Synthesis and Characterization of Homo- and Heterodinuclear Complexes Containing the N3M(μ2-SR)3MN3 Core (M = Fe, Co, Ni)

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A novel amine-thiolate ligand, H33·6HCl (N,N‘,N‘‘-tris-[2-thio-3-aminomethyl-5-tert-butylbenzyl]-1,1,1-tris(aminomethyl)ethane), has been synthesized and used in the preparation of dinuclear complexes of Fe, Co, and Ni. The nonadentate N6S3 ligand H33 is formally derived from the symmetric tridentate N2S ligand H11 (2,6-bis(aminomethyl)-4-tert-butylthiophenol). It provides two dissimilar octahedral N3S3 and N‘3S3 coordination sites to give complexes with a central N3M(μ2-SR)3MN‘3 core structure (N and N‘ denote primary and secondary amine nitrogen atoms, respectively). The complexes of H33 exist as dinuclear [MII2(3)]+, [MIIIMII(3)]2+, or [MIII2(3)]3+ species which are all accessible by chemical or electrochemical reduction/oxidation. The following complexes were isolated as microcrystalline solids: [CoIII2(3)][ClO4]3 (4b), [NiIIINiII(3)][BPh4]2 (5b), and [FeIIIFeII(3)][BPh4]2 (6b). The chemical and physicochemical properties of the respective species are very similar to those of Fe, Co, and Ni complexes of H11, [M2(1)3]n+ (M = Co, n = 3, (4a); M = Ni, n = 2 (5a); M = Fe, n = 2 (6a)), and support the formulation of 4b−6b as discrete dinuclear species with a central N3M(μ2-SR)3MN‘3 core. NMR spectra of diamagnetic cobalt complexes 4a and 4b reveal the complexes to be C3h and C3 symmetric, respectively, in the solution state. The crystal structure determination of [CoIII2(1)3][Fe(CN)6]·7MeOH·3H2O (4c) (monoclinic, space group C2/c, a = 28.037(2) Å, b = 17.861(1) Å, c = 25.727(2) Å, β = 90.24(1)°, and Z = 8) reveals 4c to consist of dinuclear [CoIII2(1)3]3+ trications featuring two fac-octahedral N3CoIII(SR)3 units bridged at the thiolate sulfur atoms. Compound 4c represents the first structurally characterized MIII2 complex of H11. The ability of H33 to form heterodinuclear complexes is demonstrated with the synthesis of [CoIIINiII(3)][BPh4]2 (7) and its linkage isomer [NiIICoIII(3)][BPh4]2 (8). All complexes undergo two electrochemically and chemically reversible one-electron-transfer reactions which convert the respective [M2(3)]n+ species. For MIII2/MIIIMII (E11/2) and MIIIMII/MII2 (E21/2): −0.40 V, −0.84 V (M = Co), +0.49 V, +0.05 V (M = Ni), +0.21 V, −0.33 V (M = Fe) vs SCE. Heterodinuclear complexes 7 and 8 also give rise to two consecutive one-electron-transfer processes at E11/2 (NiIII/II) and E21/2 (CoIII/II): +0.55 V, −0.71 V (for 7) and +0.60 V, −0.86 V (for 8), respectively. Comparison of the electrochemical properties of [CoIIINiII(3)]2+ and [NiIIINiII(3)]2+ reveals the NiIII/II redox potential in the dinuclear complexes to be influenced by the oxidation state of the adjacent metal ion. At 77 K the mixed-valent NiIIINiII (S = 3/2 spin ground states) and FeIIIFeII complexes (S = 1/2 spin ground states) exhibit localized and delocalized valencies, respectively, as indicated by UV−vis, EPR, and 57Fe Mössbauer spectroscopy.