A cobalt(II) coordination polymer-derived catalyst engineered via temperature-induced semi-reversible single-crystal-to-single-crystal (SCSC) dehydration for efficient liquid-phase epoxidation of olefins

Single-crystal-to-single-crystal (SCSC) transformations provide more avenues for phase transitions, which have piqued great interest in crystal engineering. In this work, a 3D Co(II)-based coordination polymer (CP), {Co2(OH2)8(btec)}·4H2O (1), (where (btec)4- = 1,2,4,5-benzenetetracarboxylate) undergoes SCSC transition upon heating at 180 °C to afford an anhydrous phase [Co2(btec)] (1'). Room-temperature water-vapour induced semi-reversible SCSC transformation of 1' involves condensation of two water molecules coordinating to the metal cluster, yielding a new framework [Co2(OH2)2(btec)] (2). These SCSC transitions were accomplished through a sequential bond breaking and new bond formation process which was accompanied by colour changes from orange (1) → violet (1') → pink (2). All materials were structurally elucidated by single-crystal X-ray diffraction (SCXRD) and further established by various analytical techniques. According to SCXRD data, all the frameworks possess octahedral geometries around the cobalt(II) sphere. SCXRD studies further revealed that 1 is a polymeric architecture with a binodal 4-c sql topology while 1' and 2 possess (3,6)-c kgd and (4,6)-c scu 3D nets, respectively. By virtue of multitopicity exhibited by the tetracarboxylate, the coordination number of the linker around the Co(II) sphere increased from four (in 1) to eight (in 1') and then decreased to six (in 2). Most interestingly, permanent porosity could be observed for the dihydrate 2, originated from potential void space as substantiated by dinitrogen (N2) sorption isotherm. These porous frameworks were active catalysts for the aerobic epoxidation of the model substrate cyclohexene using molecular oxygen (O2) as the final oxidant in the presence of the sacrificial i-butyraldehyde (IBA) reductant. For using the dihydrous phase 2, cyclohexene and various other olefins were catalytically oxidised to their corresponding epoxides with up to 38.5% conversion and 99.0% selectivity. The catalyst 2 can be expediently recycled in four runs without significant loss of activity. This research demonstrates that a little innovation in the active-site-engineered organic-inorganic hybrid materials can significantly enhance the catalytic performance and selectivity of coordination polymer-derived heterogeneous catalysts.

Enhancing Capacity and Stability of Anionic MOFs as Electrode Material by Cation Exchange

In this study we report on the characterization and use of the anionic metal-organic framework (MOF) JUMP-1, [(Me2NH2)2[Co3(ntb)2(bdc)]] n , alongside with its alkali-metal ion-exchanged analogs JUMP-1(Li) and JUMP-1(Na), as electrode materials for lithium and sodium batteries. Composite electrodes containing these anionic-MOFs were prepared and tested in 1 M lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in propylene carbonate (PC) and/or 1 M sodium TFSI (NaTFSI) in PC. We showed that the ion-exchanged materials JUMP-1(Li) and JUMP-1(Na) display higher capacities in comparison with the original as-prepared compound JUMP-1 (490 mA∙h∙g-1 vs. 164 mA∙h∙g-1 and 83 mA∙h∙g-1 vs. 73 mA∙h∙g-1 in Li and Na based electrolytes, respectively). Additionally, we showed that the stability of the electrodes containing the ion-exchanged materials is higher than that of JUMP-1, suggesting a form of chemical pre-alkalation works to stabilize them prior to cycling. The results of these studies indicate that the use of designed anionic-MOFs represents a promising strategy for the realization of high performance electrodes suitable for energy storage devices.