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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - ENTANGLED-TM-ALKANE (Entangled pincer ligand architectures and their application in the transition-metal-mediated activation of alkanes)

Teaser

Summary

As the major components of petroleum and natural gas, alkanes are vast and low-cost raw materials. The development of mild catalytic processes for the direct exploitation of alkanes as chemical feedstocks would revolutionise the chemical industry and, through associated energy and environmental benefits, result in significant societal impact. Such objectives are encumbered by the constituent and incredibly robust Câ€“H and Câ€“C bonds, which confer upon alkanes a remarkable degree of chemical inertness that is classically overcome only under extreme temperature regimes (>800oC) or through reactions with highly reactive superacids or free radicals. Such conditions are intrinsically associated with low product selectivity and consequently have limited practical scope.

The controlled activation of alkanes under mild conditions by well-defined molecular complexes of the transition metals has been demonstrated and represents a promising solution to this problem. The fundamental organometallic chemistry of these processes is, however, poorly understood and a much greater understanding is required before their full potential in chemical synthesis can be realised. In particular, the characterisation and reaction chemistry of sigmaâ€“alkane complexes, metal-alkane adducts formed through coordination of an intact Câ€“H bond to the metal centre, have proven to be exceedingly difficult to investigate using conventional experimental approaches as a consequence the weakly interacting nature of alkanes and their otherwise transient presence in reactions between alkanes and highly reactive metal-based compounds.

To address this knowledge gap and provide the foundation for future advancement of the field, ENTANGLED-TM-ALKANE outlines a systematic approach for the study of these pivotal sigmaâ€“alkane complexes. Inspired from supramolecular chemistry, the approach involves the innovative use of systems containing alkane substrates held in close proximity to reactive metal centres through mechanical entanglement within supporting tridentate macrocyclic â€˜pincerâ€™ ligands. Through the interwoven topology of these systems, problematic dissociation reactions of sigmaâ€“alkane complexes will be circumvented, facilitating isolation and ultimately enabling their structure and reaction chemistry to be probed in much greater detail than has been previously possible. The project objectives are to: (a) develop and use new synthetic (supramolecular) methodologies for the preparation of these mechanically interlocked metal-alkane assemblies; (b) systematically investigate the organometallic chemistry of the metal centre and its interaction with the entangled alkane; and through variation of the macromoleculesâ€™ components (macrocycle donors and geometry, alkane, metal), (c) compile a definitive and unprecedented body of qualitative and quantitative structure-activity relationships for the activation alkanes using transition metals.

Work performed

The implementation of ENTANGLED-TM-ALKANE is structured into three work packages (W1 â€“ 3), which build sequentially upon each other and correspond to the research activities required to achieve the corresponding principle objectives. The action so far has been focused exclusively on the first work package (W1); the development of the new synthetic methodologies required for the preparation of the target systems comprising a hydrocarbon substrate mechanically entrapped within the supporting tridentate macrocyclic â€˜pincerâ€™ ligand of a transition metal complex, viz. Interlocked macrocyclic Pincerâ€“Alkane Systems (IPAS). Although based on precedents from supramolecular chemistry, these IPAS represent highly challenging synthetic targets due to the lack of heteroatoms within the entangled substrate that are typically exploited in template-based construction methods.

Through our work we prepared new N-heterocyclic carbene- and phosphine- based macrocyclic ligands. The coordination and organometallic chemistry of these ligands, and a known imine-based variant, is being explored in the context of performing metal mediated Câ€“C bond coupling reactions through the macrocyclic complexes. Some of this work has been published:
1) https://doi.org/10.1039/c6dt01263a
2) https://doi.org/10.1002/anie.201807028
3) https://doi.org/10.1021/acs.organomet.8b00595

As the macrocyclic ligands are very challenging to prepare, we have used model complexes to help develop the underpinning methodology. See:
1) https://doi.org/10.1016/j.poly.2017.08.001
2) http://doi.org/10.1039/c8dt05049j
3) http://doi.org/10.1021/acs.inorgchem.9b00957
4) https://doi.org/10.1002/ejic.201900727
5) https://doi.org/10.1002/anie.201908333

Forthcoming publications will document our efforts towards the preparation of IPAS and showcase the interesting coordination and organometallic chemistry of macrocyclic ligands.

Final results

The overarching motivation of this proposal is to advance our fundamental understanding of how alkane activation reactions proceed on a molecular scale. The selective transformation of alkanes is an area of contemporary importance with wide-ranging implications for organic synthesis and the effective use of petroleum resources, and such mechanistic work will help drive chemical innovation in these areas.

As such the primarily impact of ENTANGLED-TM-ALKANE will result from knowledge generated in W2 and W3, i.e. following culmination of W1. However, realising the objectives of W1 will involve progressing the current state of the art in a number of areas, including synthetic organic, organometallic and supramolecular chemistry.