Computational and Theoretical Chemistry 1050 (2014) 83–88

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Computational and Theoretical Chemistry

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Linear monometallic cyanide cluster fullerenes ScCN@C76 and YCN@C76:A theoretical prediction

http://dx.doi.org/10.1016/j.comptc.2014.10.0192210-271X/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +86 412 2960065.E-mail address: donglaiwang@163.com (D.-L. Wang).

Qing-Ya Meng, Dong-Lai Wang ⇑, Guang Xin, Tie-Chun Li, Dong-Yan HouDepartment of Chemistry, Anshan Normal University, Anshan 114007, China

a r t i c l e i n f o

Article history:Received 15 September 2014Received in revised form 12 October 2014Accepted 15 October 2014Available online 23 October 2014

Keywords:Endohedral fullereneMCN@C76

StabilityIR and 13C NMR spectra

a b s t r a c t

Very recently, two novel monometallic cluster fullerenes YCN@Cs(6)–C82 and TbCN@C2(5)–C82 areprepared and characterized, respectively. Inspired by these findings, the possibility of encapsulatingMCN (M = Sc, Y) cluster in the C76 fullerene has been performed using density functional theory. Sixisomers of MCN@C76 are considered based on six lowest energy C76

2 � isomers. The results show thattwo non-IPR MCN@C2v(19,138)–C76 and MCN@C1(17,459)–C76 present excellent thermodynamic stabilitywith very small energy differences. Significantly, in both the structures, the entrapped MCN cluster is linear,but not triangular. This is different from the results reported on YCN@Cs(6)–C82 and TbCN@C2(5)–C82.Furthermore, IR spectra and 13C NMR spectra have also been explored to assist future experimentalcharacterization.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction fullerenes, namely YCN@C (6)–C [24] and TbCN@C (5)–C [25],

The interior spaces of fullerenes are capable of hosting atoms,ions, or clusters, forming endohedral fullerene complexes. Amongthem, endohedral metallofullerenes (EMFs), fullerenes with metalatoms or clusters encapsulated inside hollow carbon cages, haveattracted broad attention owing to their unique structures,electronic properties, and potential applications in biomedicine,electronics, photovoltaics, and materials science [1–12]. SinceLa@C82 as the first EMF was isolated [13], a large number of EMFsencaging one metal atom (e.g., Ca@C60) [14], two metal atoms(e.g., Sc2@C66) [15], metal nitride (e.g., Sc3N@Cn (n = 68, 78, 80))[16–18], metal carbide (e.g., Sc2C2@C68) [19], metal sulfide (e.g.,M2S@C82 (M = Sc, Y, Dy, and Lu)) [20], metal oxide (e.g., Sc4(l3-O)2

@C80) [21], metal hydrocarbon (Sc3CH@C80) [22], and metalcarbonitride (e.g., Sc3NC@C80) [23] have been synthesized andcharacterized.

The cluster endohedral fullerenes have attracted considerableattention because of their high production yield and high stability.It is generally accepted that monometallofullerenes (mono-EMFs)always exist as a simple form of M@C2n and multiple (two to four)metal atoms in EMFs can only be achieved by cluster formation withnonmetal atoms. For many years, monometallic cluster endohedralfullerenes were considered to be impossible and the fact that theyhave not been available through the standard production methods.Very recently, two novel monometallic cyanide cluster endohedral

s 82 2 82

have been successfully synthesized, isolated and structurally char-acterized by single-crystal X-ray diffraction. These experimentalbreakthroughs open a new route for stabilizing a cluster by a singlemetal cation within a carbon cage. Therefore, it is of great interest toexplore whether such a monometallic cyanide cluster can also con-struct other endohedral fullerenes to expand this new endohedralfullerene family.

C76 is the first higher fullerene except C60 and C70. There are19,151 geometric isomers of C76, two (D2(19,150) and Td(19,151))of them satisfying the isolated pentagon rule (IPR) [26]. Both 13CNMR and theoretical calculations have shown that D2(19,150)–C76

is much more stable than Td(19,151)–C76 and the D2(19,150)–C76

isomer is produced in higher yield [27,28]. The C76-related endohe-dral fullerenes, including Lu2@C76 [29], DySc2N@C76 [30], andLa2@C76 [31] have also been structurally identified. The Lu2@C76

endohedral fullerene is assigned to the IPR Td(19,151)–C76 cage.The carbon cage in DySc2N@C76 and La2@C76 is the non-IPRCs(17,490)–C76 cage with two pairs of pentagon adjacencies. Com-putational studies suggest that two non-IPR cages, C2v(19,138)–C76 and C1(17,459)–C76, should be the best hosts for M@C76

(M = Yb, Ca, Sr, Ba) [32,33], because the metal transfers only twoelectrons to the fullerene cage.

