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 alphabetaalphabeta and alphaalphabetabeta conformers of meso-tetrakis(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 alphabetaalphabeta 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 alphaalphabetabeta 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).