Synthesis and Structural Studies of Two Paramagnetic

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Article Cite This: Inorg. Chem. 2019, 58, 8162−8168

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Synthesis and Structural Studies of Two Paramagnetic Metallofullerenes with Isomeric C72 Cage Chong Zhao,†,‡ Mingzhe Nie,†,‡ Haibing Meng,†,‡ Chunru Wang,*,† and Taishan Wang*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China

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S Supporting Information *

ABSTRACT: We synthesized and isolated two paramagnetic metallofullerenes of La@C72 and Y@C72 with different fullerene cages, which were characterized by electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations. DFT calculations disclosed two possible isomers of La/Y@C72 with C72-C2 and C72-C2v cages, both of which have similar thermodynamic stability and one pair of fused pentagons. Their paramagnetic properties were then studied by EPR spectroscopy, and the obtained EPR signals were analyzed with very different hyperfine coupling constants, revealing distinct electron spin distributions for these two species. Furthermore, the experimental coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C72 has a C72-C2v cage and Y@C72 has a C72-C2 cage. These studies illustrate that the electron spin can be used as a probe to identify metallofullerene structure due to the susceptibility of spin-metal couplings. The successful isolation and characterizations of La@C72 and Y@C72 with such a small C72 cage reveal their stability that is important for application as paramagnetic molecule materials.

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INTRODUCTION One of the intriguing properties of endohedral metallofullerene (EMF) is its paramagnetic character, which has potential in quantum information processing (QIP) and single molecular magnet (SMM) applications.1−7 The paramagnetic metallofullerene has an open-shell electronic structure, in which the unpaired spin is originated from the odd electron in the molecules.8−17 Spin-active metallofullerenes have been found to have high sensitivity to chemical modification, temperature, local magnetic field, host, and molecular rotation, which can greatly influence the spin distributions, spin relaxation, and spin couplings.18,19 Among them, the monometal endohedral fullerenes can provide valuable information about the nature of the single metal and have brought about distinct paramagnetic properties. Since the first discovery of monometal endohedral fullerene, La@C82,20 several La@C2n molecules and their derivatives have been synthesized. However, many pristine La@C2n monometal endohedral fullerenes have not been discovered due their high reactivity, and only their nonmagnetic derivatives can be isolated, such as the La@C72− C6H3Cl2.21 Likewise, many pristine Y@C2n have not been discovered except for [email protected]−26 Furthermore, due to the low yield of monometal endohedral fullerenes, it is difficult to identify their accurate structure. Therefore, it is meaningful to explore new method to characterize these paramagnetic monometal endohedral fullerenes. It is generally acknowledged that the couplings between the unpaired electron and magnetic nucleus always show energy splittings under an external magnetic field. The coupling parameters of A and g-factor values obtained by the electron © 2019 American Chemical Society

paramagnetic resonance (EPR) spectroscopy can be used to evaluate the chemical environment and kinetic information on electron spin as well as magnetic nucleus. Typically, the spinnucleus couplings in paramagnetic metallofullerenes provide related structural information. For example, for the paramagnetic Sc3C2@C80, the hyperfine couplings between spin and Sc3 indicate three equivalent Sc nuclei.1,4,27 Similar spinnucleus couplings are also found in Y2@C79N, and its EPR spectra illustrate the susceptible Y2 rotation inside the azafullerene C79N.3,9−11 Different Y2 and Y2C2 rotations within the same C82 cage were also observed by EPR spectroscopy.28 For the uncertain position of endohedral C and N for Sc3CN@ C80 in single crystal analysis, EPR spectroscopy can confirm them through analysis of hyperfine couplings.29 Therefore, the electron spin in metallofullerenes can be used as a probe to identify metallofullerene structure due to the susceptibility of spin-metal couplings. In this work, we successfully synthesized two monometal endohedral fullerenes La@C 72 and Y@C 72 by the Krätschmer−Huffman arc-discharging method and isolation by high-performance liquid chromatography (HPLC). Their structures and paramagnetic properties were studied by EPR spectroscopy combined with ab initio calculations. The experimental EPR coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C72 has a C72-C2v cage and Y@ C72 has a C72-C2 cage. These results show that different Received: April 2, 2019 Published: May 24, 2019 8162

DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

Article

Inorganic Chemistry

Figure 1. (a, b) HPLC separation profiles of La@C72 (20 × 250 mm Buckyprep column; flow rate of 6 mL/min; toluene as eluent). (c) The experimental MALDI-TOF mass spectra of La@C72. Insets show the experimental and simulated isotopic distributions. (d, e) HPLC separation profiles of Y@C72 (20 × 250 mm Buckyprep column; flow rate of 6 mL/min; toluene as eluent). (f) The experimental MALDI-TOF mass spectra of Y@C72. Insets show the experimental and simulated isotopic distributions. (g) UV−vis absorption spectra of Y@C72 and La@C72. (h−k) The optimized structures of La@C72-C2, La@C72-C2v, Y@C72-C2, and Y@C72-C2v. The yellow balls present the pair of fused pentagons. (Figure S3), and geometry optimizations are performed on the corresponding complex structures by B3LYP/6-31G* ∼ (lanl2dz for La and Y). The results are listed in Table S2 (the single point energy is by empirical dispersion correction).31 Frequency calculations were carried out to identify the nature of the optimized molecules. Rotational−vibrational partition functions are used to elaborate the relative concentration at the temperature span of 0−3500 K. The bonding features between La/Y and C72 were analyzed with a MO interaction diagram. The ESPs are mapped on the electron density isosurface (isovalue is 0.001 au). The total electron density is employed by a .fch file transferred by the formchk module in Gaussian 09. The ESP visualization is implemented through the Mutiwfn +VMD software. The above calculations were performed using the Gaussian 09 quantum chemical program package.32 The hfcc (hyperfine coupling constant) values of aiso are performed by the ORCA program at the level of BP86/Def-2 (La ∼ SARC-TZV for the DKH2 Hamiltonian).33 The EPR spectrum was simulated by easyspin codes (http://www.easyspin.org) used in MATLAB.

fullerene cages can greatly influence the spin distributions of paramagnetic metallofullerenes. Meanwhile, the EPR spectroscopy could be used as a tool to detect the structural characteristics.

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EXPERIMENTAL SECTION

Synthesis of La@C72 and Y@C72. Metallofullerenes La@C72 and Y@C72 were prepared by the Krätschmer−Huffman arc discharge method.30 The as-prepared soot was Soxhlet-extracted with toluene for 24 h. The La@C72 and Y@C72 samples was isolated and purified by multistep high-performance liquid chromatography (HPLC), and the purity of the sample was confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF-MS). EPR Measurements. The EPR spectra were measured on a Bruker E500 instrument with a continuous-wave X band. The frequencies were 9.753 GHz for La@C72 and 9.859 GHz for Y@C72. Both spectra were obtained with the same power attenuation of 13.0 dB. All of the solution samples were dissolved in CS2 solution. Theoretical Calculations. The coordinates of C72 were generated by the revised CaGe software. There are 11190 classic isomers for C72. We selected the 42 isomers with zero to three B55 bonds (B55 means the shared C−C bond of fused pentagons) as the candidate cages. All the isomers of the selected C72 and their anions (C722−, C723−) are optimized at the PM3 and B3LYP/3-21G* level in turn. The results are listed in Table S1. Since the encaged cluster would transfer electrons to the parent cages, the stability of the anions is closely related to the corresponding metallofullerenes; the lowest energy isomers of anions are selected as candidate cages for encaging the La/ Y atom. La/Y is then put inside the cages at the typical seven sites

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RESULTS AND DISCUSSION Structural Characterization. La@C72 and Y@C72 were both synthesized by the arc-discharging method under 200 Torr He. The produced fullerenes and metallofullerenes were isolated by multistep HPLC as shown in Figure 1a, b and Figure 1d, e. The retention time of target La@C72 or Y@C72 is similar to C60O and La@C74 on the Buckyprep column. Their MALDI-TOF mass spectra are shown in Figure 1c and 1f. The highest intensity of isotopic abundance ratio appears at 1003.91 (m/z) for La@C72 and at 953.91 (m/z) for Y@C72, 8163

DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

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Inorganic Chemistry

Table 1. Calculated Relative Energies,a Distance of La/Y−C,b and EEc of the Optimized Isomers of La@C72-C2/C2v and Y@ C72-C2/C2v isomer

