Molecular insights into the ligand-based six-proton and -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 2 ) important. A  high-spin ( hs ) Fe(II) complex with  o -phenylenediamine (opda) ligands, [Fe II (opda) 3 ] 2+ ( hs - [6R] 2+ ), was reported showing photochemical H 2 evolution. In addition, a  low-spin ( ls ) [Fe II (bqdi) 3 ] 2+ (bqdi:  o -benzoquinodiimine) ( ls - [0R] 2+ ) formation by O 2 oxidation of  hs - [6R] 2+ accompanied by ligand-based six-proton and -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 6 ] 2 can be reduced by hydrazine giving  ls -[Fe II (opda)(bqdi) 2 ][PF 6 ] 2 ( ls - [2R] [PF 6 ] 2 ) and  ls -[Fe II (opda) 2 (bqdi)][PF 6 ] 2 ( ls - [4R] [PF 6 ] 2 ) 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.