Coordination-Chemistry in Metal-Organic Framework Materials
Open coordination site (OCS) on intersectional transition metal ion has shown considerable potential in its role for the applications in i.e., high ion transport and efficient gas adsorption. Thus, the activation of MOFs, the removal of precoordinated solvent molecules (typically used in during the synthesis) from the OCSs, has been thought as a prerequisite process for those applications. To date, several strategies for activating MOFs have been suggested: (i) thermal activation; (ii) freezing-drying; (iii) supercritical CO2 exchange; (iv) acid treatment. Among them, the thermal activation, which is normally performed by applying heat and vacuum, has been a unique way to remove the OCS-coordinated solvent molecule, whereas the other four methods are useful only for removing pore-filling solvents. Given that the high temperature (e.g., >150 °C) can lead to structural damage to MOFs, conceivable low-temperature process will be a considerably promising way, compared to the cumbersome, pricey heat-and-vacuum technique.
In this presentation, we will discuss how to control the activation condition for maintaining structural integrity during the activation process. Particularly, a new activation method, “chemical activation”–where methylene chloride (MC) plays a substantial role in removing both metal-coordinating and pore-filling solvents even at room temperature–will be shown. While MC treatment is surely a known method for replacing pore-filling solvent prior to thermal activation process (which is performed to lower the activation temperature), the role of MC for breaking the metal-solvent coordination has never been demonstrated. We observed that the chemical activation by MC arises via (i) replacement of pre-coordinated solvent by MC (the step before MC coordination) and (ii) spontaneous decoordination of MC (the step after MC coordination) with low activation energy which corresponds to room temperature.Also, we show that this low-temperature activation technique is more suitable for large-area MOF films.
Meanwhile, although this chemical activation method is substantially effective even in low-temperature, strongly coordinated solvents such as N,N-dimethylformamid (DMF), N,N-diethylformamid (DEF), and dimthylsulfoxide (DMSO) cannot be adaptable to this method because they are not readily replaced by MC which coordination is very weak. To exceed this limit in the chemical activation, we further developed an advanced chemical activation method, that is, “multistep chemical activation”. This process includes an intermediate coordination exchange step with moderate coordination bonding strength. More concretely, the process consists of (i) initial coordination exchange of strongly precoordinating DMF solvent molecules by other solvent molecules with moderate coordination strength such as methanol, ethanol, or acetonitrile, (ii) the exchange of the post-coordinating solvents by MC, and (iii) lastly, spontaneous decoordination of the MC molecule. With this strategy, we could completely remove the strongly coordinating DMF molecules from OCSs without structural damage.
Figure 1. Schematic representation for the “chemical activation” of open coordination site in a MOF, HKUST-1.