Confirming the Molecular Basis of the Solvent Extraction of Cadmium(II) Using 2-Pyridyl Oximes through a Synthetic Inorganic Chemistry Approach and a Proposal for More Efficient Extractants.

The present work describes the reactions of CdI ₂ with 2-pyridyl aldoxime (2paoH), 3-pyridyl aldoxime (3paoH), 4-pyridyl aldoxime (4paoH), 2-6-diacetylpyridine dioxime (dapdoH ₂ ) and 2,6-pyridyl diamidoxime (LH ₄ ). 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 [CdI ₂ (2paoH) ₂ ] ( 1 ), {[CdI ₂ (3paoH) ₂ ]} _n ( 2 ), {[CdI ₂ (4paoH) ₂ ]} _n ( 3 ) and [CdI ₂ (dapdoH ₂ ) ] ( 4 ). The reaction of CdI ₂ and LH ₄ in EtOH resulted in a Cd(II)-involving reaction of the bis(amidoxime) and isolation of [CdI ₂ (L'H ₂ )] ( 5 ), where L'H ₂ 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 ¹ H 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.