Probing the Dielectric Transition and Molecular Dynamics in the Metal–Organic Framework [(CH₃)₂NH₂]Mg(HCOO)₃ Using High Resolution NMR

Abstract

We present a detailed multinuclear (13C, 15N, 25Mg) NMR study on a metal–organic framework material with a dimethylammonium (DMA+) cation, [(CH3)2NH2]­Mg­(HCOO)3, that shows a dielectric phase transition at 270 K. The mechanism underlying this phase transition is not fully understood as there are contrasting reports attributing the phase transition to the order–disorder dynamics of the dimethylammonium cation in the cavity or to the contraction of the metal formate framework. In this work, we use high-resolution solid-state NMR to understand the mechanism of this phase transition by analyzing the motional dynamics of nitrogen and carbon. Spin–lattice relaxation time (T 1) and BPP theory yield the correlation time for nitrogen hopping of 10–9 s and activation energy of 28.62 kJ/mol. The T 1 behavior and the chemical shifts of 13C and 15N show that the phase transition has both order–disorder and displacive characteristics. 25Mg NMR spectra show a change in line width with the change in DMA+ cation dynamics, but there is no change in its chemical shift implying that its local environment remains the same.

More complex than originally thought: revisiting the origins of the relaxation processes in dimethylammonium zinc formate

“…Indeed, in combination with a wide selection of metal centers, e.g., Zn, Mn, Cu, and Ni, dozens of scientific studies reporting different aspects of DMA + cation dynamics have emerged, and their number is still growing. 2,3,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] As a part of our ongoing interest in the mechanistic aspects of phase transitions in metal formates in this work we revisit 3D zinc formate hosting DMA + cations, i.e. [(CH 3 ) 2 NH 2 ][Zn(HCOO) 3 ] (DMAZn).…”

“…24 The molecular mechanism of this phase transition has been extensively studied using a number of experimental methods such as differential scanning calorimetry (DSC), Raman spectroscopy, X-ray diffraction (XRD), broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies. 2,3,14,19,21,22,[24][25][26][27][28] Current consensus is that the low temperature phase of this compound crystallizes in the non-centrosymmetric, monoclinic Cc phase, whereas the high-temperature phase is of trigonal symmetry (space group R% 3c) and shows dynamic disorder of organic cations. Below the temperature of phase transition, the flipping of the DMA + cation between three equivalent positions was supposed to disappear, leading to a complete ordering of these cations.…”

Elastic Properties and Energy Loss Related to the Disorder–Order Ferroelectric Transitions in Multiferroic Metal–Organic Frameworks [NH4][Mg(HCOO)3] and [(CH3)2NH2][Mg(HCOO)3]

“…The ferroelectric transition temperatures of [NH 4 ][Mg(HCOO) 3 ] and [(CH 3 ) 2 NH 2 ] [Mg(HCOO) 3 ] are around 255 K and 267 K, respectively, close to room temperature, leading to promising applications. Their synthesis, structure analysis, thermal properties, dielectric properties, IR studies and Raman studies have been well studied [13][14][15][33][34][35][36][37][38]57,58]. However, their elastic properties have not been reported.…”

The Journal of Physical Chemistry C Virtual Special Issue on Advanced Characterization by Solid-State NMR and In Situ Technology and in Recognition of Michael Hunger’s 65th Birthday

“…The Mali group reported paramagnetic shifts that were correlated with the hyperfine coupling constants in paramagnetic solids such as Cu-MOFs . The Fu group used high-resolution solid-state NMR to understand the mechanism of the dielectric transition and molecular dynamics of the MOF–dimethylammonium host–guest system . Copéret and coworkers achieved surface dynamic nuclear polarization (DNP) enhancements of 40 on a 600 MHz NMR, corresponding to 1600-fold experimental time savings for Ziegler–Natta catalyst MgCl 2 supports .…”

“…2 The Fu group used high-resolution solid-state NMR to understand the mechanism of the dielectric transition and molecular dynamics of the MOF−dimethylammonium host−guest system. 3 Copeŕet and coworkers achieved surface dynamic nuclear polarization (DNP) enhancements of 40 on a 600 MHz NMR, corresponding to 1600-fold experimental time savings for Ziegler−Natta catalyst MgCl 2 supports. 4 Solid-state NMR was used by the Dubini group to describe the hydrationinduced structural transitions in biomimetic tandem repeat proteins and the cis−trans isomerization of groups at the water−protein interface.…”