Circumventing the Oxo Wall – Triazolide and Triazolylidene based ligand scaffolds for the stabilization of high-valent Co(IV)-oxo complexes

• Applicant: Dr. Benedict Elvers
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 534356777

High-valent metal-oxo complexes are of growing interest due to their diverse catalytic reactivity in C-H activation reactions, oxygen atom transfer (OAT) reactions, and other transformations of organic molecules, which are all of great interest in the chemical and pharmaceutical industry. In particular, cobalt-oxo complexes have come under increased focus due to their high reactivity and ability to activate strong C-H bonds. However, this reactivity has so far been untamed, as the electronic structure of these complexes is not yet well understood. Most Co(IV)-oxo complexes known to date exhibit high instability, which manifests itself in intramolecular (decomposition) reactions and short half-lives. This can be explained by the "oxo wall" paradigm. To circumvent this problem, the use of a templated trigonal-bipyramidal coordination geometry as well as a special class of strongly donating ligands, which maintain acceptor capabilities, is proposed here. The tripodal ligand scaffolds targeted here contain triazolide and triazolylidene donor groups, which are highly tunable in their donor/acceptor capabilities, but which have received little attention so far in the literature. This group of ligands will be comprehensively investigated in this project for their suitability to stabilize Co(IV)-oxo complexes. Particular attention will be paid to the influence of different regioisomers and substituent effects on the electronic properties of the resulting complexes. These subtle influences might allow me to investigate Co(IV)-oxo complexes side by side with their valence tautomers, the Co(III)-oxyl complexes. All complexes obtained in this work will be extensively characterized spectroscopically, structurally, and electrochemically to obtain key insights into their electronic properties, which can then be correlated with their reactivity toward C-H bonds. The proposed reactivity studies are built on seminal work by Solomon, Borovik, Anderson and others on concerted-proton electron transfer (CPET) reactions and how these mechanisms influence the catalytic efficiency of metal-oxo complexes in activating C-H bonds. These combined studies will be further complemented by comprehensive quantum-chemical (DFT) calculations, in order to establish structure-reactivity relationships for high-valent Co-oxo complexes and answer eminent question concerning, e.g., the influence of asynchronous steps and metal-oxo/oxyl basicity during the CPET reaction on the reactivity of these species.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Nicolai Lehnert