Decisive role of non-rare earth metals in high-regioselectivity addition of μ₃-carbido clusterfullerene

Abstract

Endohedral clusterfullerenes featuring encapsulation of metal clusters which transfer electrons to the outer fullerene cages show intriguing chemical properties different from empty fullerenes. Despite the extensive studies on the chemical properties of empty fullerenes, especially C₆₀, chemical functionalization of endohedral clusterfullerenes has been less explored, and previous reports are primarily limited to the well-known metal nitride and carbide clusterfullerenes. Herein, we report the first chemical functionalization of an emerging endohedral clusterfullerene μ₃-carbido clusterfullerene (abbreviated as μ₃-CCF) bearing central μ₃-C and Ti(IV) atoms forming a Ti=C double bond. A μ₃-CCF Dy₂TiC@I_h-C₈₀ is synthesized, and its molecular structure is unambiguously determined by single-crystal X-ray diffraction for the first time. A photochemical cycloaddition reaction of Dy₂TiC@I_h-C₈₀ with 2-adamantane-2,3-[3H]-diazirine (abbreviated as AdN₂) is then carried out and only one monoadduct Dy₂TiC@I_h-C₈₀-Ad is obtained, indicating its high regioselectivity. According to the X-ray single-crystal structure of Dy₂TiC@I_h-C₈₀-Ad, the Ad moiety selectively attacks the [6,6]-bond (conjunction of two fused hexagons), which is adjacent to the Ti⁴⁺ ion instead of the two Dy³⁺ ions, affording a [6,6]-open addition pattern. Theoretical calculations unveil that the Ti(IV) ion plays a decisive role in high regioselectivity, and the formation of [6,6]-open Dy₂TiC@I_h-C₈₀-Ad is thermodynamically preferred. Contrarily, a similar reaction of a Ti(III)-containing nitride clusterfullerene Y₂TiN@C₈₀ with AdN₂ is predicted to generate a different type of adduct with the addition sites adjacent to the Y³⁺ ion instead of the Ti³⁺ ion. This reveals the peculiarity of the chemical property of μ₃-CCF resulting from the existence of the non-rare earth metal Ti with a high oxidation state.

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Introduction

Chemical functionalization of fullerenes offers an opportunity to expand the properties and functionalities of the pristine fullerenes and is essential for applications of fullerene materials in versatile fields such as organic photovoltaic, biomedicine, and catalysis.^1–3 During the past three decades, extensive studies on the chemical properties of empty fullerenes, especially C₆₀, have been reported.^4,5 Upon encapsulating metal clusters into fullerene cages, endohedral clusterfullerenes form and exhibit intriguing chemical properties different from empty fullerenes owing to electron transfer from the encapsulated metal cluster to the outer fullerene cage.⁶ Among the known clusterfullerenes, metal nitride and carbide clusterfullerenes were discovered first, and thus their chemical properties have been widely studied.^7–9 Up to now, several different types of chemical reactions, including [2 + 2]-cycloadditions,^10,11 1,3-dipolar cycloaddition reactions,^12–14 disilylations,¹⁵ Bingel-Hirsch reaction,^16,17 azide addition reactions,¹⁸ Lewis acid–base addition reactions,^19–21 carbene addition,^22–25 and radical reactions,^26,27 have been applied in functionalizing metal nitride and carbide clusterfullerenes. However, chemical functionalization of other types of endohedral clusterfullerenes has been less explored due mainly to their relatively low yield compared to the metal nitride clusterfullerenes (NCFs) such as Sc₃N@C₈₀.

