The coordination structure of the extracted nickel(ii) complex with a synergistic mixture containing dinonylnaphthalene sulfonic acid and 2-ethylhexyl 4-pyridinecarboxylate ester

Abstract

In this paper, a synergist complex of Ni(II) with naphthalene-2-sulfonic acid (HNS) and n-hexyl 4-pyridinecarboxylate ester (L^I), which are corresponding short chain analogues of active synergistic extractants dinonylnaphthalene sulfonic acid (HDNNS) and 2-ethylhexyl 4-pyridinecarboxylate ester (4PC, L^II), was prepared and characterized by Nuclear Magnetic Resonance (¹H-NMR), elemental analyses, Fourier Transform Infrared Spectroscopy (FT-IR) and Electrospray Ionization Mass Spectrometry (ESI-MS) spectroscopic studies. Single crystals of the nickel synergist complex have been grown from a methanol/water (10/1) solution and analyzed by single crystal X-ray diffraction. The crystal structure of the nickel synergist complex shows that Ni(II) is coordinated by four water molecules and two monodentate L^I ligands and there is no direct interaction of the Ni(II) with sulfonic oxygen atoms of naphthalene-2-sulfonic acid anions, while hydrogen-bonded interactions of the coordinated water molecules with sulfonic oxygen atoms of naphthalene-2-sulphate anions were observed. In addition, in order to provide parallels to solvent extraction, the extracted Ni(II) complex with HDNNS and 4PC is also prepared and studied using FT-IR and ESI-MS technology. Compared with their corresponding free ligand, similar shifts assigned to the stretching vibration of the pyridine ring and SO in both the nickel synergist complex and the extracted Ni(II) complex suggest that in the non-polar organic phase, Ni(II) is also coordinated by L^II ligands, while the sulfonic oxygen atoms of dinonylnaphthalene sulfonate anions not directly bonded to Ni(II) form hydrogen bonds with water molecules (coordinated with Ni(II) or/and solubilized in the non-polar organic phase). For the ESI-MS spectrum of the extracted Ni(II) complex in non-polar organic phase, there exists a peak at m/z values of 1058.76, which indicates that the extracted Ni(II) complex in the non-polar organic phase may have a coordination structure, [Ni(H₂O)₄(L^II)₂](DNNS)₂, similar to the coordination structure of the nickel synergist complex.