Sensing of Sulfur Dioxide by Base Metal Thiolates:  Structures and Properties of Molecular NiN₂S₂/SO₂ Adducts

The cis-dithiolate N₂S₂Ni complex bismercaptoethanediazacycloheptanenickel(II), (bme-dach)Ni or Ni-1‘, takes up two equivalents of sulfur dioxide in which thiolate-sulfur to SO₂-sulfur interactions are well-defined by X-ray crystallography. Ni-1‘·2SO₂, C₉H₁₈N₂NiO₄S₄, yields monoclinic crystals belonging to the P2(1)/c space group:  a = 10.308(4) Å, b = 13.334(5) Å, c = 10.842(4) Å, α = 90°, β = 91.963(6)°, γ = 90°, and Z = 4. Further characterization by ν(SO) IR spectroscopy, thermal gravimetric analysis, electronic spectroscopy, and visual color changes upon reversible SO₂ adduct formation establish Ni-1‘ and the analogous bismercaptoethanediazacyclooctane derivative, (bme-daco)Ni, Ni-1, to be viable candidates for technical development as chemical sensors of this noxious gas. Visual SO₂ detection limits of Ni-1 and Ni-1‘ are established at 25 and 100 ppm, respectively. Both the Ni-1‘·2SO₂ adduct and the Ni-1‘ reactant are air stable. In addition, the stability of Ni-1‘·SO₂ to vacuum and removal of SO₂ by heating make Ni-1‘ a possible storage/controlled release complex for SO₂ gas.