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ENTANGLED-TM-ALKANE Report Summary

Project ID: 637313
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - ENTANGLED-TM-ALKANE (Entangled pincer ligand architectures and their application in the transition-metal-mediated activation of alkanes)

Reporting period: 2015-04-01 to 2016-09-30

Summary of the context and overall objectives of the project

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 constituent and incredibly robust C–H and C–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 sigma–alkane complexes, metal-alkane adducts formed through coordination of an intact C–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 sigma–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 ‘pincer’ ligands. Through the interwoven topology of these systems, problematic dissociation reactions of sigma–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 macromolecules’ 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 from the beginning of the project to the end of the period covered by the report and main results achieved so far

The implementation of ENTANGLED-TM-ALKANE is structured into three work packages (W1 – 3), which build sequentially upon each other and correspond to the research activities required to achieve the corresponding principle objectives. During the reporting period the action 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 ‘pincer’ ligand of a transition metal complex, viz. Interlocked macrocyclic Pincer–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.

In particular, the synthetic work so far has centred on IPAS featuring N-heterocyclic carbene- and phosphine-based macrocyclic ligands:

(a) N-heterocyclic carbene-based macrocyclic ligands.
Accelerated by preliminary work, progress in W1 using these ligands is most advanced. A series of macrocyclic ligands have been prepared and their coordination chemistry with Ag, Cu, Ni, Pd and Rh investigated (see: http://dx.doi.org/10.1039/c6dt01263a). Aided by comparison to a model acyclic complex, a range of reaction types have been explored for capturing hydrocarbon substrates within macrocyclic complexes. A promising method involving rhodium complexes that results in the entrapment of unsaturated hydrocarbons has been advanced (and will be detailed in a forthcoming high impact publication) and conversion to IPAS investigated.

(b) Phopshine-based macrocyclic ligands.
Both racemic and asymmetric routes have been explored for the multi-step synthesis of these air-sensitive ligands. After considerable experimentation, suitable procedures have been identified and enabled isolation of these ligands in low yield on small scale. Work has begun on reaction optimisation and scale up, so that their coordination chemistry may be explored. In parallel, model acyclic complexes have been used to screen reaction types for capturing hydrocarbon substrates within their macrocyclic analogues. Dissemination of these findings is planned following completion of future work associated with their subsequent use of these macrocyclic ligands in organometallic-chemistry.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

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. Within this reporting period, two examples involve:

(a) The development of transmetallation methodologies associated with the preparation of late transition metal complexes of N-heterocyclic carbenes (http://dx.doi.org/10.1039/c6dt01263a). A key feature of this work involved contrasting the effectiveness of copper and silver-based transfer agents, in particular highlighting the advantages of the former, which are less common in the literature. Helping to reinforce the academic impact, this work features in a “frontier” article in a recent Dalton Trans. article (http://dx.doi.org/10.1039/c6dt03687b).

(b) The use of macrocyclic ligands for controlling the regioselectivity and studying the mechanism of interesting C–C bond coupling reactions of alkynes. This currently unpublished work was carried out in the context of capturing interlocking molecular topologies, but will help advance the use of reactions of this type in synthetic organic chemistry.
Record Number: 193708 / Last updated on: 2017-01-24