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Quantum Chemical Study on Endohedral Heteronuclear Dimetallofullerene MM@I-C Toward Molecular Design 1

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Archana Velloth, Yutaka Imamura, Takeshi Kodama, and Masahiko Hada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08302 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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The Journal of Physical Chemistry

Quantum Chemical Study on Endohedral Heteronuclear Dimetallofullerene M1M2@Ih-C80 toward Molecular Design Archana Velloth, Yutaka Imamura, Takeshi Kodama and Masahiko Hada* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397

Email address: [email protected]

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Abstract: Structural and electronic properties on neutral and anionic species of M1M2@IhC80 including heteronuclear metal/lanthanides were theoretically investigated. Theoretical analysis demonstrates that M1M2@Ih-C80 can be categorized into three groups: i) LaM@IhC80 (M = Ce, Pr), ii) LaM@Ih-C80 (M = Sm, Eu), iii) GdM@Ih-C80 (M = Sc, Y, La, Lu). Molecular orbital analysis suggests that electron transfer from metal to cage is different for three groups because of the energy level of the metal-based molecular orbital and inherent oxidation state of metals. Thus, choosing suitable combinations of the heterometals for the encapsulation is an effective way to control the stability of M1M2@Ih-C80. The anionization can also stabilize the M1M2@Ih-C80 complexes, as confirmed for homonuclear cases. In addition, we found that the absorption spectra for the studied hetero-dimetallofullerenes are nearly independent of the metals encapsulated because the π → π* transitions on the cage are dominant even though metal orbitals are involved for the low-lying excitations.


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1. Introduction Fullerenes with metal atoms or a metal cluster confined in their cage cavities form a collection of novel molecules, which are known as endohedral metallofullerenes (EMF).1-3 Among a wide variety of fullerenes, C80 based EMFs have attracted attention due to the following reasons: i) Ih-C80 has the same symmetry as C60,4 which exhibits interesting properties such as superconductivity5 and ferromagnetism,6 ii) empty Ih-C80 is a missing fullerene, which is unstable7 and can be stable through the reduction by the encapsulation of the metal atoms so that the quadruple quasi-degenerate highest occupied molecular orbitals (LUMOs) of Ih-C80 become completely occupied.8 Therefore, the Ih-C80 carbon cage is an ideal candidate for the study of the endohedral metallofullerenes, where two encapsulated metal atoms are found and is stabilized by forming [email protected] Previous studies on dimetallofullerene (di-EMFs) have mainly focussed on the homonuclear bimetallic systems, in which two metal atoms/ions exhibit certain equivalent properties, i.e., similar metal-cage interactions.14 Conventional homonuclear di-EMFs such as La2@Ih-C80 and Ce2@Ih-C80 belong to a stable class of di-EMFs and has been studied extensively,15, 16 and there exist some unstable rare earth dimetallofullerenes (di-EMFs) such as M2@Ih-C80 (M = Sc, Y, Gd, Lu), which could be produced in the soot.17, 18 Among the wide class of EMFs, the gadolinium-based EMFs are considered as novel magnetic contrast agents.19 The advantage of the Gd-based EMFs over other Gd complexes is that the toxic Gd ions are completely confined inside the fullerene. In our previous work20, we examined five di-EMFs of M2@Ih-C80 (M = Sc, Y, La, Gd, Lu), in terms of kinetic instability, which arises due to the spin distribution over the carbon cage. We also explored for experimental observation and isolation by anionization and found that the anionization paves a way to obtain a stable di-EMF. The difference between the stable La2@Ih-C80 and unstable M2@Ih-C80 (M = Sc, Y, Gd, Lu) is determined by the energy level of the metal-based molecular orbital (MO). Hence, combining two metals from stable and unstable species might control the energy level of the metal-based MO so as to observe a stable species of di-metallofullerene. This hypothesis has inspired us to explore on the heteronuclear di-metallofullerenes. Also, from viewpoint of development for future materials, the encapsulation of the two lanthanides is attractive because applications for di-EMFs are ranged from technological21, 22 to biomedical fields23



