Functionalization of Endohedral Metallofullerenes toward Improving

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Functionalization of Endohedral Metallofullerenes toward Improving Barrier Height for the Relaxation of Magnetization for Dy2@C80-X (X = CF3, C3N3Ph2) Archana Velloth, Yutaka Imamura, and Masahiko Hada*

Inorg. Chem. Downloaded from pubs.acs.org by UNITED ARAB EMIRATES UNIV on 01/11/19. For personal use only.

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 minami-osawa, Hachioji, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: We theoretically studied the electronic and magnetic properties of the exterior functionalized endohedral metallofullerenes (EMFs) of Gd2@Ih-C80-X (where X is the exterior functional group). Molecular orbital analysis suggests that the presence of unpaired electron on the Ih-C80 cage is not favoring the observation of stable species. One of the effective strategies to address this problem is by attaching an exterior functional group to the fullerene cage. Out of the studied exterior functionalized EMFs, we were successful in finding two stable species such as Gd2@Ih-C80-CF3 and Gd2@ Ih-C80-C3N3Ph2 with no unpaired spin on the cage. Further, we utilized exterior functional groups such as −CF3 (1) and −C3N3Ph2 (2) to model and to stabilize dinuclear Dy2@Ih-C80 species, and we thoroughly investigated their magnetic properties using ab initio calculations. Within the single-ion paradigm, DyIII ions in 1 and 2 are magnetically anisotropic, and their magnetization-reversal energy barriers are estimated to be ∼698 and ∼705 cm−1, respectively. Furthermore, beyond the single-ion paradigm, i.e., considering a ferromagnetic coupling (∼30 cm−1) between the lanthanide ions and the radical spin, the energy barriers of 1 and 2 are estimated to be 79.8 and 73.0 cm−1, respectively.

■

INTRODUCTION Ever since the discovery of the first lanthanide-based single-ion magnet by Ishikawa et al.,1,2 there has been significant progress in the development of lanthanide-based single-molecule magnets (SMMs), which are molecules with bistable magnetic ground states and exhibit slow relaxation of magnetization below their blocking temperature.3−5 These anisotropic magnetic molecules have opened new avenues pertaining to the design of materials and devices for several potential applications.6,7 In fact, the SMM characteristic of a molecule is evaluated by the criteria of how slow the relaxation of magnetization is. Unfortunately, the quantum tunneling of the magnetization (QTM) and weak magnetic coupling are often observed for lanthanide ions and lead to poor SMM characteristics.3 Hence, suppressing the QTM and increasing the magnetic coupling8−10 can result in a large energy barrier (Ueff), which is the most desirable condition for efficient SMMs. In fact, two strategies were proposed to reduce the rate of QTM: (i) preserving a high symmetric environment around lanthanide ions,11−13 and (ii) coupling two or more lanthanide ions with a high spin ground state,5 introducing a radical ion between them,8,14 or coupling lanthanide ions with 3d ions, so that a large exchange coupling could be achieved in such systems.15,16 Although the above strategies are very challenging for normal lanthanide coordination complexes, they are more © XXXX American Chemical Society

feasible when two lanthanides are encapsulated inside the fullerene cage.17−19 The encapsulation of a metal or metal cluster inside fullerene cages, which are known as endohedral metallofullerenes20,21 (EMFs), is an effective approach in the design of efficient SMMs. In particular, lanthanide-encapsulated EMFs have been found to exhibit a slow electron-spin relaxation.22,23 The first EMF-based SMM, DySc2N@C80, was reported by Westerström et al. and exhibited a fast QTM under a zero field.22 Later, its analogous EMFs, such as Dy2ScN@C8024 and Dy2TiC@C80,23 were reported and are illustrative examples for the pronounced SMM behavior. A slow relaxation of magnetization was also observed for the fullerenes with a non-Kramers ions such as HoIII and TbIII; however, the relaxation time was too short compared to the DyIII containing EMFs.25,26 Beyond considering the strategies for SMMs, the stability of the EMFs is needed to be considered as well. Our previous detailed studies on the homonuclear17 and heteronuclear27 M1M2@Ih-C80 dimetallofullerenes revealed that the metal− metal (M−M) bonding molecular orbital (MO) has as spdhybrid character and the energy level of the metal-based MO is very important in designing functional materials. The instability of Ih-C80 arises due to the partial occupation (two Received: September 18, 2018

A

DOI: 10.1021/acs.inorgchem.8b02652 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. DFT computed spin density plot for ground state for Gd2@C80-X. The isodensity shown corresponds to a value of 0.001 e−/bohr3. Red and blue colors denote the positive and negative regions, respectively.

