Dependence of [FeII(Lx)]-NO binding on Aminocarboxylate Ligand structure:

Chelate complexes of Fe have been studied over the years for their ability to behave as both oxidants and antioxidants of dioxygen, as well as potential catalysts for the reduction of NO to dinitrogen. However, the first systematic study of the ligandmetal-substrate has only been done recently by Schenppensieper et al. Interestingly, they found a direct correlation of NO binding and dioxygen binding (very low selectivity for either) and were able to correlate ligand-metal structure and ligand size to reversible/irreversible binding of NO. They were also able to correlate this to the kinetics of oxidation by dioxygen shown by the faster oxidation by dioxygen corresponding to tighter and more irreversible binding of NO to the metal. Further analysis and explanation into the structure and the analysis of the d-d and LMCT transitions in relation to these values will be done based on UV-VIS and ATR-IR spectra collected. Introduction: Fe(aminocarboxylates) have been of much interest in both biological and industrial applications over the last decade. The biological roles of these complexes have been implicated in both anti and pro-oxidant processes and in the production of free radicals. In industry, iron(II/III)ethylenediaminetetraacetate (EDTA) , and others, is used as an anti-corrosion agent in high temperature steam broilers (i.e. pressure treated wood). The ability of Fe(aminocarboxylates) to behave as either oxidation or reduction agents depends on the exact aminocarboxylic ligand and the availability of the recently described seventh coordination site on both Fe and Fe. The 3d transition metals, especially in the higher oxidation states, have long been considered too small for coordination numbers above six. The classic FeEDTA complex is almost always treated as an octahedral complex because of the hexadentate nature of EDTA (Figure 1). However, crystal structures have shown that not only are all six coordinating atoms in direct contact with the iron, but a water molecule is almost always coordinated to the metal center (Figure 2). The seventh coordination site is important in explaining the reactivity of these complexes. If an octahedral, tetrahedral for some, environment is assumed, there is no viable area for any substrate interaction with the metal. In order for any chemistry to occur (as stated above) at the metal center, an arm of the ligand must un-coordinate with the metal. This becomes problematic with aminocarboxylate ligand because of the high affinity most have for iron; most are on the order of 10 or greater. This is unlikely to occur and reaction mechanisms are difficult to explain without. However, with an open seventh coordination site, or even a weakly bound water molecule, the metal center is exposed and available for reactions.