N-Heterocyclic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Lewis Acid−Base Adducts of Sc2C2@C3v(8)‑C82/N-Heterocyclic Carbene: Toward Isomerically Pure Metallofullerene Derivatives Lipiao Bao,†,‡ Muqing Chen,†,‡ Wangqiang Shen,† Le Yang,§ Peng Jin,*,§ and Xing Lu*,† †

State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China § School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China S Supporting Information *

Compared to M@C82 and M3N@C80, the chemical properties of metal carbide cluster fullerenes (CCFs) have rarely been understood.2,11,12 Taking the prototypical Sc2C2@C3v(8)-C82 as an example, 17 types of nonequivalent carbon atoms (Figure 1)

ABSTRACT: The addition of a bulky N-heterocyclic carbene (NHC) to Sc 2 C2 @C3v(8)-C 82 affords two monoadducts (2a and 2b) quantitatively and regioselectively, representing the first examples of Lewis acid−base pairs of metal carbide cluster fullerenes. 2b is likely a kinetically favorable labile product that cannot be isolated from the solution. The crystallographic results of 2a unambiguously demonstrate that one polarized C−C single bond is formed between the normal carbene site C2N of the NHC and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc2C2@ C3v(8)-C82. Theoretical calculations demonstrate that the high regioselectivity, the unexpected addition pattern, and the quantitative formation of monoadducts are synergistic results from the cage geometry and electron distribution on the cage.

Figure 1. A total of 17 types of nonequivalent carbon atoms on C3v(8)C82.

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somerically pure derivatives of fullerenes are promising materials useful in such fields as photovoltaics, optics, and semiconductors.1−3 However, it is still very challenging to generate the target adducts without tedious separation processes because chemical functionalization of fullerenes generally produces a mixture of different regioisomers with a variety of addends.1 The reaction characteristics of endohedral metallofullerenes (EMFs), that is, fullerenes with metallic species inside the cages, are more complicated because of the relatively low cage symmetry, on the one hand, and the presence of the encapsulated metallic species, on the other hand. Even for the highly symmetric Sc3N@Ih-C80, which has only two types of nonequivalent carbon atoms on the cage, its chemical reactions with different reagents normally generate two monoadduct isomers, together with numerous di- and multiadducts. For instance, the benzyne addition of Sc 3 N@C 80 afforded at least two monoadducts together with several multiadducts4,5 and, even worse, its trifluoromethylation produced a mixture of di- and multiadducts with the number of addends ranging from 2 to 18.6−9 Analogously, trifluoromethylation of the low-symmetric Y@C82 affords at least two monoadducts and three multiadducts.10 Thus, tedious separation processes are necessary for acquiring isomerically pure EMF derivatives, which hinder their future applications dramatically. Accordingly, high regioselectivity and high yield are particularly desired for the chemical reactions of EMFs. © XXXX American Chemical Society

and 25 types of nonequivalent C−C bonds are presented on the C3v(8)-C82 cage. Consequently, the reported 1,3-dipolar reaction on Sc2C2@C3v(8)-C82 generated as many as four monoadducts.13 The case of a carbene reaction of Sc2C2@C3v(8)-C82 is even worse: as many as two monoadducts in addition to three diadducts were obtained.14 The addition of (μ-H)3Re3(CO)11NCMe to Sc2C2@C3v(8)-C82 seems rather regioselective because only one adduct is formed. The reason is that the C3v(8)-C82 cage has only one “sumanene” unit to react with the Re3 complex.15 In fact, from the perspective of cage geometry (Figure 1), as many as 17 regioisomeric monoadducts for a singly bonded addition reaction are possible for Sc2C2@C3v(8)-C82, not to mention that numerous di- and multiadducts are possible. However, we demonstrate herein a highly regioselective and quantitative monoformation of a Lewis acid−base pair from the reaction between 1,3-bis(diisopropylphenyl)imidazol-2-ylene (1) and Sc2C2@C3v(8)-C82, representing a practical example to effectively obtain isomerically pure fullerene derivative(s). The remarkably high regioselectivity and quantitative formation of monoadducts are proven theoretically to be a result of both the cage geometry and electron distribution of Sc2C2@C3v(8)-C82. In a typical reaction, 3 mg (2.7 μmol) of Sc2C2@C3v(8)-C82 and 31.8 mg (82.0 μmol) of 1 were dissolved in 20 mL of Received: October 7, 2017

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DOI: 10.1021/acs.inorgchem.7b02578 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry anhydrous toluene. The reaction mixture was then stirred at ambient conditions, and the reaction process was monitored with high-performance liquid chromatography (HPLC). Figure 2

