Fused-Pentagon-Configuration-Dependent Electron Transfer of

May 26, 2017 - Host−guest electron transfer between titanium and fullerenes is found to be dependent on the fused-pentagon configuration of the bind...
3 downloads 11 Views 2MB Size
Download PDF
Article pubs.acs.org/IC

Fused-Pentagon-Configuration-Dependent Electron Transfer of Monotitanium-Encapsulated Fullerenes Jing-Shuang Dang,† Wei-Wei Wang,†,‡ Jia-Jia Zheng,†,‡ Xiang Zhao,*,† and Shigeru Nagase*,‡ †

Institute for Chemical Physics & Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, China Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan

‡

S Supporting Information *

ABSTRACT: We introduce monotitanium-based endohedral metallofullerenes (EMFs) using density functional theory calculations. Isomeric C64 fullerenes are initially employed as hosts, and Ti@C64 species show novel features on the electronic structures. Energetically, the preference of titanium residing on triplefused-pentagon subunits is proposed in theory. More importantly, different from current knowledge on mono-EMFs, electron transfer between titanium and carbon cages is not unified but is essentially dependent on the pentagon distribution of the binding sites, giving rise to variations of the cationic titanium of Ti@C64. Such selective electron-transfer character is extended to the study of the encapsulation of other neighboring metal atoms (i.e., calcium and scandium). Because of their different capabilities to accept d electrons, fullerene cages with distinct fusedpentagon motifs show selective metal encapsulation characters. In addition, some other fullerenes (C44−C48 and C82) are selected as hosts to study the electrontransfer behavior of titanium in smaller fullerenes and larger systems without pentagon adjacency.

■

INTRODUCTION Endohedral metallofullerenes (EMFs) have attracted wide interest because of their intriguing structural and electronic features.1−3 The electron-deficient fullerene cages can encapsulate metal atoms or hybrid clusters to generate unique ionic complexes via metal−cage electron transfer.4 In recent years, great efforts have been made to synthesize new EMFs with novel electronic and magnetic properties as promising candidates in photovoltaic and biomedical applications.5,6 In comparison with the well-developed lanthanide-based EMFs, group 4 metal-based (e.g., titanium) EMFs are still rarely known. In particular, to date, Ti@C28 is the only known metallofullerene with atomic titanium rather than hybrid cluster (e.g., nitrides,7−9 carbides,10−12 and sulfides13) encapsulation.14,15 The C28 fullerene was initially suggested as a Tdsymmetric cage with four unpaired electron in a quintet openshell ground state.16 Further investigations proved that C28 can be stabilized by forming EMFs with trapped tetravalent atoms, and, in particular, U@C28 was observed in the laser-furnace and carbon-arc discharge experiments.17 More recently, Ti@C28 was detected using high-resolution Fourier transform ion cyclotron resonance mass spectrometry in the gas phase. Although the exact structure of Ti@C28 has not been identified crystallographically because of its high reactivity under typical conditions, supplemental calculations suggested that a total of four valence electrons of titanium ([Ar]3d24s2) are transferred to the carbon cage, leading to the formation of Ti4+@C284−.14,15 Because Ti@C28 is the one and only known atomic titaniumbased EMF, it is of great interest to uncover the influence of titanium encapsulation on the electronic structure of fullerene. © 2017 American Chemical Society

For instance, does the encaged titanium always keep a tetravalent state in EMFs? What is the key factor determining the electron-transfer character between titanium and fullerene cages? Which kind of host fullerenes are preferred for titanium trapping? In the present work, by employing medium-sized C64 fullerenes as hosts, the encapsulation of monotitanium is explored using density functional theory (DFT) calculations. C64 species are selected because (1) so far there has been no report on C64-based EMFs and (2) C64 consists of isomeric structures with various fused-pentagon configurations (Figure 1) that are important for metal binding.18 Interestingly, we find

Figure 1. Fused-pentagon configurations.

that the trapped titanium exhibits a pentagon-distributiondependent electron-transfer nature. When titanium resides on a double fused pentagon (DFP; Figure 1a), a triple sequentially fused pentagon (TSFP; Figure 1b), or a triple directly fused pentagon (TDFP; Figure 1c) fragment, a Ti2+, Ti4+, or unconventional Ti 3+ state inside Ti@C 64 is detected, respectively, resulting in various electronic and magnetic Received: February 2, 2017 Published: May 26, 2017 6890

