Article pubs.acs.org/JACS
Unprecedented Chemical Reactivity of a Paramagnetic Endohedral Metallofullerene La@Cs‑C82 that Leads Hydrogen Addition in the 1,3Dipolar Cycloaddition Reaction Yuta Takano,† Zdenek Slanina,‡ Jaime Mateos,§ Takayoshi Tsuchiya,∥ Hiroki Kurihara,∥ Filip Uhlik,⊥ María Á ngeles Herranz,§ Nazario Martín,*,§,# Shigeru Nagase,*,∇ and Takeshi Akasaka*,∥,○,◆,¶ †
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan Department of Chemistry and Biochemistry, National Chung-Cheng University, Min-Hsiung, Chia-Yi 62199, Taiwan-ROC § Departamento de Química Orgánica I, Facultad de Química, Universidad Complutense, E-28040 Madrid, Spain ∥ Life Science Center of Tsukuba Advanced Research Alliance, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan ⊥ Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Albertov 6, 12843 Praha 2, Czech Republic # IMDEA−Nanoscience, Campus de Cantoblanco, Madrid E-28049, Spain ∇ Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan ○ Foundation for Advancement of International Science, Tsukuba, Ibaraki 305-0821, Japan ◆ Department of Chemistry, Tokyo Gakugei University, Tokyo 184-8501, Japan ¶ State Key Laboratory of Materials Processing and Die & Mold Technology, School of Materials Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, China ‡
S Supporting Information *
ABSTRACT: Synthesizing unprecedented diamagnetic adducts of an endohedral metallofullerene was achieved by using 1,3dipolar cycloaddition reaction of paramagnetic La@Cs-C82 with a simultaneous hydrogen addition. The selective formation of two main products, La@Cs-C82HCMe2NMeCHPh (2a and 2b), was first detected by HPLC analysis and MALDI-TOF mass spectrometry. 2a and 2b-O, which was readily formed by the oxidation of 2b, were isolated by multistep HPLC separation and were fully characterized by spectroscopic methods, including 1D and 2D-NMR, UV−vis-NIR measurements and electrochemistry. The hydrogen atom was found to be connected to the fullerene cage directly in the case of 2a, and the redox behavior indicated that the C−H bond can still be readily oxidized. The reaction mechanism and the molecular structures of 2a and 2b were reasonably proposed by the interplay between experimental observations and DFT calculations. The feasible order of the reaction process would involve a 1,3-dipolar cycloaddition followed by the hydrogen addition through a radical pathway. It is concluded that the characteristic electronic properties and molecular structure of La@Cs-C82 resulted in a site-selective reaction, which afforded a unique chemical derivative of an endohedral metallofullerene in high yields. Derivative 2a constitutes the first endohedral metallofullerene where the direct linking of a hydrogen atom has been structurally proven.
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power conversion efficiency than PC61BM3a and the highly efficient MRI contrast agent based on Gd3N@C80,4a just to name a few.3,6,7 For an easier accessibility to EMF-based materials, a wider availability of chemical functionalization methods on EMFs is required.1d,8 Unique properties of EMFs may lead to unique chemical reactivities, such as selective radical addition2c−e and enantioselective cycloaddition.2f In this regard, despite the
INTRODUCTION Endohedral metallofullerenes (EMFs),1 fullerenes which encapsulate metal atoms or clusters into their inner cavity, are novel materials which have attracted broad interests in a variety of research fields, such as chemistry, physics and biomaterial science.1−4 Remarkable features of EMFs are unique molecular structures and magnetic and electronic properties induced by the inside metals.5 A number of reports are recently available for demonstrating the potential uses of EMFs as promising novel materials. Some relevant examples are the preparation of organic solar cells based on Lu3N@C80 derivatives which demonstrated higher open circuit voltage and © XXXX American Chemical Society
Received: September 12, 2014
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dx.doi.org/10.1021/ja509407j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
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Second, a droplet of toluene was added into the reaction solution of o-DCB before refluxing, and the selectivity of the addition reaction was drastically improved as confirmed by the HPLC profiles (Figure 1, right). After refluxing for 1 h, the HPLC profile of the reaction mixture indicated that approximately 50% of the starting fullerene was consumed and two products were dominantly formed. This high selectivity in the reaction is notable since several adducts could be expected to form because of the 44 unequivalent carbon atoms of the La@C82 cage.12,13a The two products were successfully isolated from the unreacted starting materials and byproducts by using multistep high performance liquid chromatography (HPLC) (See the Supporting Information and Figure 