Chelating Agents Related to Ethylenediamine Bis(2-hydroxyphenyl)acetic Acid (EDDHA):  Synthesis, Characterization, and Equilibrium Studies of the Free Ligands and Their Mg²⁺, Ca²⁺, Cu²⁺, and Fe³⁺ Chelates

Abstract

Iron chelates such as ethylenediamine-N,N'-bis(2-hydroxyphenyl)acetic acid (EDDHA) and their analogues are the most efficient soil fertilizers to treat iron chlorosis in plants growing in calcareous soils. EDDHA, EDDH4MA (ethylenediamine-N,N'-bis(2-hydroxy-4-methylphenyl)acetic acid), and EDDCHA (ethylenediamine-N,N'-bis(2-hydroxy-5-carboxyphenyl)acetic acid) are allowed by the European directive, but also EDDHSA (ethylenediamine-N,N'-bis(2-hydroxy-5-sulfonylphenyl)acetic acid) and EDDH5MA (ethylenediamine-N,N'-bis(2-hydroxy-5-methylphenyl)acetic acid) are present in several commercial iron chelates. In this study, these chelating agents as well as p,p-EDDHA (ethylenediamine-N,N'-bis(4-hydroxyphenyl)acetic acid) and EDDMtxA (ethylenediamine-N,N'-bis(2-metoxyphenyl)acetic acid) have been obtained following a new synthetic pathway. Their chemical behavior has been studied to predict the effect of the substituents in the benzene ring on their efficacy as iron fertilizers for soils above pH 7. The purity of the chelating agents has been determined using a novel methodology through spectrophotometric titration at 480 nm with Fe(3+) as titrant to evaluate the inorganic impurities. The protonation constants were determined by both spectrophotometric and potentiometric methods, and Ca(2+) and Mg(2+) stability constants were determined from potentiometric titrations. To establish the Fe(3+) and Cu(2+) stability constants, a new spectrophotometric method has been developed, and the results were compared with those reported in the literature for EDDHA and EDDHMA and their meso- and rac-isomers. pM values have been also determined to provide a comparable basis to establish the relative chelating ability of these ligands. The purity obtained for the ligands is higher than 87% in all cases and is comparable with that obtained by (1)H NMR. No significant differences have been found among ligands when their protonation and stability constants were compared. As expected, no Fe(3+) complexation was observed for p,p-EDDHA and EDDMtxA. The presence of sulfonium groups in EDDHSA produces an increase in acidity that affects their protonation and stability constants, although the pFe values suggest that EDDHSA could be also effective to correct iron chlorosis in plants.

Novel chelating agents as manganese and zinc fertilisers: characterisation, theoretical speciation and stability in solution

“…These studies require knowledge of the previous chemical characterisation of the ligands, including their protonation and stability constants with metals. This type of modeling has been already applied to study the stability of several Fe chelates (Yunta et al, 2003b;López-Rayo et al, 2009), showing a good correlation between the modeling results and their behaviour in agronomics conditions. Also, different tests have been proposed López-Rayo et al, 2009 and …”

“…Most of the stability constants of the chelates with Fe, Mn and Zn were obtained from the literature (Yunta et al,201X) 2+ were determined using the potentiometric method described by Yunta et al(2003b) since they have not been reported. This method consists of the titration with HCl 0.05 M solution of the chelate formed with metal:ligand 1:1 ratio from pH 10 to pH 2.5 at 25ºC under N 2 atmosphere.…”

“…Chelating agents used in the experiment were H 4 o,o-EDDHA (Promochem), H 4 o,p-EDDHA (Syngenta Agro), Na 2 EDTA (Tritriplex III, Merck, 99%), H 5 DTPA (Tritriplex V, Merck, 99%), Na 3 HEEDTA (Sigma, 98%), Na 4 IDHA (Adob PPC, 78.1%) and Na 3 EDDS (Fluka, 35%). The titrimetric purity of H 4 o,o-EDDHA and H 4 o,p-EDDHA were determined by a photometric titration with Fe 3+ solution following the method described by Yunta et al (2003b) and corresponding to 93.9±0.6% and 93.1±0.5% respectively.…”

Iron Acquisition and Regulation in Campylobacter jejuni

“…These two chelators differ significantly in their effectiveness at binding iron, with DFO being the more powerful iron chelator (41,59). In aqueous solution, enterobactin could spontaneously break down into the trimers, dimers, and monomers of dihydroxybenzoyl serine (DHBS) (14,26).…”

Iron limitation and the role of siderophores in marine Synechococcus

“…To test the effect of desferrioxamine B on a natural microbial population, complexed to DFB in a 1:2 ratio was added to seawater at concentrations of 400:800 nM, (Yunta et al, 2003) but photolabile Fe complexes and is known to induce Fe limitation in Synechococcus (Chadd et al, 1996).…”