Role of redox active and redox non-innocent ligands in water splitting

Abstract

Transformation of naturally abundant water into energy rich feedstock can provide a solution for the current demand of sustainable and clean energy. In nature, photon energy is converted to the chemical energy in the Oxygen Evolving Center (OEC) of Photosystem II. Inspired by the cubane structure of OEC, several molecular complexes have been demonstrated for the effective water oxidation in homogeneous medium. In addition, a large number of transition metal based molecular complexes have been explored for the hydrogen evolution reaction. In all these cases, synthesis of the new molecular complexes largely depends on the ligand designing. Generally, three different types of ligands have been explored: (i) spectator ligands (ii) redox active ligands and (iii) redox non-innocent ligands. Redox active and redox non-innocent ligands play an important role in controlling the electron transfer properties of the metal centers and hence the metal bound substrates. As a result, the water oxidation and hydrogen evolution activity is largely affected by the redox properties of the ligands. In this review, we have highlighted the designing of redox active and redox non-innocent ligands, their redox properties and control over the electron transfer from the metal center. The major developments in this field and potentiality for the future progress with the redox active and redox non-innocent ligands derived molecular complexes have been described. In addition, comparison between the natural and artificial photosystem has been described for a better understanding of this subject and future development.