Three isostructural MOFs based on different metal cations: proton conductivities and SC–SC transformation leading to magnetic changes

Abstract

Three isostructural metal–organic frameworks constructed using different metal cations {[M(L)_1/2(H₂O)₂]·H₂O}_n (M = Co (1), Mn (2), Zn (3), H₄L = 5,5′-(butane-1,4-diylbis(oxy)) diisophthalic acid) were obtained and characterized. According to the results of X-ray single crystal diffraction studies, there were coordination and lattice water molecules in crystal structures, which could hydrogen bond with each other and offer a proton transfer pathway. The isostructuralism of those three compounds offered the chance to explore the effect of metal cations on the proton conduction behaviors of MOFs. The studies on proton conductivities revealed that the conductivities of MOF 1–3/Nafion composite membranes were different and adopted the order 1 (Co²⁺)/Nafion > 2 (Mn²⁺)/Nafion > 3 (Zn²⁺)/Nafion. According to thermogravimetric analysis, theoretical calculations, and crystal field theory analysis, the difference in proton conductivity of the three compounds should be attributed to the different acidities of the coordinated water molecules caused by the different radii of metal cations and valence electron configurations. Additionally, the single crystal to single crystal transformation phenomenon based on the exchange of coordination water molecules with organic small molecules was observed in compound 1. Further, this transformation could bring obvious magnetic changes.