Synthesis, crystal structures and physical properties of µ-oxo-µ-carboxylato-diiron(III) complexes with tripodal polybenzimidazole ligands containing distinct iron sites

Abstract

µ-Oxo-µ-carboxylato-diiron complexes [Fe₂O(O₂CR)L₂]X₃(R = Me or Ph, X = ClO₄ or NO₃) with the tripodal polybenzimidazole tris[(benzimidazol-2-yl)methyl]amine (L¹), tris[(N-methylbenzimidazol-2-yl)methyl]amine (L²) and tris[(N-ethylbenzimidazol-2-yl)methyl]amine (L³) have been synthesized and characterized by elemental analysis, IR, electronic, resonance-Raman and ¹H NMR spectroscopies and cyclic voltammetry. The crystal structures of [Fe₂O(O₂CMe)L²₂][ClO₄]₃H₂O·3MeCN and [Fe₂O(O₂CPh)L²₂][ClO₄]₃·H₂O·2MeCN have been determined. They show that the complexes contain two distinct iron sites. At one the benzimidazole nitrogen atom is trans to the oxo-bridge, while at the other the tertiary amine nitrogen atom is trans to the oxo-bridge. That this solid-state structure persists in solution was confirmed by ¹H NMR spectroscopy. The asymmetry is manifested in the resonance Raman spectra as an enhancement of ν_asym(Fe–O–Fe) and a large intensity ratio (I_asym/I_sym 0.19–0.26:1) of ν_asym(Fe–O–Fe) to ν_asym(Fe–O–Fe). The first complex exhibits strongly antiferromagnetic exchange with a J value of –108.22 cm^–1. The Mössbauer spectra of these two complexes and of [Fe₂O(O₂CMe)L³₂][ClO₄]₃·0.5H₂O have been recorded. The electrochemical results show that the complexes exhibit completely irreversible redox behaviour which is different from that found in µ-oxo-diiron proteins.