Synthesis, crystal structure, 57Fe Mössbauer spectroscopy and magnetic properties of high-spin iron(III) anionic complexes [Fe(azp)2]- (H2azp = 2,2́- dihydroxyazobenzene) with organic cations

Abstract Two new iron(III) complexes, (TrBA)[Fe(azp)2] (1) and (TrPA)[Fe(azp)2] (2) (H2azp = 2,2- dihydroxyazobenzene, TrBA = tributylammonium and TrPA = tripropylammonium) were prepared by reaction of the azo dye H2azp, tributylamine (or tripropylamine) and FeCl3 in methanolic solution. The crystal structure of 1 was determined by single-crystal X-ray analysis. The asymmetric unit contains one tributylamonnium cation and one [Fe(azp)2]- anion with an octahedrally coordinated Fe(III) ion with an FeN2O4 chromophore – the tridentate ligand azp2- is coordinated by two deprotonated phenolic oxygen atoms and one nitrogen atom from the azo group. The single-crystal X-ray analysis of 1 suggests that an N–H∙∙∙O hydrogen bonding interaction between the TrBA cation and the [Fe(azp)2]- anion may induce distortion of the coordination polyhedron resulting in a high-spin complex. Magnetic investigation (DC magnetic data) revealed a high-spin state in the studied temperature range (2-300 K) for both 1 and 2. AC susceptibility measurements for 1 showed a field-induced slow relaxation of magnetization with an atypical temperature dependence of the relaxation time. Moreover, the cryogenic and room temperature 57Fe Mossbauer spectroscopy parameters confirmed the high-spin state of 1. The study was also supported by theoretical calculations at the DFT level of theory.