Dinuclear cyano complexes of cobalt(III) and Iron(III) containing noninnocent 1,2,4,5-tetrakis(2-pyridinecarboxamido)benzene bridging ligands

Two new dinucleating ligands 1,2,4,5-tetrakis(2-pyridinecarboxamido)benzene, H(4)(tpb), and 1,2,4,5-tetrakis(4-tert-butyl-2-pyridinecarboxamido)benzene, H(4)(tbpb), have been synthesized, and the following dinuclear cyano complexes of cobalt(III) and iron(III) have been isolated: Na(2)Co(III)(2)(tpb)(CN)(4)] (1); N(n-Bu)(4)](2)Co(III)(2)(tbpb)(CN)(4)] (2); Co(III)(2)(tbpb(ox2))(CN)(4)] (3); N(n-Bu)(4)](2)Fe(III)(2)(tpb)(N(3))(4)] (4); N(n-Bu)(4)](2)Fe(III)(2)(tpb)(CN)(4)] (5); N(n-Bu)(4)](2)Fe(III)(2)(tbpb)(CN)(4)] (6). Complexes 2-4 and 6 have been structurally characterized by X-ray crystallography at 100 K. From electrochemical and spectroscopic (UV-vis, IR, EPR, M?ssbauer) and magnetochemical investigations it is established that the coordinated central 1,2,4,5-tetraamidobenzene entity in the cyano complexes can be oxidized in two successive one-electron steps yielding paramagnetic (tbpb(ox1))(3)(-) and diamagnetic (tbpb(ox2))(2)(-) anions. Thus, complex 6 exists in five characterized oxidation levels: Fe(III)(2)(tbpb(ox2))(CN)(4)](0) (S = 0); Fe(III)(2)(tbpb(ox1))(CN)(4)](-) (S = (1)/(2)); Fe(III)(2)(tbpb)(CN)(4)](2)(-) (S = 0); Fe(III)Fe(II)(tbpb)(CN)(4)](3)(-) (S = (1)/(2)); Fe(II)(2)(tbpb)(CN)(4)](4)(-) (S = 0). The iron(II) and (III) ions are always low-spin configurated. The electronic structure of the paramagnetic iron(III) ions and the exchange interaction of the three-spin system Fe(III)(2)(tbpb(ox1))(CN)(4)](-) are characterized in detail. Similarly, for 2 three oxidation levels have been identified and fully characterized: Co(III)(2)(tbpb)(CN)(4)](2)(-) (S = 0); Co(III)(2)(tbpb(ox1))(CN)(4)](-) (S = (1)/(2)); Co(III)(2)(tbpb(ox2))(CN)(4)](0). The crystal structures of 2 and 3 clearly show that the two electron oxidation of 2 yielding 3 affects only the central tetraamidobenzene part of the ligand.