As an emerging endohedral clusterfullerene, the μ₃-carbido cluster-fullerene (abbreviated as μ₃-CCF) discovered in 2014 appears quite special since a non-rare earth (RE) metal such as titanium (Ti) is needed, which bonds with a central μ₃-C atom via a Ti=C double bond along with two RE metals.²⁸ So far, a few Ti-based μ₃-CCFs have been isolated, including TiM₂C@C₈₀ (M = Sc, Y, Nd, Gd, Tb, Dy, Er, and Lu)^29–33 and TiSc₂C@C₇₈,²⁹ featuring the electronic configurations of [Ti⁴⁺(M³⁺)₂C⁴⁻]⁶⁺@C_2n⁶⁻ involving a Ti⁴⁺ cation. Noteworthily, this is distinctly different from the conventional metal nitride clusterfullerenes (NCFs) M₃N@C_2n in which the encapsulated RE and non-RE metals take formal oxidation states of 3+, affording electronic configurations of [(M³⁺)₃N³⁻]⁶⁺@C_2n⁶⁻.^34–36 Thus, an open question arises: given that μ₃-CCFs have similar trimetallic cluster compositions and the same charge state of the outer fullerene cage (C_2n⁶⁻) with NCFs, whether are their chemical properties similar to those of NCFs? However, to the best of our knowledge, the chemical functionalization of μ₃-CCFs has never been reported yet, and hence their chemical properties, specifically, the impact of the encapsulated Ti⁴⁺ cation, remain unknown.

Herein, we report the first chemical functionalization of μ₃-CCF by a cycloaddition reaction of Dy₂TiC@I_h-C₈₀ with 2-adamantane-2,3-[3H]-diazirine (abbreviated as AdN₂). Dy₂TiC@I_h-C₈₀ is synthesized and its molecular structure is unambiguously determined by single-crystal X-ray diffraction for the first time. After a photochemical reaction of Dy₂TiC@I_h-C₈₀ with AdN₂, only one monoadduct Dy₂TiC@I_h-C₈₀-Ad is obtained, indicating its high regioselectivity. On the basis of the X-ray single-crystal structure of Dy₂TiC@I_h-C₈₀-Ad, a [6,6]-open addition pattern is identified. Theoretical calculations are carried out to unveil the role of the Ti⁴⁺ ion within Dy₂TiC@I_h-C₈₀ μ₃-CCF in its high regioselectivity.

Results and discussion

Synthesis, isolation and X-ray crystallographic structure of Dy₂TiC@I_h-C₈₀

The synthesis procedure of Dy₂TiC@C₈₀ by a modified Krätschmer–Huffman DC-arc discharge method is similar to that of Tb₂TiC@C₈₀ we reported previously.³² Isolation of Dy₂TiC@I_h-C₈₀ was fulfilled by a three-step high performance liquid chromatography (HPLC) procedure, and its high purity was confirmed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy and HPLC (see ESI Fig. S1†).

Although Dy₂TiC@C₈₀ was reported by Popov et al. before,³³ its molecule structure has not been determined unambiguously yet. In this work, we succeeded in determining the molecular structure of Dy₂TiC@C₈₀ unambiguously by single-crystal X-ray diffraction for the first time on the basis of growing high-quality cocrystals with decapyrrylcorannulene (DPC) as the host. The relative orientation of Dy₂TiC@I_h-C₈₀ and two DPC molecules within a 2DPC·{Dy₂TiC@I_h-C₈₀} cocrystal is shown in Fig. 1a, which includes only one orientation of the fullerene cage together with the major site of the Dy₂TiC cluster for clarity (see ESI Table S1† for detailed crystallographic data). Similar to other reported clusterfullerene-DPC cocrystals,^37–39 the asymmetric crystal unit cell consists of one pair of fully ordered DPC molecules and one Dy₂TiC@I_h-C₈₀ molecule. Meanwhile, two DPC molecules within 2DPC·{Dy₂TiC@I_h-C₈₀} exhibit a V-shape geometry with a dihedral angle of 65.1°, and the nearest DPC-C₈₀ cage center distances are 7.08 Å/6.50 Å, which is evidently different from that of 2DPC·{Sc₃N@I_h-C₈₀} with a dihedral angle of 1.4° and a DPC-C₈₀ cage center distance of 6.77/6.77 Å,³⁹ predicting the distinct electronic configuration of Dy₂TiC@I_h-C₈₀, although they share the same I_h-symmetry C₈₀ cage.