Figure 3. ORTEP drawing of 2a with thermal ellipsoids set at the 20% probability level. Only one cage orientation and the major metal sites are shown. Solvent molecules and the minor metal sites are omitted for clarity. The top hexagonal ring of the cage is shown in red. The cage orientation is the same as that in Figure 1 for the ease of comparison.

with fractional occupancies as 0.37 (Sc1), 0.26 (Sc2), 0.36 (Sc3), 0.64 (Sc4), and 0.37 (Sc1′). To our surprise, one scandium atom (two disordered positions: Sc3 and Sc4) stays close to the top hexagonal ring (marked in red in Figure 3), where the addition takes place, which is different from the situations found in pristine Sc2C2@C3v(8)-C82 and its derivatives, where the two scandium ions are always distant from the top hexagonal ring of the cage.13−16 Consequently, the major Sc2C2 cluster tends to adopt a planar structure with the Sc−C2−Sc dihedral angle as 180.00°. In addition, the metal−metal distance between the two scandium atoms is remarkably elongated to a range from 4.119 to 4.562 Å compared to the values found in pristine Sc2C2@C3v(8)C82 [3.981(4) and 3.86(1) Å].16 Apparently, the geometric change of the Sc2C2 unit is a consequence of relocation of the metallic species inside the distorted cage and electron redistribution upon Lewis acid−base reaction. The high reactivity, the unexpected addition pattern, and the high monoregioselectivity observed in this reaction are remarkable, and therefore theoretical calculations are carried out to rationalize these interesting results. First, we try to understand the high reactivity of the reaction between 1 and Sc2C2@C3v(8)-C82. According to frontier molecular orbital theory, the smaller the energy difference between the highest occupied molecular orbital (HOMO) of a Lewis base and the lowest unoccupied molecular orbital (LUMO) of a Lewis acid is, the higher the reactivity of the neutralization reaction gained due to the larger stabilizing energy formed by an effective orbital interaction. Density functional theory (DFT) calculation results (Figure S2) suggest that Sc2C2@C3v(8)-C82 has a lower LUMO (−2.87 eV) than the LUMOs of Sc3N@Ih(7)-C80 (−2.37 eV) and -C60 (−2.70 eV), which gives a reasonable explanation for the higher reactivity of Sc2C2@C3v(8)-C82 than those of Sc3N@ Ih(7)-C8017 and -C6018 toward 1. Then, the high regioselectivity is interpreted by considering the p-orbital axis vector (POAV), LUMO distribution, natural population analysis (NPA) charges, thermodynamics, and nucleus-independent chemical shift (NICS) values of this endohedral. Basically, NPA could present the distribution of atomic charges,19 and NICS is used as an aromaticity criterion.20−22 Table S1 lists the LUMO distributions, NPA

Figure 2. (a) Reaction scheme of 1 with Sc2C2@C3v(8)-C82. (b) HPLC profiles of the reaction mixture probed at different times. Conditions: Buckyprep column (⌀ 4.6 mm × 250 mm); 1.0 mL min−1 toluene flow; room temperature; 330 nm detection wavelength.

shows the reaction scheme and the corresponding HPLC profiles of the reaction mixture probed at different times. After 1 min, the peak of Sc2C2@C3v(8)-C82 (41.3 min) vanished completely with two new peaks appearing at 14.7 and 15.7 min, which were attributed to the Lewis acid−base adducts 2a and 2b, respectively. It is remarkable that no di- or multiadductd were detected, although a large excess amount (30-fold) of 1 was used, revealing surprisingly high regioselectivity and the quantitative formation of monoadducts. In addition, the fast and complete consumption of Sc2C2@C3v(8)-C82 suggests that it is a very good Lewis acid to accept additional electron(s), although the cage has already gained four electrons from the encapsulated Sc2C2 cluster.14 These results are significant because most of the reported reactions of fullerenes (mainly C60 and C70) presented a yield of less than 60% and the formation of di- and/or multiadducts is generally unavoidable. The identity of 2a as a monoadduct was first verified by the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (Figure S1). To further confirm the composition of these adducts and obtain an in-depth understanding of the structures, especially the addition positions of the products, X-ray crystallographic studies of 2a were conducted. Figure 3 depicts the crystal structure of 2a, where only one cage orientation and the major metal sites are shown for clarity. The N-heterocyclic carbene (NHC) moiety links to the fullerene cage via a C−C single bond of 1.520 Å, verifying the formation of a Lewis acid−base adduct. It is surprising to see that the normal carbene site C2N of the NHC links to a specific [5,6,6]-carbon atom (type 1 in Figure 1) out of the 17 kinds of nonequivalent cage carbon atoms of Sc2C2@C3v(8)-C82. Inside the cage, the C2 unit is fully ordered with a bond length of 1.187 Å, but the two scandium atoms suffer from disorder, B


