Magnetic coupling of divalent metal centers in postsynthetic metal exchanged bimetallic DUT-49 MOFs by EPR spectroscopy

Abstract

EPR measurements at X- (9.5 GHz), Q- (34 GHz) and W-band (94 GHz) on paddlewheel (PW) type post-synthetic metal exchanged DUT-49(M,M): M- Zn, Mn, Cu MOFs are here reported (DUT–Dresden University of Technology). Temperature-dependent X-band measurements are recorded from T = 7 K to T = 170 K on monometallic DUT-49(Cu), DUT-49(Mn), and bimetallic DUT-49(Cu0.7Zn0.3), DUT-49(Cu0.5Mn0.5) MOFs. In the case of the CuII - CuII dimers in DUT-49(Cu), an isotropic exchange coupling of the metal ions (2 J = −240(11) cm−1) determined from the EPR intensity of the S = 1 spin state of the CuII–CuII dimers using the Bleaney Blowers equation. The sign of the found isotropic exchange coupling constant confirms an antiferromagnetic coupling between the cupric ions. Also, the MnII ions in the paddle wheels of DUT-49(Mn) are antiferromagnetically coupled. However, at low temperatures, EPR measurements reveal the presence of CuII and MnII monomers in DUT-49(Cu) and DUT-49(Mn), respectively, either associated with extra framework sites or defective paddle wheels. Otherwise, EPR signals observed for bimetallic DUT-49(Cu0.7Zn0.3) and DUT-49(Cu0.5Mn0.5) MOFs reveal the formation of mixed ion CuII–ZnII and CuII–MnII paddle wheels with SCuZn =1/2 and SCuMn = 2 spin states, respectively.

Mixed‐Valence CuI/CuIII Metal–Organic Frameworks with Non‐innocent Ligand for Multielectron Transfer

“…Figures 3e and 3f show normalized temperature‐dependent spectra of the two compounds when the temperature is decreased from 298 to 120 K. It is worth noting that Cu(I) (3d 10 ) and Cu(III) (3d 8 ), both in physical states, do not contribute to the X‐band EPR spectra (EPR silence) [22] . However, it is obvious from the spectra that as the temperature goes down, the signals become substantially stronger, suggesting the presence of Cu(II) (3d 9 ) ground state, with an electron spin S =1/2, interacting with the nuclear spin I Cu =3/2 of the 63,65 Cu isotopes [6d,23] . Figure S8 shows the experimental and simulated X‐band EPR spectra of NCKU‐52 and NCKU‐52‐Cu(iq) 3 at 77 K. For NCKU‐52, the parameters used for simulations in the spin Hamiltonian are g || =2.383 and $${g_ \bot }$$ =2.090, A || =126 G and $${{\rm{A}}_ \bot }$$ =20 G (33 %); g || =2.370 and $${g_ \bot }$$ =2.100, A || =148 G and $${{\rm{A}}_ \bot }$$ =40 G (67 %).…”

Mixed‐Valence CuI/CuIII Metal–Organic Frameworks with Non‐innocent Ligand for Multielectron Transfer

“…Figures 3e and 3f show normalized temperature‐dependent spectra of the two compounds when the temperature is decreased from 298 to 120 K. It is worth noting that Cu(I) (3d 10 ) and Cu(III) (3d 8 ), both in physical states, do not contribute to the X‐band EPR spectra (EPR silence) [22] . However, it is obvious from the spectra that as the temperature goes down, the signals become substantially stronger, suggesting the presence of Cu(II) (3d 9 ) ground state, with an electron spin S =1/2, interacting with the nuclear spin I Cu =3/2 of the 63,65 Cu isotopes [6d,23] . Figure S8 shows the experimental and simulated X‐band EPR spectra of NCKU‐52 and NCKU‐52‐Cu(iq) 3 at 77 K. For NCKU‐52, the parameters used for simulations in the spin Hamiltonian are g || =2.383 and $${g_ \bot }$$ =2.090, A || =126 G and $${{\rm{A}}_ \bot }$$ =20 G (33 %); g || =2.370 and $${g_ \bot }$$ =2.100, A || =148 G and $${{\rm{A}}_ \bot }$$ =40 G (67 %).…”

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 .…”