Backbonding contributions to small molecule chemisorption in a metal-organic framework with open copper(i) centers

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 Cu-I-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 Cu-I-MFU-4l via different electronic interactions, including sigma-donation, pi-backbonding, and formal electron transfer. We show that in situ Cu L-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy can elucidate pi-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 sigma-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 Cu-I-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.

Automated summary

Metal-organic frameworks show promise for gas capture, separation, and storage due to their selective adsorption of small molecules. By studying the electronic interactions between metal centers and guest molecules, such as backbonding, researchers can develop guidelines for designing solid-state adsorbents for energy-efficient separations in industry. Spectroscopic techniques and theoretical calculations are used to elucidate the molecular and electronic structures involved in these interactions, providing insights into the origins of binding energies and transition intensities.