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, H4(tpb), and 1,2,4,5-tetrakis(4-tert-butyl-2-pyridinecarboxamido)benzene, H4(tbpb), have been synthesized, and the following dinuclear cyano complexes
of cobalt(III) and iron(III) have been isolated:  Na2[CoIII2(tpb)(CN)4] (1); [N(n-Bu)4]2[CoIII2(tbpb)(CN)4] (2); [CoIII2(tbpbox2)(CN)4] (3); [N(n-Bu)4]2[FeIII2(tpb)(N3)4] (4); [N(n-Bu)4]2[FeIII2(tpb)(CN)4] (5); [N(n-Bu)4]2[FeIII2(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, Mössbauer) 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 (tbpbox1)3- and diamagnetic (tbpbox2)2- anions. Thus, complex 6 exists in
five characterized oxidation levels:  [FeIII2(tbpbox2)(CN)4]0 (S = 0); [FeIII2(tbpbox1)(CN)4]- (S = 1/2); [FeIII2(tbpb)(CN)4]2-
(S = 0); [FeIIIFeII(tbpb)(CN)4]3- (S = 1/2); [FeII2(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 [FeIII2(tbpbox1)(CN)4]- are characterized in detail. Similarly, for 2 three oxidation levels have been
identified and fully characterized:  [CoIII2(tbpb)(CN)4]2- (S = 0); [CoIII2(tbpbox1)(CN)4]- (S = 1/2); [CoIII2(tbpbox2)(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.