Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Th-Based Endohedral Metallofullerenes: Anomalous Metal Position and Significant Metal-Cage Covalent Interactions with the Involvement of Th 5f Orbitals Ying Li,† Le Yang,† Chang Liu,† Qinghua Hou,† Peng Jin,*,† and Xing Lu*,‡ †
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China State Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China
‡
S Supporting Information *
ABSTRACT: Endohedral metallofullerenes (EMFs) containing actinides are rather intriguing due to potential 5f-orbital participation in the metal−metal or metal−cage bonding. In this work, density functional theory calculations first characterized the structure of recently synthesized ThC74 as Th@D3h(14246)-C74. We found that the thorium atom adopts an unusual off-axis position inside cage due to small metal ion size and the requirement of large coordination number, which phenomenon was further extended to other Th-based EMFs. Significantly, besides the strong metal−cage electrostatic attractions, topological and orbital analysis revealed that all the investigated Th-based EMFs exhibit obvious covalent interactions between metal and cage with substantial contribution from the Th 5f orbitals. The encapsulation by fullerenes is thus proposed as a practical pathway toward the f-orbital covalency for thorium. Interestingly, the anomalous internal position of Th led to a novel three-dimensional metal trajectory at elevated temperatures in the D3h-C74 cavity, as elucidated by the static computations and molecular dynamic simulations. toward the [6,6] double bond on one C2 axis of cage,29 and the similar on-axis position was reported for most of above metals. Dramatically differing from conventional metallic complexes, the internal species of EMFs may freely move inside the cage, which phenomenon has been discovered since the first observation of circular metal motion in La2@Ih(31924)C80.46,47 Likewise, the metals in M@D3h(14246)-C74 (M = Be,16 Mg,16 Ca,16,21,22,26 Sr,16,27 Ba,16,27,29 La,34 Sm,39 Eu,41,48 Yb22,43) could hop between three equivalent sites, which are along the twofold axes in the σh plane and linked by rotation around the vertical threefold axis. The study on metal motion could help explain the disorder found for the metal atoms in the X-ray structures of EMFs. The dynamic behavior is temperature-dependent and could be finely tuned by chemical modification of the outer cage.49 The control of metal dynamics inside the cage is thus expected to find potential applications in the design of functional molecular devices with novel electronic or magnetic properties. Very recently, EMFs containing actinide elements attracted great attention since the successful synthesis and XRD characterization of Th@C3v(39717)-C82 in 2017.50 Significantly, it is the first isolated mono-EMF with four electrons formally transferred from metal to the cage. As a result, it exhibits rather different electrochemical and spectral properties compared with other types of EMFs. In the mass spectra,50
1. INTRODUCTION Endohedral metallofullerenes (EMFs) are novel hybrid molecules trapping various metallic species inside different fullerene cages.1−6 The exact metal position inside fullerenes has been intensively studied for years, since it essentially determines the nature of metal−cage interplay, which further affects the properties and functionalities of EMFs. In general, the metal tends to be situated along one symmetry axis of the cage. The most famous examples are M@C2v(39718)-C82 (M = Sc, Y, La, Ce, Eu, Gd, etc.),7−10 in which the metals are all perfectly positioned under a hexagon ring on the twofold axis. However, there are some attractive exceptions. Single-crystal Xray diffraction (XRD) studies on M@C2v(31920)-C80(M = La, Yb) reveal that the metals are located at a position beneath the hexagonal ring away from the C2 axis.11,12 The absence of hexagon along the symmetry axis and the strong will of metal to reach a large coordination number are responsible for the offaxis position. For those fullerenes violating the isolated pentagon rule (IPR),13 the metal usually resides nearby the pentagon adjacencies (PAs) to release the local cage strain by charge transfer and coordination interactions.14,15 As important members of the EMFs family, the mono-EMFs M@D3h(14246)-C74 (M = Be,16−18 Mg,16,18,19 Ca,16,18−26 Sr,16,18,25,27,28 Ba,16,18,25,27,29−32 La,33−37 Pr,38 Sm,39,40 Eu,41,42 Yb,22,32,43,44 U45) have been extensively investigated. The early combined single-crystal XRD and theoretical studies on Ba@ D3h(14246)-C74 elucidated that the Ba atom is off-centered and © XXXX American Chemical Society
Received: March 31, 2018
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DOI: 10.1021/acs.inorgchem.8b00866 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
To confirm the dynamic behavior of Th atom inside cage at elevated temperatures, ab initio molecular dynamics simulations (AIMD) were performed at the PBE/DND level of theory as implemented in the DMol3 program.67,68 Effective core potentials were used to introduce relativistic correction into the core and also reduce the computational cost. A canonical NVT ensemble was used in the simulations, and the temperatures were set to 500 and 800 K by a simple Nosé−Hoover thermostat. After equilibration at the denoted temperature for 1 ps, each simulation lasted for 4 ps with a time step of 1.0 fs.
