Tuning the Selectivity of Gd 3 N Cluster Endohedral Metallofullerene Reactions with Lewis Acids Steven Stevenson,* Khristina A Rottinger, Muska Fahim, Jessica S Field, Benjamin R Martin, and Kristine D Arvola Chemistry Department, Indiana−Purdue University at Fort Wayne, 2101 E. Coliseum Boulevard, Fort Wayne, Indiana 46805, United States *S Supporting Information ABSTRACT: We demonstrate the manipulation of the Lewis acid strength to selectively fractionate different types of Gd 3 N metallofullerenes that are present in complex mixtures. Carbon disulfide is used for all Lewis acid studies. CaCl 2 exhibits the lowest reactivity but the highest selectivity by precipitating only those gadolinium metallofullerenes with the lowest first oxidation potentials. ZnCl 2 selectively complexes Gd 3 N@C 88 during the first 4 h of reaction. Reaction with ZnCl 2 for an additional 7 days permits a selective precipitation of Gd 3 N@C 84 as the dominant endohedral isolated. A third fraction is the filtrate, which possesses Gd 3 N@C 86 and Gd 3 N@C 80 as the two dominant metallofullerenes. The order of increasing reactivity and decreasing selectivity (left to right) is as follows: CaCl 2 < ZnCl 2 < NiCl 2 < MgCl 2 < MnCl 2 < CuCl 2 < WCl 4 ≪ WCl 6 < ZrCl 4 < AlCl 3 < FeCl 3 . As a group, CaCl 2 , ZnCl 2 , and NiCl 2 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 CaCl 2 , 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, CaCl 2 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 MgCl 2 , MnCl 2 , CuCl 2 , and WCl 4 . The strongest Lewis acids (WCl 6 , ZrCl 4 , AlCl 3 , and FeCl 3 ) 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. ■ INTRODUCTION The emergence of endohedral gadolinium metallofullerenes for medical applications 1−11 has led to a desire for their isolation. Unfortunately, gadolinium soot extracts prepared under a typical dinitrogen/helium electric arc often produce complex mixtures that contain >50 types of empty-cage fullerenes (e.g., C 60 ,C 70 ,C 76 , and C 84 ), metallofullerenes with different types of endohedral clusters (e.g., Gd, Gd 2 , Gd 3 , Gd 2 C 2 , and Gd 3 N), and also their structural isomers. High-performance liquid chromatography (HPLC) has been the conventional method for endohedral separations, but chromatography is impractical given their poor solubility, low throughput, expense of solvents and waste, and difficulty resolving coeluting 12 species. To avoid HPLC purification, scientists have looked at nonchromatographic methods for fullerene separations. In 1994, Olah et al. 13 used AlCl 3 as a Lewis acid to purify C 60 from soot extract that contained only empty-cage fullerenes. Soot extract becomes further complicated by the cosynthesis of metallofullerenes and fullerenes. In 2009, Stevenson et al. 14 investigated AlCl 3 for metallofullerene separations. The selectivity was sufficient to separate Sc 3 N@C 68 , Sc 3 N@C 78 , and Sc 4 O 2 @C 80 endohedrals from contaminant endohedrals (Sc 3 N@C 80 ) and empty-cage fullerenes, whose presence dominated the fullerene distribution in the extract. 14 In 2012, Shinohara et al. 15,16 used TiCl 4 to separate monometallic (M@C 2n ), dimetallic (M 2 @C 2n ), and carbide (M 2 C 2 @C 2n ) endohedrals. In 2013, Shinohara et al. extended their use of TiCl 4 as a precipitating agent toward CF 3 - functionalized Y@C 2n derivatives. 17 In 2013, further selectivity was achieved by Stevenson and Rottinger, 18 who discovered that CuCl 2 lowered the precipitation threshold to permit resolution among erbium endohedral isomers (Er 2 @C 82 ) and selective precipitation of scandium nitrides (Sc 3 N@C 78 ) and oxides (Sc 4 O 2 @C 80 ) from fellow endohedrals, such as Sc 3 N@ C 68 and Sc 3 N@C 80 . 18 Non-Lewis acid approaches were also being developed. Other reactivity-based, nonchromatographic approaches for separating endohedrals include electrochemical reduction, 19,20 chemical redox recovery, 21,22 chemical oxidation, 23,24 host− guest complexation, 12,25−27 and reactive supports. 28−32 Narrowing the focus to nonchromatographic methods specifically for Gd 3 N@C 2n endohedrals, there is a paucity of Received: August 19, 2014 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic502024a | Inorg. Chem. XXXX, XXX, XXX−XXX