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.