Since Fischer’s seminal work on bis(benzene)chromium, metal arene complexes have become one of the most important classes of organometallic compounds. While the bound arenes are usually considered neutral in transition metal arene complexes, the more electropositive rare-earth metals tend to form complexes with reduced arene anions. Compared to readily reducible polyarenes, such as naphthalene and anthracene, the parent benzene is much more difficult to reduce(−3.42 V vs. SCE), resulting in fewer rare-earth metal reduced benzene complexes compared to other arenes. The reported rare-earth metal benzene complexes mainly contain benzene radical monoanionsor dianions. While rare-earth metal complexes of the parent benzene tetraanion remain elusive, Diaconescu and Huang reported a series of rare-earth metal biphenyl complexes (Nat. Commun. 2013, 4, 1448; Inorg. Chem. 2015, 54, 2374; Chem. Sci. 2021, 12, 227). Recently, Huang group at the College of Chemistry and Molecular Engineering (CCME) of Peking University reported the synthesis and structural characterizations of neutral inverse-sandwich rare-earth metal complexes of the parent benzene tetraanion supported by a monoanionic β-diketiminate (BDI) ligand. Further reactivity studies showed these complexes could act as four-electron reductants. This work was published in Chemical Science on 13 May 2024, titled “Neutral Inverse-Sandwich Rare-Earth Metal Complexes of the Benzene Tetraanion”.
The salt metathesis reaction between the potassium salt (BDI)K and rare-earth metal triiodides gave the trivalent metal precursors 1-Y and 1-Sm. Reduction of 1-M in benzene or para-xylene with potassium graphite yielded the neutral inverse-sandwich rare-earth metal benzene complex 2-M and para-xylene complex 3-M (Fig.1). The 1H NMR spectra of 2-Y and 3-Y are both diamagnetic. For 2-Y, the proton signal of the bound benzene shifts upfield to 2.37 ppm in C6D6 and appears as a triplet due to weak 89Y‒1H coupling (JY-H = 1.6 Hz). The 13C NMR spectrum of 2-Y also shows an upfield shifted triplet for the bound benzene at 65.7 ppm (JY-C= 4.8 Hz). The 1H and 13C NMR spectra of 3-Y exhibit similar features to 2-Y. For 2-Sm and 3-Sm, the proton signal of the bound ring could be observed at 21.7 and 26.7 ppm, respectively. Moreover, 3-Sm rapidly underwent arene exchange to form 2-Sm-d6 after dissolution in C6D6. In contrast, 2-Sm showed higher stability, with no evidence of arene exchange and decomposition in C6D6 observed even after prolonged heating at 50 °C.
Fig.1 Synthesis of 1-M, 2-M and 3-M.
The molecular structures of 2-M and 3-M were unambiguously established by single crystal X-ray diffraction (Fig. 2a). All four complexes are neutral molecules featuring an inverse-sandwich structure with a μ-η6, η6 coordination mode for the bound arene. A major difference between yttrium and samarium complexes is that the latter contains a coordinated THF molecule per samarium, likely due to the larger ionic radius of samarium. Notably, the coordination of THF causes a different conformation: the two N‒Sm‒N planes are co-planar in samarium complexes, while the two N‒Y‒N planes are almost orthogonal to each other in yttrium complexes. The structural features of the bound rings are illustrated in Fig. 2b. The average C‒C distances of 2-M and 3-M are significantly longer than that of free benzene (1.39 Å). In addition, the bound rings are significantly distorted, leading to a boat conformation for 2-Y and 3-Y, and a chair conformation for 2-Sm and 3-Sm. These structural features are consistent with the inverse-sandwich rare-earth metal biphenyl complexes, indicating the [M3+‒(arene)4−‒M3+] electronic structure for 2-M and 3-M. Moreover, the solution magnetic moments determined by the Evans method of 2-Sm(1.98 μB, 1.40 μB per samarium) and 3-Sm (2.39 μB, 1.69 μB per samarium) fall within the normal range of Sm3+ (1.3–1.9 μB), in line with the [M3+‒(arene)4−‒M3+] electronic structure.
