Formation of Spherical Aromatic Endohedral Metallic Fullerenes

Nov 22, 2017 - Formation of Spherical Aromatic Endohedral Metallic Fullerenes. Evaluation of Magnetic Properties of M@C28 (M = Ti, Zr, and Hf) from DF...
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Formation of Spherical Aromatic Endohedral Metallic Fullerenes. Evaluation of Magnetic Properties of M@C28 (M = Ti, Zr, and Hf) from DFT calculations Alvaro Muñoz-Castro*,†,‡ and R. Bruce King*,§ †

Laboratorio de Química Inorgánica y Materiales Moleculares, Universidad Autonoma de Chile, Llano Subercaseaux 2801, San Miguel, Santiago, Chile ‡ Relativistic Molecular Physics Group, Universidad Andres Bello, Republica 275, Santiago, Chile § Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: The small C28 cage has been shown experimentally to encapsulate titanium, zirconium, and hafnium (M), among other elements. Here, we explore computationally its magnetic response properties accounting for both global and local shielding tensors. Our results exhibit a continuous shielding region for M@C28 for an orientation-averaged applied field thereby differing from that observed for the hollow C28 structure. Moreover, under a specific orientation of the applied field a long-ranged shielding cone is obtained supporting the spherical aromatic behavior expected by the 2(N + 1)2 Hirsch rule for M@C28, standing for its particular abundance. The comparison between the hollow and endohedral C28 fullerenes exhibits a characteristic long-range behavior at the outside region of the structure. The particular shape of the local chemical shift anisotropy tensor at a representative carbon atom exhibits inherent patterns as a consequence of the spherical aromatic behavior. This shows the capabilities from NMR experiments to account for the nonaromatic → aromatic variation. We envisage that the current approach will be beneficial in comparative studies of aromatic and electronic structure properties, to gain a deeper understanding of the geometrical and electronic structure situation in other endohedral species beyond that available from the information provided by routine NMR measurements.

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INTRODUCTION The discovery and further isolation in useful quantities of buckminsterfullerene (C60)1−5 initiated a new era of carbon nanotechnology because of the unique electronic and physicochemical properties of such carbon clusters.6−10 Since the early recognition of the ability of fullerene cages to encapsulate metals and other species,11,12 considerable effort has been devoted to development and understanding of formation of endohedral metallofullerenes (EMFs).13−20 Such EMFs have attracted particular interest owing to their unusual properties with promising applications especially in biomedicine21,22 and photovoltaics.23 The encapsulated metal atom is able to modify the properties of fullerenes leading to increased surface charge, thereby enhancing their chemical reactivity when compared to hollow counterparts.15,24−26 The usual EMFs are based on larger fullerenes (>C60) thereby providing a substantial internal cavity for encapsulated species.15,27 Moreover, smaller endohedral fullerenes are also able to incorporate metal atoms leading to interesting high curvature structures exhibiting unusual M−Cn bonding patterns.15,28−32 Since the report by Guo and co-workers,33 and further studies by Dunk,30 M@C28 species have been © 2017 American Chemical Society

recognized as the most prominent smaller EMFs, displaying both geometric and electronic properties conferring high stability relative to other counterparts. The capability of the C28 fullerene to incorporate tetravalent atoms, such as the group 4 metals titanium, zirconium, and hafnium, has been observed from their high relative abundances relative to different cage size species.33 Their particular stability has been explained by the 32-valence electron (32-ve) principle extensively developed by Pyykkö,34,35 where the metal donates four electrons to the π-electron kernel of C28 leading to a closed-shell structure. In addition, several hypothetical examples of favorable endohedral fullerenes (Cn, n < 60) has been suggested by theoretical calculations.28,29,36−41 This magic number of skeletal electrons ensures a spherical symmetric distribution of the electronic shells, as provided by the 2(N + 1)2 Hirsch rule based on spherical harmonics corresponding to a spherical aromatic system42 favoring the stability of these structures.43 Received: October 10, 2017 Published: November 22, 2017 15251

DOI: 10.1021/acs.inorgchem.7b02611 Inorg. Chem. 2017, 56, 15251−15258

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Inorganic Chemistry

calculations for C60 and other related species.55,71,72 To evaluate the magnetic response or induced field (Bind), upon an external magnetic field (Bext) at the molecular sourrundings, a map representation of the nucleus independent shielding tensor (σij) was obtained, where Biind = −σijBjext.66,68,73−75 For convenience, the i and j suffixes are related to the x-, y-, and zaxes of the molecule-fixed Cartesian system (i,j = x,y,z). The values of Bind are given in parts per million in relation to Bext.

