Abstract
Metal–organic frameworks are promising materials for applications such as gas capture, separation, and storage, due to their ability to selectively adsorb small molecules. The metal–organic framework CuI-MFU-4l, which contains coordinatively unsaturated copper(i) centers, can engage in backbonding interactions with various small molecule guests, motivating the design of frameworks that engage in backbonding and other electronic interactions for highly efficient and selective adsorption. Here, we examine several gases expected to bind to the open copper(i) sites in CuI-MFU-4l via different electronic interactions, including σ-donation, π-backbonding, and formal electron transfer. We show that in situ Cu L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy can elucidate π-backbonding by directly probing excitations to unoccupied backbonding orbitals with Cu d-character, even for gases that participate in other dominant interactions, such as ligand-to-metal σ-donation. First-principles calculations based on density functional theory and time-dependent density functional theory additionally reveal the backbonding molecular orbitals associated with these spectroscopic transitions. The energies of the transitions correlate with the energy levels of the isolated small molecule adsorbates, and the transition intensities are proportional to the binding energies of the guest molecules within CuI-MFU-4l. By elucidating the molecular and electronic structure origins of backbonding interactions between electron rich metal centers in metal–organic frameworks and small molecule guests, it is possible to develop guidelines for further molecular-level design of solid-state adsorbents for energy-efficient separations of relevance to industry.
Highlights
Metal–organic frameworks (MOFs) are a highly diverse class of crystalline, porous solids that are promising for applications including catalysis,[1,2] molecular separations,[3,4,5,6,7,8,9,10,11] hydrogen storage,[12,13,14] and sensing.[15]
Metal–organic frameworks that contain open metal sites capable of engaging in p-complexation with guest molecules are especially promising for the capture of inert species and challenging separation schemes involving molecules of similar size, volatility, or polarizability.[22,23,24]
The speci city of 3d metal L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy to a speci c metal element, and to electronic states with 3d character, allows for quantitative characterization of backbonding interactions in complex framework systems, such as mixed-metal MOFs38 or MOFs with functional groups tethered to the open metal sites that show cooperative adsorption behavior.[33,39]
Summary
Metal–organic frameworks (MOFs) are a highly diverse class of crystalline, porous solids that are promising for applications including catalysis,[1,2] molecular separations,[3,4,5,6,7,8,9,10,11] hydrogen storage,[12,13,14] and sensing.[15]. The framework CuI-MFU-4l (Cu2Zn3Cl2(btdd), H2btdd 1⁄4 bis(1H1,2,3-triazolo[4,5-b],[40,50-i])dibenzo[1,4]dioxin), for example, features trigonal pyramidal copper(I) sites that can readily engage in p-backbonding interactions due to the lled Cu 3d orbitals, and this material has been shown to reversibly chemisorb O2, N2, and H2.29 Notably, CuI-MFU-4l exhibits the largest isosteric heat of adsorption for reversible H2 uptake of any MOF (32 kJ molÀ1),[29] motivating further exploration of p-backbonding as a means of achieving selective small molecule binding in these materials. The development of direct, in situ spectroscopic methods capable of probing backbonding interactions between metal sites and various small molecules is one key approach for identifying design rules to tune adsorption in CuI-MFU-4l and related frameworks featuring open metal sites
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