Palladium Acetate Revisited: Unusual Ring-Current Effects, One-Electron Reduction, and Metal-Metal Bonding.

Palladium(II) acetate (1) and two new complexes of the ligand α,α,α',α'-tetramethyl-1,3-benzenedipropionate (esp), C -Pd(esp) ( C-2) and C-Pd(esp) ( C-2), are studied in the solid state and in solution. Variable-temperature NMR and DFT studies of C -2 reveal an unusual shielding region above the Pd atoms. The compounds show a surprising quasi-reversible reduction between -880 and -1200 mV versus Fc/Fc, and the Pd(esp) complexes may be cleanly reduced electrochemically. EPR spectra of reduced samples show pseudo-axial signals with Pd hyperfine coupling, consistent with unprecedented, isostructural Pd species with a valence-trapped Pd-Pd-Pd electronic structure.