Synthesis and characterization of a flexible metal organic framework generated from MnIII and the 4,4′-bipyrazolate-ligand

Abstract

The synthesis and crystal structure of a novel metal organic framework, constructed from Mn^III, the 4,4′-bipyrazolate (BPZ) ligand and bridging hydroxyl groups is presented in this work. The network topology is identical to M-CFA-6 (M = Fe, Ga), but in the case of Mn-CFA-6, the structure crystallizes in the monoclinic crystal system due to the Jahn–Teller effect of the high spin d⁴ manganese(III) centers. Thermal treatment of Mn-CFA-6 leads to decomposition of the framework above 150 °C. The porosity of Mn-CFA-6 was measured by carbon dioxide sorption, since solvent exchange and removal of the occluded solvent molecules leads to a compression of the pores. Electronic spectra of Mn-CFA-6 reveal two distinct absorption bands that are assigned to the two different manganese(III) centers as revealed by the single crystal structure analysis. Furthermore, SQUID measurements exhibit ferromagnetic behaviour of Mn-CFA-6 below the magnetic ordering at 45 K and antiferromagnetic coupling above this temperature. Evaluation of the paramagnetic region by the Curie-Weiß formula reveals an effective moment of 4.8μ_B, which is very close to the spin-only value of Mn^III centers.