SYNTHESIS, ELECTROCHEMISTRY, AND MAGNETIC AND SPECTROSCOPIC PROPERTIES OF AN EXCHANGE-COUPLED FEIIINIIIFEIII COMPLEX - CRYSTAL-STRUCTURE OF [L2FE2(DMG)3NI](PF6)2.0.5CH3OH (L = 1,4,7-TRIMETHYL-1,4,7-TRIAZACYCLONONANE - DMG = DIMETHYLGLYOXIMATE(2-))

The trimetallic complex [L2FeIII2NiII(dmg)3](PF6)2.0.5CH3OH, where L is the cyclic amine 1,4,7-trimethyl-1,4,7-triazacyclononane and dmg is the dianion of dimethylglyoxime, has been synthesized and its structure determined by X-ray diffraction methods as having a tris(dimethylglyoximato)nickelate(II) bridging ligand. The complex crystallizes in monoclinic space group C2/c with cell constants a = 29.711 (7) angstrom, b = 12.569 (4) angstrom, c = 14.951 (3) angstrom, beta = 119.93 (1)-degrees, V = 4838 angstrom 3, and Z = 4. The high-spin iron(III) centers have six-coordinate N3O3 and the nickel center has six-coordinate N6 environments, with an Fe...Ni separation of 3.491 (3) angstrom and an Fe...Fe separation of 6.982 (2) angstrom. The closest intermolecular Fe...Fe separation is 7.37 angstrom. The X-ray structure confirms that a linear trinuclear complex with an Fe-Ni-Fe angle of 179.4 (2)-degrees has been formed. The compound has also been studied with infrared and electronic spectroscopy, cyclic voltammetry, and variable-temperature (4-284 K) magnetic susceptibility measurements. Analysis of the susceptibility data yields an antiferromagnetic interaction (J(Fe-Ni) between adjacent Fe(III) and Ni(II) centers and a weak antiferromagnetic interaction (J(Fe-Fe) between the terminal iron(III) centers. The following parameter values are obtained: J(Fe-Ni) = -32 (2) cm-1, J(Fe-Fe) = -5 (1) cm-1 for the perchlorate salt; J(Fe-Ni) = -29 (2) cm-1, J(Fe-Fe) = -5 (1) cm-1 for the hexafluorophosphate salt. The electronic ground state has been established to have S = 4. The effect of J(Fe-Fe), which raises the energy of the S = 4 level and lowers the energy of the S = 3 level on the energy-splitting pattern, has been discussed. The cyclic voltammograms of the FeIIINiIIFeIII complex reveal two quasi-reversible one-electron oxidation and two reversible one-electron reduction processes which have been assigned to the following equilibria: [GRAPHICS]