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 the ligands 2,2’-(2-methyl-2-(pyridine-2-yl)- propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H_2L) and 2,2’-(2-methyl-2(pyridine-2-yl)- propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H_2L~(NO_2)), namely [(FeL)_2(μ-O)] (2) and [(FeL~(NO_2))2(μ-OH)]ClO_4 (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)]BPh_4 (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 oxobridges.