Multifunctional Metal–Organic Frameworks Based on Redox-Active Rhenium Octahedral Clusters

The redox-active rhenium octahedral cluster unit [Re₆Se₈(CN)₆]^4– was combined with Gd³⁺ ions and dicarboxylate linkers in novel types of metal–organic frameworks (MOFs) that display a set of functional properties. The hydrolytically stable complexes [{Gd­(H₂O)₃}₂(L)­Re₆Se₈(CN)₆]·<i>n</i>H₂O (<b>1</b>, L = furan-2,5-dicarboxylate, fdc; <b>2</b>, L = thiophene-2,5-dicarboxylate, tdc) exhibit a 3D framework of trigonal symmetry where 1D chains of [{Gd­(H₂O)₃}₂(L)]⁴⁺ are connected by [Re₆Se₈(CN)₆]^4– clusters. Frameworks contain spacious channels filled with H₂O. Solvent molecules can be easily removed under vacuum to produce permanently porous solids with high volumetric CO₂ uptake and remarkable CO₂/N₂ selectivity at room temperature. The frameworks demonstrate an ability for reversible redox transformations of the cluster fragment. The orange powders of compounds <b>1</b> and <b>2</b> react with Br₂, yielding dark-green powders of [{Gd­(H₂O)₃}₂(L)­Re₆Se₈(CN)₆]­Br·<i>n</i>H₂O (<b>3</b>, L = fdc; <b>4</b>, L = tdc). Compounds <b>3</b> and <b>4</b> are isostructural with <b>1</b> and <b>2</b> and also have permanently porous frameworks but display different optical, magnetic, and sorption properties. In particular, oxidation of the cluster fragment “switches off” its luminescence in the red region, and the incorporation of Br^– leads to a decrease of the solvent-accessible volume in the channels of <b>3</b> and <b>4</b>. Finally, the green powders of <b>3</b> and <b>4</b> can be reduced back to the orange powders of <b>1</b> and <b>2</b> by reaction with hydrazine, thus displaying a rare ability for fully reversible chemical redox transitions. Compounds <b>1</b>–<b>4</b> are mentioned as a new class of redox-active cluster-based MOFs with potential usage as multifunctional materials for gas separation and chemical contamination sensors