Metal porous hybrids constitute an important new focus of research in material chemistry, offering potential applications in adsorption, catalysis, nonlinear optical devices and magnetic materials. Sorption capacities are often correlated to framework flexibility. Until now, this behaviour has mainly concerned coordination polymers and three-dimensional (3D) metal-oxide frameworks have been considered as rigid.^[1] This study deals with the breathing of MIL-77 (labelled MIL-n for Materials of Institut Lavoisier), a nickel(II) glutarate with a 3D inorganic subnetwork. The as-synthesized MIL-77-as or [Ni₂₀(C₅H₆O₄)₂₀(H₂O)₈].40H₂O, is cubic [S.G.: P4₃32, a = 16.5812(7) Ǻ, V = 4558.8(6) Ǻ³] and built up from helices of edge sharing Ni octahedra with water ligands.^[2]  They generate corrugated 20-membered rings and very large crossing tunnels along [111] which guest water molecules. Their evacuation renders the solid porous but, curiously, this porosity drastically decreases after the removal of the water ligands fixed on Ni atoms. To shed light on this porosity loss, we studied dehydration/rehydration processes of MIL-77 and solved the structures from powder diffraction data after removal of guest water molecules [MIL-77-evac or [Ni₂₀(C₅H₆O₄)₂₀(H₂O)₈], S.G.: P4₃32 with a =16.402(2) Å, V = 4412.9(8) ų] and ligated ones [MIL-77-anh or [Ni₂₄(C₅H₆O₄)₂₄], S.G.: P4₃32 with a = 15.643(1) Å, V = 3827.8(4) ų]. The observed flexibility of the skeleton is very unusual for a 3D oxide framework. More amazing is the reversible hopping of nickel atoms into the voids of the skeleton and the repositioning of some carboxylate groups during the transition, explaining the decrease of porosity (see Fig. 1).

References:

[1] S. Kitagawa, R. Kitaura and S.-I. Noro, Angew. Chemie  Int. Ed. 2004, 43, 2334.

[2] N. Guillou, C. Livage, M. Drillon and G. Férey Angew. Chem. Int. Ed, 2003, 42, 5314.

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