Preferential solvation of metastable phases relevant to topological control within the synthesis of metal-organic frameworks.

Abstract

A combined density functional theory and molecular dynamics study has been used to study reactions relevant to the crystallization of a model cluster based upon the metastable phase NH2-MOF-235(Al), which has been previously shown to be an important intermediate in the synthesis of NH2-MIL-101(Al). The clusters studied were of the form Al3O(BDC)6(DMF)n(H2O)m(+), where BDC(-) = NH2-benzenedicarboxylate and DMF = dimethylformamide (n = 1-3; m = {n - 3}). The ionic bonding interaction of the Al3O(7+) core with BDC(-) is much stronger than that with a coordinated solvent and is independent of the bulk solvent medium (water or DMF). The exchange reactions of a coordinated solvent are predicted to be facile, and the dynamic solvent organization indicates that they are kinetically allowed because of the ability of the solvent to migrate into the cleft created by the BDC-Al3O-BDC coordination angle. As BDC(-) binds to the Al3O(7+) core, the solvation free energy (G(solv)) of the cluster becomes less favorable, presumably because of the overall hydrophobicity of the cluster. These data indicate that as the crystal grows there is a balance between the energy gained by BDC(-) coordination and an increasingly unfavorable G(solv). Ultimately, unfavorable solvation energies will inhibit the formation of quantifiable metal-organic framework (MOF) crystals unless solution-phase conditions can be used to maintain thermodynamically favorable solute-solvent interactions. Toward this end, the addition of a cosolvent is found to alter solvation of Al3O(BDC)6(DMF)3(+) because more hydrophobic solvents (DMF, methanol, acetonitrile, and isopropyl alcohol) preferentially solvate the MOF cluster and exclude water from the immediate solvation shells. The preferential solvation is maintained even at temperatures relevant to the hydrothermal synthesis of MOFs. While all cosolvents exhibit this preferential solvation, trends do exist. Ranking the cosolvents based upon their observed ability to exclude water from the MOF cluster yields acetonitrile < DMF ∼ methanol < isopropyl alcohol. These observations are anticipated to impact the intermediate and final phases observed in MOF synthesis by creating favorable solvation environments for specific MOF topologies. This adds further insight into recent reports wherein DMF has been implicated in the reactive transformation of NH2-MOF-235(Al) to NH2-MOF-101(Al), suggesting that that DMF additionally plays a vital role in stabilizing the metastable NH2-MOF-235(Al) phase early in the synthesis.