cm−1).28 The isolation of exterior functionalized air-stable derivatives of Dy2@Ih-C80 has confirmed our hypothesis that the pristine form of M2@Ih-C80 (M = Y, Dy) could not be isolated, whereas the anionized or exterior functionalized EMFs are stable. In this paper, we first discussed on finding out the stable exterior functionalized di-EMFs. Later, from the studied stable derivatives of Gd2@Ih-C80-X, we performed ab initio complete active-space self-consistent field (CASSCF) and density functional theory (DFT) calculations to explore the magnetic properties of stable exohedral derivatives of Dy2@Ih-C80, such as Dy2@C80-CF3 (1) and Dy2@C80-C3N3Ph2 (2).

electrons) of the four-fold degenerate highest occupied molecular orbitals (HOMOs). This can be stabilized through the reduction by encapsulating the metal atoms, so that the four-fold degenerate HOMOs of Ih-C80 are completely occupied and that allow Ih-C80 to act as a six-electron acceptor to become a stable species. For instance, in the [LaM]@Ih-C80 (M = La, Ce, Pr), the metal-based MO is the lowest unoccupied molecular orbital (LUMO) and has a formal charge of [LaM]6+@[Ih-C80]6−. On the other hand, for M2@IhC80 (Y, Gd, Lu) or GdM@Ih-C80 (M = Sc, Y, La, Lu), the metal-based orbital is a singly occupied molecular orbital (SOMO), and an additional spin is delocalized over the carbon cage. Hence, the formal oxidation state of the metal atoms is +2.5. Such systems, which have an unpaired spin on the Ih-C80 cage, are prone to the external attack by reactants due to their inherent radical character, and the isolation of these molecules in their pristine form remain elusive. Thus, the chemical transformation of unstable di-EMFs to a stable form should be considered and is possible either by anionization or by an exterior functional groups. When the fullerene cage is stabilized, these di-EMFs are highly stable despite the presence of the metal−metal bonding MO. In our previous reports, we showed that anionization is an effective way to observe stable species for both homo- and heteronuclear di-EMFs.17,27 Moreover, in this paper, we focused to observe such stable species by utilizing the exterior functionalization toward exploring the SMM behavior. Recently, with the same strategy, Popov et al. have performed detailed experimental studies on M2@C80-CH2Ph (M = Y, Dy) EMFs where the Dy analogue shows SMM behavior, with a barrier height of 613 K (∼426

■

COMPUTATIONAL DETAILS

Geometry optimization was performed using the Gaussian 09 program29 using the unrestricted B3LYP functional30,31 with a double-ζ quality basis set employing the Cundari−Stevens32 (CS) relativistic effective core potential for Gd, TZVP33 basis set for Si, F, Cl, and 6-31G(d)34 basis set for all other N, C, and H atoms. The exchange couplings between the paramagnetic centers in Dy2@C80CF3 (1) and Dy2@C80-C3N3Ph2 (2) were estimated using prototypes of Gd by broken symmetry density functional theory (BS-DFT)35 calculations. The exchange parameters computed for Gd2@C80-X (X = CF3, C3N3Ph2) were given as follows: The model complexes were regarded as a three-spin system, M-rad-M, and the corresponding Hamiltonian was defined as

̂ ·SGd(1) ̂ ̂ ·SGd(2) ̂ ̂ ̂ − 2J1B Srad − 2J2 SGd(1) ·SGd(2) Ĥ = − 2J1A Srad

(1)

here, J1A or J1B is the exchange interaction between the Gd1(III) or Gd2(III) ion and the unpaired spin (rad) between them, and J2 is the exchange interaction between two Gd(III) ions, (see the Supporting Information for the BS-DFT calculations). B


Article

Inorganic Chemistry All wave function theory calculations for Dy2@C80-X (X = CF3, C3N3Ph2) were performed using MOLCAS 8.0 code.36 To compute the magnetic anisotropy of a particular Dy(III) ion, we replaced the neighboring Dy(III) ion with an isovalent closed-shell Lu(III) ion of DFT optimized structures. We used the ANO-RCC-VTZP basis set for Dy and Lu atoms, the ANO-RCC-VDZP basis set for F, N, C, and H atoms. The total number of electrons was reduced by unity in order to have a closed-shell electronic configuration because a point charge of −1e was added to the center (where the radical spin exists) of two Dy atoms in order to compensate the overall charge of the system and to simulate the electrostatic crystal field from the removed radicals electron. The ground state f-electron configuration for Dy(III) is 4f9 with 6H15/2 multiplet. First, we generated the guess orbitals and selected seven Dy(III)-based orbitals to perform the CASSCF calculations. In the CASSCF calculations, all 4f orbitals of the magnetic site are included in the active orbitals. Then, we computed 21 sextets using the configuration interaction procedure. In order to check the effects of higher excited states (quartets and doublets) on the magnetic anisotropy and low-lying energy spectrum, we have also performed RASSI calculations by mixing 21 sextets, 108 quartets, and 32 doublets which lie within 40 000 cm−1. The obtained low-lying energy for Dy(III) complexes is similar to that computed with 21 roots only. After this, we performed the RASSI-SO module to compute the spin−orbit coupled states, and utilizing these states, we performed the SINGLE_ANISO code to extract the corresponding g tensors. Then, the g tensors were computed for the eight low-lying Kramers Doublets. The Cholesky decomposition for two electron integrals is employed throughout in the calculations to reduce the disk space. The magnetic properties of the polynuclear complexes were calculated using the POLY_ANISO package.37 The exchange Hamiltonian adapted for Dy2@C80-X (X = CF3, C3N3Ph2) is shown below.

