Spin Divergence Induced by Exohedral Modification: ESR Study of Sc₃C₂@C₈₀ Fulleropyrrolidine

Abstract

The paramagnetic properties of the endohedral fullerene Sc3C2@C80 are adjustable by exohedral modification by the Prato reaction (see picture). Analysis of spin densities and endocluster dynamics reveal unique paramagnetic properties of the Sc3C2@C80 fulleropyrrolidine and provide general insight into the addend-dependent paramagnetic behavior of endohedral fullerenes. Endohedral metallofullerenes have attracted wide interest because of their novel structures and potential applications in a variety of fields, such as nanotechnology and biomedical applications.1–3 Exohedral functionalization plays a critical role not only in improving solubility and processability of metallofullerenes for expanding their practical applications, including photovoltaic cells,4 magnetic resonance imaging agents,5 and radiotracers,6 but also in controlling the position of encapsulated atoms and corresponding properties. Thus, the characteristics of endofullerenes become more diversified, and these help in designing novel materials with controllable electronic and magnetic properties.7–13 The abundant dimetal-encapsulated and trimetallic nitride template encapsulated C80 species and their derivatives have been studied in detail.1–3, 7–10 However, little was known about the derivatives and corresponding properties for the metal carbide encapsulated fullerenes, such as Sc3C2@C80, even though its pristine structure has been studied in detail since its discovery.14–20 In view of its elusive structure and alluring ESR spectrum, it is significant to investigate the effect of exohedrally functional groups on the elusive structures and properties of Sc3C2@C80.17–20 Herein, we report the spin divergence in Sc3C2@C80 fulleropyrrolidine induced by exohedral modification by the Prato reaction.21, 22 The structure of Sc3C2@C80 fulleropyrrolidine was characterized by NMR, UV/Vis, and IR spectroscopy, as well as theoretical studies. The synthesis of Sc3C2@C80 fulleropyrrolidine was carried out in o-dichlorobenzene with a solution of Sc3C2@C80, N-ethylglycine, and 13C-enriched paraformaldehyde. The product was isolated and purified by high performance liquid chromatography (HPLC) and identified as fulleropyrrolidine monoadduct by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (Figure 1). HPLC profile of reaction mixture (N-ethylglycine and 13C-enriched paraformaldehyde) treated with Sc3C2@C80 for 10 min. The insert shows the MALDI-TOF mass spectrum for the monoadduct. Although the Sc3C2@C80 fulleropyrrolidine monoadduct is paramagnetic, the 13C NMR spectrum exhibits a singlet at δ=71.62 ppm for the 13C-labeled methylene carbon (Figure 2), indicating that the low spin density distribution of cluster close to the addend does not significantly affect the pyrrolidine group, and the 5,6-ring junction is the reaction site for Sc3C2@C80 fulleropyrrolidine as in pyrrodino Sc3N@C80-Ih case.23, 24 The heteronuclear multiple quantum coherence (HMQC) spectrum verifies the non-equivalent methylene protons attached to the equivalent methylene carbon atoms, which is consistent with the 5,6-ring addition assignment of the fulleropyrrolidine (see the Supporting Information). 13C NMR spectrum of 13C-labeled Sc3C2@C80 fulleropyrrolidine monoadduct. The insert depicts the partial structure of the monoadduct. The yellow bonds show the adjacent 5- and 6-membered rings around the addend, and the red atom represents the 13C-labeled methylene carbon atom. Noticeably, although the geminal methylene protons on the pyrrolidine ring of the Sc3C2@C80 fulleropyrrolidine monoadduct exhibit similar chemical shifts and splitting pattern to that of Sc3N@C80-Ih analogue,23, 24 the chemical shift difference (Δδ=4.39–3.04=1.35) of the methylene geminal protons is larger than that of Sc3N@C80-Ih