there are signals for Th@C2n (2n = 74−96), among which the possible structures of Th@Td(19151)-C76 and Th@Cs(51578)C84 are proposed by recent theoretical calculations.51−53 Closely following the Th-EMFs, the single-crystal structures of three uranium metallofullerenes U@D3h-C74, U@C2(39714)C82, and U@C2v(39718)-C82 were obtained.45 Interestingly, theoretical calculations revealed that the valence state of the encapsulated U is changed between 3+ and 4+ depending on the isomeric fullerene cages. Very recently, the long-sought U2@Ih(31924)-C80 was successfully synthesized and characterized by single-crystal X-ray crystallography, and careful calculations suggest that it bears a weak U−U metal bond.54 All these reports show us potential fascinating properties of actinide EMFs, which still remain largely unexplored thus far. Moreover, since they are a new class of actinide complexes, the role of 5f orbitals in their bonding is one intriguing and important issue to address. Herein, during the characterization of possible structure of Th@C74 by means of density functional theory (DFT) calculations, we unexpectedly found an anomalous off-axis position for the internal Th atom. Extended investigations revealed that the unusual Th location is ubiquitous in many representative fullerenes and even in those non-IPR ones. More importantly, all the Th-based EMFs feature significant f-orbital participation in the metal−cage covalent bonding. By finding a novel three-dimensional (3D) trajectory for the Th metal in C74 cage at elevated temperatures, we demonstrate that the unique metal position could lead to unprecedented dynamic behavior.
3. RESULTS AND DISCUSSION 3.1. Optimized Geometries and Relative Energies. C74 has 14 246 conventional cage isomers consisting of pentagons and hexagons.69 To quickly find the parent cage of Th@C74, an efficient strategy is to first screen the low-energy empty cages with appropriate negative charges. Since the internal Th atom formally donates four electrons to the fullerene cage (vide infra),50−52 we optimized the tetra-anions of C74 with the number of PAs ≤ 2. Table S1 summarizes the relative energies of 18 low-lying C744− cage isomers, among which C2(13333)C74 with two PAs exhibits the lowest energy. The second lowlying isomer is C2(13290)-C74, which is closely followed by the only IPR isomer D3h(14246)-C74 with a relative energy of 8.4 kcal/mol. Our calculated energy order for C744− agrees very well with the previous reports,70 confirming the reliability of current theoretical level. To form the Th@C74 endohedrals, different metal positions inside the cage were fully considered for each of above isomers. Table S1 shows that the lowest-energy one turns out to be Th@D3h(14246)-C74, with the other isomers at least 10.8 kcal/ mol higher in energy. Th@D3h(14246)-C74 also has the largest gap energy (3.09 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), suggesting its excellent thermodynamic and kinetic stability. Figure 1 depicts the optimized structures of six lowlying Th@C74 isomers (Figure S1 summarizes other highenergy ones). Our spin-polarized optimizations at various spin states (singlet, triplet, and quintet) confirmed that all of them have closed-shell singlet ground states (Table S2). In addition, when the entropy effect was considered, Th@D3h(14246)-C74
2. COMPUTATIONAL METHODS Full geometry optimizations were performed by using the M06-2X functional55 and the standard 6-31G* basis set56 for C and the Stuttgart/Dresden relativistic effective core potential and corresponding basis set57 for the metals (denoted as M06-2X/6-31G* ∼ SDD). Harmonic vibrational frequency analyses were conducted at the same level of theory after optimizations to identify the nature of stationary points to be energy minimum (all frequencies are real) or saddle point (only one imaginary frequency) on the potential energy surface (PES). The intrinsic reaction coordinate (IRC) calculations mapped out in both directions away from each transition state (TS) were performed to verify the reaction pathway. Since the entropy effect plays a critical role in the formation of EMFs at high temperature, the molar fractions of low-energy Th@C74 isomers were calculated as a function of temperature based on the