Reactivity of Open-Shell Metallofullerene Anions: Synthesis, Crystal Structures, and Electrochemical Properties of Benzylated Gd@ C 2v -C 82

: The reactivity of the open-shell Gd@ C 2v -C 82 with different charge states towards benzyl bromide was investigated. [Gd@ C 2v -C 82 ] 3 − exhibited enhanced activity relative to Gd@ C 2v -C 82 and [Gd@ C 2v -C 82 ] − . The structural characterizations, including MALDI-TOF MS, UV-vis-NIR, and single crystal X-ray diffraction, indicate the formation of isomeric benzyl monoadducts of Gd@ C 2v -C 82 . All three monoadducts contain 1:1 mirror-symmetric enantiomers. Additionally, the addition of the benzyl group and its speciﬁc position result in distinct electrochemical behavior of the products compared to the parent Gd@ C 2v -C 82 . Theoretical studies demonstrate that only [Gd@ C 2v -C 82 ] 3 − has a HOMO energy level that matches well with the LUMO energy level of the PhCH 2 radical, providing a rationalization for the observed signiﬁcantly different reactivity.


Introduction
Endohedral metallofullerenes (EMFs) have garnered substantial attention from scientists due to their exceptional chemical and physical properties [1][2][3][4][5][6][7][8][9]. Since the discovery of the first solvent-extractable EMF, La@C 82 [10], there has been a boom in interest in EMFs. According to the electronic structure, EMFs are categorized into two main groups: closedshell EMFs and open-shell EMFs. Closed-shell EMFs possess completely filled electron shells, meaning that all the molecular orbitals are paired with electrons. Consequently, they exhibit a stable and relatively non-reactive structure. In contrast, open-shell EMFs have partially filled electron shells, resulting in the presence of unpaired electrons in their molecular orbitals. This characteristic makes them more reactive and prone to chemical reactions [11][12][13]. Nevertheless, compared with closed-shell EMFs, the reaction of open-shell EMFs is relatively less studied.

Results and Discussion
Despite Gd@C 2v -C 82 having an unpaired electron, it does not exhibit reactivity towards benzyl bromide (PhCH 2 Br) as shown in Scheme 1. As previously known, Gd@C 82 demonstrates its capability as an excellent electron acceptor, with the capacity to accept up to seven electrons [38,39]. Electrochemical reduction can be used to selectively synthesize fullerene anions by controlling the applied potential. To enhance the reactivity, Gd@C 2v -C 82 was subjected to electrochemical reduction, specifically to its monoanion, using controlled potential electrolysis (CPE). The resulting Gd@C 2v -C 82 monoanion was then subjected to the benzylation reaction. However, it was observed that the monoanion also shows no reactivity toward PhCH 2 Br (Scheme 1), indicating that the nucleophilicity of the Gd@C 2v -C 82 monoanion is not sufficiently strong to attack the carbon cation (C + ) of PhCH 2 Br. Therefore, additional electrons were added to the Gd@C 2v -C 82 cage through the CPE process. According to the cyclic voltammogram of Gd@C 2v -C 82 , the second reduction is a simultaneous two-electron transfer process [27,39]; therefore, it is rational to consider the anions obtained via CPE to be trianions. Once the electrogeneration of [Gd@C 2v -C 82 ] 3− was completed, PhCH 2 Br was added to the solution in one portion and stirred for 3h. High performance liquid chromatography (HPLC) observation confirms the occurrence of the reaction between [Gd@C 2v -C 82 ] 3− and PhCH 2 Br (Scheme 1). It is evident that [Gd@C 2v -C 82 ] 3− shows enhanced reactivity toward PhCH 2 Br.
