Preparation, structure, and properties of a mixed-valent NiIINiIII amine-selenolate complex

Two dinuclear nickel amine-selenolate complexes of the tridentate amine-selenolate ligand, 4-tert-butyl-2,6-di-(aminomethyl)selenophenol (Ib), have been synthesized and their properties characterized. The green complex [Ni-2(II)(1b)(3)]Cl (4a) is readily prepared from 1b, NiCl2 . 6 H2O, and NaOMe in methanol or by reaction of coordinatively unsaturated [Ni-2(II)(1b)(2)][BPh4](2) (2b) with another one equivalent of Ib in the presence of triethylamine. The face-sharing, bioctahedral structure of 4a is derived from UV/Vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction of its oxidation product, [(NiNiIII)-Ni-II(lb)(3)](2+). The dication was prepared by air-oxidation of 4a in MeOH and isolated as the dark-brown BPh4- salt, [(NiNiIII)-Ni-II(1b)(3)][BPh4](2) . CH3OH (4b). Mixed-valent complex 4b consists of a dinuclear, face-sharing bioctahedral dication with a central N3NiII(mu(2)-SeR)(3)(NiN3)-N-III core and well-separated tetraphenylborate anions. Distortions from D-3h symmetry suggest that 4b is a trapped-valence compound in the solid-state. On the CV time scale complex 4a undergoes two reversible one-electron oxidations at E-1/2(1) = -0.15 and at E-1/2(2) = +0.29 V vs SCE, affording 4b and the thermally not stable trication [Ni-2(III)(lb)(3)](3+), respectively. The EPR spectrum of a powdered sample of 4b reveals an S = 3/2 ground state of the mixed-valent (NiNiIII)-Ni-II complex.