Structural and electrochemical studies of Sm @ D 3 hC 74 reveal a weak metal – cage interaction and a small band gap species †

Jiangsu Key Laboratory of Thin Films and Suzhou 215006, P. R. China. E-mail: fengla Beijing National Laboratory for Molecular Materials Chemistry and Applications, Engineering, Peking University, Beijing 1008 Department of Theoretical and Computa Molecular Science, Okazaki 444-8585, Japan Department of Physical and Macromolecul University in Prague, Albertov 6, 128 43 Pra † Electronic supplementary informa 10.1039/c3nr03447j Cite this: Nanoscale, 2013, 5, 10409


Introduction
2][3][4][5] In recent years, some of abundantly produced EMFs have been well studied, and their potential applications in the elds of biomedicine, [6][7][8] material science, 9 and photovoltaics/photosynthesis [10][11][12] have been anticipated.On the other hand, to date, studies concerning the structural and electronic features of various lessknown EMFs are still going on, [13][14][15][16] which aim to better understand the EMF family and therefore to advance this eld.
Particularly, C 74 EMFs are less-known species.Despite several previously performed studies, only very few C 74 -EMFs have been unambiguously characterized.This might be due to their low solubility or low availability.For instance, the La@C 74 with a trivalent metal is almost insoluble in most common organic solvents.This feature is similar to that of pristine C 74 and has been attributed to the ultra-small band gap of the pristine cage as suggested by the related theoretical calculations. 17Upon exohedral chemical modication, the resulting derivative, that is, La@C 74 (C 6 H 3 Cl 2 ), 18 has a much wider band gap and therefore much better solubility relative to La@C 74 .Thus, the structural and electrochemical characterizations of La@C 74 (C 6 H 3 Cl 2 ) were well performed.On the other hand, the C 74 species that contain a divalent metal, namely M II @C 74 (where M ¼ Ca, 19 Ba, 20 Sr, 21 Eu, 22 Sm 23,24 or Yb 25,26 ), are more soluble than empty C 74 and La@C 74 .Specically, Ba@C 74 and Ca@C 74 , which contain a Group II metal, have been studied by means of single-crystal X-ray diffraction or NMR spectroscopy.They were revealed to have the same cage of D 3h -C 74 and both endohedral Group II metals exhibited jumps between the three symmetry-equivalent pyracylene units of the cage, reecting a featured metal-cage interaction.However, M@C 74 that contains a divalent rare-earth metal has been never characterized for clarity.Considering the different electronic features of Group II metal ions and rare-earth metal ions, we therefore guess that encapsulating a rare-earth metal instead of a Group II metal may render some difference to the endohedral structure of EMF.
In the early studies, we have detected the presence of a moderate amount of Sm@C 74 (ref.23 and 24) and Yb@C 74 (ref.25 and 26) in the soot produced by direct current arc discharge of M-Ni alloy/C composite rods under a He atmosphere.However, because of the lack of further characterization, these species are still little known.Herein, we investigate the title compound by employing a combined study of singlecrystal X-ray diffraction, DFT calculations and electrochemistry.
The following discussions will focus on its structural features as well as the metal-cage interaction and the size of band gap.

Single-crystal X-ray diffraction analysis
Black co-crystals of Sm@D 3h -C 74 /Ni II (OEP) were obtained by allowing a benzene solution of fullerene and a chloroform solution of Ni II (OEP) to diffuse together.X-ray data were collected at 90 K using a diffractometer (APEX II; Bruker Analytik GmbH) equipped with a CCD collector.The multi-scan method was used for absorption correction.The structure was resolved using direct methods (SHELXS97) and rened on F 2 using full-matrix least squares using SHELXL97. 28The intact cage was modeled via the crystallographic mirror plane in renement.Hydrogen atoms were added geometrically and rened with a riding model.

Electrochemistry
Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were carried out in o-dichlorobenzene (o-DCB) using a BAS CW-50 instrument.A conventional three-electrode cell consisting of a platinum working electrode, a platinum counter-electrode, and a saturated calomel reference electrode (SCE) was used for both measurements.0.05 M (n-Bu) 4 NPF 6 was used as the supporting electrolyte.All potentials were recorded against a SCE reference electrode and corrected against Fc/Fc + .DPV and CV were measured at a scan rate of 20 and 100 mV s À1 , respectively.

Computational method
All calculations were carried out using the Gaussian 09 program package. 29Geometry optimizations and Mulliken population analysis were performed using the density functional theory with the recently introduced M06-2X functional, 30 3-21G basis set 31 for the C atom, and the SDD basis set 32 (with the SDD effective core potential) for Ni and Sm atoms (the M06-2X/3-21G$SDD level).The orbital energies were also calculated at a higher level (the M06-2X/6-31G$SDD level).

