Tridentate Facial Ligation of Tris(pyridine-2-aldoximato)nickel(II) and\nTris(imidazole-2-aldoximato)nickel(II) To Generate Ni^IIFe^IIINi^II, Mn^IIINi^II,\nNi^IINi^II, and Zn^IINi^II and the Electrooxidized Mn^IVNi^II, Ni^IINi^III, and Zn^IINi^III\nSpecies:  A Magnetostructural, Electrochemical, and EPR Spectroscopic\nStudy

Abstract

Eight hetero- and homometal complexes <b>1</b>−<b>6</b>, containing the metal centers Ni^IIFe^IIINi^II (<b>1</b>), Mn^IIINi^II (<b>2</b>), Ni^IINi^II (<b>3a</b>−<b>c</b>\nand <b>4</b>), Zn^IINi^II (<b>5</b>), and Zn^IIZn^II (<b>6</b>), are described. The tridentate ligation property of the metal complexes tris(pyridine-2-aldoximato)nickel(II) and tris(1-methylimidazole-2-aldoximato)nickel(II) with three facially disposed pendent\noxime O atoms has been utilized to generate the said complexes. Complex <b>1</b> contains metal centers in a linear\narrangement, as is revealed by X-ray diffraction. Complexes were characterized by various physical methods including\ncyclic voltammetry (CV), variable-temperature (2−290 K) magnetic susceptibility, electron paramagnetic resonance\n(EPR) measurements, and X-ray diffraction methods. Binuclear complexes <b>2</b>−<b>6</b> are isostructural in the sense that\nthey all contain a metal ion in a distorted octahedral environment MN₃O₃ and a second six-coordinated Ni^II ion in\na trigonally distorted octahedral NiN₆ geometry. Complexes <b>1</b>−<b>4</b> display antiferromagnetic exchange coupling of\nthe neighboring metal centers. The order of the strength of exchange coupling in the isostructural Ni^II₂ complexes,\n<b>3a</b>−<b>c</b>, and <b>4</b>, demonstrates the effects of the remote substituents on the spin coupling. The electrochemical\nmeasurements CV and square wave voltammograms (SQW) reveal two reversible metal-centered oxidations, which\nhave been assigned to the Ni center ligated to the oxime N atoms, unless a Mn ion is present. Complex <b>2</b>, Mn^IIINi^II,\nexhibits a reduction of Mn^III to Mn^II and two subsequent oxidations of Mn^III and Ni^II to the corresponding higher\nstates. These assignments of the redox processes have been complemented by the X-band EPR measurements.\nThat the electrooxidized species [<b>3a</b>]⁺, [<b>3b</b>]⁺, [<b>3c</b>]⁺, and [<b>4</b>]⁺ contain the localized mixed-valent Ni^IINi^III system resulting\nfrom the spin coupling, a spin quartet ground state, <i>S</i>_t = ³/₂, has been confirmed by the X-band EPR measurements.