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry

EMFs. The origin of ground-state variation is rationalized by investigating the electron-transfer nature of Ti@C64 at a later stage. Besides the unconventional electronic state, another remarkable feature of Ti@C64 is the stability of EMFs. For empty fullerenes, the carbon skeleton is destabilized in the presence of fused pentagons with steric strain. Therefore, the most stable isomer in each series always obeys the IPR proposed by Kroto.16 As for the metallofullerenes, owing to the novel electron transfer between the trapped metal and outside carbon, the IPR sometimes fails to predict the stable structure of EMF species. Not only the number of fused pentagons but also the metal−cage interactions, the local aromaticity of anionic fullerenes, and the pentagon distribution of the carbon sphere are proven to be important for EMF stabilization.25−28 In the case of C64, owing to the energy penalty caused by highly strained fused pentagons, the most stable pristine C64 is #3451C64 with the fewest two-pentagon pairs.20 However, as listed in Table 1, three EMFs with PA = 3, Ti@#1911C64, Ti@#2983C64, and Ti@#2730C64, exhibit the lowest energy among all candidates. The strong interactions between titanium and triple fused-pentagon segments give rise to the dramatic energy change from pristine fullerenes to EMFs. Ti@#1911C64 (Figure 2) with a TDFP fragment is more stable than Ti@#3457C64 and

properties of Ti@C64 EMFs. In addition, on the basis of the proposed structure-dependent electron-transfer character, the selective metal encapsulation of mono-EMF is introduced. The divalent (calcium), trivalent (scandium), and tetravalent (titanium) metals are proven to reside on distinct hosts with different types of fused-pentagon motifs, which is important for EMF design and discovery.

■

RESULTS AND DISCUSSION C64 is known as a set of isolated pentagon rule (IPR)-violating fullerenes, and previous DFT calculations demonstrated that three species with the fewest two-pentagon adjacencies (PA = 2) exhibit the lowest energy.19,20 Herein, a total of 15 isomers with PA < 4 are employed as candidates for titanium encapsulation (labeled by the spiral code21). Optimizations on endohedral Ti@C64 at various spin states (closed-shell singlet, triplet, and quintet) are carried out at the level of (U)B3LYP/6-311G(d). The relative energies are listed in Table 1, and detailed structures are shown in Figure S1 in the Table 1. Relative Energies (ΔE) of Ti@C64 in Different Electronic States and HOMO (SOMO)−LUMO Gaps (Egap) in Ground States ΔE (kcal mol−1) labela 1911 2983 2730 3457 3451 3452 3438 3402 3425 3455 3416 3424 3423 3428 3454 a

no. of PA (fused-pentagon configuration) 3 3 3 2 2 2 3 3 3 3 3 3 3 3 3

(TDFP×1) (TSFP×1 + DFP×1) (TSFP×1 + DFP×1) (DFP×2) (DFP×2) (DFP×2) (DFP×3) (DFP×3) (DFP×3) (DFP×3) (DFP×3) (DFP×3) (DFP×3) (DFP×3) (DFP×3)

CSb

triplet

quintet

Egap (eV)

7.2 0.2 6.8 8.8 13.9 13.2 17.9 17.7 21.3 17.8 20.8 26.5 27.4 25.4 30.0

0.0 2.8 6.9 6.9 9.5 10.5 11.9 14.9 15.4 17.3 18.6 20.1 21.7 22.6 28.9

36.8 34.2 35.4 33.0 29.3 21.5 42.2 36.2 30.5 38.1 46.7 33.1 31.8 45.4 53.4

1.15 1.32 1.34 1.41 1.14 1.54 1.67 1.63 1.08 1.57 1.21 1.26 1.15 1.19 1.70

Figure 2. Structures of the two most stable Ti@C64 isomers.

other PA = 2 isomers by at least 6.9 kcal mol−1. We also take note of another isomer #2983C64 containing PA = 3 with a TSFP subunit (Figure 2). Ti@#2983C64 is isoenergetic to Ti@#1911C64 (only 0.2 kcal mol−1 higher in energy) and exhibits a larger highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap of 1.32 eV. Moreover, on the basis of equilibrium statistical thermodynamic analyses (Figure S3 in the SI), the molar fractions of six low-lying isomers in a wide temperature interval (0−3000 K) indicate that Ti@#2983C64 is predominant at elevated temperatures related to fullerene formation. Therefore, #2983C64 is a promising host for atomic titanium encapsulation because of the excellent thermodynamic stability. To further shed light onto the electronic structure and to uncover the role of pentagon distribution of host cages in the spin state and electron-transfer nature of Ti@C64, the spindensity and frontier orbital analyses are performed on three representative isomers (Ti@ #3457 C 64 , Ti@ #1911 C 64 , and Ti@#2983C64) with different fused-pentagon configurations. In the case of triplet Ti@#3457C64 with two equivalent DFPs (Figure 3a), the unpaired electrons are mainly localized on the inserted titanium with a calculated spin density of 1.35 e. The diagrams of singly occupied molecular orbital (SOMO) and SOMO−1 further indicate that the half-occupied orbitals are mainly contributed from the 3d orbitals of titanium. Moreover, the HOMO of the complex is delocalized on fullerene and is attributed to the C64-derived LUMO, suggesting that two 4s electrons of titanium are transferred to the carbon cage,

Labeled by the spiral code of ref 21. bCS: closed-shell singlet.