Figure 2. Optimized structures of the stable EMFs Gd2@Ih-C80(CF3) and Gd2@Ih-C80(C3N3Ph2).

3

Ĥ ex = − ∑ Ji . Si. Si + 1 i=1 dip

(2)

exch

(Here Ji = Ji + Ji ; i.e., Ji is the total magnetic interaction of the calculated Jidip and fitted Jiexch parameters. This describes the interaction between all the neighboring metal centers.) Using the SINGLE_ANISO and POLY_ANISO codes, we computed the transverse magnetic moments, between the connecting pairs of opposite doublet states as well as between the ground and the higher doublet states of the same and opposite magnetic moments. The numbers between the corresponding connecting states on each arrow describe the average of the matrix elements corresponding to the stationary points, and those numbers can be calculated by the following formula: ⟨i|μ|j⟩ =

i|μx |j + i|μy |j + i|μz |j 3

(3)

In this equation, μ is the average of the matrix elements of the μx, μy, μz, which is a complex value and μx, μy, and μz are the total angular moments along the main magnetic axis.

Figure 3. DFT computed energy levels for (a) Gd2@C80-CF3, (b) Gd2@C80-C3N3Ph2, and (c) [Gd2@C80]−.

RESULTS AND DISCUSSION Exploring Stable Exterior Functionalized EMFs. Reports about the exterior functionalization of the di-EMFs are relatively limited compared to mono-EMFs.21 To the best of our knowledge, only cycloaddition and radical addition reactions have been observed for the di-EMFs. The cycloaddition reaction on the di-EMFs is the most effective way to generate covalent derivatives of the fullerenes, whereas the radical addition reaction results in the formation of the singly bonded adducts of the di-EMFs. In most of the cases, the paramagnetic EMFs tend to form a closed-shell configuration so that an odd number of singly bonded adducts are always attached to the cage. In the present study, we have modeled

the prototypes based on the experimentally reported exterior functionalized crystal structures, and are given in Table S1. We investigated different reactions, which have been reported for exterior functionalized di-EMFs, and classified them into (i) disilylation (a), (ii) Prato reaction (b), and (iii) carbene addition (c−e) (Figure S1 and Table S2). We also examined complexes f−h, which are obtained from the radical addition reactions of Gd2@Ih-C80. Standard enthalpies calculated by DFT (Table S3) show that these reactions are exothermic; hence, it is expected that these reactions are thermodynamically driven. Spin density analysis (see Figure 1 and Table S4) reveals that complexes a−e and h are unstable di-EMFs because they

■

C


Article

Inorganic Chemistry

Table 1. Energies of Low-Lying KDs (in cm−1), g Tensors of DyIII Ions for 1 and 2 1 KDs

Dy1

KD1 KD2 KD3 KD4 KD5 KD6 KD7 KD8

KD1

KD2

Figure 4. Spin-density distribution for (a) Gd2@C80-CF3, (b) Gd2@ C80-C3N3Ph2, and (c) [Gd2@C80]−.

2 Dy2

Dy1

0.0 0.0 0.0 380.2 421.3 412.6 698.8 715.7 705.9 927.2 892.0 888.5 985.9 982.3 982.5 1023.8 1020.0 1025.5 1068.9 1035.3 1041.3 1104.2 1082.6 1098.2 g tensors for the ground and excited KDs gx gy gz gx gy gz

