[Cu(phen)2(fla)]ClO4 was prepared by treating [Cu(CH3CN)4]ClO4 with flavonol (flaH) in the presence of 1,10-phenanthroline (phen) as a co-ligand. Its oxygenation in DMF (or CH3CN) solution at elevated temperature gave the (O-benzoylsalicylato)copper(II) complex [Cu(phen)2(O-bs)]ClO4 (bs = benzoylsalicylato) and carbon monoxide via an endoperoxide intermediate. Crystallographic characterisation of [Cu(phen)2(O-bs)]ClO4 as the CH2Cl2 solvate [triclinic, space group P , a = 10.499(3) Å, b = 12.556(4) Å, c = 17.094(5) Å, α = 72.69(2), β = 89.35(2), γ = 69.19(2)°, V = 1999.7(10) Å3, Z = 2, R1 = 0.0962] shows that the molecule has a distorted trigonal-bipyramidal structure (τ = 0.96). The oxygenolysis was monitored by spectrophotometry, and the pseudo-first-order rate constant k′phen was found to be (2.47 ± 0.11) × 10−4 s−1 at 120 °C. Complexes of [Cu(L)(4′R-fla)2] (L = phen, bpy, TMEDA; R = H, OCH3, CH3, Cl) were also prepared by treating the complexes Cu(4′R-fla)2 with nitrogen-containing co-ligands. Their oxygenation resulted in the corresponding complexes Cu(L)(2HOpg)2 (2HOpg =2-hydroxyphenylglyoxylate) derived by spontaneous hydrolysis of Cu(L)(bpg)2 (bpg = 2-benzoatophenylglyoxylate). The (phenylglyoxylato)copper complexes were probably formed via 1,2-dioxetan intermediates, since the oxygenation of Cu(phen)(fla)2 showed chemiluminescence with bands at 506, 546, and 578 nm in the emission spectrum due to the decomposition of a 1,2-dioxetan species. Labelling experiments with an 18O2/16O2 mixture (1:3) showed the incorporation of both 18O atoms of 18O2 into the flavonolate ligand. The kinetics of the oxygenolysis of Cu(L)(fla)2 gave rate constants according to the rate law −d[Cu(L)(4′R-fla)2]/dt = k[Cu(L)(4′R-fla)2][O2]: k/M−1 s−1 = (9.50 ± 0.60) × 10−2 (L = phen), (2.40 ± 0.10) × 10−2 (L = bpy), (2.00 ± 0.10) × 10−2 (L = TMEDA) M−1 s−1 at 353.16 K. The oxygenolysis of the complexes Cu(phen)(4′R-fla)2 fits a Hammett linear free energy relationship and an increase of the electron density on the copper ion makes the oxygenation reaction faster. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)