Fig.2 (a) X-ray crystal structures of 2-Y and 2-Sm; (b) The structural features of the bound rings in 2-M and 3-M.
Density functional theory (DFT) calculations (Fig.3) showed that the ground states of 2-Y and 3-Y are closed-shell singlet. The optimized structures of 2-Sm and 3-Sm with the 11A state match better with the crystal structures than the 13A state. The highest occupied molecular orbital (HOMO) and HOMO−1 of 2-M feature δ bonding interactions between the rare-earth metals and the bound arene. For 2-Y, the HOMO and HOMO−1 are composed of slightly over 30% yttrium 4d orbitals and around 58% carbon 2p orbitals of the bound ring. The higher contribution from yttrium-based orbitals in δ bonding orbitals in 2-Y than that in yttrium biphenyl tetraanion complexes(ca. 20% of Y 4d orbitals) suggests stronger δ bonding interactions and higher covalency in the former. For 2-Sm, the αHOMO and αHOMO−1 have some additional 4f characters, which is probably due to energy degeneracy between δ bonding orbitals and 4f orbitals. For 3-M, the lower symmetry of p-xylene results in larger energy difference between HOMO and HOMO−1, which may explain the lower stability of 3-M compared to 2-M. The population analysis on 2-M and 3-M and calculated bond index further support the assignment of the [M3+–(C6H6)4−–M3+] electronic structure.
Fig.3 Kohn–Sham orbitals (isovalue 0.04) of 2-Y(a) and 2-Sm (11A) (b).
The unique [M3+–(C6H6)4−–M3+] electronic structure and the strong metal–arene δ interactions prompted the authors to explore their potential use as multielectron reductants. 2-Sm was chosen as the representative to study reactivity toward unsaturated organic substrates (Fig. 4). Treatment of 2-Sm with 2 equiv. of cyclooctatetraene (COT) quantitatively yielded a mononuclear Sm(III) product 4-Sm. Two neutral COT molecules are reduced to (COT)2− with the concomitant formation of a neutral benzene. Moreover, the reaction of 2-Sm with 2 equiv. of 1,4-diphenylbutadiyne quantitatively afforded the first rare-earth metallacyclopentatriene complex 5-Sm, along with the formation of a neutral benzene. Overall, 2-Sm can serve as a Sm(I) synthon to reduce unsaturated organic substrates by four electrons.
Fig.4 Reactivity of 2-Sm as a four-electron reductant.
In summary, this work synthesized and characterized rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes with a bulky BDI ligand. Structural, spectroscopic, and magnetic data all support a [M3+–(C6H6)4−–M3+] electronic structure with strong metal–arene interactions. DFT calculation results support the assignment of the electronic structure and unveil strong δ bonding interactions between rare-earth metals and the bound arene. Reactivity studies demonstrate that the inverse-sandwich samarium benzene complex can act as a four-electron reductant to reduce unsaturated organic substrates. These results showcase the advantages of bulky monoanionic ligands in stabilizing neutral inverse-sandwich rare-earth metal arene complexes and the potential of these metal arene complexes as multi-electron reservoirs for synthesis and reactivity. Notably, two structurally related samarium benzene complexes supported by BDI ligands were synthesized by Ankerand Harderusing different methods and reported during our manuscript preparation or at the time of our submission, highlighting the dynamic and active nature of this field (https://www.researchsquare.com/article/rs-3465325/v1 & DOI: 10.1002/anie.202405229).
Graduate students Yi Wang and Yurou Zhang are co-first authors of the paper, and Prof. Wenliang Huang is the corresponding author. This work was supported by the National Natural Science Foundation of China, Peking University and Beijing National Laboratory for Molecular Sciences. We acknowledge the Analytical Instrumentation Center of Peking University and the High-Performance Computing Platform of Peking University for their support.
Link:https://doi.org/10.1039/D4SC02491E
