Anisotropic Paramagnetic Properties of Metallofullerene Confined in a

Feb 12, 2018 - In addition, it is also a challenge to manipulate the paramagnetic properties of metallofullerene due to its wrapped spin state. The N-...
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Article Cite This: J. Phys. Chem. C 2018, 122, 4635−4640

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Anisotropic Paramagnetic Properties of Metallofullerene Confined in a Metal−Organic Framework Chong Zhao,†,‡ Haibing Meng,†,‡ Mingzhe Nie,†,‡ Li Jiang,† 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, Zhongguancun North First Street 2, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Paramagnetic metallofullerenes have a spherical molecular structure and stable unpaired spin protected by a fullerene cage and have potential applications in quantum information processing, etc. For endohedral azafullerene Y2@C79N, the N atom on the cage endows the molecule with unpaired spin and is proposed to induce the molecule orientation within the MOF pore via host−guest interaction. Angular-dependent EPR spectroscopy was employed on the Y2@C79N⊂MOF-177 complex to detect the anisotropic paramagnetic properties. These results experimentally confirmed the trend of molecule orientation of Y2@C79N within MOF-177 under certain conditions, which also induces the presence of several conformers of Y2@C79N. The additional EPR splittings in Y2@C79N⊂MOF-177 are proposed to originate from partially disordered molecules as well as the N-coupling of Y2@C79N upon implanting it into the MOF-177 pore. The Ndefect on the fullerene cage of Y2@C79N is expected to be utilized as an anchor to build a three-dimensional spin array within MOF matrix.

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INTRODUCTION Metal−organic frameworks (MOFs) are excellent materials as hosts to disperse paramagnetic molecules such as transition metal ions (Cu2+, Co2+, Mn2+),1−5 organic radicals,6−8 doped defects,9 etc. Electron paramagnetic resonance (EPR) spectroscopy is a significant method to characterize such paramagnetic guests in MOFs. The main purpose of creating such guest−host systems is to build a three-dimensional spin array that is potentially applicable in the areas such as quantum qubits and storage devices. Paramagnetic metallofullerenes, including Y2@C79N,10−12 Sc3C2@C80,13−17 M@C82 (M = La,18−20 Sc,21 Y22), and N@ C60,23−28 etc., have been expected to be ideal candidates as qubits for quantum computers in previous reports due to their sensitive electron spin. For an important paramagnetic metallofullerene Y2@C79N, its unpaired spin located on the internal Y2 cluster can respond to the temperature and other environmental conditions.12 More importantly, the N-defect on the fullerene cage of Y2@C79N could greatly change the molecule symmetry, influence the internal dynamics, and polarize the unpaired spin. Considering the porous MOF and nanoscale metallofullerene, there is a possibility to construct a host−guest system between them through a bottom-up approach. In addition, it is also a challenge to manipulate the paramagnetic properties of metallofullerene due to its wrapped spin state. The N-defect on the fullerene cage of Y2@C79N gives us a clue that this N-defect can be utilized as an anchor to build a three-dimensional spin array within the MOF matrix. Herein, angular-dependent EPR spectroscopy was employed on the Y2@C79N⊂MOF-177 © 2018 American Chemical Society

complex to detect the orientation-related paramagnetic properties. These results experimentally confirmed the trend of Y2@ C79N orientation within the MOF under certain conditions. EPR results were further simulated and calculated via DFT to describe the orientation-related paramagnetic parameters.

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EXPERIMENTAL METHODS Sample Preparation and Characterization. Y2@C79N was synthesized by the traditional arc-discharging method.29 Y2@C79N was isolated and purified by multistep high performance liquid chromatography (HPLC), and the purity of Y 2 @C 79 N was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Figure S1 shows the HPLC and MALDI-TOF-MS profiles of the purified Y2@C79N sample. The crystal of MOF-177 was synthesized by a solvothermal method as previously reported.30−33 That is, 0.5 g of Zn(NO3)2·6H2O and 40 mg of 4′,4″,4‴-benzene-1,3,5triyltribenzoic acid were dissolved in 10 mL of N,Ndiethylformamide (DEF), heated at a rate of 2 °C/min to 85 °C and held for 50 h, and cooled at a rate of 0.2 °C/min to room temperature. Y2@C79N⊂MOF-177 was prepared by an absorption method. The as-prepared MOF-177 crystals were first washed by toluene three times. Then, 20 mg of MOF-177 was immersed into the as-prepared Y2@C79N solution for several Received: November 17, 2017 Revised: February 8, 2018 Published: February 12, 2018 4635

DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640

Article

The Journal of Physical Chemistry C

Figure 1. (a) Calculated unpaired spin density distribution of Y2@C79N. (b) Schematic diagrams of the random spins of Y2@C79N in solution and aligned spins in the lattice of MOF-177 single crystal. (c) PXRD of MOF-177 from the parallelogram plane and the X-diffraction related to the (002) direction, the distance of which is 15.5 Å. (d) Schematic diagram of angular dependence of EPR measurement on Y2@C79N⊂MOF-177 single crystal.