With this background, we have examined certain combinations of the heterometals. Although more than 100 combinations from Sc, Y, and lanthanides are possible for the heteronuclear encapsulation, we restricted ourselves to the combinations of heteronuclear diEMFs such as LaM@Ih-C80 (M = Ce, Pr, Sm, Eu) and GdM@Ih-C80 (M = Sc, Y, Gd, Lu), based upon the information about the energy level of the metal-metal bonding MO in homonuclear di-EMFs.20 The theoretical examination about heterometal di-EMF, M1M2@IhC80, has never been reported as of today and is important for offering knowledge for obtaining stable di-EMF, especially because the limited availability of EMFs has hindered the experimental studies on the electronic and structural properties except for a few studies carried out by Kodama et al.24-26 The outline of this paper is as follows. First, we discussed the electronic structure and the kinetic stability of the heteronuclear di-EMFs by density functional theory (DFT). Also, we have discussed a strategy to stabilize the di-EMFs through the heterometal substitution. Further, we considered the stable anionic M1M2@Ih-C80 species. Our previous studies on the homonuclear di-EMFs suggested that, due to the intense transitions from cage to cage, the absorption spectra were found to be almost independent of the metals encapsulated except for [email protected] Thus, to probe the involvement of the metal contribution to the absorption spectra for the heteronuclear EMFs of M1M2@Ih-C80, we have also examined the absorption spectra by using time-dependent density functional theory (TDDFT).


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2. Computational Details All calculations were carried out with the Gaussian 09 program package27 using the UB3LYP functional28-30. The basis sets used were described using quasi-relativistic effective core potential developed by Stuttgart Cologne group31-33 in combination with the corresponding optimized valence basis sets for Sc, Y, La, Ce, Pr, Sm, Eu, Gd and Lu; and 631G(d)34 and 6-311+G(d)35 basis sets for C atom for neutral and anionic species, respectively. It is noted that spin-orbit interaction for f electrons is not considered because this study focuses on kinetic stability of M1M2@Ih-C80, not magnetic properties. Harmonic vibrational analysis was performed for the key stationary points to determine whether they are real minima or saddle points. Excited state calculations were carried out using TDDFT with the UB3LYP functional and the solvation effect was considered by the polarizable continuum model (PCM).36



3. Results and discussion Our previous studies on homonuclear di-EMFs have shown that the most stable configuration of M2@Ih-C80 has a D2h symmetry in which two metals are on the C2 axis penetrating through the center of hexagon rings.20 In the present work, based on our previous study on homonuclear di-EMF, we similarly incarcerated two different metals (M1M2@IhC80) in Ih-C80 fullerene and then explored the possible geometric arrangements of the metals Although other possible conformers can be explored by molecular dynamics or other methodologies as well, we restricted ourselves to stable conformers because the main trend may be explained by analysing stable conformers. But, search for all possible conformers may be required to estimate statistical physical properties. Initially, we performed calculations on LaM@Ih-C80 (M = Ce, Pr, Sm, Eu, Gd) and GdM@Ih-C80 (M = Sc, Y, La, Lu).

Figure 1. Optimized structure of a) La2@Ih-C80 b) LaCe@Ih-C80 and c) LaEu@Ih-C80 From the structural analysis (Table 1), we found that LaCe@Ih-C80 and LaPr@Ih-C80 possess a stable C2v configuration in which La and Ce/Pr atoms are on the C2 axis penetrating through the center of six-membered ring. The C2v configuration basically correspond to D2h one for M2@Ih-C80. The results are consistent with the previous experimental studies, which suggests that LaCe@Ih-C80 and LaPr@Ih-C80 possess the Ih-C80 cage.24, 25 On the other hand, for the C2v structure of LaSm@Ih-C80 and LaEu@Ih-C80, we found two imaginary frequencies for which the absolute value of 35.27 cm-1 for LaSm@Ih-C80 and 33.23 cm-1 for LaEu@Ih-C80. These modes are related to the motion of respective lanthanide ions on the σ-plane, which are displaced from the C2 axis. Hence, the location of the lanthanide ions in the C2v configuration is not stable and, as a result, a Cs configuration is obtained as a stable structure for LaSm@IhC80 and LaEu@Ih-C80. Finally, GdM@Ih-C80 (M = Sc, Y, La, Lu) possess a stable C2v configuration.26 As shown in Table S1, the energy for Cs configuration was slightly lower 6 ACS Paragon Plus Environment

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than that of C2v structure. However, the C2v and Cs configurations for GdLu@Ih-C80 were found to be almost similar in terms of the orbital energies and ordering in Table S1. Hence, we have chosen C2v configuration of GdLu@Ih-C80 for the studies. The lowering of the symmetry can be seen as moving along the lanthanide series: La2@Ih-C80 (D2h) to LaCe@IhC80 (C2v), LaEu@Ih-C80 (Cs) are depicted in Figure 1. The computed energy and the imaginary frequencies obtained for the different conformers of LaM@Ih-C80 (M = Ce, Pr, Sm, Eu) and GdM@Ih-C80 (M = Sc, Y, La, Lu) are listed in Table S1 of SI. Table 1. Ground state for the studied M1M2@Ih-C80

