Beyond cyanide: Future synthons based on the cyaphide and cyarside ions for the synthesis of designer magnetic coordination polymers

The serendipitous discovery of Prussian blue by Diesbach in 1706 had an immediate revolutionary effect on the art world, and would go on to have long-lasting repercussions in many other areas of the arts and sciences. This inexpensive, non-toxic, dark blue pigment democratized the colour blue, and features in prints and paintings from artists such as Hokusai and van Gogh. Interestingly, the versatility of this compound transcends the arts, and in contrast to most other pigments, Prussian blue has also found important medical applications. Its ability to incorporate monocationic ions into its lattice makes it a powerful sequestering agent for toxic heavy metals; pharmaceutical-grade Prussian blue is used to treat individuals who have ingested poisonous thallium or radioactive caesium.

Prussian Blue has a deceptively simple framework-like structure, consisting of iron metal centres (in the +2 and +3 oxidation states) that are bridged by cyanide anions along all three Cartesian axes, giving rise to an extended face-centred cubic (FCC) lattice. The modular nature of this structure allows for the variation of the metal ions, their charges, and consequently the number of unpaired electrons per metal site. By altering these variables, it is possible to access novel materials with a wealth of magnetic behaviours, all of them facilitated by strong intermetallic coupling through the cyanide ions. These structurally-related coordination polymers are commonly referred to as Prussian blue analogues (PBAs), and have found use as designer magnetic materials and, more recently, in the fields of gas sorption, hydrocarbon separation, water oxidation catalysis, and as battery components.
While PBAs can be readily altered by changing the nature of the metal centres, the cyanide ion is a common building block to all of them. Attempts have been made to exploit other types of highly-conjugated cyanide-containing bridging units for the synthesis of solids, however empirically it appears that intermetallic electronic communication is most pronounced for systems based on the cyanide ion. This is in part due to the short metal-metal distances available in such compounds, but also undoubtedly related to the more densely packed and ordered FCC lattice that can be achieved with a short linear bridge. This limits further developments in the field, and places a significant design constraint on the synthesis of more complex PBAs.

The goal of this project is to access novel materials related to PBAs by making use of valence isoelectronic ligands related to cyanide, such as the cyaphide and cyarside ions (where the nitrogen atom of cyanide is replaced by a phosphorus or arsenic atom, respectively). Like their lighter congener cyanide, the cyaphide and cyarside ions are ambidentate ligands capable of giving rise to close-packed structures related to Prussian blue. To date, there is no known synthetic methodology that allows for the isolation of such ions. This is the challenge we propose to address. The preference of phosphorus and arsenic for binding to heavy metals will be exploited in order to target mixed metal systems containing elements from the entirety of the periodic table. We will also extend these studies to access heteroleptic phosphorus- and arsenic-containing analogues of cyanide-based linkers such as dicyanamide and tricyanomethanide.

This proposal aims to design, synthesise and study and entirely new class of extended solids related to Prussian blue with the objective of accessing new materials with potentially transformative physical and chemical properties.

The main objective of this proposal is to generate a new family of (until now elusive) chemical reagents and to employ them for the design of chemical compounds with bespoke physical and magnetic properties. While the proposed research area is exploratory in nature, it will ultimately mature to yield results that will have an impact far beyond chemical synthesis. By analogy with the archetypal compound that serves as an inspiration for this proposal, Prussian blue, the extended solids we are targeting may have applications in a number of technologically relevant areas such as in the design of magnetic materials, sensing, catalysis, gas sorption and hydrocarbon separation, and in energy storage.

Academic: We are in a position to develop an entirely new family of materials with a myriad of important technological applications. The results of this research programme will be published in high-impact peer-reviewed journals with a global readership (Nature, Science, J. Am. Chem. Soc. etc.) and disseminated at leading international conferences. A strong emphasis will be placed on building national and international networks with researchers in the U.K. and abroad. All publications will be available in open-access journals and/or deposited in Oxford University's Research Archive (https://ora.ox.ac.uk/).

Societal: This research programme has the potential to yield a new family of technologically relevant solids and as such will have an impact on the U.K. technological and manufacturing sectors, and by extension U.K. plc., delivering employment opportunities, a more prosperous economy, and a healthier and wealthier society.

Financial: Impact on the commercial sector will be ensured by existing frameworks available at the University of Oxford including Oxford University Innovation Ltd. (https://innovation.ox.ac.uk/), the University of Oxford's technology transfer organization, who will assist with the management and exploitation of any intellectual property that arises from this research. Oxford University Innovation manages the University's intellectual property portfolio, working with University researchers on identifying, protecting and marketing technologies through licensing, spin-out company formation, consulting and material sales.

Staff development: This research will allow the PDRA to develop crucial analytical, experimental and inter-personal skills that will equip them to become a future research leader. At the same time it will permit the PI to consolidate his reputation as a leading figure in inorganic chemistry and facilitate the attainment of future research funding from national, European and international sources.

Finally, we have no doubt that this proposal will also have a significant impact where it truly matters, in the understanding and development of new, unexplored areas of chemistry. The potential societal impact of fundamental research is difficult to assess, and thus often overlooked. However, the expansion of our collective scientific knowledge is the only way we will overcome future societal challenges.

All beneficiaries will have access to details of the research via publically available website
(http://research.chem.ox.ac.uk/jose-goicoechea.aspx; http://goicoechea.chem.ox.ac.uk/) where developments, breakthroughs and publications will be openly and freely discussed.