Results and discussion
The purity and the composition of the Sm@C 74 sample were conrmed by HPLC and MALDI-TOF mass, respectively (see Fig. S1 †).The absorption spectrum of Sm@C 74 is almost identical to that reported previously, 23 as well as those of analogous species M@C 74 (M ¼ Eu, Sr, Ba and Ca) [19][20][21][22][23][24][25][26] (See Fig. S2 †).The absolute structure of Sm@D 3h -C 74 was identied via a singlecrystal X-ray diffraction (XRD) study.Co-crystals of Sm@D 3h -C 74 /[Ni II (OEP)] suitable for X-ray analysis were obtained by slow diffusion of a benzene solution of EMF into a chloroform solution of [Ni II (OEP)].The molecular structure was resolved and rened in a monoclinic space group C 2 (no.5). 33Fig. 1 shows the X-ray structure of Sm@D 3h -C 74 together with an adjacent [Ni II (OEP)] moiety.Note that the D 3h -C 74 cage has been crystallographically identied in two orientations with fractional occupancies of 0.67 and 0.33.Only the major cage is depicted in Fig. 1.The porphyrin moiety is approaching a at region of the D 3h -C 74 cage with the shortest nickel-to-cage carbon distance ranging from 2.606 to 2.867 Å, indicating their strong p-p stacking interaction.
Inside the cage of D 3h -C 74 , the Sm atom is highly disordered and several metal-sites have been rened with fractional occupancies ranging from 0.311 to 0.024, reecting a motional behavior of the endohedral Sm atom.This feature is unlike that of previously reported Ba@C 74 , in which only two different Ba positions were identied, corresponding well to the disordered cage, which is aligned in two orientations.Summarizing all the crystallographically characterized Smfullerenes that have different cage sizes and structures, it is noteworthy that such a dynamic endohedral feature is common for Sm-fullerenes, 34,35 indicating their weaker metalcage interactions relative to those of other mono-metallofullerenes. 3,36Considering the occupancies of cage and metal, we initially assigned the major Sm-site (i.e., Sm01) to the major cage, as shown in Fig. 1, while another Sm-site with the second highest occupancy (i.e., Sm02) was assigned to the minor cage, as shown in Fig. S3.† Such assignments give rise to a pair of identical conformers of Sm@C 74 , in which the metal is localized off-center, being close to one of the three symmetry-equivalent pyracylene units at the horizontal mirror plane with a distance of 2.417 Å between the Sm and the central 6 : 6 bond of that pyracylene motif.Such a metal-cage alignment is very similar to that of Ba@C 74 , conrming the previous assumption that there is a signicant interaction between the pyracylene motif and the endohedral divalent metal.Nevertheless, this Sm-pyracylene motif interaction might be weaker than that in Ba@C 74 , as reected by the fact that the Sm atom can jump to other sites where the pyracylene unit is far from.
In addition, a series of computational studies were performed at the levels of M06-2x/3-21G$SDD and M06-2X/ 6-31G*$SDD to investigate the electronic features of Sm@C 74 .As shown in Fig. 2, the DFT-optimized pristine Sm@D 3h -C 74 is in good agreement with the X-ray structure of Sm@D 3h -C 74 .Particularly, the optimized Sm site is approaching the central 6 : 6 bond of a pyracylene unit with a distance of 2.533 Å, very close to that detected in the X-ray structure.Note that there are three equivalent pyracylene units on the cage of D 3h -C 74 .However, if considering the inuence of the adjacent Ni II (OEP) moiety, these pyracylene units are chemically non-equivalent.Therefore, we further optimize the unit of Sm@D 3h -C 74 /[Ni II (OEP)] to investigate the most preferred Smsite relative to the adjacent Ni II (OEP) moiety.Two DFT-optimized congurations of Sm@D 3h -C 74 /[Ni II (OEP)] are shown in Fig. 3 and S4 † in different views.Particularly, in each conguration, the Sm site is close to one of the pyracylene units but differently aligned relative to the adjacent Ni II (OEP) moiety.Calculations further revealed that the conguration shown in the le panel, which is fully consistent with the X-ray structure of Sm@D 3h -C 74 /[Ni II (OEP)] (see Fig. 1), is 0.079 kcal mol À1 more stable than that in the right panel.Although such a difference between the relative energies of these two congurations is very small, it is detectable at the present DFT level, suggesting a weak but salient inuence of the porphyrin moiety on the location of endohedral metal.
