Reversible CO Scavenging via Adsorbate-Dependent Spin State Transitions in an Iron(II)–Triazolate Metal–Organic Framework

A new
metal–organic framework, Fe-BTTri (Fe3[(Fe4Cl)3(BTTri)8]2·18CH3OH, H3BTTri =1,3,5-tris­(1H-1,2,3-triazol-5-yl)­benzene)),
is found to be highly selective in the adsorption of CO over a variety
of other gas molecules, making it extremely effective, for example,
in the removal of trace CO from mixtures with H2, N2, and CH4. This framework not only displays significant
CO adsorption capacity at very low pressures (1.45 mmol/g at just
100 μbar), but, importantly, also exhibits readily reversible
CO binding. Fe-BTTri utilizes a unique spin state change mechanism
to bind CO in which the coordinatively unsaturated, high-spin FeII centers of the framework convert to octahedral, low-spin
FeII centers upon CO coordination. Desorption of CO converts
the FeII sites back to a high-spin ground state, enabling
the facile regeneration and recyclability of the material. This spin
state change is supported by characterization via infrared spectroscopy,
single crystal X-ray analysis, Mössbauer spectroscopy, and
magnetic susceptibility measurements. Importantly, the spin state
change is selective for CO and is not observed in the presence of
other gases, such as H2, N2, CO2,
CH4, or other hydrocarbons, resulting in unprecedentedly
high selectivities for CO adsorption for use in CO/H2,
CO/N2, and CO/CH4 separations and in preferential
CO adsorption over typical strongly adsorbing gases like CO2 and ethylene. While adsorbate-induced spin state transitions are
well-known in molecular chemistry, particularly for CO, to our knowledge
this is the first time such behavior has been observed in a porous
material suitable for use in a gas separation process. Potentially,
this effect can be extended to selective separations involving other
π-acids.

一种新型金属有机框架（metal–organic framework, MOF）Fe-BTTri，其化学式为Fe₃[(Fe₄Cl)₃(BTTri)₈]₂·18CH₃OH（其中H₃BTTri代表1,3,5-三(1H-1,2,3-三唑-5-基)苯），被证实对一氧化碳（carbon monoxide, CO）相较于多种其他气体分子具备极高的吸附选择性，使其在从氢气（hydrogen, H₂）、氮气（nitrogen, N₂）与甲烷（methane, CH₄）的混合气体中脱除痕量CO方面展现出极佳的应用效能。该框架不仅在极低压力下即具备可观的CO吸附容量（仅100 μbar时可达1.45 mmol/g），更为关键的是其结合的CO可实现便捷可逆的脱附。Fe-BTTri通过独特的自旋态转变机制实现CO结合：框架中配位不饱和的高自旋Fe(II)中心在与CO配位后，会转变为八面体构型的低自旋Fe(II)中心；而CO脱附过程可使Fe(II)位点恢复至高自旋基态，从而实现材料的简便再生与循环复用。这一自旋态转变机制已通过红外光谱（infrared spectroscopy）、单晶X射线衍射分析、穆斯堡尔谱（Mössbauer spectroscopy）以及磁化率测量等表征手段得到验证。值得注意的是，该自旋态转变对CO具有专一选择性，在H₂、N₂、二氧化碳（carbon dioxide, CO₂）、CH₄或其他烃类气体存在时均未观测到此现象，由此使得该材料在CO/H₂、CO/N₂及CO/CH₄分离体系中，以及相较于CO₂、乙烯等典型强吸附气体的优先CO吸附应用中，展现出前所未有的高吸附选择性。尽管吸附质诱导的自旋态转变在分子化学领域（尤其是针对CO）已被广泛认知，但据我们所知，这是首次在适用于气体分离工艺的多孔材料中观测到此类行为。该效应未来有望拓展至涉及其他π酸的选择性分离领域。