As observed in Figure 1a, the HPLC results show the synthesis of multiple products through the reaction of [Gd@C 2v -C 82 ] 3− with PhCH 2 Br. To separate and purify the products from the crude mixture, a three-stage HPLC procedure was performed. Initially, the reaction mixtures in toluene were separated using a Buckyprep column to isolate the main fractions 1, 2, and 3 ( Figure 1a). Subsequent chromatography employing a Buckyprep column ( Figure 1b) and a 5PBB column (Figure 1c) revealed that fraction 2 comprises four compounds: 2a, 2b, 2c, and 2d. (Figure 1c). Notably, as we only succeeded in getting single crystal structures of 2a, 2b, and 2d, the subsequent discussion in this study was only focused on these three products. Electrosynthesis has been commonly recognized as an effective strategy for the highly selective synthesis of fullerene derivations. The production of multiple adducts is likely ascribed to the delocalization of charges caused by the low symmetry of the C 82 cage and the high reactivity of trianionic Gd@C 2v -C 82 . fractions 1, 2, and 3 ( Figure 1a). Subsequent chromatography employing a Buckyprep column ( Figure 1b) and a 5PBB column (Figure 1c) revealed that fraction 2 comprises four compounds: 2a, 2b, 2c, and 2d. (Figure 1c). Notably, as we only succeeded in getting single crystal structures of 2a, 2b, and 2d, the subsequent discussion in this study was only focused on these three products. Electrosynthesis has been commonly recognized as an effective strategy for the highly selective synthesis of fullerene derivations. The production of multiple adducts is likely ascribed to the delocalization of charges caused by the low symmetry of the C82 cage and the high reactivity of trianionic Gd@C2v-C82.    Figure 1a). Subsequent chromatography employing a Buckyprep column ( Figure 1b) and a 5PBB column (Figure 1c) revealed that fraction 2 comprises four compounds: 2a, 2b, 2c, and 2d. (Figure 1c). Notably, as we only succeeded in getting single crystal structures of 2a, 2b, and 2d, the subsequent discussion in this study was only focused on these three products. Electrosynthesis has been commonly recognized as an effective strategy for the highly selective synthesis of fullerene derivations. The production of multiple adducts is likely ascribed to the delocalization of charges caused by the low symmetry of the C82 cage and the high reactivity of trianionic Gd@C2v-C82.   The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry gave preliminary speculation on the structure of the three products ( Figure 2). The mass spectra of 2a, 2b, and 2d reveal very similar molecular ion peaks, positioned around m/z = 1233. This observation indicates that they are isomers, each featuring a single benzyl group (CH 2 C 6 H 5 ) attached to the Gd@C 2v -C 82 cage, i.e., Gd@C 2v -C 82 (CH 2 C 6 H 5 ) (theoretical m/z = 1232.979). The addition pattern is consistent with that of the reported open-shell EMFs derivatives [12,[40][41][42][43][44]. This can be attributed to the paramagnetic behavior of Gd@C 2v -C 82 , which leads to specific addition patterns of functional groups in its derivatives. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry gave preliminary speculation on the structure of the three products ( Figure  2). The mass spectra of 2a, 2b, and 2d reveal very similar molecular ion peaks, positioned around m/z = 1233. This observation indicates that they are isomers, each featuring a single benzyl group (CH2C6H5) attached to the Gd@C2v-C82 cage, i.e., Gd@C2v-C82(CH2C6H5) (theoretical m/z = 1232.979). The addition pattern is consistent with that of the reported open-shell EMFs derivatives [12,[40][41][42][43][44]. This can be attributed to the paramagnetic behavior of Gd@C2v-C82, which leads to specific addition patterns of functional groups in its derivatives. The optical properties that can provide crucial insights into the electronic structures were studied through the analysis of the UV-vis-NIR absorption spectra. As shown in Figure 2, the UV-vis-NIR spectra of 2a, 2b, and 2d show remarkably different behavior compared to that of the parent Gd@C2v-C82, which is attributed to the changes in the electronic structure of the cage caused by the addition of the benzyl groups [45]. The characteristic absorption of Gd@C2v-C82 at λ = 636 nm undergoes a blue shift upon the addition of the benzyl groups, which can be explained by the reduction of the π-conjugated region in the molecule [44]. The characteristic band of Gd@C82 at 400 nm disappears in all benzyl adducts, which is ascribed to the quenching of unpaired electrons by the odd number of benzyl groups on the Gd@C82 cage. A similar phenomenon was observed in the absorption of Gd@C82 morpholine derivatives [44].