Generally, the identification of such species relies on the use of high-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS)44 to determine fullerene size and the encapsulated atoms. Further detailed characterization can be obtained by Nuclear Magnetic Resonance (NMR), as a remarkably versatile tool for the elucidation of patterns ( f ingerprint) arising from both structural and electronic characteristics. Thus, 13C NMR spectroscopy is proven to be effective in the identification of fullerenes in both solution and the solid state.45−49 The 13C NMR spectrum of C60, with its aesthetic spherical icosahedral symmetry structure and as the smallest fullerene obeying the isolated-pentagon rule (IPR),50,51 shows 60 equivalent carbon atoms as indicated by the single resonance at 143.15 ppm at room temperature.52 At lower temperatures (77° K), the molecular tumbling in C60 is hindered to a large extent, unraveling the chemical shift anisotropy (CSA) pattern,48,53,54 which provides valuable information concerning the local structural and electronic properties. This reflects a nonaxial symmetry of the chemical environment at the probe nuclei.55 From theoretical evaluation of the CSA of the 13C NMR, the formation of a spherical aromatic C60 counterpart (C6010+) reveals a characteristic variation of the CSA for carbon atoms.55 In this context, herein we report our findings using density functional calculations to evaluate the particular characteristics of the magnetic response for endohedral metallofullerenes involving the smaller C28 cage. We focus on the orientation and magnitude of the CSA at different carbon atoms and on the through-space response given by the global induced magnetic field in the C28, C284−, Ti@C28, Zr@C28, and Hf@C28 series, involving the hollow C28 cage and the respective 32-π skeletal electron endohedral counterparts. The use of the global magnetic response for evaluating the magnetic behavior and aromatic character of structures containing interstitial atoms56 offers a reliable alternative to nuclear independent chemical shift (NICS) probes inside the cluster, for which the endohedral atom can influence the NICS value obtained at the center and in the faces of the structure.

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RESULTS AND DISCUSSION The calculated structures for the smaller endohedral metallofullerenes M@C28 result in C3v symmetry for Ti and in tetrahedral symmetry (Td) for Zr and Hf. The C3v-Ti@C28 structure has been described as a noncentered endohedral metallofullerene in which the Ti atom is located close to a specific face of the cage formed by three fused pentagons displaced 0.503 Å from the center (Figure 1).37,76 This

Figure 1. Representation of the structures for the M@C28 endohedral series for M = Ti (a), Zr, and Hf (b). Designation for carbon atoms is given (right).

structure is favored by 34.7 kcal·mol−1 relative to the centered disposition in Td-Ti@C28, in accord with the results by MuletGas and co-workers (37 kcal·mol−1)76 and in previous studies.37,40 For Zr and Hf Td-M@C28 structures exhibit metal−carbon bonds that average to 2.399 and 2.463 Å, respectively, which shows a slight contraction and elongation of the centroid−C distance as compared to the isolated C28 fullerene (2.441 Å).32 Moreover, the C3v-Ti@C28 structure displays several C−Ti distances ranging from 2.064 to 2.878 Å.37,76 The M@C28 derivatives are predicted to exhibit singlet electronic ground states, in accord with previous studies, which is in contrast to the multiplet ground state for the isolated endohedral metallic atoms.28,32,34,36,40,77 The titanium, zirconium, and hafnium EMFs show significant highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gaps of 1.90, 2.20, and 2.21 eV (TZ2P/ ZORA-PBE), respectively, owing to the closed-shell electronic structure given by the 32-ve principle, with a favorable bond energy.32,40 The electronic structures for these species exhibit similar characteristics with four electrons coming from the metal atom and 28 electrons from the C28 cage. The 32-ve are accommodated in a 1s21p61d101f14 electronic configuration, where the frontier orbitals are mainly of π-C28 character. As a result of the encapsulation of the endohedral metal into the C28 cage, the chemical environments of relevant C atoms are modified leading to significant variation of their chemical shifts relative to the hollow cage.15,16,78,79 To elucidate particular patterns ( fingerprints) arising from the interstitial metal atom, we simulated the NMR spectra of M@C28. For comparison, the calculated NMR spectrum for the isoelectronic C284− and the parent C28 are given. The three expected peaks in