2). The conversion yields of the main product (2a) and the minor product (2b) are 45% and 20%, respectively, based on the consumed La@C82. These values are close to 2-fold in comparison with the best yield (24%) ever reported for the chemical functionalization of La@ C82.13b Characterization of the Main Product (2a). Matrixassisted laser desorption ionization which is coupled to a time of flight analyzer (MALDI-TOF) mass spectrum of 2a shows a molecular ion peak at 1285 m/z and a fragment peak at 1123 m/z which came from pristine La@C82 formed by the loss of the addend during laser desorption (Figure 3). It is notable that the molecular ion peak is not the expected 1284 m/z for the Prato adduct of La@C82, and peaks between 1284 to 1289 m/z are not consistent with the isotopic distribution pattern of the usual Prato adduct. This result suggests that 2a is a 1,3-dipolar cycloadduct which is accompanied by an additional hydrogen atom, and the small peak at 1284 m/z would be originated from the loss of the hydrogen atom of 2a. Generally, it is well-known that the Prato adducts of La@C2vC82 have open-shell electronic structures as well as pristine La@ C2v-C82.11 In sharp contrast with the Prato adducts of La@C2vC82 which have ever been reported, as indicated by the mass spectra, 2a has a closed-shell structure because of the addition of the hydrogen atom, and accordingly, the electron spin resonance (ESR) spectra of 2a showed no signal (data not shown). For further characterization, therefore, NMR measurements were conducted for 2a as obtained. The 1H and 13C NMR spectra, respectively, demonstrated characteristic signals of the pyrrolidine ring, phenyl group, and the hydrogen atom (Figures 4 and 5). Their assignments were confirmed by DEPT 135 and 2D-NMR measurements involving HSQC and HMBC (Figures S2−S4). The 2D-NMR spectra also provided reliable information on the addition position of the unexpected hydrogen atom. The 1H NMR signal of the hydrogen atom at 3.19 ppm shows the clear correlation with carbon atoms of the fullerene cage both in HSQC and HMBC. These results indicate the direct connection between the hydrogen atom and the fullerene cage. It is also worth mentioning that the hydrogen atom is notably upfield shifted compared to dihydro, hydroalkylated and hydroarylated derivatives of C60 (Table 1)15 or the hydrogen atoms directly connected to C60 cages after intramolecular nucleophilic additions of alcohols to C60,16 which suggests that the electron-withdrawing effect from La@ C82 is much weaker than that of C60 and the hydrogen atom on La@C82 is electron-rich. Furthermore, considering the chemical shift in the 1H NMR, it could be stated that the hydrogen atom of 2a shows a much lower acidity than those C60 derivatives listed in Table 1. Indeed, the C−H bond on La@C82 can be
former efforts, much work is still needed for a better control on the chemical reactivity of the increasing variety of EMFs. Among EMFs, paramagnetic EMFs are of particular interest because interplay between π-electron spins on the fullerene cage and inside metal atoms is expected to produce unconventional magnetic features. Its potential for spintronics devices has been demonstrated by several reports, which involve electrochemical switching of the magnetic properties,9b the high electron mobility (μ) exceeding 10 cm2 V−1 s−1 of single-crystals of a derivative of La@C2v-C823b and so on.10 However, radical reactivity that is originated from the paramagnetic property of the EMFs often leads unprecedented chemical reactivities, such as the formation of singly bonded derivative of La@C2v-C82 from the Bingel−Hirsch reaction2a or the selective radical coupling reactions2d,e of the fullerenes. For creating practical electronic and magnetic materials based on EMFs, a sophisticated management of chemical functionalization is essential on the basis of deep understanding of the unique properties of paramagnetic EMFs. La@C2v-C82 is one of the most investigated EMFs and is considered a prototype of paramagnetic EMFs, since it was demonstrated that a family of lanthanum-containing fullerenes could be produced and that extraction with toluene yielded mostly
[email protected] La@Cs-C82,12 used for the present study, is an isomer of La@C2v-C82 whose chemical derivatization has been hardly investigated.13 One reason for it is that La@Cs-C82 has demonstrated relatively poor selectivity in addition reactions, resulting low yields in previous reports because of the lower symmetry relative to La@C2v-C82 and the presence of 44 nonequivalent carbon atoms. We herein report an unprecedented chemical reactivity of a paramagnetic endohedral metallofullerene La@Cs-C82. Surprisingly, we found that the 1,3-dipolar cycloaddition reaction of the fullerene, which is referred to as the Prato reaction,14 affords two new adducts of La@Cs-C82 in a highly selective way with an unprecedented and intriguing hydrogen addition reaction.