Figure 2. Angular-dependent EPR spectra for Y2@C79N⊂MOF-177 complex at temperatures of (a) 293, (b) 253, and (c) 233 K.

equivalent 89Y nuclei, which has 100% natural abundance with a nuclear spin of 1/2. It should be noted that the Y2@C79N molecule orientation in solution is totally disordered, which cannot be utilized in quantum computing or any other systems. Therefore, the arrangement and orientation for Y2@C79N molecules are essential. The metal−organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn4O(−COO)6 and triangular planar aromatic 1′,3″,5‴-benzenetribenzoate (BTB) units. In the MOF-177 material, the Y2@C79N molecules have been characterized to be dispersed in the cage-shaped pores, whose shortest distance is about 13.99 Å between two sets of parallel quasi-planar triphenylbenzene units. Previous calculation results revealed that the “doped” N atom in Y2@C79N is favorably adjacent to the central benzene ring of 1′,3″,5‴-tris(4-carboxyphenyl) benzene.12 Therefore, in order to realize the directional arrangement of Y2@C79N molecules in MOF-177, the single crystal of MOF-177 should be a good support to realize the aligned spin as seen in Figure 1b.

days to let the Y2@C79N enter the pores of MOF-177 completely. Calculations. Y2@C79N and Y2@C79N⊂MOF-177 were first optimized using original pm6 and b3lyp/3-21g* to speed up the computational process using the Gaussian 09 quantum chemical program package,34 which was then accomplished by using the Dmol3 code35,36 with the generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof (PBE) using the materials studio 7.0 software. The computations of hfcc constants by the ORCA package37 were performed with the open-shell method of UKS at the BP86/ TZVP level using RI approximation.

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RESULTS AND DISCUSSION Y2@C79N is a spin-active metallofullerene with high stability due to its protected unpaired spin (see Figure 1a). The nitrogen atom resides at a 665 ring junction on the cage of Y2@ C79N and leads to an unpaired electron located on the internal Y2 moiety. The CW-EPR of Y2@C79N displays three symmetric lines with a 1:2:1 intensity ratio in toluene/CS2 due to the 4636

DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640

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The Journal of Physical Chemistry C

C79N⊂MOF-177 complex, it can be concluded that the varied splitting peaks at different rotation angles are ascribed to paramagnetic anisotropy for Y and N nuclei around the crystallographic axes of the sample. Calculation results revealed that the “doped” N atom in Y2@C79N is favorably adjacent to the central benzene ring of 1′,3″,5‴-tris(4-carboxyphenyl) benzene of MOF-177. Especially at low temperature, the Y2@ C79N molecules in MOF-177 do not rotate freely and tend to orientate. The directional arrangement of Y2@C79N molecules in MOF-177 crystal realize aligned spin as seen in Figure 1b. The angular-dependent EPR properties for the Y2@ C79N⊂MOF-177 complex are similar to those of previous single-crystal EPR studies dating back to the 1960s with a variation of crystal orientations.40 Angular dependence of EPR spectra of single crystals allow the determination of tensor parameters and their relative orientations. The complete spin Hamiltonian operator should include the hyperfine interaction term and the Zeeman components of the electron and nucleus as below.

First, we cultured the single crystal of MOF-177, and the PXRD data of the parallelogram surface shows that when 2θ = 5.7°, the distance is 15.5 Å, corresponding to the (002) plane (see Figure 1c). This result agrees well with the lattice data of MOF-177((002) = 15.1 Å).12,33 The as-prepared MOF-177 single crystals were then immersed into the Y2@C79N solution for 7 days to let Y2@C79N molecules enter the pores of the MOF-177 single crystal. The angular-dependent EPR measurements on the Y2@C79N⊂MOF-177 complex were subsequently performed, as illustrated in Figure 1d. In the rotating process, the (002) plane is parallel to the magnetic field direction, the rotation axis was chosen to be parallel to the crystallographic axes a and b of the hexagonal crystal, and the axis c* is oriented perpendicular to both a and b in a right-hand rule. The details of angular-dependent EPR measurements are described in the Supporting Information. At room temperature, Y2@C79N in toluene presents three peaks with a 1:2:1 intensity ratio in the EPR spectrum corresponding to two equivalent Y nuclei (Figure S4). However, the EPR spectrum of the Y2@C79N⊂MOF-177 complex at 293 K shows significant difference, in which the EPR spectrum becomes unsymmetrical, and some new hyperfine splittings emerge in the main peak (see Figures 2a and S4). Furthermore, more different characteristics of paramagnetic anisotropy were displayed under temperatures of 253 K (see Figure 2b), and there are more additional EPR peaks. As the magnetic nuclei of this molecule only contain two Y atoms and one N atom, we proposed that the additional EPR splittings in Y2@C79N⊂MOF-177 are caused by partially disordered molecules as well as the N-splitting of Y2@C79N upon implanting it into the MOF-177 pore. Briefly, the partially disordered Y2@C79N in MOF-177 may result in the anisotropic g-tensors. In addition, the strong host−guest interaction between Y2@C79N and MOF-177 and restricted molecule rotation increase the anisotropy of N-coupling that also can influence the EPR signal. Moreover, the EPR peaks’ shape and the splittings change constantly upon rotating the Y2@ C79N⊂MOF-177 complex. The angular-dependent EPR signals for the Y2@C79N⊂MOF-177 complex clearly reveals its paramagnetic anisotropy, especially at 253 and 233 K (Figure 2b,c), revealing the temperature-dependent EPR anisotropy. All of the EPR spectra in the rotation process at 253 K are listed in Figure S5. Still, the experimental EPR spectra were simulated with the Easyspin package encoded in the MATLAB platform.38,39 The axisymmetric hyperfine coupling constants of a(Y) and a(N) are shown in Figure 3, and the simulated EPR spectra also reveal the possibility of N-splittings in Y2@C79N⊂MOF-177. From the angular-dependent EPR spectra for the Y2@

