Mixed-Metal Ni²⁺–Mn²⁺ Paddle Wheels in the Metal–Organic Framework DUT-8(Ni_1–xMn_x) as Electron Paramagnetic Resonance Probes for Monitoring the Structural Phase Transition

Abstract

The metal–organic framework (MOF) DUT-8­(Ni) [Ni2(2,6-ndc)2(dabco), 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo­[2,2,2]­octane] is an important representative of flexible pillared layer MOFs, providing improved performance as an adsorbent in gas delivery and separation applications. To enable electron paramagnetic resonance (EPR) investigations, DUT-8­(Ni) was doped with Mn2+, and two variants of the compound were investigated, which differ in crystal size and flexibility. For the rigid variant, an intense EPR signal was observed, which could be assigned to the electron spin S = 3/2 state of Ni2+–Mn2+ paddle wheel (PW) units. The EPR characteristics of this magnetic Ni2+–Mn2+ PW probe open up a considerably larger temperature range for EPR-detected adsorption investigations, compared to the previously studied Cu2+ and Co2+ doping as paramagnetic probes. As a proof of concept, we measured the EPR signal of Ni2+–Mn2+ PWs of the flexible variant in situ during CO2 adsorption and desorption at 195 K to monitor the framework responsiveness of this material. In comparison to its rigid variant, the Ni2+–Mn2+ EPR signal was less intense here since a smaller amount of Mn2+ ions is incorporated into the large crystals of the flexible DUT-8 phase. However, the EPR sensitivity of the Ni2+–Mn2+ PWs was still sufficient to probe in situ the CO2 adsorption-triggered phase transition of the flexible DUT-8 material.

Understanding MOF Flexibility: An Analysis Focused on Pillared Layer MOFs as a Model System

“…Low-temperature EPR experiments [143] [65,146] On the other hand, neither Ni 2 + -Co 2 + nor Ni 2 + -Mn 2 + PW EPR signals could be observed for the cp phase. Taking advantage of this peculiarity, the intensity of the ground op/np Zn 2 + Cu 2 + 1/2 DUT-8(Ni 1À x /Co x ) [143] op Ni 2 + Co 2 + 1/2 DUT-8(Ni 1À x /Mn x ) [146] op Ni 2 + Mn 2 + 3/2 DUT-8(Ni 1À x /Cu x ) [147] op/cp Ni 2 + Cu 2 + 3/2 ). [143] Copyright 2020, American Chemical Society.…”

“…[146] Substitution of one of two nickel ions by another divalent paramagnetic TMI leads to either ferro-, or antiferromagnetically (af) coupled metal ion pairs. Low-temperature EPR experiments [143] [65,146] On the other hand, neither Ni 2 + -Co 2 + nor Ni 2 + -Mn 2 + PW EPR signals could be observed for the cp phase. Taking advantage of this peculiarity, the intensity of the ground op/np Zn 2 + Cu 2 + 1/2 DUT-8(Ni 1À x /Co x ) [143] op Ni 2 + Co 2 + 1/2 DUT-8(Ni 1À x /Mn x ) [146] op Ni 2 + Mn 2 + 3/2 DUT-8(Ni 1À x /Cu x ) [147] op/cp Ni 2 + Cu 2 + 3/2 ).…”

“…The corresponding signals of Ni 2+ /Co 2+ and Ni 2+ /Mn 2+ PWs in DUT‐8(Ni 0.98 Co 0.02 ) and DUT‐8(Ni 0.997 Mn 0.003 ) loaded with DCM and CO 2 , respectively, reveal an axial symmetry of these structural units, indicating an axially symmetric 2,6‐ndc linkers coordination to the PWs. Otherwise, in the case of DMF‐loaded samples, with a 2,6‐ndc linkers conformation displaying coordination with lower symmetry to the PWs, the corresponding EPR signals indicate an orthorhombic distortion of the local axial symmetry of the PWs [65, 146] …”

Understanding MOF Flexibility: An Analysis Focused on Pillared Layer MOFs as a Model System

“…On the other hand, neither Ni 2+ ‐Co 2+ nor Ni 2+ ‐Mn 2+ PW EPR signals could be observed for the cp phase. Taking advantage of this peculiarity, the intensity of the ground state EPR signal of the two mixed metal PWs has been used as a measure for the op/cp phase ratio in in situ EPR adsorption experiments on DUT‐8(Ni 1− x Co x ) (0.02 ≤ x ≤ 0.75) and DUT‐8(Ni 0.997 Mn 0.003 ) [143, 146] . Otherwise, the introduction of Cu 2+ into DUT‐8 leads to the formation of fe coupled Ni 2+ ‐Cu 2+ PWs having S total =3/2 as verified for DUT‐8(Ni 0.98 Cu 0.02 ) [147] …”

Monitoring the Local Structure and Magnetic Properties of the Dinuclear Cu2-Paddle Wheel Nodes in the Mesoporous Metal–Organic Framework, DUT-49(Cu), upon Adsorption-Induced Breathing Transitions

“…Therefore, we may expect a potential hydrogen bond formation between the hydrogen atoms of the organic linker in the MOF framework and the adsorbed Xe molecules. By looking for alternatives to unravel the structural details of the xenon-loaded DUT-49­(Cu), it is noticeable that the metal complexes in DUT-49­(Cu) consist of two antiferromagnetically (AFM) coupled Cu II ions in the so-called paddle-wheel (PW) unit leading to an excited electron spin state with total spin S = 1. − The characteristic magnetic signature of such PWs in the framework can be measured by electron paramagnetic resonance (EPR) spectroscopy, − and it can be used as a probe to interrogate the local structural changes of the PW units upon gas adsorption. , Specialized in situ EPR experiments during gas adsorption have been conducted only by a few groups so far. ,− In a recent study, the magnetic signature of the copper PW was used to monitor with in situ EPR the structural transitions in DUT-49­(Cu) induced by n -butane and diethyl ether at T = 298 K . In the present work, we used likewise in situ EPR spectroscopy to explore the xenon adsorption and desorption in this MOF from the local view of the PWs at T ≈ 156 K. It turns out that the experimentally derived zero-field splitting (ZFS) of the PWs behaves differently during xenon adsorption at T ≈ 156 K than it was observed for n -butane and diethyl ether adsorption at T = 298 K .…”