Syntheses, Redox Behavior, and Magnetic and Spectroscopic Properties of Cu^IICu^IICu¹¹, Cu^πNi^πCu^π, and Cu^πPd^πCu¹¹ Species. Crystal Structure of [L₂Cu₂Cu(dmg)₂Br]Cl0₄·CH₃0H (L = 1,4,7-Trimethyl-1,4,7-triazacyclononane; dmg = Dimethylglyoximato(2-))

A series of homo- and heterotrinuclear complexes Cu^IICu^IICu^II, Cu^IINi^IICu^II, and Cu^IIPd^IICu^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) Å, b = 15.972 (8) Å, c = 19.27 (1) Å, and Z = 4. The structure consists of Cu^IICu^IICu^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 (27 = -72 cm^-1) has been observed for a Cu^IIPd^IICu^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 Å. The powder EPR spectra indicate square-pyramidal geometry for the copper with a [formula omited] ground state in the homotrinuclear complexes. EPR spectra of heterotrinuclear complexes are quasi-isotropic. The cyclic voltammograms of the Cu^IICu^IICu^II complexes revealed, besides other features, a quasi-reversible one-electron oxidation corresponding to the reversible formation of the mixed-valence species Cu^IICu^IICu^II.