The cage structures of EMFs are controlled by the amount ofelectron transfer to carbon cages. The electronic state of carboncage in M@C76 (M = Yb, Ca, Sr, Ba) is the same as that ofYCN@Cs(6)–C82 [24] and TbCN@C2(5)–C82 [25]. Can the C76 fuller-ene form stable endohedral fullerene MCN@C76?

84 Q.-Y. Meng et al. / Computational and Theoretical Chemistry 1050 (2014) 83–88

In this paper, the possibility of encapsulating MCN (M = Sc, Y)cluster in the C76 fullerene has been performed using densityfunctional theory (DFT).

2. Calculation details

For the present purpose, six lowest energy C762 � isomers are

considered [32]. Various possible isomers were explored by placingthe MCN cluster with different orientations within the C76 cage.Full geometry optimizations followed by harmonic vibrationalcomputations of MCN@C76 (M = Sc, Y) were performed using theB3LYP density functional [34,35] with the 6–31G* basis set for Cand N and a double zeta basis set (Lanl2dz) [36] with the effectivecore potential (ECP) for Sc and Y (denoted here by 6-31G*-lanl2dz).Vibrational analysis confirms that all the reported structures in thiswork correspond to an energy minimum on the potential energysurface. NMR spectra were computed using gauge-independentatomic orbital (GIAO) approximation and the optimized geome-tries at the same theory level. The computed 13C chemical shiftsof MCN@C76, relative to those of C60, were converted to the tetra-methylsilane (TMS) scale using the experimental value for C60

(142.5 ppm) [37]. All the calculations were carried out using theGaussian 03 program [38].

3. Results and discussion

3.1. Relative energies and stabilities

The relative energies of the dianion C762 � and the corresponding

MCN@C76 isomers at the B3LYP/6-31G*-lanl2dz level are presentedin Table 1. The non-IPR isomer C2v(19,138) is found to be the lowestenergy isomer of C76

2 �. Two cages, Td(19,151) and C1(17,459), areenergetically comparable to cage C2v(19,138) in the dianionic state(only 2.41 and 3.21 kcal/mol higher in energy, respectively). Thestability order of the C76

2 � isomers is consistent with that predictedby the previous work [32]. When ScCN is encapsulated inside C76,DFT predicts that the most stable structure is ScCN@C2v(19,138)–C76, followed by ScCN@C1(17,459)–C76 with a very small relativeenergy of 3.40 kcal/mol (Table 1). The IPR ScCN@Td(19,151)–C76

structure is the third most stable isomer with the relative energyof 8.82 kJ/mol. For the larger YCN cluster inside, the order of isomerstability for YCN@C76 follows that for ScCN@C76. The two most sta-ble isomers, YCN@C2v(19,138)–C76 and YCN@C1(17,459)–C76, areessentially isoenergetic (relative energies 0.79 kcal/mol). Further,the binding energies are all positive value (Table 1), demonstratingthat the endohedral reactions are thermodynamically favorable. Forcomparison, the geometry of the experimentally availableYCN@Cs(6)–C82 was also optimized. The binding energies of thetwo lowest energy structures are 66.6 and 72.1 kcal/mol forYCN@C2v(19,138)–C76 and YCN@C1(17,459)–C76, respectively.Compared to the binding energies of YCN@Cs(6)–C82 (62.9 kcal/mol at the same level of theory), the encapsulation of YCN into

Table 1Relative energies (DE) and HOMO–LUMO gaps of the C76

2� and the corresponding MCN@C7

Isomersa Symmetry C762� ScCN@C76

DE (kcal/mol) Gap (eV) DE (kcal/mol)

#19,138 (PA = 1) C2v 0.00 1.40 0.00#17,459 (PA = 1) C1 3.21 1.55 3.40#19,151 (PA = 0) Td 2.41 0.76 8.82#17,646 (PA = 1) C2 11.47 1.81 13.23#19,150 (PA = 0) D2 11.06 0.89 23.13#17,894 (PA = 1) C1 11.25 1.19 11.93

a The numbering of the isomers follows the nomenclature of Fowler and Manolopoulb The binding energy (Eb) is defined as the difference between the total energy of an

cluster.

two non-IPR C76 is also highly exothermic, which indicates the sta-bility of such compounds.