Supporting Information (SI). Note that the titanium in each optimized configuration is off-centered and close to the fusedpentagon region, which is consistent with the results of Ti@ C28.14 As collected in Table 1, except for Ti@#2983C64 and Ti@#2730C64, the electronic ground states of all EMF structures are triplet rather than conventional singlet. Structural analyses further indicate that when titanium binds with a DFP (PA = 1) or TDFP (#1911C64;22−24 PA = 3) fragment, and the formed EMF is an open-shell triplet with two unpaired electrons. On the other hand, when the trapped titanium resides under a TSFP (PA = 2), the corresponding complexes (Ti@#2983C64 and Ti@#2730C64) are closed-shell singlets. Moreover, if we move the titanium from TSFP to the isolated pentagon pair of #2983 C64 and #2730C64, the stable electronic state of EMFs accordingly changes from singlet to triplet (Figure S2 in the SI), indicating that the electronic structure of Ti@C64 is strongly determined by the binding site of encaged titanium. To our best knowledge, this is the first report describing the relationship between the metal binding sites and spin states of 6891

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry

Figure 3. Frontier orbital diagrams (isovalue = 0.04 au) and spin-density distributions (isovalue = 0.004 au) of (a) Ti@#3457C64, (b) Ti@#1911C64, and (c) Ti@#2983C64.

straightforward explanation is the distinct LUMO+1 energy levels of fullerene hosts. As shown in Figure 3, the LUMO+1 values of empty #3457C64, #1911C64, and #2983C64 are −3.49, −3.65, and −3.87 eV, respectively. Such a ladder-type LUMO +1 represents the difficulty level to obtain 3d electrons of titanium. However, it is still questionable. For example, as mentioned before, the electronic structure is changed when titanium moves from the TSFP to the DFP motif of #2983C64 (Figure S2 in the SI), which means that two different electrontransfer processes take place in one host, and therefore it cannot be simply explained by the LUMO+1 level of the empty fullerene. A better explanation is to treat the titanium−fullerene electron transfer as a stepwise process. After the two 4s electrons are transferred in the first step, hybridization between the metal and cage results in a distorted complex, Ti2+@C642−. Obviously, the LUMO energy level of this divalent intermediate is crucial for subsequent 3d electron transfer. A relatively high LUMO is expected to result in a stable divalent EMF, whereas a low-lying LUMO may lead to the following transfer of 3d electrons of titanium. Similarly, if one of the two 3d electrons is transferred in the second step, the SOMO level of another intermediate, Ti3+@C643−, essentially determines the choice of the other 3d electron: to stay on titanium as a trivalent complex or to transfer to form a tetravalent structure. Therefore, the next question is, how does one obtain such distorted intermediates? Because the lower oxidation state of titanium (Ti2+ or Ti3+) has an electronic structure similar to that of the cationic state of its neighboring element (i.e., Ca2+ or Sc3+), we consider that the cationic calcium and scandium can be used as the intermediates of lower oxidized titanium. Herein, optimizations are performed on Ca@C64 and Sc@C64 in the same level of theory [B3LYP/6-311G(d)], and the results are listed in Table 2. As expected, the most feasible host for calcium encapsulation is determined as #3457C64, whereas Sc@#1911C64 is calculated as the lowest-energy isomer. Consequently, three C64 hosts with different fused-pentagon motifs are proven to be favorable for different metal atoms. Next, to elucidate such metal−cage selectivity, molecular orbital analyses on M@#3457C64, M@#1911C64, and M@#2983C64 (M = Ca, Sc, and Ti) are carried out to uncover the charge-