△E-B3LYP/TPSSh

d(La−C)-B3LYP/TPSSh

EE-B3LYP/TPSSh

La@C72-C2 La@C72-C2v isomer

0 0.17/0.67 △E-B3LYP/TPSSh

2.64/2.62 2.59/2.58 d(Y−C)-B3LYP/TPSSh

3.43/5.75 3.00/5.17 EE-B3LYP/TPSSh

Y@C72-C2 Y@C72-C2v

0 0.69/1.28

2.39/2.38 2.37/2.36

3.53/5.55 2.95/5.24

a

Unit in kilocalories per mole (kcal/mol). bThe C represents one of the B55 bond atoms, with the unit in Angstroms (Å). cEE stands for encapsulation energy, with the unit in electronvolts (eV).

Figure 2. Relative concentrations of (a) La@C72 and (b) Y@C72 isomers computed with the TPSSh/6-31G* ∼ lanl2dz level using the RRHO approximation at a temperature in the range of 0−3500 K.

Figure 3. (a, b) Experimental and simulated EPR spectra of La@C72 and Y@C72 in CS2 solution at 298 K. (c−f) Spin density distributions of La@ C72-C2, La@C72-C2v, Y@C72-C2, and Y@C72-C2v. All isovalues of spin density distributions are the same value of 0.003.

C2v, La/Y@C72-2−2, La/Y@C72-2−3, and La/Y@C72-2−8 in turn at the B3LYP/6-31G* ∼ (lanl2dz for La and Y) level, whereas the energy differences change slightly at the B3LYP/631G* and TPSSh/6-31G* levels, as shown in Table 1. It is worth noting that the most stable isomers of C72-C2 and C72C2v are with small energy differences of 0.17/0.67 kcal/mol for La@C72 and 0.69/1.28 kcal/mol for Y@C72. In the previous report for La@C72,34 the C2 one is 0.26 kcal/mol more than the C2v one with a small difference at the B3LYP/6-31G* level. In fact, the two isomers show similar characteristics; for example, both of them have one pair of fused pentagons, similar La/Y sites as the optimized structures in Figure 1h−k

which are in good agreement with the simulated isotopic distribution patterns. In addition, the UV−vis absorption spectra were also characterized (Figure 1g), and the results show that there are obvious differences between the two spectra, indicating the different isomeric structures of prepared Y@C72 and La@C72. To elucidate the structures of La@C72 and Y@C72, DFT calculations were employed to select and optimize their isomers. The calculated relative energies of the first ten isomers of C72, C722−, and C723− at the B3LYP/3-21G* level are provided in Table S1. Table S2 shows that the five lowestenergy isomers of La/Y@C72 are La/Y@C72-C2, La/Y@C728164

DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

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Inorganic Chemistry

C72; therefore, the separated out La@C72 is determined as La@C72-C2v, which is different from reported La@C72− C6H3Cl2. For Y@C72, the results are the opposite. The calculated a(Y)iso for Y@C72-C2 and Y@C72-C2v are 0.75/1.31/ 0.95 G and 0.03/0.09/0.01 G (Table 2) at the three levels, respectively. The experimental hyperfine coupling constant of Y value (1.75 G) is closer to the calculated constant of Y@C72C2 but not that of Y@C72-C2v; therefore, the separated out Y@ C72 is determined as Y@C72-C2. Considering the natural paramagnetic property of La/Y@C72 and the sensitive hyperfine coupling splitting, the EPR results can be used as the “fingerprint” of the paramagnetic metallofullerenes. Figure 3c and d shows the spin density distributions of the La@C72 and Y@C72 four isomers. The spin of La@C72-C2 is mainly polarized on the hemisphere near the fused pentagons, whereas the spin of La@C72-C2v is polarized on the opposite hemisphere. Much the same thing was found for Y@C72, in that the spin of Y@C72-C2 is mainly polarized on the hemisphere near the fused pentagons, whereas the spin of Y@C72-C2v is polarized on the opposite hemisphere (Figure 3e, f). These results indicate that the hyperfine coupling values for a(La/Y) of La/Y@C72-C2 are bigger than that of La/Y@C72C2v, which coincides with the calculated hyperfine coupling constant values and experimental EPR results. MO Properties. For La@C72-C2v, Figure 4a displays the orbital interaction diagram of C72-C2v, La@C72-C2v, and La atom. The MOs of the La atom show the valence electrons of 5d16s2 orbits, where the upper two energy levels (green) show the 6s2 orbits (occupy the α and β orbits with the same energy of −4.11 eV) and the lower line shows the 5d1 orbit (occupy the α orbit), for La atom with the energy of −4.23 eV (red). Because the frontier MO energy levels of La are higher than that of the corresponding parent cage C72-C2v (−5.16 eV), the doublet La is prone to transfer electrons to the unoccupied molecular orbitals of the parent C72-C2v cage. Meanwhile, orbital hybridization between the cluster and the cage may result in the reduction of the energy level. Figure 4a clearly shows that metal La formally transfers 3-fold electrons to the cage which is marked by red and green lines from La. For Y@ C72-C2, the orbital interaction diagram of C72-C2, Y@C72-C2, and Y atom is also provided in Figure 4b. The MOs of Y atom show the valence electrons of 4d15s2 orbits, with the lower two (green) levels showing the 5s2 orbits with the similar energy of −4.64 eV (β orbit) and −4.74 eV (α orbit) and the upper energy level showing the 4d1 orbit (occupy the α orbit) with the energy of −4.36 eV (red). The frontier MO energy levels of Y are all higher than the corresponding parent cage C72-C2 (−5.10 eV); therefore, the doublet Y is prone to transfer three electrons to the unoccupied molecular orbitals of the parent C72-C2 cage like the La@C72. Furthermore, the pictures in Figure 4a and b show that the SOMOs of La@C72-C2v and Y@ C72-C2 are consistent with the spin density distributions as shown in Figure 3d and e, respectively. In addition, the surface ESPs (electrostatic potential) of La@C72-C2v and (C72-C2v)3− (Figure 4c) and of Y@C72-C2 and (C72-C2)3− (Figure 4d) show distinctly different distributions and values; although the ion model is widely accepted, the introduction of metal clusters leads to completely different surface electrostatic potentials as shown. In detail, the ESPs of the charged (C72-C2v/C2)3− cage are much more negative than that of EMFs, which is similar to the reported ESPs for C726− and La [email protected] Besides, the fused pentagons (metalcoordinated carbon atoms) are more negative than the isolated

(the bonds of La and the near B55 for La@C72 are 2.64 and 2.59 Å, respectively, as shown in Table 1, which is about 0.22− 0.25 Å longer than those of d(Y−C) for Y@C72 at the same method of B3LYP), and similar encapsulation energies (EEs). Therefore, it is difficult to distinguish the isomers by these indistinguishable indexes. Temperature-Dependent Stability. Relative concentrations (mole fractions) can be evaluated through their partition functions qi and the enthalpies at the absolute zero temperature and ground-state energies △H (i.e., the relative potential energies corrected for the vibrational zero-point energies) by the formula as reference (rotational−vibrational partition functions were using the rigid rotator and harmonic oscillator (RRHO) approximation).35 The relative concentrations (yields) of different isomers depend on temperature since they are formed at a very high temperature. The relative yields of the five lowest-energy isomers are examined to judge the temperature effect (Figure S4). Figure 2a shows the relative concentration of the two lowest-energy La@C72 isomers from 0 to 3500 K. The lowest-energy isomer La@ C72-C2 is prevailing at the temperatures exceeding 284 K, whereas La@C72-C2v has a slight advantage at the temperatures below 284 K. It is clear that La@C72-C2v and La@C72-C2 may coexist throughout the whole temperature range, which is similar to that as previously reported for Ca@C72;35 that is, the La@C72-C2 and La@C72-C2v are with similar thermodynamic stability, and these results also coincide with the reported calculation in ref 34. For Y@C72, the lowest-energy isomer of Y@C72-C2 is prevailing throughout the whole temperature region (Figure 2b), indicating that the C2 cage is more stable than the C2v cage. EPR Analysis. Experimentally, EPR spectroscopy characterizations of produced La@C72 and Y@C72 were performed. The EPR spectrum of La@C72 (Figure 3a) shows eight clear splitting peaks; the isotropic hyperfine coupling constant of a(La)iso is 0.16 G, and the g-factor is 2.0018. Among reported mono La-based metallofullerenes, this a(La)iso value is the smallest one,17 and the g value is similar to those of La@C82 derivatives36 and La@C86−II.37 The EPR spectrum of Y@C72 (Figure 3b) shows two clear splitting peaks; the isotropic hyperfine coupling constant of a(Y)iso is 1.75 G, and the gfactor is 2.0015. Moreover, we calculated the hyperfine coupling constants (the isotropic part, dipolar part, and second-order contribution part from SOC (spin orbit coupling) are considered) at the level of BP86/B3LYP/TPSS ∼ Def-2 with the ORCA program. For La@C72, the calculation results show that a(La)iso for La@C72-C2 and La@C72-C2v are 2.02/2.16/2.04 G and 0.13/0.17/0.09 G (Table 2) at the three levels, respectively. Obviously, the calculated hyperfine coupling constants of La@C72-C2v isomer are consistent with the experimental a(La)iso value (0.16 G) for produced La@ Table 2. DFT Calculated Hyperfine Coupling Constant Values of a(La) and a(Y) by the BP86, B3LYP, and TPSS Methods isomer

