A new functional model of catechol dioxygenases: Properties and reactivity of [Fe(BLPA)DBC]BPh4

Abstract

[Fe-III(BLPA)DBC]BPh4, a new functional model for the catechol dioxygenases, has been synthesized, where BLPA is bis((6-methyl-2-peridyl)methyl)(2-pyridylmethyl)amine and DBC is 3,5-di-tert-butylcatecholate dianion. The BLPA complex has a structural feature that iron center has a six-coordinate geometry with N4O2 donor set. It exhibits EPR signals at g = 5.5 and 8.0 which are typical values for the high-spin Fe-III (S=5/2) complex with axial symmetry. The BLPA complex reacts with O-2 within a few hours to afford intradiol cleavage (75%) and extradiol cleavage (15%) products which is very unique result of all [Fe(L)DBC] complexes studied. The iron-catecholate interaction of BLPA complex is significantly stronger, resulting in the enhanced covalency of the metal-catecholate bonds and low energy catecholate to Fe-III charge transfer bands at 583 and 962 nm in CH,CN. The enhanced covalency is also reflected by the isotropic shifts exhibited by the DEC protons, which indicate increased semiquinone character. The greater semiquinone character in the BLPA complex correlates well with its high reactivity towards O-2. Kinetic studies of the reaction of the BLPA complex with 1 atm O-2 in CH3OH and CH2Cl2 under pseudo-first order conditions show that the BLPA complex reacts with O-2 much slower than the TPA complex, where TPA is tris(2-pyridylmethyl)amine. It is presumably due to the steric effect of the methyl substituent on the pyridine ring. Nevertheless, both the high specificity and the fast kinetics can be rationalized on the basis of its low energy catecholate to Fem charge transfer bands and large isotropic NMR shifts for the BLPA protons. These results provide insight into the nature of the oxygenation mechanism of the catechol dioxygenases.