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

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(mu-CH3COO)(2)(kappa O-DAPTA = O)](2) (1) and [Na(1 kappa OO';2 kappa O-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.