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

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.