When the Solvent Locks the Cage: Theoretical Insight into the Transmetalation of MOF-5 Lattices and Its Kinetic Limitations

Transmetalation is an innovative postsynthetic strategy for tailoring the properties of metal−organic frameworks (MOFs), allowing stable unprecedented metal coordination environments. Although the experimental synthetic protocol is wellestablished, the underlying mechanism for transmetalation is still unknown. In this work, we propose two different solvent-mediated reaction paths for the Ni transmetalation in ZnMOF-5 lattices through density functional theory simulations. In both mechanisms, the bond strength between the exchanged metal and the solvent is the key descriptor that controls the degree of transmetalation. We also show that the role of the solvent in this process is twofold: it initially promotes Zn exchange, but if the metal−solvent bond is too strong, it blocks the second transmetalation cycle by restricting the lattice flexibility. The competition between these two effects leads the degree of incorporation of metal into MOF-5 to display a volcano-type dependence with respect to the metal−solvent bond strength for different transition metal ions. ■ INTRODUCTION The easiness with which metal−organic frameworks (MOFs) can swap their constituents with the surrounding medium is crucial for creating novel structures with tailored properties. This process, called postsynthetic modification, can yield cations in awkward coordinations that cannot be reached by synthesis from scratch. The tremendous potential of these modified compounds with open sites in the field of catalysis, separation, and CO2 sequestration 8,9 opens an immense field of research. Generally, postsynthetic treatments occur without affecting the topology of the compound in a single-crystal-tosingle-crystal transformation, implying that the formation and cleavage of metal−ligand bonds occur while the overall MOF architecture is maintained. In particular, with postsynthetic ion metathesis (PSIM), the original cation in the MOF lattice can be replaced by divalent or even trivalent cations. The metal substitution often saturates, not reaching full ion exchange. This is the case for Ni in MOF-5, whose maximal Ni:Zn ratio is 1:3 at room temperature. Four factors have been proposed to control ion exchange: (i) the ionic radii and the coordination modes of the exchanged metals, (ii) the pore diameter and framework flexibility of the network, (iii) the difference in electronegativity between the incoming and leaving metal, and (iv) the solvent. Some solvent parameters like polarizability, donor indexes, and dielectric constants correlate poorly with the transmetalation extent. However, solvent Lewis basicity and the ligand field parameter have been shown to correlate with the rate of PSIM. Nevertheless, the dependence on the ligand field was found to be opposite for Ni inserting into MOF-5 and Co into MFU-4l. Besides PSIM in MOF, solvent also plays a critical role in linker exchange, MOF interpenetration, and reconstruction, and ion metathesis in zeolites. Simulations hold the key to unraveling the precise role of each of the contributions described above, in particular the solvent role. However, because of the large size of the MOF lattices, a plethora of competing intermediate states, and the challenges induced by the large flexibility of the lattice, no transmetalation mechanism has yet been proposed. Here we present a first-principles theoretical study of the ion exchange mechanism in MOF-5 structures and its dependence on the solvent nature. Two routes can account for the Ni transmetalation in MOF-5. In both paths, the incorporation of the first Ni is actively promoted by the medium, but solvent leftovers prevent any other metal substitution. The strength of the metal−solvent parameter dictates the kinetics of the incorporation process, thus emerging as the sole descriptor of the transmetalation process. The implications of this effect overstep the boundaries of reactivity and account for the increased robustness to moisture degradation upon trans-