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Late First-Row Transition Metal-Oxo Complexes for C–H Bond Activation

Periodic Reporting for period 3 - HurdlingOxoWall (Late First-Row Transition Metal-Oxo Complexes for C–H Bond Activation)

Reporting period: 2019-03-01 to 2020-08-31

The chemical, pharmaceutical, and materials industries rely heavily upon chemicals from oil and natural gas feed-stocks (saturated hydrocarbons) that require considerable functionalisation prior to use. Catalytic oxidative functionalisation (e.g. CH4 + [O] + cat. CH3OH), using first row transition metal catalysts, is potentially a sustainable, cheap, and green route to these high-commodity chemicals. However, catalytic oxidation remains a great modern challenge because such hydrocarbons contain remarkably strong inert C–H bonds that can only be activated with potent catalysts. We will take a Nature-inspired approach to designing and preparing powerful oxidation catalysts: we will interrogate the active oxidant, a metal-oxo (M=O) species, to guide our catalyst design. Specifically, we will prepare unprecedented Late first-row transition Metal-Oxo complexes (LM=O’s, LM = Co, Ni, Cu) that will activate the strongest of C–H bonds (e.g. CH4).

This will be accomplished using a family of novel low coordinate ligands that will support LM=O’s. Due to their expected potent reactivity we will prepare LM=O’s under unique oxidatively robust, low-temperature conditions to ensure their stabilisation. The poorly understood factors (thermodynamics, metal, d-electron count) that control the reactivity of M=O’s will be thoroughly investigated. Based on these investigations LM=O reactivity will be manipulated and optimised. We expect LM=O’s will be significantly more reactive than any early transition metal-oxo’s (EM=O’s), because they will display a greater thermodynamic driving force for C–H activation. It is thus expected that LM=O’s will be capable of the activation of the strongest of C–H bonds (i.e. CH4). Driven by the knowledge acquired from these investigations, we will design and prepare the next generation of molecular oxidation catalysts - a family of late first-row transition metal compounds capable of catalysing hydrocarbon functionalisation under ambient conditions.
"The project has progressed along the expected lines proposed initially, with a number of important research breakthroughs listed below.

The first important discovery made was the identification of a facile means to tune the reactivity of high-valent oxidants (putative reactive species in oxidation catalysis) by simple anion exchange (changing the groups that interact with the metal). The second critical breakthrough in the project was the identification of the first example of a high-valent metal-halide complex (nickel chloride) capable of performing hydrogen atom transfer oxidation (again, a process often implicated in oxidation catalysis). Both breakthroughs have opened new avenues for late transition metal oxidation catalysis and the field of oxidation catalysis as a whole. The latter, halide-based oxidation, could open up the field of oxidation catalysis to oxo-free catalysis in the future.

Hydrogen atom transfer by a high-valent nickel-chloride complex
Prasenjit Mondal, Paolo Pirovano, Ankita Das, Erik R. Farquhar, Aidan R. McDonald*, J. Am. Chem. Soc. 2018, 140, 1834–1841.

Modulation of nickel pyridinedicarboxamidate complexes to explore the properties of high-valent oxidants
Paolo Pirovano, Brendan Twamley, Aidan R. McDonald*, Chem. Eur. J. 2018, 24, 5238-5245.

Indirect evidence for a NiIII-oxyl oxidant in the reaction of a NiII complex with peracid
Paolo Pirovano, Abigail R. Berry, Marcel Swart, Aidan R. McDonald*, Dalton Trans. 2018, 47, 246-250.

Synthetic High-Valent M–O–X Oxidants
Paolo Pirovano, Aidan R. McDonald*, Eur. J. Inorg. Chem. 2018, 547–560.

Tuning the reactivity of terminal nickel(III)-oxygen adducts for C–H bond activation
Paolo Pirovano, Erik R. Farquhar, Marcel Swart, Aidan R. McDonald*, J. Am. Chem. Soc. 2016, 138, 14362–14370.

Synthesis and Characterisation of a Mesocyclic Tripodal Triamine Ligand
Andrew D. Ure, Isabel Abánades Lázaro, Michelle Cotter, Aidan R. McDonald*, Org. Biomol. Chem. 2016, 14, 483-494.!divAbstract"
See section above for progress beyond the state of the art.

The expected results until the end of the project are the same as initially proposed, but including a closer focus on high-valent metal-halides in oxidation catalysis, based on our important discovery in that area.