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Ligand Design for Nitrogen Activation

Periodic Reporting for period 1 - LiDeNiAc (Ligand Design for Nitrogen Activation)

Reporting period: 2018-06-01 to 2020-05-31

In this Marie Sklodowska Curie Individual Fellowship titled “Ligand Design for Nitrogen Activation” (LiDeNiAc) a novel class of phosphaalkene-based ligands for the preparation of electron-rich vanadium and molybdenum complexes should be synthesized. The MSC Researach Fellow conducting this research was particularly interested in advancing pyridine-based ligands with two phosphaalkene sub-units. Phosphaalkenes are systems with a PC double bond and therefore possess the ability to accept electron-density from highly reduced metal centers. Highly reduced metal centers on the other hand are necessary to facilitate the coordination and activation of atmospheric dinitrogen.
The use of N2 as a resource is of fundamental importance for society, as it is abundant and can be deemed renewable. Even though the Haber-Bosch process uses dinitrogen and hydrogen to yield ammonia, it is energy and resource intensive, while also producing greenhouse gases. It is therefore desirable to develop processes in which nitrogen is directly integrated into value-added chemicals, such as amines, pyridines or amides, all of which are important base-chemicals (usually derived from ammonia).
In this MSC action novel PDPa ligands were successfully synthesized, however they were found to be particularly sensitive to hydrolysis (towards reacting with residual water), and therefore, the direction of project was adjusted after the completion of work package 1. In the first phase the introduction of commercially available starting materials for the ligand design was tested, and it was found that in these cases three-membered phosphorus ring systems, so-called triphosphiranes, form selectively. Additionally, one of the starting materials used for the synthesis of PDPa ligands turned out to be a valuable catalyst for the reaction of alkynes with anilines, a so-called hydroamination reaction. These reactions are used in organic synthesis and play an important role in the design of novel drugs. We developed a system that works at low catalyst loadings, therefore contributing to the improvement of these hydroamination reactions. Overall the MSC fellow has shown that the proposed PDPa ligands can be synthesized and investigations of their coordination chemistry are still on-going. In addition, the impact of the action was extended into the field of hydroamination reactions and the selective synthesis of triphosphiranes is expected to allow the design of phosphide materials, used for example for the desulfurization of crude oil.

Summary of the context and overall objectives of the project (For the final period, include the conclusions of the action)
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Ligand design should be used as a tool to develop transition metal complexes for the activation and functionalization of atmospheric nitrogen. Formal objectives of this MSC Action have been to (a) develop synthetic strategies towards a variety of pyridine-diphosphaalkene (PDPa) ligands, (b) to study their coordination chemistry and (c) to investigate the nitrogen-binding towards vanadium and molybdenum complexes with PDPa ligands. The use of dinitrogen as a resource is of fundamental importance for society, as it is abundant (ca. 79% of our atmosphere) and can consequently be deemed renewable. It is therefore desirable to develop processes in which nitrogen is directly integrated into value-added chemicals, such as amines, pyridines or amides, all of which are important base-chemicals. Another important goal of this MSC IF was to foster the development and growth of the experienced researcher, through active participation in teaching activities, organization of workshops and outreach activities. The goals and objectives of this MSCA have been partially met, however, an efficient risk management helped to give the project a new direction and to enhance the overall impact of the action. A novel, selective and high-yielding route towards triphosphiranes was developed. PDPa ligands could be synthesized and their potential for the coordination, activation and functionalization of atmospheric dinitrogen is still investigated.
The work performed in this MSC action was divided in three work packages of equal length, with milestones as control points to evaluate the progress of the action and to re-adjust the direction of the research carried out at the end of each WP. The achievements of this action reflect that the experienced researcher greatly benefitted from the expertise at LIKAT, obtained effective training and in addition fostered national and international collaborations.
In the first work package a route to sterically demanding pyridine diphosphaalkene (PDPa) ligands was successfully developed. However, it was noted that these are highly susceptible towards air and moisture and therefore the planned work packages 2 and 3 were modified. Nevertheless, it was found that using the same reaction conditions to make PDPa could be adapted to selectively make triphosphiranes, a promising class of oligomeric cyclic phosphines. Therefore, the training, communication and dissemination plan could be met or exceeded.
The beginning of WP2 was used as checkpoint and it was determined that in light of the difficulties to isolate metal complexes of ligands made in WP1 it will be better to investigate the chemistry of triphosphiranes. Work Package 2 is therefore better described as “A selective route towards aryl-substituted triphosphiranes”. This also enabled the fellow to establish national and international collaborations. The work on the chemistry of triphosphiranes was published in Chemical Science. In addition, the chemistry of triphosphiranes was studied beyond their reactivity towards titanocene, and the results will soon be reported, extending the impact of the action. In work package 3 the potential of bisaminoterphenylphosphines as bulky ligands in gold-catalyzed hydroamination reactions was studied. In WP1 we found that bisaminoterphenylphosphines are valuable precursors for the synthesis of TerPCl2 (needed to make PDPa).
A novel, selective and high-yielding route towards triphosphiranes was developed. These three-membered phosphorus cycles have the potential to be used in the fabrication of single molecule precursors for the deposition of semiconductors, therefore potentially influencing the way we make electronics. This may result in specialized applications in the fabrication of displays and miniaturization in general. In addition adaption of the synthetic strategies used to access triphosphiranes to their heavier homologoues of arsenic and antimony pose the potential for accessing a variety of defined pnictide clusters, which can then be used in catalytic transformations, directly contributing to the principles of green chemistry and therefore, to a more sustainable society.
PDPa ligands could be synthesized and their potential for the coordination, activation and functionalization of atmospheric dinitrogen is still investigated. This continues to have the potential of uncovering a process that uses dinitrogen as a renewable resource under mild conditions in homogenous solution.
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