equilibrium statistical thermodynamic analyses.58 To evaluate the feasibility of forming different Th-based EMFs, we computed the binding energy using Eb = Ecage + ETh − ETh@cage, where Ecage, ETh, and ETh@cage are the total energies of isolated cage, isolated Th atom, and the endohedral, respectively. Therefore, a more positive Eb value suggests that the formation of endohedral complex is thermodynamically more favorable. For the D3h(14246)-C74 cage, our calculations showed that its singlet state is 6.2 kcal/mol higher in energy than the triplet one, which was thus used for the related emptycage calculations. To elucidate the bonding nature, we performed a quantum theory of atoms in molecules (QTAIM)59 study at the M062X/6-31G* ∼ SDD level of theory by using the Multiwfn program.60 The 13C nuclear magnetic resonance (NMR) spectra were calculated using the gauge-independent atomic orbital (GIAO) method.61 The chemical shifts were first evaluated relative to C60 and were then referenced to tetramethylsilane (TMS; δ (C60) 143.15 ppm vs TMS). The electronic absorption spectrum was simulated using the time-dependent DFT (TD-DFT) method.62 To evaluate ring aromaticity, the nucleus-independent chemical shifts (NICS, in ppm)63,64 were calculated by the GIAO method. All the above DFT calculations were performed using the Gaussian 09 software,65 and the results were visualized with the aid of the Mercury program.66
Figure 1. Optimized geometries of six low-lying Th@C74 isomers (all in singlet state) with relative energies (kcal/mol) and HOMO− LUMO gap energies (eV, in parentheses). PAs and ISs are highlighted in red and yellow, respectively. B
DOI: 10.1021/acs.inorgchem.8b00866 Inorg. Chem. XXXX, XXX, XXX−XXX
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neighboring pentagons, making each of them bear 6π electrons and thus satisfy the Hückel (4n + 2) rule. In fact, the metal positions in all D3h-C74-based EMFs reported thus far are all close to this region (Figure S3). However, the internal metal position of Th@D3h-C74 is unique. Our optimization and frequency calculations show that the metals in M@D3h-C74 (M = Ca, Sr, Ba, La, Pr, Sm, Eu, Yb, U) are all symmetrically coordinated to the [6,6] bond along the C2 axis of the cage, and all the molecules thus exhibit a perfect C2v symmetry (Figure 2d). By comparison, the Th atom deviates slightly from the C2 axis and resides closer to a hexagon of the IS region (Figure 2b,c). As a result, the whole molecule symmetry of Th@D3h-C74 reduces from C2v to Cs. To confirm the unusual Th location, we scanned the PES for the movement of Th along the C2 axis of D3h-C74 and found two minima A and B corresponding to the global minimum and TS between two global minima in a common M@C74 molecule, respectively (Figure S4). However, the two Th@C74 conformations corresponding to metal positions A and B exhibit one (30.4i cm−1) and two imaginary frequencies (317.3i cm−1 and 65.6i cm−1), respectively. To justify the TS nature of the C2v-Th@D3h-C74 conformation associated with metal position A, the optimization and frequency analysis were further performed using tight optimization criteria and an ultrafine integration grid, and the resultant imaginary frequency still remained as 26.9i cm−1. Therefore, we excluded the possibility of an artificial numerical error caused by the default integration grid. Moreover, the same calculations with the larger 6-311G* basis set for carbon also yielded an imaginary frequency of 25.5i cm−1 for C2v-Th@D3h-C74. Furthermore, the calculations using another hybrid functional, B3LYP,72,73 gave rise to an imaginary frequency of 23.3i cm−1 as well. The small value of imaginary frequency is reasonable when considering the large mass of Th atom under harmonic approximation. The comparable small imaginary frequencies are recently reported for the modes related to dimetal displacement in Ih-C80 cage and suggest the instability of the corresponding configurations.74 The imaginary mode of the unstable C2v-Th@D3h-C74 mainly corresponds to the Th displacement away from the C2 axis (Figure S5). By shifting the metal from position A to M1 in Figure 2a, we optimized and obtained the global minimum structure Cs-Th@ D3h-C74 with energy going slightly downhill by 0.02 kcal/mol (Figure 2b). The relative energy of the C2v isomer compared with the Cs minimum becomes 0.08 kcal/mol when the CRENBL basis set and effective core potential75 were used for the Th atom during the optimizations. In fact, C2v-Th@D3h-C74 is the TS connecting two stable Cs-Th@D3h-C74 structures corresponding to position M1 and its equivalent site M1′ on the other side of the σh symmetry plane (Figure 2a). Two main factors may be responsible for the off-axis Th position: metal size and coordination requirement. First, for all the M@D3h-C74 molecules reported thus far, the metal ion radii have the order of Be2+ (0.45 Å) < Mg2+ (0.72 Å) < Th4+ (0.94 Å) < Pr3+ (0.99 Å) < Ca2+ (1.00 Å) < Yb2+ (1.02 Å) < U3+ (1.025 Å)76 < La3+ (1.03 Å) < Eu2+ (1.17 Å) < Sr2+ (1.18 Å) < Sm2+ (1.22 Å) < Ba2+ (1.35 Å).77 Compared with the larger cations (Pr3+-Ba2+), which may be confined along the twofold axis when coordinating with the electronegative IS region, the small ones are expected to be more ready to escape from the axis. To justify the metal size effect, we optimized the stable structure of Be@C74 (Figure S6). With the smallest size, the Be2+ ion is positioned even farther away from the C2 axis than Th4+ and much closer to the cage. Second, since the Th(IV)
exhibits rather high thermodynamic stability and dominates within a large temperature range from 0 to 5000 K (Figure S2). For simplicity, we omit the cage spiral number and denote it as Th@D3h-C74 in the following sections. D3h-C74 has three C2 symmetry axes, one C3 symmetry axis, one S3 symmetry axis, one σh symmetry plane, and three σv symmetry planes (Figure 2a). The internal Th atom is off-
Figure 2. (a) Structure of D3h-C74. (b) Optimized geometry of Th@ D3h-C74 with selected metal−cage distances in angstroms. (c) One IS patch (two sumanenes are circled by red and blue dashed lines) of Th@D3h-C74 with the natural population analysis charges (black) of surrounding pentagons in Th@C74 and C744− cage (in parentheses); NICS values at the above ring centers (red) of Th@C74 and neutral empty cage (in parentheses). (d) Schematic structure of a conventional M@D3h-C74 molecule.
centered and resides under an interlaced-sumanene (IS) region of the cage (Figure 2b,c). Clearly, the Th4+ ion is surrounded by and interacts with the four negatively charged pentagons of the C744− cage, and its position is thus favorable due to the strong metal−cage electrostatic attractions (Figure 2c). Similar metal coordination environment can be found in the typical Th(IV) organometallic complex Th(η5-C5H5)4.71 The bonding preference to the IS region is further supported by the calculated NICS values at the ring centers. The NICS values show antiaromatic properties of the surrounding pentagons on neutral empty cage (Figure 2c). When Th atom is encapsulated, the resultant large negative NICS values signify enhanced aromaticity of the IS region. The results can be understood by considering that Th atom formally donates 4e to the four C
DOI: 10.1021/acs.inorgchem.8b00866 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
3.2. Electronic Structure and Bonding Nature. To unclose the electronic property of Th@D3h-C74, natural bond orbital analysis was performed. The natural electron configuration of the Th atom changes from [Rn]6d27s2 in the free form to [Rn]7s0.105f0.366d0.777p0.20, suggesting the complete donation of 6d and 7s electrons to the cage and the substantial back-donation from cage to the metal empty 5f orbitals. The similar electron configurations are calculated for Th@ T d (19151)-C 76 ([Rn]7s 0.08 5f 0.33 6d 0.68 7p 0.35 ) and Th@ C3v(39717)-C82 ([Rn]7s0.095f0.306d0.687p0.358s0.01). Thus, as that of Th@Td-C76 and Th@C3v-C82,50,51 the electronic structure of Th@D3h-C74 should be formally described as Th4+@C744−. The metal−cage interactions are thus dominated by ionic bonding. To further disclose the bonding nature, the interactions between the Th and C atoms in Th@D3h-C74 were studied by the QTAIM analyses and compared with that of Th@ Td(19151)-C76,Th@C3v(39717)-C82, and Th@C3v(39718)-C82 (Table S3). There are two, six, four, and two Th−C bond critical points (BCPs) located for them, respectively (Table S3). The small and positive electron density (ρBCP: 0.075 to 0.093 au) and