EMF

Symmetry

Spin state

*Relative energy in kcal/mol

LaCe@Ih-C80

LaPr@Ih-C80

LaSm@Ih-C80

LaEu@Ih-C80

GdSc@Ih-C80

GdY@Ih-C80

GdLa@Ih-C80

GdLu@Ih-C80

C2v

doublet

0.00

Cs

quartet

10.32

C2v

singlet

53.28

C2v

triplet

0.00

Cs

quartet

17.86

Cs

sextet

2.55

Cs

octet

0.00

Cs

septet

1.15

Cs

nonet

0.00

Cs

octet

2.45

C2v

dectet

0.00

Cs

octet

2.75

C2v

dectet

0.00

Cs

octet

1.66

C2v

dectet

0.00

Cs

octet

3.04

C2v

dectet

0.00

Next, we have examined the relative energy of the states with different spin multiplicities for which the geometry is optimized for each spin multiplicity. The symmetry and relative energy of the ground and excited states obtained for the studied series of M1M2@Ih-C80 are 7 ACS Paragon Plus Environment


given in Table 1. We have also analysed the spin contamination for the studied di-EMFs (see Table S2 of SI). The ground states for the studied EMFs do not have any significant spin contamination. Hence, the electronic states are reasonably described. Here, LaCe@Ih-C80, LaPr@Ih-C80, LaSm@Ih-C80, LaEu@Ih-C80 and GdM@Ih-C80 (M = Sc, Y, La, Lu) have doublet, triplet, octet, nonet and dectet ground states, respectively. Finally, we discuss the spin-orbit interaction for di-EMFs. In general, the spin-orbit effect can be important even for the description of valence orbitals of systems involving lanthanide elements. To evaluate the effect of spin-orbit interaction, we examined LaCe@IhC80 and depicted the valence orbital levels with/without spin-orbit interaction in Figure S1. It is clear that levels of cage occupied orbitals (HOMO ~ HOMO-4) hardly shift by the inclusion of spin-orbit interaction. On the other hand, LUMO energy slightly changes by approximately 0.1 eV but we believe that we can make the qualitative discussion about electron transfer and stability of M1M2@Ih-C80. Thus, we do not consider spin-orbit interaction in this study.

3.1 Electronic structure of neutral species of M1M2@Ih-C80 The electronic structure of the ground state for the heteronuclear di-EMFs can be well understood by analysing the molecular orbitals (MOs). As shown in Figure 2, the appearance of unpaired spins in the cage-based orbital and the metal-based (interstitial)37 orbital are strongly affected by the metals in M1M2@Ih-C80. The details are as follows.

Figure 2. Classification of M1M2@Ih-C80 based upon the occupation of metal-based (red arrow) and cage orbital (blue arrow). Group I: LaCe@Ih-C80 and LaPr@Ih-C80, Group II: LaSm@Ih-C80 and LaEu@Ih-C80 and Group III: GdM@Ih-C80 (M = Sc, Y, La, Lu) 8 ACS Paragon Plus Environment

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Group I: LaM@Ih-C80 (M = Ce, Pr) As shown in Figure 3, HOMO to HOMO-3 for LaCe@Ih-C80 and LaPr@Ih-C80 are mainly dominated by the cage contribution and the LUMO is mainly composed of the metal’s orbitals and is constrained in the fullerene. The cage contribution in HOMO for LaCe@Ih-C80 and LaPr@Ih-C80 is approximately 99% and the metal contribution in LUMO is 83.8% (LaCe) and 82.94% (LaPr), respectively (the orbital contribution data is summarized in Table S3 of SI). The orbital ordering for both LaCe@Ih-C80 and LaPr@Ih-C80 are almost similar to that of La2@Ih-C80. The HOMO-LUMO energy gaps of 1.21 eV for LaCe@Ih-C80 and 1.19 eV for LaPr@Ih-C80 as shown in Figure 3 suggest that LaCe@Ih-C80 and LaPr@Ih-C80 are energetically stable, which resemble the parent, La2@Ih-C80. Moreover, the absence of the spin density on the cage shows that they are kinetically stable, which is in agreement with the experimental studies.24,

25

Also the unoccupied metal-based MO clearly shows that six

electrons are effectively transferred from the metals to the Ih-C80 cage, resulting in a stable class of di-EMFs. Mulliken spin population analysis as well as Natural population analysis (NPA)38 (Table S4-S5 of SI) reveals that the sum of the electrons in the singly occupied molecular orbital (SOMO) is equivalent to the number of the f electrons from the Ce(f1) and Pr (f2), which consequently results in a doublet ground spin state for LaCe@Ih-C80 and a triplet ground spin state for LaPr@Ih-C80. The spin densities are drawn in Figure S2 and the detailed discussion about electron transfer is given in Figure S3 of SI. Thus, LaCe@Ih-C80 and LaPr@Ih-C80 can be assigned as a formal charge state of La3+M3+@Ih-C806- (M = Ce, Pr).



Figure 3. MO energy levels for LaM@Ih-C80. Red arrow represents unpaired electron.

