the research group of Huang Wenliang of the School of Chemistry and Molecular Engineering of Peking University, in cooperation with Hu Chaoshi of the Department of Chemistry of Tsinghua University and the Paula Diaconescu Research Group of the University of California, Los Angeles, reported the synthesis of anti-sandwich biphenyl compounds of samarium and ytterbium. The study of its electronic structure and bonding properties shows that the bibenzene complexes of samarium and ytterbium have very different electronic structures and bonding properties. The research results, entitled "Distinct Electronic Structures and Bonding Interaction in Inverse-Sandwich Samarium and Ytterbium Biphenyl Complexes", were published in Chemical Science, the flagship journal of the Royal Chemical Society, on October 29th, 2020.

The starting point of this study is to explore the effect of aromatic ligands on the electronic configuration of low-valent rare earth metal aromatic complexes. Fig. 1 (i) shows the changes in the electronic configuration of divalent rare earth metal ions in several known low-valent rare earth metal complexes. For most ligand systems, samarium and ytterbium are classified as traditional divalent rare earth metals with the electronic configuration of 4fn+1. Considering that aromatics and low-valent rare earth metals have good orbital energy and symmetry, they believe that it is possible to realize the transition of low-valent samarium or ytterbium electronic configuration from 4fn+1 to 4fn5d1 in the low-valent rare earth metal aromatics complex system.

The biphenyl complexes of samarium and ytterbium can be synthesized by using graphite potassium to reduce the trivalent precursor of samarium or ytterbium with a mixed solution of bibenzene (Fig. 1 (ii). The single crystal X-ray structure characterization of the reduction products shows that two samarium ions coordinate on both sides of the same benzene ring, while two ytterbium ions coordinate on both sides of different benzene rings. Reactivity, absorption spectra and X-ray absorption near-edge structure (Fig. 1 (iii)) and magnetic characterization show that samarium has a + 3-valent 4f5 configuration and biphenyl has-4 valence, while ytterbium has a + 2-valent 4f14 configuration and biphenyl has-2 valence.

Figure 1: synthesis, characterization and electronic structure of samarium and ytterbiphenyl complexes

In order to deepen the understanding of the electronic structure and bonding properties of samarium and ytterbiphenyl complexes, they analyzed them by relativistic corrected density functional theory. The results show that there is a δ interaction between Sm3+ and the bonded benzene ring, and its δ bonding orbital is mainly composed of 5d orbitals of samarium (> 20%) and π antibonding orbitals of benzene ring (~ 60%), and has considerable covalency (Fig. 1 (iv) (a));, while there is only a weak interaction between Yb2+ and bibenzene, and mainly shows ionic bonds (Fig. 1 (iv) (b). Further orbital decomposition analysis also confirmed that the δ-type orbital interaction plays an important role in the bonding of samarium with biphenyl (accounting for 39% of the mutually attractive energy composition, figure 1 (v)).

In summary, the biphenyl complexes of low-valent samarium and ytterbium were synthesized for the first time, and their electronic structures and bonding properties were elucidated by a variety of experimental characterization methods and theoretical chemical calculations. This study reveals the correlation between the electronic configuration of low-valent rare earth metal ions and the ligand environment, and provides a guide for further regulation of the electronic state of rare earth metals. Xiao Yuyuan, a doctoral student in the School of Chemistry and Molecular Engineering, Peking University, and Zhao Xiaokun, a doctoral student in the Department of Chemistry of Tsinghua University, are co-authors of this thesis. Huang Wenliang, a specially appointed researcher of Peking University, Associate Professor Hu Chaoshi of Tsinghua University, and Professor Paula Diaconescu of the University of California, Los Angeles, are co-authors. The X-ray absorption near-edge structure spectra of some samarium and ytterbium complexes in this paper were characterized by Professor Jeffrey Miller of Purdue University in the United States. The research is supported by Peking University, Beijing National Research Center for Molecular Science, Tsinghua School Talent Program, the National Natural Science Foundation of the United States, and the U.S. Department of Energy. Associate Professor Wang Bingwu of the Gao Song Research Group of Peking University and Sun Rong, a doctoral student, provided help in magnetic characterization.