Dy2 0.0 417.5 705.8 879.0 969.6 1016.9 1036.4 1082.4

Dy1

Dy2

Dy1

Dy2

0.0001 0.0002 19.983 0.0040 0.0044 17.199

0.0001 0.0003 19.989 0.0255 0.0297 17.072

0.0000 0.0001 19.989 0.0183 0.0207 17.108

0.0001 0.0003 19.991 0.0290 0.0332 17.0845

C80(C3N3Ph2). Moreover, Kareev et al. has reported the presence of Gd2@C80-CF3 species by experiment.38 To examine the geometries in more detail, possible isomers of D5h and C2v symmetries for Gd2@C80(CF3) and Gd2@ C80(C3N3Ph2) were examined (see Figure S2). The relative energies are given in Table S5 and indicated that the most stable structure is obtained when metals are incarcerated inside an Ih-C80 cage. Two types of carbon atoms exist in an Ih-C80 cage, leading to two possible configurations of Gd2@C80(CF3) and Gd2@C80(C3N3Ph2). Also, multiple isomers of Gd2@ C80(CF3) and Gd2@C80(C3N3Ph2) are possible due to the different orientations of the metal atoms inside the cage with respect to the exterior functionalized site. DFT calculations reveal that −CF3 and −C3N3Ph2 addition to a pentagon/ hexagon/hexagon junction is energetically more favorable than the addition to the carbon on a hexagon junction by 4.3 kcal/ mol for −CF3 and 3.3 kcal/mol for −C3N3Ph2 (see Figure S3). Furthermore, in order to find the lowest energy structures within the Ih-C80 cage, we considered different orientations of the metals with respect to the exterior functionalized site. There are several isoenergetic positions of the metal atoms within the cage (see Figure S3). The lowest energy configurations of the stable structure for Gd2@C80(CF3)-A, and Gd2@C80(C3N3Ph2)-B are shown in Figure 2. A and B are stable as the unpaired spin is well confined inside the Ih-C80 cage and no spin appears on the cage (see Figures 3 and 4). To gain more detailed insight into the electronic configuration, we analyzed MOs for A and B. As shown in Figure 3, the SOMO of A and B is associated with the metal-based MO as in the case of Gd2@C80 anion radical. Besides, the spin densities on the Gd atoms show that the unpaired electrons were not distributed on the cage but rather localized on the internal19 atoms (Figure 4). The energy level of the metal-based MO of A (−6.12 eV) and B (−5.67 eV) is much lower in energy than that of Gd2@C80 anion radical (−2.46 eV). The lower energy of the metal-based MO for A and B suggests the high kinetic stability. To obtain deeper interpretations of the bonding behavior in A and B, natural bond order39 (NBO) analysis was performed. The detailed electronic configurations of Gd for all the

Figure 5. Optimized Gd analogues of (a) Dy2@C80-CF3 (1) and (b) Dy2@C80-C3N3Ph2 (2) structures. The green arrows are the gzz directions of DyIII ions for the ground state.

have unpaired spin on the cage and can be easily attacked by any external reagents. Complexes a−e are obtained from cycloaddition reactions and suggest that it is not a suitable method to improve the stability of the di-EMFs. Complex h has two radical groups attached to the cage, and molecular analysis for h shows the presence of an unpaired spin on the carbon cage. Hence, complex h is kinetically unstable. Among the studied exterior functionalized di-EMFs, complexes f and g are expected to be stable as no spin appears on the cage. Therefore, quenching an unpaired spin by an organic radical (exterior functionalization) could be a good strategy in order to remove the spin from the cage. This is attributed to the functionalization with a single radical group, which makes a closed-shell structure on the cage, whereas cycloaddition results in the formation of one/two single bonds to the cage and leaves the open-shell structure intact. Therefore, we performed comprehensive studies on the radical addition prototypes of the di-EMFs such as Gd2@C80(CF3), and Gd2@ D


Article

Inorganic Chemistry

Figure 6. Magnetization blocking barrier for (a) Dy1 site in 1, (b) Dy2 site in 1, (c) Dy1 site in 2, and (d) Dy2 site in 2. The thick black line indicates the Kramers doublets (KDs) as a function of the computed magnetic moment. The green/blue arrows show the possible pathway through Orbach/Raman relaxation. The dotted red lines indicate the presence of QTM/TA-QTM between the connecting pairs. The numbers provided at each arrow are the mean absolute values for the corresponding matrix element of the transition magnetic moment. The yellow curve indicates the most likely relaxation pathway.

with the previous reports of Popov et al.42 It reveals that the C atoms involved in the Gd−C bonding belong to the neighboring hexagon of the Ih-C80 cage as shown in Figure S6. In addition, it indicates that there are inconspicuous differences between the Gd-C BCPs in terms of electronic densities and the ratio of absolute value of the potential energy density to the kinetic energy density. For A, Gd1 and Gd2 interact with a six-membered ring of the carbon cage in an η1 and η4 fashion (see Figure S6 and Table S7). However, for B, Gd1 and Gd2 interact with the junction of two-hexagon rings of the carbon cage in a η3 and η5 manner. Eventually, we were successful in finding stable species among the Gd2@C80-X. In fact, chemical functionalization is a good strategy toward attaining stability for the di-EMFs, although the functional group for exterior functionalization should be carefully chosen. Hence, it is expected that the exterior functionalized di-EMFs could extend their application in various fields such as designing SMMs. Since Dy is one of the suitable lanthanide ions, which show SMM properties in most of the complexes, we chose Dy ion for the magnetic studies. In order to estimate the exchange interaction between Dy and the radical, we utilized the isotropic Gd ion rather than the anisotropic Dy system. Thus, we further considered Dy analogues referred to as Dy2@C80(CF3) (1) and Dy2@ C80(C3N3Ph2) (2) for the magnetic behavior and studied their applications toward designing SMMs.

