Chelate Ring Size Variations and Their Effects on Coordination Chemistry and Catechol Dioxygenase Reactivity of Iron(III) Complexes † Michael Merkel, Matthias Pascaly, and Bernt Krebs* Institut fu ¨r Anorganische und Analytische Chemie, Westfa ¨lische Wilhelms-UniVersita ¨t Mu ¨nster, Wilhelm-Klemm-Strasse 8, 48149 Mu ¨nster, Germany Jo 1 rg Astner, Simon P. Foxon, and Siegfried Schindler* Institut fu ¨r Anorganische und Analytische Chemie der Justus-Liebig-UniVersita ¨t Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany Received May 6, 2005 The catechol dioxygenase reactivity of iron(III) complexes using tripodal ligands was investigated. Increasing, as well as decreasing, chelate ring sizes in the highly active complex [Fe(tmpa)(dbc)]B(C 6 H 5 ) 4 (tmpa ) tris[(2-pyridyl)- methyl]amine; dbc ) 3,5-di-tert-butylcatecholate dianion), using related ligands, only resulted in decreased reactivity of the investigated compounds. A detailed low-temperature stopped-flow investigation of the reaction of dioxygen with [Fe(tmpa)(dbc)]B(C 6 H 5 ) 4 was performed, and activation parameters of ΔH q ) 23 ± 1 kJ mol -1 and ∆S q ) -199 ± 4 J mol -1 K -1 were obtained. Crystal structures of bromo-(tetrachlorocatecholato-O,O′)(bis((2-pyridyl)- methyl)-2-pyridylamine-N,N′,N′′)-iron(III), (µ-oxo)-bis(bromo)(bis((2-pyridyl)methyl)-2-pyridylamine-N,N′,N′′,N′′′)-diiron- (III), dichloro-((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N′,N′′,N′′′)-iron(III) and (tetrachlorocatecholato-O,O′)((2- (2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N′,N′′,N′′′)-iron(III) are reported. Introduction A large variety of mononuclear nonheme iron enzymes are involved in redox processes with molecular oxygen. 1-3 For example, intradiol-cleaving catechol dioxygenases cata- lyze the insertion of dioxygen into catechols, 3 and during the oxidation reaction, substrates are converted to acyclic cis,cis-muconic acid derivatives. Mimicking the structure and function of these enzymes has been an important goal in bioinorganic chemistry during the past decade. 1,4 The crystal structures of a number of complexes of protocatechuate 3,4- dioxygenase (3,4-PCD) from Pseudomonas putida reveal an endogenous His 2 Tyr 2 donor set, 5-9 and model compounds have been prepared that contained phenolate donor groups. Although such complexes were good structural and spec- troscopic models for the active site of 3,4-PCD, they exhibited only poor catechol oxidation abilities. One of the best structural and spectroscopic models to date of the 3,4- PCD site is a model complex, based upon a modified salen ligand, prepared by Fujii and Funahashi. 10 The first functional models for catechol dioxygenases were reported by Funabiki, who observed catechol cleavage in the presence of an iron salt, pyridine, and bipyridine. However, the nature of the active species involved in the reactions is unknown. 11,12 At present, the most efficient biomimetic model compound for intradiol cleaving catechol dioxygenases is * To whom correspondence should be addressed. Fax: (+49)-641- 9934149 (S.S.); (+49)-251-8338366 (B.K.). E-mail: krebs@uni-muenster.de (B.K.); siegfried.schindler@anorg.chemie.uni-giessen.de (S.S.). † Dedicated to Professor Rudi van Eldik on the occasion of his 60th birthday. (1) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004, 104, 939. (2) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607. (3) Bugg, T. D. H.; Winfield, C. J. Nat. Prod. Rep. 1998, 15, 513. Kryatov, S. V.; Rybak-Akimova, E. V.; Schindler, S. Chem. ReV. 2005, 105, 2175. (4) Yamahara, R.; Ogo, S.; Masuda, H.; Watanabe, Y. J. Inorg. Biochem. 2002, 88, 284. (5) Ohlendorf, D. H.; Lipscomb, J. D.; Weber, P. C. Nature 1988, 336, 403. (6) Ohlendorf, D. H.; Orville, A. M.; Lipscomb, J. D. J. Mol. Biol. 1994, 244, 586. (7) Orville, A. M.; Lipscomb, J. D.; Ohlendorf, D. H. Biochemistry 1997, 36, 10052. (8) Elgren, T. E.; Orville, A. M.; Kelly, K. A.; Lipscomb, J. D.; Ohlendorf, D. H.; Que, L., Jr. Biochemistry 1997, 36, 11504. (9) Orville, A. M.; Elango, N.; Lipscomb, J. D.; Ohlendorf, D. H. Biochemistry 1997, 36, 10039. (10) Fujii, H.; Funahashi, Y. Angew. Chem., Int. Ed. 2002, 41, 3638. Inorg. Chem. 2005, 44, 7582-7589 7582 Inorganic Chemistry, Vol. 44, No. 21, 2005 10.1021/ic050708k CCC: $30.25 © 2005 American Chemical Society Published on Web 09/23/2005