Aminopyridine iron catecholate complexes as models for intradiol catechol dioxygenases. Synthesis, structure, reactivity, and spectroscopic studies

Citation
P. Mialane et al., Aminopyridine iron catecholate complexes as models for intradiol catechol dioxygenases. Synthesis, structure, reactivity, and spectroscopic studies, INORG CHEM, 39(12), 2000, pp. 2440-2444
Citations number
28
Categorie Soggetti
Inorganic & Nuclear Chemistry
Journal title
INORGANIC CHEMISTRY
ISSN journal
00201669 → ACNP
Volume
39
Issue
12
Year of publication
2000
Pages
2440 - 2444
Database
ISI
SICI code
0020-1669(20000612)39:12<2440:AICCAM>2.0.ZU;2-M
Abstract
Four new Fe(III) catecholate complexes, [(bispicMe(2)en)Fe-III(DBC)](+), [( bispicCl(2)Me(2)en)Fe-III(DBC)](+), [(trispic-Meen)Fe-III(DBC)](+), and [(B QPA)Fe-III(DBC)](+), which all contain aminopyridine ligands, were synthesi zed. The structure of [(bispicMe(2)en)Fe-III(DBC)](+) was determined by X-r ay diffraction. It crystallizes in the triclinic space group P(1)over bar w ith a = 10.666(3) Angstrom, b = 13.367(5) Angstrom, c = 17.685(2) Angstrom, alpha = 93.46(2)degrees, beta = 93.68(2)degrees, gamma = 109.0-(3)degrees, V = 2387.4 Angstrom(3), and Z = 2. All of these complexes were found to be active toward oxidation of catechol by O-2 in DMF at 20 degrees C to affor d intradiol cleavage products. The catechol was quantitatively oxidized, ma inly (90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction ra tes were measured, and for the first three (topologically similar) complexe s, a correlation of the second-order kinetic constants k with the optical p arameters of the two LMCT O(DBC) --> Fe(III) bands was found. In particular , k increases with the epsilon(max) of the charge-transfer bands. The k val ue of the complex [(BQPA)Fe-III(DBC)](+), containing a tripodal ligand, is smaller than expected on the basis of these correlations. This discrepancy could be related to steric hindrance induced by the BQPA ligand. However, t he much lower activity of the bispicen-Fe(III)-type complexes compared to t hat of the [(TPA)Fe-III(DBC)](+) complex synthesized by Jang et al. (J. Am. Chern. Sec. 1991, 113, 9200-9204), despite similar epsilon(max) values, sh ows that a knowledge of optical and NMR parameters values is not sufficient to explain the dioxygenase activity rate, in their study of protocatechuat e 3,4-dioxygenase, Orville et al. (Biochemistry 1997, 36, 10052-10066) sugg ested that asymmetric chelation of the catecholate to Fe(III) is of great i mportance in the efficiency of the intradiol dioxygenase reaction. Indeed, a comparison of the X-ray structures of [(TPA)Fe-III-(DBC)](+) and [(bispic Me(2)en)Fe-III(DBC)](+) shows that the Fe(III)-O bonds differ by 0.019 Angs trom in the former and are identical in the latter. Asymmetry could also pl ay a role in the model complexes. An alternative explanation is the possibl e existence of a low-spin state for [(TPA)Fe-III(DBC)](+), as recently iden tified in [(TPA)Fe-III(cat)](+) by Simaan et al.