Aminopyridine Iron Catecholate Complexes as Models for Intradiol Catechol Dioxygenases. Synthesis, Structure, Reactivity, and Spectroscopic Studies

Four new Fe(III) catecholate complexes, [(bispicMe2en)FeIII(DBC)]+, [(bispicCl2Me2en)FeIII(DBC)]+, [(trispicMeen)FeIII(DBC)]+, and [(BQPA)FeIII(DBC)]+, which all contain aminopyridine ligands, were synthesized. The structure of [(bispicMe2en)FeIII(DBC)]+ was determined by X-ray diffraction. It crystallizes in the triclinic space group P with a = 10.666(3) Å, b = 13.467(5) Å, c = 17.685(2) Å, α = 93.46(2)°, β = 93.68(2)°, γ = 109.0(3)°, V = 2387.4 Å3, and Z = 2. All of these complexes were found to be active toward oxidation of catechol by O2 in DMF at 20 °C to afford intradiol cleavage products. The catechol was quantitatively oxidized, mainly (90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction rates were measured, and for the first three (topologically similar) complexes, a correlation of the second-order kinetic constants k with the optical parameters of the two LMCT O(DBC) → Fe(III) bands was found. In particular, k increases with the εmax of the charge-transfer bands. The k value of the complex [(BQPA)FeIII(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, the much lower activity of the bispicen-Fe(III)-type complexes compared to that of the [(TPA)FeIII(DBC)]+ complex synthesized by Jang et al. (J. Am. Chem. Soc. 1991, 113, 9200−9204), despite similar εmax values, shows that a knowledge of optical and NMR parameters values is not sufficient to explain the dioxygenase activity rate. In their study of protocatechuate 3,4-dioxygenase, Orville et al. (Biochemistry 1997, 36, 10052−10066) suggested that asymmetric chelation of the catecholate to Fe(III) is of great importance in the efficiency of the intradiol dioxygenase reaction. Indeed, a comparison of the X-ray structures of [(TPA)FeIII(DBC)]+ and [(bispicMe2en)FeIII(DBC)]+ shows that the Fe(III)−O bonds differ by 0.019 Å in the former and are identical in the latter. Asymmetry could also play a role in the model complexes. An alternative explanation is the possible existence of a low-spin state for [(TPA)FeIII(DBC)]+, as recently identified in [(TPA)FeIII(cat)]+ by Simaan et al. (Angew. Chem., Int. Ed. Engl. 2000, 39, 196−198).