Anion-Directed Synthesis of Metal−Organic Frameworks Based on 2-Picolinate Cu(II) Complexes: A Ferromagnetic Alternating Chain and Two Unprecedented Ferromagnetic Fish Backbone Chains

Three new polynuclear copper(II) complexes of 2-picolinic acid (Hpic), {[Cu2(pic)3(H2O)]ClO4}n (1), {[Cu2(pic)3(H2O)]BF4}n (2), and [Cu2(pic)3(H2O)2(NO3)]n (3), have been synthesized by reaction of the “metalloligand” [Cu(pic)2] with the corresponding copper(II) salts. The compounds are characterized by single-crystal X-ray diffraction
analyses and variable-temperature magnetic measurements. Compounds 1 and 2 are isomorphous and crystallize
in the triclinic system with space group P1̄, while 3 crystallizes in the monoclinic system with space group P21/n.
The structural analyses reveal that complexes 1 and 2 are constructed by “fish backbone” chains through syn−anti
(equatorial−equatorial) carboxylate bridges, which are linked to one another by syn−anti (equatorial−axial) carboxylate
bridges, giving rise to a rectangular grid-like two-dimensional net. Complex 3 is formed by alternating chains of
syn−anti carboxylate-bridged copper(II) atoms, which are linked together by strong H bonds involving coordinated
nitrate ions and water molecules and uncoordinated oxygen atoms from carboxylate groups. The different coordination
ability of the anions along with their involvement in the H-bonding network seems to be responsible for the difference
in the final polymeric structures. Variable-temperature (2−300 K) magnetic susceptibility measurement shows the
presence of weak ferromagnetic coupling for all three complexes that have been fitted with a fish backbone model
developed for 1 and 2 (J = 1.74 and 0.99 cm−1; J‘ = 0.19 and 0.25 cm−1, respectively) and an alternating chain
model for 3 (J = 1.19 cm−1 and J‘ = 1.19 cm−1).

以“金属配体（metalloligand）”[Cu(pic)₂]与相应铜(II)盐反应，合成了3种2-吡啶甲酸（2-picolinic acid，Hpic）的多核铜(II)配合物：{[Cu₂(pic)₃(H₂O)]ClO₄}ₙ(1)、{[Cu₂(pic)₃(H₂O)]BF₄}ₙ(2)以及[Cu₂(pic)₃(H₂O)₂(NO₃)]ₙ(3)。采用单晶X射线衍射分析与变温磁性测试对上述化合物进行了表征。配合物1与2为同构体，均结晶于三斜晶系，空间群为P1̄；而配合物3结晶于单斜晶系，空间群为P2₁/n。结构解析表明，配合物1和2通过顺反（平伏位-平伏位）羧酸根桥联形成“鱼骨状”链，再经由顺反（平伏位-轴向）羧酸根桥联相互连接，最终构筑为矩形网格状二维网络。配合物3则由交替的顺反羧酸根桥联铜(II)原子链构成，链间通过配位硝酸根、水分子与羧酸根未配位氧原子之间的强氢键相互连接。阴离子的配位能力差异及其参与氢键网络的特性，被认为是导致最终聚合物结构差异的主要原因。变温（2~300 K）磁化率测试结果显示，三种配合物均存在弱铁磁耦合；其中针对配合物1和2采用鱼骨型模型拟合得到磁耦合参数J=1.74、0.99 cm⁻¹，J'=0.19、0.25 cm⁻¹，针对配合物3则采用交替链模型拟合得到磁耦合参数J=1.19 cm⁻¹、J'=1.19 cm⁻¹。