Probing Local Structural Changes at Cu²⁺ in a Flexible Mixed-Metal Metal-Organic Framework by in Situ Electron Paramagnetic Resonance during CO₂ Ad- and Desorption

Abstract

The flexible mixed-metal pillared-layered metal-organic framework (MOF) Zn1.9Cu0.1(BME-bdc)2(dabco) (BME-bdc2– = 2,5-bis­(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = diazabicyclo[2.2.2]­octane) with Cu2+ dopants as electron paramagnetic resonance (EPR) active probes was synthesized, and the guest-induced phase transition was studied by a newly developed in situ gas sorption EPR setup. Within the structure, the Cu2+ ions do not build any Cu2+–Cu2+ paddlewheels, and only mixed-metal Zn2+–Cu2+ paddlewheels were identified by EPR. Two distinct Zn2+–Cu2+ species (A and B) occur in the structure: Species A occurs in regions with low Cu2+ concentrations and has no adjacent mixed-metal paddlewheel units. Species B is present in domains with higher local copper concentrations, where adjacent mixed-metal paddlewheel units alter the EPR signal. The parent monometallic Zn MOF is known to undergo phase transitions from a narrow pore (np) to a large pore (lp) phase due to host-guest interactions with CO2. In situ EPR measurements during CO2 ad- and desorption at T = 195 K reveal the reversible transition from an np phase at low pressures to an lp phase at high pressures. The EPR experiments suggest an increase of the Cu2+ equatorial ligand field as well as a decrease of the rhombic distortion of the mixed-metal paddlewheel unit upon the CO2 adsorption triggered phase transition. The transition shows a large hysteresis typical for flexible materials, and the corresponding transition pressures are almost identical to those reported for the parent monometallic compound by gas adsorption isotherms. This result indicates that doping the monometallic material with a small amount of a Cu2+ probe does not change its responsiveness significantly and helps gather information on the local structure during the phase transition. The results demonstrate that the newly built in situ EPR setup is sensitive to small structural changes in the Cu2+ coordination environment induced by guest molecules which can hardly be resolved by other experimental methods.

Chemistry of Soft Porous Crystals: Structural Dynamics and Gas Adsorption Properties

“…Instead, in recent studies inhomogeneity is rationally introduced by mixing topologically related organic or inorganic building blocks to synthesize identical framework materials. Although this altered the guest‐responsive properties, little information was gained on the size, composition, and effects of domains that form within these multivariant SPCs . Beyond changes in composition and the presence of structural defects, the size and morphology of the crystals were found to strongly influence the behavior of SPCs.…”

Structural Transitions of the Metal–Organic Framework DUT-49(Cu) upon Physi- and Chemisorption Studied by in Situ Electron Paramagnetic Resonance Spectroscopy

“… 10 , 43 − 45 In particular, structural transitions in flexible MOFs such as MIL-53 and some others, were addressed. 28 , 46 − 51 Several promising applications of in situ EPR to MOFs were recently reported, 52 − 54 including a study of Cu-doped DUT-8(Ni). 47 DUT-49(Cu), to the best of our knowledge, was never studied using EPR.…”

“…EPR spectroscopy has not yet become a routine tool for studying MOFs; however, its applications in this field are rapidly developing. − Bare MOFs with paramagnetic metals, embedded or adsorbed radicals, were actively studied over the past decade. ,− In particular, structural transitions in flexible MOFs such as MIL-53 and some others, were addressed. ,− Several promising applications of in situ EPR to MOFs were recently reported, − including a study of Cu-doped DUT-8­(Ni) . DUT-49­(Cu), to the best of our knowledge, was never studied using EPR.…”

Die Chemie verformbarer poröser Kristalle – Strukturdynamik und Gasadsorptionseigenschaften

“…Stattdessen zielen einige Studien auf das rationale Einmischen von strukturell verwandten Liganden oder anorganischen Knotenpunkten in ein inhomogenes Kristallgitter ab. Während damit eine Modifizierung des gastresponsiven Verhaltens nachgewiesen werden konnte, ist wenig über die Größe, Zusammensetzung und Effekte der entstehenden Kristalldomänen bekannt . Zusätzlich zu Unterschieden in der Zusammensetzung und der Gegenwart von Defekten konnten die Größe und Morphologie als weitere Einflussparameter für das Verhalten von SPCs identifiziert werden.…”