whereas the two 3d electrons still locate on encaged titanium. The triplet state can be attributed to the unpaired 3d electrons of divalent Ti2+. As for Ti@#1911C64 with a highly strained TDFP fragment, although the triplet ground state is the same as Ti@#3457C64, a different electron-transfer nature is observed. As depicted in Figure 3b, the unpaired electrons of the triplet complex are separately distributed on the inside titanium and fullerene surface, with the spin-density ratio (i.e., SC64/STi) of 0.95/1.05. Moreover, according to the frontier orbitals of Ti@#1911C64 shown in Figure 3b, the single electron of SOMO is distributed on C64, whereas SOMO−1 is contributed to the 3d electron of the titanium atom. Accordingly, a formal trivalent electronic configuration of titanium in triplet Ti@#1911C64 can be identified. Different from Ti@C28,14 the four valence electrons of atomic titanium did not completely but partially transferred to the carbon cage. One electron residing on the Ti 3d-derived orbital remains localized on titanium to form the SOMO−1 of the complex after encapsulation. On the other hand, the occupied HOMO and half-occupied SOMO of the complex are attributed to the C 64 -derived LUMO and LUMO+1, respectively. For the transferred titanium electrons, two of the three are paired, and thus the outside cage contains only one single electron. The triplet ground state complex can be attributed to the ferromagnetic coupling between the inner and outer electrons, and Ti@#1911C64 should be expressed as an abnormal electronic structure of Ti3+@#1911C643−. For singlet Ti@#2983C64 with a TSFP fragment (Figure 3c), orbital interaction analysis indicates that the HOMO and HOMO−1 originated from the LUMO and LUMO+1 of pristine #2983C64, whereas the unoccupied orbitals (e.g., the LUMO in Figure 3c) are localized on encaged titanium, suggesting that all four valence electrons of titanium are transferred to carbon skeleton. As expected, the strong electron transfer between titanium and C64 should be the reason why #2983 C64 with PA = 3 is more stable than the fullerenes with smaller PA values. On the basis of the discussions of molecular orbitals of Ti@ C64, we can conclude that titanium exhibits distinct ionic states in C 64 hosts with different fused-pentagon motifs. A 6892

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry

and is 0.64 eV lower in energy than that of Sc@#1911C64, implying that further electron ejection is necessary to stabilize the TSFP fragment. Finally, turn back to titanium. As depicted in Figure 3c, the titanium can perfectly donate four electrons to #2983 C64 and form the tetravalent complex. On the basis of the analyses of frontier orbital diagrams, it is clear that because of their different capabilities to accept electrons, the fullerene hosts with different types of fusedpentagon fragments show selective charge-transfer and metal encapsulation character. Such selective host−guest hybridization is important for EMF design and discovery. For example, the divalent calcium is feasible for single pentagonpair stabilization, and, consequently, it is expected that calcium can be captured in larger-size fullerenes (>C70) with isolated PA (C70 is the smallest cage with PA = 1). In theory, #10612C72 with PA = 1 has been suggested as the most feasible host for calcium encapsulation,29 which is consistent with our prediction. On the other hand, in the case of titanium with two additional d electrons, our calculations suggest that PA = 1 is not sufficient for the transfer of four valence electrons and TSFP is found to be an essential motif for titanium stabilization. It is predictable that titanium can be trapped in smaller fullerenes, in which the distribution of pentagon adjacencies is more compact. Recently, Poblet and co-workers studied the growth and structures of titanium-encapsulated small EMFs.15 Interestingly, the proposed lowest-energy Ti@C2n (2n < 50) structure in each series always contains a TSFP fragment (e.g., 197C48, #116C46, #89C44, etc.), which agrees well with our results. Herein, to clarify the role of the TSFP motif in the electron transfer of those small titanium-trapped EMFs, the electron configurations of the low-lying Ti@C2n (2n = 44, 46, 48) structures are investigated at the B3LYP/6-311G(d) level of theory. As listed in Table S2 in the SI, each selected EMF shows a closed-shell singlet ground state, which is consistent with the TSFP-containing Ti@#2983C64. Moreover, on the basis of the frontier orbital diagrams shown in Figures S5−S7 in the SI, the HOMO and HOMO−1 of each EMF are mainly contributed from the LUMO+1 and LUMO of the corresponding carbon cage, suggesting that not only the 4s but also the two 3d electrons of titanium are transferred to the cages. Therefore, we consider that the outcomes based on C64 calculations are valid, in principle, for other fullerene systems. In addition, the HOMO−LUMO gaps (Egap) of different EMFs are also collected in Table S2 in the SI. Note that the Egap value of Ti@C46 (1.68 eV) is slightly larger than that of Ti@C44 (1.62 eV), but in laser ablation studies, Ti@C44 rather than Ti@C46 is detected as the most abundant EMF besides [email protected],15 Such inconsistency can be rationalized by the kinetic stability of small EMFs during bottom-up growth. Among the three C2 addition reactions Ti@C42 + C2 → Ti@ C44, Ti@C44 + C2 → Ti@C46, and Ti@C46 + C2 → Ti@C48, kinetic calculations by Poblet et al. indicated that the second reaction exhibits the highest activation barrier compared to the other two processes,15 which reveals that Ti@C44 is easy to obtain from smaller Ti@C42 but difficult to react with foreign C2 to generate a larger Ti@C46, resulting in an abundance of Ti@C44. On the contrary, Ti@C46 is difficult to form but the following C2 addition based on Ti@C46 easily takes place with a much smaller energy barrier. Therefore, in addition to the strong metal−cage interaction, the outstanding kinetic stability of Ti@C44 should be another reason why it can be captured in experiments.

