Monitoring and Understanding the Paraelectric–Ferroelectric Phase Transition in the Metal–Organic Framework [NH₄][M(HCOO)₃] by Solid‐State NMR Spectroscopy

Abstract

The paraelectric-ferroelectric phase transition in two isostructural metal-organic frameworks (MOFs) [NH4 ][M(HCOO)3 ] (M=Mg, Zn) was investigated by in situ variable-temperature (25) Mg, (67) Zn, (14) N, and (13) C solid-state NMR (SSNMR) spectroscopy. With decreasing temperature, a disorder-order transition of NH4 (+) cations causes a change in dielectric properties. It is thought that [NH4 ][Mg(HCOO)3 ] exhibits a higher transition temperature than [NH4 ][Zn(HCOO)3 ] due to stronger hydrogen-bonding interactions between NH4 (+) ions and framework oxygen atoms. (25) Mg and (67) Zn NMR parameters are very sensitive to temperature-induced changes in structure, dynamics, and dielectric behavior; stark spectral differences across the paraelectric-ferroelectric phase transition are intimately related to subtle changes in the local environment of the metal center. Although (25) Mg and (67) Zn are challenging nuclei for SSNMR experiments, the highly spherically symmetric metal-atom environments in [NH4 ][M(HCOO)3 ] give rise to relatively narrow spectra that can be acquired in 30-60 min at a low magnetic field of 9.4 T. Complementary (14) N and (13) C SSNMR experiments were performed to probe the role of NH4 (+) -framework hydrogen bonding in the paraelectric-ferroelectric phase transition. This multinuclear SSNMR approach yields new physical insights into the [NH4 ][M(HCOO)3 ] system and shows great potential for molecular-level studies on electric phenomena in a wide variety of MOFs.

Structural distortions in the high-pressure polar phases of ammonium metal formates

“…8 and 24) and [NH 4 ][M(HCOO) 3 ]. 2,16,25 The trend in T C is thought to arise from the variation in the hydrogen bonding strength that is linked to the degree of covalency in the M-O bonding, 15,24,26 as well as the mass and size of the M 2+ cation. 16 However, the observed spontaneous polarisation (P s ) values for [NH 4 ][M II (HCOO) 3 ] (M = Mn, Fe, Co, Zn) measured between 0.97-2.2 μC cm −2 could not be correlated with the M 2+ cation used (estimates of P s based upon the crystal structure resulted in a much narrower range of values: 0.94-1.03 μC cm −2 ).…”

Complete multinuclear solid‐state NMR of metal‐organic frameworks: The case of α‐Mg‐formate

“…There have been many recent reports that include the use of metal SSNMR to characterize MOFs . Although metal centers are ubiquitous within MOFs and should serve as a convenient spectroscopic handle for SSNMR characterization, many MOFs feature multiple non‐equivalent metal sites that reside in similar chemical environments, which give rise to overlapping signals and pose a severe obstacle to metal SSNMR characterization.…”

“…Research in our group has been focused on MOF characterization by multinuclear NMR in several areas: interrogating the metal centres, probing linker atoms, monitoring the behavior of guest molecules in the MOFs, and understanding interactions between guest species and the framework . In this article, we aim to use the α‐Mg‐formate (Mg 3 (HCOO) 6 MOF as an example to illustrate the power of SSNMR for MOF characterization, as it features metal ( 25 Mg), linker ( 1 H, 13 C, 17 O), and guest ( 13 C, 2 H) isotopes that are all NMR‐active.…”

Looking into Metal-Organic Frameworks with Solid-State NMR Spectroscopy

“…Very recently, first studies of 67 Zn and 25 Mg centers have also been described [24][25][26][27][28][29]. In these studies, 67 Zn and 25 Mg NMR spectroscopy mostly served as a tool for the verification of structural models of MOFs, which had been proposed by X-ray diffraction analyses.…”