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(Cu0.7Zn0.3), DUT-49(Cu0.5Mn0.5) MOFs. In the case of the Cu-II - Cu-II dimers in DUT-49(Cu), an isotropic exchange coupling of the metal ions (2J = -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(Cu0.7Zn0.3) and DUT-49(Cu0.5Mn0.5) 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.