TRINUCLEAR COPPER(II) HYDROXO AND HEXANUCLEAR COPPER(II) OXO COMPLEXES WITH THE LIGAND 3-(BENZYLIMINO)BUTANONE 2-OXIME - SYNTHESES AND SPECTRAL, STRUCTURAL, AND REDOX CHARACTERISTICS

The syntheses, structures, and properties of a trinuclear copper(II) hydroxo complex and a hexanuclear copper(II) oxo complex containing the same imino-oximato ligand are reported. The pair [Cu3(mu-3-OH)L3(ClO4)2(H2O)] (1) and [Cu6(mu-4-O)2L6(H2O)] (ClO4)2.0.5H2O (2) displays the first example of tri- and hexacopper(II) complexes built with the same ligand, 3-(ben-zylimino)butanone 2-oxime (LH). The X-ray crystallographic structures have been determined. 1 crystallizes in the monoclinic space group P2(1)/c (a = 15.350 (1) angstrom, b = 12.508 (1) angstrom, c = 21.439 (2) angstrom, beta = 96.81 (1)-degrees, Z = 4). The molecular structure shows that the copper ions are at the corners of an equilateral triangle, separated by 3.25 A, held together by a mu-3-hydroxo group and three oxime bridges. An unusual triply coordinated perchlorate ion interacts in axial position with the three copper ions. 2 crystallizes in the monoclinic space group P2(1)/n (a = 15.622 (2) angstrom, b = 27.025 (2) angstrom, c = 18.864 (4) angstrom, beta = 97.38 (2)-degrees, Z = 4). Here the [Cu6(mu-4-O)2L6]2+ cation contains two triangular [Cu3O] units, bridged by their central oxygen atoms. The electron-transfer properties of 1 and 2 in propylene carbonate solution have been studied by voltammetry, coulometry, and spectrophotometry. 1 is electroreducible into a [Cu(I)Cu(II)2(OH)]+ species at -0.41 V versus SCE, followed by a fast chemical step leading to another [Cu(I)Cu(II)2(OH)]+ entity, 3, which is reversibly oxidizable at +0.42 V versus SCE. 2 is reversibly electrooxidizable at +0.42 V versus SCE into a [Cu(III)Cu(II)2(O)]2+ species, 4, which transforms slowly through an homogeneous reaction into 1. Reversible and quantitative interconversion between 1 and 2 is achieved by protonation/deprotonation of the mu-3-oxygen. This proton gated reaction grants to the species either a mild oxidant (1) or a mild reductant (2) character. The similarity of the redox potential for the couples [Cu(I)Cu(II)2(OH)]+/[Cu(II)Cu(II)2(OH)]2+ involving 3 and [Cu(II)3(O)]+/[Cu(III)Cu(II)2(O)]2+ involving 2 makes 1 act as a chemical switch between two loops of redox reaction chains.