Fig. 1 Crystal structures of (a) 2DPC·{Dy₂TiC@I_h-C₈₀} and (d) 2DPC·{Dy₂TiC@I_h-C₈₀-Ad}. Geometric configurations of (b) Dy₂TiC@I_h-C₈₀ and (e) Dy₂TiC@I_h-C₈₀-Ad along with the metal positional disorder of the Dy₂TiC cluster within (c) Dy₂TiC@I_h-C₈₀ and (f) Dy₂TiC@I_h-C₈₀-Ad.

The ordered I_h-C₈₀ cage and the major Dy/Ti sites with the largest occupancies (0.82, 0.91, and 0.80 for Dy1, Dy2, and Ti1, respectively) were extracted from the cocrystal for clarity (ESI Table S2†), showing that the Ti atom is located beneath a [5,6] bond, while the two Dy atoms are positioned beneath a hexagon and a [5,5,6] junction, respectively. Meanwhile, the shortest distances of the encapsulated metals and the outer cage carbons are 2.399 (9) Å, 2.344 (7) Å, and 2.132 (6) Å for Dy1–C75, Dy2–C11, and Ti–C42, respectively (ESI Table S3†), indicating strong metal–cage interactions which were verified by the high occupancy of two Dy atom and one Ti atom disorders. Furthermore, the bond length of Ti–C81 (1.824 (6) Å) is much shorter than those of Dy1–C81 (2.144(6) Å) and Dy2–C81 (2.175(6) Å) (Fig. 2b), indicating the existence of a Ti=C double bond as the characteristic of μ₃-CCF.^28,33 It is intriguing to investigate whether the encapsulated Dy₂TiC cluster takes a planar or pyramidal configuration. We find that the sum value of ∠Dy1–C81–Ti, ∠Dy2–C81–Ti, and ∠Dy1–C81–Dy2 is 355.4° (Fig. 2c), indicating that the encapsulated Dy₂TiC cluster is pyramidal, in which the C81 atom is 0.25 Å deviated from the plane composed of the TiDy₂ unit. This is similar to the case of Tb₂TiC@C₈₀ but different from the planar cluster found in Lu₂TiC@C₈₀, in which the central carbon is 0.07 Å deviated from the trimetallic plane,²⁸ revealing the influence of the size of RE metal on the geometry of the M₂TiC cluster.

Fig. 2 The crystallographic orientation of the encapsulated Dy₂TiC cluster inside (a and b) Dy₂TiC@I_h-C₈₀ and (d and e) Dy₂TiC@I_h-C₈₀-Ad, as well as (c and f) the configurations of the Dy₂TiC cluster inside both fullerene cages, are also shown. The blue and cyan regions indicate the coordination interactions between the encapsulated metals and the fullerene cage, and the C1–C2 bond is highlighted in red to show the difference before and after the Ad addition.

