Copper(ii)-flavonolate complexes of sterically hindering 3N ligands as enzyme-substrate models for copper(ii) quercetin 2,4-dioxygenase: experimental and computational study on the dioxygenation reactivity

Abstract

Four new copper(II)-flavonolate complexes of the type [Cu(L)(fla)](ClO₄) 1–4, where L is the 3N ligand 4-methyl-(1-pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1), 4-methyl-(6-methyl(1-pyrid-2-ylmethyl))-1,4-diaza-cycloheptane (L2), 4-methyl-1-(N-methylimidazol-2-ylmethyl)-1,4-diazacycloheptane (L3) or 4-methyl-1-(quinol-2-ylmethyl)-1,4-diazacycloheptane (L4), and H(fla) is 3-hydroxyflavone, have been prepared as functional models for the Cu(II)-containing quercetin 2,4-dioxygenase (2,4-QueD) enzyme. The single crystal X-ray structure of [Cu(L2)(fla)](ClO₄) 2 comprises the CuN₃O₂ chromophore adopting a trigonal bipyramidal distorted square pyramidal coordination geometry around Cu(II) (TBDSP, τ = 0.47). The rate of dioxygenation of the ES model complexes, determined in DMF solution at 80 °C (k_obs: 1 (2.03 ± 0.04) > 2 (0.58 ± 0.04) < 3 (9.19 ± 0.02) > 4 (0.56 ± 0.05 × 10⁻³ s⁻¹)), reveals that 3 reacts much faster than the other complexes in the presence of excess dioxygen. The replacement of the pyridyl nitrogen in 1 by an imidazolyl nitrogen with higher basicity to get 3 increases the π-back bonding of flavonolate with Cu(II) located in a trigonally distorted square pyramidal coordination geometry, and enhances the activation of the flavonolate towards dioxygen. In contrast, the 6-methyl (2) and benzo (4) groups on the pyridyl moiety (1) lower the reaction rate by sterically hindering the approach of molecular oxygen. A computational study supports a reaction pathway involving a single-electron transfer (SET) from flavonolate to dioxygen to generate a superoxide radical. The latter reacts rapidly with the activated flavonoxy radical intermediate to give the dioxygenated products.