Coordination Modes in the Formation of the Ternary Am(III), Cm(III), and Eu(III) Complexes with EDTA and NTA:  TRLFS, ¹³C NMR, EXAFS, and Thermodynamics of the Complexation

Abstract

The formation and the structure of the ternary complexes of trivalent Am, Cm, and Eu with mixtures of EDTA+NTA (ethylenediamine tetraacetate and nitrilotriacetate) have been studied by time-resolved laser fluorescence spectroscopy, 13C NMR, extended X-ray absorption fine structure, and two-phase metal ion equilibrium distribution at 6.60 m (NaClO4) and a hydrogen ion concentration value (pcH) between 3.60 and 11.50. In the ternary complexes, EDTA binds via four carboxylates and two nitrogens, while the binding of the NTA varies with the hydrogen ion concentration, pcH, and the concentration ratios of the metal ion and the ligand. When the concentration ratios of the metal to ligand is low (1:1:1-1:1:2), two ternary complexes, M(EDTA)(NTAH)(3-) and M(EDTA)(NTA)(4-), are formed at pcH ca. 9.00 in which NTA binds via three carboxylates, via two carboxylates and one nitrogen, or via two carboxylates and a H2O. At higher ratios (1:1:20 and 1:10:10) and pcH's of ca. 9.00 and 11.50, one ternary complex, M(EDTA)(NTA)(4-), is formed in which NTA binds via three carboxylates and not via nitrogen. The two-phase equilibrium distribution studies at tracer concentrations of Am, Cm, and Eu have also confirmed the formation of the ternary complex M(EDTA)(NTA)(4-) at temperatures between 0 and 60 degrees C. The stability constants (log beta111) for these metal ions increase with increasing temperature. The endothermic enthalpy and positive entropy indicated a significant effect of cation dehydration in the formation of the ternary complexes at high ionic strength.

Mixed Ligand Complexes of Am3+, Cm3+ and Eu3+ with HEDTA and HEDTA + NTA—Complexation Thermodynamics and Structural Aspects

“…The solution EXAFS (Extended X-ray Absorption Fine Structure) studies of Eu(EDTA)(NTA) 4− at pH ∼ 9.0 confirmed three binding possibilities for NTA, such as: (1) via three carboxylates, (2) via two carboxylates and one H 2 O, and (3) via two carboxylates and one nitrogen. This is supported by the TRLFS and 13 C NMR data [31]. However, at pH = 11.5, NTA binds via three carboxylate groups but not with nitrogen, with EDTA binding via hexadentation.…”

“…In the formation of the ternary complex M(EDTA)(NTA) 4− , where M 3+ = Am 3+ , Cm 3+ and Eu 3+ , EDTA binds via four carboxylates and two nitrogens, while the nature of the binding of NTA varies with the hydrogen ion concentration [31]. The solution EXAFS (Extended X-ray Absorption Fine Structure) studies of Eu(EDTA)(NTA) 4− at pH ∼ 9.0 confirmed three binding possibilities for NTA, such as: (1) via three carboxylates, (2) via two carboxylates and one H 2 O, and (3) via two carboxylates and one nitrogen.…”

Understanding the Solution Behavior of Minor Actinides in the Presence of EDTA4–, Carbonate, and Hydroxide Ligands

“…12 Any remaining metal ion coordination sites in the EDTA complex are typically occupied by water molecules in aqueous solutions, or by other ligands that are present in solution. 13 Secondary ligands are able to bind to f-ion complexes of EDTA 4-by displacing some of the inner sphere water molecules and result in the formation of a ternary complex. This has been shown to occur for actinide and lanthanide complexes of EDTA 4-where the secondary ligand is nitrilotriacetate (NTA), iminodiacetate (IDA), oxydiacetate (ODA) and citrate (see Figure 1).…”

“…This has been shown to occur for actinide and lanthanide complexes of EDTA 4-where the secondary ligand is nitrilotriacetate (NTA), iminodiacetate (IDA), oxydiacetate (ODA) and citrate (see Figure 1). [13][14][15][16] However, few studies have observed the formation of ternary complexes of the actinides or lanthanides with carbonate. 17 Actinide complexes of EDTA 4-can be soluble at both acidic and alkaline pHs, and may be used as a model to understand the migration and speciation of radionuclides in the environment, as well as the behaviour of actinides in various effluents and matrices.…”

Effects of Concentration of Some Lanthanide(III) Complexes of 1,7‐Bis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane on Bis(p‐nitrophenyl)phosphate Hydrolysis

“…It was found previously that 1:1 and 1:2 Eu III -EDTA complexes and similar complexes have spectral maxima close to each other at high pH. [13][14][15] Thus, it is possible that the In another paper by Merbach et al, [16] the absorption spectrum of EuDO2A + has been reported to contain two peaks, one at 579.0 nm and the other at 579.8 nm, which are in equilibrium at pH 6.4 with an equilibrium constant, K, of 4.0. The former, major peak was assigned to EuDO2A(H 2 O) 3 + and the latter, minor one to EuDO2A-(H 2 O) 2 + .…”