SYNTHESIS, STRUCTURE, AND REACTIVITY OF THE NICKEL(II) COMPLEX OF 7-AZA-1,4-DITHIACYCLONONANE

The “missing” member of the series of ligands, L1-L4, based on cyclononane [9]aneNxSy (x + y = 3), namely 7-aza-1,4-dithiacyclononane, L3, [9]aneNS2, has been synthesized and characterized. A key step in the synthesis is the final detosylation, upon which comments are made. The Ni(II) complex has been prepared as a perchlorate salt and investigated by X-ray diffraction. Octahedral symmetry is observed, with each nitrogen in an axial position. The species [Ni(L3)2](ClO4)2 crystallizes in the monoclinic space group P21/a (No. 14) with a = 16.607 (3) Å, b = 8.837 (1) Å, c = 7.465 (1) Å, β = 93.3 (2)°, V = 1093.7 Å3, and Z = 2. Of 1929 reflections, 1645 were used (F > 3σ(F)), and the structure was refined to R(Fo) = 0.0517 and Rw = 0.0591. Stepwise replacement of N by S has a pronounced effect on the geometry of the nickel(II) complexes. Electrochemical oxidation in dry CH3CN yields [Ni(L3)2]3+ (ESR active, axial spectrum, g± = 2.121, gt = 2.019), which is stable for several hours. However, the Ni(III) species is very sensitive to moisture. Cyclic voltammetry in acetonitrile shows two features. The oxidation to Ni(III) occurs at E1/2 = 1.044 V (vs Ag+/0). An irreversible reduction wave at -1.52 V is associated with an oxidation wave at -0.122 V. Reduction of [Ni(L3)2]3+ in deoxygenated water using sodium borohydride yields a brownish yellow species, ESR inactive, that is unstable and deposited Ni(0) on standing. On immediate aeration, this species is reoxidized to the starting Ni(II) complex. Chemical reduction of [Ni(L3)2]2+ in acetonitrile media produces an ESR-active Ni(I) species that differed in the presence of triphenylphosphine. On the basis of electronic and ESR spectroscopy, it is concluded that such species adopt a tetrahedral geometry but may be involved in equilibria with other 5-coordinate complexes. Although the NiS4 chromophore is analogous with some hydrogenases based on that metal center, it appears that the completely saturated macrocycles and 6-coordinated Ni(II) centers do not represent models for such activity. © 1990, American Chemical Society. All rights reserved.