Isolation and Characterisation of Two Sc₃N@C₈₀ Isomers

Abstract

Endohedral fullerenes exhibit an intriguing variety of carbon cages and encaged atoms, ions or clusters. For example, the isolated pentagon rule (IPR) obeying missing cage C 74 and the non-IPR fullerene C 66 are accessible only in endohedral form, [1,2] new C 82 isomers have been stabilised by the encapsulation of one Tm 2 + ion and even a Sc 3 3 + cluster has been found inside a C 82 fullerene. [3,4] The discovery of Sc 3 N@C 80 in 1999 opened the gate to a new class of fullerenes with endohedral trimetal nitride cluster molecules.[5] Such four-atomic species have never been prepared before. Several promising material properties make trimetal nitride clusterfullerenes outstanding, for example, unusually high yields relative to empty, mono-and di-metallofullerenes, high thermal stability and long term stability in air. [5][6][7][8][9] The use in solar cells and as local magnets are presently the most discussed applications of fullerene compounds. Regarding the latter possibility, magnetic moments up to three times larger than those of well-known mono-metallofullerenes are expected for trimetal nitride cluster fullerenes.Currently scandium is the most commonly used metal in the synthesis of trimetal nitride cluster fullerenes. The Sc 3 N cluster has also been encapsulated in C 68 and C 78 . [10,11] The mixedmetal cluster fullerene ErSc 2 N@C 80 has been separated and characterised by X-ray diffraction.[12] Yttrium, [9] holmium, [7] terbium, [13] and lutetium [14] have been encaged as trimetal nitrides (abbreviated: M 3 N) in C 80 . Very recently the presence of a second cage isomer has been reported for samples which were previously considered as pure Sc 3 N@C 80 (I h ) isomer.[15] It was estimated, that this sample contains approximately 10 % of the second isomer.[15] Due to its crucial role in the class of cluster fullerenes, the isolation and characterisation of isomeric pure Sc 3 N@C 80 is of great importance. This is the precondition for the detailed analysis of optical and vibrational properties of both isomers. In this context we have to question, whether the presence of the second Sc 3 N@C 80 isomer could account for the near-IR (NIR) absorptions reported in the first paper on Sc 3 N@ C 80 . [5] Another open question is whether the structural properties of the encaged Sc 3 N cluster differ in both isomers.Herein, the isolation of the second Sc 3 N@C 80 isomer is reported for the first time. The electronic transitions and optical energy gap of both isomer pure Sc 3 N@C 80 structures have been analysed by Vis/NIR spectroscopy. In addition, the electronic properties of Sc 3 N@C 80 (I) were studied by cyclic voltammetry. IR and Raman spectroscopy, in combination with group theory, were used to examine the vibrational structure of both Sc 3 N@C 80 isomers. Based on this analysis the cage symmetry of the Sc 3 N@C 80 (II) isomer and the geometric structure of the encaged Sc 3 N cluster in Sc 3 N@C 80 (II) were determined. Experimental SectionThe fullerene soot was prepared by a modified Krätschmer-Huffman arc-bur...

Violating the Isolated Pentagon Rule (IPR): The Endohedral Non‐IPR C70 Cage of Sc3N@C70

“…[3,5,11,12] Separation of the cluster fullerenes was performed by two-step HPLC (HewlettPackard 1100) with toluene as the eluent. In the first step, a linear combination of two analytical Buckyprep columns (4.6 250 mm; Nacalai Tesque) was applied.…”

“…Sample preparation and experimental details for UV/Vis/ NIR and FTIR spectroscopic measurements were described previously. [3,5,11,12] DFT computations were performed with the PRIRODA package [28] employing the PBE density functional [29] and the implemented TZ2P-quality basis set with effective-core potential for Sc atoms. [30] Received: August 10, …”

“…The latter exhibits a large variety of cavity sizes ranging from C 60 to C 100 . [1][2][3][4][5][6][7][8][9][10][11][12] There have even been cages reported that do not obey the isolated pentagon rule (IPR). These non-IPR cages include Sc 2 @C 66 , [8] Sc 3 N@C 68 , [9, 11c] Sc 2 C 2 @C 68 , [13] and La 2 @C 72 .…”

“…In the first-step HPLC, Sc 3 N@C 70 was eluted on Buckyprep column together with C 84 , which was subsequently removed in the second-step HPLC on a Buckyclutcher column (see the Supporting Information). HPLC and MS analysis showed the purity of the isolated Sc 3 N@C 70 (t R = 21.5 min, m/z 989) to be higher than or equal to 99 %, (Figure 1 The UV/Vis/NIR spectrum of Sc 3 N@C 70 dissolved in toluene shows a spectral onset of 960 nm (Figure 2), which corresponds to an optical band gap of 1.3 eV [1][2][3][4][5][6][7][8][9][10][11][12] and indicates that Sc 3 N@C 70 is a stable fullerene. The optical band gap of Sc 3 N@C 70 is larger than those of Sc 3 N@C 68 (1.1 eV) and Sc 3 N@C 78 (1.0 eV) but smaller than those of Sc 3 N@C 80 (I) (1.7 eV) and Sc 3 N@C 80 (II) (1.6 eV).…”

“…[14] C 70 fullerenes are a peculiar species because up to now only noble-gas atoms (He, Ne, Ar, Kr, Xe), [15] N, [16] and P [16a] have been encapsulated inside the C 70 cage. The metal atoms that typically form endohedral fullerenes with other cages [1][2][3][4][5][6][7][8][9][10][11][12] have not yet been entrapped in C 70 . Besides Sc 3 N@C 68 , the trimetallic nitride endohedral fullerenes (cluster fullerenes) exhibit a large variety of cage sizes; [3,5,11,12] however, a M 3 N@C 70 species is missing.…”