A Ligand Field Molecular Mechanics Study of CO2-Induced Breathing in the Metal-Organic Framework DUT-8(Ni)

Flexible metal-organic Frameworks (MOFs) are an interesting class of materials due to their diverse properties. One representative of this class is the layered-pillar MOF DUT-8(Ni). This MOF consists of Ni-2 paddle wheels interconnected by naphthalene dicarboxylate linkers and dabco pillars (Ni-2(ndc)(2)(dabco), ndc = 2,6-naphthalene-dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane). DUT-8(Ni) undergoes a volume change of over 140% upon adsorption of guest molecules. Herein, a ligand field molecular mechanics (LFMM) study of the CO2-induced flexibility of DUT-8(Ni) is presented. LFMM is able to reproduce experimental and DFT structural features as well as properties that require large simulation cells. It is shown that the transformation energy from a closed to open state of the MOF is overcompensated fivefold by the host-guest interactions. Structural characteristics of the MOF explain the shape of the energy profile at different loading states and provide useful insights to the interpretation of previous experimental results.