SYNTHESES, REDOX BEHAVIOR, AND MAGNETIC AND SPECTROSCOPIC PROPERTIES OF CUIICUIICUII, CUIINIIICUII, AND CUIIPDIICUII SPECIES - CRYSTAL-STRUCTURE OF [L2CU2CU(DMG)2BR]CLO4.CH3OH (L=1,4,7-TRIMETHYL-1,4,7-TRIAZACYCLONONANE-DMG=DIMETHYLGLYOXIMATO(2-))

A series of homo- and heterotrinuclear complexes Cu(II)Cu(II)Cu(II), Cu(II)Ni(II)Cu(II), and Cu(II)Pd(II)Cu(II) containing the dimethylglyoximato dianion as bridging ligands and either 1,4,7-trimethyl-1,4,7-triazacyclononane or N,N,N',N'-tetramethylethylenediamine as capping ligands have been synthesized and characterized on the basis of IR, electronic, and EPR spectroscopy and variable-temperature (4-290 K) magnetic susceptibility measurements. The crystal and molecular structure of the compound [L2Cu2(dmg)2CuBr]ClO4.CH3OH has been established by X-ray diffraction. It crystallizes in orthorhombic system, space group Pnam with lattice constants a = 14.006 (7) angstrom, b = 15.972 (8) angstrom, c = 19.27 (1) angstrom, and Z = 4. The structure consists of Cu(II)Cu(II)Cu(II) trinuclear cations, uncoordinated perchlorate anions, and a methanol molecule of crystallization. Each copper ion is in a square-pyramidal environment. The Cu(dmg)2(2-) dianion functions as a bridge between two terminal copper ions through its deprotonated oximate oxygen and represents the central copper atom in the trinuclear unit. The central copper atom is coordinated also to a bromide ion. The terminal copper ions achieve a CuN3O2 chromophore structure by coordinating to three nitrogen atoms of the cyclic amine and the deprotonated oxygen atoms of dimethylglyoxime dianion. A strong antiferromagnetic interaction (J = -448 cm-1) between adjacent Cu(II) ions has been found for the aforementioned compound, showing that dimethylglyoxime is a good mediator for spin-exchange interactions. A moderately strong antiferromagnetic interaction (2J = -72 cm-1) has been observed for a Cu(II)Pd(II)Cu(II) system, with a diamagnetic square-planar Pd(II) as the central ion, although the Cu...Cu separation is expected to be ca. 7.6 angstrom. The powder EPR spectra indicate square-pyramidal geometry for the copper with a (d(x2-y2))1 ground state in the homotrinuclear complexes. EPR spectra of heterotrinuclear complexes are quasi-isotropic. The cyclic voltammograms of the Cu(II)Cu(II)Cu(II) complexes revealed, besides other features, a quasi-reversible one-electron oxidation corresponding to the reversible formation of the mixed-valence species Cu(II)Cu(III)Cu(II).