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COMPUTATIONAL DETAILS Geometry optimizations and subsequent calculations were performed using scalar relativistic density functional theory (DFT) methods employing the ADF code57 with the allelectron triple-ζ Slater basis set plus the double-polarization (STO-TZ2P) basis set in conjunction with the Perdew− Burke−Ernzerhof (PBE) functional58,59 within the generalized gradient approximation (GGA). Vibrational analyses were performed to verify that the optimized structures are local minima. Relativistic effects were considered through the ZORA Hamiltonian.60 Open-shell structures were calculated according to the related unrestricted GGA-DFT level of theory. Evaluation of the van der Waals forces via PBE-D361,62 reveals a contribution of ∼0.2% in the stabilization of the endohedral M@C28 structures.32 In addition, the polarizable continuum model was incorporated by considering a conductor-like screening model treatment via the COSMO module63,64 with tetrahydrofuran (THF) as solvent, to take into account the effects of polar solvents in C284−, as employed in the formation of anionic fullerenes.65 The nuclear and nucleus-independent shielding tensors66−68 were calculated within the GIAO formalism, employing the OPBE58,69,70 functional and an all-electron STO-TZ2P basis set, which shows a good agreement with chemical shift 15252

DOI: 10.1021/acs.inorgchem.7b02611 Inorg. Chem. 2017, 56, 15251−15258

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Inorganic Chemistry THF as solvent for C284− are located at 187.5, 152.7, and 136.0 ppm with a relative intensity of 3:3:1 (Figure 2 and Table 1)

