Crystallographic characterization of the metal-organic framework Fe(bdp) upon reductive cation insertion.

Precisely locating extra-framework cations in anionic metal-organic framework compounds remains a long-standing, yet crucial, challenge for elucidating structure-performance relationships in functional materials. Single-crystal X-ray diffraction is one of the most powerful approaches for this task, but single crystals of frameworks often degrade when subjected to post-synthetic metalation or reduction. Here, we demonstrate the growth of sizable single crystals of the robust metal-organic framework Fe(bdp) (bdp = benzene-1,4-dipyrazolate) and employ single-crystal-to-single-crystal chemical reductions to access the solvated framework materials AFe(bdp)·THF (A = Li, Na, K). X-ray diffraction analysis of the sodium and potassium congeners reveals that the cations are located near the center of the triangular framework channels and are stabilized by weak cation-π interactions with the framework ligands. Freeze-drying with benzene enables isolation of activated single crystals of NaFe(bdp) and LiFe(bdp) and the first structural characterization of activated metal-organic frameworks wherein extra-framework alkali metal cations are also structurally located. Comparison of the solvated and activated sodium-containing structures reveals that the cation positions differ in the two materials, likely due to cation migration that occurs upon solvent removal to maximize stabilizing cation-π interactions. Hydrogen adsorption data indicate that these cation-framework interactions are sufficient to diminish the effective cationic charge, leading to little or no enhancement in gas uptake relative to Fe(bdp). In contrast, MgFe(bdp) exhibits enhanced H affinity and capacity over the non-reduced parent material. This observation shows that increasing the charge density of the pore-residing cation serves to compensate for charge dampening effects resulting from cation-framework interactions and thereby promotes stronger cation-H interactions.