Solvent-Free Synthesis of ZIFs: A Route toward the Elusive Fe(II) Analogue of ZIF‑8

Herein we report the synthesis of an elusive metal–organic framework, the iron­(II) analogue of ZIF-8 with the formula Fe­(2-methylimidazolate)2, here denoted as MUV-3. The preparation of this highly interesting porous material, inaccessible by common synthetic procedures, occurs in a solvent-free reaction upon addition of an easily detachable template molecule, yielding single crystals of MUV-3. This methodology can be extended to other metals and imidazolate derivatives, allowing the preparation of ZIF-8, ZIF-67, and the unprecedented iron­(II) ZIFs Fe­(2-ethylimidazolate)2 and Fe­(2-methylbenzimidazolate)2. The different performance of MUV-3 toward NO sorption, in comparison to ZIF-8, results from the chemisorption of NO molecules, which also causes a gate-opening behavior. Finally, the controlled pyrolysis of MUV-3 results in a N-doped graphitic nanocomposite that exhibits extraordinary performance for the oxygen evolution reaction (OER), with low overpotential at different current densities (316 mV at 10 mA cm–2), low Tafel slope (37 mV per decade), high maximum current density (710 mA cm–2 at 2.0 V vs RHE), and great durability (15 h).

本文报道了一种难以合成的金属有机框架（metal-organic framework, MOF）的合成工作：该材料为ZIF-8的铁(II)类似物，分子式为Fe(2-甲基咪唑啉盐)₂，本文中将其命名为MUV-3。这种极具研究价值的多孔材料无法通过常规合成路径制备，其制备可通过无溶剂反应实现：向体系中加入易于脱除的模板分子后，即可得到MUV-3的单晶体。该合成策略可推广至其他金属与咪唑酯衍生物体系，进而实现ZIF-8、ZIF-67以及此前未见报道的铁(II)基ZIF材料——Fe(2-乙基咪唑啉盐)₂与Fe(2-甲基苯并咪唑啉盐)₂的制备。与ZIF-8相比，MUV-3对一氧化氮（NO）的吸附性能存在显著差异，这源于NO分子的化学吸附，该过程同时引发了门控打开行为。最后，对MUV-3进行可控热解可得到氮掺杂石墨基纳米复合材料，该材料在析氧反应（oxygen evolution reaction, OER）中展现出优异性能：在不同电流密度下均具有较低过电位（10 mA·cm⁻²时为316 mV）、极低的塔菲尔斜率（37 mV/dec）、极高的最大电流密度（相对于可逆氢电极（reversible hydrogen electrode, RHE）在2.0 V时达710 mA·cm⁻²）以及出色的循环耐久性（15 h）。