Trapping an unprecedented Ti₃C₃ unit inside the icosahedral C₈₀ fullerene: a crystallographic survey

Abstract

The sub-nanometer cavity of fullerene cages is an ideal platform to accommodate otherwise unstable species for accurate structural characterization with, for example, rather accurate single crystal X-ray diffraction (XRD) crystallography. Herein, we report the successful entrapment of an isolated Ti₃C₃ moiety inside the icosahedral-C₈₀ cage to form Ti₃C₃@I_h-C₈₀via an arc-evaporation process in the gas phase. The single crystal XRD crystallographic results unambiguously reveal that the C₃-unit adopts an unprecedented cyclopropane-like structure which coordinates with the three titanium atoms in an unexpected fashion where the triangular C₃-unit is nearly perpendicular to the Ti₃-plane. The intercalation of a cyclopropanated C₃-unit into the titanium layer is thus unambiguously confirmed. The theoretical results reveal that the Ti₃C₃ cluster transfers six electrons to the I_h-C₈₀ cage so that each titanium atom has a positive charge slightly above +2 and the C₃-unit is negatively charged with about −1. It is noteworthy that this is the first observation of the cyclopropane-coordination fashion in any reported organometallic complex, providing new insights into coordination chemistry.

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Introduction

Fullerene cages serve as a protector to host a variety of isolated metal atom(s) or otherwise unstable metallic units to form so-called endohedral metallofullerenes (EMFs). By virtue of the solubility of EMFs, high-quality single crystals can be obtained and thus the interplay between the metallic species within the clusters, in addition to the metal–cage interactions, is possible to study by the sophisticated single crystal X-ray diffraction (XRD) approach. Up to now, a collection of EMFs containing one to three pure metal atoms, which are called mono-, di- and tri-EMFs, respectively, has been crystallographically confirmed.^1–4 Interesting results such as direct bonding between the repulsive metal ions (e.g., Lu²⁺–Lu²⁺, U³⁺–U³⁺ or Y²⁺–Y²⁺) have been achieved taking advantage of the confinement effect of the fullerene cages.^5–7

In addition, various metallic clusters, such as metal nitride,⁸ carbide,⁹ oxide,¹⁰ sulfide¹¹ and even cyanide,¹² have been successfully trapped inside fullerene cages. It is noteworthy that only with the protection of the fullerene cages are the metallic clusters able to exist and they transfer a certain number of electrons to the surrounding cages. As typical examples, an improbable monometallic cluster YCN¹² was found in C_s(6)-C₈₂ featuring a two-electron-transfer configuration but the I_h-C₈₀ is able to host a variety of metallic species such as La₂,¹³ Sc₃N,⁸ Sc₃C₂,¹⁴ Sc₄C₂,¹⁵ TiSc₂C₂,¹⁶ TiDy₂C,¹⁷ Sc₃CH¹⁸ and Sc₃C₂CN¹⁹ which donate six electrons to the cage. It's noted that encapsulation of a carbide cluster consisting of either an odd or an even number of carbon atoms is feasible. Impressively, Popov et al. theoretically revealed Sc₄C₃@C₈₀ encapsulating a triangular aromatic C₃ cluster with two pairs of Sc atoms coordinating with the C₃ moiety.²⁰ Recently, Chen and coworkers²¹ reported the structure of U₂C@C₈₀ where a non-bridged uranium-carbon double bond was identified. More interestingly, Popov and coworkers²² reported that a titanium–carbon double bond (Ti=C) exists in TiLu₂C@I_h-C₈₀, representing the first example of EMFs with a multiple bond between the metal and the non-metal atom of the endohedral cluster. It is thus expected that novel and even unprecedented bonding features are possibly observed in EMFs containing different metallic clusters.

Herein, we report the preparation and systematic characterization of a brand-new titanium-containing EMF, namely Ti₃C₃@I_h-C₈₀, which is one of the rare examples of EMFs that do not contain group-III elements. X-ray crystallographic results reveal that the C₃-unit of the Ti₃C₃ cluster resembles cyclopropane with an average C–C distance of 1.380 Å, perpendicularly coordinated with the Ti₃-moiety, exhibiting a totally unexpected coordination fashion in organometallic chemistry.

