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.

Abstract

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.