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

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(Cu 0.7 Zn 0.3 ), DUT-49(Cu 0.5 Mn 0.5 ) MOFs. In the case of the Cu II - Cu II 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 Cu II –Cu II 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 Mn II ions in the paddle wheels of DUT-49(Mn) are antiferromagnetically coupled. However, at low temperatures, EPR measurements reveal the presence of Cu II and Mn II 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(Cu 0.7 Zn 0.3 ) and DUT-49(Cu 0.5 Mn 0.5 ) MOFs reveal the formation of mixed ion Cu II –Zn II and Cu II –Mn II paddle wheels with S CuZn = 1/2 and S CuMn = 2 spin states, respectively.


I. INTRODUCTION
Owing to the potential applications such as gas storage and separation, catalysis, heat storage, liquid purification, supercapacitors, drug delivery and dielectrics, metal-organic frameworks (MOFs) have attracted much attention in the present decade. 1,2Among MOFs, DUT-49 (DUT-Dresden University of Technology) has gained much attention due to its breathing effect on gas adsorption 3 and negative gas adsorption 4 (NGA).Recently, NuMOF technologies first commercialized a MOF product for toxic gas storage. 5urthermore, understanding the magnetic properties of MOFs can help drive innovation in the field of molecular magnetism. 6Electron paramagnetic resonance (EPR) spectroscopy is a unique tool to investigate the local geometric and electronic structure as well as the magnetic properties of paramagnetic ions in MOF materials. 1,3,7PR has been employed on MOF materials to understand catalytic properties, 1 intra-and intermolecular interactions, 8 structural changes upon gas 3 and liquid adsorption, magnetic properties, 7,8 post-ion exchange modification, 9 and photochromism due to UV laser irradiation. 10Moreover, a series of paddle wheel (PW) based M II -M II dimers were investigated by EPR in DUT-49(Cu) upon in situ n-butane and Et 2 O adsorption, 3 DUT-8(Ni 1−x Mnx) upon CO 2 adsorption, 11 DUT-8(Ni) upon N 2 and NO adsorption, 12,13 post ion exchange modified FexCu (3−x) (BTC) 2 9 and parent Cu 3 (BTC) 2 7 upon nitroxide radical adsorption, 14 and bimetallic ZnxCu (3−x) (BTC) 2 15 upon olefin adsorption. 16n this work, we investigated the magnetic coupling of monoand binuclear Cu II , Zn II , and Mn II dimer centers in the PW units of the DUT-49(M,M) MOFs (Figure S1).The temperature-dependent EPR data at the X-and Q-band frequencies evidence the excited state antiferromagnetic exchange interaction of the metal dimers upon increasing temperature.