Single crystal X-ray diffraction measurement is widely accepted as the most reliable technique for analyzing the structure of isomers [46][47][48][49]. It provides solid evidence for the structure confirmation of 2a, 2b, and 2d. We made efforts to obtain single crystals for all the products, unfortunately, only products 2a, 2b, and 2d were successfully crystallized. Figure 3 displays the single crystal structures of the three isomers, namely, 2a, 2b, The optical properties that can provide crucial insights into the electronic structures were studied through the analysis of the UV-vis-NIR absorption spectra. As shown in Figure 2, the UV-vis-NIR spectra of 2a, 2b, and 2d show remarkably different behavior compared to that of the parent Gd@C 2v -C 82 , which is attributed to the changes in the electronic structure of the cage caused by the addition of the benzyl groups [45]. The characteristic absorption of Gd@C 2v -C 82 at λ = 636 nm undergoes a blue shift upon the addition of the benzyl groups, which can be explained by the reduction of the π-conjugated region in the molecule [44]. The characteristic band of Gd@C 82 at 400 nm disappears in all benzyl adducts, which is ascribed to the quenching of unpaired electrons by the odd number of benzyl groups on the Gd@C 82 cage. A similar phenomenon was observed in the absorption of Gd@C 82 morpholine derivatives [44].
Single crystal X-ray diffraction measurement is widely accepted as the most reliable technique for analyzing the structure of isomers [46][47][48][49]. It provides solid evidence for the structure confirmation of 2a, 2b, and 2d. We made efforts to obtain single crystals for all the products, unfortunately, only products 2a, 2b, and 2d were successfully crystallized. Figure 3 displays the single crystal structures of the three isomers, namely, 2a, 2b, and 2d. Notably, 2a and 2b were co-crystallized with decapyrrylcorannulene (DPC) molecules [50]. The DPC molecules were found to form a V-shaped structure when combined with 2a and 2b, with the angle between the DPC molecules being 64.9 • and 64.8 • , respectively. The arrangement of these two molecules is very similar to other reported structures [51][52][53][54]. In 2a·2DPC, the distances between the centroid of the C 82 cage and the central five-membered rings of DPC molecules are 7.361 Å and 7.394 Å, while they are 7.439 Å and 7.474 Å in 2b·2DPC. From there, it can be deduced that the distance between the carbon cage and the DPC is approximately 2.8-3.2 Å, indicating the presence of π-π interactions between the carbon cage and DPC molecules. These π-π interactions play a significant role in stabilizing the co-crystallized structure of 2a and 2b with DPC molecules. All three crystals feature a monoclinic P2 1 /c space group. The carbon cages and embedded Gd atoms (in cyan color) display varying degrees of disorder. An interesting observation is that, irrespective of the location of the benzyl addition, the Gd atoms with major occupancy are consistently found underneath a hexagonal ring along the C 2 axis of the C 2v -C 82 cage (marked in orange in Figure 3). And, the distances between Gd and the adjacent carbon atoms, ranging from 1.9 Å to 2.4 Å, indicate a strong interaction between the endohedral Gd and the outer C 2v -C 82 cage. and 2d. Notably, 2a and 2b were co-crystallized with decapyrrylcorannulene (DPC) molecules [50]. The DPC molecules were found to form a V-shaped structure when combined with 2a and 2b, with the angle between the DPC molecules being 64.9° and 64.8°, respectively. The arrangement of these two molecules is very similar to other reported structures [51][52][53][54]. In 2a·2DPC, the distances between the centroid of the C82 cage and the central five-membered rings of DPC molecules are 7.361 Å and 7.394 Å, while they are 7.439 Å and 7.474 Å in 2b·2DPC. From there, it can be deduced that the distance between the carbon cage and the DPC is approximately 2.8-3.2 Å, indicating the presence of π-π interactions between the carbon cage and DPC molecules. These π-π interactions play a significant role in stabilizing the co-crystallized structure of 2a and 2b with DPC molecules. All three crystals feature a monoclinic P21/c space group. The carbon cages and embedded Gd atoms (in cyan color) display varying degrees of disorder. An interesting observation is that, irrespective of the location of the benzyl addition, the Gd atoms with major occupancy are consistently found