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Metal Chelates Based on Isoxazoline[60]fullerenes Armando Ramírez-Monroy and Timothy M. Swager* Department of Chemistry and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
bS Supporting Information ABSTRACT: Fullerene C60 reacts with a variety of nitrile oxides, providing access to an array of fullerene-fused heterocycles bearing covalently linked chelate moieties. The improved chelating property of the newly synthesized isoxazoline[60]fullerene adducts toward transition metals allows the syntheses of octahedral and square-planar organometallic compounds of rhenium, iridium, and platinum. This new approach has great potential as a general route to other novel derivatives containing catalytically active transition metals.
T
he chemistry of fullerenes has enabled the formation of new carbon-based materials, supramolecular structures and guided efforts directed at the functionalization of carbon nanotubes. The electrochemical and photophysical properties of the fullerene are often tuned by covalent functionalization, and hence the addition of metal centers to binding domains has excellent prospects for creating functional systems. Previous studies have revealed the virtues of creating systems combining π-electron structures and multi redox-active organometallic or coordination complexes in artificial photosynthetic systems, catalysts for organic reactions, or new molecular devices in materials science. These studies established the critical nature of the linker between the π-electron system and the metal center to elicit the desired function. The most extensively studied fullerene systems are the porphyrin-C60 and ferrocenyl-C60 complexes that function as donoracceptor dyads and triads. Moreover, nitrogen-containing mono- and multidentate ligands, such as pyridine, bipyridine, o-phenanthroline, terpyridine, arene pincer ligands (NCN), and ortho-metalated complexes (C, N), have been attached to [60]fullerene and applied in ruthenium, copper, and platinum chemistry.1 Fullerenes can be functionalized by many methods. However, because of the complexities of multiple addition reactions and uncontrolled regiochemistry, only a few methods are convenient (selective) enough to be commonly used. The most studied reactions are with azomethine ylides, diazoalkene, nitrilimine, nitrile oxides, and azides.2 Cycloaddition reactions with 1,3dipoles of nitrile oxides are useful to install organic functional groups by the construction of heterocyclic five-membered rings on the fullerene surface.3 One of the primary reasons for the interest in nitrile oxide cycloadditions to C60 is the potential to create a variety of regiochemically defined 1,2-heterobifunctional derivatives upon chemical reduction of the isoxazoline[60]fullerenes. However, the presence of the strongly electron withdrawing fullerene moiety causes a significant decrease in the nucleophilicity of the nitrogen in the isoxazoline, making the r 2011 American Chemical Society
heterocycle less susceptible to reduction than typical non-fullerene isoxazolines.3e Nevertheless, in the presence of Mo(CO)6, Cu(OTf)2, or DIBAL-H retrocycloaddition has been observed; consequently, nitrile oxide cycloaddition reactions have been used as protection/deprotection of [60]fullerene or for solubilization purposes.4 As a consequence, the coordination of transition metals to isoxazoline[60]fullerenes has not been achieved to this point. Here we report a facile synthesis of transition-metal compounds of isoxazoline[60]fullerenes containing pendant metal-chelating groups such as pyridine and phenol that enhance the coordinative capability of the nitrogen in the isoxazoline ring. Isoxazoline[60]fullerenes (2ad, Scheme 1) were synthesized by dehydrohalogenation of hydroximoyl chlorides or bromides (1ad). The former compounds and the resultant nitrile oxide intermediates were prepared in situ starting from the corresponding oximes. Alternatively, the hydroximoyl chlorides 1a,b were also isolated, and their reactions with C60 provided the corresponding isoxazoline[60]fullerenes 2a,b in improved yields. Reactions were carried out at room temperature, and monoadducts 2ad were isolated in 2847% yields after chromatography. Multiple addition reactions are possible with a single C60 core; however, careful control of the stoichiometry of the reagents and of the reaction conditions allowed the isolation of monoaddition products preferentially along with unreacted fullerene. Demethylation of compound 2b with BBr3 gives compound 3 with the corresponding hydroxyl group (Scheme 2). Nevertheless, attempts at demethylation of compound 2c failed under the same reaction conditions and 2d displays limited solubility after purification. Reactivity studies of isoxazoline[60]fullerenes 2a,b with organometallic compounds confirm the potential of these complexes as ligands. Reaction of 2a with Re(CO)5Cl produces compound 4 (Scheme 3) as a fac isomer, as has been corroborated by Received: March 18, 2011 Published: April 11, 2011 2464
dx.doi.org/10.1021/om200238a | Organometallics 2011, 30, 2464–2467
Organometallics Scheme 1. Synthesis of Isoxazoline[60]fullerenes 2ad
Scheme 2. Deprotection of 2b