For Zr and Hf, three different types of carbon atoms are observed owing to their more symmetrical Td structures. In contrast to the values for the hypothetical empty isoelectronic species C284−, the peaks predicted for C1 and C3 in the endohedral M@C28 derivatives nearly overlap at 149.2 and 149.3 ppm for Zr and at 150.3 and 149.6 ppm for Hf. C2 is shifted toward a more shielding region at 175.9 and 176.5 ppm for Zr and Hf, respectively, with Δδ = −11.6 and −11.0 ppm relative to C284−. C3 also is sligthly shielded by −3.5 and −3.1 ppm (Δδ), respectively, with C1, deshielded with Δδ values of +13.2 and +14.3 ppm. Overall, these spectra exhibit characteristic patterns ( f ingerprint) owing to encapsulation of metal atoms leading to a shielding in the 13C NMR signals closest to the metal atom. This observation can be useful as initial suggestions of EMF structures from NMR experiments as has been used previously for larger fullerenes.15,80,81 Low-temperature solid-state NMR can provide information related to the principal components of the chemical shift tensor, namely, δ11, δ22, and δ33, accounting for the CSA pattern. This can be used to provide a deeper understanding of the electronic structure (Figure 3 and Table 1).45−49 The CSA values are relevant parameters that are reduced to a single value when the isotropic chemical shift (δ) is used. For example, for the C60 fullerene these values have been measured to be 220, 186, and 40 ppm at 77 K,48,53,54 denoting a typical CSA pattern for the aromatic chemical shift tensor.53,82,83 However, in a theoretical comparison between fullerene and its hypothetical truly aromatic counterpart (C6010+),55 the CSA pattern for such highly symmetrical spherical aromatic species is expected to be 241.9, 234.3, and 8.8 ppm. This indicates main differences between the δ11 and δ22 components lying at the cage surface, owing to the axial symmetry of the chemical shift tensor (δ11 ≈ δ22 ≠ δ33). For Ti@C28, Zr@C28, and Hf@C28, the 32-ve count indicates spherical aromaticity as anticipated by the 2(N + 1)2 Hirsch rule. The approach based on the CSA patterns offers an experimentally verifiable estimation of the aromatic character of carbon structures. In this sense, the terms given by the Haeberlen convention83,84 are particularly useful for determining the anisotropy and the degree of axial symmetry. Thus, aromatic carbon atoms exhibit a characteristic axially symmetric tensor with a large anisotropy in the CSA.82 The anisotropic term component defined by δaniso = δ33 − (δ11 + δ22)/2 accounts for the characteristic shift to a higher value of δ33 for the typical aromatic chemical shift tensor numbers. In addition the asymmetry parameter η = (δ22 − δ11)/(δ33 + δiso) (0 ≤ η ≤ 1) is related to the difference between the axial components. The CSA values for M@C28 (Ti, Zr, and Hf) indicate significant anisotropy consistent with the sp2 aromatic character of the cage carbon atoms. Moreover, a variable axial asymmetry is observed resulting from the strain introduced by the curvature of C28 surface, owing to the violation of the isolated-pentagon-rule (IPR). For Zr@C28 and Hf@C28, the asymmetry parameter η decreases drastically to nearly zero (η = 0.01) in contrast to the η value of 0.56 for the neutral parent C28 cage. This indicates that the chemical environment at the surfaces in Zr@C28 and Hf@C28 are more axially symmetric, similar to that found for the spherical aromatic C6010+.55 However, for the acentric Ti@C28 structure, larger asymmetry parameters are found showing that the location of the endohedral atom affects significantly the 13C−CSA, where the atoms closest to the metal center show smaller asymmetry parameters. Interestingly, between the parent C28 and the

Figure 2. Calculated 13C NMR spectra in color code for C28 and related 32-π electron cages, using the isotropic chemical shielding (δiso) of C60 as a secondary reference (143.15 ppm vs TMS).

and were found accounting for three different types of carbon atoms in such a tetrahedral cage. The carbon atom located at the middle of each triplet of directly fused pentagons is the most shielded at 136.0 ppm (C1), whereas the carbon atom located next to C1 is deshielded to 187.5 ppm (C2). Finally, the carbon atoms located at the rim of the triplet of directly fused pentagons (C3) are slightly less shielded than C1, with an expected peak at 187.5 ppm. Such signals are deshielded in comparison to the parent neutral Td-C28 in its preferred quintuplet ground state.76 Upon encapsulation of Ti, the less symmetrical C3v-Ti@C28 structure exhibits eight different types of carbon atoms owing to the decrease in symmetry from Td to C3v. Thus, the four equivalent C1 atoms split into two groups, namely, C1 and C1′, with a 1:3 ratio; the nine equivalent C2 atoms split into three groups designated as C2, C2′, and C2″ (3:3:3 ratio), and the 15 equivalent C3 atoms split into three groups, designated as C3, C3′, and C3″ in a 6:6:3 ratio (Table 1). For the face containing the triplet of directly fused pentagons closest to Ti, the most shielded atom is C1 at 152.8 ppm. C2 is calculated at 166.8 ppm, in contrast to the related deshielded peak located at 187.5 ppm in C284−. The C3 carbon atoms are predicted at 155.3 ppm, corresponding to an ∼3 ppm deshielding relative to C284−. The average chemical shifts for the different types of C1, C2, and C3, are 159.4, 173.4, and 158.3 ppm, respectively. This indicates a global shielding of both C2 by −14.1 and a deshielding of both C1 (+23.4 ppm) and C3 (+5.6 ppm) (Δδ), to the isoelectronic hollow C284− counterpart. 15253

