RESEARCH ON THE ADSORPTION OF O2 IN METAL–ORGANIC FRAMEWORKS WITH OPEN MANGANESE(II) COORDINATION SITES

Three different metal–organic frameworks (MOFs), specifically Mn2 (dobdc)(DMF)4 ( H2 dobdc=[2,5-dihydroxyterephthalic acid]; compound (1), Mn5 ( btac )4(μ3- OH )2( EtOH )2⋅ DMF⋅3EtOH⋅3H2O ( H2 btac=[benzotriaole-5-carboxylicacid]; compound (2), and Mn3 (2,6-ndc)3⋅4DMF ( H 2ndc=[2,6-naphthalenedicarbo-xylic acid]; compound (3), have been synthesized, the channels of which are lined with coordinatively unsaturated Mn II centers. The adsorption of O2 in these MOFs has been measured using a gravimetric method at different temperatures (-78°C, -5°C, and 25°C) at a pressure of 1 bar. Gas adsorption isotherms of compounds 1 and 2 at 298 K indicated that they bind O2 by chemisorption at low pressure, with capacities of 1.2 wt.% and 2.14 wt.%, respectively, for the first cycle, with reversible oxygen binding for compound 1 and partially irreversible oxidation for compound 2. However, compound 3 binds O2 by physisorption, with a capacity of just 0.21 wt.%. This difference between the three compounds stems from the different coordination environments of the respective Mn II centers, which give rise to differences in electron density. The results suggest that there must be an optimal electron density around the exposed Mn II center for partial charge transfer from this center to the bound O2 molecule; if the electron density is too high or too low, reversible chemisorption of O2 is not favored.