3.2. Electronic and geometric structures

The HOMO–LUMO (highest occupied molecular orbital and low-est unoccupied molecular orbital, respectively) energy gap valuesare also given in Table 1. The HOMO–LUMO gaps for the C76

2 �

anions with D2 and Td symmetry are 0.89 and 0.72 eV, respectively.Smaller energy gaps for two IPR C76

2 � isomers may result in theirlower kinetic stability. The calculated HOMO–LUMO gaps are1.35, 1.38, 1.69 and 1.70 eV for ScCN@C2v(19,138)–C76, YCN@C2v

(19,138)–C76, ScCN@C1(17,459)–C76 and YCN@C1(17,459)–C76,respectively. The HOMO–LUMO gap values reported for stableM@C76 (M = Ca, Sr, Ba) EMFs at the same theory level are 1.42,1.41, and 1.40 eV for Ca@C2v(19,138)–C76, Sr@C2v(19,138)–C76,and Ba@C2v(19,138)–C76, respectively; 1.71, 1.70, and 1.68 eV forCa@C1(17,459)–C76, Sr@C1(17,459)–C76, and Ba@C1(17,459)–C76,respectively [33]. Note that HOMO–LUMO gaps for C2v(19,138)and C1(17,459) isomers are altered slightly by changing the inter-nal metal or cluster. The larger gaps for MCN@C2v(19,138)–C76

and MCN@C1(17,459)–C76 accounts for their substantial kineticstabilities. Thus, the synthesis of MCN@C2v(19,138)–C76 andMCN@C1(17,459)–C76 seems feasible.

Fig. 1 depicts the optimized molecular structures of twoMCN@C76 isomers. Some key geometric parameters are listed inTable 2. For both MCN@C2v(19,138)–C76 and MCN@C1(17,459)–C76 structures, the metal atom locates upon the fused pentagons,the C atom is located in the center of the cage, and the N atomresides on one side of cage. The entrapped MCN cluster is linear(with the M–C–N angles of 180.0� in the lowest energy conformer),but not triangular. This is different from the results reported onYCN@Cs(6)–C82 [24] and TbCN@C2(5)–C82 [25]. The C–N bondlength of the trapped MCN moiety is about 1.17 Å in each case,which is longer than that found in YCN@Cs(6)–C82 (0.935 Å) [24]and TbCN@C2(5)–C82 (0.945 Å) [25]. It is interesting to note thatthe C–N bond found in the present study is in good agreement withthe results reported for traditional inorganic metal cyanide com-pounds (e.g., 1.168 Å in CuCN, 1.167 Å in AgCN, and 1.168 Å inAuCN) [39] or cyano coordination complexes (e.g., 1.172 Å in[Au(CN)2]�) [40]. The calculated distances between M and the CNunit carbon are 2.216–2.232 Å for ScCN@C76 and 2.345–2.368 Åfor YCN@C76, which are much longer than that in monomeric tran-sition-metal cyanides TM(CN) (e.g., 1.778 Å in CuCN, 2.014 Å inAgCN, and 1.920 Å in AuCN) [39].

The HOMO and LUMO orbitals of the non-IPR C2v(19,138) andC1(17,459) isomers of MCN@C76 resemble that of the correspondingempty C76

2 � dianions (Fig. S1 in the Supporting Information). There-fore, the valence state of MCN@C76 can be approximately describedas [M3+(CN)�]2+@[C76]2� (M = Sc, Y), as in the case of YCN@Cs(6)–C82

[24] and TbCN@C2(5)–C82 [25] EMFs. Calculations were also carriedout to compute their vertical electron affinity (VEA), vertical ioniza-

6 (M = Sc, Y) isomers at B3LYP/6-31G(d)-lanl2dz level of theory.

