Holding Open Micropores with Water: Hydrogen-Bonded Networks Supported by Hexaaquachromium(III) Cations

To form any porous solid requires payment of an energetic penalty to create voids through either strong bonds or many weak interactions working in concert. Here, we present a porous solid that is composed, after pore activation, of 27 wt % water and sustained by charge-assisted hydrogen bonds between hexaaquachromium(III) cations and organophosphonate anions. The network forms a pillared layered motif including guest solvents, as confirmed crystallographically. Although aquo ligands are an integral part of the hydrogen-bonding network that sustains the pores, activation under high vacuum is possible, and the network demonstrates reversible CO₂ sorption. The stability of the network is attributed to the inertness of the octahedral d³ Cr(III) center in combination with high degrees of complementarity and cooperativity of hydrogen bonds. The sub-nanometer pores are guest selective on the basis of size, shape, and chemical functionality. Separation processes are ubiquitous in industrial processes. Porous solids can be employed in, for example, fixed beds or membranes, to reduce the energy for separations in relation to processes such as distillation. Energy reduction reduces costs and greenhouse gas emissions. For any separation process, there will be a best-suited porous solid in terms of pore size and surface chemistry. As such, there is ongoing interest in developing new porous solids. Traditional approaches focus on robust solids with strong bonding interactions. Solids sustained by weak bonding, such as hydrogen bonding, offer different opportunities because they can order in a more facile manner; the challenge is to have sufficiently strong interactions to sustain pores. Solids with water molecules as part of the sustaining network are unprecedented. The present study introduces a member of a family of porous solids and shows CO₂ capture, as well as shape and functional-group selectivity, for guest molecules. A hydrogen-bonded solid has pores sustained by complementary interactions between hexaaqua metal cations and phosphonate anions. The pores are templated by guest molecules of a certain size and chemical functionality, but these guests are removable. Even though the activated form of the solid is 27% water by weight, reversible gas sorption is demonstrated. The framework shows adaptability to different guest species.