Intramolecular Ligand Hydroxylation: Mechanistic High-Pressure Studies on the Reaction of a Dinuclear Copper(I) Complex with Dioxygen

We provide a mechanistic study of a monooxygenase model system and detail low-temperature stopped-flow kinetics studies in acetone as solvent, employing both the use of rapid-scanning diode-array observation and variable high-pressure (20-100 MPa) techniques. The dicopper(I) complex employed is [Cu(2)(H-XYL-H)](2+) (1), with the H-XYL-H ligand wherein a m-xylyl group links two bis[2-(2-pyridyl)ethyl]amine units. This reacts with O(2) reversibly (k(1)/k(-)(1)) giving a peroxo-dicopper(II) intermediate [Cu(2)(H-XYL-H)(O(2))](2+) (2), which thereupon irreversibly (k(2)) reacts by oxygen atom insertion (i.e., hydroxylation) of the xylyl group, producing [Cu(2)(H-XYL-O(-))(OH)](2+) (3). Activation parameters are as follows: k(1), DeltaH() = 2.1 +/- 0.7 kJ/mol, DeltaS() = -174 +/- 3 J/(K mol); k(-)(1), DeltaH() = 80.3 +/- 0.8 kJ/mol, DeltaS() = 77 +/- 3 J/(K mol); k(2), DeltaH() = 58.2 +/- 0.2 kJ/mol, DeltaS() = -5.8 +/- 0.9 J/(K mol). These values are similar to values obtained in a previous study in dichloromethane. At low temperatures and higher concentrations, the situation in acetone is complicated by a pre-equilibrium of 1 to an isomer form. The present study provides the first determination of activation volumes for individual steps in copper monooxygenase reactions. The data and analysis provide that DeltaV()(k(1)) = -15 +/- 2.5 cm(3)/mol and DeltaV()(k(-)(1)) = +4.4 +/- 0.5 cm(3)/mol for formation and dissociation of 2, respectively, while DeltaV()(k(2)) = -4.1 +/- 0.7 cm(3)/mol; a volume profile for the overall reaction has been constructed. The significance of the findings in the present study is described, and the results are compared to those for other systems.