Journey to the Holy Grail of a coordination saturated buckyball

Abstract

Exohedral metallofullerenes are increasingly gaining attention from both academia and industry due to their unique structural and electronic properties. C₆₀ provides an incredible opportunity to bind up to 30 metals on the surface to realize an icosidodecahedral C₆₀@M₃₀ supramolecular complex, the Holy Grail of a coordination saturated buckyball. Recently, the Li and Zhan groups reported a rhombicuboctahedral C₆₀@Cu₂₄ core–shell structure, a giant leap on their journey toward the Holy Grail.

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Dandan Chu

Dandan Chu was born in Anhui province, China. She received her BS degree from Anhui Normal University in 2014. Then, she joined the research group of Prof. Yong Cui at Shanghai Jiao Tong University. Her research interest focuses on chiral metal–organic cages.

Yuhao Liu

Yuhao Liu obtained his MS degree from Fuzhou University in 2016, where he worked on the synthesis of 2D nanosheets under the supervision of Dr Ling Wu. Then, he joined the research group of Prof. Yong Cui at Shanghai Jiao Tong University. His research interest is focused on the synthesis, properties, and applications of supramolecular materials.

Yingguo Li

Yingguo Li was born in Henan province, China. He received his BS degree from Xuchang University in 2013 and MS degree in 2016 at Lanzhou University. Then, he joined the research group of Prof. Yong Cui group at Shanghai Jiao Tong University. His research interest focuses on metal–organic cages for catalysis and separation.

Yan Liu

Yan Liu received her BS degree in applied chemistry in 2006 and PhD in chemistry in 2011 from Shanghai Jiao Tong University. She conducted her postdoctoral research at the University of North Carolina (2011–2013). She joined Shanghai Jiao Tong University in 2014, where she is now a professor of chemistry. Her research interest focuses on supramolecular chemistry and coordination chemistry.

Yong Cui

Yong Cui received his PhD in physical chemistry in 1999 from Fujian Institute of Research on the Structure of Matter, CAS. He conducted his postdoctoral research at the University of Science and Technology of China, the University of North Carolina and the University of Chicago from 1999 to 2005. He joined Shanghai Jiao Tong University in 2005, where he is now a chair professor of chemistry. His research interest focuses on the fields of molecular assembly, porous materials and chiral materials.

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Since the discovery in 1985,¹ fullerene-C₆₀ (C₆₀, buckyball) and its derivatives have always been a research hotspot because of their unique structures and physicochemical properties.^2–17 One particular direction is to exploit the exohedral metallofullerenes,^7–9,18,19 coordination complexes of transition metals (M) spreading over the surface of C₆₀ binding to its C=C bonds in η² fashion, resulting in various C₆₀ derivatives with intriguing photofunctional/electronic properties.⁴ As a spherical polyene system, the 30 C=C bonds of C₆₀ provide an incredible opportunity to bind up to 30 metal atoms in a μ₃₀-(η²)₃₀ fashion and realize the C₆₀@M₃₀ core–shell structure, an icosidodecahedral supramolecular complex featuring 20 vertex-shared M₃-trigons, the Holy Grail of a coordination saturated buckyball (Scheme 1).

Scheme 1 Formation of η²-fashion metal-olefin complex (a), the proposed icosidodecahedral C₆₀@M₃₀ structure in a μ₃₀-(η²)₃₀ coordination mode (b), and the icosidodecahedron polyhedron (c). For clarity, the coordination interactions between M and C=C are omitted in (b).

Simple as it may seem, the highest nuclearity of the reported exohedral metallofullerenes to date is merely 6,^8,9 including C₆₀@M₆ structures with six mononuclear clusters in η² fashion (octahedron geometry)²⁰ and two trinuclear clusters in (μ₃–η²:η²:η²) fashion.²¹ Intuitively, metallofullerenes with nuclearity of more than 6 have been, until now, difficult to synthesize, most likely due to the steric crowding between bulky stabilizing ligands.