fulleropyrrolidine (Δδ=4.07–2.81=1.26). Besides the different ring currents derived from the adjacent 5- and 6-membered rings,24, 25 the different encapsulated metal clusters also should contribute to this large degree of deshielding/shielding effect. Fourier transform infrared (FTIR) spectroscopy was employed to elucidate the structures of Sc3C2@C80 and Sc3C2@C80 fulleropyrrolidine (see the Supporting Information). For Sc3C2@C80 fulleropyrrolidine monoadduct, signals at around 693 and 714 cm−1 represent the antisymmetric ScC stretching vibrations, which are different from that of pristine Sc3C2@C80 with single signal at 670 cm−1. This change can be ascribed to the changed configuration of Sc3C2 cluster resulting from the exohedral addend. The geometry of the Sc3C2@C80 fulleropyrrolidine monoadduct was further investigated by DFT calculations.26, 27 The results demonstrate that the molecule has a mirror plane splitting the pyrrolidine ring, and the Sc3C2 endocluster has C2v symmetry, reduced from C3v symmetry in pristine Sc3C2@C80 (Figure 3).17–20 For the inner Sc3C2 cluster, one Sc atom is located at bottom of the cage and far away from the addend; and the other two Sc atoms are close to the pyrrolidine ring and positioned symmetrically. Such an assignment of inner cluster is consistent with the electrostatic potential map for the calculated 5,6-adduct of [C80(CH2)2NH]6−,28 for which the electrostatic potentials have a minimum at the bottom of the cage and far away from the addend, thus leading to one Sc atom located near the energy minimum and the other two Sc atoms repulse from each other to minimize the energy further.28 Therefore, the chemical functionalization on metallofullerene Sc3C2@C80 can change the configuration of the inner Sc3C2 cluster effectively and can be used to control the location of Sc atoms intentionally. Optimized structure (left; N blue, Sc yellow, C gray, H white) and calculated spin density distribution (right) of the Sc3C2@C80 fulleropyrrolidine monoadduct. Figure 4 shows the experimental and simulated ESR spectra of the Sc3C2@C80 fulleropyrrolidine monoadduct. Surprisingly, the spectra of monoadduct and pristine Sc3C2@C80 differ substantially (Table 1).14–16 Splitting values of 8.602 (one nucleus) and 4.822 G (two nuclei) and a g value of 2.0007 were observed for the Sc3C2@C80 fulleropyrrolidine monoadduct, compared with 6.256 G (three nuclei) and 2.0006, respectively, for Sc3C2@C80. This ESR pattern of Sc3C2@C80 fulleropyrrolidine is also different from that of Sc3C2@C80(Ad) (Ad=adamantylidene) (20.55, 5.479 MHz). As the hyperfine coupling constants (hfcc) of Sc nuclei are related to the spin density near the Sc atoms, the spin density distributions were calculated to elucidate the paramagnetic property of Sc3C2@C80 fulleropyrrolidine. As shown in Figure 3, the spin density for Sc3C2@C80 fulleropyrrolidine is localized exclusively on the Sc3C2 endocluster inhomogeneously; the higher spin density is localized on a unique Sc nucleus far away from the pyrrolidine addend, whereas the lower spin density is localized on the other two Sc nuclei homogeneously. In contrast, the pristine Sc3C2@C80 has the most spin localization on the Sc3C2 cluster as well but each Sc nucleus has the same spin density.19 Therefore, remarkably, the spin divergence in Sc3C2@C80 fulleropyrrolidine was induced by exohedral modification, and these unique spin density distributions derived from the exohedral modification can well explain the coupling constants of Sc nuclei in Sc3C2@C80 fulleropyrrolidine monoadduct. Notably, although Sc3C2@C80 fulleropyrrolidine is paramagnetic, the very low spin density on fullerene cage close to the addend could explain the 13C NMR signal at δ=71.62 ppm for the 13C-labeled methylene carbon. Again, chemical modification has been proved to be a powerful technique not only to change the configuration of inner