Series of Structural and Functional Models for the ES (Enzyme–Substrate) Complex of the Co(II)-Containing Quercetin 2,3-Dioxygenase

A series of mononuclear Co^II–flavonolate complexes [Co^IIL^R(fla)] (L^RH = 2-{[bis­(pyridin-2-ylmethyl)­amino]­methyl}-p/m-R-benzoic acid; R = p-OMe (1), p-Me (2), m-Br (4), and m-NO₂ (5); fla = flavonolate) were designed and synthesized as structural and functional models for the ES (enzyme–substrate) complexes to mimic the active site of the Co­(II)-containing quercetin 2,3-dioxygenase (Co–2,3-QD). The metal center Co­(II) ion in each complex shows a similar distorted octahedral geometry. The model complexes display high enzyme-type dioxygenation reactivity (oxidative O-heterocyclic ring opening of the coordinated substrate flavonolate) at low temperature, presumably due to the attached carboxylate group in the ligands. The reactivity exhibits a substituent group dependent order of −OMe (1) > −Me (2) > −H (3) > −Br (4) > −NO₂ (5), and the Hammett plot is linear (ρ = −0.78). This can be explained as the electronic nature of the substituent group in the ligands may influence the conformation and redox potential of the bound flavonolate and finally bring different reactivity. The structures, properties, and reactivity of the model complexes show some dependence on the substituent group in the supporting model ligands, and there is some relationship among them. This study is the first example of a series of structural and functional ES models of Co–2,3-QD, with focus on the effects of the electronic nature of substituted groups and the carboxylate group of the ligands to the dioxygenation reactivity, that will provide important insights into the structure–property–reactivity relationship and the catalytic role of Co–2,3-QD.