One of the key requisites to achieve a coordination saturated buckyball is adopting a vertex-shared M₃-trigon coordination configuration. However, it seems like an impossible mission as the multiple auxiliary ligands surrounding the M₃ unit prevent other M₃ units from hovering over the neighbor hexagons and thus restrict the total number of M₃ units on the C₆₀ surface, particularly for these open-shell transition metals with a coordination number no less than 4.

The studies of metallofullerenes can be dated back to the early 1990s, when several metal-coated fulleride clusters with high nuclearity such as C₆₀@Li₁₂,^22,23 C₆₀@Ba₃₂, C₆₀@Ca₃₂,²⁴ and others^25,26 were speculated to exist under specific conditions. Ab initio molecular dynamics calculation revealed that C₆₀@Li₁₂ could be a stable cluster. However, their exact structures were not confirmed due to the lack of analytical techniques, preventing further investigation of their functions.

Recently, Li's group at Jinan University and Zhan's group at Shantou University jointly reported a rhombicuboctahedral C₆₀@Cu₂₄, a metallofullerene of unprecedented high nuclearity.²⁷ It is a giant leap toward complete coordination on the C₆₀ surface. Overall, the ingenious strategies in their endeavor toward high nuclearity include: (i) adopting closed-shell Cu(I) with a lower coordination number instead of open-shell metals and (ii) adopting anionic bridge ligands instead of terminal ligands. Both of these strategies were intended to minimize the steric crowd of the M₃ coordination sphere. As a continuous effort of the two research groups to explore metallofullerene complexes, four exohedral Cu(I)–C₆₀ (cuprofullerene) carboxylate complexes with nuclearities of 6, 12, and 24 were successfully synthesized by solvothermal reactions of fluorocarboxylic/bicarboxylic acid, Cu₂O, and C₆₀ in aromatic solvents. From a structure perspective, all of the cuprofullerene complexes embody the same coordination mode of [μ₃–η²:η²:η²)-C₆₀]-Cu₃-[(μ₂–κ¹:κ¹)-COO]₃ (Fig. 1a) with a Cu₃ : C₆₀ ratio of 2 : 1, 4 : 1, and 8 : 1 (Fig. 1b–d). Particularly, the 24-nuclear cuprofullerene represents a rhombicuboctahedral C₆₀@Cu₂₄ core–shell structure with 8 Cu₃ units canopying on the 8 independent hexagons of C₆₀ (Fig. 1d), paving the road toward the completely coordinated C₆₀. For detailed structural analysis, please refer to their recently published paper.²⁷

Fig. 1 The simplified structures of the synthesized cuprofullerenes. (a) Cu₃ unit, (b) C₆₀@Cu₆, (c) C₆₀@Cu₁₂ and (d) C₆₀@Cu₂₄. Color code: red, Cu; cyan, O; black, C; green, F; gray, H.

As opposed to the reported exohedral M₃-metallofullerenes, in which the monodentate auxiliary ligands such as phosphine, CO, and isocyanide were frequently used,^7–9 the synergistic associations of (μ₂–κ¹:κ¹)-carboxylate and (μ₃–η²:η²:η²)-hexagon could significantly enhance the stability of the Cu₃ units. Furthermore, the low coordination of Cu(I) and the outward orientation of carboxylates far away from the buckyball could endow the Cu₃ unit with much relieved steric crowding on the surface of C₆₀, which provides an opportunity to squeeze in more Cu₃ units on the adjacent hexagons to increase the overall nuclearity.

