Synthesis and characterization of carbene derivatives of Th@ C 3v(8)-C82 and U@ C 2v(9)-C82: exceptional chemical properties induced by strong actinide-carbon cage interaction

Abstract

Chemical functionalization of endohedral metallofullerenes (EMFs) is essential for the application of these novel carbon materials. Actinide EMFs, a new EMF family member, have presented unique molecular and electronic structures but their chemical properties remain unexplored. Here, for the first time, we report the chemical functionalization of actinide EMFs, in which the photochemical reaction of Th@C _3v(8)-C₈₂ and U@C _2v(9)-C₈₂ with 2-adamantane-2,3'-[3H]-diazirine (AdN₂, 1) was systematically investigated. The combined HPLC and MALDI-TOF analyses show that carbene addition by photochemical reaction afforded three isomers of Th@C _3v(8)-C₈₂Ad and four isomers of U@C _2v(9)-C₈₂Ad (Ad = adamantylidene), presenting notably higher reactivity than their lanthanide analogs. Among these novel EMF derivatives, Th@C _3v(8)-C₈₂Ad(I, II, III) and U@C _2v(9)-C₈₂Ad(I, II, III) were successfully isolated and were characterized by UV-vis-NIR spectroscopy. In particular, the molecular structures of first actinide fullerene derivatives, Th@C _3v(8)-C₈₂Ad(I) and U@C _2v(9)-C₈₂Ad(I), were unambiguously determined by single crystal X-ray crystallography, both of which show a [6,6]-open cage structure. In addition, isomerization of Th@C _3v(8)-C₈₂Ad(II), Th@C _3v(8)-C₈₂Ad(III), U@C _2v(9)-C₈₂Ad(II) and U@C _2v(9)-C₈₂Ad(III) was observed at room temperature. Computational studies suggest that the attached carbon atoms on the cages of both Th@C _3v(8)-C₈₂Ad(I) and U@C _2v(9)-C₈₂Ad(I) have the largest negative charges, thus facilitating the electrophilic attack. Furthermore, it reveals that, compared to their lanthanide analogs, Th@C _3v(8)-C₈₂ and U@C _2v(9)-C₈₂ have much closer metal-cage distance, increased metal-to-cage charge transfer, and strong metal-cage interactions stemming from the significant contribution of extended Th-5f and U-5f orbitals to the occupied molecular orbitals, all of which give rise to their unusual high reactivity. This study provides first insights into the exceptional chemical properties of actinide endohedral fullerenes, which pave ways for the future functionalization and application of these novel EMF compounds.

Fig. 1. (a) HPLC tracing of the reaction between Th@C_3v(8)-C₈₂ with 1 at different times (black line: the signal of the pristine compound Th@C_3v(8)-C₈₂ before irradiation, red line: the signal of the resulting products after irradiation for 15 minutes). (b) HPLC tracing of the reaction between U@C_2v(9)-C₈₂ with 1 at different times (black line: the signal of the pristine compound U@C_2v(9)-C₈₂ before irradiation, red line: the signal of the resulting products after irradiation for 10 minutes). HPLC conditions: Buckyprep column (10 mm × 250 mm): flow rate, 4 mL min⁻¹; toluene as the mobile phase; wavelength 310 nm.

Fig. 2. (a) Recycling HPLC profile for the separation of U@C_2v(9)-C₈₂Ad(I), U@C_2v(9)-C₈₂Ad(II), U@C_2v(9)-C₈₂Ad(III) and U@C_2v(9)-C₈₂Ad(IV). (b) HPLC chromatogram of isolated U@C_2v(9)-C₈₂Ad(I), U@C_2v(9)-C₈₂Ad(II), U@C_2v(9)-C₈₂Ad(III) and U@C_2v(9)-C₈₂Ad(IV). HPLC conditions: Buckyprep column (10 mm × 250 mm); flow rate, 4 mL min⁻¹; toluene as the mobile phase; wavelength 310 nm.

Fig. 3. UV-vis-NIR absorption spectra of (a) Th@C_3v(8)-C₈₂ and Th@C_3v(8)-C₈₂Ad(I, II, III) and (b) U@C_2v(9)-C₈₂ and U@C_2v(9)-C₈₂Ad(I, II, III) in CS₂.

Fig. 4. Cyclic voltammogram of (a) Th@C_3v(8)-C₈₂Ad(I) and (b) U@C_2v(9)-C₈₂(I) in o-dichlorobenzene (0.05 M (n-Bu)₄NPF₆; scan rate 100 mV s⁻¹ for CV).

Fig. 5. (a) HPLC traces of isomerization of Th@C_3v(8)-C₈₂Ad(I, II, III) and (b) recycling HPLC traces of isomerization of U@C_2v(9)-C₈₂Ad(I, II, III) at room temperature. HPLC conditions: Buckyprep column (10 mm × 250 mm); flow rate, 4 mL min⁻¹; toluene as the mobile phase.

Fig. 6. ORTEP drawings of (a) Th@C_3v(8)-C₈₂Ad(I) and (b) U@C_2v(9)-C₈₂Ad(I) showing thermal ellipsoids at the 50% probability level. (c and d) The view showing the interaction of the metal ion with the closest cage portion.

Fig. 7. Different types of carbon atoms in (a) Th@C_3v(8)-C₈₂ and (b) U@C_2v(9)-C₈₂.

Fig. 8. Optimized structures of the Th@C_3v(8)-C₈₂Ad(I–III) isomers with relative energies (kcal mol⁻¹). The distances between the two addition site C atoms are given in Å.

Fig. 9. Frontier molecular orbitals of Th@C_3v(8)-C₈₂ and Th@C_3v(8)-C₈₂Ad(I) (occupied: black; unoccupied: cyan). For selected orbitals, the participation (%) of the Th AOs is first given, followed by two numbers showing the % contribution of the Th-5f (red), -6d (blue), or -7s (green) AOs to this metal hybrid orbital.

Fig. 10. Optimized structures of the U@C_2v(9)-C₈₂Ad(I–IV) isomers with relative energies (kcal mol⁻¹). The distances between the two addition site C atoms are given in Å.

Fig. 11. Frontier molecular orbitals of U@C_2v(9)-C₈₂ and U@C_2v(9)-C₈₂Ad(I) (occupied: black; unoccupied: cyan). For selected orbitals, the participation (%) of the U AOs is first given, followed by two numbers showing the % contribution of the U-5f (red), or -6d (blue) AOs to this metal hybrid orbital.

Fig. 12. Charge density values of different carbon atoms in Th@C_3v(8)-C₈₂ and M@C_2v(9)-C₈₂ (M = U, Sc, La). Please refer to Fig. 7 for the atom numbering.

Scheme 1. Reaction of 1 with Th@C_3v(8)-C₈₂ or U@C_2v(9)-C₈₂.