Biggins Naomi, Ziebel Michael E, Gonzalez Miguel I, Long Jeffrey R
Department of Chemistry, University of California Berkeley California 94720 USA
Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley California 94720 USA.
Chem Sci. 2020 Aug 10;11(34):9173-9180. doi: 10.1039/d0sc03383a.
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.
精确确定阴离子金属有机骨架化合物中的骨架外阳离子位置，对于阐明功能材料的结构-性能关系而言，仍然是一个长期存在但至关重要的挑战。单晶X射线衍射是完成这项任务最有效的方法之一，但骨架的单晶在进行后合成金属化或还原时往往会降解。在此，我们展示了坚固的金属有机骨架Fe(bdp)（bdp = 苯-1,4-二吡唑酸酯）的大尺寸单晶的生长，并采用单晶到单晶的化学还原方法来制备溶剂化骨架材料AFe(bdp)·THF（A = Li、Na、K）。对钠和钾同系物的X射线衍射分析表明，阳离子位于三角形骨架通道的中心附近，并通过与骨架配体的弱阳离子-π相互作用得以稳定。用苯进行冷冻干燥能够分离出NaFe(bdp)和LiFe(bdp)的活性单晶，以及首次对其中骨架外碱金属阳离子也进行了结构定位的活性金属有机骨架进行结构表征。对溶剂化和活化的含钠结构的比较表明，两种材料中的阳离子位置不同，这可能是由于在去除溶剂时发生阳离子迁移以最大化稳定的阳离子-π相互作用所致。氢气吸附数据表明，这些阳离子-骨架相互作用足以降低有效阳离子电荷，导致相对于Fe(bdp)而言气体吸收几乎没有增强或没有增强。相比之下，MgFe(bdp)相对于未还原的母体材料表现出增强的H亲和力和容量。这一观察结果表明，增加孔内阳离子的电荷密度有助于补偿阳离子-骨架相互作用导致的电荷衰减效应，从而促进更强的阳离子-H相互作用。