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RESULTS AND DISCUSSION Synthesis and Isolation of the Two Adducts. First, the 1,3-dipolar cycloaddition reaction of La@Cs-C82 (denoted as La@C82 hereafter for simplicity) using an azomethine ylide as 1,3-dipole was conducted based on a standard procedure by using o-dichlorobenzene (o-DCB) as solvent (Scheme 1). After Scheme 1. Synthesis of the La@Cs-C82 Derivatives
refluxing the reaction solution for 15 min, the HPLC profiles of the resulting solution showed different peaks with consumption of the starting La@C82 (Figure 1, left). The appearance of several shoulder peaks indicated that the selectivity of the addition reaction to the fullerene was quite low under these conditions. B
dx.doi.org/10.1021/ja509407j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
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Figure 1. HPLC profiles of the 1,3-dipolar cycloaddition (left) without and (right) with the addition of droplet of toluene. The latter reaction afforded 2a and 2b selectively. Conditions: Buckyprep column (ϕ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm; temperature, 40 °C.
Figure 2. HPLC profiles of isolated (left) 2a and (right) 2b-O. Conditions: Buckyprep column (ϕ 4.6 × 250 mm); eluent, toluene; flow rate, 1.0 mL/min; wavelength, 330 nm; temperature, 40 °C.
Figure 3. MALDI-TOF mass spectra of 2a accompanied by its magnified view and simulated spectra for 2a, in negative liner mode using 1,1,4,4-tetraphenyl-1,3-butadiene as a matrix.
readily oxidized (vide inf ra) as well as in the case of
[email protected] Characterization of the Minor Product (2b). Just after the isolation by HPLC, 2b mainly showed a molecular ion peak at 1285 m/z and a fragment peak at 1123 m/z in MALDI-TOF mass measurement as well as 2a (Figure 6). However, after a few minutes from the isolation under ambient conditions, the minor product showed a different spectral pattern which is attributed to an oxidized 2b (2b-O) at 1301 m/z (Figure 6) instead of 2b. This oxidation process was also confirmed by HPLC analysis in which the elution time of 2b (12.8 min) (Figure 1) was extended as the oxidation on 2b progressed to afford 2b-O (13.2 min) (Figure 2). In contrast to 2b, 2a is much more stable, and its spectra were almost unchanged after 3 days stored under ambient conditions without light. Although
Figure 4. 1H NMR spectra of 2a and 2b-O in CS2/CD2Cl2 = 3/1 (v/ v).
the reasons for the significant different stabilities of 2a and 2b are still not completely clear, it may be explained by the fact that the stability and electronic structures of the compounds can be influenced by the different direction of the addend, in particular the phenyl ring, in 2a and 2b, as indicated by DFT calculations (vide inf ra). Although the instability of 2b made it hard to accomplish further characterization, investigation on 2b-O provided the sufficient structural information which is associated with 2b. 2bO has also a closed-shell structure as well as 2a, and therefore, C
dx.doi.org/10.1021/ja509407j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
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Figure 5. 13C NMR spectra of 2a and 2b-O in CS2/CD2Cl2 = 3/1 (v/v).
Table 1. 1H NMR Chemical Shifts of Hydrogen Atoms Directly Linked to C60 in Dihydro[60]fullerene and Other Hydroalkylated and Hydroarylated Derivatives of C60 Compared to 2a compound
shift in ppm
solvent (v/v)
ref
2a C60H2 C60HCN C60H(CH2Ph) C60H(C4H7O) C60H(C4H8O) C60H(C2F5)
3.19 7.00 7.19 7.84 6.61 6.34 5.30
CD2Cl2/CS2 = 1/3 CDCl3/CS2 = 1/1 C2D2Cl4/CS2 = 1/1 CDCl3 CDCl3/CS2 = 1/1 CDCl3 CDCl3
This work 15e 15b 15g 15d 15f 15g
between the hydrogen atom and the carbon atoms of the C82 cage. This result suggested that the oxidized position is the bond between the hydrogen atom and the adjacent carbon atom of the cage, and a hydroxyl moiety was formed as the result of the oxidation. This fact was confirmed by the 1H NMR measurement of 2b-O following mixing of the sample solution of 2b-O with D2O (Figure S6). The disappearance of the proton signal of the hydrogen atom unambiguously indicates that the hydrogen atom belongs to a hydroxyl group and formation of an epoxide is excluded as the result of the oxidation (Scheme 2). Scheme 2. Formation of 2b-O
Discussion on the Reaction Mechanism and Molecular Structures of 2a and 2b-O. In the present reaction, the hydrogen addition was a key for affording 2a and 2b-O in relatively high yields. Because the reaction without addition of toluene before heating afforded a vast number of adducts as demonstrated in Figure 1 (left), some Prato adducts were possibly formed, and they were somehow unstable and evolved to a variety of different compounds. It is also indicated by the shorter reaction time (
