Structural Insights into the Coordination and Extraction Mechanism of Nickel(II) with Dinonylnaphthalene Sulfonic Acid and n-Hexyl 3-Pyridinecarboxylate Ester as Extractants

In this work, a nickel(II) synergist complex with methyl isonicotinate (BI, a short chain analog of n-hexyl 3-pyridinecarboxylate ester) and naphthalene-2-sulfonic acid (HNS, a short chain analog of dinonylnaphthalene sulfonic acid) was synthesized and studied by single-crystal X-ray diffraction. The nickel(II) complex crystallizes in the monoclinic P2(1)/n space group with the composition [Ni(H2O)(4)(BI)(2)](NS)(2)center dot 2H(2)O. The Ni(II) ions of these crystallographically independent molecules lie on an inversion center, forming a trans-form distorted octahedral coordination structure. The nickel(II) ions can coordinate with four water molecules and two BI ligands, resulting in a monometallic structure [Ni(H2O)(4)(BI)(2)](2+). There is no direct interaction between nickel(II) and sulfonic oxygen atoms of the sulfonate anions, but hydrogen bonds form between sulfonic oxygen atoms and water molecules in the synergist complex. In order to further elucidate the solution structure of the nickel(II) complexes with the actual synergistic mixture containing n-hexyl 3-pyridinecarboxylate ester and dinonylnaphthalene sulfonic acid in the nonpolar organic phase, the nickel(II) complexes were studied by electrospray ionization mass spectrometry. The results indicated that the extracted nickel(II) complexes in the nonpolar solvent have a similar coordination structure as that of the crystalline nickel(II) synergist complex.