YCN@C76

Gap (eV) Ebb (kcal/mol) DE (kcal/mol) Gap (eV) Eb

b (kcal/mol)

1.35 82.0 0.00 1.38 66.61.69 85.0 0.79 1.70 72.11.24 67.0 2.69 1.14 57.71.94 82.3 9.10 1.93 71.01.20 47.6 17.70 1.14 35.71.44 56.0 8.39 1.47 46.0

os [26]. PA is the number of pentagon adjacencies.endofullerene and the sum of the total energies of the C76 cage and encapsulated

Fig. 1. Top view (left) and side view (right) of optimized structures of ScCN@C2v(19,138)–C76 (a) and ScCN@C1(17,459)–C76 (b). Green balls represent Sc atoms. Orange andblue balls denote C and N atoms of the inner CN unit, respectively, and the fused pentagon pair is highlighted in red. The YCN@C76 has similar structures. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2Optimized structural parameters, VIP, VEA, AIP, AEA, and DE(S–T) for MCN@C2v(19,138)–C76 and MCN@C1(17,459)–C76 (M = Sc, Y).

EMFs C–N bond (Å) M–C bond (Å) M–C–N angle (�) VIP (eV) VEA (eV) AIP (eV) AEA (eV) DE(S–T) (eV)

ScCN@C2v(19,138)–C76 1.168 2.216 180.0 6.19 2.63 6.15 2.69 0.48YCN@C2v(19,138)–C76 1.165 2.345 180.0 6.21 2.63 6.17 2.69 0.51ScCN@C1(17,459)–C76 1.169 2.232 177.1 6.30 2.40 6.24 2.47 0.82YCN@C1(17,459)–C76 1.167 2.368 174.0 6.31 2.41 6.26 2.49 0.83

Q.-Y. Meng et al. / Computational and Theoretical Chemistry 1050 (2014) 83–88 85

tion potential (VIP) as well as adiabatic electron affinity (AEA) andadiabatic ionization potential (AIP) (Table 2). The calculated EA(VEA 2.63 eV, AEA 2.69 eV for ScCN@C2v(19,138)–C76; VEA 2.63 eV,AEA 2.69 eV for YCN@C2v(19,138)–C76; VEA 2.40 eV, AEA 2.47 eVfor ScCN@C1(17,459)–C76; VEA 2.41 eV, AEA 2.49 eV forYCN@C1(17,459)–C76) and the IP (VIP 6.19 eV, AIP 6.15 eV forScCN@C2v(19,138)–C76; VIP 6.21 eV, AIP 6.17 eV for YCN@C2v

(19,138)–C76; VIP 6.30 eV, AIP 6.24 eV for ScCN@C1(17,459)–C76;VIP 6.31 eV, AIP 6.26 eV for YCN@C1(17,459)–C76) suggest that theC2v(19,138) and C1(17,459) isomers of MCN@C76 have good electronaccepting capacity. Note that the predicted EAs or IPs of ScCN@C76

and YCN@C76 with the same cage structure are almost the same.The internal cluster influence is minor.

The triplet states for MCN@C2v(19,138)–C76 andMCN@C1(17,459)–C76 are higher in energy than their singlet states(Table 2), indicating that the ground state for two isomers is thesingle state. The singlet–triplet splitting energies DE(S–T) ofMCN@C1(17,459)–C76 are about 0.32 eV above these of MCN@C2v

(19,138)–C76. The result is in excellent agreement with previoustheoretical data for M@C76 (M = Yb, Ca, Sr, Ba) [32,33].

3.3. IR and 13C NMR spectra

Infrared (IR) spectroscopy serves as one useful tool for structuralelucidation and compound identification. The IR spectra of isomersMCN@C2v(19,138)–C76, MCN@C1(17,459)–C76, as well as their

160 150 140 130 1200

1

2

3

4

5

ScCN@C2v(19138)-C76

Deg

ener

acy

Chemical Shifts (ppm)

160 150 140 130 1200

1

2

3

4

5

YCN@C2v(19138)-C76

Deg

ener

acy

Chemical Shifts (ppm)

2

86 Q.-Y. Meng et al. / Computational and Theoretical Chemistry 1050 (2014) 83–88

corresponding empty charged cages C762 � were simulated at the 6-

31G*-lanl2dz level of theory. As shown in Fig. 2, the spectra ofScCN@C76 and YCN@C76 are very similar to the spectra of C76

2 � withthe same C76 cage, especially in the ranges dominated by the carboncage vibrations (200–1000 cm�1 and 1000–1700 cm�1). The IR spec-tra of the internal MCN units show the C–N stretching frequencies(mCN) at 2257, 2280, 2250, and 2266; the M–CN stretching frequen-cies (mM–CN) at 478, 399, 472, and 379 cm�1 for ScCN@C2v(19,138)–C76, YCN@C2v(19,138)–C76, ScCN@C1(17,459)–C76 and YCN@C1

(17,459)–C76, respectively. Both mCN and mM–CN have very weakabsorption intensities. These values are consistent with previousassertions for inorganic metal cyanide MCN compounds (mCN at2170, 2164, 2236; mM–CN at 591, 480, 598 cm�1 for M = Cu, Ag, andAu, respectively) [41]. The IR spectra of MCN@C76 clearly show thepresence of the cyano groups.

