A linear correlation between energy of LMCT band and oxygenation reaction rate of a series of catecholatoiron(III) complexes: initial oxygen binding during intradiol catechol oxygenation

The oxygen reactivity of catecholatoiron(III) complexes has been examined using a series of catecholate ligands as the substrate. All the complexes examined here, [Fe III(TPA)(R-Cat)]BPh 4 ( 1– 9) (TPA: tris(pyridin-2-ylmethyl)amine; R-Cat: substituted catecholate ligand, R = 3,5- t Bu 2 ( 1), 3,6- t Bu 2 ( 2), 3,5-Me 2 ( 3), 3,6-Me 2 ( 4), 4- t Bu ( 5), 4-Me ( 6), H ( 7), 4-Cl ( 8) and 3-Cl ( 9)), exclusively afforded the intradiol cleaving products of the catecholate ligands upon exposure to O 2. It was revealed that 1– 7 can be categorized into two classes based on their electrochemical properties; i.e., the complexes having the dialkyl-substituted (group A) and the mono- or non-substituted (group B) catecholate ligands. In spite of their classification, these two groups show a linear correlation between the logarithm of the reaction rate constant with O 2 and the energy of the catecholate-to-iron(III) LMCT band, although 2 shows a large negative deviation from the correlation line. Based on this LMCT-energy dependent reactivity of 1 and 3– 9 as well as the very low reactivity of 2, we have discussed on the mechanisms of the reaction of [Fe III(TPA)(R-Cat)]BPh 4 with O 2.