Controlling phosphonate availability in amorphous metal(IV) phosphonates through structural modification

Metal(IV) phosphonates are a type of organic-inorganic coordination polymer material with promise for a variety of applications including catalysis, sorption, separations, proton exchange and energy storage. The extent of phosphonate coordination to the metal(IV) can vary in these materials, which has substantial implications on their functionality. This study demonstrates how the choice of tetravalent metal, the type of metal(IV) precursor, synthesis duration and synthesis temperature can all be used to control phosphonate availability, which was characterised via solid-state NMR of eight different amorphous metal(IV) phosphonate sorbents. It was found that Zr chloride (compared to Zr propoxide and Zr tert-butoxide) was the most effective Zr precursor for inducing a greater extent of phosphonate coordination. Increasing reaction time also increased phosphonate coordination, but increasing synthesis temperature had the largest impact. Changing the metal(IV) from Zr to Ti resulted in a substantially reduced reactivity with the phosphonate group. Sorbents prepared with Ti resulted in lower ligand content, a high proportion of free phosphonate environments, formation of anatase and even crystallisation of unreacted 2,6-bis(1,2,3-triazol1-ethanephosphonate)pyridine ligand, for which a novel crystal structure was identified. This work also reveals a novel means of assessing phosphonate availability using x-ray absorption spectroscopy (XAS), which opens new avenues for studying metal phosphonate materials given XAS is not restricted to the rigid measurement conditions of solid-state NMR. • Metal(IV) phosphonate synthesis conditions control phosphonate availability. • Low temperature, alkoxide precursors & short synthesis increase free phosphonate. • Zr(IV) reacts more effectively with phosphonate than Ti(IV). • Novel phosphonate crystal structure identified. • New XAS method to assess phosphonate availability developed.