X-ray crystallographic and absorption spectroscopic analyses of structures of catecholato (pyridine)iron chloride complexes in relevance to functional model complexes for catechol 1,2-dioxygenases

Abstract

Catecholatoiron complexes were isolated in the crystalline or non-crystalline form from the reaction solution of catechols with FeCl 3 in the presence of pyridine in THF; they form a functional model system for catechol dioxygenases. Two types of complex were obtained, depending on substituent on catechol and pyridine: [FeCl (catechol)(pyridine)](pyridine) and [FeCl 2(catechol)] −·2[pyridine·H] +. An X-ray quality single crystal was obtained with the 3.5-di-tert-butycatecholato(2,6-lutidine)iron chloride complex: it is monoclinic, space group C2 m , with cell dimensions a = 11.947(3), b = 16.122(3), c = 14.725(2) A ̊ , β = 100.38(2)°, and Z = 2. The [Fe 2(catechol) 2Cl 4] 2− anion forms a dinuclear complex with two crystallographically equivalent iron centers, each metal ion of which is in a five-coordinated distorted square-pyramidal geometry with two chloride anions and two oxygen atoms of two bridging catechol molecules. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) studies have revealed that the other isolated complexes are also dimeric in the solid state and become monomeric in the THF solution. The presence of FeCl bonding and its conversion to Fe-pyridine in solution are shown by the Br K-edge EXAFS analysis of the analogous bromide complex.