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N-Heterocyclic Carbene Ligands Bearing Hydrogen Atom Transfer Moieties: Synthesis and Catalytic Potential

English title N-Heterocyclic Carbene Ligands Bearing Hydrogen Atom Transfer Moieties: Synthesis and Catalytic Potential
Applicant Ward Thomas R.
Number 140242
Funding scheme Project funding
Research institution Institut für Anorganische Chemie Universität Basel
Institution of higher education University of Basel - BS
Main discipline Inorganic Chemistry
Start/End 01.04.2012 - 31.08.2015
Approved amount 214'240.00
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Keywords (7)

oxygen evolving complex; hydrogen atom transfer; non-innocent ligands; bio-inorganic chemistry; catalysis; oxidation; N-heterocyclic carbene ligands

Lay Summary (English)

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Lay summary

N-Heterocyclic Carbene Ligands Bearing Hydrogen Atom Transfer Moieties: Synthesis and Catalytic Potential

Lay Summary

Over the course of evolution, Nature has devised metalloenzymes that convert plentiful solar energy into chemical energy: Photosynthesis produces carbohydrates and dioxygen from carbon dioxide, water and solar energy. Carbohydrates are energy-loaded molecules that can be stored and consummed (to release energy) upon demand. Energy is stored within a molecule in the form of chemical bonds which consists of pairs of electrons between atoms. Redistributing electron-pairs between atoms allows to control the flow of energy. To orchestrate this delicate process, catalysts contribute to canalize the flow of electrons. For this purpose, catalysts temporarily store and release pairs of electrons, thus favouring the reshuffling of bonds and energy.

In a biomimetic spirit, it is proposed to develop catalysts that are capable of storing pairs of electrons both on an organic moiety and a central metal. In this context, catalysts consisting of abundant metals (iron, copper, manganese etc.) will be tested for the oxidation of water or methane to produce dioxygen or methanol.

It is anticipated that this approach will lead to the development of more efficient catalytic systems to produce chemical energy from solar energy, thus contributing to lower the cost and the environmental impact of these artificial photosynthesis catalysts.


Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

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Publications

Publication
N-Heterocyclic carbene ligands bearing a naphthoquinone appendage: Synthesis and coordination chemistry
E. A. Milopolska, M. Kuss-Petermann, M. Neuburger, O. Wenger, T. R. Ward (2016), N-Heterocyclic carbene ligands bearing a naphthoquinone appendage: Synthesis and coordination chemistry, in Polyhedron, 103, 261-266.
Redox-active ligands in catalysis
Praneeth Vijayendran K K, Ringenberg Mark R., Ward Thomas R. (2012), Redox-active ligands in catalysis, in Angewandte Chemie - International Edition, 51(41), 10228-10234.

Abstract

Overcoming Mankind’s addiction to fossil fuel is perhaps the greatest scienfic challenge of the twenty-first century. Critical analysis of the key chemical reactions that may allow to address this issue, reveals that these reactions require the rigorous shuttling of multiple electrons and protons. For this purpose, Nature relies on highly evolved metalloenzymes which incorporate both metal- and organic cofactors.In a biomiemtic spirit, it is proposed to explore the synthesis, coordination chemistry and the catalytic properties of non-innocent, redox-active ligands. For this purpose, N-Heterocyclic Carbene ligands (NHC) will be derivatized with well documented Hydrogen Atom Transfer moieties (HAT, NHetCHAT for the ligand). Such HAT moieties include: sterically hindered hydroxylamines, phenols, catechols, hydrochinones, dihydropyrazines and dihydropyridines. According to the Marcus theory, fine tuning the distance between the donor-acceptor moieties as well as the thermochemical driving force of the HAT transfer may lead to improved reaction rates and contribute to lower the applied overpotential to drive the reaction. Additionally, removing some of the oxidative load from the catalytically competent metal by delocalizing it on the HAT moiety may contribute to increase the total turnover number of the catalytic systems.The following reactions will be scrutinized: alcohol oxidation, alcane oxidation as well as water oxidation. Initially, we will rely on well documented NHC-bearing organometallic catalysts precursors (typically, Ru, Pd and Ir complexes) which will be prepared and tested with NHetCHAT ligands. Having gained understanding and confidence with such systems, first-row transition metal complexes incorporating NHetCHAT ligands will be scrutinized.We believe that the approach delineated herein will yield highly relevant insight into fundamental aspects of catalytic processes relying on the transfer of multiple electrons and protons. Ultimately, it may lay the basis for novel catalytic systems for alcane or water oxidation.
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