Abstract
The present work describes the reactions of CdI2 with 2-pyridyl aldoxime (2paoH), 3-pyridyl aldoxime (3paoH), 4-pyridyl aldoxime (4paoH), 2-6-diacetylpyridine dioxime (dapdoH2) and 2,6-pyridyl diamidoxime (LH4). The primary goal was to contribute to understanding the molecular basis of the very good liquid extraction ability of 2-pyridyl ketoximes with long aliphatic chains towards toxic Cd(II) and the inability of their 4-pyridyl isomers for this extraction. Our systematic investigation provided access to coordination complexes [CdI2(2paoH)2] (1), {[CdI2(3paoH)2]}n (2), {[CdI2(4paoH)2]}n (3) and [CdI2(dapdoH2)] (4). The reaction of CdI2 and LH4 in EtOH resulted in a Cd(II)-involving reaction of the bis(amidoxime) and isolation of [CdI2(L’H2)] (5), where L’H2 is the new ligand 2,6-bis(ethoxy)pyridine diimine. A mechanism of this transformation has been proposed. The structures of 1, 2, 3, 4·2EtOH and 5 were determined by single-crystal X-ray crystallography. The complexes have been characterized by FT-IR and FT-Raman spectra in the solid state and the data are discussed in terms of structural features. The stability of the complexes in DMSO was investigated by 1H NMR spectroscopy. Our studies confirm that the excellent extraction ability of 2-pyridyl ketoximes is due to the chelating nature of the extractants leading to thermodynamically stable Cd(II) complexes. The monodentate coordination of 4-pyridyl ketoximes (as confirmed in our model complexes with 4paoH and 3paoH) seems to be responsible for their poor performance as extractants.
Highlights
Organic matter and heavy toxic metals are the main pollutants of wastewaters, the threat from the latter being more serious [1–6]
The complexant and the metal ion source are insoluble in the aqueous phase, the complexation reaction occurs at the interphase region and the metal species is transferred into the organic phase
In the third type of liquid-liquid extraction, the extractant is soluble in the organic phase and the source of the metal ion is soluble only in the aqueous phase; after the complexation reaction that occurs at the interphase surface, the metal-extractant coordination complex is transferred into the organic phase
Summary
Organic matter and heavy toxic metals are the main pollutants of wastewaters, the threat from the latter being more serious [1–6] This is due to the non-biodegradable and non-decomposable nature of the toxic metals, making the development of efficient approaches for their removal and uptake extremely important [7]. The complexant and the metal ion source are insoluble in the aqueous phase, the complexation reaction occurs at the interphase region and the metal species is transferred into the organic phase. In the third type of liquid-liquid extraction, the extractant is soluble in the organic phase (hexane, kerosene, and chloroform for laboratory experiments) and the source of the metal ion is soluble only in the aqueous phase; after the complexation reaction that occurs at the interphase surface, the metal-extractant coordination complex is transferred into the organic phase. It is firmly established that an effective extractant should [14]: (a) selectively coordinate to the toxic metal ion having no or very weak affinity for alkali and alkaline metal ions (e.g., Na+, Ca2+, etc.) which are present at higher concentrations in waste and natural waters; (b) give somewhat thermodynamically stable complex with the toxic metal ion; (c) have fast kinetics with the metal ion to be extracted; (d) resist hydrolysis; and (e) result in a reversible complexation allowing for the easy and complete recovery of the metal ion without destruction or decomposition of the extractant
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