Easily Switchable 18 pi-, 19 pi-, and 20 pi-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes

Ni-II 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to Pd-II ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic Pd-II centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20 pi antiaromatic nature of the two-electron reduction species was confirmed by H-1 NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.