Regioselective Polyamination of Gd@C2v(9)-C82 and Non-High

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Regioselective Polyamination of Gd@C2v(9)‑C82 and Non-High Performance Liquid Chromatography Rapid Separation of Gd@C82(morpholine)7 Huan Huang,†,⊥ Lele Zhang,∥,⊥ Xuejiao J. Gao,‡ Xihong Guo,† Rongli Cui,† Binggang Xu,# Jinquan Dong,† Yanbang Li,§ Liangbing Gan,§ Fei Chang,∥ Xingfa Gao,*,‡ and Baoyun Sun*,† †

CAS Key Lab for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ Key Laboratory of Functional Small Organic Molecule, Ministry of Education, and Jiangxi’s Key Laboratory of Green Chemistry, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China § College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ∥ College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot 010021, China # School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China S Supporting Information *

E

adduct.22 Only two bisadducts with selectivity have been reported via photochemical reaction of Sc3N@(C80-Ih(7))14 and Bingel−Hirsch reaction of La@C8219 (Scheme 1b). When polyaddition forming single bonds to EMFs were involved, more complex mixtures of multiadducts were obtained. What is more, each multiadduct contains very many isomers and only part of them can be separated by high performance liquid chromatography (HPLC) (Scheme 1c). For example, addition of hydroxyl and amino groups formed inseparable mixtures.23,24 Perfluoroalkylation of La@C8225 afforded seven isomers of La@ C82(C8F17)2. Similarly, trifluoromethylation of Y@C82 afforded a series of adducts Y@C82(CF3)n (n = 1, 3, 5) and two isomers of Y@C82(CF3)5 were separated.26 Further example is trifluoromethylation of Sc3N@(C80-Ih(7)), which yielded mixtures with multiple CF3 groups added to the carbon cage.26−28 In a word, selectivity is usually difficult to achieve and any application is limited in time-expensive chromatographic separations. Recently, we have reported the regioselective additions of secondary amines to empty fullerenes, such as C60 and C70.29,30 It is proposed that secondary amines are a class of the most effective reagents for selective multiaddition reactions, because they are not only nucleophile reagent but also electrophilic reagent and their reactivity is suitable for regioselectivity. Gd@ C82 is chosen to represent the abundant EMFs, M@C82. An isomerically pure hepta-amino adduct for Gd@C82, Gd@ C82(morpholine)7, is synthesized with morpholine in the presence of N-fluorobenzenesulfonimide (NFSI) with high regioselectivity (Scheme 1d). Amination of Gd@C82 was carried out in the presence of NFSI (5 equiv) and an excess amount of morpholine (30 equiv) in o-dichlorobenzene (o-DCB) at room temperature for 20 h. New color strips were directly detected by thin layer chromatography (TLC). The major product (Figure 1a) was

ndohedral metallofullerenes (EMFs) are fullerenes with metal atoms or metallic clusters trapped inside their hollow interiors.1−3 Their unique structures and electronic properties give rise to potential applications in materials research, photovoltaics, and biomedicine.4−11 To satisfy emerging applications, exohedral chemical functionalization of EMFs is required. Among these, derivatives formed by single covalent bonds have been investigated in recent years because of their different properties from those of pristine EMFs.1,12 For instance, derivatives with a singly bonded substituent, such as monoadduct La@C2n (C6H3Cl2) (2n = 72, 74, 80, 82)13−16 made the insoluble EMFs dissolve. However, due to many equivalent reaction sites containing on EMFs carbon cage, an obvious challenging of monoadduction is regioselectivity (Scheme 1a). For instance, four isomers of benzyl adducts, Scheme 1. Single Bond Additions of EMFs

La@C82(CH2C6H5), were synthesized when La@C82 reacted with toluene.17 Also, the Bingel−Hirsch reaction of La@C82 constructed four isomers of monadducts.18,19 Only the benzyl radical, generated from benzyl bromide, afforded one monoadduct when reacting with La2@C8020 and Sc3C2@IhC80,21 respectively. Very recently, a N-heterocyclic carbine reacted with Sc3N@Ih-C80 and formed a singly bonded [6,6,6]© 2017 American Chemical Society

Received: September 8, 2017 Revised: December 15, 2017 Published: December 15, 2017 64

