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π-Arene/Cation Structure and Bonding. Solvation versus Ligand Binding in Iron(III) Tetraphenylporphyrin Complexes of Benzene, Toluene, p-Xylene, and [60]Fullerene

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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.

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