The synthesis of novel metal-organic frameworks (MOFs) is increasingly driven by the desire to develop robust, functional materials. In this context, MOFs based on the oxophilic group IV metal ions (e.g. Zr4+ and Ti4+), are receiving more and more attention. For Ti4+ however, few three-dimensional porous carboxylate frameworks have been reported to date due to the tendency of the Ti-alkoxide precursors to form ill-defined oxyhydroxides during synthesis. By using the hydrolytically more stable titanocene dichloride and trans-1,4-cyclohexanedicarboxylic acid (H2cdc), we were able to synthesize COK-69, a topological analogue of MIL-88 featuring an unprecedented Ti3(µ3-O)(COO)6O2(H2O) cluster. The photoactivity of COK-69 was investigated by proton-coupled electron transfer studies, EPR, DRS and molecular modeling, which revealed that up to one Ti4+ per cluster can be reduced to Ti3+ under UV irradiation in ethanol. Furthermore, due to the aliphatic nature of the cdc2- linker, COK-69 displays a distinct breathing behavior. By guest-induced switching between the axial a,a‘ and equatorial e,e‘ conformations, the framework adopts a closed or open pore form, respectively. A similar situation is encountered in the cdc-analogue of the archetypical Zr MOF UiO-66. The fully reversible breathing of this material between its guest-loaded crystalline state and its evacuated, nearly amorphous form has been studied by solid state NMR, total scattering analysis (PDF) and molecular modeling. From these data, we concluded that upon guest-removal only long-range crystalline order is lost, while the local structure is retained. The underlying mechanism behind this reversible amorphisation relies on two thirds of the cdc2- linkers to change from the elongated e,e'-conformation to the shorter a,a‘-conformation, which induces a tetragonal distortion of the initially cubic unit cell. However, this contraction occurs randomly along any of the three crystallographic axes, resulting in the formation of tetragonal nanodomains within the crystallites. The broad diffraction peaks of the empty material could be simulated by applying a cluster growth model, which revealed that the tetragonally distorted domains comprise approximately 5-7 unit cells.

Flexible Ti- and Zr-MOFs based on 1,4-trans-cyclohexanedicarboxylate linkers

251st National Meeting (2016)

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