Copper(II) and Sodium(I) Complexes based on 3,7-Diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane-5-oxide: Synthesis, Characterization, and Catalytic Activity

Abstract

The reaction of 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) with metal salts of Cu^II or Na^I /Ni^II under mild conditions led to the oxidized phosphane derivative 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane-5-oxide (DAPTA=O) and to the first examples of metal complexes based on the DAPTA=O ligand, that is, [Cu^II (μ-CH₃ COO)₂ (κO-DAPTA=O)]₂ (1) and [Na(1κOO';2κO-DAPTA=O)(MeOH)]₂ (BPh₄ )₂ (2). The catalytic activity of 1 was tested in the Henry reaction and for the aerobic 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated oxidation of benzyl alcohol. Compound 1 was also evaluated as a model system for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechol as the substrate. The kinetic data fitted the Michaelis-Menten equation and enabled the obtainment of a rate constant for the catalytic reaction; this rate constant is among the highest obtained for this substrate with the use of dinuclear Cu^II complexes. DFT calculations discarded a bridging mode binding type of the substrate and suggested a mixed-valence Cu^II /Cu^I complex intermediate, in which the spin electron density is mostly concentrated at one of the Cu atoms and at the organic ligand.

Keywords: copper; enzymes; homogeneous catalysis; oxidation; phosphane ligands.