Unique Reactivity of a Tetradentate N₂S₂ Complex of Nickel: Intermediates in the Production of Sulfur Oxygenates

Complex 1 [(N,N‘-dimethyl-N,N‘-bis(2-sulfanylethyl)ethylenediamine)nickel(II)], previously shown to react with H2O2
to produce the fully oxygenated disulfonate 5 [diaqua(N,N‘-dimethyl-N,N‘-bis(2-sulfonatoethyl)ethylenediamine)nickel(II)], has been explored in detail to explain the observed reactivity of this compound and to discern intermediates
in the oxygenation reaction. Reaction of 1 with 1 equiv of methyl iodide results in the monomethylated square-planar nickel complex 2 {[(N,N‘-dimethyl-N-(2-sulfanylethyl)-N‘-(2-methylthioethyl)(ethylenediamine)nickel(II)] iodide},
while a slight excess of methyl iodide results in the dimethylated complex 3 [diiodo(N,N‘-dimethyl-N,N‘-bis(2-methylthioethyl)ethylenediamine)nickel(II)], an X-ray structure of which has shown that the nickel ion is in an octahedral
N2S2I2 environment. Crystal data of 3:  monoclinic, a = 8.865(3) Å, b = 14.419(4) Å, c = 14.389(6) Å, β =
100.19(3)°, V = 1810.2(12) Å3, space group P21/n, Z = 4. The equatorial positions are occupied by the two
cis-amine N-atoms and the coordinated iodides, while the axial positions are occupied by the thioether sulfur
atoms. In organic solvents, the dithiolate complex 1 reacts with molecular oxygen or H2O2 to produce the mixed
sulfinato/thiolato complex 4 [(N,N‘-dimethyl-N-(2-sulfanylethyl)-N‘-(2-sulfinatoethyl)(ethylenediamine)nickel(II)], and
the fully oxidized product 5. X-ray analysis of complex 4 reveals a square-planar geometry in which the nickel ion
is coordinated by two cis-amine nitrogens, one thiolate sulfur donor, and one sulfinato sulfur donor. Crystal data
of 4:  orthorhombic, a = 11.659(2) Å, b = 13.119(3) Å, c = 16.869(3) Å, V = 2580.2(9) Å3, space group Pbca,
Z = 8. This complex is the only intermediate in the oxygenation reaction that could be isolated, and it is shown
to be further reactive toward O2 to yield the fully oxidized product 5. For a better understanding of the reactivity
observed for 4, DFT calculations have been undertaken, which show a possible reaction path toward the fully
oxidized product 5.