Quercetin 2,3-Dioxygenase Mimicking Ring Cleavage of the Flavonolate Ligand Assisted by Copper. Synthesis and Characterization of Copper(I) Complexes [Cu(PPh3)2(fla)] (fla = Flavonolate) and [Cu(PPh3)2(O-bs)] (O-bs = O-Benzoylsalicylate)

Cu(PPh 3 ) 2 (fla) has been prepared by reacting copper(I) chloride with sodium flavonolate in tetrahydrofuran solution. Crystallographic characterization of the complex (orthorhombic, P 2 1 2 1 2 1, a = 9.588(1) Å, b = 17.364(3) Å, c = 24.378(3) Å, V = 4058.6(10) Å 3, Z = 4, R = 0.049) has shown that the coordination geometry of the molecule is tetrahedral. Oxygenation of Cu(PPh 3 ) 2 (fla) in a methylene chloride solution at ambient conditions gives the O -benzoylsalicylato copper complex Cu(PPh 3 ) 2 ( O -bs) and carbon monoxide. Labeling experiments with an 18 O 2 − 16 O 2 mixture (1:4) evidenced the incorporation of both 18 O atoms of 18 O 2 into the O -bs ligand. IR and MS studies of labeled O -bs confirmed the incorporation of 18 O 2 while the released CO remained unlabeled. Crystallographic characterization of Cu(PPh 3 ) 2 ( O -bs) on crystals obtained as the acetone solvate (triclinic, P 1̄, a = 13.154(1) Å, b = 17.991(1) Å, c = 20.495(1) Å, α = 80.01(1)°, β = 88.02(1)°, γ = 71.83(1)°, V = 4537.5(5) Å 3, Z = 4, R = 0.0403) shows that the molecule has a distorted tetrahedral structure. The oxygenolysis was followed by spectrophotometry, and the rate constants were determined according to the rate law −d[Cu(PPh 3 ) 2 (fla)]/d t = k 2 [Cu(PPh 3 ) 2 (fla)][O 2 ]. The rate constant, activation enthalpy, and entropy at 363.16 K are as follows: k 2 /M - 1 s - 1 = 4.16 ± 0.48, Δ H ⧧ /kJ mol - 1 = 102 ± 7, Δ S ⧧ /J mol - 1 K - 1 = −13.0 ± 21. The reaction fits a Hammett linear free energy relationship for 4‘-substituted flavonolates, and an increase of the electron density on copper makes the oxygenation reaction faster.