Figure 4. MO energy levels for a) LaSm@Ih-C80 and b) LaEu@Ih-C80. Red arrow represents unpaired electron. 10 ACS Paragon Plus Environment

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Group II : LaM@Ih-C80 (M = Sm, Eu) Figure 4 depicts MOs of LaSm@Ih-C80 and LaEu@Ih-C80. The HOMO is singly occupied molecular orbital (SOMO) and dominated by the cage contribution, whereas the HOMO-1, HOMO-2 and HOMO-3 are fully occupied and primarily contributed from the IhC80 cage. The other SOMOs are dominated by the 4f orbitals of the respective lanthanides. NPA reveals that there are six f electrons in LaSm@Ih-C80 and seven f electrons in LaEu@IhC80 (see Table S5 of SI), which suggests that the heterometal Sm and Eu exist in a lower oxidation state of M2+ (see Table S4-S5 of SI). Although the oxidation state of M2+ should be more examined by considering spin-orbit interaction and electron correlation, the lower oxidation state of M2+ for di-EMFs of Sm39,

40

(Sm2@C88, Sm2@C90 and Sm2@C92) and

mono-EMFs and most of the reported EMFs of Eu41 has been reported theoretically and experimentally before. Therefore, we think that more detailed analysis seems to be beyond the scope of this paper and do not go into details. Due to the inherent divalent state of Sm and Eu, only five valence electrons, which consists of three electrons (5d1 6s2) from La3+ and two electrons (6s2) from Sm2+/Eu2+ for LaM@Ih-C80, are transferred to the metals. Therefore, instead of the expected Ih-C806- cage, the Ih-C805- cage is formed, and the unpaired spin on the cage appears and leads to kinetic instability. (see more detailed discussion with Figure S4 in SI). Thus, the formal charge state of LaSm@Ih-C80 and LaEu@Ih-C80 can be described as La3+M2+@Ih-C805- (M = Sm, Eu).

Group III: GdM@Ih-C80 (M = Sc, Y, La, Lu) GdM@Ih-C80 (M = Sc, Y, La, Lu) heteronuclear di-EMFs are derived from Gd2@Ih-C80, by replacing one of the Gd with a heterometal (M = Sc, Y, La, Lu). Table 1 shows that all the studied GdM@Ih-C80 (M = Sc, Y, La, Lu)possess a dectet ground state with seven singly occupied f electrons on Gd(III), one unpaired spin on the carbon cage, and one unpaired spin localized between the metal atoms as in the case of Gd2@Ih-C80 (17- et) reactive species.20 Thus, the sum of the electrons in the singly occupied MOs is equivalent to the nine net spin electrons, which results in the dectet ground state.



Figure 5. Molecular orbitals for GdM@Ih-C80 (M = Sc, Y, La, Lu). The seven singly occupied f electrons for GdM@Ih-C80 is omitted for brevity. Red arrow represents unpaired electron. The MOs for the ground state of GdM@Ih-C80 (M = Sc, Y, La, Lu) are given in Figure 5. Here, the singly-occupied molecular orbitals (SOMO) for 4f electrons in GdM@IhC80 are omitted for brevity. From Figure 5, it is clear that the orbital ordering in GdLa@Ih-C80 is different from other GdM@Ih-C80 (M = Sc, Y, Lu). Careful examination on the orbitals reveals that the metal-based MO for GdLa@Ih-C80 is α-HOMO-1. On the other hand, those for all other GdM@Ih-C80 (M = Sc, Y, Lu) are buried below the cage-based orbitals; namely, the metal-based MO is α-HOMO-4 for GdSc@Ih-C80 and GdY@Ih-C80 and α-HOMO-7 for GdLu@Ih-C80. All the valence electrons from the metal dimer are not transferred to the cage, which is evident from an unpaired spin on the cage (see Figure S5-S6 of SI). Thus, GdM@IhC80 series also exhibit a radical character as in the case of parent Gd2@Ih-C80 and can be assigned as a formal charge state of Gd2.5+M2.5+@Ih-C805- (M = Sc, Y, La, Lu)