Table 2. Relative Energies (ΔE) and HOMO (SOMO)− LUMO Gaps (Egap) of Ca@C64 and Sc@C64 Ca@C64

a

Sc@C64

labela

ΔE (kcal mol−1)

Egap (eV)

ΔE (kcal/mol)

Egap (eV)

1911 2983 2730 3457 3451 3452

1.8 4.2 6.4 0.0 4.9 3.8

2.01 1.63 1.80 1.85 1.15 1.39

0.0 2.5 8.0 10.7 12.9 12.5

1.32 1.67 1.43 1.17 0.90 1.25

Labeled by the spiral code of ref 21.

transfer nature of EMFs. As shown in Figure 4, in the case of Ca@C64, the HOMOs of EMFs (originating from the LUMO

Figure 4. HOMO and LUMO orbital diagrams (isovalue = 0.04 au) of Ca@C64.

of the corresponding C64 hosts) are distributed on carbon cages, suggesting the two 4s electrons of calcium are transferred to the unoccupied orbital of fullerene cages. The LUMO energy level of Ca@#3457C64 is 0.12 and 0.51 eV higher than that of Ca@#1911C64 and Ca@#2983C64, respectively, indicating that #1911 C642− and #2983C642− are more easily obtain additional electrons for stabilization. Compared to calcium, one more d electron exists in scandium, resulting in a doublet spin state of each Sc@C64. As seen in Figure 5, in the case of Sc@#3457C64,

Figure 5. SOMO orbital diagrams (isovalue = 0.04 au) of Sc@C64.

partial distribution of SOMO on scandium can be detected to exhibit a d orbital nature, revealing that the 3d electron is not transferred completely to the host because of the relatively higher LUMO level of #3457C642− as mentioned. On the contrary, the SOMOs of both Sc@#1911C64 and Sc@#2983C64 are distributed around fullerene cages, and there is no contribution from the trapped scandium, suggesting that all of the valence electrons of scandium are transferred after encapsulation. Moreover, the TDFP fragment of Sc@#1911C64 is saturated by metal−cage hybridization, and the SOMO is delocalized away from the fused-pentagon region. On the other hand, the SOMO of Sc@#2983C64 still locates around the TSFP segment 6893

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry

sensitivity to pentagon distributions. When the atomic titanium binds on a double or triple fused-pentagon subunit of fullerenes, the formed EMFs exhibit entirely different electron-transfer character. Energetically, we find that titanium prefers to reside on the triple fused-pentagon subunits rather than the common isolated pentagon pairs because of stronger metal−cage interactions. To date, owing to the thermodynamic instability, studies on fullerenes with triple fused pentagons have always been neglected.37 However, the present study implies that the encapsulation of atomic titanium is a feasible approach to stabilizing such highly strained fragments, which may be valuable in the search for unconventional fullerenes for practical applications. In addition, we find that the proposed structure-dependent electron-transfer character can be extended to the study of metal-selective encapsulation. Calcium, scandium, and titanium are proven to reside on distinct hosts with different types of fused-pentagon motifs. Hence, the current study provides a potential route for the design and target-driven synthesis of EMFs.

Thus, our discussions focus on the relationship between the electron transfer of trapped titanium and fused-pentagon motifs of fullerenes. Next, we turn to another interesting question: when an IPR fullerene is selected as the host molecule, how does electron transfer operate between titanium and the carbon cage without fused pentagon? Here C82 species are employed as the IPR hosts for titanium encapsulation because the C82-based EMFs have been intensively studied.30−36 In particular, the Cs-symmetric #6C82 and C3v-symmetric #8C82 are two IPR-obeying isomers that have been proven as feasible cages to accept four electrons from endohedral metal or clusters.31−34 In addition, the C2v IPR structure # 9C82 has been proposed as the host for trivalent element encapsulation.35,36 On that basis, in the present work, the three isomeric C82 (#6C82, #8C82, and #9C82) are selected as host molecules for titanium encapsulation. Optimizations of those Ti@C82 structures at both singlet and triplet states are performed at the level of B3LYP/6-311G(d) and all three EMFs show triplet ground states (Table S3 in the SI). Furthermore, the calculated spin-density distributions (Figure 6) reveal that the unpaired electrons are mainly localized on the