DOI: 10.1021/acs.chemmater.7b03787 Chem. Mater. 2018, 30, 64−68

Communication

Chemistry of Materials

typically more reactive than [6,6,6]-junctions according to the previous reports.20,32 Furthermore, 1,4-additions of morpholines are observed. The addition sites C2 through C7 on carbon cage show mirror symmetry, and the exohedral morpholine group attached to C4 features conformation disorder with two torsion positions, which are shifted slightly by 1.96 Å. Interestingly, morpholine groups form a continuous ribbon of edge-sharing para-C6(C4H8NO)2 hexagons (each shared edge is a cage C(sp3)−C(sp2) bond) apart from the hexagonal ring along the C2 axis of the C2v(9)-C82 cage.33 In sharp contrast, the addition sites of empty fullerenes, e.g., the amination of C6030 and C70,29 are around a pentagon. Besides novel external addition sites, internal metal Gd of Gd@C82(morpholine)7 also shows different features. Previously, the M atom is situated near the hexagonal ring along the C2 axis.33,34 Though Gd in Gd@C82(morpholine)7 has one major site and six less occupied sites (Figure 2b and Table S2). The shortest three Gd−C distances between the dominated Gd site (0.70) and carbon cage are Gd−C77 (2.35 Å), Gd−C98 (2.35 Å), and Gd−C82 (2.47 Å) (Figure S2). This phenomenon suggests that Gd prefers to reside on the (5,6,6) patch apart from the C2 axis, which is further supported by theory caculations (Figure S5 and Table S3). The different occupancy of Gd inside the cage is caused by the new electronic environment after the addition of seven morpholine groups. Thus, hepta-amination of Gd@C82 is an effective way to tune the electronic properties of fullerene cores. As presented in Figure 3, the UV−vis-NIR spectrum of Gd@ C82(morpholine)7 changes considerably. The two characteristic

Figure 1. (a) Reaction scheme between Gd@C82 and morpholine, (b) HPLC profile for isolated Gd@C82(morpholine)7 on Buckyprep column. Column: Buckyprep (φ10 × 250 mm). Eluent: toluene 5 mL/ min at 25 °C. Detector: UV 330 nm.

separated using flash chromatography on silica gel column. Interestingly, the isolated fraction showed only one peak on Buckyprep column, indicating that only one isomer exists in the fraction (Figure 1b). Black single crystals of the isolated fraction were grown by evaporation from a CS2/hexane solution. As shown in the single-crystal structure,31 addition of morpholines provides a hepta-adduct, Gd@C82(morpholine)7 (Figure 2). A pair of

Figure 3. UV−vis-NIR spectra of Gd@C 8 2 and Gd@ C82(morpholine)7 in toluene.

bands of Gd@C82 at 620 and 714 nm are blue-shifted to 570 and 695 nm, respectively, which can be ascribed to the reduction of the π-conjugated region. The band at around 1000 nm suggests the major π-system of Gd@C82 remains in Gd@ C82(morpholine)7 in coincidence with its X-ray structure. The band of Gd@C 8 2 at 400 nm disappears in Gd@ C82(morpholine)7 as in Gd@C82(−),35 which confirms seven morpholines are attached to Gd@C82 by single bonds and quench the unpaired electron on the fullerene core. MALDITOF MS (positive ion mode) shows a strong signal at m/z = 1658 (Figure S3), which corresponds to a 1:6 adduct but not a 1:7 adduct. The absence of molecular cation peak suggests the [Gd@C82(morpholine)6]+ species is formed by detachment of a morpholine moiety under laser irradiation.

Figure 2. X-ray structure of rac-Gd@C82(morpholine)7 (a) and an enantiomer (b). All sites are shown. Solvent molecules and their hydrogen atoms are omitted for clarity. C, gray; N, blue; O, red; Gd, light blue. The distance between C8 and C8′ is 1.96 Å.

enantiomers is present in the unit cell and the two enantiomers exist as a head to tail dimeric structure (Figure 2a). The range of C−N bond lengths between nitrogen atoms and seven adjacent cage carbon atoms is 1.48−1.50 Å, indicating all morpholines attach the carbon cage by single bonds. Also, they are linked to [5,6,6]-junctions on the Gd@C82 cage, which are 65

DOI: 10.1021/acs.chemmater.7b03787 Chem. Mater. 2018, 30, 64−68

Communication

Chemistry of Materials Table 1. Redox Potentials of Gd@C82(morpholine)7 and Gd@C82a Compound Gd@C82 Gd@C82(morpholine)7

ox

E1

0.095 0.047

red

red

E1

−0.405 −1.354

E2

−1.415 −1.703

red

E3

−1.94

red

E4

−2.145

red

E5

−2.434

a

El/2 values (V vs Fc/Fc+) of the redox couples of Gd@C82 and Gd@C82(morpholine)7, detected by DPV (scan rate 50 mV/s) in o-DCB (0.1 M (nca Bu)4NClO4) at room temperature. Solutions were deaerated by N2 purge prior to the experiments. El/2 = (Ean peak + Epeak)/2.

