CAS: Spectroscopy and Reactivity of Isolated Metal-oxo Complexes

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

With this grant, the Chemical Structure, Dynamics, and Mechanisms-B program is supporting the research of Professor Etienne Garand at the University of Wisconsin-Madison. The project focuses on improving the efficiency of fundamental processes involved in the conversion of solar energy into chemical fuels. Artificial photosynthesis is a promising option for the storage of solar energy. The water oxidation process is key to artificial photosynthesis, it includes the splitting water (H2O) into oxygen (O2) and protons (H+). The research challenge in water splitting resides in lowering its substantial extra energy needed to perform this reaction. To reduce this energy, a large number of catalysts have been reported. The search for catalysts underscores the need for understanding the catalytic water oxidation itself, such that rational catalyst design can be applied. This research project utilizes a combination of tools - mass spectrometry and laser spectroscopy - to probe critical catalytic complexes which are intermediates in the water oxidation reaction. The results from these studies clarify the structures and reactivity of these reactive complexes and provide stringent benchmarks for computational chemistry methods, which can then be used to design future catalysts. In addition, this research project benefits a diverse and competitive STEM workforce by providing mentorship, high-level training and career development for students. The project will support the team's participation in three programs aimed at increasing diversity in STEM; namely, the ACS Bridge to the doctorate which aims at increasing the pool of competitive underrepresented students applying to graduate program; CHOPS which provides an opportunity for students to visit the chemistry department and learn about the graduate program, and CATALYST which provides individual mentoring and support to targeted first-year graduate students. This research project addresses the mechanism for water oxidation using solar energy. A critical step in this mechanism involves nucleophilic attack of a water molecule on a metal-oxo species and the formation of an O-O bond. Building on methodology developed with earlier NSF support, the experimental approach is based on sequential cryogenic ion traps for the controlled manipulation and characterization of ions and clusters. This provides the capabilities to generate highly reactive intermediates and directly probe their structure and reactivity. Precise chemical structure characterizations of the complexes are obtained via cryogenic ion predissociation spectroscopy. This project: 1) Characterizes the electronic structure and spin state of metal-oxo complexes and their relationship with the nature of the metal center and the influence of the ligands; 2) Explores the intrinsic reactivity and interactions between metal-oxo complexes and water molecules, with particular focus on the propensity to form O-O bond and its relationship to the electronic structure of the M=O group; and 3) Defines the role of bases and an extended water solvent network in inducing structural changes and interactions that promote and facilitate the critical O-O formation step. This research promotes the development of a diverse and competitive workforce by providing training and career development at the graduate level. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.