Iron chelates: a challenge to chemists and Mössbauer spectroscopists

Abstract

The speciation of iron in aqueous solutions containing Fe 3+ and selected chelates such as EDTA, EDDA, CDTA and HEDTA has been studied using transmission 57 Fe Mössbauer spectrometry in frozen solutions. The protonation of various complexes as well as binuclear complex formation could be detected as a function of pH. Autoreduction of Fe 3+ to Fe 2+ was observed in several cases. Reaction with hydrogen peroxide proved to be rather different for the four ligands, while the dihapto complex [XFe(η 2 -O 2 )] 3− had surprisingly identical Mössbauer parameters for X = EDTA, CDTA or HEDTA. Paramagnetic spin relaxation observed in Z. Hommonay et al. the Mössbauer spectra was found to be strongly influenced by the identity of the chelating ligand, despite the basically spin-spin origin of the phenomenon.

Influence of pH, Iron Source, and Fe/Ligand Ratio on Iron Speciation in Lignosulfonate Complexes Studied Using Mössbauer Spectroscopy. Implications on Their Fertilizer Properties

“…The occurrence of the Fe 3+ doublet in the frozen solution Mössbauer spectra also indicates that Fe 3+ –LS complexes have dimeric/oligomeric structure, and no monomeric Fe 3+ species can be found in them . This hints at a significant difference compared to Fe 3+ chelates formed with EDTA or analogous ligands in the same concentration and pH range. , Taking into account literature data on the dimerization of Fe 3+ species in aqueous solutions, , one can assume the presence of −OH/O-bridged μ-(OH/O) x Fe y –LS type complexes.…”

“…36 This hints at a significant difference compared to Fe 3+ chelates formed with EDTA or analogous ligands in the same concentration and pH range. 37,38 Taking into account literature data on the dimerization of Fe 3+ species in aqueous solutions, 37,38 one can assume the presence of −OH/O-bridged μ-(OH/O) x Fe y −LS type complexes.…”

Characterization of Fe–Leonardite Complexes as Novel Natural Iron Fertilizers

“…The Mossbauer spectrum of the Fe 3+ −LN complex in aqueous solution at pH 7 shows a well-defined quadrupole doublet (Figure 7B), which indicates that Fe 3+ −LN complexes have an oligomeric structure and no monomeric Fe 3+ species can be found in the solution. 33,46 This indicates a significant difference compared to Fe 3+ chelates formed with EDTA or analogous ligands in the same concentration and pH range, 47,48 where mainly monomeric Fe 3+ species were found. On the basis of literature data on the dimerization of Fe 3+ species in aqueous solutions, 33,47 the presence of OH/O-bridged μ-(OH/ O) x Fe y LN-type complexes can be assumed, in good agreement with the data on the solid Fe 3+ −LN samples discussed in the previous section.…”

“…33,46 This indicates a significant difference compared to Fe 3+ chelates formed with EDTA or analogous ligands in the same concentration and pH range, 47,48 where mainly monomeric Fe 3+ species were found. On the basis of literature data on the dimerization of Fe 3+ species in aqueous solutions, 33,47 the presence of OH/O-bridged μ-(OH/ O) x Fe y LN-type complexes can be assumed, in good agreement with the data on the solid Fe 3+ −LN samples discussed in the previous section.…”

Functional characterization of the chloroplast ferric chelate oxidoreductase enzyme

“…With regard to the measurements of pH dependence of the FCR reaction, Fe(III)‐EDTA is often thought to lose its stability above pH 7.0. However, according to previous Mössbauer spectroscopy measurements, this does not mean that Fe(III) would be released from the chelate and form insoluble FeOH 3 ; instead, the monomeric Fe(III)‐EDTA is transformed into the dimeric Fe(III) 2 ‐EDTA 2 form (in a 0.01 M Fe(III)‐EDTA 1 : 1.1 solution, a 100% dimeric complex was found; Homonnay et al ., ). A similar effect was also shown at pH 7.0 (Stein & Marinsky, ).…”