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)(OH2)(TFA)(2), exhibits an S-bound DMSO ligand. H-1 and F-19 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.