underneath a hexagonal ring along the C2 axis of the C2v-C82 cage (marked in orange in Figure 3). And, the distances between Gd and the adjacent carbon atoms, ranging from 1.9 Å to 2.4 Å, indicate a strong interaction between the endohedral Gd and the outer C2v-C82 cage. As depicted in Figure 3, the benzyl group was added in the form of a single bond at the [5,6,6]-ring junction of the C2v-C82 cage. Despite the addition, the C2v symmetry of the cage is preserved, indicating that the attachment of the benzyl group did not disrupt the overall symmetry of the molecule ( Figure 4). Interestingly, all three crystal structures of 2a, 2b, and 2d contain mirror-symmetric enantiomers, suggesting that each compound exists in a pair of structures that are mirror images of each other. To gain a better understanding of the addition sites and the arrangement of the benzyl groups in each enantiomer, a Schlegel diagram of the C2v-C82 cage was mapped. In this diagram, the addition sites of the benzyl groups in each enantiomer are marked (1-2 for 2a, 3-4 for 2b, and 5-6 for 2d) (Figure 4a), providing valuable insights into the spatial orientation and symmetry of the derivatives. It can be observed that the addition sites in structures that are mutually enantiomeric are mirror-symmetric. The denoted sites 1 and 2 refer to cage 1 and cage 2 of 2a, sites 3 and 4 refer to cage 1 and cage 2 of 2b, and sites 5 and 6 refer to cage 1 and cage 2 of 2d. Figure 4b displays the mirror-symmetric enantiomers for 2a, 2b, and 2d. For every isomer, there is an identical structure that is a mirror image of the original isomer. Owing to the existence of the C2 axis, the addition sites labeled as 1′ to 6′ on the cage are equivalent to the corresponding sites of 1 to 6.
The crystal packing and molecular organization for each isomer were studied. As shown in Figure 5, the smallest asymmetric unit for the three products consists of four fullerene cages. Furthermore, 2a and 2b are arranged in a head-to-tail mode with DPC As depicted in Figure 3, the benzyl group was added in the form of a single bond at the [5,6,6]-ring junction of the C 2v -C 82 cage. Despite the addition, the C 2v symmetry of the cage is preserved, indicating that the attachment of the benzyl group did not disrupt the overall symmetry of the molecule (Figure 4). Interestingly, all three crystal structures of 2a, 2b, and 2d contain mirror-symmetric enantiomers, suggesting that each compound exists in a pair of structures that are mirror images of each other. To gain a better understanding of the addition sites and the arrangement of the benzyl groups in each enantiomer, a Schlegel diagram of the C 2v -C 82 cage was mapped. In this diagram, the addition sites of the benzyl groups in each enantiomer are marked (1-2 for 2a, 3-4 for 2b, and 5-6 for 2d) (Figure 4a), providing valuable insights into the spatial orientation and symmetry of the derivatives. It can be observed that the addition sites in structures that are mutually enantiomeric are mirror-symmetric. The denoted sites 1 and 2 refer to cage 1 and cage 2 of 2a, sites 3 and 4 refer to cage 1 and cage 2 of 2b, and sites 5 and 6 refer to cage 1 and cage 2 of 2d. Figure 4b displays the mirror-symmetric enantiomers for 2a, 2b, and 2d. For every isomer, there is an identical structure that is a mirror image of the original isomer. Owing to the existence of the C 2 axis, the addition sites labeled as 1 to 6 on the cage are equivalent to the corresponding sites of 1 to 6. tion along the c-axis. The distances between the two back-to-back DPC molecule the range of 3.4-3.8 Å, indicating the existence of π-π interactions. These inter lead to favorable π-electron overlap and stabilize the crystal packing. The crysta ture of 2d without DPC co-crystallization reveals the formation of a one-dime zigzag supramolecular chain along the b-axis. The detailed crystallographic d these compounds can be found in the experimental section. The addition of functional groups on the fullerene cages can greatly alter th trochemical properties of the compounds. Cyclic voltammetry (CV) measuremen conducted for each isomer and compared with the CV curves of the pristine Gd@ [27]. As reported in the literature, the pristine Gd@C2v-C82 shows three pairs of rev redox peaks and two irreversible redox processes in its CV curves, and the seco the fourth reduction are two-electron processes [27]. However, the addition of the group to form isomers 2a, 