Tuning the Selectivity of Gd₃N Cluster Endohedral Metallofullerene Reactions with Lewis Acids

Abstract

We demonstrate the manipulation of the Lewis acid strength to selectively fractionate different types of Gd3N metallofullerenes that are present in complex mixtures. Carbon disulfide is used for all Lewis acid studies. CaCl2 exhibits the lowest reactivity but the highest selectivity by precipitating only those gadolinium metallofullerenes with the lowest first oxidation potentials. ZnCl2 selectively complexes Gd3N@C88 during the first 4 h of reaction. Reaction with ZnCl2 for an additional 7 days permits a selective precipitation of Gd3N@C84 as the dominant endohedral isolated. A third fraction is the filtrate, which possesses Gd3N@C86 and Gd3N@C80 as the two dominant metallofullerenes. The order of increasing reactivity and decreasing selectivity (left to right) is as follows: CaCl2 < ZnCl2 < NiCl2 < MgCl2 < MnCl2 < CuCl2 < WCl4 ≪ WCl6 < ZrCl4 < AlCl3 < FeCl3. As a group, CaCl2, ZnCl2, and NiCl2 are the weakest Lewis acids and have the highest selectivity because of their very low precipitation onsets, which are below +0.19 V (i.e., endohedrals with first oxidation potentials below +0.19 V are precipitated). For CaCl2, the precipitation threshold is estimated at a remarkably low value of +0.06 V. Because most endohedrals possess first oxidation potentials significantly higher than +0.06 V, CaCl2 is especially useful in its ability to precipitate only a select group of gadolinium metallofullerenes. The Lewis acids of intermediate reactivity (i.e., precipitation onsets estimated between +0.19 and +0.4 V) are MgCl2, MnCl2, CuCl2, and WCl4. The strongest Lewis acids (WCl6, ZrCl4, AlCl3, and FeCl3) are the least selective and tend to precipitate the entire family of gadolinium metallofullerenes. Tuning the Lewis acid for a specific type of endohedral should be useful in a nonchromatographic purification method. The ability to control which metallofullerenes are permitted to precipitate and which endohedrals would remain in solution is a key outcome of this work.

Isolation of CeLu2N@Ih‐C80 through a Non‐Chromatographic, Two‐Step Chemical Process and Crystallographic Characterization of the Pyramidalized CeLu2N within the Icosahedral Cage

“…CaCl 2 , ZnCl 2 , and NiCl 2 were the most selective and had the lowest precipitation thresholds (<+0.19 V). Intermediate precipitation thresholds (i.e., between +0.19–0.4 V) were found for MgCl 2 , MnCl 2 , CuCl 2 , and WCl 4 69. The lowest selectivity for individual metallofullerenes was achieved with the most reactive Lewis acids (WCl 6 , ZrCl 4 , AlCl 3 , and FeCl 3 ), which had the highest precipitation onsets (> +0.5 V).…”

“…For example, CuCl 2 could selectively precipitate metallic oxide fullerenes, metallic nitride fullerenes, and separate two structural isomers for Er 2 @C 82 68. For the MRI‐active Gd‐based metal nitride species, the reactivity of Gd 3 N@C 78–92 with eleven Lewis acids (CaCl 2 <ZnCl 2 <NiCl 2 <MgCl 2 <MnCl 2 <CuCl 2 <WCl 4 ≪WCl 6 <ZrCl 4 <AlCl 3 <FeCl 3 ) was investigated 69. CaCl 2 , ZnCl 2 , and NiCl 2 were the most selective and had the lowest precipitation thresholds (<+0.19 V).…”

“…With this broad spectrum of Lewis acid strengths, it was possible to control which metallic nitride fullerenes would remain in solution and which endohedrals would precipitate. In this manner, an enriched sample of Gd 3 N@C 88 was obtained with ZnCl 2 69…”

The Technology and Setup for High-Throughput Synthesis of Endohedral Metal Fullerenes

“…In order to enrich of FM with EMF, the complexation Fig. 4 Mass spectral (positive ion) analysis of fullerenes, detached from CC synthesis under 64.8 chamber pressure reaction with Lewis acids was used [12,13]. The resulting filtrate is a mixture of EMF and empty fullerenes, which accounted for most of the EMF.…”

2‐Aminoethanol Extraction as a Method for Purifying Sc3N@C80 and for Differentiating Classes of Endohedral Fullerenes on the Basis of Reactivity

“…Chromatography on silica gel followed by HPLC then produces the purified TN endohedral 16. Stevenson and co‐workers have recently published a method for TN endohedral fullerene enrichment that is based on precipitation of relatively electron‐rich metallofullerenes with Lewis acids 17–20. Unlike the previously mentioned techniques, this process leaves the empty cage unreacted and in solution.…”