Phenolate-bonded bis(μ-oxido)-bis-copper(III) intermediates: hydroxylation and dehalogenation reactivities

Area of Science:

• Bioinorganic Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Copper-catalyzed phenolate ortho-hydroxylation mechanisms are debated, involving either a peroxo-bis-copper(II) or a bis(μ-oxido)-bis-copper(III) species.
• The bis(μ-oxido)-bis-copper(III) species are thermally unstable, limiting their characterization and mechanistic studies.

Purpose of the Study:

• To expand the scope and characterization of bis(μ-oxido)-bis-copper(III) species with ligated phenolates.
• To elucidate the mechanism of exogenous phenolate ortho-hydroxylation by copper-oxygen species.

Main Methods:

• Synthesis and characterization of a novel bis(μ-oxido)-bis-copper(III) complex using N,N'-di-tert-butyl-1,3-propanediamine (DBPD) ligand.
• Low-temperature oxygenation of a copper(I) complex to form a peroxo intermediate, followed by reaction with phenolates.
• Spectroscopic analysis, X-ray crystallography, and Density Functional Theory (DFT) calculations.

Main Results:

• Formation of a stable bis(μ-oxido)-bis-copper(III) species (A) with two ligated phenolates.
• Regiospecific ortho-hydroxylation of phenolates upon thermal decomposition of species A, with C-H bond hydroxylation being faster than C-X (halide) displacement.
• DFT and experimental evidence supporting an electrophilic attack mechanism of an oxide ligand onto the phenolate π-system.

Conclusions:

• The study demonstrates the utility of flexible diamine ligands like DBPD in stabilizing reactive copper-oxo species.
• A hydroxylation mechanism involving O-O bond cleavage of a peroxo intermediate upon phenolate ligation, preceding aromatic hydroxylation, is supported.
• This work provides crucial insights into copper-mediated oxygen activation and hydroxylation pathways relevant to both synthetic and biological systems.