Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX pubs.acs.org/JACS Synthesis and Characterization of Non-Isolated-Pentagon-Rule C ‑ Actinide Endohedral Metallofullerenes U@ 1(17418) C76, C ‑ C ‑ U@ 1(28324) C80, and Th@ 1(28324) C80: Low-Symmetry Cage Selection Directed by a Tetravalent Ion † § ⊥ ‡ ⊥ † ⊥ † ∥ † Wenting Cai, , , Laura Abella, , Jiaxin Zhuang, , Xingxing Zhang, Lai Feng, Yaofeng Wang, ‡ § # § Roser Morales-Martínez, Ronda Esper, Mauro Boero, Alejandro Metta-Magaña, ‡ ‡ § † Antonio Rodríguez-Fortea, Josep M. Poblet,*, Luis Echegoyen,*, and Ning Chen*, † Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, PR China ‡ Departament de Química Física i Inorganica,̀ Universitat Rovira i Virgili, c/Marcel·lí Domingo 1, 43007 Tarragona, Spain § Department of Chemistry, University of Texas at El Paso, 500 W University Avenue, El Paso, Texas 79968, United States # University of Strasbourg, CNRS, Institut de Physique et Chimie des Materiaux́ de Strasbourg UMR 7504, 23 rue du Loess, F-67034 Strasbourg, France ∥ Soochow Institute for Energy and Materials InnovationS (SIEMIS), College of Physics, Optoelectronics and Energy & Collaborative, Soochow University, Suzhou, Jiangsu 215006, PR China *S Supporting Information ABSTRACT: For the first time, actinide endohedral metal- lofullerenes (EMFs) with non-isolated-pentagon-rule (non-IPR) carbon cages, U@C80,Th@C80,andU@C76, have been successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray diffractometry, UV−vis−NIR and Raman spectroscopy, and cyclic voltammetry. Crystallo- graphic analysis revealed that the U@C80 and Th@C80 share the same non-IPR cage of C1(28324)-C80, and U@C76 was assigned to non-IPR U@C1(17418)-C76. All of these cages are chiral and have never been reported before. Further structural analyses show that enantiomers of C1(17418)-C76 and C1(28324)-C80 share a significant continuous portion of the cage and are topologically connected by only two C2 insertions. DFT calculations show that the stabilization of these unique non-IPR fullerenes originates from a four-electron transfer, a significant degree of covalency, and the resulting strong host−guest interactions between the actinide ions and the fullerene cages. Moreover, because the actinide ion displays high mobility within the fullerene, both the symmetry of the carbon cage and the Downloaded via UNIV OF TEXAS AT EL PASO on December 21, 2018 at 16:51:24 (UTC). possibility of forming chiral fullerenes play important roles to determine the isomer abundances at temperatures of fullerene See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. formation. This study provides what is probably one of the most complete examples in which carbon cage selection occurs through thermodynamic control at high temperatures, so the selected cages do not necessarily coincide with the most stable ones at room temperature. This work also demonstrated that the metal−cage interactions in actinide EMFs show remarkable differences from those previously known for lanthanide EMFs. These unique interactions not only could stabilize new carbon cage structures, but more importantly, they lead to a new family of metallofullerenes for which the cage selection pattern is ff q+ q− di erent to that observed so far for nonactinide EMFs. For this new family, the simple ionic A @C2n model makes predictions less reliable, and in general, unambiguously discerning the isolated structures requires the combination of accurate computational and experimental data. 1. INTRODUCTION cage isomers are very limited, whereas non-IPR fullerenes Fullerenes are all-carbon molecules consisting of 12 pentagonal containing fused pentagons (pentalene units) are common and 1 carbon rings and a variable number of hexagons. So far, most numerous. In contrast to IPR fullerenes, the non-IPR isomers experimental results showed that the pentagons on pristine empty fullerenes are surrounded by five hexagons, thus obeying Received: September 27, 2018 the isolated pentagon rule (IPR).2 However, the numbers of IPR Published: November 20, 2018 © XXXX American Chemical Society A DOI: 10.1021/jacs.8b10435 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX Journal of the American Chemical Society Article fi fl Figure 1. HPLC chromatograms of puri ed (a) Th@C80, (b) U@C80, and (c) U@C76 on a Buckyprep column with toluene as the eluent at ow rate of 4.0 mL·min−1. Insets show the positive mode MALDI-TOF mass spectra and expansions of the experimental and theoretical isotopic distributions of Th@C80, U@C80, and U@C76. are strongly destabilized due to the enhanced local steric strain In particular, different from the common Ln3+ charge state, on the fused pentagons caused by enforced bond angles actinides were found to adopt variable charge states depending accompanied by higher