Dioxygen Reduction by a Pd(0)–Hydroquinone Diphosphine Complex

Abstract

A novel p-terphenyl diphosphine ligand was synthesized with a noninnocent hydroquinone moiety as the central arene (1-H). Pseudo-tetrahedral 4-coordinate Ni0 and Pd0–quinone (2 and 3, respectively) complexes proved accessible by metalating 1-H with the corresponding M­(OAc)2 precursors. O2 does not react with the Pd0–quinone species (3) and protonation occurs at the quinone moiety indicating that the coordinated oxidized quinonoid moiety prevents reactivity at the metal. A 2-coordinate Pd0–hydroquinone complex (4-H) was prepared using a one-pot metalation with PdII followed by reduction. The reduced quinonoid moiety in 4-H shows metal-coupled reactivity with small molecules. 4-H was capable of reducing a variety of substrates including dioxygen, nitric oxide, nitrous oxide, 1-azido adamantane, trimethylamine n-oxide, and 1,4-benzoquinone quantitatively producing 3 as the Pd-containing reaction product. Mechanistic investigations of dioxygen reduction revealed that the reaction proceeds through a η2-peroxo intermediate (Int1) at low temperatures followed by subsequent ligand oxidation at higher temperatures in a reaction that consumed half an equivalent of O2 and produced water as a final oxygenic byproduct. Control compounds with methyl protected phenolic moieties (4-Me), displaying a AgI center incapable of O2 binding (7-H) or a cationic Pd–H motif (6-H) allowed for the independent examination of potential reaction pathways. The reaction of 4-Me with dioxygen at low temperature produces a species (8-Me) analogous to Int1 demonstrating that initial dioxygen activation is an inner sphere Pd-based process where the hydroquinone moiety only subsequently participates in the reduction of O2, at higher temperatures, by H+/e– transfers.

Stabilized Carbenium Ions as Latent, Z‐type Ligands

“…Carbenium ions,which have already received interest as ligands for transition metals, [9] may also present ah igher chemical resilience,i np articular to oxidative conditions.W ith av iew to capitalize on these attributes,w e thus decided to reengage in carbenium chemistry [10] and explore the synthesis and properties of carbenium-based analogues [11] of known L/Z ambiphilic boron-containing ligands ( Figure 1C). [3a] Thec arbenium-based systems described herein bear similarities with phosphine ligands functionalized by carbon-based redox non-innocent moieties [12] while also adding to ongoing efforts in the chemistry of cationic phosphines as ligands for late-transition-metal catalysts. [13] Bearing in mind that the high reactivity of carbenium ions may complicate these studies,w ed ecided to focus on the incorporation of stabilized examples of such moieties.T othis end, 1-lithio-2-diphenylphosphinobenzene was quenched with N-methyl acridone or xanthone leading to the formation of the phosphinocarbinols 1 and 2,respectively (Scheme 1).…”

A Molecular Boroauride: A Donor–Acceptor Complex of Anionic Gold

“…Agapie et al. have recently used metal complexes of related noninnocent terphenyl diphosphines for the activation of small molecules, including CO and O 2 . The phosphine donors are poised to stabilize a linear Au I cation, while the intrinsic redox activity and Lewis acidity of the DBA core give it the ability to both serve as an electron reservoir and engage in acceptor interactions with a highly reduced metal atom as needed.…”

Pillarquinone‐Based Porous Polymer for a Highly‐Efficient Heterogeneous Organometallic Catalysis

“…There are also reports for unorthodox ligands to load metal‐catalyst such as urea, graphene and so on. Quinone has been reported as ligand for palladium and other precious metal in solution phase . We envision that quinone may be explored for metal‐catalyst loading for heterogeneous catalysis.…”

“…Quinone has been reported as ligand for palladium and other precious metal in solution phase. [16] We envision that quinone may be explored for metal-catalyst loading for heterogeneous catalysis. This unorthodox ligand could be conveniently converted from oxidation of hydroquinone, and is relatively inert in many catalytic reaction types.…”