PROTON SPONGES

Since the review by Staab and Saupe (Angew. Chem., Int. Ed. Engl., 27 (1988) 865), the field of ''proton sponges'' has seen great progress, partly a result of the efforts of Staab himself. The present work updates the literature references up to the end of 1993 and has a different scope since it is mainly based on crystallographic results. A collection of nearly 70 Xray structures has been examined and their geometrical parameters discussed. An effort has been made to relate the geometrical characteristics of the solid-state structures to solution properties such as pK(a) and rate constants for intermolecular proton transfer. The relationships between the chemical structure and the basicity of proton sponges are examined for naphthalene, fluorene, phenanthrene, biphenyl and benzene derivatives. The theoretical as well as experimental data for the intramolecular hydrogen bonded [N-H ... N](+) system are discussed with special emphasis on crystallographic data. The paradigmatic case of 1,8-bis(dimethylamino)naphthalene and its cation, has been studied by means of semiempirical AM1 calculations. Finally, the most relevant structural conclusions are summarized.