comparison of the 1H NMR and IR data of the analogues [facRe(2,20 -bipyridine)(CO)3Cl]5 and [fac-Re(pyridine-2-aldoxime)(CO)3Cl].6 Compounds 5 and 6 were obtained by the reaction of 3 with the organometallic derivatives [Pt(2-phenylpyridine)(SEt2)Cl] and [Ir(2-phenylpyridine)2Cl2]2, respectively, in a toluene H2Oethanol mixture as solvent (Scheme 4). Compounds 5 (65%) and 6 (97%) were isolated by recovery of the organic phase and further purification by Celite filtration or chromatography on silica gel. Once isolated, compound 5 showed reduced solubility in deuterated chloroform or toluene solvents due to strong intermolecular forces present in the solid state (vide infra). With regard to 6, coordination of the organometallic moiety was established by 1H NMR spectrum, which shows 15 different signals which integrate for 20 hydrogen atoms present in the asymmetric chemical structure. Red-brown monocrystals of 5 were obtained by indirect diffusion of pentane into a toluene solution at room temperature. Crystal structures for compounds 2a,b were also obtained, allowing us further structural comparison. As expected, the isoxazoline ring for both 2a and 2b is placed on the 66 fullerene junction. For both compounds, bond lengths and bond angles of the isoxazoline ring are very similar and are in the same range for other isoxazoline[60]fullerenes previously reported.3b,d,7 All isoxazoline[60]fullerene crystal structures support clearly an O1N1 single bond (e.g., 1.395(3) Å for 2a and 1.414(2) Å for 2b) and a N1dC61 double bond (e.g., 1.282(4) Å for 2a and 1.272(2) Å for 2b) in the isoxazoline ring. For compound 5, two crystallographically independent molecules are present in the unit cell (5A,B) displaying quite similar crystallographic data. The crystal structure of 5 establishes that the isoxazoline ring coordination is enhanced by the cooperative influence of the
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Scheme 3. Synthesis of Isoxazoline[60]fullerene Rhenium Complex
additional chelating hydroxyl group. This deduction is obtained from the comparison of crystal structures of compounds 2b and 5. In 5 the isoxazoline nitrogen is strongly bonded to Pt, as revealed by a shorter Pt1N1 bond length (1.993(3) Å) as compared to the trans pyridine (Pt1N2 = 2.015(3) Å). Additionally, there is a marked enhanced affinity of the chelating hydroxyazomethinic moiety for the metal ion in 5 clearly seen in the bond lengths for N1C61 (1.304(8) Å), C61C62 (1.463(9) Å), C62C67 (1.433(8) Å), and C67O2 (1.314 (8) Å), pointing to a contribution from a resonance structure: longshortlongshort. The aromaticity in the phenoxy ring is also partially disrupted, displaying localization on the internal carbon atoms induced by coordination of the oxygen to the metal: C62C63 (1.409(9) Å), C63C64 (1.373(8) Å), C64C65 (1.420(8) Å), C65C66 (1.353(9) Å), and C66C67 (1.415(9) Å). For reference, 2b displays a typical NdC double bond and a delocalized aromatic ring. There are also significant ππ intermolecular contacts in the crystal structure. The strong intermolecular interactions between the organometallic and fullerene moieties present in 5 force the molecule to bend 35° from the idealized planes formed by the two ligands bonded to the metallic center. Electronic spectra were systematically performed for compounds 2a and 4 along with 2b, 3, and 5 at the same molar concentration (Figure 1). Due to limited solubility, electrochemical experiments were performed only on 2a and 4 and compared with the corresponding pristine C60 cyclic voltammogram (Figure 2). The results indicate that original fullerene properties are retained with the organic or organometallic fragments attached to the fullerene surface. Consequently, organometallic fullerene complexes have prospects to create functional C60 substitutes in applications. For instance, the principal C60 absorption bands in the UV region are present in the isoxazoline[60]fullerenes and their corresponding organometallic complexes. These bands are slightly shifted to lower wavelength for all the compounds. Cyclic voltammetric studies carried out on 2a show four successive electrochemically reversible redox waves, which can be assigned to the first four reduction steps of the fullerene fragment followed by an irreversible peak assigned to the isoxazoline fragment. The cyclic voltammogram of compound 4 displays a reversible wave in the cathodic direction followed by an irreversible peak and three more reversible redox waves assigned to the fullerene moiety. Two additional irreversible peaks are shown for the reduction of the organometallic fragment, and in the anodic direction an irreversible oxidation peak is assigned to the Re(I)Re(II) redox pair. 2465
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Scheme 4. Synthesis of Isoxazoline[60]fullerene Platinum and Iridium Complexes
catalysts, and materials. Our current efforts are focused on realizing these opportunities.
’ ASSOCIATED CONTENT Supporting Information. Text, figures, tables, and CIF files giving experimental details of the preparation and characterization of the products, crystallographic data for 2a,b and 5, NMR and absorption spectra, and electrochemical measurement data. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 1. UVvis of C60 and derivatives 2b, 3, and 5 (6.25 106 M in CH2Cl2 at 25 °C).
’ ACKNOWLEDGMENT We thank Dr. Peter M€uller and Dr. M. K. Takase for help in X-ray structure analysis. This work was sponsored in part by the National Science Foundation (No. DMR-1005810). A.R.-M. thanks the CONACYT Mexico for a postdoctoral fellowship. We are grateful for the use of the facilities in the Institute for Soldier Nanotechnologies, supported by the U.S. Army Research Office. A.R.-M. thanks Ms. Grace Han for her assistance in obtaining MS and IR analyses. ’ REFERENCES
Figure 2. Cyclic voltammetry of C60 and derivatives 2a and 4 (argon atmosphere; 5 104 M in toluene/acetonitrile (4/1) and 0.1 M TBAPF6; scan rate 0.1 V/s; 10 °C; reference electrode Ag/AgCl, counter electrode platinum wire, working electrode glassy carbon).
In summary, we report a highly efficient synthesis of a series of isoxazoline[60]fullerenes bearing covalently linked chelating moieties by 1,3-dipolar cycloaddition reactions of nitrile oxides to C60. Our fullerene-fused heterocycles have established that isoxazoline[60]fullerenes can be coordinated to metal centers, providing that they are endowed with a pendant chelating group. Octahedral rhenium and iridium adducts, as well as a squareplanar platinum compound, are obtained in good yields. The chelating properties of these new fullerene adducts have prospects for creating functional supramolecular complexes,
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