First Examples of Dinickel Complexes Containing the N(3)Ni(&mgr;(2)-SR)(3)NiN(3) Core. Synthesis and Crystal Structures of [L(2)Ni(2)][BPh(4)](2) and [L(3)Ni(2)][BPh(4)](2) (L = 2,6-Di(aminomethyl)-4-tert-butyl-thiophenolate).

Abstract

A coordinatively unsaturated dinuclear Ni(II) complex of the tridentate ligand 2,6-di(aminomethyl)-4-tert-butyl-thiophenol (HL) has been synthesized and investigated in the context of ligand binding and oxidation state changes. The starting complex [L(2)Ni(2)][BPh(4)](2) (1) is readily prepared from NaL, NiCl(2).6H(2)O, and NaBPh(4) in methanol. Compound 1.CH(3)CN.CH(3)OH crystallizes from an acetonitrile/methanol mixed-solvent system in monoclinic space group P2(1)/n with a = 21.940(4) Å, b = 13.901(3) Å, c = 23.918(5) Å, beta = 110.00(3) degrees, and Z = 4. The structure consists of dinuclear [L(2)Ni(2)](2+) cations with two distorted planar cis-N(2)S(2)Ni coordination units joined by the thiophenolate sulfur atoms. The molecule has idealized C(2)(v)() symmetry. Complex 1 readily adds another 1 equiv of HL to afford the pale green complex [L(3)Ni(2)]Cl (2). The dinuclear structure and its formulation as a 3:2 complex (six-coordinate Ni ions) is derived from UV spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction of its oxidation product, [L(3)Ni(2)](2+). The dication was prepared by chemical oxidation of 2 with iodine in DMF and isolated as the dark brown BPh(4)(-) salt, [L(3)Ni(2)][BPh(4)](2).CH(3)OH (3), which crystallizes in monoclinic space group P2(1)/c with a = 23.678(5) Å, b = 20.090(4) Å, c = 16.797(3) Å, beta = 106.16(3) degrees, and Z = 4. Complex 3 is the first structurally characterized example that features a bioctahedral N(3)Ni(&mgr;(2)-SR)(3)NiN(3) core. Distortions from D(3)(h)() symmetry suggest that 3 is a trapped-valence Ni(II)Ni(III) compound. The Ni-S and Ni-N bond lengths vary from 2.2975(9) to 2.4486(12) Å and from 2.027(3) to 2.120(3) Å, respectively. On the CV time scale complex 2 undergoes two reversible electron-transfer reactions at E(1/2) = -0.02 and +0.44 V vs SCE, affording 3 and the transient dark green trication [L(3)Ni(2)](3+) (tau(1/2) approximately 15 min at 295 K), respectively. While 2 is EPR silent, the EPR spectrum of a powdered sample of 3 reveals g( perpendicular) = 4.0 and g( parallel) = 2.09 at 77 K, consistent with an S = (3)/(2) spin state of the mixed-valent Ni(II)Ni(III) complex.