Photochemical reactions of Dy₂TiC@I_h-C₈₀ with AdN₂

Among the chemical functionalization methods developed for metal nitride and carbide clusterfullerenes, a cycloaddition reaction using AdN₂ as a reagent has been demonstrated to exhibit high reactivity and regioselectivity.^{22,24,25,40–43} Hence, in our present work we managed to investigate the cycloaddition reaction of Dy₂TiC@I_h-C₈₀ μ₃-CCF with AdN₂. The photochemical reaction of Dy₂TiC@I_h-C₈₀ with AdN₂ is shown in Fig. 3a. A mixture solution of Dy₂TiC@I_h-C₈₀ and excess AdN₂ in toluene was irradiated with a high-pressure mercury lamp, and the reaction process was monitored by analytical HPLC (Fig. 3b). Before the reaction, the HPLC profile of the pristine mixture solution showed two peaks at 3.6 and 39.8 min which were assigned to the AdN₂/solvent peak and the pristine Dy₂TiC@I_h-C₈₀, respectively. After light irradiation for 5 min, a new peak appeared at 16.5 min, indicating the generation of the monoadduct Dy₂TiC@I_h-C₈₀-Ad as confirmed by MALDI-TOF mass spectroscopic characterization. After prolonging the reaction time up to 60 min, the peak at 16.5 min continued to increase along with a decrease of the peak corresponding to the pristine Dy₂TiC@I_h-C₈₀, indicating the promoted conversion of Dy₂TiC@I_h-C₈₀ to the monoadduct Dy₂TiC@I_h-C₈₀-Ad. Finally, the reaction was stopped when an obvious peak of bisadduct at 13.3 min appears. Noteworthily, only one monoadduct Dy₂TiC@I_h-C₈₀-Ad was obtained even after the reaction proceeded for 60 min, indicating its high regioselectivity. The resultant solution was then subjected to preparative HPLC separation, and the purity of Dy₂TiC@I_h-C₈₀-Ad was verified by analytical HPLC (ESI Fig. S2a†). The purified Dy₂TiC@I_h-C₈₀-Ad product was characterized by MALDI-TOF mass spectroscopy, showing an intense mass peak at m/z = 1479.8 (ESI Fig. S2b†).

Fig. 3 (a) Synthetic route of Dy₂TiC@I_h-C₈₀-Ad through the photochemical reaction of Dy₂TiC@I_h-C₈₀ with AdN₂; (b) HPLC profiles of the reaction mixtures during the photochemical reaction process of Dy₂TiC@I_h-C₈₀ with AdN₂ with different reaction times. Conditions: Buckyprep column (∅ 4.6 mm × 250 mm), 20 μL injection volume, 1.0 mL min⁻¹ toluene flow. The inverted triangle and asterisk present the bisadduct and impurity, respectively.

To identify the addition pattern of Dy₂TiC@I_h-C₈₀-Ad, we grew high-quality cocrystals using DPC as the host similar to the case of the pristine Dy₂TiC@I_h-C₈₀, and successfully determined the molecular structure of Dy₂TiC@I_h-C₈₀-Ad unambiguously by single-crystal X-ray diffraction. As illustrated in Fig. 1d, the asymmetric crystal unit cell of the 4DPC·2{Dy₂TiC@I_h-C₈₀-Ad} cocrystal contains two pairs of fully ordered DPC molecules and two Dy₂TiC@I_h-C₈₀-Ad molecules. DPC molecules within 2DPC·{Dy₂TiC@I_h-C₈₀-Ad} exhibit a V-shape geometry with a dihedral angle of 66.3° similar to the pristine 2DPC·{Dy₂TiC@I_h-C₈₀} cocrystal and the nearest DPC-C₈₀ cage center distances (7.19 Å/6.47 Å) exhibit only minor changes. However, different from the case of the pristine Dy₂TiC@I_h-C₈₀ with multiple position disorders of Dy and Ti atoms, the encapsulated Ti atom within Dy₂TiC@I_h-C₈₀-Ad exhibits no positional disorders while the two Dy atoms still show several disorders (Fig. 1f, Table S2†). This indicates that the attachment of the Ad group between the C1–C2 bond which is fractured effectively traps the underneath Ti atom as a result of the strengthened Ti-cage interaction as discussed in detail below. Furthermore, the main occupancies of the endohedral two Dy atom disorder are 0.42 and 0.43 for Dy₂TiC@I_h-C₈₀-Ad, which are obviously less than those (0.82 and 0.91) of pristine Dy₂TiC@I_h-C₈₀, suggesting the weakened interaction of Dy atoms and the I_h-C₈₀ cage after carbene addition.

