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[CO2III(tpb)(CN)(4)] (1); [N(n-Bu)(4)](2)[Co-2(III)(tbpb)(CN)(4)] (2); [Co-2(III)-(tbpb(ox2))(CN)(4)] (3); [N(n-Bu)(4)](2)[Fe-2(II)(tpb)(N-3)(4)] (4); [N(n-Bu)(4)](2)[Fe-2(III)(tpb)(CN)(4)] (5); [N(n-Bu)(4)](2)[Fe-2(III)(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, Mossbauer) 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-2(III)(tbpb(ox2)) (CN)(4)](0) (S = 0); [Fe-2(III)(tbpb(ox1))(CN)(4)](-) (S = 1/2); [Fe-2(III)(tbpb)(CN)(4)](2-) (S = 0); [(FeFeII)-Fe-III(tbpb)(CN)(4)](3-) (S = 1/2); [Fe-2(II)(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-2(III)(tbpb(ox1))(CN)(4)](-) are characterized in detail. Similarly, for 2 three oxidation levels have been identified and fully characterized: [Co-2(III)(tbpb)(CN)(4)](2-) (S = 0); [Co-2(III)(tbpb(ox1))(CN)(4)](-) (S = 1/2); [Co-2(III)(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.