Redox and acid–base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes
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) (H2LNO2), namely [(FeL)2(μ-O)] (2) and [(FeLNO2)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 pKa of the hydroxo bridge is estimated to be about 20.4. We attribute this high pKa 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.