Strong Proton-Electron Coupling in π-Planar Metal Complex with Redox-Active Ligands

Proton-coupled electron transfer (PCET) of metal complexes has been widely studied, especially in biochemistry and catalytic chemistry. Although metal complexes bearing redox-active ligands play a part in these research areas, those with pi-planar structure remain entirely unexplored, which are vital for future development of iono-electronics. Here, proton-electron coupling of a pi-planar nickel complex bearing redox-active N,S-ligands, Ni(itsq)(2), was investigated by combining experimental and theoretical approaches. Strong proton-electron coupling was manifested in a large potential shift, which is twice greater than that of a typical PCET-type pi-planar metal complex with redox-inactive ligands, [Ni(dcpdt)(2)](2-). Theoretical calculations affirmed that the stabilization of frontier orbitals by protonation is greater in Ni(itsq)(2) than that in [Ni(dcpdt)(2)](2-). These results indicate that pi-planar metal complexes with redox-active ligands are promising for developing novel PCET-type materials.