Results and discussion

Soot containing titanium-EMFs was synthesized by an arc discharge method using TiC as the precursor. SnCl₄ was first chosen to react with the extract for the enrichment of Ti-EMFs followed by multiple-stage HPLC separation to obtain a pure sample of Ti₃C₃@C₈₀ (Fig. S1 and S2†). Its MALDI-TOF mass spectrum shows a single peak at m/z 1139.8 (Fig. 1a) which is perfectly consistent with the calculated results of Ti₃C₈₃. Interestingly, the retention time of Ti₃C₃@I_h-C₈₀ in a Buckyprep column is much longer than that of Sc₃N@I_h-C₈₀ (Fig. S3†). Previous studies have suggested a correlation between the HPLC retention time and the molecular geometry of fullerenes.²³ Generally, the retention mechanism is proportional to the cage polarizability and is dominated by π–π interactions with the stationary phase. This phenomenon may suggest a different electronic configuration of the title compound from that of Sc₃N@I_h-C₈₀. Consistently, the absorption spectrum of Ti₃C₃@I_h-C₈₀ (Fig. 1b) shows a small bandgap of 0.82 eV (onset at 1500 nm) in comparison with the large one of Sc₃N@I_h-C₈₀ (1.70 eV),²⁴ suggesting a relatively low stability and unique electronic configuration of Ti₃C₃@I_h-C₈₀. In addition, no signal is observed in the EPR spectrum of Ti₃C₃@I_h-C₈₀ even at 100 K (Fig. S4†), indicative of its diamagnetic property with a closed-shell electronic configuration. Compared to the previously reported Ti-containing mixed-metal EMFs, such as TiSc₂N@I_h-C₈₀ (ref. 25) and TiY₂N@C₈₀,²⁶ which feature obvious EPR signals at low temperatures, it can be concluded that our title compound has a different electronic structure, which is presumably attributed to the difference in the spin distribution of the embedded clusters.

Fig. 1 (a) MALDI-TOF mass spectrum of purified Ti₃C₃@I_h-C₈₀. The insets show the experimental and theoretical isotopic distributions of Ti₃C₈₃. (b) Vis-NIR absorption spectrum of Ti₃C₃@I_h-C₈₀ recorded in CS₂. A photograph of the corresponding solution is shown in the inset.

The structure of Ti₃C₈₃ is unambiguously established as Ti₃C₃@I_h-C₈₀, instead of any other form such as Ti₃C@C₈₂, by a single crystal XRD study performed on a black crystal obtained by slow diffusion of a benzene solution of Ni^II(OEP) (OEP is the dianion of octaethyl porphyrin) into the CS₂ solution of the EMF. The Ti₃C₃@I_h-C₈₀·Ni^II(OEP)·2C₆H₆ cocrystal falls into the monoclinic C2/m space group, where a half of Ni^II(OEP) and a half of Ti₃C₃@I_h-C₈₀ are included in its asymmetric unit cell. The crystallographic mirror plane coincides with one of the symmetric planes of the I_h-C₈₀ cage, resulting in a fully ordered I_h-C₈₀ cage by combining the existing half with its mirror image.

Inside the cage, disorder is observed for the Ti₃C₃ cluster (see ESI Fig. S5 and Table S1 for details†). Specifically, one carbon atom in the C₃-unit exhibits two disordered positions generated by the crystallographic mirror plane, whereas the other two located in the crystallographic mirror plane are fully ordered. Eleven Ti sites are distinguished for the three titanium atoms with occupancy values ranging from 0.127 to 0.506. In fact, three of the disordered sites apart from Ni^II(OEP) belong to one Ti atom, with Ti2 as the major one (occupancy = 0.505). Meanwhile, two major sites out of the remaining eight disordered Ti-positions close to Ni^II(OEP), namely Ti1 and Ti1A (occupancy = 0.506), can be assigned to the other two Ti atoms.

Fig. 2 displays the molecular structures of Ti₃C₃@I_h-C₈₀ showing only the major metal site together with the co-crystallized Ni^II(OEP) molecule. The porphyrin moiety nestles the fullerene cage with the shortest Ni–cage–carbon distance of 2.950 Å, suggesting substantial π–π interactions. Interestingly, the three carbon atoms stay in the center of the cage to form cyclopropane with an average bond length of 1.380 Å. The Ti–Ti distances among the three titanium atoms are 2.976 Å for Ti1–Ti1A and 3.604 Å for Ti1–Ti2 and Ti1A–Ti2, respectively, which are comparable to the values observed in organotitanium complexes, indicating weak metal–metal interactions between the titanium atoms.^27–29 From a geometric point of view, the three titanium atoms intercalate between the cage and the C₃-unit, preventing the encapsulated carbon atoms from being merged with the cage framework.

