A novel organometallic Pd(III) complex with a pyridine-based macrocyclic ligand can react to form C–C bonds, opening up a new area of monopalladium chemistry (J. Am. Chem. Soc., DOI: 10.1021/ja103001g).
Calling the ligand design "very clever," Melanie S. Sanford, a chemistry professor at the University of Michigan who was not involved in the work, says it "represents a really exciting start to what should be a very fruitful exploration of the reaction chemistry of Pd(III)."
The chemistry of palladium in the 0, +1, +2, and +4 oxidation states is well-known, but complexes involving the +3 state are rare. Only three other monometallic complexes have been reported: PdF63– and two Pd(III) complexes with the macrocylic ligands triazacyclononane and trithiacyclononane, which have only Pd–N and Pd–S bonds, respectively. Other monomeric Pd(III) complexes have also been proposed as intermediates in several Pd-catalyzed reactions, but none has been isolated.
In the new work, a research group led by Liviu M. Mirica, a chemistry professor at Washington University in St. Louis, studied three octahedral Pd(III) complexes. In these, N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (abbreviated N4) provides four nitrogen ligands to the Pd atom. The remaining two ligands in the complexes are –CH3, –C6H5, or –Cl. The team prepared the complexes in the Pd2+ oxidation state, then oxidized them electrolytically or chemically to Pd3+.
Mirica and colleagues found that a solution of [(N4)PdIIICH3Cl]+ (shown) produced ethane when irradiated with light, likely through a reaction mechanism involving homolytic cleavage of the Pd–CH3 bond to form CH3·. Replacing the methyl with a phenyl ligand led to the formation of biphenyl. A mixture of methyl and phenyl Pd complexes produced toluene. The third complex studied, [(N4)PdIII(CH3)2]+, also produced ethane.
Study of the complexes and their chemistry should be helpful for developing catalysts for Pd-catalyzed, oxidative C–C bond formation, including conversion of methane to ethane, Mirica says.