structures are given in Table S6. For A and B structures, Gd atoms possess an electronic configuration of 6s0.224f7.05d0.566p0.27. In the view of the 4f75d16s2 configuration of the neutral Gd atom, the significant decrease in 6s and 5d orbitals in the Gd2 dimers is consistent with the picture that five electrons were detached from the Gd atoms and were transferred to the carbon cage, and one electron is delocalized between the metal atoms and is used for the metal-based MO (with an spd character). Besides, the electron back-donation from the carbon cage is another factor leading to the large electronic populations in the 5d and the 6p orbitals of the Gd atom. In addition, Mayer bond order40 analyses on Gd−C were carried out to evaluate the interactions between Gd and carbon cages (Table S7 and Figure S5). The values of Gd−C bonds in A and B fall into the interval of 0.206−0.292 and 0.203−0.271, respectively, and are in positive correlation with the Gd−C distances. The interaction between the Gd and the cage atoms shows delocalization features. Furthermore, to investigate the bonding nature of Gd and the C cages, the bonding critical point (BCP) indicators based on the quantum theory of atoms in molecules (QTAIM) of A and B were used and examined with the MULTIWFN 3.3.7 program.41 The BCPs referred to the Gd cage are shown in Figure S6, and the corresponding parameters are given in Table S7. Small and the positive electron density and Laplacian values in all the BCPs show similarities to those of the other EMFs.42 Moreover, the BCP analysis is in good agreement E


Article

Inorganic Chemistry

Figure 7. Low-lying exchange spectrum in (a) 1a and (b) 2a. The exchange states are placed on the diagram according to their magnetic moments (black bold lines). The red arrows show the QTM/TA-QTM values. The green/blue arrows show the Orbach/Raman relaxation. The numbers between the corresponding connecting states are the averaged transition moment in μB.

EMFs toward Exploring SMM Behavior. The DFT optimized structures of the investigated di-EMFs 1 and 2, which are dysprosium analogues of Gd2@C80(CF3) (A) and Gd2@C80(C3N3Ph2) (B), are shown in Figure 5. The atomic ground state for DyIII is 6H15/2, that is, eight Kramers doublets (KDs). For 1 and 2, the state with mJ = ± 15/2 was found to be stabilized as the ground state (see Tables S8 and S9). The computed energy spectrum for the eight low-lying KDs are in the range of 1104−1098 cm−1 (see Table 1 and Figure 6). The ab initio calculations show that the ground KD at each DyIII center is strongly axial. This is evident from the very small values of the obtained transversal g tensors (see Tables S8 and S9). In both 1 and 2, the first and the second excited KDs are also Ising in nature, whereas the third excited KD has significant transverse anisotropy. The transition probability within the ground and first excited KDs is small, which suggests that the QTM via the ground KD and TA-QTM (temperature assisted-QTM) via the first excited KD are quenched. This finding in conjunction with the small angle between the ground and the first excited KDs (see Tables S7 and S8) stimulates the relaxation via the second excited state. The Orbach/Raman process related to the ground and the first excited state is found to be small, but within the same spin states (−1 → −2) are found to be large (1.8 μB), suggesting spin relaxation (−1 → −2) as a possible pathway. The computed mechanism suggests the relaxation occurs via the second excited state KD (−1 → −2 → −3 → +3 → +2 → +1)

for all Dy ions in both complexes. Thus, the single-ion relaxation is expected to occur via the second excited state with the energy barrier of 698.8 cm−1 for Dy1 site and 715.7 cm−1 for Dy2 site in 1. Similarly, for the Dy1 and Dy2 sites in 2, the relaxation occurs via the second excited state with the energy barrier of 705.5 and 705.8 cm−1, respectively. To understand the mechanism of magnetic relaxation as a full molecule, we considered the exchange coupling between the paramagnetic centers. On the basis of the hypothetical BSDFT calculations and the conventional approach,15,43 the DyIII···radical exchanges for 1 and 2 were estimated to be +152.1 cm−1 and +142.9 cm−1 and the DyIII···DyIII interactions were found to be −0.9 cm−1 and −1.3 cm−1, respectively. The predicted exchange coupling for [Gd3+-e-Gd3+] fullerenes in the present work is in agreement with the experimental estimations.44,45 However, on the basis of the experimental data from Popov et al.,28 for a Dy2@C80-CH2Ph EMF, the Dy···radical interaction was experimentally reported to be +32 cm−1, whereas a J value of [Gd3+-e-Gd3+] obtained by BS-DFT was +181−184 cm−1.28 It indicates that the experimentally determined J value is 5−6 times less than the calculated value for these type of di-EMFs. Keeping this in mind, we also used 5 times less Dy···radical J value (+30.4 cm−1 for 1 and +28.6 cm−1 for 2) than that of the BS-DFT obtained J value in order to obtain the exchange coupled states mechanism using POLY_ANISO program (Figure 7). Previous computational studies have predicted the relaxation of magnetization via the F