2b, and 2d has led to changes in the electrochemical cha istics of the compounds. Moreover, the addition position of the benzyl group was to have effects on the electrochemical behavior [46,55]. As illustrated in Figure 6, curves of 2a, 2b, and 2d exhibit more redox signals compared with the pristine G The crystal packing and molecular organization for each isomer were studied. As shown in Figure 5, the smallest asymmetric unit for the three products consists of four fullerene cages. Furthermore, 2a and 2b are arranged in a head-to-tail mode with DPC molecules along the b-axis. Additionally, the DPC molecules adopt an S-shape configuration along the c-axis. The distances between the two back-to-back DPC molecules are in the range of 3.4-3.8 Å, indicating the existence of π-π interactions. These interactions lead to favorable π-electron overlap and stabilize the crystal packing. The crystal structure of 2d without DPC co-crystallization reveals the formation of a one-dimensional zigzag supramolecular chain along the b-axis. The detailed crystallographic data for these compounds can be found in the experimental section. on the electronic structure of the Gd@C2v-C82 cage. In addition, compared to the redox behavior of Gd@C2v-C82, the first reduction potentials of 2a, 2b, and 2d show negative shifts, and the first oxidation potentials show positive shifts, which indicates a decrease in electron-donating/accepting properties after modification with the benzyl group. The enlarged electrochemical gaps in the derivatives 2a, 2b, and 2d demonstrate the enhanced stability of the derivatives relative to the parent Gd@C2v-C82. The redox potentials for Gd@C2v-C82 and its benzylated products are summarized in Table 1. The addition of functional groups on the fullerene cages can greatly alter the electrochemical properties of the compounds. Cyclic voltammetry (CV) measurements were conducted for each isomer and compared with the CV curves of the pristine Gd@C 2v -C 82 [27]. As reported in the literature, the pristine Gd@C 2v -C 82 shows three pairs of reversible redox peaks and two irreversible redox processes in its CV curves, and the second and the fourth reduction are two-electron processes [27]. However, the addition of the benzyl group to form isomers 2a, 2b, and 2d has led to changes in the electrochemical characteristics of the compounds. Moreover, the addition position of the benzyl group was found to have effects on the electrochemical behavior [46,55]. As illustrated in Figure 6, the CV curves of 2a, 2b, and 2d exhibit more redox signals compared with the pristine Gd@C 2v -C 82 , suggesting the presence of distinct electron transfer processes, which is attributed to the addition of the benzyl group and its influence on the electronic structure of Gd@C 2v -C 82 . In the case of 2a, the CV curves display irreversible cathodic electrochemical behavior, with peak potential (E pc ) values observed at −0.97, −1.51, −1.66, −1.93, and −2.39 V vs. Fc/Fc + (Table 1). On the other hand, 2a also shows one quasi-reversible oxidation peak with an E pa value of +0.14 V vs. Fc/Fc + and two irreversible oxidations with E pa values of +0.74 and +1.03 V vs. Fc/Fc + . For compound 2b, there are three quasi-reversible reductions with E pc values of −1.06, −1.89, and −2.11 V vs. Fc/Fc + , while the second reduction is found to be irreversible, with an E pc value of −1.58 V vs. Fc/Fc + . The electrochemically anodic behavior of 2b exhibits irreversible characteristics. CV measurements for 2d show two irreversible reductions with E pc values of −0.86 and −1.52 V vs. Fc/Fc + and two quasi-reversible reductions with E pc values of −1.77 and −2.04 V vs. Fc/Fc + . Moreover, compound 2d exhibits electrochemically irreversible anodic behavior, which is similar to the behavior observed for 2d. The distinct electrochemical characteristics of 2a, 2b, and 2d may be attributed to the specific addition position of the benzyl group and its influence on the electronic structure of the Gd@C 2v -C 82 cage. In addition, compared to the redox behavior of Gd@C 2v -C 82 , the first reduction potentials of 2a, 2b, and 2d show negative shifts, and the first oxidation potentials show positive shifts, which indicates a decrease in electron-donating/accepting properties after modification with the benzyl group. The enlarged electrochemical gaps in the derivatives 2a, 2b, and 2d demonstrate the enhanced stability of the derivatives relative to the parent Gd@C 2v -C 82 . The redox potentials for Gd@C 2v -C 82 and its benzylated products are summarized in Table 1.    