The extra-framework sub-lattice of the metal-organic framework MIL-110: a solid-state NMR investigation

A changeable character: Differences in the dynamics of occluded moieties within the large pores (see graphic) of aluminium 1,3,5-benzene tricarboxylate framework solid MIL-110 are a function of the synthesis pH. Host-guest proton-transfer processes lead to a reversible change in the character of the framework from cationic to neutral, depending on the nature of the extra-framework moieties.The aluminium 1,3,5-benzene tricarboxylate framework solid MIL-110, which crystallises either at pH approximately 0 or at pH approximately 4, has been investigated by solid-state NMR spectroscopy. At pH approximately 0 the solid (MIL-110{pH0}) is the unique stable thermodynamic product, whereas at pH approximately 4 MIL-110{pH4} is observed as a kinetic product in competition with the MIL-96 phase. Diffraction studies and (27)Al NMR spectroscopy prove that the framework is identical in both cases. The nature and dynamics of occluded moieties within the large pores look quite different for the two compounds. MIL-110{pH4} and MIL-110{pH0} both show the presence of occluded 1,3,5-benzene tricarboxylate (btc), with additional nitrates and water molecules. However, the proportions of btc, nitrates and water are functions of the pH, leading to an identical framework and a quite different extra-framework. With the extra-framework moieties, the framework undergoes a proton transfer which is a function of the synthesis pH. Washing the MIL-110{pH0} phase with water produces a different extra-framework structure, richer in water and poorer in btc and nitrates. The hydroxyl groups of the inorganic aluminium cluster of the framework are involved in a proton transfer, which leads for all cases to a cationic framework and an anionic extra-framework. (1)H-(1)H DQ 2D NMR spectra (DQ=double quantum) give evidence for the interaction of extra-framework btc with the terminal water carried by Al(2,3) and shows their proximity to the closest hydroxyl groups. A structure for the non-diffracting extra-framework is proposed as the most plausible topology. It provides an efficient picture for the creation of many substituted MIL-110 compounds that would have a large number of applications.