Fig. 2 ORTEP drawing of Ti₃C₃@I_h-C₈₀·[Ni^II(OEP)] with 15% thermal ellipsoids. Only the major Ti₃C₃ site is shown. Solvent molecules and H atoms are omitted for clarity.

Fig. 3 illustrates the configuration of the major Ti₃C₃ site showing its relationship to the adjacent cage frameworks. Each titanium atom is centered above a respective [5,6]-ring junction with average Ti–C (cage) distances of 2.12, 2.12, and 2.13 Å, respectively, which fall in the range of typical Ti–C single bond lengths in reported organotitanium compounds (usually 2.1 Å),^30,31 suggestive of strong metal–cage interactions (see also Table S2, ESI†). In contrast, the Ti–C distances within the Ti₃C₃ cluster are spread in a wide range from 1.838 Å to 3.090 Å, indicative of a rather complicated coordination environment. Interestingly, the plane of the C₃-unit is nearly perpendicular to that of the Ti₃-moiety with a dihedral angle of 83.43°, presenting a unique coordination fashion between carbon and titanium. To the best of our knowledge, such an unusual coordination pattern for cyclopropane interacting with metal ions has never been reported in any organometallic complexes, despite the above-mentioned theoretical possibility of a triangular C₃ unit inside the Sc₄C₃@C₈₀.²⁰

Fig. 3 Orthogonal views showing the relative position of the major Ti₃C₃ site to the closest cage portions.

We have performed density functional theory (DFT) calculations at the B3LYP/3-21G∼SSD level to understand the unique coordination environment in Ti₃C₃@I_h-C₈₀. The calculated Wiberg indices for the bonds in the cyclopropane ring are 1.18, 0.80, and 0.75 while their calculated lengths are 1.39 Å, 1.58 Å, and 1.62 Å, respectively, a bit longer than the corresponding X-ray results. As for the Ti₃-unit, there are six bond indices between the ring carbons and Ti atoms larger than 0.5, namely 0.88, 0.67, 0.64, 0.60, 0.59, and 0.58. There is one (weak) C–C double bond not bridged by Ti, one (weak) single bond bridged by one Ti, and one (still weaker) single bond bridged by two Ti atoms. In this simplified picture (Fig. 4a), each Ti atom is bonded to the ring via two Ti–C bonds, supporting the results of the geometry of the embedded Ti₃C₃ cluster (Fig. 4b) obtained by the single crystal X-ray method. Moreover, the electron distribution in Ti₃C₃@I_h-C₈₀ is visualized in Fig. S6, ESI† for more graphical information about bonds between the carbons of the cage and also bonds in the encapsulate, showing a selected isosurface of electron density (i.e., every point of the surface has the same prescribed value of the electron density). At the selected constant electron density of 0.086 a.u., the bonds in the cage and especially in the Ti₃C₃ cluster are rather well visible, where the near-vertical coordination environment of the Ti₃-unit and C₃-unit was found. Such a clear picture further provides us with unique bonding information. The B3LYP/3-21G∼SDD Mulliken charges in Ti₃C₃@I_h-C₈₀ on Ti atoms are 2.77, 2.48, and 2.44 while on the ring carbons they are −0.49, −0.41, and −0.22. The Mulliken-charge choice is supported³² by the fact that it is in good agreement with the available observed charges³³ for metallofullerenes. However, unless there are charges deduced from observations for a class of clusterfullerenes, it cannot really be decided which particular charge definition is the best³⁴ for the class. Thus, the results for the isolated Ti₃C₃@I_h-C₈₀ system demonstrate a charge transfer of about six electrons to the cage, matching the charge state of common (I_h-C₈₀)^6– reflected by the Vis-NIR spectrum.

Fig. 4 (a) The optimized Ti₃C₃ configuration at the B3LYP/3-21G∼SDD level and (b) geometry configuration of the major site of the Ti₃C₃ cluster from X-ray results.

