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.

旨在探究所选金属螯合物对其过氧化物酶活性的构象和宏观环效应，本研究合成了以乙二胺四乙酸（EDTA）的宏观环衍生物和对苯二胺（简称edtaodH2）及其新型开链类似物（edtabzH2）为配体的Cu2+和Fe3+络合物。其中，edtaodH2的Fe3+络合物表现出类似过氧化物酶的活性，而edtabzH2的络合物则不具备此活性。X射线晶体学研究表明，前者形成了二聚分子{[Fe-(edtaod)]2O}，其中每个金属离子以八面体配位占据宏观环腔之上，由于刚性宏观环骨架的作用，形成了Fe-O-Fe桥；中央金属离子暴露于环境之中，有利于氧气的捕获以驱动生物模拟活性。而过氧化物酶无活性的Fe3+络合物由单核复杂离子[Fe-(edtabz)-(H2O)]+构成，其中金属离子适合于畸变的五角双锥构型，从而免受环境氧气的侵害。与铁(III)络合物相比，具有单核结构且热力学稳定性更高的铜(II)络合物未表现出过氧化物酶活性。空间效应在生物模拟活性中起着至关重要的作用。