Dioxygen binding to Fe-MOF-74: Microscopic insights from periodic QM/MM calculations

Accurate MOF-FF parameter sets were determined for the ferrous and ferric forms of an iron-based metal-organic framework (MOF) called Fe-MOF-74. For this purpose, density functional theory (DFT) calculations were applied to truncated cluster models of Fe-MOF-74, and the DFT-calculated geometries and energy derivatives were used for the force-field parameterization. The resultant parameter sets performed remarkably well in reproducing the experimentally determined structure of the MOF. We also performed periodic quantum mechanics (QM) / molecular mechanics (MM) calculations employing a subtractive scheme called ONIOM, with the optimized MOF-FF parameters used for the MM calculations, in an attempt to evaluate the binding energies between O₂ and several Fe-MOF-74 variants. The calculated binding energy for Fe-MOF-74 agreed very well with the experimental value, and QM/MM geometry optimization calculations confirmed that the O₂-bound complex has a side-on geometry. Our calculations also predicted that, when the two neighboring iron ions around the O₂-binding site are replaced with other metal ions (Mg²⁺, Ni²⁺, Zn²⁺, Co²⁺, or Mn²⁺), there are noticeable variations in the binding energy, indicating that these substituted metal ions affect the O₂ binding indirectly.