Theoretically, the Ti₃C₃ encapsulation is also favorable from the calculated energetics point of view.³⁵ The B3LYP/6-31G*∼SDD encapsulation-energy gain per one encapsulated atom for Ti₃C₃@I_h-C₈₀ is calculated to be about 18 kcal per mol per atom which is even larger than a relative energy gain computed for the recently isolated Y₂C₂@C₁(1660)-C₁₀₈.³⁶ The stability calculations indeed support the finding that a novel EMF containing a unique Ti₃C₃ cluster was experimentally obtained.

The electrochemical properties of Ti₃C₃@I_h-C₈₀ were measured in o-dichlorobenzene (o-DCB) by cyclic voltammetry (CV) and the profile is shown in Fig. 5. In the anodic region, Ti₃C₃@I_h-C₈₀ displays a reversible oxidation step at 0.30 V and an irreversible one at 0.77 V whereas in the cathodic region, three reversible reduction steps are observed at −0.62 V, −1.22 V and −2.31 V. The electrochemical gap is thus calculated to be 0.92 V which is quite smaller than those of the other I_h-C₈₀ based Ti-containing EMFs, namely, TiLu₂C@I_h-C₈₀ (ref. 22), TiSc₂C₂@I_h-C₈₀ and TiSc₂C@I_h-C₈₀,¹⁶ suggesting a lower kinetic stability of Ti₃C₃@I_h-C₈₀ which is also consistent with the Vis-NIR result. In addition, the irreversible oxidation process of Ti₃C₃@I_h-C₈₀ has not been reported for the other Ti-based EMFs^4,17,22,26 (Table 1). These results suggest that we can tune the electronic properties of Ti-EMFs by varying the number of redox-active Ti atoms. Furthermore, the frontier molecular orbital of Ti₃C₃@I_h-C₈₀ visualized in Fig. S7 (ESI†) shows that the predominant localization of the LUMO is on the Ti₃C₃ cluster (mainly on the Ti atoms), suggesting that the first reduction step is ascribed to the alteration of the reduction behavior of the inner cluster, similar to the case of Sc₂TiC₂@I_h-C₈₀.²⁷

Fig. 5 CV curve of Ti₃C₃@I_h-C₈₀. Conditions: working electrode, glassy carbon electrode; counter electrode, Pt wire; reference electrode, Ag wire; supporting electrolyte, 0.05 M TBAPF₆ in o-DCB with ferrocene as the internal standard. Scan rate: 100 mV s⁻¹.

Table 1 Redox potentials of Ti₃C₃@I_h-C₈₀ and typical Ti-based EMFs^a

EMFs	^ox E 2	^ox E 1	^red E 1	^red E 2	^red E 3	ΔEgap^b	Ref.
a Half-cell values in V versus Fe(Cp)2+/0 unless otherwise noted. b ΔEgap = ^oxE1 − ^redE1. c Irreversible peak value.
Ti3C3@Ih-C80	0.77^c	0.30	−0.62	−1.22	−2.31	0.92	This work
TiLu2C@Ih-C80	—	0.64	−0.91	—	—	1.55	22
TiSc2C@Ih-C80	—	0.66	−0.67	−1.51	−1.66	1.33	16
Sc2TiC2@Ih-C80	—	0.53	−0.76	−1.01	−1.96	1.26	16
Dy2TiC@Ih-C80	—	0.61	−0.97	−1.62	−1.87	1.58	17
Dy2TiC2@Ih-C80	—	0.47	−1.14	−1.58	−2.21	1.61	17

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Conclusions

An unprecedented Ti₃C₃ cluster has been successfully entrapped inside the I_h-C₈₀ cage to form a stable compound. X-ray results reveal a novel coordination fashion by which the cyclopropane-like C₃-unit perpendicularly coordinates to the Ti₃-moiety. The electronic configuration of (Ti₃C₃)⁶⁺@C₈₀⁶⁻ was confirmed according to the Vis-NIR spectroscopic and electrochemical characterization in addition to DFT calculations. Successful entrapment of the Ti₃C₃ cluster inside fullerene cages and identification of the unprecedented cyclopropane-coordination fashion may stimulate intensive interest in creating new hybrid molecules with fullerenes as protectors and discovering new reactivities in coordination chemistry.

