Elucidating The Formation And Transformation Mechanisms Of The Switchable Metal-Organic Framework Elm-11 By Powder And Single-Crystal Epr Study

The effect of the synthesis conditions on the structure and guest-responsive properties of a "gate pressure" metal-organic framework (MOF) with composition [Cu(4,4'-bipy)(2) (BF4)(2)](n) (4,4'-bipy = 4,4'-bipyridine), also known as ELM-11 (ELM = elastic layer material) was investigated. Two different batches of ELM-11, synthesized from water- methanol and water-acetonitrile solutions, have been entirely characterized by PXRD, nitrogen (77 K) and carbon dioxide (195 K) physisorption, elemental analysis, DRIFT, TG, and SEM. Both ELM-11 samples were studied by electron paramagnetic resonance (EPR) spectroscopy in order to follow the change in the local structure of the copper ion during the activation and resolvation. Continuous wave X-band EPR measurements on powder samples provided an elongated octahedral coordination symmetry of the cupric ions and revealed different axial ligands in the as-synthesized and activated forms in both bulk samples of ELM-11. One of the procedures was amended in order to slow down the crystallization that allows isolation of single crystals of two polymorphic modifications of Cu-4,4'-bipyridine coordination polymers, namely [Cu(4,4'-bipy)(2)(CH3CN)(2)](BF4)(2) and [Cu-2(4,4'-bipy)(3) (CH3CN)(4)](BF4)(2), one of which shows a crystal structure similar to that of ELM-11. Further single-crystal EPR experiments on the as-synthesized material [Cu(4,4'-bipy)(2)(CH3CN)(2)](BF4)(2) revealed the orientation of the g tensor of the cupric ions and proved that layers of acetonitrile-synthesized ELM-11 are arranged perpendicularly to the crystallographic c axis.