Naturally assembling cocrystallates of C 60 and C 70 fullerenes with tetraphenylporphyrins (H 2 TPP‚C 60 ‚3 toluene, 1; H 2 T 3,5-dibutyl PP‚C 60 , 2; H 2 T 3,5-dimethyl PP‚1.5C 60 ‚2 toluene, 3; H 2 T piv PP‚C 60 , 4; H 2 T 3,5-dimethyl PP‚C 70 ‚4 toluene, 5; ZnTPP‚C 70 , 6; NiT 4-methyl PP‚2C 70 ‚2 toluene, 7) show unusually short porphyrin/fullerene contacts (2.7-3.0 Å) compared with typical π-π interactions (3.0-3.5 Å). In the C 60 structures, an electron-rich, 6:6 ring juncture, C-C bond lies over the center of the porphyrin ring. In the C 70 structures, the ellipsoidal fullerene makes porphyrin contact at its equator rather than its poles; a carbon atom from three fused six-membered rings lies closest to the center of the porphyrin. These structures provide an explanation for the manner in which tetraphenylporphyrin-appended silica stationary phases effect the chromatographic separation of fullerenes. The interaction of the curved π surface of a fullerene with the planar π surface of a porphyrin, without the need for matching convex with concave surfaces, represents a new recognition element in supramolecular chemistry. NMR measurements show that this interaction persists in toluene solution, suggesting a simple way to assemble van der Waals complexes of donor-acceptor chromophores.

Abstract:A new class of low-melting N,N′-dialkylimidazolium salts has been prepared with carborane counterions, some of the most inert and least nucleophilic anions presently known. The cations and anions have been systematically varied with combinations of 1-ethyl-3-methyl-(EMIM + ), 1-octyl-3-methyl-(OMIM + ), 1-ethyl-2,3-dimethyl-(EDMIM + ), and 1-butyl-2,3-dimethyl-(BDMIM + ) imidazolium cations and CB 11 H 12 -, CB 11 H 6 Cl 6 -, and CB 11 H 6 Br 6 -carborane anions to elucidate the factors which affect their melting points. From trends in melting points, which range from 156°C down to 45°C, it is clear that the alkylation pattern on the imidazolium cation is the main determinant of melting point and that packing inefficiency of the cation is the intrinsic cause of low melting points. C-Alkylation of the anion can also contribute to low melting points by the introduction of a further packing inefficiency. Nine of the thirteen salts have been the subject of X-ray crystal structural determination. Notably, crystallographic disorder of the cation is observed in all but one of these salts. It is the most direct evidence to date that packing inefficiency is the major reason unsymmetrical N,N′-dialkylimidazolium salts can be liquids at room temperature.

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes) 3 Si][H-CB 11 Me 5 Br 6 ]·C 6 H 6 (where Mes is 2,4,6-trimethylphenyl). Free (Mes) 3 Si + cations are well separated from the carborane anions and benzene solvate molecules. Ortho -methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29 Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in “gas phase” calculations, indicating that three-coordination can be preserved in all phases.

An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of approximately 0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

When partnered with carborane anions, arenium ions are remarkably stable. Previously investigated only at subambient temperatures in highly superacidic media, protonated benzene is readily isolated as a crystalline salt, thermally stable to >150 °C. Salts of the type [H(arene)][carborane] have been prepared by protonating benzene, toluene, m-xylene, mesitylene, and hexamethylbenzene with the carborane superacid H(CB11HR5X6) (R ) H, Me; X ) Cl, Br). They have been characterized by elemental analysis, X-ray crystallography, NMR and IR methods. Solid-state 13 C NMR spectra are similar to those observed earlier in solution, indicating that lattice interactions are comparable to solution solvation effects. The acidic proton(s) of the arenium cations interact weakly with the halide substituents of the anion via ion pairing. This is reflected in the dependence of the C-H stretching frequency on the basicity of the carborane anion. Bond lengths in the arenium ions are consistent with predominant cyclohexadienyl cation character, but charge distribution within the cation is less well represented by this resonance form. Structural and vibrational comparison to theory is made for the benzenium ion (C 6H7 + ) with density functional theory at B3LYP/6-31G* and B3P86/6-311+G(d,p) levels. The stability of these salts elevates arenium ions from the status of transients (Wheland intermediates) to reagents. They have been used to bracket the solutionphase basicity of C60 between that of mesitylene and xylene.

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Porphyrins and fullerenes are spontaneously attracted to each other. This new supramolecular recognition element is explored in discrete, soluble, coordinatively linked porphyrin and metalloporphyrin dimers. Jawlike clefts in these bis-porphyrins are effective hosts for fullerene guests. X-ray structures of the Cu complex with C60 and free-base complexes with C70 and a pyrrolidine-derivatized C60 have been obtained. The electron-rich 6:6 ring-juncture bonds of C60 show unusually close approach to the porphyrin or metalloporphyrin plane. Binding constants in toluene solution increase in the order Fe(II) < Pd(II) < Zn(II) < Mn(II) < Co(II) < Cu(II) < 2H and span the range 490−5200 M-1. Unexpectedly, the free-base porphyrin binds C60 more strongly than the metalated porphyrins. This is ascribed to electrostatic forces, enhancing the largely van der Waals forces of the π−π interaction. The ordering with metals is ascribed to a subtle interplay of solvation and weak interaction forces. Conflicting opinions on the relative importance of van der Waals forces, charge transfer, electrostatic attraction, and coordinate bonding are addressed. The supramolecular design principles arising from these studies have potential applications in the preparation of photophysical devices, molecular magnets, molecular conductors, and porous metal-organic frameworks.

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