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 αβαβ and ααββ conformers of meso-tetrakis(aminophenyl)porphyrin, and the coordination studies of their affinity for O₂ 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 αβαβ model, the carboxylic acid groups are maintained in an apical location at a fixed distance from the distal oxygen atom of iron-bound O₂. A particular emphasis is placed on the ααββ 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 O₂.