Insight into understanding dielectric behavior of a Zn-MOF using variable-temperature crystal structures, electrical conductance, and solid-state ¹³C NMR spectra

A Zn-based metal-organic framework (MOF)/porous coordination polymer (PCP), (EMIM)[Zn(SIP)] (1) (SIP^3- = 5-sulfoisophthalate, EMIM⁺ = 1-ethyl-3-methylimidazolium), was synthesized using the ionothermal reaction. The Zn²⁺ ion adopts distorted square pyramid coordination geometry with five oxygen atoms from three carboxylates and one sulfo group. One of two carboxylates in SIP^3- serves as a μ₂-bridge ligand to link two Zn²⁺ ions and form the dinuclear SBU, and such SBUs are connected by SIP^3- ligands to build the three-dimensional framework with rutile (rtl) topology. The cations from the ion-liquid fill the channels. This MOF/PCP shows two-step dielectric anomalies together with two-step dielectric relaxations; the variable-temperature single-crystal structure analyses disclosed the dielectric anomaly occurring at ca. 280 K is caused by an isostructural phase transition. Another dielectric anomaly is related to the dynamic disorder of the cations in the channels. Electric modulus, conductance, and variable-temperature solid-state ¹³C CP/MAS NMR spectra analyses revealed that two-step dielectric relaxations result from the dynamic motion of the cations as well as the direct-current conduction and electrode effect, respectively.