Highly Efficient Bioinspired Molecular Ru Water Oxidation Catalysts with Negatively Charged Backbone Ligands

Lele Duan §, Lei Wang §, Fusheng Li §, Fei Li ¶, and Licheng Sun *§¶

§ Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

¶ State Key Lab of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China

Acc. Chem. Res., Article ASAP

DOI: 10.1021/acs.accounts.5b00149

Publication Date (Web): July 1, 2015

Copyright © 2015 American Chemical Society

*E-mail: lichengs@kth.se.

 Special Issue

Published as part of the Accounts of Chemical Research special issue “Synthesis in Biological Inorganic Chemistry”.

Biography

Lele Duan is a researcher at KTH Royal Institute of Technology. After receiving his Ph.D. degree under the supervision of Professor Licheng Sun at KTH in 2011, he started his postdoctoral career in the same group and later in 2013 joined in the Etsuko Fujita group at Brookhaven National Laboratory. His current research focuses on the development of artificial photosynthetic systems for fuel production.

Biography

Lei Wang is a graduate student under the supervision of Professor Licheng Sun in Department of Chemistry at KTH. His research focuses on the development of molecular WOCs and water splitting devices.

Biography

Fusheng Li joined the group of Professor Licheng Sun at KTH as a Ph.D. student in September 2011. He focuses on the design of molecular devices for both electrodriven and light-driven water splitting.

Biography

Fei Li is an associate professor at Dalian University of Technology (DUT), China. His research interests include photocatalytic and electrocatalytic water splitting with molecular catalysts.

Biography

Licheng Sun is a full Professor at KTH Royal Institute of Technology and a distinguished professor at DUT. His research interests cover solar cells and solar fuels, in particular, artificial photosynthesis, bioinspired water oxidation using molecular catalysts, and hydrogen generation using FeFe-hydrogenase model complexes. In addition, he is also interested in the development of photoelectrochemical cells based on molecular catalysts and nanostructured semiconductors.

Abstract

Conspectus

The oxygen evolving complex (OEC) of the natural photosynthesis system II (PSII) oxidizes water to produce oxygen and reducing equivalents (protons and electrons). The oxygen released from PSII provides the oxygen source of our atmosphere; the reducing equivalents are used to reduce carbon dioxide to organic products, which support almost all organisms on the Earth planet. The first photosynthetic organisms able to split water were proposed to be cyanobacteria-like ones appearing ca. 2.5 billion years ago. Since then, nature has chosen a sustainable way by using solar energy to develop itself. Inspired by nature, human beings started to mimic the functions of the natural photosynthesis system and proposed the concept of artificial photosynthesis (AP) with the view to creating energy-sustainable societies and reducing the impact on the Earth environments. Water oxidation is a highly energy demanding reaction and essential to produce reducing equivalents for fuel production, and thereby effective water oxidation catalysts (WOCs) are required to catalyze water oxidation and reduce the energy loss.

X-ray crystallographic studies on PSII have revealed that the OEC consists of a Mn4CaO5cluster surrounded by oxygen rich ligands, such as oxyl, oxo, and carboxylate ligands. These negatively charged, oxygen rich ligands strongly stabilize the high valent states of the Mn cluster and play vital roles in effective water oxidation catalysis with low overpotential. This Account describes our endeavors to design effective Ru WOCs with low overpotential, large turnover number, and high turnover frequency by introducing negatively charged ligands, such as carboxylate. Negatively charged ligands stabilized the high valent states of Ru catalysts, as evidenced by the low oxidation potentials. Meanwhile, the oxygen production rates of our Ru catalysts were improved dramatically as well. Thanks to the strong electron donation ability of carboxylate containing ligands, a seven-coordinate RuIV species was isolated as a reaction intermediate, shedding light on the reaction mechanisms of Ru-catalyzed water oxidation chemistry. Auxiliary ligands have dramatic effects on the water oxidation catalysis in terms of the reactivity and the reaction mechanism. For instance, Ru-bda (H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts catalyze CeIV-driven water oxidation extremely fast via the radical coupling of two RuV═O species, while Ru-pda (H2pda = 1,10-phenanthroline-2,9-dicarboxylic acid) water oxidation catalysts catalyze the same reaction slowly via water nucleophilic attack on a RuV═O species. With a number of active Ru catalysts in hands, light driven water oxidation was accomplished using catalysts with low catalytic onset potentials. The structures of molecular catalysts could be readily tailored to introduce additional functional groups, which favors the fabrication of state-of-the-art Ru-based water oxidation devices, such as electrochemical water oxidation anodes and photo-electrochemical anodes. The development of efficient water oxidation catalysts has led to a step forward in the sustainable energy system.