Mixed 3d-3d’-Metal Complexes: A Dicobalt(III)Iron(III) Coordination Cluster Based on Pyridine-2-Amidoxime

In the present work, we describe the use of the potentially tridentate ligand pyridine-2-amidoxime (NH2paoH) in Fe-Co chemistry. The 1:1:3 FeIII(NO3)3·9H2O/CoII(ClO4)2·6H2O/NH2paoH reaction mixture in MeOH gave complex [CoIII2FeIII(NH2pao)6](ClO4)2(NO3) (1) in ca. 55% yield, the cobalt(II) being oxidized to cobalt(III) under the aerobic conditions. The same complex was isolated using cobalt(II) and iron(II) sources, the oxidation now taking place at both metal sites. The structure of 1 contains two structurally similar, crystallographically independent cations [CoIII2FeIII(NH2pao)6]3+ which are strictly linear by symmetry. The central high-spin FeIII ion is connected to each of the terminal low-spin CoIII ions through the oximato groups of three 2.1110 (Harris notation) NH2pao− ligands, in such a way that the six O atoms are bonded to the octahedral FeIII center ({FeIIIO6} coordination sphere). Each terminal octahedral CoIII ions is bonded to six N atoms (three oximato, three 2-pyridyl) from three NH2pao− groups ({CoIIIN6} coordination sphere). The IR and Raman spectra of the complex are discussed in terms of the coordination mode of the organic ligand, and the non-coordinating nature of the inorganic ClO4− and NO3− counterions. The UV/VIS spectrum of the complex in EtOH shows the two spin-allowed d-d transitions of the low-spin 3d6 cobalt(III) and a charge-transfer NH2pao− → FeIII band. The δ and ΔΕQ 57Fe-Mössbauer parameter of 1 at 80 K show the presence of an isolated high-spin FeIII center. Variable-temperature (1.8 K–300 K) and variable-field (0–7 T) magnetic studies confirm the isolated character of FeIII. A critical discussion of the importance of NH2paoH and its anionic forms (NH2pao−, NHpao2−) in homo- and heterometallic chemistry is also attempted.