Solid-State Host-Guest Chemistry of Cyanoborate-Bridged Cubane-Type Tetranuclear Ruthenium Complexes: Solvate Crystal Formation and Reversible Sorption of Volatile Organic Compounds.

Small-molecule sorption by porous metal-organic frameworks has attracted considerable attention but remains largely unexplored in nonporous crystals composed of discrete metal complexes. Here, we report cyanoborate-bridged cubane-type tetranuclear ruthenium complexes, [Ru4(Cp)4{BR(CN)3}4] (R = F (1), Me (2), H (3)), as adaptive crystalline hosts for volatile organic compounds (VOCs). Recrystallization of 1 from solvents with widely varying polarities, sizes, and shapes afforded solvate crystals containing one to three guest molecules per cubane unit. The guest molecules were accommodated around the shallow concave faces of the cubane units, and the packing structures were classified into three types largely governed by guest size and shape. The desolvated solid of 1 reversibly sorbed various VOC vapors, with the sorption amounts largely consistent with the solvent contents of the corresponding solvate crystals, indicating guest-dependent rearrangement of the intermolecular packing. Complexes 2 and 3 exhibited substituent-dependent CH2Cl2 sorption-desorption behavior. Although 1 showed only very limited N2 and H2 uptake at 77 K, it exhibited appreciable low-temperature CO2 uptake. These results expand the scope of adaptive crystalline hosts to discrete metal-complex systems with rigid, three-dimensionally shaped molecular units.