Perovskite-Like Polar Lanthanide Formate Frameworks of [NH2NH3][Ln(HCOO)4] (Ln = Tb–Lu and Y): Synthesis, Structures, Magnetism, and Anisotropic Thermal Expansion

A series of isostructural hydrazinium lanthanide (Ln) formate framework compounds of [NH2NH3]­[Ln­(HCOO)4] for Ln3+ ions from Tb3+ to Lu3+ and Y3+ have been successfully prepared by utilizing NH2NH3+. The compounds crystallize in orthorhombic polar space group Pca21, with cell parameters at 180 K of a = 18.2526(7)–18.1048(5) Å, b = 6.5815(2)–6.5261(2) Å, c = 7.6362(3)–7.5044(2) Å, and V = 917.33(6)–886.67(4) Å3, showing the effect of lanthanide contraction. The compounds possess polar perovskite-like structures incorporating the hydrazinium cations in the cavities of the NaCl-like framework, in which the Ln3+ ions in a bicapped trigonal prism are connected by anti–anti and syn–anti formate groups. The N–H···Oformate hydrogen-bonding interactions are between the hydrazinium cations and the anionic framework. One anti–anti formate group is frustrated by the competitive N–H···Oformate hydrogen-bonding interactions. It thus twists or flips upon warming, resulting in large anisotropic thermal expansion and negative thermal expansion below 180 K. A comparison with the transition metal and magnesium analogues revealed that the structural compactness, tighter binding of the hydrazinium cation by the framework, and symmetrically better match between the framework and ammonium cation for Ln compounds could inhibit the occurrence of phase transition in the series. The IR spectroscopic, thermal, and magnetic properties are investigated.以肼鎓阳离子（hydrazinium cation，NH₂NH₃⁺）为结构导向剂，成功合成了一系列通式为[NH₂NH₃][Ln(HCOO)₄]的同构肼鎓镧系（lanthanide, Ln）甲酸盐骨架化合物，其中Ln³+离子涵盖Tb³+至Lu³+以及Y³+。该系列化合物结晶于正交晶系极性空间群Pca2₁，在180 K下的晶胞参数为a=18.2526(7)~18.1048(5) Å，b=6.5815(2)~6.5261(2) Å，c=7.6362(3)~7.5044(2) Å，晶胞体积V=917.33(6)~886.67(4) ų，呈现出典型的镧系收缩（lanthanide contraction）效应。此类化合物具有类钙钛矿（perovskite-like）极性骨架结构，肼鎓阳离子填充于类氯化钠（NaCl-like）骨架的空腔中；其中Ln³+离子处于双帽三棱柱（bicapped trigonal prism）配位环境，并通过反-反（anti–anti）和顺-反（syn–anti）两种甲酸根基团实现桥连。肼鎓阳离子与阴离子骨架之间存在N–H···O甲酸根氢键相互作用，其中一个反-反甲酸根基团因竞争性的N–H···O甲酸根氢键作用发生构象畸变，升温时会发生扭转或翻转，进而使材料在180 K以下表现出显著的各向异性热膨胀（anisotropic thermal expansion）与负热膨胀（negative thermal expansion）行为。通过与过渡金属（transition metal）及镁的同构类似物对比可知，镧系化合物的骨架更紧凑，骨架对肼鎓阳离子的结合作用更强，且骨架与铵类阳离子的对称性匹配度更佳，这一特性可抑制该系列化合物发生相变。本研究还对该系列化合物的红外光谱（IR spectroscopic）、热学及磁学性能进行了系统表征与分析。