First examples of dinickel complexes containing the N3NiNi(μSR)3NiN3 core.: Synthesis and crystal structures of [L2Ni2][BPh4]2 and [L3Ni2][BPh4]2 (L = 2,6-di(aminomethyl)-4-tert-butyl-thiophenolate)

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 [L2Ni2][BPh4](2) (1) is readily prepared from NaL, NiCl2. 6H(2)O, and NaBPh4 in methanol. Compound 1 . CH3CN . CH3OH crystallizes from an acetonitrile/methanol mixed-solvent system in monoclinic space group P2(1)/n with a = 21.940(4) Angstrom, b = 13.901(3) Angstrom, c = 23.918(5) Angstrom, beta = 110.00(3)degrees, and Z = 4. The structure consists of dinuclear [L2Ni2](2+) cations with two distorted planar cis-N2S2Ni coordination units joined by the thiophenolate sulfur atoms. The molecule has idealized C-2v symmetry. Complex 1 readily adds another 1 equiv of HL to afford the pale green complex [L3Ni2]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, [L3Ni2](2+). The dication was prepared by chemical oxidation of 2 with iodine in DMF and isolated as the dark brown BPh4- salt, [L3Ni2][BPh4](2). CH3OH (3), which crystallizes in monoclinic space group P2(1)/c with a = 23.678(5) Angstrom, b = 20.090(4) Angstrom, c = 16.797(3) Angstrom, beta = 106.16(3)degrees, and Z = 4. Complex 3 is the first structurally characterized example that features a bioctahedral N3Ni(mu(2)-SR)(3)NiN3 core. Distortions from D-3h symmetry suggest that 3 is a trapped-valence (NiNiIII)-Ni-II compound. The Ni-S and Ni-N bond lengths vary from 2.2975(9) to 2.4486(12) Angstrom and from 2.027(3) to 2.120(3) Angstrom, 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 [L3Ni2](3+) (tau(1/2) approximate to 15 min at 295 K), respectively. While 2 is EPR silent, the EPR spectrum of a powdered sample of 3 reveals g(perpendicular to) = 4.0 and g(parallel to) = 2.09 at 77 K, consistent with an S = 3/2 spin state of the mixed-valent (NiNiIII)-Ni-II complex.