Article

Atomic Nitrogen Encapsulated in Fullerenes: Realization of a Chemical Faraday Cage

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Abstract

Fullerenes, C60 and C70, are ideal containers for atomic nitrogen. We will show by electron paramagnetic resonance (EPR) experiments that nitrogen in C60 keeps its atomic ground state configuration and resides in the center of the cage. This is the first time that atomic nitrogen is stabilized at ambient conditions. The inert shell of the fullerene protects the highly reactive nitrogen from undergoing chemical reactions with the surroundings. The fullerene cage is the chemical analogue of the Faraday cage in case of electrical fields, i.e. it shields off the chemical reactivity. As for the free nitrogen atom, the spins of the three p-electrons of nitrogen in C60 are parallel (S = 3/2) and the atom has spherical symmetry. Due to the center position of nitrogen in C60, extremely sharp EPR lines are observed. This reflects the absence of a strong host–guest interaction and shows that the individuality of nitrogen in the fullerenes is preserved. Further evidence for the almost interaction-free suspension of nitrogen in the fullerene cages is provided by g-factor measurements. These investigations show that magnetic shielding of the host molecules can account for the observed differences between N@C60 and N@C70. The fullerene cage can be chemically modified without destroying the endohedral complex. The chemical modifications change the symmetry of the molecule which is observed through an additional fine structure in the EPR spectrum. Influences of the modifications on the stability of N@C60 will be discussed.

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Laser vaporization of graphite in a high-pressure supersonic nozzle produced a remarkable stable C60 molecule in high yield with striking and exciting new properties. Specifically it was found that the stability of the new species arises from its unique ability to close into a spheroidal, aromatic molecule in the form of a truncated icosahedron. This should be an exceptionally strong binding site for a wide range of atoms. Evidence is presented for clusters of carbon atoms which form very stable complexes with lanthanum atoms, particularly C60 La. Such aromatic egg-shell complexes of metal atoms may be stable enough to survive in normal condensed-phase chemical environments.
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Three series of regioisomeric bisadducts of C60, namely, C62(anisyl)4 and the mixed systems C62(anisyl)2-(COOEt)2, and C61(COOEt)2(NCOOEt), were synthesized starting from the 1,2-monoadducts C61(COOEt)2 (1), C61-(anisyl)2 (2), and C60(NCOOEt) (4) by using the Bingel and Bamford-Stevens reactions, and nitrene additions. In the case of C61(COOEt)2(NCOOEt) the complete series of nine possible regioisomers were isolated for the first time. For steric reasons the cis-1 isomers of C62(anisyl)4 and C62(anisyl)2(COOEt)2 were not formed. The transannular [6,6] bonds in the cis-1 isomer 42 of C61(COOEt)2(NCOOEt) are closed. The properties and regioselectivities of formation of these bisadducts and their monoadduct precursors were compared with those of the series C62-(COOEt)4 and C60(NCOOEt)2, which we synthesized previously. In the additions to 1, 2, and 4 the preferred positions of attack are e and trans-3 for sterically demanding addends (e.g., combinations of C(anisyl)2 and C(COOEt)2) and cis-1, e, and trans-3 for sterically less demanding addends (e.g., combinations of N(COOEt) and C(COOEt)2). A detailed analysis of the MO structures, the experimental and calculated geometries of monoadduct precursors, and the stabilities of reaction products leads to the conclusion that the addend-independent cage distortion itself is responsible for the observed regioselectivities of bisadduct formations.
Article
Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C60 have been studied. Because of the high symmetry of the cage, most of the conventional relaxation mechanisms otherwise affecting electron spins in solution are absent, resulting in an exceptional homogeneous ERP linewidth of only 2.5 kHz. Apparently, solvent collision-induced deformations of the carbon shell which modulate the zero-field-splitting sensed by the quartet spin state of N@C60 are the dominant spin relaxation mechanism.
Article
The synthesis and EPR spectroscopic investigations of a family of six endohedral fullerenes, namely, N@C-60 (1), N@C-61(COOC2H5)(2) (2), N@C-66(COOC2H5)(12) (3), N@C-66(COOC2D5)(12) (4), N@C-61(COOC2D5)(2) (5), and N@C-70 (6), containing atomic nitrogen in the S-4(3/2) ground state is described. The parent systems N@C-60 (1) and N@C-70 (6) were synthesized by nitrogen ion implantation. The syntheses of the C-2 upsilon-symmetric monoadducts 2 and 5 and the Ti, symmetric hexaadducts 3 and 4 exhibiting well-defined cage distortions were accomplished via cyclopropanation with the corresponding malonates, With respect to these additions the reactivity of 1 is indistinguishable from that of empty C-60 The quartet electronic spin of the encapsulated N-atoms is a very sensitive probe for cage modifications. In the monoadducts 2 and 5 a permanent zero field splitting (ZFS) tensor with rigidly aligned axes was revealed reflecting the intrinsic droplet like cage distortion. In contrast, no fine structure due to intrinsic distortions was monitored in the ESR spectra of the highly symmetric hexaadducts 3 and 4. In these cases only matrix-induced distortions of the cage lead to ZFS interactions. In solution fluctuations of the ZFS tensor are the major source of spin relaxation. The root-mean-square value of this collision-induced fluctuating ZFS interaction as estimated from relaxation data fur N@C-60 (1) and the hexaadduct 3 is in the range of the ZFS interaction measured for the monoadduct 2.
Article
A new paramagnetic defect in solid C60 was produced by nitrogen implantation in solid C60. The hyperfine splitting and the isotope effect unambiguously show that the paramagnetic center contains one nitrogen nucleus. The hyperfine interaction is isotropic, its value is comparable to that of the free nitrogen atom, and the spin of the electron system is S = 3/2, as in atomic nitrogen. The complex responsible for this center is soluble in toluene and CS2 and is stable. We suggest that the complex consists of nitrogen inside C60.