(315i) Characterizing Open-Metal Site Density and Speciation in Metal Organic Framework Materials: The Case of MIL-100(Cr)

Metal–organic framework materials (MOFs) have been the focus of extensive research over the past couple of decades owing to their utility in enhancing performance in a range of applications including but not limited to gas separations, heterogeneous catalysis, and sensing. A rigorous understanding of the role of open-metal sites in molecular processes pertinent to these applications is first and foremost reliant on accurate estimates of both their density and speciation under specific sets of experimental conditions. Existing methods for quantifying open-metal site density exhibit drawbacks originating from unselective adsorption, use of high pressures and/or low temperatures, or the handling of hazardous reagents. We report herein for the first time the use of room-temperature water adsorption isotherms for the quantification of MOF open-metal site density. We show that the quantity of water adsorbed irreversibly at room temperature on MIL-100 represents the open-metal site density under a given set of activation conditions. Crucially, this approach circumvents the need for assumptions relating to the identity of open-metal sites and the degree of adsorbate saturation, while also obviating risks associated with the use of hazardous reagents. In addition, we also present the use of probe molecules endowed with empty antibonding orbitals for the characterization and quantification of reduced open-metal sites proposed to be key to several catalysis and separations applications. Given the near-universal presence of water as a labile ligand in the first coordination sphere of possible MOF open-metal sites, and the pertinence of reduced sites in critical applications, we envision that the protocols presented here could represent an approach to characterizing open-metal site density and speciation that is broadly applicable within (and maybe even beyond) the field of MOF research.