Redox and acid–base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes

摘要: The diiron unit is commonly found as the active site in enzymes that catalyze important biological transformations. Two μ-(hydr)oxo-diiron(III) complexes with ligands 2,2′-(2-methyl-2-(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L) and 2,2′-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2LNO2), namely [(FeL)2(μ-O)] (2) [(FeLNO2)2(μ-OH)]ClO4 (5) were synthesized characterized. In solid state, both structures are asymmetric, unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of ligand NH groups to phenolate O atoms hold cores a bent configuration (Fe–O–Fe angle 143.7° for 2 140.1° 5). A new bridged diferrous complex, [(FeL)2] (4), was Upon exposure air 4 complex oxidized diferric 2. Cyclic voltammetry at different scan rates chemical reduction [(FeL)2(μ-OH)]BPh4 (1) cobaltocene revealed disproportionation followed by proton transfer, mixed-valence species could not be trapped. Subsequent molecular oxygen results formation Electrochemical studies 5 indicate easier diiron(III/III) state than 1. protonation benzoic acid form [(FeL)2(μ-OH)]+ only changes Fe–O–Fe 5° (from 138.6°), pKa hydroxo bridge estimated about 20.4. We attribute this high partly stabilization benzoate ligand's amine proton. Magnetic susceptibility on samples 1 yielded values antiferromagnetic exchange coupling constants, J, these S = 5/2 dimers −13.1 cm−1 −87.5 cm−1, respectively, typical such hydroxo- oxo-bridges.