Energetics of formation and hydration of a porous metal organic nanotube

Hybrid materials, such as metal organic nanotubes (MON), are of interest because of their chemical tunability and permanent porosity. While an increasing number of compounds is being reported, very little is known about their thermodynamic stability. Herein, the energetics of a MON, (C₄H₁₂N₂)_0.5[(UO₂)(H_ida)(H_2ida)]·2H₂O (UMON, C₁₀H₂₁N₃UO₁₂) (ida = iminodiacetate), that possesses unique water exchange and uptake has been investigated by acid solution calorimetry, thermal analysis, and water adsorption calorimetry. The enthalpy of formation of UMON, C₁₀H₂₁N₃UO₁₂ (ΔH_f,rxn), from the dense components (uranium oxide (UO₃), piperazine (C₄H₁₀N₂), and iminodiacetic acid (C₄H₇NO₄) was ∼55.3 ± 0.9 kJ/mol, which was similar to values for other metal organic framework materials. The dehydration enthalpy to form an anhydrous UMON and gaseous H₂O at 37 °C from thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) experiments was 57.8 ± 1.9 kJ/mol of water. This value is somewhat higher than the vaporization enthalpy of water (44 kJ/mol) and suggests modest bonding interactions of H₂O with the inner walls of the nanotubes. Water adsorption calorimetry of (C₄H₁₂N₂)_0.5[(UO₂)(H_ida)(H_2ida)]·2H₂O indicated that the water molecules are confined inside the UMON material in two thermally distinct positions. The ice-like arrangement of the confined water molecules inside the nanotube impacts the energetics of the material and adds to the stabilization of the structure.