Designing non-innocent ligands for multiproton-multielectron chemistry: Reactivity of Pd-hydroquinone complexes

The use of redox-active and non-innocent ligand frameworks has been shown to expand or improve the reactivity and catalytic behavior of transition metal complexes. While ligands capable of storing electrons or protons are well precedented few have been shown to shuttle both protons and electrons to and from the metal center. A ligand framework capable of facilitating multiple proton and electron transfers was targeted. A para-terphenyl diphosphine ligand contg. a redox-active hydroquinone moiety was successfully synthesized. Metalation with M(OAc)_2 (M = Ni, Pd) readily afforded the reduced zero-valent metal center coordinated to the quinone form of the ligand. A Pd(0)-hydroquinone complex was synthesized and found to readily reduce a variety of substrates, including dioxygen and organoazides, to quant. yield the Pd(0)-quinone complex. A Ag(I)-hydroquinone complex and a Pd(0) complex supported by a redox-inactive para-methoxy substituted central arene were utilized as probes for ligand-only and Pd-only reactivity, resp. The mechanism of substrate activation was studied to analyze the role of the hydroquinone moiety as a two electron and two proton reservoir.