Charged States of Sc 3 N@C 68 : An In Situ Spectroelectrochemical Study of the Radical Cation and Radical Anion of a Non-IPR Fullerene Peter Rapta, †,‡ Alexey A. Popov, †,§ Shangfeng Yang, †,¥ and Lothar Dunsch* ,† Group of Electrochemistry and Conducting Polymers, Leibniz-Institute for Solid State and Materials Research Dresden, D-01171 Dresden, Germany, Department of Physical Chemistry, Faculty of Chemical and Food Technology, SloVak UniVersity of Technology, SK-81237 BratislaVa, SloVak Republic, and Chemistry Department, Moscow State UniVersity, Moscow 119992, Russia ReceiVed: March 27, 2008 The redox behavior of Sc 3 N@C 68 is studied systematically by means of electrochemistry, in situ ESR/Vis-NIR spectroelectrochemistry, and detailed theoretical treatment. Formation of the negatively and positively charged paramagnetic species for the same trimetallic nitride endohedral fullerene is demonstrated for the first time. The electrochemical study of Sc 3 N@C 68 exhibits two electrochemically irreversible but chemically reversible reduction steps and two reversible oxidation steps. A double-square reaction scheme is proposed to explain the observed redox reaction at cathodic potentials involving the reversible dimerisation of the Sc 3 N@C 68 monoanion. The spin state of the radical cation and the radical anion is probed by ESR spectroscopy, indicating that in both states, the large part of the unpaired spin is delocalized on the fullerene cage. The charged states of the non-isolated pentagon rule fullerene are characterized furthermore by in situ absorption spectroscopy. The interpretation of experimental data is supported by the density functional theory (DFT) calculations of the spin distribution in the anion and cation radicals of Sc 3 N@C 68 and time-dependent DFT calculations of the absorption spectra of the charged species. Introduction Endohedral fullerenes with an encaged trimetallic nitride cluster (clusterfullerenes) have been attracting great interest since their discovery in 1999 1 because of the feasibility of tuning the trapped metal atoms and stabilizing a large variety of cage sizes with different isomeric structures. 2–13 It has been recently discussed how much the variation of the cluster size can be extended 3,14 and how the leading forces for the stabilization of endohedral structures have been revealed. 15 Sc 3 N@C 68 discov- ered in 2000 16 is a unique member of the clusterfullerene family because it was the first clusterfullerenes with the cage violating the isolated pentagon rule (IPR; note, however, that several new non-IPRnitrideclusterfullereneshavebeendiscoveredrecently). 9,10,12,17 The non-IPR carbon cage with D 3 symmetry (isomer 6140) was proven for this clusterfullerene by means of NMR 16 and single- crystal X-ray studies. 18 However, despite the increased interest in such unusual structures, only few spectroscopic and theoreti- cal studies of Sc 3 N@C 68 have been reported up to now. 19–22 The electronic structure and vibrational spectra of Sc 3 N@C 68 were studied in detail experimentally and with theoretical computations in our group. 22 Although the electronic structure of trimetallic nitride clusterfullerenes is formally conceived as a result of a six-fold electron transfer from the cluster to the carbon cage, it was shown that the effect of Sc 3 N encapsulation in the C 68 cage is more complicated, with a significant hybridization of the cluster and the cage orbitals. 20,22 The cluster-to-cage electron transfer in the clusterfullerenes implies that the surplus electron density is localized on the fullerene and hence raises the question of the redox behavior of these molecules. Systematic electrochemical studies of clusterfullerenes including Sc 3 N@C 80 (I), 6,23,24 Tm 3 N@C 80 (I), 25 Dy 3 N@C 2n (2n ) 78, 80), 26 Y 3 N@C 80 , 27 Er 3 N@C 80 , 27 Gd 3 N@C 2n (2n ) 80, 84, 88), 28 and Sc 3 N@C 78 24 were reported in the past few years by our group 6,25,26 and by the others. 23,24,27,28 A strong dependence of the electrochemical redox behavior and frontier molecular orbitals (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) on the cage size and the structure and a significant contribution of the cluster to both the LUMO and the HOMO were found. By decreasing the fullerene cage size to C 68 and by encapsulating the trimetallic nitride cluster in such a small cage, a substantial change in the LUMO and the HOMO is expected as compared to larger cage sizes. Thus, the detailed analysis of different Sc 3 N@C 68 redox states appears an interesting point to be studied. Recently, we reported the electrochemically generated stable radical cation of Sc 3 N@C 68 as confirmed by in situ ESR/Vis-NIR spectroelectrochemistry. 29 The spin state of such radical cation was probed, indicating that the large part of the unpaired spin is delocalized over the fullerene cage. The anodic redox behavior of Sc 3 N@C 68 is quite different from that in the cathodic range, where a more complex redox behavior was found. The complex redox behavior in the cathodic part, with strongly shifted reoxidation peaks both for the anion and dianion, appears to be typical for nonfunctionalized M 3 N@C 2n endohedral fullerenes, 6,23,25–28 and the observation of the cor- responding paramagnetic anions of M 3 N@C 2n endohedral fullerenes is up to now limited only to the [Sc 3 N@C 80 ] •-30 and [Y 3 N@C 80 C 4 H 9 N] •- . 31 In this work, we performed a detailed spectroelectrochemical and theoretical study on the charge transfer at Sc 3 N@C 68 with the focus on the formation of both * To whom correspondence should be addressed. E-mail: L.Dunsch@ ifw-dresden.de. † Leibniz-Institute for Solid State and Materials Research Dresden. ‡ Slovak University of Technology. § Moscow State University. ¥ New address: Hefei National Laboratory for Physical Sciences at Microscale & Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China. J. Phys. Chem. A 2008, 112, 5858–5865 5858 10.1021/jp802655f CCC: $40.75  2008 American Chemical Society Published on Web 06/11/2008