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

Abstract

The reaction of [Ni-2(OH)(2)(Me-2-tpa)(2)](2+) (1) (Me-2-tpa = bis(6-methyl-2-pyridylmethyl) (2-pyridyl methyl)amine) with H2O2 causes oxidation of a methylene group on the Me-2-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(mu-oxo)dinickel(Ill) ([Ni-2(O)(2)(Me-2-tpa)(2)](2+) (2)), a bis(mu-superoxo)dinickel(II) ([Ni-2(O-2)(2)(Me-2-tpa)(2)](2+) (3)), a (mu-hydroxo)(mu-alkylperoxo)dinickel(II) ([Ni-2(OH)(Me-2-tpa)(Me-tpa-CH2OO)](2+) (4)), and a bis(mu-alkylperoxo)dinickel(II) ([Ni-2(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 degrees 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 degrees C. Thermal decomposition of 2 under N-2 generated an N-dealkylated ligand Me-dpa ((6-methyl-2-pyridyl methyl) (2-pyridyl m ethyl) a mine) 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(mu-oxo)dimetal complexes. In contrast, the decomposition of 2 and 3 in the presence Of 02 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 O-2 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 O-2 at -20 degrees 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 alclehyde gives a carboxylate and an alkoxide via the Cannizzaro reaction.