Dioxygen Reduction by a Pd(0)-Hydroquinone Diphosphine Complex.

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry

Background:

• Development of novel ligands is crucial for advancing catalysis.
• Noninnocent ligands offer unique reactivity pathways in metal complexes.

Purpose of the Study:

• To synthesize and characterize a novel p-terphenyl diphosphine ligand with a hydroquinone moiety.
• To investigate the reactivity of resulting metal complexes, particularly with small molecules.
• To elucidate the mechanism of dioxygen reduction by a 2-coordinate palladium-hydroquinone complex.

Main Methods:

• Synthesis of a p-terphenyl diphosphine ligand featuring a hydroquinone core.
• Metalation reactions with Ni(0) and Pd(0) precursors to form metal-quinone complexes.
• Preparation of a 2-coordinate Pd(0)-hydroquinone complex via one-pot metalation and reduction.
• Reactivity studies with various small molecules including O2, NO, N2O, and organic oxides.
• Mechanistic investigations using control compounds and low-temperature studies.

Main Results:

• A novel hydroquinone-containing diphosphine ligand was synthesized.
• Pseudo-tetrahedral Ni(0) and Pd(0)-quinone complexes were formed.
• A 2-coordinate Pd(0)-hydroquinone complex exhibited metal-coupled reactivity towards small molecules.
• This complex quantitatively reduced O2, NO, N2O, and organic oxides, yielding the Pd(0)-quinone product.
• Dioxygen reduction proceeds via a eta(2)-peroxo intermediate followed by ligand oxidation.

Conclusions:

• The hydroquinone moiety in the palladium complex plays a critical role in small molecule activation and reduction.
• The 2-coordinate Pd(0)-hydroquinone complex is a potent reductant for various substrates.
• Dioxygen activation is an inner sphere Pd-based process, with the hydroquinone ligand participating in subsequent reduction steps.