Metal-​ligand cooperativity in Pd-​hydroquinone complexes for dioxygen reduction

The development of selective catalysts for the multiproton-multielectron redn. of small mol. substrates is of significant interest. Metal-ligand cooperativity has been utilized to expand the reactivity of transition metals. To facilitate proton and electron transfer during dioxygen redn., a ligand contg. a redox-active hydroquinone moiety was prepd. A Pd(0)-hydroquinone complex was synthesized and found to rapidly react with dioxygen at room temp. to quant. yield the corresponding Pd(0)-quinone complex. Slower reactivity with dioxygen was obsd. with a (Pd-H)-hydroquinone complex at room temp. to cleanly yield the protonated Pd(0)-quinone complex. Structure-reactivity studies were conducted using a Ag(I)-hydroquinone complex and a Pd(0) complex with a redox-inactive para-methoxy-substituted central arene as probes for ligand-only and Pd-only reactivity resp. The mechanism of dioxygen reaction and product selectivity was studied by variable temp. NMR, gas quantification by Toepler pump, and UV/Vis and soln. IR spectroscopy