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3.2 Control of metal-based orbital level We performed a detailed analysis on the metal-based MO, which is the key for kinetic stability and depends upon the metals/lanthanides encapsulated. The metal-metal bonding orbitals in di-EMFs have s, p, and d orbital nature. For example, the metal-based MOs of Sc and Y are mainly composed of 4s and 5s orbitals plus (3d, 4p) and (4d, 5p) orbitals, respectively, whereas those for lanthanides are composed of 6s plus (5d, 6p) orbitals. Here, the metal-based MO for Group I is LUMO, whereas that for Group III is SOMO (see Figure 6a). As explained in Sec. 3.1, the electron transfer mechanism for Group II is different from those of Group I and III; hence, we do not mention Group II in this section. The energy level of the metal-based MO in both homo- and heteronuclear di-EMFs are depicted in Figure 6a. The metal-based MO energies for the heteronuclear di-EMFs lie in between those for the corresponding homonuclear di-EMFs. For example, the metal-based MO energy of GdLa@Ih-C80 is in between the homonuclear La2@Ih-C80 and Gd2@Ih-C80 (see Figure 6a) and is highest among those of the occupied ones for other analogues of Group III (Figure 5). Hence, comparison of energy level of the metal-based MO in analogues di-EMFs could be one of effective ways to design a stable di-EMF such as LaCe@Ih-C80 and [email protected], 25 In the previous work, Popov et al. have analysed the nature of metal-metal bonding using the energy levels of the metal dimers and dimetallofullerenes.42, 43 Based on Popov’s study, we have further compared the metal-metal bonding MO in the respective dimers of the encapsulated metals. The energy levels of the metal dimers along with the energy level of the cage LUMO were shown in Figure 6b. The metal dimers such as La2, Ce2 and Pr2 have a high energy of the three highest occupied MOs in their dimeric form, which is well above the LUMO of Ih-C80 cage, and hence the transfer of six electrons to the cage would result in a stable class of di-EMFs, M2@Ih-C80 (M = La, Ce, Pr). This is applicable to the heteronuclear combinations of LaCe@Ih-C80 and LaPr@Ih-C80 as well (three highest occupied MOs are above the cage-based MO). In contrast, the energy level of the metal-metal bonding MOs of Sc2, Y2, Gd2 and Lu2 dimers is deep below the energy level of the empty Ih-C80 LUMOs, and hinders the transfer of six electrons to the cage. Thus, if the metal-based MO is deep below the Ih-C80 cage orbitals, the transfer of six electrons to the cage is not feasible and may result in an unpaired spin on the cage. The same situation arises for the combinations of the other heterometals such as GdM (M = Sc, Y, Gd, Lu) for which the metal-based MO is deep below



the cage MO as reflected in their di-EMF form (Figure 6a). Thus, choosing suitable combinations of the heteronuclear metals is important towards attaining the chemical stability. We discuss metal-metal and metal-cage interactions. In the studied di-EMFs, the M-M distance was found to be more than 3.75 Å. The detailed structural parameters for the M-M/M-C bond distances have been tabulated in Table S6. Analysis on the metal-cage interactions suggests that for the studied di-EMFs with a C2v configuration, [LaCe@Ih-C80, LaPr@Ih-C80 and GdM@Ih-C80 (M = Sc, Y, La, Lu)] metal and cage interact in a η6 fashion and are connected to hexagonal ring of the fullerene cage. On the other hand, for the di-EMFs such as LaSm@Ih-C80 and LaEu@Ih-C80, which have a Cs configuration, the metal-cage interaction was found to be different and the metals are dislocated from the metal position of the C2v configuration. Hence, depending upon the metals encapsulated and the metal-cage interaction, the interaction of the di-EMFs varies and is consistent with the previous reports by Rajaraman et al.44, 45


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Figure 6. a) DFT computed MO energy of metal-based orbital in homonuclear (M2@Ih-C80; M = Sc, Y, La, Ce, Pr, Gd, Lu) and heteronuclear M1M2@Ih-C80 di-EMFs. b) Energy levels for metal-metal bonding MO in metal dimers compared to that of C80 cage. Metal based-MOs in dimers are encircled in magenta and the LUMO energy level for the empty Ih-C80 cage is green.



3.3 Anionization of M1M2@Ih-C80 Group I compounds, LaM@Ih-C80 (M = Ce, Pr) form a series of stable di-EMFs such as LaCe@Ih-C80 and LaPr@Ih-C80. In contrast, neutral species of LaM@Ih-C80 (M = Sm, Eu) of Group II and GdM@Ih-C80 (M = Sc, Y, La, Lu) of Group III are kinetically unstable, i.e., reactive because of the unpaired spin on the fullerene cage. In order to experimentally obtain a di-EMF, one of the effective means is the reduction of the neutral species to form an anionic species so that all the cage orbitals become fully occupied. Thus, we examined the anionic species of LaM@Ih-C80 (M = Sm, Eu) and GdM@Ih-C80 (M = Sc, Y, La, Lu), whose optimized geometries are C2v.

Figure 7. Molecular orbitals for [GdM@Ih-C80]- (M = Sc, Y, La, Lu). The seven singly occupied f electrons for GdM@Ih-C80 is omitted for brevity.