HOMO and LUMO distributions of Sm@D 3h -C 74 were calculated at a M06-2X/3-21G$SDD level, as shown in Fig. 4. Specically, the HOMO is uniformly delocalized on the D 3h -C 74 framework, while the LUMO is mostly localized on the cage area where the Sm is close to.The calculated Mulliken charge on the endohedral Sm atom is 2.04, indicative of a formal two-electron transfer from the metal to the cage.Thus, an ionic model of Sm 2+ @(D 3h -C 74 ) 2À has been veried, which well describes the electronic feature of Sm@D 3h -C 74 .Also, because of the off-center position of the endohedral divalent metal, the metal-donated charges are mainly delocalized on the pyracylene motif and the nearby area, as suggested by the Mulliken population analysis.In particular, the carbons on the central 6 : 6 bond of that pyracylene motif are much more negatively charged than others (see Fig. S3 and Table S1 †), indicating their higher nucleophilic affinity.
Furthermore, a comparison study between D 3h -C 74 , (D 3h -C 74 ) 2À and Sm@D 3h -C 74 was performed to estimate the role of the endohedral Sm atom in stabilizing the otherwise unstable D 3h -C 74 cage.The DFT-calculated HOMO/LUMO energy levels of D 3h -C 74 , (D 3h -C 74 ) 2À and Sm@D 3h -C 74 are summarized in Fig. 5.The ultra-small band gap of D 3h -C 74 (i.e., 1.65 eV) and the high HOMO/LUMO energy levels of (D 3h -C 74 ) 2À shall account for their low stabilities.In contrast, Sm@D 3h -C 74 shows a band gap (i.e., 2.76 eV) larger than that of D 3h -C 74 but smaller than that of (D 3h -C 74 ) 2À .Besides, the lower-lying HOMO/LUMO of Sm@D 3h -C 74 as compared with those of (D 3h -C 74 ) 2À is in line with its higher stability relative to the other.Thus, our computational studies show that the encapsulation of a divalent rare-earth metal can denitely improve the stability of a D 3h -C 74 cage by rationalizing the size of band gap as well as the HOMO/ LUMO energy levels.
The electrochemical properties of Sm@D 3h -C 74 were investigated by means of cyclic voltammetry (CV) and differential pulse voltammetry (DPV).As shown in Fig. 6, both CV and DPV proles suggest four reversible one-electron reductive steps on    the cathodic side and two reversible one-electron oxidative steps on the anodic side.All the redox potentials obtained from CV are summarized in Table 1 and compared with those of other Sm-fullerenes such as Sm@C 2v (3)-C 80 34 and Sm@C 2 (5)-C 82 . 35It is noteworthy that the rst oxidation potential of Sm@D 3h -C 74 is 220-230 mV lower than those of middle-sized Sm-fullerenes, indicating a much stronger electron-donating ability.On the other hand, only a minor difference is found among the rst reduction potentials of these Sm-fullerenes.Thus, the electrochemical potential gap of Sm@D 3h -C 74 is 0.97 eV, substantially smaller than those of other Sm-fullerenes (i.e., 1.28 eV for Sm@C 2v (3)-C 80 and 1.26 eV for Sm@C 2 (5)-C 82 ), indicative of a smaller band gap relative to the others.This has been reected by the fact that Sm@D 3h -C 74 shows lower solubility in common organic solvents.As a result, the resolution of the 13 C NMR spectrum of Sm@D 3h -C 74 was too low to be acceptable even aer long-time scans because of the insufficient solubility of Sm@D 3h -C 74 in CS 2 .

Conclusions
In conclusion, we have studied a divalent metallofullerene Sm@D 3h -C 74 structurally and electrochemically.Specically, single-crystal X-ray diffraction analysis revealed that the endohedral Sm atom mainly binds to one pyracylene motif on the cage, which has been conrmed by computational studies.Nevertheless, the Sm-cage interaction is weaker than that in Ba@C 74 , as reected by the fact that the Sm atom more or less jumps to an other site far from the pyracylene motif.Thus, this study has conrmed that the variety of endohedral metal indeed imposes a salient inuence on the metal-cage interaction and therefore affects the endohedral behavior.In addition, electrochemical studies demonstrated a smaller electrochemical potential gap of Sm@D 3h -C 74 relative to those of other Smfullerenes, indicative of a small band gap species.

Fig. 1
Fig. 1 Ortep drawing of Sm@D 3h -C 74 $Ni II (OEP) with 25% thermal ellipsoids, showing the intermolecular interaction.The major Sm site with fractional occupancy of 0.311 and the major cage position with fractional occupancy of 0.673 are depicted.The solvent molecules are omitted for clarity.