Cation Control of Pore and Channel Size in Cage-Based Metal−Organic Porous Materials

Porous materials resembling zeolites that are composed of organic and inorganic building units were synthesized
and characterized. Control of pore and channel size was achieved by using different-sized cations. The metal-assembled, anionic cage molecule, Co4128-, with a hydrophobic cavity and four carboxylate rich arms, was used
as a structural unit for the formation of materials with pores and channels. When assembled into a solid material
with dications (Mg2+, Ca2+, Sr2+, and Ba2+), Co4128- arranges into sheets of cages linked together by cations. The
series of materials based on Co4128- and containing alkaline earth cations was characterized using X-ray
crystallography. The magnesium material packs with cages close together, has small channels, and has cation−carboxylate linkages in three dimensions. The calcium material has cages packed with voids between them and
has 5 × 10 Å channels and 10 × 21 Å pores. The strontium and barium materials also pack with voids between
the cages and similarly to each other. They have 11 × 13 Å and 11 × 11 Å channels and 10 × 27 Å and 9 × 27
Å pores, respectively. Each of these materials has many (20−50) solvent water molecules associated with each
cage. The associated water can be removed from and adsorbed by the materials. The heat of water binding has
been measured to be −52 kJ/mol (Mg4Co412); −47 kJ/mol (Ca4Co412); −48 kJ/mol (Sr4Co412); −49 kJ/mol (Ba4Co412).