Biomimetic catalysis of catechol cleavage by O₂ in organic solvents - Role of accessibility of O₂ to Fe^III in 2,11-diaza[3,3](2,6)pyridinophane-type catalysts

Three new complexes, [Fe(LN₄H₂)Cl₂]⁺, [Fe(LN₄H₂)(Cat)]⁺, and [Fe(LN₄H₂)(DBC)]₊, were synthesized by using the tetradentate macrocyclic ligand LN₄H₂ (where LN₄H₂, Cat, and DBC stand for 2,11-diaza[3,3](2,6) pyridinophane, catecholate, and 3,5-di-tert-butylcatecholate, respectively). The structure of [Fe(LN₄H₂)Cl₂]⁺ was determined by X-ray diffraction. It crystallizes in the monoclinic space group C2/c with a = 9.613(1), b = 11.589(1), c = 14.063(2) Å, β=110.20(2)°, V = 1541.9(3) ų and Z = 4. These complexes were found to catalyze the oxidation of catechol groups using O₂. This was performed in various organic solvents at 20 °C. The reaction rates were measured for the stoichiometric complexes [Fe(LN₄H₂)(Cat)]⁺ and [Fe(LN₄H₂)(DBC)]⁺. It was found that despite the relatively high energy of the ligand-to-metal charge transfer O(DBC or Cat)→Fe^III, their activity was comparable to that of the fast TPA systems [TPA indicates tris(2-pyridylmethyl)amine]. The oxidation products of DBCH₂ have been studied. It has then been shown that the LN₄H₂ systems catalyse by means of both intra- and extradiol cleavage of catechol groups. The existence of multiple reactive pathways can account for the fast reactivity observed.