density Laplacian (∇2ρBCP: 0.141 to 0.164 au), negative total energy density (HBCP: −0.037 to −0.022 au), the ratios of absolute value of potential energy density (|VBCP|) to ρBCP (1.373 to 1.493) of greater than 1 and those of kinetic energy density (GBCP) to ρBCP (0.759 to 0.831) of less than 1, as well as large delocalization index in fuzzy atomic space (δ(Th,C): 0.718 to 0.817) at these BCPs all suggest considerable Th−C covalency.79,80 Consistently, the corresponding Wiberg bond order (WBO) values (0.609 to 0.761) for their Th−C interactions are also large. The similar bonding parameters as those of optimized one were also obtained for the single-crystal structure of Th@C3v(39717)-C82 (Table S3), strongly confirming the considerable Th−C covalent interactions in the real system. For comparison, further topological analysis was performed for the reported M@D3h(14246)-C74 (Ca, Sr, Ba, La, Pr, Sm, Eu, Yb, U), La@Td(19151)-C76, La@
ion has a rather strong coordination ability (coordination numbers from 4 to 15),78 the slight deviation from axis may be because it tries to approach to a sumanene-type hexagon to pursue a larger coordination number. Similar metal environments were recently observed in the single-crystal structures of M@C2v(31920)-C80(M = La, Yb), both of which feature off-axis metal atoms (Figure S7).11,12 The unusual off-axis position of Th in D3h-C74 stimulated us to further investigate its preferential location in other representative parent cages of mono-EMFs. We selected the Td(19151)-C76, C3v(39717)-C82, and C2v(39718)-C82 cages as examples. Figure 3 shows that in all the optimized structures
Figure 3. Optimized structures of Th@Td(19151)-C76, Th@ C3v(39717)-C82, and Th@C2v(39718)-C82 with selected metal−cage distances (Å) viewed along the symmetry axes.
the Th atom deviates slightly from the C3, C3, and C2 axis of these cages, respectively. In addition, the reported single-crystal structure of Th@C3v(39717)-C82,50 which is very similar to our optimized one, also shows an off-axis metal position. Therefore, we believe that the off-axis location is ubiquitous and could be found in more Th-based EMFs. The binding energy (Eb) of Th@C74 is calculated to be as large as 243.6 kcal/mol, while the Eb values are 270.3 and 260.8 kcal/mol for Th@C3v(39717)-C82 and Th@Td(19151)-C76, respectively. The Eb values are consistent with their relative yields (Th@C82 > Th@C76 > Th@C74) in the experiments.50
Figure 4. MO diagrams for some Th-based complexes. For selected orbitals, the participation (%) of the metal AOs is first given, followed by two consecutive numbers representing the contribution (%) of the Th-6d (blue) and Th-5f (red) AOs to this metal hybrid orbital. D
DOI: 10.1021/acs.inorgchem.8b00866 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry C3v(39717)-C82, and M@C2v(39718)-C82 (M = Sc, Y, La, Ce, Eu, Gd, U). The results reveal that the covalent interactions between metal and cage for the Th-based EMFs are stronger (indicated by larger ρBCP, δ(M,C), and WBO; more negative HBCP) than all the ones containing other metals regardless of cage size and structure. In addition, the covalent bonding interactions in Th-based EMFs is stronger than that for the common organothorium complexes such as Th(η5-C5H5)471 and Th(η8-C8H8)281 as well (Table S3). The metal−cage interactions in the Th-based EMFs were also disclosed by analyzing their occupied molecular orbitals (MOs). Figure 4 shows that there are significant overlaps between Th and cage orbitals in the HOMOs, indicating substantial covalent interactions between Th and the cage besides the dominating ionic ones. Both Th-6d and Th-5f atomic orbitals (AOs) contribute to the bonding interactions in their HOMOs. In particular, Th atom participates in some HOMOs completely via its 5f AOs (Figure 4; please see Figure S8 for enlarged ones). For example, the metal orbitals in HOMO, HOMO−2, and HOMO−11 of Th@D3h-C74 are pure 5fxz2, 5fz(x2−y2), and 5fxyz orbitals, respectively. The involvement of 5f orbitals in the covalent bonding was also found in other Th-based EMFs as well as Th(η5-C5H5)4 and Th(η8-C8H8)2 (Figure 4). Our results thus demonstrate that the encapsulation by carbon cages could serve as a new pathway toward the forbital covalency long sought