Structural snapshots of a dynamic coordination sphere in model complexes for catechol 1,2-dioxygenases

A series of mononuclear iron(III) complexes as models for intradiol cleaving catechol dioxygenases has been synthesized. In all six compounds, the iron(III) core is coordinated by the ligand [(6-bromo-2-pyridyl)methyl]bis[(2-pyridyl)methyl]amine (brtpa) which mimics the endogenous ligands of this enzyme class and their dynamic behavior. Its steric hindrance pushes the alpha-substituted pyridine arm of the ligand away from the coordination site in the presence of nucleophiles. This effect turns the iron-brtpa system into an excellent model system for the first step in the reaction mechanism of intradiol cleaving chatechol dioxygenases. The proton acceptor abilities of the bromopyridyl arm are underlined by a hydrogen bond to a coordinated methanol molecule in one of the new compounds and by base dependent kinetic investigations. The presented model system is an effective catalyst that requires between 0.5 and 1.0 fewer equivalents of external base than most other complexes due to the integration of an internal base. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.