Preferential Encapsulation and Stability of La₃N Cluster in 80 Atom Cages: Experimental Synthesis and Computational Investigation of La₃N@C₇₉N

Abstract

Trimetallic nitride clusters, M 3 N, where M = Group IIIB and 4f-block metals, can be encapsulated in all-carbon cages (e.g., C 80 , C 88 , C 96 ) to form metallic nitride fullerenes (MNFs). [1][2][3][4] Of these metals, Sc has the smallest ionic radius, and Sc 3 N@C 80 is readily produced as the dominant member of the MNF family of compounds (e.g., Sc 3 N@C 68 ,5 Sc 3 N@C 78 6 ). The ease of synthesizing Sc 3 N@C 80 is in stark contrast to those rare-earth metals having larger radii. Efforts to synthesize larger metal atom trimetallic nitride clusters (i.e., La 3 N@C 80 ) in C 80 cages have been unsuccessful. Adjacent to La and shown in Figure 1, neighboring metal MNFs such as Ce 3 N@C 80 , Pr 3 N@C 80 and Nd 3 N@C 80 with C 80 cages are not the preferred compounds; rather the cage size increases to the preferred C 88 cage. [7][8][9] The difficulty in entrapping these bulky clusters in C 80 cages has been attributed to larger ionic radii. For La 3 N clusters, the preferred cage size shifts beyond C 88 , and La 3 N@C 96 becomes the dominant MNF. 10 In the reverse direction, from left to right (i.e., Gd, Tb, Dy, Ho, Er, Tm, and Lu), the ease and yield of making rare-earth C 80 MNFs increases as the ionic radius decreases. The smallest 4f-block based MNF, Lu 3 N@C 80 , is synthesized in high yield and is the dominant MNF.In this communication, we also report the electronic stabilization of La 3 N@C 79 N, a molecule which represents a new class of metallic nitride azafullerenes (MNAFs).The synthesis of La 3 N@C 79 N is achieved via the CAPTEAR approach (Chemically Adjusting Temperature, Energy, and Reactivity). 11 In this method, a 0.5 inch graphite rod is core-drilled to 3/16" inch and packed with a ratio of 1.25 g Sc 2 O 3 to 3.75 g La2O3. The oxidizing atmosphere and CAPTEAR conditions are achieved via addition of 2 torr/min air into the plasma reactor. Our experiments for synthesizing La 3 N@C 79 N are performed with less air (2 torr/min) added to the reactor relative to previously published CAPTEAR conditions for synthesis of Sc 3 N@C 80 (6 torr/min). 11 Other reactor conditions include a He flow rate of 630 mL/min, 220 A, 36 V gap, and dynamic flow at 300 torr. Resulting soot (11.3 g) is harvested and extracted with carbon disulfide. Upon solvent removal, the residue is washed with ether and 20 mg of extract is obtained. A MALDI mass spectrum of this material is shown in Figure 2. Figure 2B). For the La 3 N cluster, this reduction of cage size from 96 to 80 atoms reflects the significance and role of electronic effects in lieu of ionic radius.In order to understand geometric and electronic properties of the largest metallic nitride azafullerene (M 3 N@C 79 N, M = La) reported so far, we performed a series of density functional theory (DFT) calculations using the spin unrestricted mPW1PW91 method 12 with 6-31G(d) for C and N, and the relativistic effective core potential basis SDD 13 for La, as implemented in the Gaussian 03 program. 14 We have used this approach in prior investigations of various late trans...

Gd2@C79N: Isolation, Characterization, and Monoadduct Formation of a Very Stable Heterofullerene with a Magnetic Spin State of S = 15/2

“…This is also consistent with the optimized formation of the (La 3 N) 5+ cluster in the (C 79 N) 5- cage (La 3 N@C 79 N), but not in the isoelectronic (C 80 ) 6- cage. 8 …”

“…Because of the recent experimental success in preparing stable heterofullerenes with (C 79 N) 5- cages with different endohedral clusters (M 2 ) 5+ and (La 3 N) 5+ 7,8 . We have explored the stability of the heterofullerene cage alone and with two Gd +3 encapsulated ions by density functional theory (DFT) computations using the spin-unrestricted B3LYP functional in the Gaussian 03 program.…”

“…6 More recently, our laboratory has reported the isolation and characterization of the paramagnetic dimetallic endohedral heterofullerenes (EHFs), Y 2 @C 79 N and Tb 2 @C 79 N. 7 The EPR studies of Y 2 @C 79 N indicate that the unpaired spin density is mainly localized between the two equivalent yttrium ions and not on the heterofullerene cage. 7 Also, the trimetallic nitride cluster (La 3 N) 6+ has been reported by Stevenson and coworkers in the same C 79 N cage, namely, La 3 N@C 79 N. 8 Surprisingly, it was found that the large La 3 N cluster was preferentially encapsulated in a C 79 N cage, but not in a C 80 cage (e.g., La 2 @C 80 ). 8 However, most studies of EHFs are based on computational studies because experimental isolation and detailed characterization is extremely difficult because of the small amount of sample usually available and the difficulty in the separation and characterization of EHFs.…”

“…9-16 Herein, we report the preparation, purification, and spectroscopic studies of the first gadolinium based EHF, namely, Gd 2 @C 79 N. All theoretical and experimental studies confirm that this unique EHF displays an unusual high stability with a impressive yield compared with other EHFs reported to date. 7,8 The magnetic properties for this paramagnetic molecule are also very interesting since the seven unpaired f electrons for each Gd 3+ ion within the cage can interact with an additional unpaired electron formally provided by the nitrogen atom on the heterofullerene C 79 N cage. Additionally, the first successful functionalized gadolinium based EHF is also reported and isolated.…”

Endohedral fullerenes: the importance of electronic, size and shape complementarity between the carbon cages and the corresponding encapsulated clusters

“…One reported example seems to deviate and almost contradict these observations, a compound with the formula La 3 N@C 79 N, which was detected by matrix‐assisted laser desorption/ionization mass spectrometry as the preferentially templated cage using the so‐called chemically adjusting plasma temperature, energy, and reactivity synthetic method . The authors tried to explain this apparent contradiction with the other results on the basis of electronic, as opposed to size, effects, given that the size of the C 79 N cage should be very similar to that of C 80 .…”