∧

∧

∧

∧

∧

H = gβeH ·S + S ·A ·I − gnβnH ·I

(1)

among which the hyperfine parameters should include a (3 × 3) matrix, and the A includes the isotropic A0 and the traceless, anisotropic part of the T matrix, The traceless tensor describes the hyperfine interaction energy (E) upon the classical interaction of the electronic and nuclear dipoles as below. E MS , MI =

gβγMI MS 2

1 − 3cos2α r3

(1 − 3cos2 θ) av

(2)

where r is the distance from the nucleus to the electron, α is the angle between the line r and principal axis of the tensor, and θ is the angle between H0 and the principal axis. Obviously, the E will zero for the s-electrons but nonzero if the unpaired electron has p- or d- character. From the angular-dependent EPR spectra for the Y2@C79N⊂MOF-177 complex, it can be concluded that the varied EPR peaks at different rotation angles are ascribed to paramagnetic anisotropy for Y and N nuclei around the crystallographic axes of the sample. The three main peaks represent the Y-couplings, and there is no significant change for the magnetic position and symmetry, indicating that the unpaired electron is mainly distributed in the 5s- orbital, and the paramagnetic anisotropy is weak for Y2 nuclei. The splittings in the main peaks are ascribed to the N-couplings, and moreover, the splitting number and shape are varying along the rotation of the crystal, indicating that the unpaired electron is mainly located in the 2p-orbital according to the discussion about eq 2. The hyperfine couplings contain three main contributions corresponding to the hyperfine interaction term (the second term in eq 1): (a) the Fermi contact term that arises from the finite electron spin density on the nucleus for s-electrons; (b) the spin dipole−dipole part that arises from the dipole interaction between the magnetic moment of the electron and the magnetic nucleus; (c) the spin−orbit coupling part that is the second order contribution compared to the above two.41−43 The isotropic coupling mainly comes from the Fermi constant, whereas the anisotropic coupling arises from the dipole−dipole interaction, when in the rapid rolling of magnetic particles with a low viscosity liquid, the anisotropic dipole− dipole interaction tends to be averaged, just like the Y2@C79N molecule in toluene. However, in a rigid ordered system, the

Figure 3. Experimental and simulated EPR spectra of Y2@ C79N⊂MOF-177 complex at 75°. 4637

DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640

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The Journal of Physical Chemistry C Table 1. DFT-Computed a(Y), a(N), and g-Values in Y2@C79N and Y2@C79N⊂MOF-177a Y2@C79N/mT Y1 Y2 N g

a(FC)isoc a(SD)xx,yy,zzd a(FC)iso a(SD)xx,yy,zz a(FC)iso a(SD)xx,yy,zz gxx,yy,zz

0.43 0.43 −0.046 1.958

−6.43 0.39 −6.43 0.39 0.314 −0.046 1.963

Euler rotationb

Y2@C79N⊂MOF-177/mT −0.82

0.39

−0.82

0.39

0.093 2.000

−0.064 1.959

−6.61 0.39 −6.36 0.39 0.329 −0.064 1.963

−24.9°

4.6°

−70.3°

−3.9°

87.3°

−15.1°

−1.9°

18.8°

23.2°

−0.82 −0.82 0.107 2.001

a

Computations were performed with the open-shell method of UKS at the BP86/TZVP level using RI approximation. bEuler rotation coordinate system is relative to the g-tensor, and the order of Euler rotation is by alpha around Z(first rotation), beta around Y′(second rotation), gamma around Z″(third rotation). ca(FC)iso refers to the coupling contribution from isotropic Fermi contact. da(SD) means the coupling contribution from anisotropic spin dipole−dipole interaction.