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Table 1. Calculateda CSA Parameters Involving the Isotropic, Anisotropic, and Asymmetry Terms According to the Absolute Shielding (σij), and the Relative Shielding Tensor (δiso) ij C28 C1 C2 C3 C284− C1 C2 C3 Ti@C28 C1 C2 C3 C2′ C1′ C3′ C3″ C2″ Zr@C28 C1 C2 C3 Hf@C28 C1 C2 C3

11

22

33

iso

anisob

asymc

δisod

10.0 −25.2 −57.4

10.0 −6.0 −5.4

101.0 144.2 106.8

40.4 37.7 14.7

90.9 159.8 138.2

0.00 0.18 0.56

140.5 143.2 166.2

2.7 −90.2 −42.2

2.7 −85.3 −12.3

129.1 155.4 138.8

44.8 −6.7 28.1

126.4 243.1 166.0

0.00 0.03 0.27

136.0 187.5 152.7

−10.5 −53.8 −41.5 −76.2 −51.7 −43.6 −59.6 −104.6

−9.8 −52.2 −11.6 −28.2 −9.5 −19.7 −36.4 −73.2

104.4 148.2 129.6 154.0 118.8 136.7 133.6 152.4

28.0 14.1 25.5 16.5 19.2 24.5 12.5 −8.5

114.5 201.2 156.2 206.2 149.4 168.3 181.5 241.4

0.01 0.01 0.29 0.35 0.42 0.21 0.19 0.20

152.8 166.8 155.3 164.3 161.6 156.4 168.3 189.3

−11.0 −72.5 −23.2

−11.0 −64.9 −15.5

116.9 152.2 133.3

31.6 4.9 31.6

127.9 221.0 152.7

0.00 0.05 0.08

149.2 175.9 149.3

−13.3 −75.3 −24.8

−13.3 −64.6 −15.9

118.2 152.9 134.3

30.5 4.3 31.2

131.5 222.8 154.7

0.00 0.07 0.09

150.3 176.5 149.6

a

Values in parts per million. bAnisotropic term, see text. cAsymmetry term, see text. dRelative shielding tensor (δiso) obtained by using C60 as secondary reference according to, for a given carbon A, δAcalc = δC60exp + σC60calc − σAcalc, with δC60exp = 143.15 ppm relative to TMS,48 and σC60Calc = 37.68 ppm. The same procedure can be applied to express a specific eigenvalue from CSA (σij) in relative shielding tensor (chemical shift) δij.

describes the orientation, magnitude, and sign of the local response relative to its own principal axis system (PAS)49,54,72,85 (Figure 3). The spatial representation of σij, instead of δij provides a clearer and intuitive description of the size and direction of each component related to the magnitude of the response.55 For the three different types of carbon atoms in the Td structures, related to the triplet of directly fused pentagons motif (C1, C2, and C3), σ33 is oriented toward the center of the structure, with σ11and σ22 lying at the cage surface. Strong differences between the CSA patterns are observed, accounting for the different isotropic chemical shifts discussed above. The sizable deshielding of the axial component of C2 is the largest and is comparable to that obtained for C60 (σ33 = 146.4; δ33 = 39.1; δexp33 = 4048,55) exhibiting similar pyramidalization angles (POAV) θp ≈ 13.1° and 11.6°, respectively. The increases in the POAV θp parameters for C1 and C3 of ∼20.9 and ∼17.9°, respectively, reduce the anisotropy of the CSA tensor indicating a decrease in the axial symmetry. This leads to a more shielded chemical shift in the 13C NMR spectrum for C1 and C3, compared with that of C2. Comparison between the isoelectronic Td symmetric systems C284−, Zr@C28, and Hf@C28 reveals clearly that the most significant terms in the CSA arising from the encapsulation of the metal center are the components tangential to the fullerene surface (i.e., σ11 and σ22). Finally, the asymmetry parameter (η) indicates a minor difference between the latter components, as designated for Zr@C28 and Hf@C28. However, for Ti@C28, the

Figure 3. Graphical representation of the shielding tensors (σij, Table 1) for representative carbon atoms, namely, C1, C2, and C3, concerning its orientation and magnitude in its PAS (±: blue and orange), in 32-π electron spherical aromatic cages. σ33 is oriented perpendicularly to the cage surface. For Ti@C28 only C1, C2, and C3 are shown.

