Formation, characterization, and reactivity of bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands

Bis(mu -oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyrid yl)ethyl]amine ligands have been successfully generated by treating the cor responding bis(mu -hydroxo)dinickel(II) complexes or bis(mu -methoxo)dinick el(II) complex with an equimolar amount of H2O2 in acetone at low temperatu re. The bis(mu -oxo)dinickel(III) complexes exhibit a characteristic UV-vis absorption band at similar to 410 nm and a resonance Raman band at 600-610 cm(-1) that shifted to 570-580 cm-1 upon O-18-substitution. Kinetic studie s and isotope labeling experiments using O-18(2) imply the existence of int ermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(mu -oxo)dinickel(III) complex. The bis(mu -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 decomp ose, leading to benzylic hydroxylation of the ligand side arm (phenethyl gr oup). The kinetics of the ligand hydroxylation process including kinetic de uterium isotope effects (KIE), p-substituent effects (Hammett plot), and ac tivation parameters (DeltaH(H)(double dagger) and DeltaS(H)(double dagger)) indicate that the bis(mu -oxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(mu -oxo)dicopper(III) complex. Such a reactivity of bis(mu -oxo)dinicke l(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(mu -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 aro matic ligand hydroxylation occurred, showing a sharp contrast with the reac tivity of the (mu-eta (2):eta (2)-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic ar omatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems ar e discussed on the basis of the detailed comparison of these two systems wi th the same ligand.