Ligand effect on reversible conversion between copper(I) and bis(μ-oxo)dicopper(III) complex with a sterically hindered tetradentate tripodal ligand and monooxygenase activity of bis(μ-oxo)dicopper(III) complex

A new sterically hindered tetradentate tripodal ligand (Me-2-etpy) and its labeled analogue having deuterated methylene groups (d(4)-Me-2-etpy) were synthesized, where Me-2-etpy is bis(6-methyl-2-pyridylmethyl)(2-pyridylethyl)amine. Copper(I) complexes [Cu(Me-2-etpy or d(4)-Me-2-etpy)](+) (1 and 1-d(4), respectively) reacted with dioxygen at -80 degreesC in acetone to give bis(mu-oxo)dicopper(III) complexes [Cu-2(O)(2)(Me-2-etPY or d(4)-Me-2-etpy)(2)](2+) (1-oxo and 1-d(4)-oxo, respectively), the latter of which was crystallographically characterized. Unlike a bis(mu-oxo)dicopper(III) complex with a closely related Me-2-tpa ligand having a 2-pyridylmethyl pendant, 1-oxo possessing a 2-pyridylethyl pendant is not fully formed even under 1 atm of O-2 at -80 degreesC and is very reactive toward the oxidation of the supporting ligand. Thermal decomposition of 1-oxo gave an N-dealkylated ligand in yield similar to80% based on a dimer and a corresponding aldehyde. The deuterated ligand d(4)-Me-2-etpy greatly stabilizes the bis(mu-oxo)dicopper(III) complex 1-d(4)-oxo, indicating that the rate determining step of the N-dealkylation is the C-H bond cleavage from the methylene group. The reversible conversion between 1-d(4) and 1-d(4)-oxo in acetone is dependent on the temperature, and the thermodynamic parameters (DeltaH and DeltaS) of the equilibrium were determined to be -53 +/- 2 kJ mol(-1) and -187 +/- 10 J mol(-1) K-1, respectively. The effect of the 2-pyridylethyl pendant in comparison with the 2-pyridylmethyl and 6-methyl-2-pyridylmethyl pendants on the physicochemical properties of the copper(I) and bis(mu-oxo)dicopper(III) species is discussed.