Ligand-Derived Effects in N , C -Cyclometalated Palladium(II) Acetate Dimers

Abstract N,C-Cyclometalated palladium(II) acetate dimers were synthesized as structurally related families differing by aryl ring substitution Pd1 or azole modification Pd2–5. X-ray study confirmed an acetate-bridged dimeric scaffold, a pronounced structural trans influence, and systematic differences in steric congestion between the two series. Solution NMR studies showed that Pd1 complexes exist predominantly as a single transoid isomer, whereas Pd2–5 exhibits transoid/cisoid mixtures. Analysis of structural parameters and NMR data indicates that steric effects and trans influence differentiation govern the transoid/cisoid ratio. In the cisoid form, the trans influence is concentrated on a single acetate bridge, weakening this trans-OAc ligand. Electrochemical studies (DPV/CV) revealed pronounced sensitivity of the first oxidation process to ligand substitution, particularly within the Pd1 series, where an anodic shift of 0.37 V was observed. Correlation of oxidation potentials with Hammett parameters and DFT-calculated frontier-orbital energies revealed substantial involvement of the cyclometalated aryl ring in oxidation. DFT calculations showed greater aryl contribution to the HOMO in Pd1 than in Pd2–5, providing an electronic basis for the strong substituent dependence of oxidation. Positive correlations between oxidation potential and Pd–Pd distance (or interplanar angle) indicate that steric effects opposing Pd–Pd contraction disfavor oxidation.