Sequential Reaction Intermediates in Aliphatic C−H Bond Functionalization Initiated by a Bis(μ-oxo)dinickel(III) Complex

The reaction of [Ni2(OH)2(Me2-tpa)2]2+ (1) (Me2-tpa = bis(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) with H2O2
causes oxidation of a methylene group on the Me2-tpa ligand to give an N-dealkylated ligand and oxidation of a
methyl group to afford a ligand-based carboxylate and an alkoxide as the final oxidation products. A series of
sequential reaction intermediates produced in the oxidation pathways, a bis(μ-oxo)dinickel(III) ([Ni2(O)2(Me2-tpa)2]2+
(2)), a bis(μ-superoxo)dinickel(II) ([Ni2(O2)2(Me2-tpa)2]2+ (3)), a (μ-hydroxo)(μ-alkylperoxo)dinickel(II) ([Ni2(OH)(Me2-tpa)(Me-tpa-CH2OO)]2+ (4)), and a bis(μ-alkylperoxo)dinickel(II) ([Ni2(Me-tpa-CH2OO)2]2+ (5)), was isolated and
characterized by various physicochemical measurements including X-ray crystallography, and their oxidation pathways
were investigated. Reaction of 1 with H2O2 in methanol at −40 °C generates 2, which is extremely reactive with
H2O2, producing 3. Complex 2 was isolated only from disproportionation of the superoxo ligands in 3 in the absence
of H2O2 at −40 °C. Thermal decomposition of 2 under N2 generated an N-dealkylated ligand Me-dpa ((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine) and a ligand-coupling dimer (Me-tpa-CH2)2. The formation of (Me-tpa-CH2)2
suggests that a ligand-based radical Me-tpa-CH2• is generated as a reaction intermediate, probably produced by
H-atom abstraction by the oxo group. An isotope-labeling experiment revealed that intramolecular coupling occurs
for the formation of the coupling dimer. The results indicate that the rebound of oxygen to Me-tpa-CH2• is slower
than that observed for various high-valence bis(μ-oxo)dimetal complexes. In contrast, the decomposition of 2 and
3 in the presence of O2 gave carboxylate and alkoxide ligands, respectively (Me-tpa-COO- and Me-tpa-CH2O-),
instead of (Me-tpa-CH2)2, indicating that the reaction of Me-tpa-CH2• with O2 is faster than the coupling of Me-tpa-CH2• to generate ligand-based peroxyl radical Me-tpa-CH2OO•. Although there is a possibility that the Me-tpa-CH2OO• species could undergo various reactions, one of the possible reactive intermediates, 4, was isolated from
the decomposition of 3 under O2 at −20 °C. The alkylperoxo ligands in 4 and 5 can be converted to a ligand-based
aldehyde by either homolysis or heterolysis of the O−O bond, and disproportionation of the aldehyde gives a
carboxylate and an alkoxide via the Cannizzaro reaction.