Proton conductivity studies on five isostructural MOFs with different acidity induced by metal cations†
Five isostructural metal–organic frameworks, [M1.5(L)(bpy)(DMF)]·H2O (M = Mn(1), Co(2), Ni(3), Zn(4), Cd(5); L = 3-(3,5-dicarboxylpheny)-5-(3-carboxylphenyl)-1H-1,2,4-triazole, bpy = 2,2′-bipyridine) have been synthesized and characterized. The same structure and similar proton transfer pathway of these compounds offer the chance to explore the relationship between the structure and the proton conductivity deeply and precisely. It is found that the conductivities of compounds 1–5 are very different and adopt the order 1 (Mn2+) > 4 (Zn2+) > 5 (Cd2+) > 2 (Co2+) > 3 (Ni2+). In order to explore the main reason for this difference, the influencing factors on proton conduction, such as morphology, water absorption, proton density, mobility of proton carrier, and acidity of all compounds have been studied via SEM, water uptake tests, and theory calculations, respectively. The results suggest that all compounds display a similar morphology, water absorption, proton density, and mobility of the proton carrier. The Gaussian calculations reveal that the acidities of these compounds are very different. This may be caused by the different coordination ability between the metal cations and the L3− ligand. The order of acidities of the compounds is very consistent with that of the proton conductivity, which suggests that the acidity is the deciding factors of the proton conduction of this series MOFs.