Inorganic Chemistry

■

first exchange-excited state in [Tb-N23−Tb] with the barrier height of 207.0 cm−146 and Dy2@C79N with the barrier height of 531.0 cm−1.18 Chibotaru et al.46 have developed a more reliable model of the exchange interactions, which showed that the mixing of the crystal field (CF) states increases the probability of the direct transition from the ground state to the first excited state. On the basis of these findings, we conclude that the magnetic relaxation occurs via the low exchange doublets, resulting in the energy barrier as 79.8 and 73.0 cm−1 for 1 and 2, respectively, which are less than those for the single Dy(III) ions without the radical counterpart. We also examined the exchange coupled state mechanism, considering isotropic exchange, i.e., using the exact J values of BS-DFT, as shown in Figure S7 in the Supporting Information. The strong magnetic coupling based on the BS-DFT (J = +152.1 cm−1 and +142.9 cm−1 for 1 and 2, respectively) suggests that the presence of an unpaired spin in between the metal atoms causes the strong ferromagnetic coupling between them, and the mixing of the CF states leads to the large magnetic moment and the observation of high relaxation barrier of magnetization in the range of 351 and 325.6 cm−1 for 1 and 2, respectively. Therefore, the exchange coupling between metals affects the relaxation mechanism and should be designed so as to suppress magnetic relaxation.

■

CONCLUSIONS

■

ASSOCIATED CONTENT

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yutaka Imamura: 0000-0002-9527-6813 Masahiko Hada: 0000-0003-2752-2442 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.V. is grateful to Prof. Takeshi Kodama, Tokyo Metropolitan University, Japan, for the useful discussions and Dr. Kuduva R. Vignesh, Texas A&M University, USA, for the help received with respect to POLY_ANISO analysis and insightful discussions. A.V. is also thankful to the Asian Human Resource Fund for the scholarship provided. We would like to thank the Research Center for Computational Science, Okazaki, Japan, for providing the computing resources. This study was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination Asymmetry” from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP17H05380.

■

Toward the observation and isolation of the stable di-EMFs, we theoretically explored the introduction of various exohedral functional groups to the Ih-C80 cage of Gd2@C80-X di-EMFs. We were successful in finding the stable derivatives of the diEMFs such as Gd2@C80-CF3 and Gd2@C80-C3N3Ph2 so as to achieve the disappearance of the radical spin on the cage and the confinement of the unpaired electron inside the cage. Exterior functionalization is in fact a good strategy toward the observation and isolation of the stable di-EMFs, although the choice of the functional group is crucial. Further, we utilized exterior functional groups such as −CF3 and −C3N3Ph2 for Dy2@C80 and performed ab initio calculations on stable di-EMFs of Dy2@C80-CF3 and Dy2@ C80-C3N3Ph2 to understand the mechanism of magnetic relaxation. The detailed magnetic studies on Dy2@C80-CF3 and Dy2@C80-C3N3Ph2 suggest that the presence of an unpaired spin in between the metal atoms is crucial for the observation of a relaxation barrier for magnetization reversal and paves a way for the new generation SMMs.

REFERENCES

(1) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.-Y.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (2) Tanaka, D.; Inose, T.; Tanaka, H.; Lee, S.; Ishikawa, N.; Ogawa, T. Proton-Induced Switching of the Single Molecule Magnetic Properties of a Porphyrin Based Tbiii Double-Decker Complex. Chem. Commun. 2012, 48, 7796−7798. (3) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013, 113, 5110−5148. (4) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic Bistability in a Metal-Ion Cluster. Nature 1993, 365, 141. (5) Habib, F.; Murugesu, M. Lessons Learned from Dinuclear Lanthanide Nano-Magnets. Chem. Soc. Rev. 2013, 42, 3278−3288. (6) Parks, J. J.; Champagne, A. R.; Costi, T. A.; Shum, W. W.; Pasupathy, A. N.; Neuscamman, E.; Flores-Torres, S.; Cornaglia, P. S.; Aligia, A. A.; Balseiro, C. A.; et al. Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect. Science 2010, 328, 1370. (7) Leuenberger, M. N.; Loss, D. Quantum Computing in Molecular Magnets. Nature 2001, 410, 789. (8) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Strong Exchange and Magnetic Blocking in N23−-Radical-Bridged Lanthanide Complexes. Nat. Chem. 2011, 3, 538. (9) Long, J.; Habib, F.; Lin, P.-H.; Korobkov, I.; Enright, G.; Ungur, L.; Wernsdorfer, W.; Chibotaru, L. F.; Murugesu, M. Single-Molecule Magnet Behavior for an Antiferromagnetically SuperexchangeCoupled Dinuclear Dysprosium(III) Complex. J. Am. Chem. Soc. 2011, 133, 5319−5328. (10) Le Roy, J. J.; Ungur, L.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. Coupling Strategies to Enhance Single-Molecule Magnet Properties of Erbium−Cyclooctatetraenyl Complexes. J. Am. Chem. Soc. 2014, 136, 8003−8010. (11) Rinehart, J. D.; Long, J. R. Exploiting Single-Ion Anisotropy in the Design of F-Element Single-Molecule Magnets. Chem. Sci. 2011, 2, 2078−2085. (12) Gupta, S. K.; Rajeshkumar, T.; Rajaraman, G.; Murugavel, R. An Air-Stable Dy(III) Single-Ion Magnet with High Anisotropy Barrier and Blocking Temperature. Chem. Sci. 2016, 7, 5181−5191. (13) Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z. On Approaching the Limit of Molecular Magnetic Anisotropy: A

