Backbonding Contributions to Small Molecule Adsorption in a Cu^I-Containing Porous Framework for Gas Separations

Nanoporous metal–organic frameworks are promising for gas capture, separation, and storage due to their high surface area and tunable chemistry and are amenable to incorporation into membranes. The framework Cu^I-MFU-4l, which contains coordinatively unsaturated Cu^I centers, can engage in backbonding interactions with small molecule guests. We use in situ near edge X-ray absorption fine structure spectroscopy coupled with theory to examine several gases expected to bind to the open Cu^I sites via different electronic interactions, including σ-donation and π-backbonding. We show that in situ Cu L-edge X-ray absorption spectroscopy of Cu^I-MFU-4l in the presence of a gas can elucidate the π-backbonding contributions by directly probing excitations to unoccupied backbonding orbitals with Cu d-character, even for gases where other interactions, such as ligand-to-metal σ-donation, dominate. First-principles calculations based on time-dependent density functional theory 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 adsorbate, and the transition intensities are proportional to the binding energies of the guest molecules with Cu^I-MFU-4l.