respective tetraanion (C284−) the anisotropy term increases, and the asymmetric parameter η decreases, as expected for aromatic sp2 carbons.55,82 To exploit further the information provided by CSA in these 32-π electron spherical aromatic systems, a graphical representation of the absolute shielding (σij, i,j = 1, 2, 3) 15254

DOI: 10.1021/acs.inorgchem.7b02611 Inorg. Chem. 2017, 56, 15251−15258

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Inorganic Chemistry acentric location of the metal atom leads to significant asymmetry in the more distant carbon atoms. To unravel the global magnetic behavior in the structures of interest, the response under an averaged and a specific orientation of the applied field were obtained. This provides global criteria for evaluating aromaticity of clusters containing interstitial atoms for which the traditional method using the nuclear independent chemical shift (NICS) method in the center of the cluster cannot be applied. In such systems the atoms close to the central probe can influence the NICS value at the center and faces of the structure. We have shown that the global magnetic response accounts for the shielding cone inherent to planar and spherical aromatic structures.56 The isotropic magnetic response given by the orientationaveraged term Bisoind = −(1/3)(σxx + σyy + σzz)Bjext accounts for the in-solution molecular tumbling. First, we explore the Bisoind for the isolated neutral Td-C28 fullerene in the quintet spin state with four unpaired electrons, which exhibits slightly deshielded regions at the center of each face similar to nonaromatic fullerenes.86,87 Interestingly, encapsulation by a tetravalent atom modifies strongly the resulting behavior leading to a continuous shielding region. This is similar to that found for C202+, C32, C50, and C6010+, corresponding to 18-, 32-, and 50-π electron Hirsch aromatic structures, exhibiting characteristic features for spherical aromatic compounds.56,87 Comparison between Td-C28 and Zr@C28 in Figure 4 shows the effect of the endohedral atom on the global magnetic response of the C28 fullerene, leading from a nonaromatic to a spherical aromatic fullerene.

through the main C3-axis and x- and y- perpendicular to it, the neutral Td-C28 exhibits a short-range response, which is located close to the C28 backbone owing to the diminished response from its nonaromatic character.55,87 In strong contrast, for Zr@ C28 a long-range shielding response under a perpendicularly oriented applied field (Bzext) is found, involving a complementary deshielding region at the perpendicular plane. Such behavior nicely resembles the long-range shielding cone characteristic for planar aromatic rings,67,68,82,88 indicating a direct relation between planar and spherical aromatic compounds.87,89 Furthermore, for spherical aromatic species, the long-range shielding cone is obtained in relation to the specific orientation of the external field. This differs from planar aromatic rings, where only an induced perpendicular field can give rise to such characteristic.67,68,82,88 In addition, the response for a field oriented in not matching with any symmetry axis is given for Zr@C28 at the Supporting Information, showing that these findings are obtained in any direction for spherical aromatic compounds, which is similar to the found for C202+, C32, C50, and C6010+ Hirsch aromatic fullerenes.56,87 The contour plot representation on Figure 5 depicts the variation of the induced field at both the center and the

Figure 4. Three-dimensional representation of the magnetic response Bind for Td-C28 and Zr@C28 as a representative example of M@C28, in relation to the orientation-averaged external field (Bisoext; iso), and to specific orientations of the applied field (Bzext, Bxext, and Byext, respectively) indicated by arrows. Isosurface values set at −2 and +2 ppm relative to the external field, denoting shielding regions in blue and deshielding regions in red.

