First Examples of Dinickel Complexes Containing the N3Ni(μ2-SR)3NiN3 Core. Synthesis and Crystal Structures of [L2Ni2][BPh4]2 and [L3Ni2][BPh4]2 (L = 2,6-Di(aminomethyl)-4-tert-butyl-thiophenolate)

1998-07-07T00:00:00Z (GMT) by Berthold Kersting Dieter Siebert
A coordinatively unsaturated dinuclear NiII 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, NiCl2·6H2O, and NaBPh4 in methanol. Compound 1·CH3CN·CH3OH crystallizes from an acetonitrile/methanol mixed-solvent system in monoclinic space group P21/n with a = 21.940(4) Å, b = 13.901(3) Å, c = 23.918(5) Å, β = 110.00(3)°, 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 C2v 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 P21/c with a = 23.678(5) Å, b = 20.090(4) Å, c = 16.797(3) Å, β = 106.16(3)°, and Z = 4. Complex 3 is the first structurally characterized example that features a bioctahedral N3Ni(μ2-SR)3NiN3 core. Distortions from D3h symmetry suggest that 3 is a trapped-valence NiIINiIII 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 E1/2 = −0.02 and +0.44 V vs SCE, affording 3 and the transient dark green trication [L3Ni2]3+1/2 ≈ 15 min at 295 K), respectively. While 2 is EPR silent, the EPR spectrum of a powdered sample of 3 reveals g = 4.0 and g = 2.09 at 77 K, consistent with an S = 3/2 spin state of the mixed-valent NiIINiIII complex.