Besides adopting low coordination Cu(I) and carboxylate as auxiliary ligands, the use of fluorocarboxylate as ligands might also play an essential role in the formation of cuprofullerene complexes of high nuclearity. Compared with the non-fluorinated carboxylate, the poorly coordinated fluorocarboxylate facilitates relatively strong association of the weak basic olefin C₆₀ with Cu(I), as reported in the tris(η²-ethene)-Cu(I) cationic complexes.²⁸ As a matter of fact, using non-fluorinated n-butylcarboxylate under similar conditions did not produce a complex of similar nuclearity. Inspired by the successful evolution of nuclearity from C₆₀@Cu₆ to C₆₀@Cu₁₂ through increasing the concentration of heptafluorobutyrate auxiliary ligands, the C₆₀@Cu₂₄ structure was eventually realized by using dicarboxylate, hexafluoroglutarate and glutarate, as bridging ligands, which is vital to further increase the local concentration of carboxylates around the surface of the buckyball. As a result, the bridging dicarboxylate provides extra bridging interactions beyond μ₂-H₂O associating eight Cu₃ units, further stabilizing the rhombicuboctahedral C₆₀@Cu₂₄ core–shell structure. It is worth pointing out that the authors were also able to obtain the extremely high-nuclear structure with non-fluorinated glutarate in 1-chloronaphthalene, in which the high solubility of C₆₀ might have played an important role. It exemplifies the intricacy and complexity of the crystallization process, which leads us to believe that the final mission, the complete coordination of C₆₀, can be accomplished by the fine tuning of the reaction systems. In addition, by closely looking into the geometry of the obtained C₆₀@Cu₂₄ molecule, it appears there is enough space above the 6 vacant C=C bonds to allow additional metal atoms and complete the proposed C₆₀@Cu₃₀ icosidodecahedron, provided the suitable auxiliary ligands are used. We look forward to the realization of the great Holy Grail by the authors as soon as possible.

Note that solvothermal conditions were utilized for the first time to prepare exohedral metallofullerene in this work. The high-boiling-point aromatic solvents make it possible to run the solvothermal reactions at rather high temperatures, and thus to overcome the energy barriers toward these high-nuclear cuprofullerenes.

The initial photophysical/photochemical properties were studied for the above-obtained Cu₃-metallofullerenes. Their solid-state UV-Vis diffuse reflectance spectra exhibit absorption of the whole UV-visible area, indicating more efficient absorption of visible light than the pristine C₆₀ due to the complexation of Cu(I). Theoretical calculations confirmed that the complexation could reduce the symmetry and disturb the π-system of C₆₀, resulting in the symmetry-allowed excitation in the visible-light region. With the increase of nuclearity, the lowest absorptions of the four complexes show a hypsochromic shift, suggesting the potential influence of nuclearity on their electronic structures. Compared with the pristine C₆₀ material, the cuprofullerene complexes show not only much higher efficient absorption of visible light, but also more controllable HOMO–LUMO gap, which implies an attractive prospect in utilizing solar energy.

As organocopper reagents, we could also expect these cuprofullerene complexes to function as novel catalysts in preparing organic fullerene derivatives,²⁹ opening a new door for organic fullerene chemistry.

Conclusions

In short, the Li and Zhan groups have successfully prepared a series of exohedral cuprofullerene complexes, featuring Cu₃ units canopying on the C₆₀ surface in a (μ₃–η²:η²:η²) fashion, by judiciously selecting fluorocarboxylate/dicarboxylate as auxiliary ligands and solvothermal reaction conditions. The ratio of M₃ : C₆₀ has been expanded from 2 : 1 to 4 : 1 and 8 : 1, forming the unprecedented 12- and 24-nuclear cuprofullerenes. Particularly, the 24-nuclear complex, a rhombicuboctahedral C₆₀@Cu₂₄ core–shell structure, represents an exohedral metallofullerene of the most nuclearity, a giant leap toward the coordination saturated C₆₀@M₃₀. This work greatly enriches the structural diversity of exohedral metallofullerenes and is expected to bring advanced applications such as in the photoelectric conversion and catalytic synthesis of organic fullerene derivatives. We look forward to the realization of C₆₀@M₃₀ icosidodecahedron and its intriguing applications in the near future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key Basic Research Program of China (2016YFA0203400), the Key Project of Basic Research of Shanghai (17JC1403100 and 18JC1413200) and the Shanghai Rising-Star Program (19QA1404300).

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