According to a close analysis of the single crystal structure of Dy₂TiC@I_h-C₈₀-Ad, the Ad moiety selectively attacks the [6,6]-bond (conjunction of two fused hexagons, C1–C2) which is adjacent to the Ti⁴⁺ ion instead of the two Dy³⁺ ions shown in Fig. 2d and e. Intriguingly, the distance (2.216(2) Å) of the addition sites (C1, C2) is much longer than that of the conventional C–C single bond (1.54 Å), indicating an open-cage addition pattern.^22,41 Hence, a [6,6]-open addition pattern of the Ad moiety can be deduced, which is similar to the cases of photochemical cycloaddition reactions of Ad with metal nitride and carbide clusterfullerenes.^22,24,25,40 Based on the largest occupancy, Dy1, Dy2, and Ti atoms within Dy₂TiC@I_h-C₈₀-Ad locate underneath a [5,6] bond, a [5,6]-bond, and a fractured [6,6]-bond, respectively (Fig. 2d and e). It is obvious that Ad addition leads to rotation of the Dy₂TiC cluster from the coordinated blue region to the cyan region (see Fig. 2ab, d and e) due to the self-adaptive interaction between the endohedral cluster and the outer cage. For the encapsulated Dy₂TiC cluster within Dy₂TiC@I_h-C₈₀-Ad, the bond lengths of Dy1–C81 (2.242 (5) Å), Dy2–C81 (2.140(5) Å), and Ti–C81 (1.829 (5) Å) are close to those within the pristine Dy₂TiC@I_h-C₈₀ as discussed above (ESI Table S3†). These results indicate that the μ₃-carbido nature of the encapsulated Dy₂TiC cluster remains unchanged after grafting the Ad moiety. Moreover, the sum value of ∠Dy1–C81–Ti, ∠Dy2–C81–Ti and ∠Dy1–C81–Dy2 becomes 359.3°(Fig. 2f), which is larger than that observed for the pristine Dy₂TiC@I_h-C₈₀ (355.4°), indicating that the cycloaddition of the Ad moiety to μ₃-CCF induces an obvious geometric change of the encapsulated μ₃-carbido cluster. A plausible explanation is that the grafted Ad moiety results in the deformation of the I_h-C₈₀ cage with an enlargement of the confined internal space, and thus the strain of the Dy₂TiC cluster is reduced.^22,24,25,40

Theoretical calculation of Dy₂TiC@I_h-C₈₀-Ad

To understand the high regioselectivity of Ad addition to Dy₂TiC@I_h-C₈₀ μ₃-CCF and unveil the role of the non-RE metal Ti within the μ₃-carbido cluster, we performed comprehensive theoretical studies. Density functional theory (DFT) computations on the pristine Dy₂TiC@I_h-C₈₀ μ₃-CCF with S = 5 spin ground state (spin multiple state 2S + 1 = 11) were carried out, verifying that the encapsulated Dy₂TiC cluster takes a μ₃-carbido structure bearing a Ti⁴⁺ ion and Ti=C double bond (1.79 Å) (ESI Tables S4–S5, Fig. S3–S4†). Besides, the electronic configuration of [(Dy³⁺)₂Ti⁴⁺C⁴⁻]⁶⁺@[I_h(31924)-C₈₀]⁶⁻ is confirmed similar to the case of the first μ₃-CCF Lu₂TiC@I_h-C₈₀.²⁸

The high regioselectivity of Ad addition around Ti instead of Dy can be understood by further analyzing the geometric and electronic configuration of Dy₂TiC@I_h-C₈₀. First, the shorter Ti–C (cage) distances than those of Dy–C (cage) suggest much stronger interactions between Ti⁴⁺ with the I_h-C₈₀ cage (ESI Table S6†), which is also confirmed by the larger Mayer bond order (MBO), density of electrons, and the Laplacian of the electron density between the Ti⁴⁺ ion and the I_h-C₈₀ cage than those of Dy³⁺ ions with the I_h-C₈₀ cage (ESI Tables S6 and S7†). Besides, the Ti⁴⁺ ion and its adjacent cage carbons with much more negative charges also contribute to the stronger interactions of Ti⁴⁺-C (cage) than that of Dy³⁺-C (cage) (ESI Fig. S4d†).

