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Periodic Report Summary 2 - STU POSTERMAC (Studies of Polynuclear Clusters for Biomass Conversion)

Summary of the project objectives
There are four main objectives: 1) Study the chemical performance of the multidentate amino ligand supported tri- and hexanuclear iron clusters developed by the Betley group. 2) Construct novel cluster designs by ligand manipulations. 3) Apply the accumulated knowledge on cluster chemistry and their unique properties to use them as mediators for actual organic chemical group transformations with focus on biomass conversion, especially ethanol and monomeric carbohydrates conversion. 4) Development of catalytic versions of these organic transformations.
Altered objectives
Objectives 3) and 4) are the objectives for the second period back at the Technical University of Denmark in Europe, which is being reported on in this report. However, as described in the report for the first period at Harvard University in USA, objective 3) has been altered and objective 4) has been realised to be of too optimistic character at the current stage of knowledge within designed homogeneous polynuclear cluster chemistry and has, as such, been removed.
As described in the first phase report, the return phase objective is to design a new ligand that might result in novel cluster chemistry. Moreover, because the research fellow has limited access to Mössbauer spectroscopic and SQUID magnetometry measurements at the Technical University of Denmark, we have decided to focus the objectives objective on making compounds not too depending on these types of analyses. The focus of the objective of making new ligands is therefore to find novel chemistry within the realm of making network systems of the clusters that, initially, may rely more on crystallographic data.
Because of this change in objectives, the original objectives have not been met, and the second part of the project, the return phase, might be considered partially unsuccessful from that perspective. However, it is our strong belief that the knowledge and experience the research fellow has gained during the stay at Harvard University in the first phase, by far exceeds in value the importance of achieving the originally defined research objectives. This argument is based on the fact that those original objectives are still feasible albeit further out in the future than initially anticipated. The argument is also based on the fact that the research fellow is able to transfer knowledge on this very new and extremely important topic of chemical research to Europe. This is orders of magnitude more valuable than the scientific objectives described above. As such, an important part of this Marie Curie fellowship is to have the research fellow transferring this utterly novel knowledge to Europe.
Laboratory work performed
The main piece of the work has been the study of oxidative addition of various substrates to (HL)2Fe6(thf)2 and its oxidised congeners. A large range of substrates were investigated in order to obtain a fundamental insight on the behaviour of the cluster when submitted to different types of oxidising agents.
The stereoselectivity seems to be guided by the proposed molecular orbitals of the SOMO’s. As such, the t1u set comprises the three molecular orbitals responsible for directing the ligation of the substrates.
In total, it has been shown that both X-X and R-X type reagents oxidatively adds to the cluster in a trans stereoselective manner. Moreover, all hybridisations on carbon were shown to be feasible as reagents. As such, aliphatic, aryl, and alkynyl carbon atoms all took part of the oxidative addition reaction, and they all followed the trans stereoselectivity of addition to the cluster (Figure 1, and see first period report for more details).
Based on this information, it was conceived that using biscoordinating ligands might lead to a situation where we can not only link the clusters together into networks but also control the spatial setup of these networks. Indeed, this turned out to be the case and the research fellow has shown that it is feasible to create all the dimension types of networks (1D, 2D, and 3D). This is illustrated in Figure 2, upper part.
Key to these creations was using the triply oxidised congener of (HL)2Fe6(thf)2 which take up nitrile-based L type ligands on all of its six coordination sites. Depending on the choice of ligand and ligation process it is then possible to control which dimension order of the network. As such, succinonitrile is used for creating the 1D and 2D networks and glutaronitrile is used for the 3D network. Differentiation between making the 1D and 2D networks is achieved by the choice of reaction conditions.
Another approach to gain access to networks based on the (HL)2Fe6(thf)2 cluster is to alter the ligand framework in such a way that it connects two ligands together (see Figure 2, lower left part). Since each cluster consists of two ligands, a 1D network ought to be created by this method. One unique potential of this setup is that it might lead to a situation where one can gain access to a string of clusters containing a sequence of different oxidation states, which can be expected to have very conductive properties. This is especially promising since it is known that oxidising (HL)2Fe6(thf)2 with one equivalent of FcPF6 in MeCN leads to equal amounts of (HL)2Fe6 and [(HL)2Fe6(NCMe)4)2+. Another potential might be an easy differentiation between other dimensions if higher dimension networks are desired. Hence, the research fellow am currently looking into this aspect, and he is expected to have results on this in the near future.

Overall results and their potential impact
The main results from this project have been the revealing of 1) the rich organometallic type chemistry that these clusters can partake in and 2) the stereoselectivity by which these clusters undergo, for example, oxidative addition. Both results are very important, and they are both expected to be made public knowledge in several publications in due time. So far, no publications have yet been submitted concerning this work, because of the extremely complicated and challenging chemistry it involves. Moreover, normally projects based on these hexanuclear clusters stretch over long periods (up to >5 years), so this situation is not surprising.
These achievements are, to the best of our knowledge, the first examples of this type of chemistry. Hence, because of the novelty of the results found in this project and because of the enormous potential this chemistry holds from a chemical point-of-view, this project has the potential to show a great impact in the long run. In academia, where it might evolve to become a novel field in itself that focusses on cluster-specific catalysis and in industry that might benefit from, for example, new methodologies to construct otherwise elusive materials and/or molecules. This might, in turn, benefit the European society as a whole, which naturally is in the interest of the European Union.
One example of future potential impact this chemistry holds is within the network materials. Because one can control the molecular setup in each dimension, it might be feasible to construct a material that is, for example, highly conducting in one dimension and very insulating in another dimension. Alternatively, one can imagine a material that be switched on and off with respect to conductivity simply by changing the surrounding chemical conditions, such as changing the solvent, the pH, the temperature, solution salts (if any present), L-type or X type ligands, or network concentration (if soluble).

Practical objectives and connection to the future
The research fellow took part of the Destination Europe Boston conference in February 2015. This was done in order to take advantage of his position as a European researcher with experience at a Boston based university (Harvard University). He therefore had the opportunity talk to people from that area, about the possibilities of working in Europe and differences between Europe and America. In addition, it was to seek out the conference as a potential platform for attracting, and ultimately hiring, PhD students and postdocs in the future.
Furthermore, by invitation the research fellow attended a PhD dissertation in Boston of a former Harvard University co-worker. Besides using this event as an opportunity to discuss chemistry with his former supervisor at Harvard University (Professor Theodore Alexander Betley), we are of the strong opinion that it is of his utmost professional interest to keep in contact with his American-based colleagues. Not only does this open up for Cross-Atlantic international cooperation, these people are also some of the absolutely most talented researchers in the world which provides the research fellow with a second-to-none network.
A more practical objective, but still major with respect to working time consumption, is the transfer of knowledge to Europe, more specifically the Technical University of Denmark, and setting up the necessary everyday laboratory requirements for this kind of chemistry. In this regard, it should be mentioned that designed homogeneous polynuclear cluster chemistry is a very young and skills-demanding research field and, therefore, entails an extremely steep learning curve. The timeline for this knowledge transfer and setting up the laboratory for homogeneous polynuclear cluster chemistry is therefore longer than one would usually expect for knowledge transferring.
It is, nevertheless, the ambition and immediate goal for the research fellow’s future career to implement this knowledge in Europe. He has therefore applied for a European Research Council Starting Grant together with the Technical University of Denmark based on this type of chemistry, for which the current status is that he is invited to the interview. Furthermore, he is in the process of applying for other grants as well. These include both national and international grants.

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