Two-Dimensional Heisenberg Antiferromagnets: Syntheses, X-ray Structures, and Magnetic Behavior of [Cu(pz)₂](ClO₄)₂, [Cu(pz)₂](BF₄)₂, and [Cu(pz)₂(NO₃)](PF₆)

We report on the syntheses, crystal structures, and magnetic susceptibilities of a family of copper pyrazine (pz)-based antiferromagnets with moderate in-plane magnetic exchange. These materials fall into two classes: monoclinic complexes [Cu(pz)2]A2 for A = ClO4 (1) or BF4 (2) and the tetragonal complex [Cu(pz)2(NO3)]PF6 (3). Compound 1 and its deuterated version [Cu(pz-d4)2](ClO4)2 (1a) crystallize in the space group C2/m at room temperature with disordered perchlorate anions. For both 1 and 2, the C centering of the Cu(II), S = 1/2, site yields four equivalent nearest neighbors, producing layers of Cu(II) ions bridged by the pz molecules, which map onto a square magnetic lattice. The layers are offset such that Cu(II) ions lie above and below the holes of adjacent layers. Compound 3 crystallizes in the space group I4/mcm with a layer structure similar to those of 1 and 2 but with Cu(II) ions of adjacent layers stacked above each other and bridged by semicoordinate NO3- ions. The variable-temperature susceptibilities in these compounds approximate a two-dimensional Heisenberg antiferromagnet with J values within the layers of 17.5(3) K (1), 15.3(3) K (2), and 10.8(3) K (3). Ordering transitions are observed in the magnetic data at 4.2(3) and 4.3(5) K for 1 and 2, respectively.本工作报道了一类具有中等面内磁交换作用的、以吡嗪（pyrazine，缩写pz）为配体的铜基反铁磁材料的合成、晶体结构与磁化率特性。该类材料可分为两类：其一为单斜晶系配合物[Cu(pz)₂]A₂（其中A⁻为ClO₄⁻（化合物1）或BF₄⁻（化合物2））；其二为四方晶系配合物[Cu(pz)₂(NO₃)]PF₆（化合物3）。化合物1及其氘代产物[Cu(pz-d₄)₂](ClO₄)₂（标记为1a）在室温下以空间群C2/m结晶，其高氯酸根阴离子存在无序分布。对于化合物1和2，自旋S=1/2的铜(II)离子的C心晶格位点拥有4个等价的近邻离子，由此形成由吡嗪分子桥联的铜(II)离子层，该层可对应于正方形磁晶格。各层之间存在错位，使得相邻层的孔洞上方与下方均排布有铜(II)离子。化合物3以空间群I4/mcm结晶，其层状结构与化合物1、2相似，但相邻层的铜(II)离子呈堆叠排布，并由半配位硝酸根（NO₃⁻）离子桥联。上述化合物的变温磁化率数据符合二维海森堡反铁磁模型，其层内磁交换耦合常数J分别为：化合物1的17.5(3) K、化合物2的15.3(3) K以及化合物3的10.8(3) K。磁化率测试数据显示，化合物1与2分别在4.2(3) K与4.3(5) K处出现磁有序转变。