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02652. Computational details. Tables of experimentally reported exterior functionalized adducts, DFT computed versus expected spin contamination values, total energies, Mulliken spin population, relative energies, natural population analysis, interatomic distance, Mayer bond order, BCP parameters, energies and g tensors, lowest exchange doublets. Figures of optimized structures for the exterior functionalized EMFs, DFToptimized molecular structure of configurations, molecular orbitals, low-lying exchange spectrum (PDF) G


Article

Inorganic Chemistry

B.; Petersson, G. A.; et al. Gaussian 09, Revision D.1; Gaussian, Inc.: Wallingford, CT, 2009. (30) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785− 789. (31) Becke, A. D. Density-Functional Thermochemistry. Iii. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (32) Cundari, T. R.; Stevens, W. J. Effective Core Potential Methods for the Lanthanides. J. Chem. Phys. 1993, 98, 5555−5565. (33) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (34) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. chim. acta. 1973, 28, 213−222. (35) Noodleman, L. Valence Bond Description of Antiferromagnetic Coupling in Transition Metal Dimers. J. Chem. Phys. 1981, 74, 5737− 5743. (36) Aquilante, F.; De Vico, L.; Ferré, N.; Ghigo, G.; Malmqvist, P.å.; Neogrády, P.; Pedersen, T. B.; Pitoňaḱ , M.; Reiher, M.; Roos, B. O.; et al. MOLCAS 7: The Next Generation. J. Comput. Chem. 2010, 31, 224−247. (37) Chibotaru, L. F.; Ungur, L. F. Program Poly_Aniso; University of Leuven: Leuven, Belgium, 2008. (38) Kareev, I. E.; Bubnov, V. P.; Yagubskii, E. B. Endohedral Gadolinium-Containing Metallofullerenes in the Trifluoromethylation Reaction. Russ. Chem. Bull. 2008, 57, 1486−1491. (39) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural Population Analysis. J. Chem. Phys. 1985, 83, 735−746. (40) Bridgeman, A. J.; Cavigliasso, G.; Ireland, L. R.; Rothery, J. The Mayer Bond Order as a Tool in Inorganic Chemistry. J. Chem. Soc., Dalton Trans. 2001, 2095−2108. (41) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (42) Popov, A. A.; Dunsch, L. Bonding in Endohedral Metallofullerenes as Studied by Quantum Theory of Atoms in Molecules. Chem. - Eur. J. 2009, 15, 9707−9729. (43) Vieru, V.; Ungur, L.; Chibotaru, L. F. Key Role of Frustration in Suppression of Magnetization Blocking in Single-Molecule Magnets. J. Phys. Chem. Lett. 2013, 4, 3565−3569. (44) Velkos, G.; Krylov, D. S.; Kirkpatrick, K.; Liu, X.; Spree, L.; Wolter, A. U. B.; Büchner, B.; Dorn, H. C.; Popov, A. A. Giant Exchange Coupling and Field-Induced Slow Relaxation of Magnetization in Gd2@C79N with a Single-Electron Gd−Gd Bond. Chem. Commun. 2018, 54, 2902−2905. (45) Hu, Z.; Dong, B.-W.; Liu, Z.; Liu, J.-J.; Su, J.; Yu, C.; Xiong, J.; Shi, D.-E.; Wang, Y.; Wang, B.-W.; et al. Endohedral Metallofullerene as Molecular High Spin Qubit: Diverse Rabi Cycles in Gd2@C79N. J. Am. Chem. Soc. 2018, 140, 1123−1130. (46) Vieru, V.; Iwahara, N.; Ungur, L.; Chibotaru, L. F. Giant Exchange Interaction in Mixed Lanthanides. Sci. Rep. 2016, 6, 24046.

