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^2-), C_s-Pd₃(esp)₃ (C_s-2) and C_3h-Pd₃(esp)₃ (C_3h-2), are studied in the solid state and in solution. Variable-temperature NMR and DFT studies of C_s-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 Pd₃(esp)₃ complexes may be cleanly reduced electrochemically. EPR spectra of reduced samples show pseudo-axial signals with ¹⁰⁵Pd hyperfine coupling, consistent with unprecedented, isostructural Pd₃ ⁵⁺ species with a valence-trapped Pd^II-Pd^II-Pd^I electronic structure.