Structure–Activity Relationship Insights for Organophosphonate Hydrolysis at Ti(IV) Active Sites in Metal–Organic Frameworks

Abstract

Organophosphorus nerve agents are among the most toxic chemicals known and remain threats to humans due to their continued use despite international bans. Metal-organic frameworks (MOFs) have emerged as a class of heterogeneous catalysts with tunable structures that are capable of rapidly detoxifying these chemicals via hydrolysis at Lewis acidic active sites on the metal nodes. To date, the majority of studies in this field have focused on zirconium-based MOFs (Zr-MOFs) that contain hexanuclear Zr(IV) clusters, despite the large toolbox of Lewis acidic transition metal ions that are available to construct MOFs with similar catalytic properties. In particular, very few reports have disclosed the use of a Ti-based MOF (Ti-MOF) as a catalyst for this transformation even though Ti(IV) is a stronger Lewis acid than Zr(IV). In this work, we explored five Ti-MOFs (Ti-MFU-4l, NU-1012-NDC, MIL-125, Ti-MIL-101, MIL-177(LT), and MIL-177(HT)) that each contains Ti(IV) ions in unique coordination environments, including monometallic, bimetallic, octanuclear, triangular clusters, and extended chains, as catalysts to explore how both different node structures and different linkers (e.g., azolate and carboxylate) influence the binding and subsequent hydrolysis of an organophosphorus nerve agent simulant at Ti(IV)-based active sites in basic aqueous solutions. Experimental and theoretical studies confirm that Ti-MFU-4l, which contains monometallic Ti(IV)-OH species, exhibits the best catalytic performance among this series with a half-life of roughly 2 min. This places Ti-MFU-4l as one of the best nerve agent hydrolysis catalysts of any MOF reported to date.