■

COMPUTATIONAL METHODS

■

ASSOCIATED CONTENT

All calculations are performed with the Gaussian 09 program.38 The (U)B3LYP functional in conjunction with the basis set of 6-311G(d) is employed for all structural optimizations and vibrational frequency calculations. To clarify the influence of DFT methods on the stability, multiplicity, and electron-transfer nature of Ti@C64, further singletpoint calculations are performed at the levels of BP86/6-311+G(d,p), TPSSh/6-311+G(d,p), and B3LYP/6-311+G(d,p) (Table S1 in the SI). The results (Figure S4 in the SI) show that the ground state of Ti@#3457C64 is sensitive to functionals with different exact exchange parameters, but the metal−cage electron-transfer nature is not changed.

Figure 6. Spin-density distributions (isovalue = 0.004 au) of triplet Ti@#6C82, Ti@#8C82, and Ti@#9C82.

S Supporting Information *

trapped titanium, further suggesting that only two 4s electrons transfer to the cages. Such a divalent nature is different from the known tetravalent Th@C82 and trivalent U@C82,34,35 implying that fused pentagon may be important for stronger titanium− fullerene interactions. For comparison, a Cs-symmetric isomer #31759 C82, which contains a TSFP motif, is randomly selected as the non-IPR host among 669 isomeric C82 with two-pentagon pairs. Calculations show that a singlet Ti@#31759C82 is obtained and, similar to the case of Ti@#2983C82, both the HOMO and HOMO−1 are contributed from the fullerene cage rather than the inside titanium, indicating that all four electrons are transferred to form a tetravalent ion pair (Figure S8 in the SI). It should be mentioned that, although Ti@#31759C82 shows a stronger electron-transfer behavior, the IPR-based EMFs are still at least 54.8 kcal mol−1 more stable than the non-IPR EMF with two-pentagon adjacencies. Therefore, we can conclude that monotitanium is difficult to stabilize in larger fullerene systems. For the stable IPR cages, the 3d electron of atomic titanium is hard to transfer to the outer sphere. For the nonIPR cages, even a stronger titanium cage can be achieved, and the steric effect caused by fused pentagons may result in a thermodynamically unfavored EMF. This should be the reason why most of the titanium-containing EMFs are observed in the form of clusters rather than monotitanium-trapped complexes.7−13

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00284. Structures, energies, and molecular orbitals of Ti@C2n (2n = 44, 46, 48, 64, and 82) (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). *E-mail: [email protected] (S.N.). ORCID

Xiang Zhao: 0000-0003-3982-4763 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (Grants 21503157 and 21573172), the China Postdoctoral Science Foundation (Grant 2014M562402), and the Specially Promoted Research Grant (Grant 22000009) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

■

■

CONCLUSIONS In the present work, we introduce the novel character of titanium-encapsulated fullerenes. One of the most interesting features of trapping titanium into a fullerene cage is the

REFERENCES

(1) Lu, X.; Feng, L.; Akasaka, T.; Nagase, S. Current status and future developments of endohedral metallofullerenes. Chem. Soc. Rev. 2012, 41, 7723−7760.

