High-valent Pd coordination compounds

Palladium holds a privileged position in inorganic and organometallic chemistry. It is virtually synonymous with transition metal-catalyzed cross-coupling chemistry, a strategy that allows the construction of carbon-carbon bonds with relative ease. This was recognized in 2010, when Heck, Negishi, and Suzuki were awarded the Nobel Prize in Chemistry for developing “Palladium Catalyzed Cross-Couplings in Organic Synthesis.” The success can be, in large part, attributed to the extensive mechanistic investigations into the elementary steps of these catalytic cycles, nearly all of which feature Pd⁰ and Pd^II oxidation states and thus can be called ‘low-valent’ palladium chemistry.¹ Careful investigation revealed the interplay between the steric and electronic effects of the ligands, which helped fine-tune the reactivity. However, low-valent palladium chemistry is not suitable for certain transformations such as oxidative carbon-heteroatom bond formations and is susceptible to decomposition pathways such as b-hydride elimination. In the past couple of decades, the field of (Diagram Presented)Fig. 1 Pictorial representation of d electronic configuration of a Pd center in the þ2, þ3, and þ4 oxidation states. ‘high-valent’ Pd catalysis has received a lot of attention from several research groups, and Pd^III and Pd^IV intermediates have been invoked in the various proposed catalytic cycles. In the 1980s and 1990s, several studies reported well-defined organometallic Pd^III and Pd^IV coordination complexes, and the interest has grown extensively in developing this high-valent chemistry as a complement to the traditional low-valent chemistry.^2–4 To understand the reactivity of high-valent Pd centers, it is worthwhile to look at their electronic structure in....