Hydrothermal Synthesis and Structural Characterization of Metal Organophosphonate Oxide Materials: Role of Metal-Oxo Clusters in the Self Assembly of Metal Phosphonate Architectures

Two new metal organophosphonate oxide materials with formulas [CuII4CuI2(L)2(2,2′-bpy)6(HPW12O40)]n·4nH2O (1) and [Cu­(2,2′-bpy)­VO2(OH)­(H2L)]n (2) have been synthesized starting from the Cu­(II) salts, 2,2′-bipyridine (2,2′-bpy), p-xylylenediphosphonic acid (H4L), and sodium tungstate (for 1)/ammonium metavanadate (for 2). Both the compounds 1 and 2 are characterized by routine elemental analyses, IR spectroscopy, thermogravimetric (TG) analysis, and unambiguously characterized by single crystal X-ray crystallography. The crystal structure of compound 1 consists of 2D copper phosphonate layers connected by the Keggin heteropolyanion to form a three-dimensional (3D) framework. The copper phosphonate layers in compound 1 are fabricated by the rare copper hexanuclear clusters in which the four terminal Cu­(II) centers form two eight-membered Cu-dimer (Cu2P2O4) rings (top and the bottom) that are connected to each other by the two central Cu­(I) atoms of four-membered Cu2O2 rings. These hexanuclear assemblies are connected to each other along the plane through the p-xylyl linkers to form a two-dimensional (2D) layer. Compound 1 is a unique example in terms of the existence of a hexanuclear copper phosphonate cluster in the 3D coordination matrix. Compound 2 has a 2D structure, in which the one-dimensional [Cu­(2,2′-bpy)­(H2L)]n chains are connected by the VO2OH subunits to from a 2D layer. The formation of VO2OH in compound 2 ceases the formation of eight-membered Cu-dimer rings. The self-assembly of the polyoxometalates plays an important role in the formation of the metal organophosphonate phases.

本研究以二价铜盐、2,2'-联吡啶（2,2′-bipyridine，简称2,2′-bpy）、对苯二亚甲基二膦酸（H4L）以及钨酸钠（用于合成化合物1）/偏钒酸铵（用于合成化合物2）为原料，成功合成了两种新型金属有机膦酸盐氧化物材料，其分子式分别为[CuII4CuI2(L)2(2,2′-bpy)6(HPW12O40)]n·4nH2O（化合物1）与[Cu­(2,2′-bpy)­VO2(OH)­(H2L)]n（化合物2）。通过常规元素分析、红外（IR）光谱、热重（TG）分析对两种化合物进行了表征，并借助单晶X射线衍射技术完成了明确的结构解析。化合物1的晶体结构由二维铜膦酸盐层构成，该层通过Keggin型杂多阴离子（Keggin heteropolyanion）桥联形成三维（3D）骨架。化合物1中的铜膦酸盐层由罕见的六核铜簇构筑而成，该簇中4个端基二价铜（CuII）中心形成两个八元铜二聚体（Cu2P2O4）环（分别位于上下两侧），而两个四元Cu2O2环的中心一价铜（CuI）原子将这两个八元环相互连接。这些六核组装体通过对苯二亚甲基连接基沿平面相互桥联，最终形成二维（2D）层状结构。就三维配位骨架中存在六核铜膦酸盐簇而言，化合物1属于首例此类结构。化合物2呈现二维层状结构，其中一维[Cu­(2,2′-bpy)­(H2L)]n链通过VO2OH结构单元桥联，进而形成二维层状结构。化合物2中VO2OH结构单元的形成，抑制了八元铜二聚体环的生成。多金属氧酸盐（polyoxometalates）的自组装过程在金属有机膦酸盐物相的形成中发挥着关键作用。