A Ni-based MOF for selective detection and removal of Hg²⁺ in aqueous medium: a facile strategy

The uncoordinated sulfur atom of the thiocyanato ligand of a 3D metallo-organic framework (MOF), [Ni(3-bpd)₂(NCS)₂]_n (1), where 3-bpd is 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene, has been explored for selective visual detection and effective removal of Hg²⁺ in aqueous medium. [Ni(3-bpd)₂(NCS)₂]_n has been synthesized under ambient conditions and characterized by elemental analysis, FT-IR spectroscopy, UV-vis spectroscopy, FE-SEM and EDX, XPS and X-ray single crystal diffraction analysis. 1 shows two thiocyanato moieties bound to the hexacoordinated metal center with N atoms whereas the sulfur end remains uncoordinated. The strategy for binding and removal of Hg²⁺ is very simple. Hg²⁺ is known for its affinity for strong bond formation with the soft center, S. The sulfur atoms form bonds with this heavy metal ion. The color of 1 changes from green to grey as Hg atoms form coordinate bonds with S atoms of the SCN^- group. It has been found that Hg is present in grey materials in a 2 : 1 ratio (Hg/Ni). 1 is selective for Hg as other metals/metalloids, including heavy metal ions, present in water are unable to change the color of 1 as they do not form any bond with it. Binding of Hg with the S end of the thiocyanato moiety has been confirmed by a number of techniques, e.g. IR and UV-vis spectra, FESEM, EDX, and XPS. Simple filtration is done to separate grey materials for the removal of Hg²⁺ ion from the water medium. A theoretical calculation has been performed to rationalize the fact.