Experimental

Synthesis and isolation of Ti₃C₃@I_h-C₈₀

Soot containing titanium-EMFs was produced by evaporating graphite rods packed with mixed TiC and graphite powder (mole ratio of Ti : C = 1 : 10) under a 400 mbar He atmosphere and 100 A current. The soot was extracted with CS₂, and the filtered extract was treated with SnCl₄ for the enrichment of Ti-EMFs, followed by multi-stage HPLC separation conducted on an LC-908 machine (Japan Analytical Industry Co. Ltd.) sequentially employing a 5PBB column (∅ 20 × 250 mm) and a Buckyprep column (∅ 20 × 250 mm). More details about the SnCl₄ treatment and HPLC separations are present in the ESI.†

Spectroscopic and electrochemical studies

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) (MICROFLEX, Bruker, Germany) was used for the mass spectroscopic characterization. The Vis-NIR spectrum was measured on a PE Lambda 750S spectrophotometer in CS₂. Cyclic voltammetry (CV) was performed in o-dichlorobenzene using a CHI-660E instrument with 0.05 M TBAPF₆ as the electrolyte. The electron paramagnetic resonance (EPR) spectrum of Ti₃C₃@I_h-C₈₀ was recorded with a Bruker A300 spectrometer in toluene.

Crystallographic characterization

Cocrystals of Ti₃C₃@I_h-C₈₀ were obtained by slow diffusion of a benzene solution of Ni^II(OEP) into a CS₂ solution of the EMF in a glass tube at room temperature for one month. Single-crystal X-ray data were collected at 100 K using a radiation wavelength of 0.6525 Å with a MarCCD detector at beamline BL17B of the Shanghai Synchrotron Radiation Facility. A multi-scan method was used for absorption corrections. The structures were solved with direct methods and were refined with SHELXL-2014.³⁷

Crystal data of Ti₃C₃@I_h-C₈₀·Ni^II(OEP)·2(C₆H₆)

Black block, 0.31 × 0.24 × 0.20 mm, monoclinic, space group C2/m, a = 25.0964(9) Å, b = 15.1544(5) Å, c = 19.7269(7) Å, β = 95.3360(10)°, V = 7470.0(5) ų, F_w = 1888.20, λ = 0.6525 Å, Z = 4, D_calc = 1.679 Mg m⁻³, μ = 0.498 mm⁻¹, T = 100 K, R₁[I > 2σ(I)] = 0.1079, wR₂ (all data) = 0.2972, GOF (on F²) = 1.068. The maximum residual electron density is 1.953 e Å⁻³. Crystallographic data have been deposited in the Cambridge Crystallographic Data Center (CCDC number: 1883920).†

Computational study of Ti₃C₃@I_h-C₈₀

The structure optimization started with a combined basis set, the standard 3-21G basis³⁸ for C atoms and SDD basis³⁹ with the SDD effective core potential for Ti atoms, applied within the density functional theory (DFT) approach, namely using Becke's three parameter functional⁴⁰ with the non-local Lee–Yang–Parr correlation functional⁴¹ (denoted as B3LYP/3-21G∼SDD). Herein, analytical harmonic vibrational analysis is then carried out in order to confirm that the local-energy minimum was indeed localized. The B3LYP/3-21G∼SDD optimized structure was further refined by performing re-optimizations with the standard 6-31+G* basis set⁴² for C atoms (B3LYP/6-31+G*∼SDD). Moreover, in the latter structure the encapsulation energy⁴³ (the energy gain upon encapsulation of three Ti and three C atoms into the C₈₀ cage) was evaluated at the B3LYP/6-31G*∼SDD level with inclusion of the basis set superposition error⁴⁴ (BSSE/CP7) and the so-called steric correction that reflects the cage distortion upon encapsulation.⁴⁵ All the computations were carried out with the Gaussian 09 program package.⁴⁶

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the NSFC (51672093, 51602112 and 51602097) is gratefully acknowledged. We thank the staff from the BL17B beamline of the National Center for Protein Science Shanghai at the Shanghai Synchrotron Radiation Facility for assistance during data collection and the Analytical and Testing Center in the Huazhong University of Science and Technology for all related measurements.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1883920. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc04315b

‡ These authors contributed equally to this work.

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