Synthesis of Well‐Defined High‐Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

Abstract

The last decade has witnessed the rapid development of high‐valent Pd‐involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one‐electron and two‐electron oxidants have been employed to give access to high‐oxidation‐state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd‐mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined PdIII/IV complexes starting from their PdII precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O2; 7) sulfonyl chlorides; and 8) others are covered. A “greener” electrooxidation manner has also been reviewed.

Rapid Electrochemical Methane Functionalization Involves Pd–Pd Bonded Intermediates

“…Given the key role of M−M bonding in high-valent Pd oxidation catalysis, 2 addressing these structural and mechanistic knowledge gaps is critical for the rational design of oxidative electrocatalysis with high-valent Pd intermediates. 27 Herein, we establish the core structure of the electrochemically generated Pd III 2 and provide a structural basis for its unique mechanism of formation. Since the Pd III 2 complex cannot be isolated from the sulfuric acid medium, we combine X-ray absorption and Raman spectroscopies to establish that it contains a Pd−Pd bond with each Pd atom coordinated by five O atoms.…”

“…In contrast, our Pd III 2 complex is generated via sequential one-electron electrochemical oxidation from a simple mononuclear Pd II (sulfate) complex, leaving open the critical questions of whether a M–M bond exists in the Pd III 2 species and what role it plays in fostering the unusual ECE electrochemical oxidation mechanism. Given the key role of M–M bonding in high-valent Pd oxidation catalysis, addressing these structural and mechanistic knowledge gaps is critical for the rational design of oxidative electrocatalysis with high-valent Pd intermediates …”

Structurally-Responsive Ligands for High-Performance Catalysts

“…It follows that oxidation and reduction reactions can be promoted by changes in coordination numbers. For example, reduction of six-coordinate octahedral Pd­(IV) or Pt­(IV) complexes occurs more readily after ligand loss affords a five-coordinate intermediate. − There are several studies of oxidation/reduction chemistry with complexes containing SRLs that change coordination mode depending on the oxidation state of the metal. − Even challenging small molecule reduction (e.g., of N 2 , PhN = NPh or O 2 ) can be achieved by metals (e.g., Mo­(III) or W­(II)), in which the SRL stabilizes a masked low-coordinate structure. , …”

Cycloneophylpalladium(IV) Complexes: Formation by Oxidative Addition and Selectivity of Their Reductive Elimination Reactions

“…Yield: 78% (0.023 g, 0.031 mmol). (6). Upon addition of bromine (2 μL, 0.04 mmol), an immediate color change from orange to green was observed.…”

“…It is now well established that Pd­(II)/Pd­(IV) cycles are important in catalysis by palladium complexes, especially in reactions involving strong oxidants such as halogens, dioxygen, and peroxides. , For example, the catalytic reaction of iodine with alkanes or arenes to form iodo-containing hydrocarbons is thought to involve alkane C–H bond activation to form an alkylpalladium­(II) complex, followed by oxidative addition of iodine and then reductive elimination from palladium­(IV). − These impressive advances have stimulated further efforts to understand the factors that affect reactivity and selectivity in oxidative addition at palladium­(II) and reductive elimination from palladium­(IV) complexes. − In studies of selectivity in reductive elimination, the cycloneophylpalladium­(IV) complexes have played an important role as a model to distinguish between reactivity at C­(sp 2 ) or C­(sp 3 ) centers. ,,,,− When the palladium­(IV) complexes ( A ) are sufficiently stable to be isolated, the mechanisms of reductive elimination have been studied and, in most cases, have been shown to involve five-coordinate intermediates such as B (Scheme ). − , External nucleophilic attack on such intermediates occurs selectively at the CH 2 group and can lead, for example, to C–O or C–N bond formation (Scheme a).…”