Crystallographic characterization of the metal–organic framework Fe2(bdp)3 upon reductive cation insertion

Abstract

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 Fe₂(bdp)₃ (bdp²⁻ = benzene-1,4-dipyrazolate) and employ single-crystal-to-single-crystal chemical reductions to access the solvated framework materials A₂Fe₂(bdp)₃·yTHF (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 Na_0.5Fe₂(bdp)₃ and Li₂Fe₂(bdp)₃ 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 Fe₂(bdp)₃. In contrast, Mg_0.85Fe₂(bdp)₃ exhibits enhanced H₂ 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–H₂ interactions.