Host-guest dynamics in copper-based metal-organic frameworks

[摘要] ENGLISH ABSTRACT: A vast number of metal-organic framework materials (MOFs) and structures have been reported in the literature. These materials are of interest as they exhibit a wide range of remarkable properties such as porosity, magnetism, chromism, thermal expansion and dynamic phase-change behaviour. However, many of the reports do not include investigations of all these possible properties, which provides an opportunity to revisit such materials. This study reports three sets of related MOFs that were studied in order to gain insight into their properties and dynamic host-guest interactions.In the first section three isoreticular copper(II)glutarate based 3D MOFs, whose pore dimensions vary according to the length of the 4,4´-bipyridyl linkers, allows exploration of the effect of increasing pore dimensions on the sorption behaviour of the frameworks. These MOFs capture CO2 under supercritical conditions and retain the gas under ambient conditions. The retention of the CO2 makes it possible to model the positions of the molecules within the channels of the frameworks using single-crystal X-ray diffraction analysis (SCXRD). Comparison of the CO2 gas sorption isotherms with the trend in thermal evolution of the CO2 adducts provided insight into the dynamics of the host-guest interactions within the MOFs. Theoretical models constructed from crystallographic data were used to calculate framework electrostatic potential maps through DFT and semi-empirical methods. The maps yielded a detailed picture of the electrostatic interactions prevailing at the sorption sites, which shows an electrostatic complementarity between the sorption sites and the CO2 molecules for two of the MOFs. The δ+ carbon of the CO2 molecules interacts with the δ– carboxylate moieties of the framework. Furthermore, both δ– oxygen atoms of each of the CO2 molecules interact with several δ+ hydrogen atoms of the framework. This electrostatic complementarity between CO2 and the sorption site results in a strong interaction, which stabilises the gas within these MOFs.The second set of MOFs studied comprises two 3D Cu(II)-glutarate-based MOFs with flexible linkers, [Cu2(glu)2(bpa)] and [Cu2(glu)2(bpp)], that undergo spontaneous phase changes upon solvent loss at room temperature. These MOFs are an extension of the isoreticular series of the previous section. Using SCXRD, we show that the phase changes result in new narrow-channel phases, with a large reduction in solvent accessible volume as compared with the original wide-channel phases. Moreover, the [Cu2(glu)2(bpa)] MOF displays a stepped sorption isotherm upon CO2 sorption at RT. This is indicative of the framework reverting to the wide-channel phase. The positions of the CO2 molecules in the channels of the frameworks were determined using SCXRD analysis of crystals exposed to supercritical CO2. Finally, a scan of the potential energy surface using molecular mechanics was conducted to elucidate the mechanism by which the phase change occurs. This appears to be a direct enantiomeric conversion of the glutarate ligands as a result of structural constraints.The third section describes a one-dimensional porous coordination polymer (PCP) [Cu2(acetate)2(dptz)]n•2CHCl3, that possesses pleochroic properties, i.e. the crystals appear to have different colours depending on their orientation. Solvent exchange with acetonitrile and nitromethane reveals that crystals of this PCP are also solvatochromic. This allows the solvent exchange process to be monitored using optical microscopy with polarised light. The experiment revealed an unexpected double V-shaped pattern in the crystals, which could be explained using an overlay of the measured crystal facets with the crystal structure, along with modelling of crystal surface adsorption at the different crystal facets. Furthermore, we identified that the crystals must be growth twins subject to the double re-entrant corner effect.

[发布机构] Stellenbosch University