C82 and 2a, 2b, and 2d. Density functional theory calculations at the M06-2X/6-31G*~SDD level were then carried out to disclose the underlying reactivity of Gd@C2v-C82 and its anions. Figure 7 illustrates the optimized structures of 2a, 2b, and 2d, which are consistent with the corresponding single crystal structures. For pristine Gd@C2v-C82, it is well known that it has an unpaired electron distributed on the outer cage due to the 3e − transfer from the internal metal to the cage. However, our calculations with different spin multiplicities reveal that all three monoadducts have the octet ground state completely stemming from the   -C 82 and 2a, 2b, and 2d. Density functional theory calculations at the M06-2X/6-31G*~SDD level were then carried out to disclose the underlying reactivity of Gd@C 2v -C 82 and its anions. Figure 7 illustrates the optimized structures of 2a, 2b, and 2d, which are consistent with the corresponding single crystal structures. For pristine Gd@C 2v -C 82 , it is well known that it has an unpaired electron distributed on the outer cage due to the 3e − transfer from the internal metal to the cage. However, our calculations with different spin multiplicities reveal that all three monoadducts have the octet ground state completely stemming from the Gd-4f 7 electron configuration of their internal Gd 3+ cation (Table 2). Clearly, the unpaired electron (spin) on the outer cage of Gd@C 2v -C 82 has been effectively quenched as a result of the formation of a single bond with the benzyl group, which thus leads to the excellent stability of the adducts. In addition, the reactivity of different charged forms of Gd@C2v-C82 was elaborated through DFT simulations. Figure 8 shows the calculated energy levels of frontier molecular orbitals of Gd@C2v-C82 with various charge states (0, −1, −3). Consistent with our experimental findings, only [Gd@C2v-C82] 3− demonstrates HOMO (highest occupied molecular orbital) energy levels that are comparable to the LUMO (lowest unoccupied molecular orbital) of PhCH2 radical, thus enhancing the orbital interactions and charge transfer during the addition reaction. Through the process of electrochemical reduction, the orbital energy levels of Gd@C2v-C82 are greatly elevated, resulting in a significant improvement in its reactivity.    In addition, the reactivity of different charged forms of Gd@C 2v -C 82 was elaborated through DFT simulations. Figure 8 shows the calculated energy levels of frontier molecular orbitals of Gd@C 2v -C 82 with various charge states (0, −1, −3). Consistent with our experimental findings, only [Gd@C 2v -C 82 ] 3− demonstrates HOMO (highest occupied molecular orbital) energy levels that are comparable to the LUMO (lowest unoccupied molecular orbital) of PhCH 2 radical, thus enhancing the orbital interactions and charge transfer during the addition reaction. Through the process of electrochemical reduction, the orbital energy levels of Gd@C 2v -C 82 are greatly elevated, resulting in a significant improvement in its reactivity.

Materials and Methods
Gd@C 2v -C 82 was purchased from Xiamen Funano New Material Technology Company LTD, Xiamen, China. PhCH 2 Br, anhydrous o-DCB, 1,1,4,4-Tetraphenyl-1,3-butadiene (TPB), toluene, CDCl 3 , and CS 2 were purchased from Aladdin and used as received. Tetrabutylammonium perchlorate (TBAP) and Tetra-n-butylammonium hexauorophosphate (TBAPF 6 ) were recrystallized from absolute ethanol and dried under vacuum before use. The purity of the products was verified by HPLC (LaboACE LC-5060, Japan Analytical Industry Co., Ltd., Tokyo, Japan) equipped with Buckyprep and 5PBB columns, with toluene used as the eluent. UV-vis-NIR spectra were recorded using a SHIMADZU UV-3600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). MALDI-TOF MS measurements were conducted using a Bruker autoex speed mass spectrometer. CV studies were conducted in a solution of o-DCB containing 0.05 M TBAPF 6 in a one-compartment cell connected to a CHI 760E workstation, Shanghai, China. A 2 mm diameter glassy carbon disk, a Pt wire, and a Ag wire were used as the working, counter, and pseudoreference electrodes, respectively. Ferrocene (Fc) was introduced into the solution at the end of each experiment as an internal standard. All reactions were carried out inside a glove box under Ar protection.