Considering the simultaneously appeared [5,6]-adduct and [6,6]-adduct isomers in the reaction of M₃N@I_h-C₈₀ (M = Sc, Lu) with carbene Ad: under light irradiation,²² it is interesting that our experimental finding shows that only [6,6]-adduct around the Ti⁴⁺ ion obtained with C1 and C2 as the addition sites instead of C2 and C3 ([5,6]-adduct isomer). The theoretical calculations indicate that the [6,6]-adduct has 3.3 kcal mol⁻¹ potential energy (Fig. 4) lower than the [5,6]-adduct. Furthermore, statistical thermodynamic analysis considering the entropy–enthalpy effect shows that [6,6]-open Dy₂TiC@I_h-C₈₀-Ad (C1, C2) has the 99.97% distribution relative to that (0.03%) of [5,6]-open Dy₂TiC@I_h-C₈₀-Ad (C2-C3) at 298.15 K. Thus, the [6,6]-open Dy₂TiC@I_h-C₈₀-Ad obtained in experiment is the thermodynamic product, which is the same as the reactions of AdN₂ with several endohedral metallofullerenes previously reported.^23,43

Fig. 4 Relative energies (ΔE, in kcal mol⁻¹) of [5,6]-Dy₂TiC@I_h-C₈₀-Ad (in blue) and [6,6]-Dy₂TiC@I_h-C₈₀-Ad (in green). The cyan, brown, and light gray balls represent the titanium, dysprosium and carbon atoms, respectively.

In order to consolidate the universal role of the Ti(IV) within the μ₃-carbido cluster in determining the regioselectivity and reaction mechanism of μ₃-CCF, we further computed similar cycloaddition products of the analogously reported μ₃-CCF Y₂TiC@I_h-C₈₀ ³³ and nitride clusterfullerene Y₂TiN@C₈₀ bearing Ti(III)^44,45 with AdN₂, in which replacing Dy³⁺ with Y³⁺ is to ensure the validity of μ₃-CCF clusterfullerenes. Fig. 5 shows that without the presence of Ti(IV), the Ad moiety of Ti(III)-based [6,6]-Y₂TiN@I_h-C₈₀-Ad with the lowest potential energy is located nearly around the Y atom. However, similar to Dy₂TiC@I_h-C₈₀-Ad, the Ad moiety selectively attacks the [6,6]-bond of I_h-C₈₀ adjacent to the Ti(IV) instead of the Y atom to form a Y₂TiC@I_h-C₈₀-Ad adduct with energy superiority. Further frontier molecular orbital (FMO) computations in Fig. S6† show that in the presence of Ti(IV), the FMO occupancy of Dy₂TiC@I_h-C₈₀ and Y₂TiC@I_h-C₈₀ are similar to each other (ESI Fig. S5†), but different to that of Y₂TiN@I_h-C₈₀ bearing Ti(III). This reveals the decisive role of the Ti⁴⁺ ion with a high formal oxidation state (IV) in the electronic configuration of μ₃-CCF M₂TiC@I_h-C₈₀, which directly affects the reactivity. Thus, the cycloaddition reaction of Y₂TiC@I_h-C₈₀ with AdN₂ is expected to exhibit similar regioselectivity and addition pattern to that of Dy₂TiC@I_h-C₈₀.

Fig. 5 Relative energies (ΔE, in kcal mol⁻¹) of Dy₂TiC@I_h-C₈₀ (a), Y₂TiC@I_h-C₈₀ (b) and Y₂TiN@I_h-C₈₀ (c) with different addition sites of Ad. The green, brown, purple and light gray balls represent the titanium, dysprosium, yttrium and carbon atoms, respectively.