Communication

Inorganic Chemistry

multiadducts even when a large excess amount of NHC (1) has been used. Figure S4 shows the LUMO distribution of 2a, and the cage carbon atoms holding pronounced LUMO distributions or less negative NICS values (Figure 4) are all localized around the area close to the addition position and are covered by the large NHC moiety. Accordingly, further additions are prevented by the steric hindrance of the existing bulky NHC moiety. In summary, the reaction between Sc2C2@C3v(8)-C82 and 1 proceeds in a highly regioselective manner, which affords only two adducts (2a and 2b) quantitatively. The NHC moiety links to a specific [5,6,6]-carbon atom of Sc2C2@C3v(8)-C82 using its normal carbene site (C2N) in the final product 2a, which is rationalized theoretically as a consequence of the synergistic effect of electron distribution and cage geometry. In addition, 2b is also theoretically predicted to be a normal carbene monoadduct, with the addition site locating at one of the type 2 cage carbon atoms of Sc2C2@C3v(8)-C82. Our current work provides a practical strategy to obtain isomerically pure fullerene derivative(s) in a highly regioselective and quantitative manner, which may illuminate the applications of fullerenes/EMFs in such fields as catalysis and materials science.

charges, and relative energies of the corresponding singly bonded monoadducts of the five different types of carbon atoms with the highest POAV values. It is obvious that type 1 carbon atoms (Figure 1) possess the highest POAV values and largest LUMO distributions, in addition to the most positive NPA charges. Accordingly, they are the most reactive sites toward the NHC moiety, and thus it is not surprising that 2a was found in an exclusively high yield. In addition, type 2 carbon atoms bear the second largest LUMO distributions, relatively high positive NPA charges, and high POAV values. Therefore, we speculate that the addition site of 2b is at a carbon atom of type 2. Further thermodynamic results (Table S1) reveal that all of the C2Nbonding isomers (normal carbene adducts) are more stable than the corresponding C5N-bonding isomers (abnormal carbene adducts) by an energy difference of at least 2.9 kcal mol−1. Because 2a takes the normal carbene structure, it is reasonable to speculate that 2b should also have a structure with the normal carbene center linking to a type 2 carbon atom (see Figure S3d for the proposed structure of 2b). Thus, our theoretical results are in perfect agreement with the experiments. According to the recently proposed predictive aromaticity criteria (PAC), which is reflected by the NICS value of the pentagons and hexagons on the cages,20−22 the attack should occur at particular sites to maintain the cage aromaticity maximally. Figure 4 illustrates the average NICS values at the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02578. Further experimental details, vis−near-IR spectra, orbital diagrams, relative energies, optimized structures, and LUMO plots (PDF) Accession Codes

CCDC 1504915 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 4. Average NICS values at the three surrounding ring centers of the cage carbon atoms of Sc2C2@C3v(8)-C82 (filled stars) and 2a (hollow squares). The numbering schemes for the carbon atoms are shown in Figure 1. Data is obtained from the optimized structures of Sc2C2@C3v(8)-C82 and 2a at the M06-2X/6-31G*∼LanL2DZ level.

ORCID

Peng Jin: 0000-0001-6925-9094 Xing Lu: 0000-0003-2741-8733 Author Contributions

three surrounding ring centers of each nonequivalent cage carbon atom of the pristine Sc2C2@C3v(8)-C82 and 2a. Obviously, all of the carbon atoms of Sc2C2@C3v(8)-C82 adopt negative NICS values, consistent with its high aromaticity. In particular, the carbon atoms at the sites of addition C1 have the smallest negative NICS values, and undoubtedly the NHC (1) moiety tends to attack these particular carbon atoms to pursue the minimal disruption of the aromaticity of the EMFs. The theoretical NICS results (Figure 4) of the monoadduct 2a indeed show no obvious differences compared to those of the corresponding pristine Sc2C2@C3v(8)-C82. The quantitative formation of only monoadducts is also brilliant in the Lewis acid−base complexation reaction. Our calculation results reveal that the lack of suitable sites for the second or further additions owing to the steric hindrance of the bulky NHC moiety in 2a is the reason for the absence of di- or

‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the NSFC (Grants 51472095, 51672093, 51602112, and 51602097) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1014) is gratefully acknowledged. We thank the Analytical and Testing Center in Huazhong University of Science and Technology for all related measurements.

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REFERENCES

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