in the organothorium complexes.82−84 3.3. Metal Motion. Considering the D3h symmetry of C74 cage, the Th atom has six symmetrically equivalent sites linked by the rotation of C3 axis and separated by the σh plane (M1, M2, and M3 forming layer 1 and M1′, M2′, and M3′ forming layer 2) due to its off-axis position (Figure 2a). We then searched the TS for metal migration between two minima M1 and M2 in the same layer. Surprisingly, two TS structures TS1 and TS2 (imaginary frequencies: 44.6i cm−1 and 44.7i cm−1, respectively) were located apart from layer 1 and interconnected by an intermediate (INT; Figure 5a). Our IRC calculations verified that the two TSs indeed connect the INT and corresponding energy minimum structures (Figure S9). Therefore, the migration of Th from M1 to M2 is an unusual “two-step reaction” with free energy barrier of 49.9 kcal/mol. According to the D3h cage symmetry, there are amount to 6 energy minima (Cs-Th@D3h-C74), 12 intralayer TSs, 3 interlayer TSs (C2v-Th@D3h-C74), and 6 INTs along the minimum-energy pathway for metal movement. Therefore, our static DFT computations suggest that the Th atom deviates from the C2 axis of D3h-C74 and oscillates between two adjacent stable sites belonging to two layers (M1/ 2/3 ↔ M1′/2′/3′ in Figure 2a) at low temperatures (energy barrier: 0.02 kcal/mol). Besides this interlayer one, its trajectory may further contain the intralayer M1↔M2↔M3↔M1 and M1′↔M2′↔M3′↔M1′ motions at elevated temperatures. The whole dynamic trajectory (Figure 5b) is thus 3D. This scenario is different from that of any conventional M@D3h-C74 molecule, which has three equivalent stable metal sites linked by three TSs on the σh plane, and the metal motion is mainly restricted in the two-dimensional (2D) space (Figure 5b).16,21,22,27,29,34,43,48 Furthermore, AIMD simulations were performed at the PBE/DND level of theory to vividly demonstrate the dynamic behavior of internal Th atom at higher temperatures. Figure 5c shows the trajectory obtained after 5 ps duration starting from the lowest-energy Cs-Th@D3h-C74 configuration at 800 K (the
Figure 5. (a) Energy profile (both relative energies and Gibbs free energy at 298.15 K in parentheses) for Th to transfer between two symmetrically equivalent sites in the same layer. (b) A comparison of metal trajectory of Th@D3h-C74 with that of a conventional M@C74 molecule (two views). The energy minima (M), transition state (TS), and intermediate (INT) are represented by purple, red, and yellow balls, respectively. (c) 5 ps AIMD trajectory of Th@D3h-C74 at 800 K.
one at 500 K was shown in Figure S10). The Th atom showed oscillations between two neighboring energy minima (e.g., M1↔M1′) for most of the simulation time. It then started to circulate along a 3D pathway, which essentially resembled that predicted from the above DFT calculations (Figure 5b). Therefore, static calculations and molecular dynamics (MD) simulations consistently suggest the novel 3D metal motion at elevated temperatures. To the best of our knowledge, this is the first report on the dynamic behavior of an actinide EMF molecule. 3.4. Th Position in non-IPR Cages. It is well-accepted that the trapped metal prefers to coordinate with the PAs of nonIPR fullerenes to release the local cage strain by charge transfer. Indeed, the Th atom in most of the non-IPR C74 cages tends to stay beneath the PAs (Figures 1 and S1). However, for Th@ C1(14049)-C74 (PA = 1), the Th atom prefers to locate under the IS region rather than PA (Figure 1). To rationalize this discrepancy, we optimized the structures of M@C1(14049)-C74 (M = Li, Ca, La, Th) containing typical monovalent to tetravalent metals. By putting each metal under the PA or IS fragment, we optimized and calculated the energy differences between the two resultant configurations (Figure 6). Ca@ C1(14049)-C74 shows the lowest PA-IS relative energy, which E
DOI: 10.1021/acs.inorgchem.8b00866 Inorg. Chem. XXXX, XXX, XXX−XXX
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For the UV/vis spectrum, the main absorption peaks might appear at 418, 427, 493, 606, and 747 nm (Figure S14). These spectral features are mainly ascribed to the electron transitions from the HOMO, HOMO−2, and HOMO−3 to the LUMO, LUMO+1, and LUMO+5 orbitals.