pyramidalization of the carbon on the cluster and the cage structures. For instance, four 3−5 ff atoms. Two strategies have shown practical e ectiveness in electrons of the Th atom are formally transferred to the C3v(8)- 38 generating non-IPR fullerene cages. One is exohedral halogen- C82 cage in Th@C3v(8)-C82. The oxidation state of U in U@ ation so as to release the high bond strain of the fused D (1)-C and two U@C isomers, U@C (5)-C and U@ 6−11 3h 74 82 2 82 pentagons. For instance, some non-IPR fullerenes can be C2v(9)-C82, can be 3+ or 4+, depending on the isomeric cage obtained as their chlorinated derivatives or perfluoroalkylated 36 9,12−14 structure. The very recent discovery of U2C@C80 even showed fullerenes. However, the functionalized cages are severely that U can take a formal 5+ charge state in the encapsulated distorted by the added groups, and some structures undergo cluster.39 In contrast to the corelike 4f orbitals of the skeletal transformations. lanthanides, the 5f orbitals of the actinides are chemically Endohedral metal doping is another practical way for the accessible.42,43 Thus,weanticipatethatsomepreviously stabilization of non-IPR fullerenes without changing the cage undiscovered fullerene cage structures can be stabilized by framework. In these structures, substantial electron densities encapsulating actinide metals, which may provide a new transferred from the encapsulated metallic species to the cages paradigm for metal−cage interactions and possibly alter some are significantly localized on the fused pentagons, and long-established principles based on the knowledge of stabilization results from coordination of metal ions with the 1,15,16 lanthanide EMFs. pentalene units. To date, various non-IPR endohedral Herein, we report the synthesis, isolation and systematic metallofullerenes (EMFs) with metallic clusters (i.e., M ,MN, 2 3 characterization of three new non-IPR actinide EMFs, U@ Sc2S, Sc2O; M = group 3 elements and most lanthanides) have been isolated and structurally characterized with X-ray C1(17418)-C76,U@C1(28324)-C80, and Th@C1(28324)-C80.It 17 is rather surprising to observe that two unanticipated non-IPR crystallography such as Sc3N@D3(6140)-C68, Sc2C2@ 18 19 cage isomers C1(17418)-C76 and C1(28324)-C80, which have C2v(6073)-C68, Sc3N@C2v(7854)-C70, Sc2O@C2(7892)- 20 21 never been observed before, were stabilized as actinide EMFs C70, Sc2S@D2(10528)-C72, M2@D2(10611)-C72 (M = La, 22,23 24 with a relatively high product yield. Combined experimental and Ce), DySc2N@Cs(17490)-C76, M3N@C2(22010)-C78 (M 25,26 27,28 computational studies have been performed to rationalize these = Dy, Gd), M3N@Cs(39663)-C82 (M = Y, Gd), and 29 Tb3N@C5(51365)-C84. In contrast, most studies of non-IPR unexpected results, which provide deeper understanding and cages stabilized by single metallic species are limited to the realm new insights into the metal−cage interactions and stabilization 30 of theoretical predictions, such as Ca@C72, M@C1(17459)- of EMFs. 31,32 C76 (M = Yb, Ca, Sr, Ba), M@C2v(19138)-C76 (M = Sm, Yb, 31−33 34 Ca, Sr, Ba), and Th@C1(17418)-C76, whereas exper- 2. RESULTS AND DISCUSSION imental studies are extremely rare. To the best of our knowledge, 2.1. Preparation, Purification, and Spectroscopic the only pristine mono-EMF possessing a non-IPR cage whose Characterizations of U and Th EMFs. Th@C80, U@C80, structure was unambiguously elucidated by single-crystal X-ray fi ff 33 and U@C76 were synthesized by a modi ed direct current arc- di raction is Sm@C2v(19138)-C76. In addition, a non-IPR 44 La@C isomer was obtained in the form of La@C (10612)- discharge method. The graphite rods packed with U3O8/ 72 2 graphite powder and ThO /graphite powder (molar ratio of M/ C72(C6H3Cl2), the result of synergistic stabilization from 2 endohedral metal doping and exohedral derivatization.35 This C = 1:24) were annealed and then vaporized in the arcing rare observation of non-IPR mono-EMFs seems to be ascribed chamber under a 200 Torr helium atmosphere. The collected fl to the relatively weak metal−cage interactions. Due to the very raw soot was re uxed in CS2 under an argon atmosphere for 12 limited experimental results available, a deeper understanding of h. Multistage HPLC separation procedures were employed to this phenomenon has never been fully addressed. isolate and purify Th@C80, U@C80 and U@C76 (see Figures − Recent success in the synthesis and characterization of a series S1 S3). The purity of the isolated Th@C80, U@C80, and U@ fi of actinide EMFs demonstrated that actinide EMFs show C76 were con rmed by the observation of single peaks by HPLC substantially different electronic and chemical properties from (Figure 1a−c). The positive-ion mode MALDI-TOF mass 34,36−41 fi those of the most extensively studied lanthanide EMFs.