Dual-active Hf(iv)–organic framework for the detection of FOX-7 and as a heterogeneous catalyst for Knoevenagel condensation

Abstract

A functionalized Hf(IV)-based metal–organic framework (MOF) was synthesized, characterized, and successfully employed as a highly sensitive and selective fluorometric sensor for (1,1-diamino-2,2-dinitroethylene) FOX-7, which is a significant nitro explosive. The fluorescent sensor (MOF), denoted as CSMCRI-KNC′, exhibited a turn-off fluorescence signal towards FOX-7 in a water medium even in the presence of other potentially competitive explosives. The limit of detection (LOD) value for the detection of FOX-7 is fairly low (189 nM), and the response time is extremely rapid (<20 s). The sensing mechanism was investigated in detail and found to be based on non-covalent interactions, fluorescence resonance energy transfer (FRET), and photo-induced electron transfer (PET) processes. The sensing medium (water), detection time, sensitivity, and selectivity demonstrate the great potential of CSMCRI-KNC′ in the fluorescence-based sensor field. Additionally, the MOF was utilized as an efficient, recyclable, and heterogeneous catalyst for Knoevenagel condensation between barbituric acid and aromatic aldehydes in the presence of ethanol in an eco-friendly reaction medium. The present catalytic methodology has several attractive features, including mild reaction conditions, a short reaction time, high atom economy, and tolerance of various functional moieties. The catalyst could be recovered and reused successively without any remarkable loss in its catalytic activity. Moreover, the “hot filtration method (Sheldon's test)” has also been established to ensure the heterogeneity of the catalyst. Furthermore, the plausible mechanism of the catalysis is well presented.