Selective Gas Adsorption in Permanently Microporous Coordination Cages with Exposed Metal Sites

Porous coordination cages (PCCs), molecular analogs of metal-organic frameworks, offer modular platforms for studying the adsorption properties of small molecules, with coordinatively unsaturated metal centers playing a pivotal role in tuning these behaviors. In this work, we present the synthesis, activation, and detailed gas adsorption studies of second-row transition metal-based M₂₄L₂₄ cuboctahedral cages, specifically Mo₂₄(bdc)₂₄, Rh₂₄(bdc)₂₄, and [Ru₂₄(bdc)₂₄]Cl₁₂. These materials represent rare examples of Mo-, Rh-, and Ru-based hybrid porous solids. The synthesis and activation of these cages were optimized to maximize porosity, yielding BET surface areas of up to 832 m²/g. Gas adsorption studies with CO₂ and CO reveal distinctive uptake behaviors linked to the metal cations, with Mo₂₄(bdc)₂₄ demonstrating the highest gravimetric CO₂ uptake (2.12 mmol/g at 298 K) and [Ru₂₄(bdc)₂₄]Cl₁₂ exhibiting the strongest CO binding (-75 kJ/mol). Additionally, we explore the selective adsorption of unsaturated hydrocarbons, such as ethylene and propylene, revealing strong binding interactions at low pressures as a result of strong metal-hydrocarbon interactions based on pi-backbonding interactions.