Formation, Characterization, and Reactivity of Bis(μ-oxo)dinickel(III) Complexes Supported by A Series of Bis[2-(2-pyridyl)ethyl]amine Ligands

Bis(μ-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands have been successfully generated by treating the corresponding bis(μ-hydroxo)dinickel(II) complexes or bis(μ-methoxo)dinickel(II) complex with an equimolar amount of H2O2 in acetone at low temperature. The bis(μ-oxo)dinickel(III) complexes exhibit a characteristic UV−vis absorption band at ∼410 nm and a resonance Raman band at 600−610 cm-1 that shifted to 570−580 cm-1 upon 18O-substitution. Kinetic studies and isotope labeling experiments using 18O2 imply the existence of intermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(μ-oxo)dinickel(III) complex. The bis(μ-oxo)dinickel(III) complexes supported by the mononucleating ligands (L1 X = para-substituted N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X = OMe, Me, H, Cl) gradually decompose, leading to benzylic hydroxylation of the ligand side arm (phenethyl group). The kinetics of the ligand hydroxylation process including kinetic deuterium isotope effects (KIE), p-substituent effects (Hammett plot), and activation parameters (ΔH H ⧧ and ΔS H ⧧) indicate that the bis(μ-oxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(μ-oxo)dicopper(III) complex. Such a reactivity of bis(μ-oxo)dinickel(III) complexes has also been suggested by the observed reactivity toward external substrates such as phenol derivatives and 1,4-cyclohexadiene. The thermal stability of the bis(μ-oxo)dinickel(III) complex is significantly enhanced when the dinucleating ligand with a longer alkyl strap is adopted instead of the mononucleating ligand. In the m-xylyl ligand system, no aromatic ligand hydroxylation occurred, showing a sharp contrast with the reactivity of the (μ-η2:η 2-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic aromatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems are discussed on the basis of the detailed comparison of these two systems with the same ligand.