Large Gadolinium Nitride Cluster Encapsulated inside a Non‐IPR Carbon Cage: A Theoretical Characterization on Gd₃N@C₇₈

Abstract

Dedicated to Professor Shigeru Nagase on the occasion of his 65th birthday With metal atoms or clusters entrapped in their inner spaces, endohedral metallofullerenes (EMFs) are viewed as unique compounds with a variety of potential applications. [1][2][3][4][5][6][7][8][9][10] These EMFs are commonly divided into several forms by virtue of the encapsulated moieties.[8] The tri-metallic nitride templated endohedral metallofullerenes (TNT EMFs) often attract much attention due to their high stability and yield resulting from strong metal-cage interactions. [5,6,[10][11][12] Particularly, many efforts have been devoted to gadolinium nitride cluster fullerenes, because they can be used as excellent contrast agents for magnetic resonance imaging (MRI). [13,14] To date, six such EMFs, Gd 3 N@C 2n (2n = 78~88), have been successfully synthesized. [15][16][17][18][19][20] Although the gadolinium nitride cluster fullerenes have been discussed with several experimental methods, [15][16][17][18][19][20][21][22] only a few theoretical studies have been carried out on these significant chemical compounds. [23À 26] As the smallest Gd 3 Ncontaining EMFs, Gd 3 N@C 78 was revealed by X-ray diffraction to have the C 2 (22010)ÀC 78 cage OK which violates the isolatedpentagon rule (IPR) and has two pairs of pentagon adjacencies.[15] (The fullerene isomers are coded by the spiral number, which was proposed by Fowler and Manolopoulous. [27] ) Because of its relatively small carbon cage and large metal cluster, Gd 3 N@C 2 (22010)ÀC 78 is a good candidate for revealing the structures, electronic properties and metal-cage interactions in gadolinium nitride cluster fullerenes.Many other C 78 -based EMFs have been extracted successfully and most of them were determined to possess the IPR carbon cage D 3h (24109)ÀC 78 , such as La 2 @C 78 , [28,29] Ce 2 @C 78 , [30] Ti 2 C 2 @C 78 , [31][32][33][34] and Sc 3 N@C 78 . [35,36] In contrast, the major isomers of Dy 3 N@C 78 and Tm 3 N@C 78 were proposed to bear the same non-IPR carbon cage with Gd 3 N@C 78 . [37][38][39] Thus, as the only isolated and characterized non-IPR carbon cage, C 2 (22010)-C 78 is an exceptional structure among 24109 classical C 78 isomers.[27] Herein, we report-for the first time-a thorough quantum chemical investigation on the electronic structures, electron affinity, ionization potential, thermodynamic stability, and IR vibrational frequencies of Gd 3 N@C 78 endohedral metallofullerene systems including both IPR and non-IPR cages.Several electronic states with different spin multiplicities were calculated to search for the ground state of Gd 3 N@C 2 (22010)-C 78 . The results revealed that a high-spin state with S = 21/2 has the lowest relative energy, indicating that seven 4f electrons of each Gd 3 + ion are ferromagnetically coupled in the ground state. Interestingly, the relative energy of the low-spin state with S = 7/2, which represents seven 4f electrons in one Gd 3 + ion that are non-ferromagnetically coupled with fourteen 4f electrons on other two Gd 3 + ion...

Structural interconnections and the role of heptagonal rings in endohedral trimetallic nitride template fullerenes

“…For YSc 2 N@C 78 , conversely, the isomer with the lowest energy also has a large HOMO‐LUMO gap (1.37 eV) and is a classical, but non‐IPR cage, C 78 :22010 with two pentagon adjacencies. The same cage has been reported as the parent in Gd 3 N@C 78 , Y 3 N@C 78 , and GdSc 2 N@ [13c] ; in these cases, as for YSc 2 N, the central TN is larger than Sc 3 N. The isomer with the second lowest energy is nonclassical but lies only 1.8 kcal mol −1 higher, so could be expected to have significant population in an equilibrium mixture at 1000 K. LaSc 2 N@C 78 is predicted to have the same most favored cage as YSc 2 N@C 78 , but a second isomer based on a (different) nonclassical cage ( :185809) that also has two pentagon adjacencies, and lies 9.0 kcal mol −1 above the best classical isomer.…”

Full Exploration of the Diels–Alder Cycloaddition on Metallofullerenes M3N@C80 (M=Sc, Lu, Gd): The D5h versus Ih Isomer and the Influence of the Metal Cluster

“…[33] It was also recently published that the low-spin state in the case of the related compound Gd 3 N@C 78 is 4.5 meV ( % 0.1 kcal mol À1 ) lower in energy than the highspin state. [34] All calculations reported here have been performed considering the antiferromagnetic coupling (i.e., S = 7 = 2 ). In practice, the analytical vibrational frequencies for the TSs of the gadolinium-based EMFs were not calculated due to the high computational cost and SCF convergence difficulties related with the high number of unpaired electrons, and based on the similarities of the TS structures with those of the other metal clusters.…”

“…The difference in energy between the latter two configurations was found to be extremely low (0.1 kcal mol −1 ) 33. It was also recently published that the low‐spin state in the case of the related compound Gd 3 N@C 78 is 4.5 meV (≈0.1 kcal mol −1 ) lower in energy than the high‐spin state 34. All calculations reported here have been performed considering the antiferromagnetic coupling (i.e., S =7/2).…”

A benchmark study of DFT methods on the electronic properties of lanthanofullerenes: a case study of Ce@C2v(9)-C82 anion

“…[57][58][59] We note that CEP-4G basis set has been used in previous studies of EMFs and was shown to perform well, and a few examples are the studies of Dy 2 @C 100 , 60 Yb@C 76 , 61 Gd 3 N@C 78 (ref. 62) and Gd 2 @C 98 . 63 The calculations with BS2 basis set gives an RMSD of 10.56 ppm using BHandHLYP method, which is 4.55 ppm larger than that at the BS1 level, suggesting CEP-4G may be not a good choice for an open-shell molecular system.…”