Effect of Ad addition on the electronic properties of Dy₂TiC@I_h-C₈₀

In order to unveil the effect of Ad addition on the electronic properties of Dy₂TiC@I_h-C₈₀, we carried out UV-vis-NIR spectroscopic characterization and electrochemical studies (Fig. 6). Their characteristic absorption data and redox potentials along with those of other analogous clusterfullerenes are summarized (ESI Table S8†). The absorption spectrum of Dy₂TiC@I_h-C₈₀ in the range of 300–1600 nm is featureless, with two broad shoulder peaks at 390 and 676 nm. After grafting an Ad moiety, the shoulder peak at 390 nm becomes more apparent in the absorption spectrum of Dy₂TiC@I_h-C₈₀-Ad (Fig. 6a) and the shoulder peak at 676 nm exhibits a negligible change. These results indicate that Ad addition induces little influence on the optical absorption of Dy₂TiC@I_h-C₈₀.

Fig. 6 (a) UV–vis–NIR spectra of Dy₂TiC@I_h-C₈₀ and Dy₂TiC@I_h-C₈₀-Ad dissolved in toluene. Inset: Enlarged spectral region of 350–800 nm. Cyclic voltammograms of (b) Dy₂TiC@I_h-C₈₀ and (c) Dy₂TiC@I_h-C₈₀-Ad in 1,2-dichlorobenzene solution with ferrocene (Fc) as the internal standard under a nitrogen atmosphere. Scan rate: 100 mV s⁻¹, tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte. Each redox step is marked with a solid dot to aid comparison; the asterisk denotes the oxidation peak of ferrocene.

Cyclic voltammometry (CV) and differential pulse voltammetry (DPV) of the pristine Dy₂TiC@I_h-C₈₀ and Dy₂TiC@I_h-C₈₀-Ad are then compared as illustrated in Fig. 6b and c. The pristine Dy₂TiC@I_h-C₈₀ exhibits two reversible reduction steps, one irreversible reduction step and two reversible oxidation steps which are similar to those reported by Popov et al.³³ After Ad addition, all three reduction steps become irreversible, and the first reduction potential shifts negatively from −0.93 V (half-wave potential) to −1.10 V (peak potential), whereas the second and third reduction potentials exhibit a negligible cathodical shift. Only one reversible oxidation step and one irreversible oxidation peak are observed for Dy₂TiC@I_h-C₈₀-Ad, with the first oxidation potential cathodically shifting from 0.57 V to 0.49 V relative to that of the pristine Dy₂TiC@I_h-C₈₀ (ESI Table S8†). This cathodical shift of redox potentials in Dy₂TiC@I_h-C₈₀ after carbene functionalization is similar to those of Ad adducts of M₃N@C₈₀ (M = Sc, Lu) reported previously.²²

Conclusion

In summary, the first chemical functionalization of μ₃-CCF exemplified by Dy₂TiC@I_h-C₈₀ is studied, revealing the decisive role of the non-rare earth metal Ti(IV) in high-regioselectivity addition. The molecular structure of the pristine Dy₂TiC@I_h-C₈₀ is unambiguously determined by single-crystal X-ray diffraction for the first time, revealing the feature of the Ti=C double bond and the influence of the size of rare earth metals on the geometry of the M₂TiC cluster. A photochemical cycloaddition reaction of Dy₂TiC@I_h-C₈₀ with I₂ affords only one monoadduct Dy₂TiC@I_h-C₈₀-Ad, indicating its high regioselectivity. On the basis of the X-ray single crystal structure of Dy₂TiC@I_h-C₈₀-Ad, the Ad moiety selectively attacks the [6,6]-bond which is adjacent to the Ti⁴⁺ ion instead of the two Dy³⁺ ions, and the presence of a [6,6]-open addition pattern is confirmed. According to theoretical calculations, Dy₂TiC@I_h-C₈₀-Ad is thermodynamically preferred, and the Ti(IV) ion plays a decisive role in high regioselectivity. In contrast, a similar reaction of a Ti(III)-containing nitride clusterfullerene Y₂TiN@C₈₀ with AdN₂ is predicted theoretically to generate a different type of adduct with the addition site adjacent to the Y³⁺ ion instead of the Ti³⁺ ion. Hence, the peculiarity of the chemical properties of μ₃-CCF resulting from the existence of the non-rare earth metal Ti with a high oxidation state is unveiled. As the first chemical functionalization of μ₃-carbido clusterfullerene with high regioselectivity, our finding on the decisive role of non-rare earth metal titanium in the regioselectivity fulfills an in-depth understanding of the fascinating chemical properties of endohedral fullerenes.