Figure 5. Contour plot representations of the magnetic response Bind for the studied series, in relation to the orientational-averaged external field (Bisoext; iso), and two specific orientations of the applied field (Bzext and Bxext, denoted by arrows).

surroundings of the C28 cage. Interestingly, a related response is found for Ti, Zr, and Hf endohedral fullerenes, owing to the metal → fullerene charge transfer upon metal encapsulation and bond formation,32 where the valence electrons from the interstitial atom are transferred to the carbon shell. This indicates that the related 32-π electron aromatic fullerenes, showing a related global magnetic response, can be expected to resemble larger fullerenes satisfying the Hirsch rule. In such series, the isotropic representation shows a continuous shielding region contained in the cage. The long-range shielding cone, obtained from two different orientations of

This aromatic behavior enhanced the stability in the studied EMF derived from C28,43 which has been evaluated via other criteria by Miralrio and Sansores recently.40 They found a substantial aromatic character by using structural, magnetic, and energetic criteria.40 The characteristics of the induced magnetic field can be unraveled under a specific orientation of the applied field. Thus, upon different orientations of the field, with z-axis pointing 15255

DOI: 10.1021/acs.inorgchem.7b02611 Inorg. Chem. 2017, 56, 15251−15258

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the applied field, exhibits a shielding region of −5.0 ppm at 5.6 and 6.0 Å for Zr@C28 and Hf@C28, respectively. For Ti@C28, the acentric location of Ti induces a slightly longer range for the face closer to Ti (5.9 Å) than the face on the opposite side (5.6 Å). A shielding region of −10 ppm is found at 4.6 and 4.8 Å for the heavier counterparts and at 4.7 and 4.4 Å for the two different sides of Ti@C28, respectively. This observation accounts for the long-range characteristics of the shielding cone in the spherical C28 cage with 32-π valence electrons. This is similar to that found for the two preferred isomers for the C32 fullerene, and it is also observed for the 18π and 50π electron fullerene structures.87 Thus, the encapsulation of tetravalent elements into C28 leads to truly spherical aromatic compounds, which can be expected for similar larger species.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (A.M.-C.) *E-mail: [email protected]. (R.B.K.) ORCID

Alvaro Muñoz-Castro: 0000-0001-5949-9449 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the reviewers for their useful comments. A.M.-C. thanks the financial support of FONDECYT 1140359 and Millennium Project No. RC120001.

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CONCLUSIONS The endohedral M@C28 metallofullerenes are the smallest metal-encapsulated species characterized experimentally. The inclusion of tetravalent elements leads to 32-π electron spherical aromatics satisfying the Hirsch rule and exhibiting long-range characteristics of truly spherical aromatic fullerenes. The behavior of the isolated neutral C28 fullerene strongly contrasts with that found for the M@C28 series (M = Ti, Zr, and Hf), exhibiting a short-range behavior, with a shielding response at the cage backbone and deshielding regions in the center of each face. After the endohedral atom is allowed for, a homogeneous shielding region is found for the orientational averaged response. Upon a field oriented along a specific axis, a parallel long-range shielding response is generated, with a complementary deshielding region contained in the perpendicular plane, resembling the characteristic shielding cone property of aromatic species, standing for its particular abundance. In addition, the isotropic chemical shielding can differentiate between the species, exhibiting a deshielding shift when 32π electron Hirsch aromatic systems are formed. Interestingly, for Td-M@C28 (M = Zr, Hf), two peaks overlap showing two main signals corresponding to the (C1;C3) and C2 carbons. For C3vTi@C28, eight chemically different atoms are obtained, exhibiting a strong dependence of the 13C NMR patterns on the endohedral atom position. Analysis of the local CSA tensor shows a major trend to axial symmetry in the aromatic structures, with an increase in their anisotropies (aniso) and a decrease in the asymmetry (η) of the tangential components. These provide useful parameters to gain a deeper understanding of the variation of the chemical environment for the cage atoms in the nonaromatic → aromatic transition. We anticipate that the current approach will benefit comparative studies of aromaticity and electronic structure in endohedral fullerenes thereby providing a better understanding than that obtained by simple routine NMR measurements.

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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.7b02611. Three-dimensional representation of the magnetic response Bind for Zr@C28 in relation to a specific orientation of the applied field oriented along the C3 symmetry axis, and avoiding any symmetry axis (PDF) 15256

DOI: 10.1021/acs.inorgchem.7b02611 Inorg. Chem. 2017, 56, 15251−15258

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