Molecular Insights into the Ligand-Based Six-Proton- and Six-Electron-Transfer Processes Between Tris-ortho-Phenylenediamines and Tris-ortho-Benzoquinodiimines.

The global demand for energy and the concerns over climate issues renders the development of alternative renewable energy sources such as hydrogen (H ) important. A high-spin (hs) Fe complex with o-phenylenediamine (opda) ligands, [Fe (opda) ] (hs-[6R] ), was reported showing photochemical H evolution. In addition, a low-spin (ls) [Fe (bqdi) ] (bqdi: o-benzoquinodiimine) (ls-[0R] ) formation by O oxidation of hs-[6R] , accompanied by ligand-based six-proton and six-electron transfer, revealed the potential of the complex with redox-active ligands as a novel multiple-proton and -electron storage material, albeit that the mechanism has not yet been understood. This paper reports that the oxidized ls-[0R][PF ] can be reduced by hydrazine giving ls-[Fe (opda)(bqdi) ][PF ] (ls-[2R][PF ] ) and ls-[Fe (opda) (bqdi)][PF ] (ls-[4R][PF ] ) with localized ligand-based proton-coupled mixed-valence (LPMV) states. The first isolation and characterization of the key intermediates with LPMV states offer unprecedented molecular insights into the design of photoresponsive molecule-based hydrogen-storage materials.