II. EXPERIMENTAL TECHNIQUES
See supplementary material Sec.S2 for the synthesis 17 and EPR instrumentation part.

III. RESULTS AND DISCUSSION
Temperature-dependent CW-X band EPR measurements were performed in the range 7 K < T <170 K for four MOFs, illustrated in Figs.1(a Where the first term corresponds to the Zeeman splitting between electron spin S = 1/2 of Cu II ion and applied magnetic field B (β e -Bohr magneton, ĝg tensor), and the second term represents the Cu II electron-nuclear hyperfine (hf) interaction ( Â -hyperfine splitting (hfs) tensor and I Cu = 3/2 nuclear spin of 63,65 Cu).The simulated spin Hamiltonian parameters of Cu II monomer [Fig.2(a)] for the monometallic DUT-49(Cu) are gxx = 2.052(3), gyy = 2.060(2), gzz = 2.335(4), Axx,yy = 30(5) MHz, Azz = 545(8) MHz which could be assigned to the defective Cu-Cu paddlewheel units or extra framework cupric ions. 7,9he spectral features illustrated in Fig. 1 2) from the parent sample or defects. 12The total spin states of Cu II -Cu II , Cu II -Zn II , Cu II -Mn II , and Mn II -Mn II metal dimers in the PW units at a temperature of 7 K (LS-low spin) and 160 K (HS-high spin) are mentioned in Table S1 (supplementary material).The Cu II -Cu II dimers in the PW units are well separated by the long linker (H 4 BBCDC), which prevents the inter dimer exchange interactions.Moreover, an EPR signal intensity of the S = 1 spin state of these magnetically coupled Cu II -Cu II dimers in the PW units, which is proportional to the magnetic susceptibility, was extracted from temperature-dependent X-band EPR data for all MOFs [Fig.2(d)].The isotropic coupling constant of the Cu II -Cu II dimer can be estimated using the Bleaney-Bowers susceptibility equation of exchange coupled identical dimer species with S 1 = 1/2 and S 2 = 1/2 Here I EPR is the EPR intensity extracted from the double integration of B xy2 and B z2 parts of the S = 1 signal, kB is the Boltzmann's constant, and 2J is the isotropic coupling constant.The 2J -value is found to be −240( 11 the S = 0 singlet state corresponds to the ground state.Such an antiferromagnetic (AFM) exchange coupling is the characteristic behavior of Cu II -Cu II PW species. 3,7,19igure 1(c) shows the temperature-dependent X-band EPR spectra of DUT-49(Mn).The low-temperature spectrum at T = 7 K shows a signal at g = 2.007 with a resolved hfs into six lines [Fig.S2(a)].The g-value and the hfs sextet are characteristics of isolated Mn II ion 20 with an S = 5/2 high spin state and a hyperfine interaction (hfi) with the 55 Mn nucleus having I Mn = 5/2 characterised by an isotropic hyperfine coupling constant of Aiso = 240 MHz.The spectrum exhibits only the central MS = 1 /2 ↔ − 1 /2 spin transitions, whereas the outer MS = ± 1 /2 ↔ ±3/2 and MS = ±3/2 ↔ ±5/2 transitions are not resolved presumably because of large D strain effects.For spectra recorded at T > 30 K [Figs.1(c) and S2(a)], a new multiline spectrum emerges, covering a broad field range of ∼150 mT < B < ∼550 mT at X-band frequencies.The intensity of this spectrum increase with rising temperatures.A comparable behavior is observed in the temperature-dependent Qband spectra of DUT-49(Mn) [Fig.3(a)].Based on the multiline characteristic and temperature dependence, we may assign this spectrum to AFM-coupled Mn II -Mn II dimers.In this case of AFM coupled Mn II dimers, we expect total spin states S = 0, 1, 2, 3, 4, and 5 (S = 0 singlet ground state), which is increasingly populated with rising temperatures.However, the poor resolution of the X-and Q-band spectra prevented a determination of the Spin Hamiltonian parameters of the Mn II -Mn II dimers here by simulation or fitting procedures.CW W-band spectra did not provide a better resolution and suffered from poor signal-to-noise ratios.
Meanwhile, the low-temperature X-band [Figs.1(d) and S2(b)] and Q-band [Fig.3(b)] EPR spectra of DUT-49(Cu 0.5 Mn 0.5 ) show the coexistence of Cu II and Mn II monomer species with the emergence of Cu II -Cu II dimer as well upon increasing temperature above 80 K. Some characteristic spectral features of the Mn II -Mn II dimers at about ∼1450 mT, ∼1200 mT, and ∼900 mT are likewise distinguishable at Q-band whereas the X band spectra suffer here from low signal to noise ratios and poor resolution except for the Cu II -Cu II dimer spectrum.An additional low field signal at ∼700 mT in the Q-band spectrum indicates the existence of further magnetic species in DUT-49(Cu 0.5 Mn 0.5 ), which we tendentially assign to an AFM coupled Cu II -Mn II dimer with possible total spin states S = 2 and S = 3.

IV. CONCLUSION
EPR spectroscopy revealed for both transition metal dimers, Cu II -Cu II and Mn II -Mn II , in the paddle wheel units of MOFs DUT-49(Cu) and DUT-49(Mn) an antiferromagnetic coupling.Besides these metal dimers in the regular paddle wheel units, monomeric paramagnetic Cu II and Mn II species are observed, which most likely indicates the presence of defective paddle wheels with a missing metal ion.In the case of the mixed metal ion MOF DUT-49(Cu 0.7 Zn 0.3 ), the formation of binuclear paramagnetic Cu II -Zn II dimers besides the antiferromagnetic coupled Cu II -Cu II dimers in the paddlewheel units could be confirmed.More complicated spectra have been obtained for MOF DUT-49(Cu 0.5 Mn 0.5 ) that allowed for the unambiguous identification of Cu II -Cu II dimers and further indicated the presence of Mn II -Mn II dimers and AFM coupled Cu II -Mn II dimers.Our results confirm the feasibility of the post-synthetic metal ion exchange of Co II in the DUT-49 framework by other divalent transition metal ions such as Cu II , Zn II , and Mn II through the magnetic coupling of the divalent metal centers.

SUPPLEMENTARY MATERIAL
See supplementary material for the structure of the paddle wheel unit and the comparison of CW X-band EPR experiments at low and 160 K temperatures.
)-1(d).No dominant EPR signals of Co(II) could be detected after the post-synthetic metal ion exchange procedure in the four studied samples.The species M (∼260 mT < B < ∼360 mT) in Figs.1(a) and 1(b) indicates the Cu II monomer having a 3d 9 ground state with an electron spin S = 1/2 interacting with the nuclear spin I Cu = 3/2 of the two copper isotopes 63,65 Cu.The spin Hamiltonian for the Cu II monomer M species can be written as Ĥ = βeBĝS + S ÂI Cu (1)