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The current interest in porous crystalline metal-organic frameworks (MOFs) [1] is largely due to their wide range of compositions and structure types with low framework densities, their tunability, and the possibility of accessible, coordinatively unsaturated metal sites (CUS). The existence of CUS can strongly modify interactions with gases [2] or liquid adsorbates, [3] and is thus of particular importance gas storage and separations.The redox properties of transition-metal-substituted zeolites and mesoporous materials have been extensively studied and used for selective catalysis in liquid-phase oxidation, [4] removal of nitrogen oxides, [5] and photocatalytic reactions.[6]These features are very rare for MOFs containing 3d metals, in particular with respect to the reducibility of the framework metal ions. [7] For this reason, we examine herein both the conditions of generation of iron CUS with mixed valence Fe II

The uptake and adsorption enthalpy of carbon dioxide at 0.2 bar have been studied in three different topical porous MOF samples, HKUST-1, UiO-66(Zr), and MIL-100(Fe), after having been pre-equilibrated under different relative humidities (3, 10, 20, 40%) of water vapor. If in the case of microporous UiO-66, CO(2) uptake remained similar whatever the relative humidity, and correlations were difficult for microporous HKUST-1 due to its relative instability toward water vapor. In the case of MIL-100(Fe), a remarkable 5-fold increase in CO(2) uptake was observed with increasing RH, up to 105 mg g(-1) CO(2) at 40% RH, in parallel with a large decrease in enthalpy measured. Cycling measurements show slight differences for the initial three cycles and complete reversibility with further cycles. These results suggest an enhanced solubility of CO(2) in the water-filled mesopores of MIL-100(Fe).