6894

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry

(21) Fowler, P. W.; Manolopoulos, D. E. An atlas of fullerenes; Oxford University Press: Oxford, U.K., 1995. (22) Han, X.; Zhou, S. J.; Tan, Y. Z.; Wu, X.; Gao, F.; Liao, Z. J.; Huang, R. B.; Feng, Y. Q.; Lu, X.; Xie, S. Y.; Zheng, L. S. Crystal structures of saturn-like C50Cl10 and pineapple-shaped C64Cl4: geometric implications of double- and triple-pentagon-fused chlorofullerenes. Angew. Chem., Int. Ed. 2008, 47, 5340−5343. (23) Shan, G. J.; Tan, Y. Z.; Zhou, T.; Zou, X. M.; Li, B. W.; Xue, C.; Chu, C. X.; Xie, S. Y.; Huang, R. B.; Zhen, L. S. C64Cl8: A strain-relief pattern to stabilize fullerenes containing triple directly fused pentagons. Chem. - Asian J. 2012, 7, 2036−2039. (24) Dang, J.-S.; Wang, W.-W.; Zhao, X.; Nagase, S. Unconventional electronic structure and chlorination/dechlorination mechanisms of #1911 C64 fullerene. Inorg. Chem. 2016, 55, 6827−6829. (25) Rodríguez-Fortea, A.; Alegret, N.; Balch, A. L.; Poblet, J. M. The maximum pentagon separation rule provides a guideline for the structures of endohedral metallofullerenes. Nat. Chem. 2010, 2, 955− 961. (26) Campanera, J. M.; Bo, C.; Poblet, J. M. General rule for the stabilization of fullerene cages encapsulating trimetallic nitride templates. Angew. Chem., Int. Ed. 2005, 44, 7230−7233. (27) Garcia-Borràs, M.; Osuna, S.; Swart, M.; Luis, J. M.; Solà, M. Maximum aromaticity as a guiding principle for the most suitable hosting cages in endohedral metallofullerenes. Angew. Chem., Int. Ed. 2013, 52, 9275−9278. (28) Garcia-Borràs, M.; Osuna, S.; Luis, J. M.; Swart, M.; Solà, M. The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes. Chem. Soc. Rev. 2014, 43, 5089−5105. (29) Kobayashi, K.; Nagase, S.; Yoshida, M.; O̅ sawa, E. Endohedral metallofullerenes. are the isolated pentagon rule and fullerene structures always satisfied? J. Am. Chem. Soc. 1997, 119, 12693−12694. (30) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. Fullerenes with metals inside. J. Phys. Chem. 1991, 95, 7564−7568. (31) Yang, H.; Jin, H.; Wang, X.; Liu, Z.; Yu, M.; Zhao, F.; Mercado, B. Q.; Olmstead, M. M.; Balch, A. L. X-ray crystallographic characterization of new soluble endohedral fullerenes utilizing the popular C82 bucky cage. Isolation and structural characterization of Sm@C3v(7)-C82, Sm@Cs(6)-C82, and Sm@ C2(5)-C82. J. Am. Chem. Soc. 2012, 134, 14127−14136. (32) Mercado, B. Q.; Chen, N.; Rodríguez-Fortea, A.; Mackey, M. A.; Stevenson, S.; Echegoyen, L.; Poblet, J. M.; Olmstead, M. M.; Balch, A. L. The shape of the Sc2(μ2-S) unit trapped in C82: crystallographic, computational, and electrochemical studies of the isomers, Sc2(μ2-S) @Cs(6)-C82 and Sc2(μ2-S)@C3v(8)-C82. J. Am. Chem. Soc. 2011, 133, 6752−6760. (33) Valencia, R.; Rodríguez-Fortea, A.; Poblet, J. M. Understanding the stabilization of metal carbide endohedral fullerenes M2C2@C82 and related systems. J. Phys. Chem. A 2008, 112, 4550−4555. (34) Wang, Y.; Morales-Martínez, R.; Zhang, X.; Yang, W.; Wang, Y.; Rodríguez-Fortea, A.; Poblet, J. M.; Feng, L.; Wang, S.; Chen, N. Unique four-electron metal-to-cage charge transfer of Th to a C82 fullerene cage: complete structural characterization of Th@C3v(8)C82. J. Am. Chem. Soc. 2017, 139, 5110−5116. (35) Liu, X.; Li, L.; Liu, B.; Wang, D.; Zhao, Y.; Gao, X. Theoretical study on the ground state structure of Uranofullerene U@C82. J. Phys. Chem. A 2012, 116, 11651−11655. (36) Hachiya, M.; Nikawa, H.; Mizorogi, N.; Tsuchiya, T.; Lu, X.; Akasaka, T. Exceptional chemical properties of Sc@C2v(9)−C82 probed with adamantylidene carbene. J. Am. Chem. Soc. 2012, 134, 15550−15555. (37) Wang, Y.; Díaz-Tendero, S.; Martín, F.; Alcamí, M. Key structural motifs to predict the cage topology in endohedral metallofullerenes. J. Am. Chem. Soc. 2016, 138, 1551−1560. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;

