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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - RotaxHEC (Click to Lock: Mechanically Interlocked Architectures as Hydrogen Evolving Catalysts)

Teaser

Summary

Context

With the world population predicted to reach 11.2 billion by 2100, the global energy demand will inevitably likewise increase. The need for new sustainable and â€œgreenâ€ sources of energy are as such an urgent requirement. Photocatalytic water splitting is an attractive solution for a number of reasons. To begin with solar power is an abundant resource and entirely renewable. Secondly, water is an abundant feed stock for this process, generating storable hydrogen and oxygen gas. The hydrogen evolution side of water splitting involves the reduction of protons to hydrogen gas. A number of catalysts have been developed for this purpose; combining these with a photosensitiser unit can generate photocatalytic systems. However, effective electron transfer is diffusion limited; attempts to enforce the proximity of these two species through connective coordination bonds have failed to produce especially more efficient catalysts. Dissociation of the photosensitisers and degradation of the hydrogen evolving catalysts (HECs) are the chief explanations for this.

Mechanically interlocked molecules (MIMs) have recently emerged as novel ligands for coordination to metal ions. Early work by Sauvage and co-workers revealed that coordination complexes of catenane ligands can have quite different properties to their non-interlocked analogues, including increased stability due to the inability of the constituent ligand components to dissociate from one another. We propose that metal ion complexes of MIMs could thus be utilised as more stable HECs, due to both the enforced proximity of the ligand components, and protection of the metal centre from nucleophilic attack.

Objectives

My objective was to investigate MIM complexes as the HEC component of water splitting. To achieve my objectives I developed new chemistry to synthesise suitable MIMs and investigated their properties when combined with metal ions.

Work performed

Results

i) Scalable methods for the synthesis of MIM ligands (Chem. Sci. 2016, 7, 3154)
Whilst a number of high-yielding methodologies have been developed for the synthesis of MIMs, these usually focus on the final mechanical bond forming step. However, the synthesis of the precursor units, such as appropriate macrocycles, can often be laborious and low yielding. To rectify this short-coming we developed a more efficient route towards these key precursors. After optimisation the reactions were found to give the desired macrocycle compounds in approximately 70% yield across a range of macrocycle structures, and allowed access to these in gram scale. We are now in talks with a company to make these molecules available to the wider community.

ii) New Methods for the Synthesis of Polynuclear MIM Ligands (J. Am. Chem. Soc. 2016, 138, 16329; Molecules 2017, 22, 89)
It was foreseeable that for our photocatalytic systems we might require two cavities formed by mechanical bonds capable of binding metal ions. Oligomeric [n]rotaxanes, where n = >2, would thus be a desirable target. We developed the first route to oligomeric rotaxanes, where the precise length and macrocycle order could be controlled exactly with precise control of the order of different macrocycles.

iii) Rotaxane Ligands for Early Transition Metals and their Hydrogen Evolving Capabilities (manuscripts in preparation)
We have investigated the electrochemical and spectroscopic properties of first row transition metal complexes of tri-, tetra- and penta-dentate rotaxane ligands. Our results showed that in many instances the ligand environment and coordination behaviour of the rotaxane ligands, including their electrochemical properties, was distinct from their non-interlocked counterparts. A manuscript describing this work is currently in preparation in collaboration with Roessler at QMUL. We also examined our complexes for their ability to act as electrocatalysts in the production of H2 in acidic solutions. Unfortunately, thus far none of our complexes outperform existing HECs.

iv) New Approaches to Catenane Ligands (manuscripts in preparation)
Reports of scalable syntheses of catenanes in the literature are relatively limited compared to rotaxanes and none were suited to our desire to investigate crowded structures. To overcome this we developed a new approach to generate suitable catenanes in quantitative yield. Our method allows access to catenanes using small molecule building blocks that are readily accessible from commercially available materials in a minimal number of steps, including building blocks containing pyridine and other ligands for 1st row transition metal ions.

v) Collaborations and Reviews (Chem. Sci. 2017, 8, 6679; Chem. Commun. 2017, 53, 298. Chem. Soc. Rev. 2017, 46, 2577)
In addition to the results described above I have also contributed to collaborations between my host group and other researchers and contributed to literature reviews in collaboration with Prof Goldup on the subject of metal coordination in MIM synthesis and properties. These reviews have been extremely well received and allowed me to broaden my influence on the field by showcasing my personal views on the opportunities presented by MIM ligands.
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Dissemination
Over the course of my fellowship I have produced four research publications and two literature reviews. Three further manuscripts are in preparation. All of these articles were published in open access form and raw data made available through the UoS Repository. In addition to these 9 publications, I have presented my results at national and international meetings including the RSC Macrocyclic and Supramolecular Chemistry annual meeting, the MASC Early Career meeting, the International Symposium on Macrocyclic and Supramolecular Chemistry and the Telluride Workshop on Switches and Motors.

Final results

The synthetic methodologies that we have developed for the synthesis of MIM ligands and their precursor materials go well beyond the state of the art. We have also shown that interlocked ligands can be used to generate heteroleptic metal complexes that are otherwise impossible to obtain and that the MIM ligand alters the electrochemical and spectroscopic properties of the complex compared to non-interlocked analogues. As such it is envisaged that further work would allow tuning of properties through careful design of the interlocked ligand framework.Future work will focus on tuning the ligand environment further using the synthetic approaches developed and diversifying the structural features of the MIM ligands based on our preliminary results.

Overall the project has been extremely successful, delivering results for 9 publications in well regarded scientific journals and providing the wider field with important new knowledge and methods. The results of this action not only bring MIM-based HECs several steps closer, they also dramatically expand the range of structures available for study in a range of applications including catalysis, sensing and materials chemistry. Furthermore, the success of the Action, as judged both by scientific results and academic publications, combined with training and mentoring from my Host, Prof Goldup, positioned me to apply for independent research positions. Ultimately, I was offered a Leverhulme Trust Early Career Fellowship and an Imperial College Junior Research Fellowship. I accepted the position at Imperial College and began my independent research career in October 2017.