Synthesis, Structural Analysis, and Magnetic Properties of Ethylmalonate-Manganese(II) Complexes

Five manganese(II) complexes of formulas [Mn2(Etmal)2(H2O)2(L)]n (1–4) and {[Mn(Etmal)2(H2O)][Mn(H2O)4]}n (5) with H2Etmal = ethylmalonic acid (1–5) and L = 1,2-bis(4-pyridyl)ethane (bpa) (1), 4,4′-azobispyridine (azpy) (2), 4,4′-bipyridyl (4,4′-bpy) (3), and 1,2-bis(4-pyridyl)ethylene (bpe) (4) were synthesized and structurally characterized by single crystal X-ray diffraction. Their thermal behavior and variable-temperature magnetic properties were also investigated. The structure of the compounds 1–4 consists of corrugated layers of aquamanganese(II) units with intralayer carboxylate-ethylmalonate bridges in the anti-syn (equatorial-equatorial) coordination mode which are linked through bis-monodentate bpa (1), azpy (2), 4,4′-bpy (3), and bpe (4) ligands to build up a three-dimensional (3D) framework. The structure of compound 5 is made up by zigzag chains of manganese(II) ions with a regular alternation of [Mn(H2O)4]2+ and chiral (either Δ or λ enantiomeric forms) [Mn(Etmal)2(H2O)]2– units within each chain. In contrast to the bidentate/bis-monodentate coordination mode of the Etmal ligand in 1–4, it adopts the bidentate/monodentate coordination mode in 5 with the bridging carboxylate-ethylmalonate also exhibiting the anti-syn conformation but connecting one equatorial and an axial position from adjacent metal centers. The manganese–manganese separation through the carboxylate-ethylmalonate bridge in 1–5 vary in the range 5.3167(4)–5.5336(7) Å. These values are much shorter than those across the extended bis-monodentate N-donors in 1–4 with longest/shortest values of 11.682(3) (3)/13.9745(9) Å (4). Compounds 1–5 exhibit an overall antiferromagnetic behavior, where the exchange pathway is provided by the carboxylate-ethylmalonate bridge. Monte Carlo simulations based on the classical spin approach (1–5) were used to successfully reproduce the magnetic data of 1–5.

本研究合成了通式分别为[Mn2(Etmal)2(H2O)2(L)]n（1~4）与{[Mn(Etmal)2(H2O)][Mn(H2O)4]}n（5）的5种锰(II)配合物，其中H2Etmal为乙基丙二酸（ethylmalonic acid，1~5），配体L分别为1,2-双(4-吡啶基)乙烷（1,2-bis(4-pyridyl)ethane，bpa，对应配合物1）、4,4'-偶氮双吡啶（4,4′-azobispyridine，azpy，对应配合物2）、4,4'-联吡啶（4,4′-bipyridyl，4,4'-bpy，对应配合物3）与1,2-双(4-吡啶基)乙烯（1,2-bis(4-pyridyl)ethylene，bpe，对应配合物4）；所有配合物均通过单晶X射线衍射（single crystal X-ray diffraction）完成结构表征。同时还对其热行为与变温磁学性质展开了研究。配合物1~4的结构为波纹状层状结构，层内包含以反-顺（平伏-平伏）配位模式形成的羧酸盐-乙基丙二酸桥联水合锰(II)结构单元，该层再通过双单齿配体bpa、azpy、4,4'-bpy与bpe桥联，构筑得到三维（3D）骨架。配合物5的结构由锰(II)离子形成的锯齿形链构成，每条链内存在[Mn(H2O)4]2+与手性（Δ或λ对映异构体形式）[Mn(Etmal)2(H2O)]2–单元的规则交替排布。与配合物1~4中Etmal配体的双齿/双单齿配位模式不同，其在配合物5中采取双齿/单齿配位模式；此时桥联的羧酸盐-乙基丙二酸仍保持反-顺构象，但连接相邻金属中心的一个平伏位与一个轴向位。配合物1~5中通过羧酸盐-乙基丙二酸桥联的锰-锰间距介于5.3167(4)~5.5336(7) Å之间，该间距远短于配合物1~4中通过延伸的双单齿氮供体配体的锰-锰间距，后者的最长/最短间距分别为11.682(3)（对应配合物3）与13.9745(9) Å（对应配合物4）。配合物1~5整体表现出反铁磁行为，其磁交换途径由羧酸盐-乙基丙二酸桥联配体提供。基于经典自旋模型的蒙特卡洛（Monte Carlo）模拟被成功用于重现配合物1~5的磁学实验数据。