New Family of Ferric Spin Clusters Incorporating Redox-Active <i>ortho</i>-Dioxolene Ligands

Seven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe3(OAc)3(L)3] (1), where LH2 = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe3(OAc)(Cat)(L)3] (2), [Fe6(OAc)2(Cat)4(L)4] (3), [Fe4(3,5-DBCat)2(L)4] (4), [Bu4N][Fe4(OAc)(3,5-DBCat)4(L)2] (5a, 5− is the complex monoanion [Fe4(OAc)(3,5-DBCat)4(L)2]−), [Fe4(OAc)(3,5-DBCat)3(3,5-DBSQ)(L)2] (6), [Fe2(Cl4Cat)2(L)(LH2)(H2O)] (7), and [Et3NH]2[Fe2(Cl4Cat)2(L)2] (8a, 82− is the complex dianion [Fe2(Cl4Cat)2(L)2]2−), where CatH2 = catechol; 3,5-DBCatH2 = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl4CatH2 = tetrachlorocatechol. While compounds 2−4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV−visible spectroscopy, voltammetry, UV−visible spectroelectrochemistry, and magnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5−, and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5− is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ− ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibility measurements for compounds 3, 4, 5a, 6, 7, and 8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5−, 7 and 82−, and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxo-bridged ferric complexes.以三核前驱体[Fe3(OAc)3(L)3]（记为化合物1，其中LH2=2-[[(2-羟乙基)亚氨基]苯甲基]苯酚）为起始原料，合成了7种包含多齿希夫碱（Schiff base）与不同取代儿茶酚根配体的新型双核、三核、四核及六核铁配合物。上述配合物分别以如下形式分离得到：[Fe3(OAc)(Cat)(L)3]（2）、[Fe6(OAc)2(Cat)4(L)4]（3）、[Fe4(3,5-DBCat)2(L)4]（4）、[Bu4N][Fe4(OAc)(3,5-DBCat)4(L)2]（5a，其中5⁻为单核配合物阴离子[Fe4(OAc)(3,5-DBCat)4(L)2]⁻）、[Fe4(OAc)(3,5-DBCat)3(3,5-DBSQ)(L)2]（6）、[Fe2(Cl4Cat)2(L)(LH2)(H2O)]（7）以及[Et3NH]2[Fe2(Cl4Cat)2(L)2]（8a，其中8²⁻为双核配合物二阴离子[Fe2(Cl4Cat)2(L)2]²⁻），其中CatH2=儿茶酚（catechol）；3,5-DBCatH2=3,5-二叔丁基儿茶酚（3,5-di-tert-butyl-catechol）；3,5-DBSQH=3,5-二叔丁基半醌（3,5-di-tert-butyl-semiquinone）；Cl4CatH2=四氯儿茶酚（tetrachlorocatechol）。其中化合物2−4、5a、7及8a可通过将前驱体1与对应取代儿茶酚直接反应制得，而化合物6则通过对5a进行化学氧化合成。所有上述配合物均通过单晶X射线衍射（single crystal X-ray diffraction）、红外光谱（infrared spectroscopy）、紫外−可见光谱（UV−visible spectroscopy）、伏安法（voltammetry）、紫外−可见光谱电化学（UV−visible spectroelectrochemistry）以及磁化率（magnetic susceptibility）与磁化强度（magnetization）测量进行了完整表征。对三种四核配合物（4、5⁻及6）的电化学研究表明，除了对应三价铁中心还原为二价铁的还原过程外，邻二氧杂环戊烯配体（o-dioxolene ligands）还存在多组可逆氧化还原过程。通过对化合物5a的化学氧化过程开展伏安法研究，并结合对其对应电化学氧化过程的光谱电化学分析，结果显示单电子氧化产物存在两种异构体形式。一种寿命较短的中性物种（5）与配合物5⁻具有完全一致的配体排布模式，是化学氧化与电化学氧化共同的动力学产物。数小时后，该物种会发生显著的结构重排，转化为配合物6；这一转化过程主要由3,5-DBSQ⁻配体倾向于以非桥联模式配位的特性所驱动。对化合物3、4、5a、6、7及8a的变温磁化率（variable temperature magnetic susceptibility）测量结果表明，其磁行为主要由成对反铁磁交换相互作用主导：配合物3的基态自旋S = 0的能级隔离度较差，配合物4、5⁻、7及8²⁻的基态自旋S = 0均具有良好的能级隔离度，而配合物6的基态自旋S = 1/2同样具备优异的能级隔离度。上述基态自旋数值通过低温变场磁化强度（low temperature variable field magnetization）测量得到了验证。采用合适的哈密顿量（Hamiltonians）对化合物3、4、5a、6、7及8a的磁化率随温度变化曲线进行了拟合与/或模拟，推导得到了J值，该结果与此前报道的烷氧基桥联铁(III)配合物的磁结构相关性一致。