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

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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.