Fluorescent heterometallic MOFs: Tunable framework charges and application for explosives detection

A series of three-dimensional fluorescent heterometallic MOFs based on rod- and cluster-like building blocks were readily prepared under solvothermal conditions. Among them, compounds 1-4 were constructed from rod-like building blocks {Cd-O-M}_n (M = Na, K, Na-K, and Mg) and BDC^2- ligands, featuring right-hand and left-hand channels with a cap topology. The framework charges of 1-4 can be tuned from negative to neutral via a strategy of changing the secondary metal centre (Na⁺, Na⁺-K⁺, K⁺ and Mg²⁺). It is worth noting that the rod-like building block of compound 2 contained three types of metal ion centres (Cd²⁺, Na⁺ and K⁺). Different from compounds 1-4, the topology of 5 changed from a cap net to a 6¹⁰ net in the presence of Ca²⁺ as a secondary metal centre. Interestingly, the anionic framework of 5 features two types of linear trinuclear clusters as the building blocks, i.e. {Cd₂CaO₁₇} and {Cd₂CaO₁₆}, which are relatively rare in known heterometallic open frameworks. Moreover, fluorescent compound 5 with a microporous structure displayed sensing properties towards some nitro-containing compounds, suggesting its potential application as a fluorescent sensor for the sensitive detection of explosives.