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

Catecholatoiron complexes were isolated in the crystalline or non-crys talline form from the reaction solution of catechols with FeCl3, in th e presence of pyridine in THF; they form a functional model system for catechol dioxygenases. Two types of complex were obtained, depending on substituents on catechol and pyridine: [FeCl (catechol) (pyridine)] (pyridine) and [FeCl,(catechol)](-) .2[pyridine.H](+). An X-ray quali ty single crystal was obtained with the 3,5-di-tert-butylcatecholato(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) Angs trom, beta=100.38(2)degrees and Z=2. The [Fez(catechol)(2)Chl(4)](2-) anion forms a dinuclear complex with two crystallographically equivale nt iron centers, each metal ion of which is in a five-coordinated dist orted square-pyramidal geometry with two chloride anions and two oxyge n 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 dim eric in the solid state and become monomeric in the THF solution. The presence of Fe-Cl bonding and its conversion to Fe-pyridine in solutio n are shown by the Br K-edge EXAFS analysis of the analogous bromide c omplex. (C) 1998 Elsevier Science S.A. All rights reserved.