The electrochemical properties of Gd@C82(morpholine)7 and Gd@C82 were compared by differential pulse voltammetry (DPV) (Figure S4). Their redox potentials are listed in Table 1. Gd@C82(morpholine)7 exhibits one oxidation process and five reduct io n st ep s. The redo x po t en t ials of Gd @ C82(morpholine)7 are shifted cathodically, proving morpholine group has the ability of donating electrons. Also, potential gap between its first oxidation and reduction wave (1.40 V) is much larger than that for Gd@C82 (0.50 V), which agrees with the blue shift of the characteristic bands of Gd@C82. Therefore, morpholine addition dramatically modifies the electronic structure of Gd@C82. The large potential gap of Gd@ C82(morpholine)7 suggests its high chemical stability. The C82 cage contains 24 symmetrically nonequivalent carbons and 19 symmetrically nonequivalent 6−6 bonds, thus the addition of seven morpholines to the carbon cage could afford many possible adducts. However, this rule is broken here in this reaction. In order to further understand the regioselectivity in morpholine addition, density functional theory (DFT) calculations are performed. Reportedly, the addition of morpholine to empty fullerene cages in the presence of NFSI is initiated by 1,2-addition of the NFSI species, and the subsequent addition of morpholine follows the 1,4-addition pattern.30 Therefore, the initiatial addition of NFSI plays the key role in controlling the regioseletivity. There are 35 symmetrically unique C−C bonds in Gd@C82 (labeled by a−z and a′−i′ in Figure S7). The addition of NFSI to these 35 bonds lead to 66 possible isomers for the Gd@C82−NFSI adducts. According to DFT calculations, Gd@C82−NFSI int1 (Figure 4) has the largest thermodynamic stability among the 66 isomers. Thus, int1 is chosen as the precursor to study the subsequent addition of morpholine. Sites 1, 2, 3, and 4 in int1, which are located in the vicinity of the F atom with less steric hindrance, are the possible addition sites for the addition of the nucleophilic morpholine (Figure 4). DFT calculations suggest that site 1 of int1 is more energetically favored for morpholine attack than the other three sites by over 10 kcal/mol and thus is the most probable site for the subsequent reactions. The subsequent reactions follow the same mechanism that has been proposed for morpholine addition in empty fullerenes. Namely, the addition of morpholine to site 1 causes the leaving of the F atom and yields structure int2. Then, the second morpholine adds to int2 at the para-position of the first morpholine, resulting in the leaving of the NFSI group and the formation of int3. The third morpholine adds to int3 at the para-position of the second morpholine, based on which the addition of the next morpholine occurs until the formation of the experimental hepta-adduct Gd@C82(morpholine)7. Interestingly, the addition of morpholine initiated by the second, third, and fourth most stable Gd@C82−NFSI as the precursors leads to the same hepta-adduct Gd@C82(morpholine)7 (Figure S8). The local strain on cage carbons plays an important role in determining the reactivity. After fixing the previous two morpholine groups by Gd@C82−NFSI precursors, the following morpholines tends to attack strained carbons around the symmetry plane of the

Figure 4. Possible mechanism to generate Gd@C82(morpholine)7.

C82 cage and far away from Gd metal by 1,4-addition. (see more details in SI). This may explain why only one unique hepta-adduct structure was obtained and separated rapidly only with silica gel column. In summary, the first regioselective multiamination of Gd@ C82 was presented and only one isomer of the hepta-adduct was separated without time-exhausted HPLC. X-ray results unambiguously confirm seven morpholine groups add to the carbon cage in a 1,4-addtion pattern and form a continuous ribbon of edge-sharing para-C6(morpholine)2 hexagons apart from the hexagonal ring along the C2 axis. Additionally, Gd resides near a (5,6,6) patch deviated from C2 axis. UV−vis-NIR spectrum and electrochemical property of Gd@ C82(morpholine)7 prove most π-system remains and morpholines have donating electron ability to Gd@C82. A nucleophilic addition initiated by Gd@C82−NFSI intermediate has been proposed. The same product Gd@C82(morpholine)7 has been obtained from four possible path according to DFT results, which determinates the regioselectivity and easy separation. In light of this paper, selective multiaddition of Gd@C82 provides valuable clues for further synthesis and characterization of other multiadditional adducts in the future.

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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.chemmater.7b03787. 66

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Experimental, including and theoretical calculation details (PDF) Crystallographic data for C110H56GdN7O7 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*Baoyun Sun. E-mail: [email protected]. *Xingfa Gao. E-mail: [email protected]. ORCID

Liangbing Gan: 0000-0001-6646-3452 Xingfa Gao: 0000-0002-1636-6336 Baoyun Sun: 0000-0003-1542-4642 Author Contributions ⊥

Huan Huang and Lele Zhang contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We cordially thank Dr. Zengqiang Gao (Beijing Synchrotron Radiation Facility) for his kind help with the X-ray data collection on beamline 3W1A of the Beijing Synchrotron Radiation Facility in the Institute of High Energy Physics, Chinese Academy of Sciences. This work is supported by the National Basic Research Program of China (2016YFA0203200) and National Natural Science Foundation of China (U1632113, 21402202, 11705211, and 11505191).

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