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 present study illustrates the importance of the oxidation state of iron within the mesoporous iron trimesate [{Fe(3)O(H(2)O)(2)F(0.81)(OH)(0.19)}{C(6)H(3)(CO(2))(3)}(2)] denoted MIL-100(Fe) (MIL= Material from Institut Lavoisier) during adsorption of molecules that can interact with the accessible metal sites through π-back donation. Adsorption of CO has been first followed by FTIR spectroscopy to quantify the Lewis acid sites in the dehydrated Fe(III) sample, outgassed at 150 °C, and on the partially reduced Fe(II/III), outgassed at 250 °C. The exposure of MIL-100(Fe) to CO(2), propane, propene and propyne has then been studied by FTIR spectroscopy and microcalorimetry. It appears that π-back donating molecules are strongly adsorbed on reduced iron(II) sites despite the weaker Lewis acidity of cus Fe(2+) sites compared to that of Fe(3+) ones, as shown by pyridine adsorption.