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

Abstract

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 the 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) (H₂L^NO2), namely [(FeL)₂(μ-O)] (2) and [(FeL^NO2)₂(μ-OH)]ClO₄ (5) were synthesized and characterized. In the solid state, both structures are asymmetric, with unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of the ligand NH groups to the phenolate O atoms hold the diiron cores in a bent configuration (Fe–O–Fe angle of 143.7° for 2 and 140.1° for 5). A new phenolate bridged diferrous complex, [(FeL)₂] (4), was synthesized and characterized. Upon exposure to air the diferrous 4 complex is oxidized to the diferric 2. Cyclic voltammetry at different scan rates and chemical reduction of [(FeL)₂(μ-OH)]BPh₄ (1) with cobaltocene revealed disproportionation followed by proton transfer, and a mixed-valence species could not be trapped. Subsequent exposure to molecular oxygen results in the formation of 2. Electrochemical studies of 5 indicate easier reduction of the diiron(III/III) to the mixed-valence state than for 1. The protonation of 2 by benzoic acid to form [(FeL)₂(μ-OH)]⁺ only changes the Fe–O–Fe angle by 5° (from 143.7° to 138.6°), and the pK_a of the hydroxo bridge is estimated to be about 20.4. We attribute this high pK_a partly to stabilization of the benzoate by hydrogen bonding to the ligand's amine proton. Magnetic susceptibility studies on solid samples of 1 and 2 yielded values of the antiferromagnetic exchange coupling constants, J, for these S = 5/2 dimers of −13.1 cm⁻¹ and −87.5 cm⁻¹, respectively, typical of such unsupported hydroxo- and oxo-bridges.