Observation of an Intermediate to H2 Binding in a Metal–organic Framework

Coordinatively-unsaturated metal sites within certain zeolites and metal–organic frameworks can strongly adsorb various molecules. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through electrostatic interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that H₂chemisorption at the trigonal pyramidal Cu⁺sites in the metal–organic framework Cu^I‑MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. This finding demonstrates that adsorption at framework metal sites does not always follow a concerted pathway and underscores the importance of probing kinetics in the design of next-generation adsorbents.