For anionic species of Group II of LaM@Ih-C80 (M = Sm, Eu), Sm and Eu have six and seven unpaired f electrons, respectively, with the unoccupied metal-based MO. This reveals that the reduction occurs in the cage (see Figure S8-S9 of SI). Thus, the quadruple quasi-degenerate fullerene HOMOs of Ih-C80 are now fully occupied and result in stable anionic species, La3+M2+@Ih-C806- (M = Sm, Eu). Furthermore, the anions for Group III, GdM@Ih-C80 (M = Sc, Y, La, Lu) contain seven unpaired electrons from the Gd (III) and an unpaired electron on the metal-based MO (see Figure S9 of SI). Hence, the formal charge 16 ACS Paragon Plus Environment

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state of them can be described as Gd2.5+M2.5+@Ih-C806- (M = Sc, Y, La, Lu), and the corresponding spin multiplicity is nonet. For the anionic species of GdM@Ih-C80, SOMO is well protected by the fullerene cage (see Figure 7) and it is expected that the studied anions are kinetically stable. Thus, it is anticipated that [GdM@Ih-C80]- could be possibly isolated, which is in agreement with the experimental observation of the [GdM@Ih-C80]- (M = Y, La) species.26 3.4 Absorption spectra of M1M2@Ih-C80

Figure 8. Absorption spectra of (a) La2@Ih-C80 (b) LaCe@Ih-C80 and c) LaPr@Ih-C80. All of the spectra (a-c) are interpolated using the Lorentzian function with half width at half maximum = 0.05 eV In order to identify M1M2@Ih-C80 through absorption spectra, we investigated excited states of the neutral M1M2@Ih-C80. Specifically, we examined La2@Ih-C80, LaCe@Ih-C80 and LaPr@Ih-C80. The similarity in this class of di-EMFs is that they transfer six electrons to the cage resulting in an electronic structure of M13+M23+@Ih-C806-. In general, encapsulation of two metal atoms lowers the molecular symmetry. In the present work, geometry optimization demonstrated that the most stable configuration for La2@Ih-C80 is of D2h symmetry and those 17 ACS Paragon Plus Environment


for LaCe@Ih-C80 and LaPr@Ih-C80 are of C2v symmetry. Therefore, only these configurations were studied by TDDFT calculations. The absorption spectra of LaM@Ih-C80 (M = La, Ce, Pr) are drawn in Figure 8a-8c and the numerical data of transitions with oscillator strength over 0.0040 in the energy range over 400 nm are given in Table 2-4.

Table 2. Selected excitation energies with strong oscillator strengths (f) and their main configurations for La2@Ih-C80 (H = HOMO, L = LUMO) E/nm (eV)

f

Main composition(contribution)

555.39 (2.23)

0.0126

H

507.39 (2.44)

0.0238

H-2 → L+1 (0.48), H → L+3 (0.39), H-3 → L+4 (0.32)

467.47 (2.65)

0.0803

H-2 → L+3 (0.57), H-2 → L+4 (-0.33)

466.83 (2.66)

0.0135

H

460.56 (2.74)

0.0648

H-3 → L+4 (0.58)

452.08 (2.74)

0.0812

H-2 → L+4 (0.38), H-1→ L+3 (0.25), H-1→ L+1 (0.25)

430.39 (2.74)

0.0069

H-7 → L (0.61)

406.19 (3.05)

0.0942

H

→ L+1 (0.69)

→ L+6 (0.63), H-2 → L+4 (0.24)

→ L+9 (0.55)


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Figure 9. Selected molecular orbitals for La2@Ih-C80 obtained using DFT with B3LYP (H = HOMO, L = LUMO) Figure 9 shows the selected molecular orbitals for La2@Ih-C80. All the HOMO and other occupied MOs near HOMO are distributed over fullerene cage. LUMO, LUMO+1, LUMO+2, LUMO+8 are mainly distributed over the encaged metal atoms. The remaining unoccupied orbitals up to LUMO+9 (LUMO+3 to LUMO+7 and LUMO+9) are formed by mixing of La and cage orbitals. TDDFT calculations suggest that the HOMO → LUMO transition (630 nm) has approximately zero oscillator strength and is forbidden (b2g → ag). Two transitions at 555 and 507 nm contribute to the tail in energy range of 500 - 600 nm in the absorption spectrum. The dominant peak around 460 nm consists of four transitions, namely, HOMO→ LUMO+6, HOMO-2→ LUMO+3, HOMO-2→ LUMO+4, and HOMO3→ LUMO+4 as shown in Table 2. The peak around 406 nm can be assigned to HOMO → LUMO+9. Also, from Table 2 it is clear that most of the transitions occurs from cage to metal orbitals as well as to the orbitals formed by mixing of La and cage. It is noted that the oscillator strength for the transitions involving cage to metal orbitals are small compared to that of the transitions to metal-cage mixing orbitals. The contribution from all other



