A rational design and green synthesis of 3D metal organic frameworks containing a rigid heterocyclic nitrogen-rich dicarboxylate: structural diversity, CO2 sorption and selective sensing of 2,4,6-TNP in water.

A heterocyclic fluorogenic linker, thiadiazole dicarboxylate (H2tdz), offering diverse coordination modes has been utilized to synthesize two three-dimensional (3D) metal organic frameworks {[Cd2(tdz)2(4,4'-bpy)2]·6.5H2O}n (1) and [Cu(tdz)(4,4'-bpy)]n (2) from a one-pot self-assembly reaction under ambient conditions in high yields. MOFs 1 and 2 are structurally characterized by elemental analysis, FTIR and UV-Vis spectroscopy, single crystal and powder X-ray diffraction, and thermogravimetric analysis. Their single crystal molecular structures reveal that 1 contains a permanent pore along the b-axis with dimensions of 14.576 × 17.567 Å2 while 2 forms a 3D two-fold interpenetrated structure with an extensively reduced pore size. The effect of interpenetration has been further elaborated by a comparison of their CO2 uptake capacities at 263 K, 273 K and 298 K. The uptake value of CO2 for non-interpenetrated 1 (24.8 cm3 g-1) is much higher compared to that of interpenetrated 2 (5 cm3 g-1) at 298 K. Furthermore, to obtain an insight into the adsorbate-adsorbent interaction owing to the presence of the polar thiadiazole moiety and polarizable CO2 molecules, the isosteric heat of adsorption (Qst) was calculated for 1 (29.1 kJ mol-1) and 2 (23.4 kJ mol-1). Thermal and chemical stabilities of 1 in water were ascertained by variable temperature powder X-ray diffraction (VT-PXRD) analysis. MOF 1 having a d10 metal center was utilized to harness the availability of nitrogen atoms in the tdz2- linker for the selective sensing of nitrophenols in water with a detection limit for TNP as low as 1.4 ppm. Moreover, 1 is found to be stable and recyclable up to four cycles.