二维镨-镉异核金属配合物的水热合成、晶体结构及谱学性能研究

利用水热合成法，以柔性的乙二胺四乙酸（H₄EDTA）为配体、氧化镨以及氯化镉为金属源合成了一种镨-镉异金属-有机配合物Pr₂Cd₃(EDTA)₃(H₂O)₁₁]·14H₂O （1）。通过X射线单晶衍射确定化合物1的结构，该化合物属单斜晶系的C2空间群，a=16.154(3) Å，b=14.863(3) Å，c=14.875(3) Å，β=115.855(3)°，V=3214.2(9) Å3, Z=2，化合物1的结构中存在纳米尺寸大小的“心形”Pr₆Cd₆O₁₂轮簇。其中Cd2+的配位数为7，采取单帽三棱柱的配位构型，而Pr3+采取十配位双帽四方反棱柱的构型。EDTA4-配体的四个羧基全部去质子化，与一个Cd2+和两个Pr3+配位，其中4个羧基氧原子和2个氮原子都与Cd2+配位，两个羧基分别桥连1个Pr3+。Pr3+和Cd2+通过μ₂-O氧原子交替连接形成Pr₆Cd₆O₁₂轮簇，每个Pr₆Cd₆O₁₂轮簇与附近的6个Pr₆Cd₆O₁₂轮簇共边连接，从而形成一个二维（6, 3）层状结构。二维层再通过…AAA…类型的堆积方式形成三维超分子结构。游离水分子填充在二维层空隙中，与羧基以及配位水分子之间形成比较强的O-H…O氢键，这些氢键有利于结构的稳定。通过热重分析、稳态荧光光谱、热微扰二维红外相关光谱（2D-IR COS）、固体紫外-可见漫反射光谱等手段进一步对化合物1的谱学性能进行表征。在红外光谱上，由于存在大量的氢键，化合物的红外光谱在3 680～2 640 cm-1波数范围内出现宽而强的吸收谱带。同时配体H₄EDTA中的羧基脱去了质子氢，并且与金属离子发生配位，因此化合物1中羧基的C=O双键的伸缩振动吸收峰与未配位配体中的C=O双键的伸缩振动吸收峰相比，向低波数移动，在1 527 cm-1波数处出现吸收峰。固体荧光测试显示在325 nm的紫外光照射下，配合物1能发出强的360 nm左右的荧光，主要是由能量在Cd2+和EDTA4-之间发生明显的LMCT跃迁转移引起的，因此化合物1可作为一类潜在的发光材料。热微扰下的二维红外光谱显示，由于水分子与羧基及水分子之间存在氢键，使得O-H的伸缩振动吸收峰对热的微扰响应比较敏感。紫外-可见光谱测试显示化合物1在位于216 nm处出现很强的紫外吸收峰，归属于化合物1中EDTA4-配体的中n→σ*跃迁及π→π*，位于444, 468和484 nm的弱吸收峰，归属为Pr3+的f-f跃迁。

The Hydrothermal Synthesis,Crystal Structure and Spectral Properties of a 2D Pr-Cd Heterometal Complex

A 2D Pr-Cd heterometal-organic compound, Pr₂Cd₃(EDTA)₃(H₂O)₁₁]·(H₂O)₁₄(1) (H₄EDTA=ethylene diamine tetraacetic acid) has been successfully prepared by the H₄EDTA ligand, Pr₆O₁₁ and CdCl₂·2.5H₂O. The structure of 1 was determined by X-ray single-crystal diffraction. Compound 1 crystallizes in the monoclinic space group C2, a=16.154(3) Å，b=14.863(3)Å，c=14.875(3)Å，β=115.855(3)°，V=3 214.2(9)Å3, Z=2. There are nanosized heart-like Pr₆Cd₆O₁₂ wheel-clusters in the structure. The coordination geometries for the two seven-coordinated Cd2+ ions are both close to that of a monocapped trigonal prism. The Pr3+ ion is ten-coordinate and described as seriously distorted dicapped square antiprism. The completely deprotonated EDTA4- ligand link one Cd2+ and two Pr3+ ions. Four carboxylate O and two N atoms of the EDTA4- ligand are all coordinated to the Cd2+ cation and the remaining carboxylate groups connect one Pr3+ ion, respectively. The Pr3+ and Cd2+ cations are bridged by ₂-O alternatively to form a Pr₆Cd₆O₁₂ wheel-clusters. Each Pr₆Cd₆O₁₂ is linked to six surrounding wheels by sharing Pr3+, forming a highly ordered layered network. The 2D layers are further packed in …AAA… stacking mode and the free water molecules are suspended between the layers. There are strong O-H…O hydrogen-bond interactions between water molecules and carboxylate groups, and the O…O distance ranges from 2.666 to 3.050 Å. The hydrogen-bond interactions play an important role in stabling the structure. At the same time, PXRD, TG/DSC, IR and 2D IR correlation spectroscopy, solid Luminescent spectrum and UV-Visible absorption spectrum are studied. Because there are strong O-H…O hydrogen bonds, the IR spectrum of compound 1 shows broad bands around 3 680～2 640 cm-1. The four carboxylic acid groups of EDTA4- ligand are completely deprotonated, The C=O stretching vibrations peak of carboxylate groups of compound 1 shift lower wavenumber compared to H₄EDTA ligand. Compound 1 shows emission peak at 360 nm that can be assigned to LMCT transition of between Cd2+and EDTA4- when it is excited at 325 nm. Compound 1 is a potential luminescent material. 2D IR correlation spectrum of 1 indicates that the stretching vibrations of O-H are sensitive with the thermal perturbation because of strong hydrogen-bond interactions between water molecules and carboxylate groups. The UV-Visible absorption spectrum of compound 1 shows the absorption bands of n→σ* and π→π* transitions of EDTA4- ligand and f→f transition of Pr3+.