Structural insights into the extraction mechanism of cobalt(II) with dinonylnaphthalene sulfonic acid and 2-ethylhexyl 4-pyridinecarboxylate ester

In this work, to elucidate the synergistic extraction mechanism of cobalt(II) with dinonylnaphthalene sulfonic acid (HDNNS) and 2-ethylhexyl 4-pyridinecarboxylate ester (L), hexaaquacobalt(II) naphthalene-2-sulfonate (compound 1) was prepared using naphthalene-2-sulfonic acid (HNS, the short chain analog of HDNNS) and di-methyl isonicotinate tetraaquacobalt(II) naphthalene-2-sulfonate (compound 2) was prepared using methyl isonicotinate (L^I, a short chain analog of 2-ethylhexyl 4-pyridinecarboxylate ester) and HNS; the compounds were studied by single crystal X-ray diffraction. Moreover, 2 and the actual extracted cobalt(II) complex were further investigated by Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS). The results indicated that the actual extracted cobalt(II) complex possesses a similar coordination structure as 2. Combined with the results obtained by single crystal X-ray diffraction of 1 and 2, FT-IR and ESI-MS of 2 and the actual extracted cobalt(II) complex, it is reasonable to conclude that the extracted cobalt(II) complex with the actual synergistic mixture is much more stable than the cobalt(II) complex with HDNNS alone. Therefore, the extraction selectivity cobalt(II) is effectively enhanced with the addition of 2-ethylhexyl 4-pyridinecarboxylate ester to HDNNS.