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

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journal contribution
posted on 03.03.2002, 00:00 by Vincent E. Kaasjager, Elisabeth Bouwman, S. Gorter, Jan Reedijk, Craig A. Grapperhaus, Joseph H. Reibenspies, Jason J. Smee, Marcetta Y. Darensbourg, Agnes Derecskei-Kovacs, Lisa M. Thomson
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.