Structurally Diverse Diazafluorene-Ligated Palladium(II) Complexes and Their Implications for Aerobic Oxidation Reactions

Abstract

4,5-Diazafluoren-9-one (DAF) has been identified as a highly effective ligand in a number of Pd-catalyzed oxidation reactions, but the mechanistic basis for its utility has not been elucidated. Here, we present the complex coordination chemistry of DAF and palladium(II) carboxylate salts. Multiple complexes among an equilibrating mixture of species have been characterized by (1)H and (15)N NMR spectroscopy and X-ray crystallography. These complexes include monomeric and dimeric Pd(II) species, with monodentate (κ(1)), bidentate (κ(2)), and bridging (μ:κ(1):κ(1)) DAF coordination modes. Titration studies of DAF and Pd(OAc)2 reveal the formation of two dimeric DAF/Pd(OAc)2 complexes at low [DAF] and four monomeric species at higher [DAF]. The dimeric complexes feature two bridging acetate ligands together with either a bridging or nonbridging (κ(1)) DAF ligand coordinated to each Pd(II) center. The monomeric structures consist of three isomeric Pd(κ(1)-DAF)2(OAc)2 complexes, together with Pd(κ(2)-DAF)(OAc)2 in which the DAF exhibits a traditional bidentate coordination mode. Replacing DAF with the structurally related, but more-electron-rich derivative 9,9-dimethyl-4,5-diazafluorene (Me2DAF) simplifies the equilibrium mixture to two complexes: a dimeric species in which the Me2DAF bridges the two Pd centers and a monomeric species with a traditional κ(2)-Me2DAF coordination mode. The use of DAF in combination with other carboxylate ligands (CF3CO2(-) or tBuCO2(-)) also results in a simplified collection of equilibrating Pd(II)-DAF complexes. Collectively, the results highlight the ability of DAF to equilibrate rapidly among multiple coordination modes, and provide valuable insights into the utility of DAF as a ligand in Pd-catalyzed oxidation reactions.