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(PPh3)(2)(fla) has been prepared by reacting copper(I) chloride with sodium flavonolate in tetrahydrofuran solution. Crystallographic characterization of the complex (orthorhombic, P2(1)2(1)2(1), a = 9.588(1) Angstrom, b = 17.364(3) Angstrom, c = 24.378(3) Angstrom, V = 4058.6(10) Angstrom(3), Z = 4, R = 0.049) has shown that the coordination geometry of the molecule is tetrahedral. Oxygenation of Cu(PPh3)(2)(fla) in a methylene chloride solution at ambient conditions gives the O-benzoylsalicylato copper complex Cu(PPh3)(2)(O-bs) and carbon monoxide. Labeling experiments with an O-18(2)-O-16(2) mixture (1:4) evidenced the incorporation of both O-18 atoms of O-18(2) into the O-bs ligand. IR and MS studies of labeled O-bs confirmed the incorporation of O-18(2) while the released CO remained unlabeled. Crystallographic characterization of Cu(PPh3)(2)(O-bs) on crystals obtained as the acetone solvate (triclinic, P (1) over bar, a = 13.154(1) Angstrom, b = 17.991(1) Angstrom, c = 20.495(1) Angstrom, alpha = 80.01(1)degrees, beta = 88.02(1)degrees, gamma = 71.83(1)degrees, V = 4537.5(5) Angstrom(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(PPh3)(2)(fla)]/dt = k(2)[Cu(PPh3)(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, Delta H double dagger/kJ mol(-1) = 102 +/- 7, Delta S double dagger/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.