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 (Me2-etpy) and its labeled analogue having deuterated methylene
groups (d4-Me2-etpy) were synthesized, where Me2-etpy is bis(6-methyl-2-pyridylmethyl)(2-pyridylethyl)amine. Copper(I) complexes [Cu(Me2-etpy or d4-Me2-etpy)]+ (1 and 1-d4, respectively) reacted with dioxygen at −80 °C in acetone
to give bis(μ-oxo)dicopper(III) complexes [Cu2(O)2(Me2-etpy or d4-Me2-etpy)2]2+ (1-oxo and 1-d4-oxo, respectively),
the latter of which was crystallographically characterized. Unlike a bis(μ-oxo)dicopper(III) complex with a closely
related Me2-tpa ligand having a 2-pyridylmethyl pendant, 1-oxo possessing a 2-pyridylethyl pendant is not fully
formed even under 1 atm of O2 at −80 °C and is very reactive toward the oxidation of the supporting ligand.
Thermal decomposition of 1-oxo gave an N-dealkylated ligand in yield ∼80% based on a dimer and a corresponding
aldehyde. The deuterated ligand d4-Me2-etpy greatly stabilizes the bis(μ-oxo)dicopper(III) complex 1-d4-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-d4 and 1-d4-oxo in acetone is dependent on the temperature, and the thermodynamic
parameters (ΔH and ΔS) 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(μ-oxo)dicopper(III) species is
discussed.