cluster, but also to tune the electronic and paramagnetic properties of an endofullerene. Experimental and simulated ESR spectra of the Sc3C2@C80 fulleropyrrolidine monoadduct. Sample αSc [G] g value Sc3C2@C80 6.256 (6.51[b]) 2.0006 (1.9985[b]) monoadduct 8.602; 4.822; 4.822 2.0007 Comparisons of the spin density distributions between paramagnetic trimetallic endofullerenes are helpful to understand the unique ESR properties of Sc3C2@C80 fulleropyrrolidine. The Y3N@C80-Ih fulleropyrrolidine monoanion was reported to be ESR active with hfcc patterns of 6.26 G (two nuclei) and 1.35 G (one nucleus),29 and the unpaired spin was delocalized both on the fullerene cage and on the internal Y3N cluster.29 The Sc3N@C80 anion radical was reported to be ESR active with peculiar hfcc pattern of 55.7 G (three nuclei), and the exclusive unpaired spin was proposed to be on the Sc3N cluster.30, 31 In contrast, the Sc3N@C68 cation radical was reported with hfcc of 1.289 G (three nuclei), and its unpaired spin was delocalized both on the C68 cage and on the endocluster as in the Y3N@C80-Ih fulleropyrrolidine monoanion case.32 From the above examples, we can conclude that the unpaired spin distributions are relevant to the kind of endoclusters and type of fullerene cages. For Sc3C2@C80, the Sc3C2 cluster can rotate freely as confirmed by ESR, MEM/Rietveld, and 13C NMR spectroscopic experiments, as well as DFT calculations. This dynamic Sc3C2 cluster along with the equivalent coupling between the three Sc atoms and the unpaired electron leads to symmetrical hyperfine splitting of 22 lines.14–20 In contrast, the splitting pattern for Sc3C2@C80 fulleropyrrolidine suggests that the exohedral addend prominently hinders the free rotation of the endocluster, which leads to inhomogeneous spin density distributions on internal cluster. The peculiar chemical shift difference between the methylene geminal protons also suggests a nonhomogeneous C80 cage caused by hindered Sc3C2 rotation. Unlike the proposed jumping motion for the three Sc atoms in pristine Sc3C2@C80, intrafullerene motion of Sc atoms in Sc3C2@C80 fulleropyrrolidine was suggested as oscillation modes by DFT calculation (see the Supporting Information). The Sc3C2 cluster oscillates randomly around the equilibrium position; for example, the Cs plane of Sc3C2 cluster can swing to and fro, left and right, as well as around within 15 ° for the dihedral angles. Therefore, there is no doubt that the unpaired spin is also closely connected to the intrafullerene motion of the endocluster. In summary, the paramagnetic properties of Sc3C2@C80 were tuned by exohedral modification. The 5,6-ring junction was determined to be the reaction site for the Sc3C2@C80 fulleropyrrolidine monoadduct, and the endohedral Sc3C2 cluster was deformed by the pyrrolidine addend. Most importantly, the pyrrolidine addend changes the spin density distributions and alters the paramagnetic properties of Sc3C2@C80 fulleropyrrolidine as a result. The dynamics of Sc3C2 endocluster were also discussed to elucidate the variable properties caused by the exohedral addend. Such controllable molecular paramagnetism is of great significance to the construction of novel molecular devices. The synthesis of Sc3C2@C80 fulleropyrrolidine was carried out in a solution of Sc3C2@C80 (5 mg) in o-dichlorobenzene (o-DCB) with an excess of N-ethylglycine (8 mg) and 13C-enriched (99 %) paraformaldehyde (8 mg) at 108 °C for 10 min. The product was isolated and purified by HPLC. The 13C NMR and HMQC spectra were measured in CS2 with D2O inside a capillary as an internal lock. All the ESR experiments were performed at room temperature in o-DCB. DFT calculations were investigated by Perdew, Burke, and Enzerhof (PBE)/double numerical plus polarization using the DMol3 code in Accelrys Materials Studio.26, 27 Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.