Pyrrolidinium-Based Cyanides: Unusual Architecture and Dielectric Switchability Triggered by Order-Disorder Process.

Monika Trzebiatowska,Mirosław Mączka,Anna Gagor,Adam Sieradzki

doi:10.1021/acs.inorgchem.0c00637

Abstract

Two three-dimensional metal–organic compounds of the formula Pyr2KM(CN)6, where M = Co, Fe and Pyr = pyrrolidinium ((CH2)4NH2+), have been found to crystallize at room temperature in a monoclinic structure, space group P21/c. They are cyano-bridged compounds with an unprecedented type of architecture containing pyrrolidinium cations in the voids. The materials have been investigated by X-ray diffraction, dielectric, and spectroscopic methods as a function of temperature in order to determine their properties and the mechanism of the reversible phase transitions occurring at ca. 345–370 K. The phase transitions in both crystals are first order and are associated with a symmetry increase to a rhombohedral structure (space group R3®m) as well as a significant disorder of organic cations above Tc. On the basis of Raman scattering and IR spectroscopy it has been assumed that the phase transition in both crystals is triggered by thermally induced pseudorotation of the organic cation and large out-of-plane motions of its atoms followed by a “click-in” of the cyanide bridges. The materials have been proposed as possible switchable dielectrics due to their respective high differences in dielectric permittivities across the phase transition.

Highlights

The hybrid organic−inorganic perovskites based on diatomic and multiatomic bridges, which are built of MIIO6 or MIIN6 octahedra (MII = Cd, Mg, Zn, Mn, Co, Fe, Cu, Ni) connected by short organic linkers (for instance, HCOO−, N(CN)2−, and HPO2−), have been extensively
A2KCo(CN)[6] (A = methylammonium (MA), dimethylammonium (DMA), azetidinium (AZE), formamidinium (FM)) and eight A2KFe(CN)[6] (A = MA, DMA, trimethylammonium (TrMA), tetramethylammonium (TMA), guanidinium (GA), acetamidinium (Ace), imidazolium (HIm), trimethylammonsyiunmthesoizxeidde.15−((23CHT3h)e3NreOseHa,rchTroMnOM))A, cyanides TrMA, have TMA, been AZE, HIm, TrMO, and FM analogues has shown that these compounds exhibit single order−disorder phase transitions (PTs) accompanied by a distortion of the metal cyanide frameworks.[15−21] These transitions lead to steplike dielectric anomalies with a change in the real part of the dielectric permittivity ε′ that is typically below 10.15,19,21 Much larger changes, about 18 at 1 MHz and 30 at 2 MHz, were reported only for (Him)2KFe(CN)[6] and (MA)2KCo(CN)[6], respectively.[17,20]
Two metal−organic compounds of the formula [Pyr2KM(CN)6], where M = Co, Fe, have been found to crystallize in an unusual architecture, with channels expanding in the b direction filled with Pyr+ cations

Summary

Introduction

In the RT phase (I) the cations are ordered and anchored in the structure through N−H···N HBs to the cyanide nitrogen atoms. Such behavior in the frameworks based on cyanide ligands is related to structural changes starting from the phase where organic cations are frozen to the orientationally disordered phase, which leads to a significant change in the dielectric constant.[14,20−32] the difference in the observed frequency dispersions of 1Co and 2Fe indicates the disparate range of resonant frequencies attributed to the dynamic motion of the organic cation.

Results

Conclusion