4. CONCLUSIONS In this work, we first added a new member to the growing actinide EMFs family by characterizing the observed ThC74 as the IPR-satisfying Th@D3h(14246)-C74. Differing from all the other M@D3h-C74 molecules, the Th atom oscillates between two equivalent positions slightly away from the C2 axis with 4e formally transferred to the cage. The unusual metal position was further found for the Th encapsulation in other IPR and non-IPR cages, suggesting its universality. Significantly, besides the strong metal−cage electrostatic attractions, we found substantial covalent Th−C interactions in all the Th-based EMFs, which are partially contributed by the metal’s 5f AOs. Interestingly, both static calculations and dynamic simulations revealed that the Th atom could hop along a novel 3D trajectory inside D3h-C74 at elevated temperatures. Finally, various spectra of Th@D3h-C74 were simulated to help future experimental identification. The current work not only sheds important light into the largely unexplored metal−cage interactions of the rising actinide EMFs but also provides a new approach to achieve fcovalency for actinides. The disclosed temperature-dependent dynamic behavior may also be useful for the design of novel molecular devices with tunable properties.
Figure 6. Optimized lowest-energy structures of M@C1(14049)-C74 (M = Li, Ca, La, Th) with energy differences (ΔΔE (PA-IS)) in parentheses.
can be understood by considering that the PA is greatly stabilized by accepting 2e from the Ca atom, and achieves 10π electrons to fulfill the Hückel (4n + 2) rule. By comparison, since Th formally transfers 4e to the cage, IS with four pentagons is the best candidate to accommodate the electrons to enhance local aromaticity (vide supra). Obviously, electrostatic interaction originated from the metal-to-cage electron transfer plays a primary role in stabilizing the entire EMF structure. Interestingly, although C1(13771)-C74 (PA = 2) and C1(13393)-C74 (PA = 1) also bear IS regions, they energetically favor the metal positioned under the PA (Figure 1, Table S4). In fact, we note that Th@D3h-C74 and Th@C1(14049)-C74 can transform to each other via a single step of Stone−Wales (SW) rotation with the metal position almost intact (Figure S11). Thus, the particular metal position of Th@C1(14049)-C74 should also originate from its close cage connectivity with the experimentally obtained Th@D3h-C74. Therefore, to determine the position of highly charged metallic species in non-IPR cages, the sumanene or IS patches should also be considered for the metal coordination besides the common PAs. 3.5. Spectra Simulation. Finally, IR, NMR, and UV/vis spectra of Th@D3h-C74 were simulated to assist the structural characterization in future experiments. The IR spectrum of Th@C74 can be mainly divided into three regions (Figure S12). The region from 0 to 100 cm−1 is very weak and attributed to the vibrations of encapsulated Th atom. The other two regions are contributed by fullerene cages. The metal doping redshifts the strongest peak of empty C74 from 1514 cm−1 (B) to 1509 (A′) cm−1. Some other relatively strong absorption peaks may appear at 1418, 1436, and 1531 cm−1. The 13C NMR spectrum of Th@C74 was simulated by using the GIAO method. In our previous work, the simulated 13C NMR spectra for various EMFs have perfectly reproduced the experimental observations.9,85 D3h-C74 exhibits nine lines from 132.6 to 149.2 ppm (Figure S13). With the lower Cs symmetry, Th@C74 covers broader chemical shifts ranging from 117.6 to 155.7 ppm. When the possible metal movement at the NMR time scale is considered, the spectrum may show nine lines again (128.1 to 147.4 ppm) mainly stemming from the cage symmetry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00866. Structures and relative energies of more Th@C74 isomers, QTAIM parameters, MO and IRC plots, simulated spectra, and optimized Cartesian coordinates (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. (P.J.) *E-mail:
[email protected]. (X.L.) ORCID
Peng Jin: 0000-0001-6925-9094 Xing Lu: 0000-0003-2741-8733 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from NSFC (Nos. 21103224, 51472095, 51672093, 51602112, 51602097) and Program for Changjiang Scholars and Innovative Research Team in University (IRT1014) is gratefully acknowledged.
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