near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2016, 55, 16071−16074. (14) Rajeshkumar, T.; Rajaraman, G. Is a Radical Bridge a Route to Strong Exchange Interactions in Lanthanide Complexes? A Computational Examination. Chem. Commun. 2012, 48, 7856−7858. (15) Vignesh, K. R.; Langley, S. K.; Murray, K. S.; Rajaraman, G. Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {Tm4IIIDy4III} (Tm = Co and Cr) SingleMolecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling. Chem. - Eur. J. 2017, 23, 1654−1666. (16) Langley, S. K.; Wielechowski, D. P.; Vieru, V.; Chilton, N. F.; Moubaraki, B.; Abrahams, B. F.; Chibotaru, L. F.; Murray, K. S. A {Cr2IIIDy2III} Single-Molecule Magnet: Enhancing the Blocking Temperature through 3d Magnetic Exchange. Angew. Chem., Int. Ed. 2013, 52, 12014−12019. (17) Velloth, A.; Imamura, Y.; Kodama, T.; Hada, M. Theoretical Insights into the Electronic Structures and Stability of Dimetallofullerenes M2@Ih-C80. J. Phys. Chem. C 2017, 121, 18169−18177. (18) Singh, M. K.; Yadav, N.; Rajaraman, G. Record High Magnetic Exchange and Magnetization Blockade in Ln2@C79N (Ln = Gd(III) and Dy(III)) Molecules: A Theoretical Perspective. Chem. Commun. 2015, 51, 17732−17735. (19) Fu, W.; Zhang, J.; Fuhrer, T.; Champion, H.; Furukawa, K.; Kato, T.; Mahaney, J. E.; Burke, B. G.; Williams, K. A.; Walker, K.; et al. Gd2@C79N: Isolation, Characterization, and Monoadduct Formation of a Very Stable Heterofullerene with a Magnetic Spin State of S = 15/2. J. Am. Chem. Soc. 2011, 133, 9741−9750. (20) Popov, A. A.; Yang, S.; Dunsch, L. Endohedral Fullerenes. Chem. Rev. 2013, 113, 5989−6113. (21) Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current Status and Future Developments of Endohedral Metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723−7760. (22) Westerström, R.; Dreiser, J.; Piamonteze, C.; Muntwiler, M.; Weyeneth, S.; Brune, H.; Rusponi, S.; Nolting, F.; Popov, A.; Yang, S.; et al. An Endohedral Single-Molecule Magnet with Long Relaxation Times: DySc2N@C80. J. Am. Chem. Soc. 2012, 134, 9840−9843. (23) Junghans, K.; Schlesier, C.; Kostanyan, A.; Samoylova, N. A.; Deng, Q.; Rosenkranz, M.; Schiemenz, S.; Westerström, R.; Greber, T.; Büchner, B.; et al. Methane as a Selectivity Booster in the ArcDischarge Synthesis of Endohedral Fullerenes: Selective Synthesis of the Single-Molecule Magnet Dy2TiC@C80 and Its Congener Dy2TiC2@C80. Angew. Chem., Int. Ed. 2015, 54, 13411−13415. (24) Krylov, D. S.; Liu, F.; Avdoshenko, S. M.; Spree, L.; Weise, B.; Waske, A.; Wolter, A. U. B.; Buchner, B.; Popov, A. A. Record-High Thermal Barrier of the Relaxation of Magnetization in the Nitride Clusterfullerene Dy2ScN@C80-Ih. Chem. Commun. 2017, 53, 7901− 7904. (25) Liu, F.; Gao, C.-L.; Deng, Q.; Zhu, X.; Kostanyan, A.; Westerström, R.; Wang, S.; Tan, Y.-Z.; Tao, J.; Xie, S.-Y.; et al. Triangular Monometallic Cyanide Cluster Entrapped in Carbon Cage with Geometry-Dependent Molecular Magnetism. J. Am. Chem. Soc. 2016, 138, 14764−14771. (26) Dreiser, J.; Westerström, R.; Zhang, Y.; Popov, A. A.; Dunsch, L.; Krämer, K.; Liu, S.-X.; Decurtins, S.; Greber, T. The Metallofullerene Field-Induced Single-Ion Magnet HoSc2N@C80. Chem. Eur. J. 2014, 20, 13536−13540. (27) Velloth, A.; Imamura, Y.; Kodama, T.; Hada, M. Quantum Chemical Study on Endohedral Heteronuclear Dimetallofullerene M1M2@Ih-C80 toward Molecular Design. J. Phys. Chem. C 2017, 121, 27700−27708. (28) Liu, F.; Krylov, D. S.; Spree, L.; Avdoshenko, S. M.; Samoylova, N. A.; Rosenkranz, M.; Kostanyan, A.; Greber, T.; Wolter, A. U. B.; Büchner, B.; et al. Single Molecule Magnet with an Unpaired Electron Trapped between Two Lanthanide Ions inside a Fullerene. Nat. Commun. 2017, 8, 16098. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, H


Functionalization of Endohedral Metallofullerenes toward Improving

Recommend Documents