Benzene, toluene, p-xylene, and [60]fullerene are shown to be weak ligands to a hard metal such as iron(III) in Fe(TPP)+ cation (TPP = tetraphenylporphyrinate). X-ray crystal structures of [Fe(TPP)(C6H6)][CB11H6Br6] · 3.5C6H6 (1), [Fe(TPP)(C7H8)][CB11H6Cl6] · 2C7H8 (2), [Fe(TPP)(C8H10)][Ag(CB11H6Br6)2] · arene (3), and [Fe(TPP)(C60)][F20-BPh4] · 2.5dichlorobenzene (4) show distinctively short Fe · · · C contacts, in the range 2.65-2.95 Å, which distinguish ligation from π-π cocrystallized solvation. Dihedral angles between the arene and metalloporphyrin planes are also diagnostic of ligand versus solvate roles. The essential features of the arene coordination can be faithfully reproduced using density functional theory. In contrast to alkali metal cation/π-arene interactions, a component of the covalent bonding can be recognized. In a broader context, this study suggests that solvents such as benzene should always be viewed as potential ligands in the presence of coordinatively unsaturated cations. The common use in supramolecular chemistry of the term 'noncovalent' to describe entities with metal-ligand bonds is viewed as misleading.

Naturally assembling cocrystallates of C60 and C70 fullerenes with tetraphenylporphyrins (H2TPP · C60 · 3 toluene, 1; H2T(3,5-dibutyl)PP · C60, 2; H2T(3,5-dimethyl)PP · 1.5C60 · 2 toluene, 3; H2T(piv)PP · C60, 4; H2T(3,5-dimethyl)PP · C70 · 4 toluene, 5; ZnTPP · C70, 6; NiT(4-methyl)PP · 2C70 · 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 C60 structures, an electron-rich, 6:6 ring juncture, C-C bond lies over the center of the porphyrin ring. In the C70 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.

Large, inert, weakly basic carborane anions of the icosahedral type CHB(11)R(5)X(6)(-) (R = H, Me; X = Cl, Br) allow ready isolation and structural characterization of discrete salts of the solvated proton, [H(solvent)(x)][CHB(11)R(5)X(6)], (solvent = common O-atom donor). These oxonium ion Brønsted acids are convenient reagents for the tuned delivery of protons to organic solvents with a specified number of donor solvent molecules and with acidities leveled to those of the chosen donor solvent. They have greater thermal stability than the popular [H(OEt(2))(2)][BAr(F)] acids based on fluorinated tetraphenylborate counterions because carborane anions can sustain much higher levels of acidity. When organic O-atom donors such as diethyl ether, tetrahydrofuran, benzophenone, and nitrobenzene are involved, the coordination number of the proton (x) in [H(solvent)(x)()](+) is two. A mixed species involving the [H(H(2)O)(diethyl ether)](+) ion has also been isolated. These solid-state structures provide expectations for the predominant molecular structures of solvated protons in solution and take into account that water is an inevitable impurity in organic solvents. The O.O distances are all short, lying within the range from 2.35 to 2.48 A. They are consistent with strong, linear O.H.O hydrogen bonding. Density functional theory calculations indicate that all H(solvent)(2)(+) cations have low barriers to movement of the proton within an interval along the O.H.O trajectory, i.e., they are examples of so-called SSLB H-bonds (short, strong, low-barrier). Unusually broadened IR bands, diagnostic of SSLB H-bonds, are observed in these H(solvent)(2)(+) cations.