Dinuclear tetrakis Schiff base complexes for electrocatalytic water splitting

Abstract

Solar energy is one of the most promising sources of sustainable renewable energy which has the potential to meet the daunting energy demands of our growing society. However, the future of sustainable renewable energy supply depends largely on whether there will be breakthroughs enabling us to efficiently convert this solar energy into a storable form of energy such as a liquid fuel. In this thesis we investigate aspects of artificial photosynthesis that could become pathways for catalytic fuel generation. The described techniques involve catalytic conversion of solar energy to fuel using water and/or carbon dioxide as a starting material, mimicking natural photosynthesis in plants.

In the later part of this thesis we describe the second part of artificial photosynthesis, which is catalytic reduction of the protons to H2 (fuel). We developed a class of dicobalt Schiff base complexes with analogous ligand environment to the water oxidation catalyst described earlier. The structural and electrochemical properties of these complexes were studied by X-ray crystallography, infrared spectroscopy, cyclic and differential pulse voltammetry, UV-visible spectroscopy etc. Electrocatalytic investigation of proton reduction indicates reduction of protons to hydrogen gas. The catalytic reduction of protons by this particular complex showed a first order dependence on the catalyst and a second order dependence on the proton source. This class of dinuclear water oxidation and proton reduction catalyst described here plays an important role in understanding the mechanistic pathways of artificial photosynthesis. To the best of our knowledge this is a unique example of a catalytic complex where merely changing the central metal, can achieve both catalytic water oxidation and reduction of protons.

Herein, we study a modular approach towards artificial photosynthesis. The photosynthetic apparatus primarily consists of a light harvesting system connected to a catalytic oxidative site and a catalytic reductive site. I first describe our efforts towards developing the catalytic site where water can be oxidized to generate protons and oxygen. The natural water oxidation catalyst found in photosystem II of green plants and algae, has one calcium and four manganese centers connected via μ-oxo linkages. Using this natural catalyst as a blue print, we developed a μ-carboxylate connected dimanganese tetrakis Schiff base complex since it has promising structural, redox, and photophysical properties. We have also synthesized an analogous dimanganese Schiff base complex with open manganese coordination sites for binding water. A detailed investigation of the electrocatalytic and photocatalytic properties of such dimanganese Schiff base complexes showed evidence of catalytic water oxidation. Furthermore, the results indicate that an open coordination site to the manganese center enhances catalytic activity. This class of dimanganese complexes also shows signs of proton coupled electron transfer (PCET), which is essential to prevent charge buildup during oxidation of water.