Palladium(III) Debut

For the first time, chemists have demonstrated that palladium complexes in the +3 oxidation state are intermediates in catalytic bond-forming processes. This finding puts a new twist on the chemistry of palladium and could someday lead to new reactions.

Palladium catalyzes a wide variety of reactions, and the chemistry of its oxidation states of 0, +1, +2, and +4 is well-known. Graduate student David C. Powers and assistant professor of chemistry Tobias Ritter of Harvard University made the new discovery while investigating the mechanism of a small set of Pd-catalyzed carbon-heteroatom bond-forming reactions. These types of reactions have been thought to always proceed via a catalytic cycle that switches between Pd(II) and Pd(IV), where a lone Pd metal center gives up two electrons to form the new bond.

On the basis of kinetic experiments, they instead propose that complexes containing two Pd(III) metal centers are intermediates in the reactions, with each Pd contributing one electron to a nascent C-heteroatom bond (Nat. Chem., DOI: 10.1038/nchem.246).

It's not certain that the newly discovered mechanism will prove to be generally applicable, but Ritter's team hopes to explore the bimetallic Pd(III) catalyst's potential in a variety of reactions. Nature makes use of bimetallic catalysis in a variety of enzyme active sites, and bimetallic catalysts made from rhodium, palladium's next-door neighbor on the periodic table, are versatile tools for synthesis, Ritter notes.

The team "has provided a timely and fascinating contribution to the current rapid development of higher oxidation state organopalladium chemistry and a stimulating evaluation of the potential involvement of such states in organic synthesis," says Allan J. Canty of the University of Tasmania, in Australia. Canty recently showed that analogous platinum(III) complexes can serve as reaction intermediates (J. Am. Chem. Soc. 2009, 131, 7236).

The advance builds on work by the late F. Albert Cotton of Texas A&M University, who first demonstrated that these types of Pd(III) complexes could be made in the laboratory, says organometallic chemist Melanie S. Sanford of the University of Michigan.