Characterization of DMSO Coordination to Palladium(II) in Solution and Insights into the Aerobic Oxidation Catalyst, Pd(DMSO)2(TFA)2

Recent studies have shown that Pd(DMSO)(2)(TFA)(2) (TFA = trifluoroacetate) is an effective catalyst for a number of different aerobic oxidation reactions. Here, we provide insights into the coordination of DMSO to palladium(II) in both the solid state and in solution. A crystal structure of Pd(DMSO)(2)(TFA)(2) confirms that the solid-state structure of this species has one O-bound and one S-bound DMSO ligand, and a crystallographically characterized mono-DMSO complex, trans-Pd(DMSO)(OH(2))(TFA)(2), exhibits an S-bound DMSO ligand. (1)H and (19)F NMR spectroscopic studies show that, in EtOAc and THF-d(8), Pd(DMSO)(2)(TFA)(2) consists of an equilibrium mixture of Pd(S-DMSO)(O-DMSO)(TFA)(2) and Pd(S-DMSO)(2)(TFA)(2). The O-bound DMSO is determined to be more labile than the S-bound DMSO ligand, and both DMSO ligands are more labile in THF relative to EtOAc as the solvent. DMSO coordination to Pd(II) is substantially less favorable when the TFA ligands are replaced with acetate. An analogous carboxylate ligand effect is observed in the coordination of the bidentate sulfoxide ligand, 1,2-bis(phenylsulfinyl)ethane to Pd(II). DMSO coordination to Pd(TFA)(2) is shown to be incomplete in AcOH-d(4) and toluene-d(8), resulting in Pd(II)/DMSO adducts with <2:1 DMSO/Pd(II) stoichiometry. Collectively, these results provide useful insights into the coordination properties of DMSO to Pd(II) under catalytically relevant conditions.