Ligand Degradation Study of Unsymmetrical β-Diketiminato Copper Dioxygen Adducts: The Length Chelating Arm Effect.

An investigation on the reactivity of O 2 binding to unsymmetrical β-diketiminato copper(I) complexes by spectroscopic and titration analysis was performed. The length of chelating pyridyl arms (pyridylmethyl arm vs pyridylethyl arm) leads to the formation of mono- or di-nuclear copper-dioxygen species at -80 °C. The pyridylmethyl arm adduct ( L 1 CuO 2 ) forms mononuclear copper-oxygen species and shows ligand degradation, resulting in the formation of (2 E ,3 Z )- N -(2,6-diisopropylphenyl)-4-((( E )-pyridin-2-ylmethylene)amino)pent-3-en-2-imine, which slowly converts to its cyclization isomer 1-(2,6-diisopropylphenyl)-4,6-dimethyl-2-(pyridin-2-yl)-1,2-dihydropyrimidine after addition of NH 4 OH at room temperature. On the other hand, the pyridylethyl arm adduct [( L 2 Cu) 2 (μ-O) 2 ] forms dinuclear species at -80 °C and does not show any ligand degradation product. Instead, free ligand formation was observed after the addition of NH 4 OH. These experimental observations and product analysis results indicate that the chelating length of pyridyl arms governs the Cu/O 2 binding ratio and the ligand degradation behavior.