Figure 4. Optimized structures of (a) Y2@C79N and (b,c) Y2@C79N⊂MOF-177. The marked distance is in Å.

directions in the unit cell, such as a symmetry axis, and we have not found such a special angle in our angular paramagnetic spectrum except the collinear position at 180°; that is because in the ab plane, the guest Y2@C79N has broken the symmetry of the MOF crystal. Further calculation shows that the π−π interaction between Y2@C79N and MOF-177 effectively changes the distance of the Y2 cluster from 3.876 Å (for Y2@C79N) to 3.950 Å (for Y2@ C79N⊂MOF-177) (see Figure 4). The “doped” N atom is 3.587 Å far from the nearest C atom in the central benzene of H3BTB (4′,4″,4‴-benzene-1,3,5-triyl-tribenzoic acid), a very comfortable and stable distance for the fullerene based host−guest complex as reported previously (3.5−3.8 Å).44,45 We also analyzed the curve map of the DOS (density of states) about the Y2@C79N (Figure S7). We divided Y2@C79N into two parts, Y2 and C79N. According to the partial population density of states, the orbits below the HOMO are almost originated from the C79N cage. For the orbits of LUMO + n, the orbital contribution ratio of Y2 gradually occupies a large part. That is to say, for the guest molecule of Y2@C79N in the MOF-177, the surrounding crystal field could affect the electronic structure of Y2 cluster as well as the unpaired spin. It may be a reason to answer why the EPR spectrum of Y2@C79N is sensitive to the framework of MOF-177.

dipole−dipole interaction and the spin−orbit coupling strongly depend on the direction of the magnetic field. The above contributions are described in detail in the Supporting Information based on the spin density matrix. In order to assist in explaining anisotropic paramagnetic properties of the Y2@C79N⊂MOF-177 complex, we performed theoretical studies on the EPR anisotropy contributions by ORCA. Table 1 lists the Fermi constant contribution and spin dipole−dipole contribution about the Y and N nuclei of Y2@ C79N and Y2@C79N⊂MOF-177 models. For the Y atoms, the two a(FC)iso values vary from 6.43 mT (for Y2@C79N) to 6.61 and 6.36 mT (for Y2@C79N⊂MOF-177). Conversely, the a(SD) values are nearly the same for Y2@C79N and Y2@ C79N⊂MOF-177, and the values are much smaller (0.39 to 0.82 mT) than the hfcc of a(FC)iso (6.36 to 6.43 mT). It is disclosed that the coupling of Y2@C79N molecules in MOF-177 mainly originates from the isotropic Fermi contribution rather than the spin dipole−dipole interaction. For the spin−orbit coupling in Y2@C79N⊂MOF-177, the hfcc values of Y and N nuclei are all less than 0.01 mT, much smaller than the a(FC) and a(SD). It is concluded that the anisotropy of Y2@C79N⊂MOF-177 mainly arises from the spin dipole−dipole interaction but not the spin−orbit coupling. Indeed, the DFT computed directional tensors of a(Y1)xx,yy,zz and a(Y2)xx,yy,zz are close to coaxial differing by about 5.8° (the angle value is obtained according to the Euler rotation in Table 1, Figure S6 and Table S1). Furthermore, the angle between the z axes (gzz and a(Y1,Y2)zz) of the g and a tensor is about 42.2°, indicating that both tensors are totally noncollinear and that is also a reason leading to the complex splitting in our EPR spectra. In general, the spectra from the different sites will not superimpose unless the magnetic field of the spectrometer is parallel to some special

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CONCLUSION In conclusion, a host−guest system between Y2@C79N and MOF with controllable paramagnetic properties has been realized through a bottom-up approach. The angular-dependent EPR spectroscopy was employed on a Y2@C79N⊂MOF-177 complex to detect the orientation-related paramagnetic properties. These results experimentally confirmed the trend of Y2@ 4638

DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640

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C79N orientation within the MOF under certain conditions. Furthermore, we proposed that the orientation-related EPR splittings are caused by partially oriented molecules as well as the N-splitting of Y2@C79N upon implanting it into MOF-177 pore. The N-defect on the fullerene cage of Y2@C79N is expected to be utilized as an anchor to build arrayed spin within the MOF matrix.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11353. Experimental method, HPLC data, MALDI-TOF-MS data, EPR spectrum, and calculation results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

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 (61227902, 51672281, 51472248) and the Beijing Natural Science Foundation (2162050). T.W. particularly thanks the Youth Innovation Promotion Association of CAS (2015025).

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DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640

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DOI: 10.1021/acs.jpcc.7b11353 J. Phys. Chem. C 2018, 122, 4635−4640