Comparative Studies of Structures and Peroxidase-like Activities of Copper(II) and Iron(III) Complexes with an EDTA-Based Phenylene-Macrocycle and Its Acyclic Analogue

With the objective of studying the conformational and macrocyclic effects of selected metal chelates on their peroxidase activities, Cu²⁺ and Fe³⁺ complexes were synthesized with a macrocyclic derivative of ethylenediaminetetraacetic acid and o-phenylenediamine (abbreviated as edtaodH₂) and its new open-chain analogue (edtabzH₂). The Fe³⁺ complex of edtaodH₂ has a peroxidase-like activity, whereas the complex of edtabzH₂ does not. The X-ray study of the former shows the formation of a dimeric molecule {[Fe­(edtaod)]₂O} in which each metal with an octahedral coordination is overposed over the macrocyclic cavity, as a result of rigid macrocyclic frame, to form an Fe–O–Fe bridge; the exposure of the central metal to the environment facilitates the capture of oxygen to drive the biomimetic activity. The peroxidase-inactive Fe³⁺ complex consists of a mononuclear complex ion [Fe­(edtabz)­(H₂O)]⁺, the metal ion of which is suited in a distorted pentagonal bipyramid to be protected from environmental oxygen. The copper­(II) complexes, which have mononuclear structures with high thermodynamic stability compared with the iron­(III) complexes, show no peroxidase activity. The steric effects play a fundamental role in the biomimetic activity.