A New Series of Chiral Metal Formate Frameworks of [HONH3][MII(HCOO)3] (M = Mn, Co, Ni, Zn, and Mg): Synthesis, Structures, and Properties

We report the synthesis, crystal structures, IR, and thermal, dielectric, and magnetic properties of a new series of ammonium metal formate frameworks of [HONH3]­[MII(HCOO)3] for M = Mn, Co, Ni, Zn, and Mg. They are isostructural and crystallize in the nonpolar chiral orthorhombic space group P212121, a = 7.8121(2)–7.6225(2) Å, b = 7.9612(3)–7.7385(2) Å, c = 13.1728(7)–12.7280(4) Å, and V = 819.27(6)–754.95(4) Å3. The structures possess anionic metal formate frameworks of 49·66 topology, in which the octahedral metal centers are connected by the anti–anti formate ligands and the hydroxylammonium is located orderly in the channels, forming strong O/N–H···Oformate hydrogen bonds with the framework. HONH3+ with only two non-H atoms favors the formation of the dense chiral 49·66 frameworks, instead of the less dense 412·63 perovskite frameworks for other monoammoniums of two to four non-H atoms because of its small size and its ability to form strong hydrogen bonding. However, the larger size of HONH3+ compared to NH4+ resulted in simple dielectric properties and no phase transitions. The three magnetic members (Mn, Co, and Ni) display antiferromagnetic long-range ordering of spin canting, at Néel temperatures of 8.8 K (Mn), 10.9 K (Co), and 30.5 K (Ni), respectively, and small spontaneous magnetizations for the Mn and Ni members but large magnetization for the Co member. Thermal and IR spectroscopic properties are also reported.

本工作报道了一系列通式为[HONH3][MⅡ(HCOO)3]的新型甲酸铵金属骨架材料的合成、晶体结构、红外光谱、热学、介电及磁学性质，其中M分别为Mn、Co、Ni、Zn和Mg。该系列材料为同构体，均结晶于非极性手性正交晶系空间群P2₁2₁2₁，晶胞参数a=7.8121(2)~7.6225(2) Å，b=7.9612(3)~7.7385(2) Å，c=13.1728(7)~12.7280(4) Å，晶胞体积V=819.27(6)~754.95(4) ų。其结构包含具有4⁹·6⁶拓扑的阴离子型甲酸金属骨架，其中八面体配位的金属中心由反-反构型的甲酸配体桥连，羟胺阳离子（hydroxylammonium）有序填充于骨架孔道中，并与骨架形成强O/N-H···O甲酸根氢键。由于羟胺阳离子仅含两个非氢原子，尺寸较小且可形成强氢键，因此其更易形成致密的手性4⁹·6⁶骨架，而非其他含2~4个非氢原子的单铵盐所偏好的低致密性4¹²·6³型钙钛矿（perovskite）骨架。但相较于铵根阳离子（NH4+），羟胺阳离子尺寸更大，导致该系列材料仅表现出简单的介电行为，且未发生相变。其中含Mn、Co、Ni的三个磁性体系均表现出自旋倾斜型反铁磁长程有序，奈尔温度（Néel temperature）分别为8.8 K（Mn）、10.9 K（Co）与30.5 K（Ni）；Mn和Ni体系的自发磁化强度较小，而Co体系则具有较大的自发磁化强度。本工作同时报道了该系列材料的热学与红外光谱性质。