Methane as a Selectivity Booster in the Arc-Discharge Synthesis of Endohedral Fullerenes: Selective Synthesis of the Single-Molecule Magnet Dy₂TiC@C₈₀ and Its Congener Dy₂TiC₂@C₈₀

The field of endohedral metallofullerene (EMF) research was revolutionized in 1999, when it was discovered that the presence of small amounts of nitrogen gas in the arc-discharge generator afforded Sc₃N@C₈₀, a new type of EMF with a trimetalnitride cluster inside the carbon cage.¹ The use of NH₃ as a reactive gas instead of molecular nitrogen resulted in much higher selectivity in the synthesis of nitride clusterfullerenes as the yield of empty fullerenes in such conditions decreased dramatically.² Discovery of nitride clusterfullerenes triggered exhaustive studies of other clusterfullerenes, resulting in a variety of EMF families with endohedral S,³ O,⁴ C₂,⁵ CH,⁶ CN,⁷ and other nonmetal units.⁸

One of the advantages of the trimetallic cluster in nitride clusterfullerenes is the possibility of combining two or even three different metals within one EMF molecule. Mixed-metal nitride clusterfullerenes may exhibit new properties not present in homometallic nitride clusterfullerenes. Examples include unusual redox behavior,⁹ stabilization of unconventional carbon cages,¹⁰ and strong variation of chemical reactivity¹¹ and magnetization behavior¹² depending on the number of lanthanide ions in the cluster. However, a disadvantage of the mixed-metal EMFs is the increased complexity of their chromatographic separation.

Whereas nitride clusterfullerenes are usually formed with Group III metals, such as Sc, Y, and trivalent lanthanides,¹³ Yang et al. demonstrated that a single Ti ion can be introduced into the mixed-metal nitride cluster together with Sc or Y.¹⁴ Due to the trivalent Ti, M₂TiN@C₈₀ clusterfullerenes have unusual electronic and chemical properties.¹⁵ Recently, in an attempt to obtain Ti-based nitride clusterfullerenes with Lu using NH₃ as a reactive gas or melamine as a solid organic nitrogen source, we have discovered a new type of clusterfullerene, Lu₂TiC@C₈₀, which has an endohedral μ₃-carbide ion and a TiC double bond.¹⁶ The molecule is an isostructural analogue of Lu₂ScN@C₈₀, in which the Sc–N fragment is replaced by the isoelectronic TiC fragment. Unfortunately, in the Lu/Ti/NH₃ and Lu/Ti/melamine syntheses, Lu₂TiC@C₈₀ is only a minor by-product; the products are predominantly Lu₃N@C_2n nitride clusterfullerenes, which precludes further exploration of this new type of clusterfullerenes. Herein we demonstrate that 1) M₂TiC@C₈₀ clusterfullerenes can be synthesized with high selectivity for many lanthanides using methane as a reactive gas; 2) this synthetic route also affords appreciable amounts of another cage isomer of M₂TiC@C₈₀ as well as a new type of clusterfullerenes, M₂TiC₂@C₈₀; 3) Dy₂TiC@C₈₀ exhibits single-molecule magnet (SMM) behavior, whereas the SMM properties of Dy₂TiC₂@C₈₀ are are less pronounced owing to the presence of the second carbon atom in the central unit.

EMFs were obtained by arc-discharge synthesis under He atmosphere with a small amount of CH₄ gas using graphite rods packed with a mixture of Ti, lanthanide (Y, Ce, Nd, Gd, Dy, Er, or Lu), and a graphite powder. The role of CH₄ in this type of synthesis is similar to that of NH₃² in the synthesis of nitride clusterfullerenes: the reactive gas increases selectivity of the process by suppressing the formation of empty fullerenes and making the EMFs with desired central atom(s) the main products. Figure 1 shows that under optimized conditions, carbide clusterfullerenes are the most abundant EMF products for Lu, Dy, Er, Y, and Gd. M₂TiC@C₈₀-I (Roman number denotes the isomer) is the major or the sole component of the fraction eluting at 36 min. Thus, pure M₂TiC@C₈₀-I (Lu, Dy, Gd) was obtained from the EMF extract in a single HPLC separation step. For other metals, the main fraction also included a small amount of M₂C₈₂, which could be then easily removed at the second HPLC step (see the Supporting Information (SI), Figures S2 and S4).

The ionic radius of the lanthanide ion (R³⁺) plays a crucial role in the absolute yield of EMFs. Lu (R³⁺=0.86 Å), Er (0.90 Å), and Dy (0.91 Å) afford similar amounts of M₂TiC@C₈₀-I per synthesis, the yields of Gd₂TiC@C₈₀-I (0.94 Å) and Nd₂TiC@C₈₀-I (0.98 Å) are roughly 6 and 20 times lower than that of Dy₂TiC@C₈₀-I, respectively, whereas Ce₂TiC@C₈₀ (1.01 Å) is not produced at all. Yttrium (R³⁺=0.90 Å) stands apart as the yield of Y₂TiC@C₈₀-I is about 4 times lower than that of Dy₂TiC@C₈₀-I, despite the similar ionic radii of Dy³⁺ and Y³⁺.

