Coordination Studies of Bis-Strapped-Hanging-Carboxylate Porphyrins. X-ray Characterization of a Five-Coordinate Iron(II) Complex with a Built-in Axial Base

The comparison of two series of preorganized strapped porphyrins, derived from both the alpha beta alpha beta and alpha alpha beta beta conformers of mesotetrakis(aminophenyl)porphyrin, and the coordination studies of their affinity for O-2 are reported. Both series of ligands bear a proximal built-in axial base and a polar distal side defined by either a hanging malonyl ester or a hanging malonic acid residue. In the alpha beta alpha beta model, the carboxylic acid groups are maintained in an apical location at a fixed distance from the distal oxygen atom of iron-bound O-2. A particular emphasis is placed on the alpha alpha beta beta series, for which the crystal structure of the five-coordinate (S = 2) iron(II) ester complex was determined. In this series, the distal strap can adopt either a tilted conformation or a vertical position over the Fe-II core, allowing changes in the distance between the carboxylic groups and the bound O-2.