(2) Akasaka, T.; Lu, X. Structural and electronic properties of endohedral metallofullerenes. Chem. Rec. 2012, 12, 256−269. (3) Popov, A. A.; Yang, S.; Dunsch, L. Endohedral fullerenes. Chem. Rev. 2013, 113, 5989−6113. (4) Popov, A. A.; Dunsch, L. Electrochemistry in cavea: endohedral redox reactions of encaged species in fullerenes. J. Phys. Chem. Lett. 2011, 2, 786−794. (5) Rudolf, M.; Wolfrum, S.; Guldi, D. M.; Feng, L.; Tsuchiya, T.; Akasaka, T.; Echegoyen, L. Endohedral metallofullerenesfilled fullerene derivatives towards multifunctional reaction center mimics. Chem. - Eur. J. 2012, 18, 5136−5148. (6) Partha, R.; Conyers, J. L. Biomedical applications of functionalized fullerene-based nanomaterials. Int. J. Nanomed. 2009, 4, 261− 275. (7) Yang, S.; Chen, C.; Popov, A. A.; Zhang, W.; Liu, F.; Dunsch, L. An endohedral titanium(III) in a clusterfullerene: putting a non-groupIII metal nitride into the C80-Ih fullerene cage. Chem. Commun. 2009, 6391−6393. (8) Popov, A. A.; Chen, C.; Yang, S.; Lipps, F.; Dunsch, L. Spin-flow vibrational spectroscopy of molecules with flexible spin density: electrochemistry, ESR, cluster and spin dynamics, and bonding in TiSc2N@C80. ACS Nano 2010, 4, 4857−4871. (9) Chen, C.; Liu, F.; Li, S.; Wang, N.; Popov, A. A.; Jiao, M.; Wei, T.; Li, Q.; Dunsch, L.; Yang, S. Titanium/yttrium mixed metal nitride clusterfullerene TiY2N@C80: synthesis, isolation, and effect of the group-III metal. Inorg. Chem. 2012, 51, 3039−3045. (10) Cao, B. P.; Hasegawa, M.; Okada, K.; Tomiyama, T.; Okazaki, T.; Suenaga, K.; Shinohara, H. EELS and 13C NMR characterization of pure Ti2@C80 metallofullerene. J. Am. Chem. Soc. 2001, 123, 9679− 9680. (11) Cao, B. P.; Suenaga, K.; Okazaki, T.; Shinohara, H. Production, isolation, and EELS characterization of Ti2@C84 dititanium metallofullerenes. J. Phys. Chem. B 2002, 106, 9295−9298. (12) Svitova, A. L.; Ghiassi, K. B.; Schlesier, C.; Junghans, K.; Zhang, Y.; Olmstead, M. M.; Balch, A. L.; Dunsch, L.; Popov, A. A. Endohedral fullerene with μ3-carbido ligand and titanium-carbon double bond stabilized inside a carbon cage. Nat. Commun. 2014, 5, 3568. (13) Li, F.-F.; Chen, N.; Mulet-Gas, M.; Triana, V.; Murillo, J.; Rodríguez-Fortea, A.; Poblet, J. M.; Echegoyen, L. Ti2S@D3h(24109)C78: a sulfide cluster metallofullerene containing only transition metals inside the cage. Chem. Sci. 2013, 4, 3404−3410. (14) Dunk, P. W.; Kaiser, N. K.; Mulet-Gas, M.; Rodríguez-Fortea, A.; Poblet, J. M.; Shinohara, H.; Hendrickson, C. L.; Marshall, A. G.; Kroto, H. W. The smallest stable fullerene, M@C28 (M = Ti, Zr, U): stabilization and growth from carbon vapor. J. Am. Chem. Soc. 2012, 134, 9380−9389. (15) Mulet-Gas, M.; Abella, L.; Dunk, P. W.; Rodríguez-Fortea, A.; Kroto, H. W.; Poblet, J. M. Small endohedral metallofullerenes: exploration of the structure and growth mechanism in the Ti@C2n (2n = 26−50) family. Chem. Sci. 2015, 6, 675−686. (16) Kroto, H. W. The stability of the fullerenes Cn, with n = 24, 28, 32, 36, 50, 60 and 70. Nature 1987, 329, 529−531. (17) Guo, T.; Diener, M. D.; Chai, Y.; Alford, M. J.; Haufler, R. E.; McClure, S. M.; Ohno, T.; Weaver, J. H.; Scuseria, G. E.; Smalley, R. E. Uranium stabilization of C28: a tetravalent fullerene. Science 1992, 257, 1661−1664. (18) Tan, Y. Z.; Li, J.; Zhu, F.; Han, X.; Jiang, W. S.; Huang, R. B.; Zheng, Z.; Qian, Z. Z.; Chen, R. T.; Liao, Z. J.; Xie, S. Y.; Lu, X.; Zheng, L. S. Chlorofullerenes featuring triple sequentially fused pentagons. Nat. Chem. 2010, 2, 269−273. (19) Thilgen, C.; Diederich, F. Structural aspects of fullerene chemistry-a journey through fullerene chirality. Chem. Rev. 2006, 106, 5049−5135. (20) Yan, Q.-B.; Zheng, Q.-R.; Su, G. Structures, electronic properties, spectroscopies, and hexagonal monolayer phase of a family of unconventional fullerenes C64X4 (X = H, F, Cl, Br). J. Phys. Chem. C 2007, 111, 549−554. 6895

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896

Article

Inorganic Chemistry Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.

6896

DOI: 10.1021/acs.inorgchem.7b00284 Inorg. Chem. 2017, 56, 6890−6896