Significant amounts of M₂C₈₂ and M₂C₈₄ (presumably dimetallofullerenes M₂@C₈₂ and carbides M₂C₂@C₈₂) are formed for Lu and Er. Besides, the presence of traces of nitrogen in the generator leads to the formation of nitride clusterfullerene M₃N@C₈₀. However, the yield of these EMFs decreases with the ionic radius faster than the yield of M₂TiC@C₈₀-I, and Dy₂TiC@C₈₀-I can be produced with a high degree of selectivity and relatively high absolute yield. Equally high selectivity is also achieved for Y and Gd (Figure 1), but their overall yield is lower.

The molecular structure of Lu₂TiC@C₈₀ determined by single-crystal X-ray diffraction and ¹³C NMR spectroscopy was reported earlier.¹⁶ The Lu₂TiC cluster rotates inside the C₈₀-I_h(7) fullerene resulting in a simple two-line ¹³C NMR spectrum of the cage carbons. The use of ¹³C powder for the synthesis of Lu₂TiC@C₈₀-I in this work allowed detection of the ¹³C resonance of the central carbon atom at 340.98 ppm (Figure 2). This value is much more positive than the chemical shift of endohedral carbon atoms reported before: 220–260 ppm in M₂C₂@C_2n clusterfullerenes,^8c 292.4 ppm in YCN@C₈₂,¹⁷ and 328.3 ppm in the Sc₃C₂@C₈₀⁻ anion.¹⁸ Note that large downfield shifts for metal-bonded carbon atoms are not unusual in Ti alkylidenes with a TiC bond.¹⁹ Particularly large shifts in the range of 400–600 ppm are reported for α-C atoms in μ₃-bridging alkylidynes of titanium in titanocubane frameworks.²⁰

The high selectivity makes Dy₂TiC@C₈₀-I a suitable synthetic target for further studies of its properties. Comparison of the Vis/NIR and IR absorption spectra prove that the Dy and Lu compounds are isostructural (Figure 2). Both have very similar Vis/NIR spectra with the lowest energy band at 910 nm (Figure 2 b) and almost identical vibrational pattern of the fullerene cage (Figure 2 c). However, the larger ionic radius of Dy³⁺ pushes the central carbon atom closer to Ti, which results in a higher TiC stretching frequency in Dy₂TiC@C₈₀-I (834 cm⁻¹ versus 821 cm⁻¹ in Lu₂TiC@C₈₀-I). Analogous variations of the vibrational frequency with an increase of the lanthanide ionic radius was observed for the ScN stretching mode in M₂ScN@C₈₀.²¹ In a similar fashion, isostructurality of other M₂TiC@C₈₀-I EMFs can be established via their absorption spectra (Figure S7). Besides, all compounds show similar redox behavior with the reversible Ti-based reduction near −1.0 V (vs. Fe(Cp)₂^+/0 couple) and cage-based oxidation at +0.6 V (Figure 3, Figure S8, and Table 1). Note that reduction potential depends on the size of the lanthanide, whereas the oxidation process is metal-independent.

Owing to the higher selectivity in the synthesis with CH₄ as the reactive gas, other Ln–Ti carbide clusterfullerenes could be detected; this was not possible in the first report on Lu₂TiC@C₈₀-I.¹⁶ Mass spectrometry studies proved the formation of M₂TiC@C_2n with larger cages (C₈₂, C₈₄), albeit in rather small amounts making their isolation impractical at this moment (see SI). More importantly, we identified the second isomer of M₂TiC@C₈₀ and a new type of M-Ti-carbide cluster with one more carbon atom in the structure, M₂TiC₂@C₈₀ (the new clusterfullerene also has two isomers, see Figures S1 b and S2 a). M₂TiC@C₈₀-II and M₂TiC₂@C₈₀-I have very similar retention times and elute in one fraction (Figures 1 and 4). Dy and Lu EMFs were separated by recycling HPLC as shown in Figures S1 and S3. Figure 4 compares HPLC traces and mass spectra of isolated Dy₂TiC@C₈₀-II and Dy₂TiC₂@C₈₀-I (each sample still contains a few percent of the other compound). Absorption spectra of M₂TiC@C₈₀-II and M₂TiC₂@C₈₀-I (M=Dy, Lu) shown in Figure 4 b,c lack well-defined features and extend to ca. 1000 nm for the former and ca. 1100 for the latter. The close similarity of the M₂TiC and M₂ScN clusters suggest that M₂TiC@C₈₀-II is likely to have a D_5h(6) cage. The ¹³C NMR spectrum of Lu₂TiC₂@C₈₀-I (Figure 4 c inset) has two ¹³C signals at 143.20 and 136.05 ppm (versus 143.46 and 136.60 ppm in Lu₂TiC@C₈₀-I). Thus, the cage symmetry of M₂TiC₂@C₈₀-I can be assigned as I_h(7).