The 13C NMR spectra of MCN@C2v(19,138)�C76 andMCN@C1(17,459)�C76 EMFs are also computed (Fig. 3). When theMCN cluster is encapsulated, two molecules still maintain the sym-metry of empty cage fullerenes, C2v and C1, respectively. Conse-quently, MCN@C2v(19,138)–C76 exhibits 24 lines, 15 with fullintensity peaks, 8 with half intensity peaks and one peak with 1/4 the intensity; MCN@C1(17,459)–C76 has 77 NMR peaks withequal intensity. The chemical shifts of the entrapped CN carbonatoms are 123, 120, 124, and 120 ppm for ScCN@C2v(19,138)–C76,YCN@C2v(19,138)–C76, ScCN@C1(17,459)–C76 and YCN@C1

(17,459)–C76, respectively. These values are different from that ofYCN@Cs(6)–C82 (292 ppm) [24], which encapsulates a triangularYCN cluster. Certainly such a difference is caused by the differentchemical environment of the internal cluster. Previous studiesindicate that the 13C cyanide chemical shift primarily influencedby the CN binding mode. Typical values of 13C chemical shifts forcyanide groups range from around 120 ppm for organic nitriles

0 500 1000 1500 2000 2500

*

ScCN@C2v(19138)-C76

Frequency/cm-1

*

YCN@C2v(19138)-C76

C2v(19138)-C76

2-

0 500 1000 1500 2000 2500

*

ScCN@C1(17459)-C76

Frequency/cm-1

*

YCN@C1(17459)-C76

C1(17459)-C76

2-

Fig. 2. Comparison of the IR spectrum of charged empty cage C762� and the

corresponding MCN@C76 (M = Sc, Y). The asterisks mark mCN stretching frequencies.

160 150 140 130 1200

1

ScCN@C1(17459)-C76

Deg

ener

acy

Chemical Shifts (ppm)

160 150 140 130 1200

1

2

YCN@C1(17459)-C76

Deg

ener

acy

Chemical Shifts (ppm)

Fig. 3. The computed 13C NMR spectra. Intensities are given in atoms per cage. Theblue lines denote chemical shifts of the CN carbon atoms. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

Q.-Y. Meng et al. / Computational and Theoretical Chemistry 1050 (2014) 83–88 87

[42], to 144.4–169.0 ppm for diamagnetic mercury cyanides [43].The calculated chemical shifts of the entrapped CN moiety ofMCN@C76 are consistent with those previously observed for cya-nides bound to Au(III) ([Au(CN)4]�, 105.2 ppm) [40,44]. The pre-sented results are expected to be helpful for the futureexperimental characterization.

4. Conclusions

In summary, the geometrical structures and electronic proper-ties of six endohedral MCN@C76 (M = Sc, Y) metallofullerenes havebeen studied by means of density functional theory (DFT) at theB3LYP/6-31G*-lanl2dz level of theory. The results reveal that theC2v(19,138) cage with a pair of adjacent pentagons is the lowestenergy isomer of C76

2 �. Two non-IPR MCN@C2v(19,138)–C76 andMCN@C1(17,459)–C76 have excellent thermodynamic stabilitywith very small energy differences. Significantly, in both the struc-tures, the entrapped MCN cluster is linear, but not triangular. Thisis different from the results reported on YCN@Cs(6)–C82 andTbCN@C2(5)–C82. The calculated C–N bond length of the trappedMCN moiety is in good agreement with the results reported for tra-ditional inorganic metal cyanide compounds. The calculated mCN

and mM–CN stretching frequencies, as well as 13C chemical shifts fur-ther confirm the existence of a single C�N environment inMCN@C2v(19,138)–C76 and MCN@C1(17,459)–C76 EMFs. It isexpected that the present results will enlarge and enrich theresearch area of monometallic cyanide cluster fullerenes.

Acknowledgements

This work was supported by Program for Liaoning Excellent Tal-ents in University (Grant No. LR2012040) and Natural ScienceFoundation of Liaoning Province, China (No. 2013020096).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.comptc.2014.10.019.

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