excitations to the orbitals formed by the mixing of La and cage states is negligibly small in the absorption features because of the (approximately) zero oscillator strengths. Next, we examine LaCe@Ih-C80 and LaPr@Ih-C80 to compare the absorption spectra with that of La2@Ih-C80 (Figure 8). The selected molecular orbitals of LaM@Ih-C80 (M = Ce, Pr) are depicted in Figures 10a and 10b. It is noted that α-HOMO and α-LUMO are used for α-spin electron and β-HOMO and β-LUMO are used for β-spin electrons for clarity. We showed only α-MOs of the di-EMF in Figure 10 for brevity. Table 3 and 4 show the transitions for αas well as β-orbitals. The absorption spectra for LaCe@Ih-C80 and LaPr@Ih-C80 show similar absorption spectra (Figures 8b, 8c). The main features in the range of 400 to 600 nm are four peaks around 409, 441, 473, 526 nm for LaCe@Ih-C80, which can be observed around 406, 449, 485, 530 for LaPr@Ih-C80. Most of the dominant transitions takes place to α-LUMO+7, α-LUMO+9, α-LUMO+10 for LaCe@Ih-C80 and α-LUMO+8, α-LUMO+9, α-LUMO+12 for LaPr@Ih-C80. The MOs to which the transition occur are mainly formed by mixing the metal atoms and the cage as shown in Figure 10a-10b. Since a D2h symmetry of La2@Ih-C80 is lowered to

C2V for LaM@Ih-C80 (M = Ce, Pr), more transitions are symmetry-allowed;

consequently, more allowed transitions for LaCe@Ih-C80 and LaPr@Ih-C80 appear in a wider range compared to La2@Ih-C80 (Figure 8). However, transitions for LaCe@Ih-C80 and LaPr@Ih-C80 are still similar to those of La2@Ih-C80 as confirmed by Table 2-4 and Figure 8a-8c. Thus, even though the metal orbitals are contributed to the low-lying excitations, it cannot bring about significant changes in the absorption spectra, which suggests that the absorption spectra of M1M2@Ih-C80 of both homo- and heteronuclear di-EMFs have a predominant feature of [Ih-C80]6-.


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Figure 10. Selected molecular orbitals for a) LaCe@Ih-C80 and b) LaPr@Ih-C80 obtained using DFT with B3LYP (H = HOMO, L = LUMO)



Table 3. Selected excitation energies with strong oscillator strengths (f) and their main configurations for LaCe@Ih-C80 (H = HOMO, L = LUMO)

E/nm (eV)

f

Main composition(contribution)

526.31 (2.38) 0.0084 α-H-1 → α-L+7 (0.73), β-H-2 → β-L+3 (-0.54) 520.19 (2.38) 0.0182 α-H-1 → α-L+7 (0.61), β-H-2 → β-L+3 (0.51) 498.32 (2.49) 0.0120 α-H-2 → α-L+6 (0.57) , β-H → β-L+7 (-0.30) 489.34 (2.53) 0.0153 α-H-2 → α-L+9 (0.52), α-H-3 → α-L+6 (0.42), β-H-3 → β-L+1(0.35) 474.77 (2.61) 0.0221 α-H-2 → α-L+9 (-0.42), β-H-2 → β-L+7 (0.53) 473.49 (2.62) 0.0226 α-H-3 → α-L+9 (0.45), β-H-3 → β-L+7 (0.58), 449.66 (2.75) 0.0213 α-H-2 → α-L+10 (0.36), α-H-2 → α-L+9 (0.21), β-H-2 → β-L+7 (0.47) 443.48 (2.79) 0.0108 β -H-1 → α-L+10 (0.56), β-H → β-L+11 (0.32) 441.30 (2.80) 0.0283 α-H-2 → α-L+9 (0.22), β-H-2 → β-L+10 (0.47) 435.94 (2.84) 0.0171 α-H-6 → α-L+3 (0.48), α-H-1 → α-L+12 (0.23) β-H-3 → β-L+8 (0.34), β-H-3 → β-L+10 (-0.36) 429.53 (2.89) 0.0062 α-H-7 → α-L+3 (0.71), β-H-7 → β-L (0.43) 409.01 (3.03) 0.0283 α-H-7 → α-L+7 (0.45), α-H→ α-L+12 (-0.36), α-H-6 → α-L+7 (-0.34), β-H-1 → β-L+14 (0.39), β-H → β-L+12 (-0.35)


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Table 4. Selected excitation energies with strong oscillator strengths (f) and their main configurations for LaPr@Ih-C80 (H= HOMO, L= LUMO) E/nm (eV)

f

Main composition (contribution)

