Kinetic studies on the copper(II)-mediated oxygenolysis of the flavonolate ligand. Crystal structures of [Cu(fla)2] (fla = flavonolate) and [Cu(O-bs)2(py)3] (O-bs = O-benzoylsalicylate)†

Abstract

The complex [Cu(fla)₂] has been prepared by treating copper(II) chloride with sodium flavonolate in tetrahydrofuran solution. Crystallographic characterisation of the complex [Cu(fla)₂]·2CHCl₃ has shown that the co-ordination geometry around the copper(II) ion is square planar. Oxygenation of [Cu(fla)₂] in dimethylformamide solution at ambient conditions gives [Cu(O-bs)₂] (O-bs = O-benzoylsalicylate) and carbon monoxide. Crystallographic characterisation of [Cu(O-bs)₂] on crystals obtained from pyridine as [Cu(O-bs)₂(py)₃]·0.91py revealed that the co-ordination geometry of the copper(II) ion is square pyramidal with trans O atoms of O-bs and N atoms of py ligands in basal and an N atom of py in apical position. The oxygenolysis of [Cu(fla)₂] in DMF was followed by electron spectroscopy and the rate constants were determined according to the rate law –d[Cu(fla)₂]/dt = k[Cu(fla)₂][O₂]. The rate constant, activation enthalpy, entropy and free energy at 373 K are as follows: k/mol dm^–3 s^–1 = (1.57 ± 0.08) × 10^–2, ΔH ^‡/kJ mol^–1 = 53 ± 6, ΔS ^‡/J K^–1 mol^–1 = –138 ± 11, ΔG ^‡/kJ mol^–1 = 105 ± 2. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper results in a faster oxygenation reaction.