a(La)-BP86

a(La)-B3LYP

a(La)-TPSS

La@C72-C2 La@C72-C2v isomer

2.02 G 0.13 G a(Y)-BP86

2.16 G 0.17 G a(Y)-B3LYP

2.04 G 0.09 G a(Y)-TPSS

Y@C72-C2 Y@C72-C2v

0.75 G 0.03 G

1.31 G 0.09 G

0.95 G 0.01 G 8165

DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

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Inorganic Chemistry

Figure 4. (a) Molecular orbital interaction diagrams for C72-C2v, La@C72-C2v, and La. (b) Molecular orbital interaction diagram for C72-C2, Y@C72C2, and Y. The molecular orbitals of empty cages C72-C2v and C72-C2 in (a) and (b) are calculated with the fragments of the corresponding endohedral fullerenes without further optimization. (c) The calculated ESP for the optimized La@C72-C2v and (C72-C2v)3−. (d) The calculated ESP for the optimized Y@C72-C2 and (C72-C2)3−. Detailed values of ESPs are given in the Supporting Information.

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pentagons fragments (IPR fragments) for both the EMFs and the −3 anion cages. In conclusion, the distribution of pentagons determines the relative intensity of the electrostatic potential, while the charge number of the carbon cage determines the absolute intensity of the surface electrostatic potential.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00954.

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CONCLUSION

Two paramagnetic metallofullerenes La@C72 and Y@C72 have been discovered. Although monometal endohedral fullerenes have a high reactivity, the successful isolation and characterizations of La@C72 and Y@C72 reveal their stability even with such a small C72 cage. The two most advantageous isomers of La/Y@C72-C2 and La/Y@C72-C2v are with similar thermodynamics stability via DFT calculations. However, their different EPR coupling constants revealed varied electron spin distributions and structures for these two species. The electron spin in paramagnetic metallofullerenes provides much structural information as the spin-metal couplings are sensitive to the subtle structural differences. The experimental EPR coupling constants were compared with those of calculated coupling constants, and comparison results revealed that the produced La@C72 has a C72-C2v cage and Y@C72 has a C72-C2 cage. The MO analysis further proves the ionic model as (La)3+@(C72)3− and (Y)3+@(C72)3−. Therefore, this study further confirms that the electron spin in metallofullerenes can be used as a probe to identify metallofullerene structures. The successful isolation and characterizations of La@C72 and Y@ C72 reveal their stability, which is important for their application as paramagnetic molecule materials.

Experimental details, HPLC chromatogram, and calculation details (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.W.). *E-mail: [email protected] (C.W.). ORCID

Haibing Meng: 0000-0002-9538-3352 Chunru Wang: 0000-0001-7984-6639 Taishan Wang: 0000-0003-1834-3610 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51672281, 61227902, 51832008). T.W. particularly thanks the Youth Innovation Promotion Association of CAS (2015025).

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DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

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DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168

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DOI: 10.1021/acs.inorgchem.9b00954 Inorg. Chem. 2019, 58, 8162−8168