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

Three new complexes, [Fe(LN4H2)Cl-2](+), [Fe(LN4H2)(Cat)](+), and [Fe(LN4H2 )(DBC)](+), were synthesized by using the tetradentate macrocyclic ligand L N4H2 (where LN4H2, Cat, and DBC stand for 2,11-diaza[3,3](2,6) pyridinophan e, catecholate, and 3,5-di-tert-butylcatecholate, respectively). The struct ure of [Fe(LN4H2)Cl-2](+) was determined by X-ray diffraction. It crystalli zes in the monoclinic space group C2/c with a = 9.613(1), b = 11.589(1), c = 14.063(2) Angstrom, beta =110.20(2)degrees, V = 1541.9(3) Angstrom (3), a nd Z = 4. These complexes were found to catalyze the oxidation of catechol groups using O-2. This was performed in various organic solvents at 20 degr eesC. The reaction rates were measured for the stoichiometric complexes [Fe (LN4H2)(Cat)](+) and [Fe(LN4H2)(DBC)](+). It was found that despite the rel atively 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 hav e been studied. It has then been shown that the LN4H2 systems catalyse by m eans of both intra- and extradiol cleavage of catechol groups. The existenc e of multiple reactive pathways can account for the fast reactivity observe d.