The dissociation constants of diprotonated chalcogenobispyridines: Transmission of electronic effects across chalcogen bridges

The dissociation constants of the diprotonated chalcogenobispyridines have been determined using potentiometric titrations to establish a method for the measurement of the ability of a bridging ligand to relay electronic effects. The relationship between pK(a) and structure of the chalcogenobispyridines results from a balance between inductive, mesomeric, and steric effects. Delocalization of cationic charge onto the bridgehead increases the apparent electronegativity of the bridging atom, thereby relaying a strong base-weakening effect to the site of first deprotonation. Such delocalization was found to be a function of both the substitution site (4-X > 2-X >> 3-X) and orbital overlap requirements (S > O approximate to Se > Te).