SULFUR-LIGATED NICKEL OXIDATION-STATES - CHEMISTRY OF A FAMILY OF NIZS2N4(Z=+2,+3,+4) COMPLEXES INCORPORATING HEXADENTATE THIOETHER-IMINE-OXIME BINDING

The title ligands RC(= NOH)C(Me) = N(CH2)2S(CH2)xS(CH2)2N = C(Me)C(= NOH)R (R = Me, Ph; x = 2, 3) abbreviated as H2RxS (1) afford the brown, paramagnetic (S = 1) complexes Ni(II)(H2RxS)(ClO4)2.pH2O (R = Me, p = 1; R = Ph, p = 0). Structure determination of the two R = Me, p = 1 complexes has revealed hexadentate distorted octahedral NiS2N4 coordination: a = 7.382 (1) angstrom, b = 17.598 (4) angstrom, c = 19.646 (5) angstrom, beta = 93.28 (2)-degrees, V = 2548.2 (10) angstrom 3, R = 0.0865, and R(w) = 0.0790 for x = 2 and a = 7.536 (2) angstrom, b = 17.925 (7) angstrom, c = 19.579 (7) angstrom, beta = 91.73 (3)-degrees, V = 2643.5 (15) angstrom 3, R = 0.0741, and R(w) = 0.0738 for x = 3; space group P2(1)/c and Z = 4 for both complexes. Upon oxidation of the nickel(II) complexes with concentrated HNO3 in the cold, violet diamagnetic Ni(IV)(RxS)2+ is formed. The salts Ni(IV)(MexS)(ClO4)2(x = 2, 3) decompose slowly in moist environments regenerating the nickel(II) complexes via Ni(III)(HMexS)2+ and/or Ni(III)(MexS)+. The EPR spectra of the latter in frozen solution (77 K) correspond to g perpendicular-to > g parallel-to (d(z)2(1) ground state). The following couples (illustrated with R = Me, x = 2) are observed in buffered aqueous media: couple A, Ni(IV)(Me2S)2+ + 2H+ + 2e- half-arrow-left-over-half-arrow-right Ni(II)(H2Me2S)2+, E-degrees = 0.82 V; couple B, Ni(IV)(Me2S)2+ + e- half-arrow-left-over-half-arrow-right Ni(II)(Me2S)+, E-degrees = 0.56 V; couple C, Ni(III)(Me2S)+ + mH+ + e- half-arrow-left-over-half-arrow-right Ni(II)(H(m)Me2S)m+ (m = 0, E-degrees = 0.33 V; m = 1, E-degrees = 0.79 V; m = 2, E-degrees = 1.16 V). Couple A operates below pH 5. At higher pH this is replaced by couples B and C. Comparison with related NiN6 species has revealed that replacement of amine coordination by thioether coordination increases E-degrees. This is attributed to the lower thermodynamic stability of the Ni-S(thioether) bond.