529.86 (2.34) 0.0136 α-H-2 → α-L+8 (0.22), α-H → α-L+5 (0.16) β-H-2 → β-L+64 (0.65), β-H → β-L+3 (0.26) 507.71 (2.44) 0.0077 α-H-1 → α-L+9 (0.29), β-H-1 → β-L+6 (0.83) 487.82 (2.54) 0.0119 α-H-3 → α-L+7 (0.50), α-H-2 → α-L+9 (0.43), β-H-3 → β-L+5 (0.46) 485.97 (2.55) 0.0196 α-H-3 → α-L+9 (0.52), α-H → α-L+8 (0.40), β-H→ β-L+7 (0.53) 459.62 (2.70) 0.0075 α-H-9 → α-L+2 (0.51), α-H-3 → α-L+9 (-0.32), β-H-1 → β-L+8 (-0.50) 455.74 (2.72) 0.0152 α-H-3 → α-L+9 (-0.36), β-H-7 → β-L (0.41), β-H-4 → β-L+1 (0.21) 455.67 (2.73) 0.0272 α-H-2 → α-L+8 (0.45), α-H-4→ α-L+4 (-0.31), β-H-2 → β-L+7 (0.62) 449.35 (2.76) 0.0527 α-H-2 → α-L+9 (0.47), β-H→ β-L+11 (0.35) 445.39 (2.78) 0.0115 α-H-6 → α-L+3 (0.50), α-H-3→ α-L+9 (-0.38), β-H-7 → β -L(-0.46) 444.79 (2.79) 0.0130 α-H-4 → α-L+4 (0.76) 406.1 (3.05) 0.0354 α-H→ α-L+12 (-0.34), β-H-1 → β-L+13 (0.46), β-H → β-L+11 (0.35)



4. Conclusion We have examined the electronic structures of neutral and anionic M1M2@Ih-C80 using DFT and TDDFT. Our comparative studies categorize a series of M1M2@Ih-C80 into three groups: (1) For LaM@Ih-C80 (M = La, Ce, Pr), the two metals donate six electrons to the cage for forming a stable electronic structure of La3+M3+@Ih-C806-. (2) For LaM@Ih-C80 (M = Sm, Eu), La donates three electrons to the cage, whereas Sm and Eu atoms prefer the oxidation state of +2, and thus donate only two electrons to the cage, resulting in a configuration of [La3+M2+]@Ih-C805-. (3) For GdM@Ih-C80 (M = Sc, Y, La, Lu), the two metals donate only five electrons to the cage, resulting in a configuration of [Gd2.5+M2.5+]@IhC805-. Thus, for LaM@Ih-C80 (M = Sm, Eu) and GdM@Ih-C80 (M = Sc, Y, La, Lu), one of the quadruple quasi-degenerate HOMOs of Ih-C80 is singly occupied and results in an unpaired spin on the cage. Any M1M2@Ih-C80 heteronuclear di-EMF, having an unpaired spin on the cage, remains unstable with respect to an attack by the external reactive species, which is consistent with the previous experimental fact that these unstable species have not been obtained. From the above results, the combination of two metals affects the level of metalbased orbital and leads to two di-EMF groups of LaM@Ih-C80 (M = La, Ce, Pr) and GdM@Ih-C80 (M = Sc, Y, La, Lu). On the other hand, the electron configuration for LaSm@Ih-C80 and LaEu@Ih-C80 is different from the others because of the inherent lower valence state (divalent) in contrast to the usual trivalent state of the other examined metals. Anionization, which leads to the disappearance of the unpaired electron on the cage and the confinement of the unpaired electrons inside the cage, could be an effective method to stabilize the di-EMFs. We expect that the anions of the novel di-EMFs could be successfully synthesised. Moreover, the present study may serve as a reference for the studies of other novel heteronuclear di-EMFs with potential applications. We have also investigated the absorption spectra for the heteronuclear di-EMFs and found that similarity in the absorption spectra of the homo- and the heteronuclear di-EMFs arises due to the predominant cage to cage transitions. In this study, we examined electronic structure of the fullerenes possessing heterometals and explored the possibility for the experimental observation and isolation


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through the anionization and encapsulation of heterometals. We will study heterometallo fullerenes with feasible exohedral functional groups, which would be interesting, and report elsewhere soon.

Acknowledgments The calculations were performed at the Research Center for Computational Science, Okazaki, Japan. This research is supported by Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology (JST) Agency, “Creation of Innovative Functions of Intelligent Materials in the basis of the Element Strategy”. A.V. would like to thank Tokyo Metropolitan Government for Asian Human Resource fund and Mukesh Kumar Singh at IITB for the helpful discussions.

Supporting Information Relative energies of the optimized structures in different spin configurations; Spin density plot for M1M2@Ih-C80 and [M1M2@Ih-C80]- (M1M2= LaCe, LaPr, LaSm, LaEu, GdSc, GdY, GdLa, GdLu)

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