DFT calculations show that the M₂TiC₂ cluster has two conformations with similar energies. In the most stable one, the C₂ unit is perpendicular to the M₂Ti plane and has μ₂-coordination with all metal atoms (Figure 3). In the second one (9 kJ mol⁻¹ higher in energy), the C₂ fragment is tilted out of the M₂Ti plane, has μ₂-coordination with Ti and one lanthanide, and μ₁-coordination with another lanthanide. C₂ is likely to exhibit fluxional motion at room temperature. The structure and dynamics of the M₂TiC₂ cluster are similar to those of M₃C₂ clusters in Sc₃C₂@C₈₀²² and Lu₃C₂@C₈₈.²³

M₂TiC@C₈₀-I has a cluster-localized LUMO with large contribution of Ti and a fullerene-based HOMO,¹⁶ and similar spatial distribution of the frontier MOs is predicted for M₂TiC@C₈₀-II. In Lu₂TiC₂@C₈₀-I, both HOMO and LUMO are predominantly localized on the endohedral cluster and have a large contribution of the acetylide fragment. The LUMO of Lu₂TiC₂@C₈₀ is to a large extent localized on Ti, whereas in the HOMO both Lu and Ti have comparable contributions (Figure 3). Thus, both reduction and oxidation of M₂TiC₂@C₈₀-I are cluster-based processes. Dy₂TiC₂@C₈₀-I exhibits two reversible reductions and one oxidation; in comparison to Dy₂TiC@C₈₀-I, the redox potentials are shifted cathodically by ca. 0.15 V (Figure 3, Table 1).

Recently we have discovered that mixed-metal nitride clusterfullerenes Dy_xSc_−-xN@C₈₀ (x=1–3) exhibit slow relaxation of the magnetization with a temperature-dependent decay rate, that is, they behave as single-molecule magnets (SMMs).¹², ²⁴ Of the three nitride clusterfullerenes, Dy₂ScN@C₈₀ is the strongest SMM due to the ferromagnetic exchange and dipolar interaction of the two Dy³⁺ ions. Unfortunately, selective synthesis of mixed-metal nitride clusterfullerenes is not possible. The arc-discharge synthesis in the Dy–Sc system always produces a mixture of Dy_xSc_3−xN@C_2n (x=0–3) compounds with different isomeric structures as well as other cage sizes,²⁵ and isolation of Dy₂ScN@C₈₀ is quite a tedious process. Therefore, the selective synthesis of Dy₂TiC@C₈₀-I, which is isostructural to Dy₂ScN@C₈₀-I, opens a more convenient route to EMF-based Dy-SMMs as along as the magnetic properties of the two EMFs are similar.

Figure 5 displays the magnetization curve of Dy₂TiC@C₈₀-I recorded for different temperatures at a field sweep rate of 5 mT s⁻¹. The system shows magnetic hysteresis with a temperature-dependent opening. The observation indicates that the system exhibits a slow relaxation of the magnetization with a decay rate that depends on the temperature. At a constant temperature, the opening is also determined by the field sweep rate as demonstrated in Figure S9. Similar behavior was observed for Dy₂ScN@C₈₀ and is characteristic for SMMs. The stability of the remnant magnetization can be quantified by determining the temperature at which the zero-field magnetization relaxation time is 100 s, the so-called 100 s blocking temperature (T_B100).²⁶ In the present case, we obtain T_B100=1.7 K for Dy₂TiC@C₈₀-I as compared to T_B100=3.6 K for Dy₂ScN@NC₈₀.¹², ^26c The higher T_B100 value for the nitride clusterfullerene indicates that the nitride ion in the endohedral cluster is advantageous for stronger SMMs.

Isolation of Dy₂TiC@C₈₀-II and Dy₂TiC₂@C₈₀-I allows us to study how the cage isomerism and the carbide cluster composition affect magnetic properties. Corresponding magnetization curves measured at 1.8 K are shown in Figure 5. The two isomers of Dy₂TiC@C₈₀ have a similar field dependence and the same T_B100 value to within 0.1 K. In contrast, Dy₂TiC₂@C₈₀ exhibits a narrower hysteresis than the Dy₂TiC@C₈₀ isomers. The T_B100 value of Dy₂TiC₂@C₈₀ is below 1.5 K demonstrating that it is the softest magnet in the series. Thus, substitution of a single carbide ion by an acetylide unit in the endohedral cluster has an obvious deleterious effect on the SMM properties.

This work shows that lanthanide–Ti carbide clusterfullerenes can be synthesized with high selectivity using methane as a reactive gas. In particular, Dy₂TiC@C₈₀-I is the first mixed-metal EMF obtained as a main fullerene product and separated without elaborate chromatographic procedures. Similar to its Dy₂ScN@C₈₀ analogue, Dy₂TiC@C₈₀-I is a single-molecule magnet. Thus, the class of EMF-SMMs is expanded to carbide clusterfullerenes. The study of the second isomer of Dy₂TiC@C₈₀ showed that the carbon cage isomerism has little effect on the magnetic behavior as the two isomers behave similarly. At the same time, we found a strong dependence of SMM properties on the central nonmetal unit: the strength of Dy₂-SMMs is decreases in the series Dy₂ScN>Dy₂TiC>Dy₂TiC₂.