Functional Models for the Dioxygen-Activating Cu(II)-2,4-QueD Enzymes: Incorporation of Diimine Coligands in...

Five copper(II)-flavonolate complexes [Cu(diimine)(fla)](ClO₄) 1-5, where diimines are 2,2'-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3), 3,4,7,8-tetramethyl-1,10-phenanthroline (4), and dipyrido[3,2-d:2',3'-f]-quinoxaline (5), and H(fla) is 3-hydroxyflavonone, have been prepared and studied as functional models for Cu(II) quercetin 2,4-dioxygenase. The complex 1 is monomeric, while 2 and 3 are centrosymmetric dimers (EPR: ΔM_s, ±2, 1600 G), all containing a square pyramidal CuN₂O₂O' coordination sphere (τ: 1, 0.09; 2, 0.10; 3, 0.13). In DMF solution, all of the Cu(II)-flavonolate adducts decompose at 80 °C, upon exposure to pure dioxygen, to produce CO and depside, which is hydrolyzed to give the corresponding acids. The dioxygenase reactivities of the adducts are much higher than those of the model complexes reported so far, and the rates vary as 1 (k_O2: 1, 6.13 ± 0.02) < 2 (1.80 ± 0.02) < 3 (8.39 ± 0.01) > 4 (2.87 ± 0.03) > 5 (3.81 ± 0.01 M^-1 s^-1). The decreased π-backbonding of bpy (1) and 5,6-dmp (3) with Cu(II) enhances the π-backbonding of the antibonding π*-orbitals of conjugated C═O of fla^- with Cu(II), causing an increase in their reactivity toward O₂. The X-ray structures and DFT, FT-IR, and EPR spectral studies of the model complexes support the presence of the π-backbonding.