Functionalization of Microporous Lanthanide-Based Metal–Organic Frameworks by Dicarboxylate Ligands with Methyl-Substituted Thieno[2,3‑<i>b</i>]thiophene Groups: Sensing Activities and Magnetic Properties

From a methyl-substituted thieno­[2,3-b]­thiophene dicarboxylate, three types of three-dimensional (3-D) microporous lanthanide-based metal–organic frameworks, {[Ln­(DMTDC)1.5­(H2O)2]·DEF}n (type I, Ln = Eu 1, Tb 2), {[Ln­(DMTDC)1.5­(H2O)2]·0.5DMF·0.5H2O}n (type II, Ln = Gd 3, Dy 4, Er 5), and {[Ln4(DMTDC)6­(DMF)2]·0.5DMF·1.5H2O}n (type III, Ln = Er 6) (H2DMTDC = 3,4-dimethylthieno­[2,3-b]­thiophene-2,5-dicarboxylic acid, DEF = N,N′-diethylformamide, DMF = N,N′-dimethylformamide), have been solventhermally synthesized. Types I and II are isostructural, which exhibit 1-D triangular channels constructed by double-stranded rod-shaped {Ln­(CO2)2}n chains. Type III demonstrates an intriguing framework with triple-stranded rod-shaped {Ln­(CO2)3}n chains arranged along the (1,1,0) and (1,–1,0) axes and possesses two kinds of triangular channels along two axes, respectively. Immobilization of the Lewis basic sites of thiophene groups induced gas adsorption and sensing properties into these microporous frameworks. Complexes 5­(Er) and 6­(Er) display moderate adsorption properties toward N2 and CO2, and the Qst of CO2 are as high as 36.3 and 34.8 kJ mol–1, respectively. Complexes 1­(Eu) and 2­(Tb) exhibit sensing properties toward nitrobenzene, acetone, and the Cu2+ ion in both DMF and aqueous solution. Complex 3­(Gd) shows a significant magnetocaloric effect with ΔSm = 24.3 J·kg–1·K–1 at 3.0 K and 7 T. Complex 4­(Dy) exhibits slow magnetic relaxation with the energy barrier Δ/kB of 48.29 K.以甲基取代噻吩并[2,3-b]噻吩二羧酸盐为原料，通过溶剂热法合成得到三类三维（3D）微孔镧基金属有机框架（metal–organic frameworks，MOFs），具体包括：{[Ln(DMTDC)1.5(H2O)2]·DEF}n（I型，Ln=Eu 1、Tb 2）、{[Ln(DMTDC)1.5(H2O)2]·0.5DMF·0.5H2O}n（II型，Ln=Gd 3、Dy 4、Er 5）以及{[Ln4(DMTDC)6(DMF)2]·0.5DMF·1.5H2O}n（III型，Ln=Er 6）。其中，H2DMTDC=3,4-二甲基噻吩并[2,3-b]噻吩-2,5-二羧酸（3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid），DEF=N,N′-二乙基甲酰胺（N,N′-diethylformamide），DMF=N,N′-二甲基甲酰胺（N,N′-dimethylformamide）。I型与II型为同构结构，均呈现由双链棒状{Ln(CO2)2}n链构筑的一维三角孔道。III型框架具有独特拓扑结构：沿(1,1,0)与(1,-1,0)轴方向排布有三链棒状{Ln(CO2)3}n链，并分别沿两个轴方向存在两类三角孔道。噻吩基团的路易斯碱性位点被固定后，赋予此类微孔框架气体吸附与传感性能。配合物5(Er)与6(Er)对氮气（N2）和二氧化碳（CO2）具有中等吸附性能，其二氧化碳的等量吸附热（Qst）分别高达36.3 kJ·mol–1与34.8 kJ·mol–1。配合物1(Eu)与2(Tb)在DMF及水溶液中，均对硝基苯、丙酮与铜离子（Cu2+）展现出传感性能。配合物3(Gd)具有显著的磁热效应：在3.0 K、7 T条件下，其磁熵变ΔSm为24.3 J·kg–1·K–1。配合物4(Dy)则表现出慢磁弛豫行为，其能垒Δ/kB为48.29 K。