Synthesis and Characterization of Cu–Ni Mixed Metal Paddlewheels Occurring in the Metal–Organic Framework DUT-8(Ni_0.98Cu_0.02) for Monitoring Open-Closed-Pore Phase Transitions by X-Band Continuous Wave Electron Paramagnetic Resonance Spectroscopy

Abstract

A Cu2+-doped metal–organic framework (DUT-8­(Ni0.98Cu0.02), M2(NDC)2­DABCO, M = Ni, Cu, NDC = 2,6-napththalene dicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]­octane, DUT = Dresden University of Technology) was synthesized in the form of large (>1 μm) and small crystals (<1 μm) to analyze their switchability by X-band continuous wave (cw) electron paramagnetic resonance (EPR) spectroscopy. The large crystals are flexible and in a porous open pore (op) phase after solvation in N,N-dimethylformamide (DMF), but in the activated solvent-free form, a nonporous closed pore (cp) phase forms. EPR measurements of the rigid Ni-free DUT-8­(Cu) show a characteristic electron spin S = 1 room temperature signal of the antiferromagnetically coupled Cu2+-Cu2+ paddlewheel building unit of this metal–organic framework. None of the mixed metal DUT-8­(Ni0.98Cu0.02) materials showed comparable signals, indicating the absence of dimeric Cu2+-Cu2+ paddlewheel units in the materials. Instead, characteristic electron spin S = 3/2 signals are detected for all DUT-8­(Ni0.98Cu0.02) samples at temperatures T < 77 K, which can be assigned to ferromagnetically coupled mixed metal Ni2+-Cu2+ paddlewheel units. Those signals differ characteristically for the op and cp phase and enable monitoring the reversible op–cp transition during the de-/adsorption of DMF.

Towards modeling spatiotemporal processes in metal–organic frameworks

“…To characterize dynamic phenomena in MOFs, such as switching transitions and linker flipping, one often resorts to 129 Xe and 2 H NMR [110]. A less frequently used but sometimes valuable technique is electron paramagnetic resonance (EPR), which has been successfully applied to monitor phase transitions in MOFs [111].…”

A Ligand Field Molecular Mechanics Study of CO2‐Induced Breathing in the Metal–Organic Framework DUT‐8(Ni)

“…Our goal is to present simulations using the example of CO 2 ‐induced breathing in the pillared layer MOF DUT‐8(Ni) (Ni 2 (ndc) 2 (dabco), ndc = 2,6‐naphthalenedicarboxylate, dabco = 1,4‐diazabicyclo‐[2.2.2]‐octane, see Figure ) using an LFMM approach. Motivated by its outstanding properties, DUT‐8(Ni) is being investigated extensively recently . Some examples of the properties of interest are the enormous volume change upon opening (more than 140 %), its isomorphism, the property dependence on the metal center, and crystallite size effects .…”

New insights into solvent-induced structural changes of 13C labelled metal–organic frameworks by solid state NMR

“…Paramagnetic effects can thus not be the dominating source of the high isotropic chemical shift in the as made samples. This conclusion is supported by the absence of EPR signals, 35,40 calculations 46 predicting an antiferromagnetic coupling in the Ni 2 cluster, and the temperature dependence of the carboxylate 13 C NMR signal (Fig. S10, ESI †).…”

“…The flexibility of 1 was characterized from a fundamental point of view in several studies. 22,[32][33][34][35][36][37][38][39][40] In contrast, DUT-8(Ni) with crystal sizes below 1 mm, further denoted as compound 2, is rigid and remains in a metastable open pore phase even after guest removal ( Fig. S1-S3, ESI †).…”