Modulation of Water Vapor Sorption by a Fourth-Generation Metal–Organic Material with a Rigid Framework and Self-Switching Pores

Hydrolytically stable adsorbents
are needed for water vapor sorption
related applications; however, design principles for porous materials
with tunable water sorption behavior are not yet established. Here,
we report that a platform of fourth-generation metal–organic
materials (MOMs) with rigid frameworks and self-switching pores can
adapt their pores to modulate water sorption. This platform is based
upon the hydrolytically stable material CMOM-3S, which
exhibits bnn topology and is composed of rod building
blocks based upon S-mandelate ligands, 4,4-bipyridine
ligands, and extraframework triflate anions. Isostructural variants
of CMOM-3S were prepared using substituted R-mandelate ligands and exhibit diverse water vapor uptakes (20–67
cm3/g) and pore filling pressures (P/P0, 0.55–0.75). [Co2(R-4-Cl-man)2(bpy)3]­(OTf) (33R) is of particular interest because of its unusual isotherm. Insight
into the different water sorption properties of the materials studied
was gained from analysis of in situ vibrational spectra,
which indicate self-switching pores via perturbation of extraframework
triflate anions and mandelate linker ligands to generate distinctive
water binding sites. Water vapor adsorption was studied using in situ differential spectra that reveal gradual singlet
water occupancy followed by aggregation of water clusters in the channels
upon increasing pressure. First-principles calculations identified
the water binding sites and provide structural insight on how adsorbed
water molecules affect the structures and the binding sites. Stronger
triflate hydrogen bonding to the framework along with significant
charge redistribution were determined for water binding in 33R. This study provides insight into a new class of fourth-generation
(self-switching pores) MOM and the resulting effect upon water vapor
sorption properties.

水蒸气吸附相关应用亟需水解稳定吸附剂，但目前尚未建立具备可调谐水蒸气吸附行为的多孔材料设计原则。在此，我们报道一类兼具刚性骨架与自切换孔道的第四代金属有机材料（metal–organic materials, MOMs）平台，该类材料可通过调控孔道结构实现水蒸气吸附行为的精准调制。该平台基于水解稳定材料CMOM-3S，其具有bnn拓扑结构，以基于S-扁桃酸配体、4,4'-联吡啶配体及骨架外三氟甲磺酸根阴离子的棒状结构基元构筑而成。
通过使用取代的R-扁桃酸配体，我们制备得到CMOM-3S的同构变体，这些变体展现出跨度广泛的水蒸气吸附量（20~67 cm³/g）与微孔填充压力（相对压力P/P₀区间为0.55~0.75）。其中，配合物[Co₂(R-4-Cl-man)₂(bpy)₃](OTf)（标记为33R）因其特殊的吸附等温线而受到特别关注。
通过对原位振动光谱的分析，我们深入解析了所研究材料的水蒸气吸附性能差异：研究表明，骨架外三氟甲磺酸根阴离子与扁桃酸连接配体发生扰动，进而形成独特的水结合位点，这正是材料实现自切换孔道的内在机制。
我们借助原位差分光谱开展水蒸气吸附动力学研究，结果揭示了孔道内吸附过程的动态演化规律：随着压力升高，孔道内首先发生单分子水占据，随后逐渐形成水分子团簇。第一性原理计算明确了材料中的水结合位点，并从结构层面解析了吸附水分子如何影响材料骨架与水结合位点。针对33R中的水结合过程，研究发现其存在更强的三氟甲磺酸根与骨架间氢键作用，同时伴随显著的电荷重新分布。
本研究为新一代（自切换孔道型）第四代金属有机材料的开发及其水蒸气吸附性能调控机制提供了全新的理论与实验认识。