Synthesis and reactivity of novel molecules and solids derived from heavier group 15 cyanate analogues

Metal pnictides (compounds containing nitrogen, phosphorus, arsenic, antimony and/or bismuth) are a varied family of solids that adopt a wealth of structures and stoichiometries.[1,2] Amongst their many applications they have been employed as fumigants, pesticides, flares and, perhaps most notably, as semiconductors.[3] The diversity of their physical and chemical properties makes such compounds interesting for a range of technological applications. However, while many such species can be readily synthesised using a variety of methods, these often come at a high energetic cost. Moreover, our ability to access well defined nanoscale materials is still significantly limited by existing technological methods. This is the challenge we propose to address.
Our goal is to study the use of alternative pnictogen-containing reagents for the synthesis of novel molecules, clusters, nanoparticles and structured solids. With this in mind, we will explore the reactivity of heavier pnictogen-containing analogues of the cyanate anion, PnCO- (Pn = P-Bi), with transition- and post-transition metal salts. Preliminary studies have shown that such species readily undergo decarbonylation reactions acting as E- sources.
The 2-phosphaethynolate ion (PCO-) was first reported by Becker and co-workers in 1992 and was the subject of limited study until very recently.[8,9] In 2012, two separate reports by Grutzmacher and Cummins revived the interest in this remarkable anionic species.[10,11] More recently, we observed that PCO- can also be obtained by direct carbonylation of solutions of K3P7 in moderate to good yields.[12] A novel state-of-the-art synthesis of this anion was reported in 2014 allowing for the isolation of sodium salts in multi-gram quantities (> 100 grs.) starting from inexpensive precursors (Scheme 1).[13] We recently extended this research to afford the heavier arsenic-containing analogue (AsCO-) which can also be isolated in high yields from elemental arsenic and sodium metal.[14] These synthetic breakthroughs have transformed these anions from mere chemical curiosities to viable synthetic feedstocks for the synthesis of novel phosphorus- and arsenic-containing molecular species. That being said, due to their relative novelty, the use of these compounds for the synthesis of solids and molecular clusters remains largely unexplored.
Our goal is to carry out solution-phase metathesis reactions between heavier group 15 cyanate analogues (PnCO-) with a variety of metal salts (in the presence and absence of strongly coordinating ligands and/or surfactant molecules), with the aim of developing a novel bottom-up approach to metal pnictides. Our first objective is to carry out proof-of-concept reactions between alkali metal salts of PnCO- (Pn = P, As) with the lower halides of the group 13 elements (e.g. GaI) with the objective of identifying a novel route towards III/V semiconductor materials (such as GaP). III-V semiconductors such as GaAs, Ga1-xInxP, and InP are considered attractive candidates for optoelectronic devices due to their optimal band gaps and high absorption coefficients. In addition to photovoltaic applications,[15] they have been widely utilized in solid state lasers, LEDs, and optical waveguides.[16-19] The current availability of two heavier cyanate analogues also opens up the possibility of synthesising mixed pnictide species such as GaAs1-xPx, which is a compound of enormous industrial interest for alpha-numeric and graphical displays.
This project falls within the EPSRC Physical Sciences research theme (specifically the Chemical Synthetic Methodology area).