Experimental

Detailed experimental process

A toluene solution (25 mL) containing approximately 5 mg of Dy₂TiC@I_h-C₈₀ and excessive AdN₂ was first degassed for about 15 minutes and later was irradiated with a high-pressure mercury-arc lamp (cutoff <350 nm) at room temperature. The reaction process was monitored by analytical HPLC. The detailed separation procedure of monoadduct Dy₂TiC@I_h-C₈₀-Ad using preparative HPLC is presented in the ESI† (LC-908 instrument, Japan Analytical Industry Co., Ltd, toluene as the mobile phase). MALDI-TOF MS was performed on a BIFLEX III (Bruker, Germany) with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) as the matrix.

Spectroscopic and electrochemical studies

UV-Vis-NIR spectra were recorded on a UV 3150 (Shimadzu, Japan) in toluene. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were obtained in 1,2-dichlorobenzene with 0.1 M (n-Bu)₄NPF₆ as the supporting electrolyte using a CHI660E workstation. Pt disc, Pt wire and silver wire were used as the working electrode, reference electrode and auxiliary electrode, respectively. The scan rate for CV was 20 mV s⁻¹. Conditions for DPV: pulse amplitude, 50 mV; scan rate, 20 mV s⁻¹.

X-ray crystallographic study

Crystal blocks of Dy₂TiC@I_h-C₈₀ and Dy₂TiC@I_h-C₈₀-Ad were obtained by slow volatilization of a toluene solution of DPC and Dy₂TiC@I_h-C₈₀/Dy₂TiC@I_h-C₈₀-Ad in a glass tube. After two weeks, black block crystals suitable for crystal measurements were formed. Single-crystal XRD measurements of Dy₂TiC@I_h-C₈₀ and Dy₂TiC@I_h-C₈₀-Ad crystals were conducted at 100 K at the BL17B station of Shanghai Synchrotron Radiation Facility. The crystal structures were solved with the ShelXT structure solution program using the Intrinsic Phasing method, later refined with the ShelXL⁴⁶ refinement package embedded within OLEX2.⁴⁷ CCDC 2036864 and 2036865† contain the supplementary crystallographic data for Dy₂TiC@I_h-C₈₀ and Dy₂TiC@I_h-C₈₀-Ad, respectively.

Author contributions

M. C. and Y. Z. contributed equally to this work. M. C. performed the chemical reaction of Dy₂TiC@C₈₀ with AdN₂ and confirmed the crystal structures of Dy₂TiC@C₈₀ and Dy₂TiC@C₈₀-Ad by the crystal growth, crystal measurements and resolution. Meanwhile, M. C. wrote the original draft. Y. Z., M. L. and X. Z. carried out the theoretical calculation and analysis of Dy₂TiC@C₈₀ and Dy₂TiC@C₈₀-Ad. F. J., J. X. and R. G. synthesized and separated Dy₂TiC@C₈₀ for crystal growth and further chemical functionalization. Q. Z. provided decapyrrylcorannulene (DPC) which was used as the host to obtain high-quality single crystals of Dy₂TiC@C₈₀ and Dy₂TiC@C₈₀-Ad. S. X., G. W. and S. Y. provided the discussion and article revision.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was partially supported by the National Key Research and Development Program of China (2017YFA0402800) and the National Natural Science Foundation of China (52172053, 51925206, 21772189, U1932214, 21773181, 21573172 and 91961113). We thank the staff from BL17B1 at Shanghai Synchrotron Radiation Facility for assistance during data collection. We thank the staff from the BL17B1beamline of the National Facility for Protein Science in Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility, for assistance during data collection.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2036864 and 2036865. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01442d

‡ These authors contributed equally to this work.

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