Synthesis of 2a, 2b, and 2d
A 2 mg (1.75 mmol) sample of Gd@C 2v -C 82 was electrochemically reduced at −1.36 V versus a silver wire reference electrode in 8 mL of o-DCB containing 0.1 M TBAP as the supporting electrolyte under an argon atmosphere. The potentiostat was turned off once the electrogeneration of [Gd@C 2v -C 82 ] 3− was completed. Subsequently, 100 equivalents of PhCH 2 Br (21.5 µL) were added into the solution in one portion under stirring. The reaction was allowed to proceed for 2 h with stirring, and, finally, it was electrochemically oxidized at a potential of 0 V. The solvent was evaporated under a vacuum, and the crude was washed with plenty of methanol to remove the unreacted PhCH 2 Br and the supporting electrolyte. The slurry was separated by filtration. The residue was dried at 70 • C under a vacuum and dissolved in toluene. The soluble fraction was then purified by HPLC using Buckyprep and 5PBB columns.

Single Crystal X-ray Crystallography
The single crystals of 2a and 2b were obtained by co-crystallization with DPC molecules. In a 5.0 mL centrifuge tube, 0.5 mg of 2a was dissolved in 0.5 mL of carbon disulfide. Then, a toluene solution containing DPC was added to the tube, with a molar ratio of DPC to fullerene in the mixed solution being 2:1. After around three weeks, black sheet-like crystals were formed. A crystal of 0.4 mm × 0.25 mm × 0.03 mm dimension was mounted in the 100 K nitrogen cold stream on an XtaLAB PRO MM007HF diffractometer with Cu-Kα radiation (λ = 1.5406 Å). The CrystalClear software package (Rigaku) was used for data collection, cell refinement, and data reduction. The crystal structures were solved by direct methods and refined by the full-matrix method based on F 2 using the SHELXLTL software package. All the non-hydrogen atoms were refined anisotropically, and the positions of the hydrogen atoms were generated geometrically.
Crystal The crystal of 2d was grown through solvent diffusion in which a near-saturated carbon disulfide solution was injected into the glass tube followed by slow injections of hexane, and the fullerene solution was layered at a height of about 5 cm. At this point, a clear delamination between hexane and carbon disulfide solution appears, creating a diffusion interface that provides favorable conditions for crystal growth. The tube is closed with a cap and left at a low temperature for several weeks until the solvent diffusion is complete and several crystals are attached to the tube walls. The crystal of 2d (0.1 mm × 0.08 mm × 0.06 mm) was tested using the same instruments and methods as 2b.
Crystal The supplementary crystallographic data for this paper can be obtained free of charge from the Cambridge Crystallographic Data Centre with CCDC numbers 2281483, 2283118, and 2283119 via www.ccdc.cam.ac.uk/data_request/cif (accessed on 1 July 2023).

Computational Details
The calculations were carried out at the DFT level with the M06-2X [56] function, the 6-31G* all-electron basis set of C and H atoms, the SDD basis set, and the corresponding effective core potentials of Gd [57][58][59]. All the above DFT calculations were performed using the Gaussian 09 software package [60]. The results were visualized with CYLview [61] and VMD [62] software.

Conclusions
In summary, this study presents the first successful synthesis of open-shell Gd@C 2v -C 82 benzyl monoadducts using the electrochemical synthesis method. Three Gd@C 2v -C 82 (PhCH 2 ) isomers (2a, 2b, and 2d) were synthesized, isolated, and characterized. The crystal structures revealed that the benzyl group is attached to the [5,6,6] junction of the cage through a single bond and each isomer contains a mirror-symmetric enantiomer. The addition of the benzyl group and its specific position have notable effects on the electrochemical behavior. DFT calculations reveal that [Gd@C 2v -C 82 ] 3− has a HOMO energy level that aligns effectively with the LUMO energy levels of the PhCH 2 radical, rationalizing the higher reactivity towards benzyl. The electrochemical method shows great potential in regulating the chemical reactivity of fullerenes, opening up possibilities for creating a wide range of new fullerene products.