Solvent-Controlled Syntheses, Structure, and Magnetic Properties of Trinuclear Mn(II)-Based Metal–Organic Frameworks

Solvothermal reactions of manganese­(II) salts with hexa­[4-(carboxyphenyl)­oxamethyl]-3-oxapentane acid (H6L) afforded a family of porous metal–organic frameworks, namely, Mn3(L)­(DMA)4·2DMA (1, C2/c), Mn3(L)­(H2O)2(DMF)2·8DMF (2, Cc), and Mn3(L)­(H2O)2(DMF)·4DMF (3, P21/c). All compounds have been characterized by elemental analysis and thermogravimetric analysis and structurally confirmed by single-crystal X-ray diffractions. Their structures consist of three types of trinuclear MnII subunits, which are further bridged by the carboxylic ligand, resulting in two types of topological nets (pts and sra). All of the MnII3 subunits are terminally coordinated by solvent molecules. The structure of the MnII3 core in 1 is symmetric with an inversion center, whereas those in 2 and 3 display a symmetry-breaking phenomenon. Their magnetic behaviors exhibit interesting variations, in which the local net magnetization at low temperature increases gradually from 1 to 3. Such magnetic evolution behavior in trinuclear subunits has never been observed previously.

以锰(II)盐与六[4-(羧基苯基)草酰甲基]-3-氧杂戊烷酸（hexa[4-(carboxyphenyl)oxamethyl]-3-oxapentane acid, H6L）进行溶剂热反应（solvothermal reaction），得到一系列多孔金属有机框架（metal–organic frameworks, MOFs），分别为Mn3(L)(DMA)4·2DMA（1，空间群C2/c，其中DMA为N,N-二甲基乙酰胺（N,N-Dimethylacetamide））、Mn3(L)(H2O)2(DMF)2·8DMF（2，空间群Cc，其中DMF为N,N-二甲基甲酰胺（N,N-Dimethylformamide））以及Mn3(L)(H2O)2(DMF)·4DMF（3，空间群P21/c）。所有化合物均通过元素分析（elemental analysis）与热重分析（thermogravimetric analysis）进行表征，并经单晶X射线衍射（single-crystal X-ray diffraction）确证其晶体结构。其晶体结构包含三种三核锰(II)次级结构单元（trinuclear Mn(II) subunits），这些单元进一步被羧酸配体（carboxylic ligand）桥连，最终形成两类拓扑网络（topological nets，pts与sra）。所有三核锰(II)次级结构单元均由溶剂分子进行端基配位。化合物1中的三核锰(II)核心结构具有对称中心对称性，而化合物2与3中的对应结构则呈现对称性破缺（symmetry-breaking）现象。三者的磁学行为展现出有趣的变化规律：低温下的局部净磁化强度从化合物1至3逐渐递增。这种基于三核次级结构单元的磁演化行为此前尚未见诸报道。