Polar Sulfone-Functionalized Oxygen-Rich Metal–Organic Frameworks for Highly Selective CO2 Capture and Sensitive Detection of Acetylacetone at ppb Level

A rational combination of an oxygen-rich pyridyl substituted tetrapodal ligand, tetrakis­(4-pyridyloxymethylene)­methane (TPOM), and a polar sulfone-functionalized conjugated bent dicarboxylate linker, dibenzothiophene-5,5′-dioxide-3,7-dicarboxylic acid (H2(3,7-DBTDC)), with d10 metal centers, Zn­(II) and Cd­(II), has led to the construction of two new three-dimensional (3D) metal–organic frameworks,{[Zn2(TPOM)­(3,7-DBTDC)2]·7H2O·DMA}n (1) and {[Cd2(TPOM)­(3,7-DBTDC)2]·6H2O·3DMF}n (2). Single-crystal X-ray analysis indicates that 1 is a 3D framework with a dinuclear repeating unit having two different Zn­(II) centers (tetrahedral and square pyramidal) and 2 is a 3D framework comprised of a dinuclear repeating unit with one crystallographically independent distorted pentagonal bipyramidal Cd­(II) coordinated to chelating/bridging carboxylates and nitrogen atoms of the TPOM ligand. In both cases, the pores are aligned with oxygen atoms of the TPOM ligand and decorated with polar sulfone moieties. On the basis of the stability established by thermogravimetric analysis and powder X-ray diffraction (PXRD) and the presence of large solvent accessible voids (25.4% for 1 and 40.6% for 2), gas sorption studies of different gases (N2, CO2, and CH4) and water vapor have been explored for both 1 and 2. The CO2 sorption isotherm depicts type I isotherm with an uptake of 93.6 cm3 g–1 (for 1) and 100.6 cm3 g–1 (for 2) at 195 K. Additionally, sorption of CO2 is highly selective over that of N2 and CH4 for both 1 and 2 due to the strong quadrupolar interactions between sulfone moieties and CO2 molecules. Configurational bias Monte Carlo (CBMC) molecular simulation has further justified the highly selective CO2 capture. On the other hand, the luminescence nature of 1 and 2 has been employed for highly selective detection of acetylacetone in aqueous methanol with a limit of 59 ppb in 1 and 66 ppb in 2, which are among the best reported values so far in the literature. The Stern–Volmer plots, spectral overlap, density functional theory calculations, CBMC simulation, and time-resolved lifetime measurements have been utilized for an extensive mechanistic study. The exclusive selectivity for acetylacetone in 1 and 2 have been confirmed by competitive selectivity test. Both exhibited good recyclability and stability after sensing experiments analyzed by fluorescence, PXRD, and field emission scanning electron microscopy studies.

将富氧吡啶取代的四足配体四(4-吡啶氧甲基)甲烷（tetrakis(4-pyridyloxymethylene)methane, TPOM）与极性砜基功能化共轭弯折型二羧酸连接体二苯并噻吩-5,5'-二氧化物-3,7-二羧酸（dibenzothiophene-5,5′-dioxide-3,7-dicarboxylic acid, H₂(3,7-DBTDC)）与d¹⁰金属中心Zn(II)和Cd(II)合理组合，成功构筑了两种新型三维（3D）金属有机框架：{[Zn₂(TPOM)(3,7-DBTDC)₂]·7H₂O·DMA}ₙ（1）与{[Cd₂(TPOM)(3,7-DBTDC)₂]·6H₂O·3DMF}ₙ（2）。单晶X射线衍射分析表明，配合物1为具有双核重复单元的三维框架，该单元包含两种不同配位环境的Zn(II)中心（四面体构型与四方锥构型）；配合物2则为由双核重复单元构成的三维框架，其中包含1个晶体学独立的畸变五角双锥构型Cd(II)，该中心与TPOM配体的螯合/桥联羧基及氮原子配位。两种配合物的孔道均由TPOM配体的氧原子排布而成，并被极性砜基官能团修饰。基于热重分析与粉末X射线衍射（powder X-ray diffraction, PXRD）验证的稳定性，以及二者均具备较大溶剂可及孔隙率（配合物1为25.4%，配合物2为40.6%），我们对1和2开展了不同气体（N₂、CO₂、CH₄）及水蒸气的吸附研究。CO₂吸附等温线呈现I型吸附特征，在195 K下，配合物1的吸附量为93.6 cm³·g⁻¹，配合物2为100.6 cm³·g⁻¹。此外，由于砜基官能团与CO₂分子间存在强四极相互作用，两种配合物对CO₂的吸附选择性均远高于N₂与CH₄。构型偏置蒙特卡洛（configurational bias Monte Carlo, CBMC）分子模拟进一步证实了其对CO₂的高选择性捕获能力。另一方面，配合物1和2的发光特性被用于甲醇水溶液中乙酰丙酮的高选择性检测，配合物1的检出限为59 ppb，配合物2为66 ppb，这两项数值均为目前文献报道的最优值之一。我们通过斯特恩-沃尔默（Stern–Volmer）曲线、光谱重叠分析、密度泛函理论（density functional theory, DFT）计算、CBMC模拟以及时间分辨寿命测量等手段开展了详尽的机理研究。竞争选择性实验证实了1和2对乙酰丙酮的专属选择性。经过荧光测试、PXRD及场发射扫描电子显微镜分析后发现，两种配合物在传感实验后仍具备良好的可回收性与稳定性。