Structures, Magnetochemistry, Spectroscopy, Theoretical Study, and Catechol Oxidase Activity of Dinuclear and Dimer-of-Dinuclear Mixed-Valence MnIIIMnII Complexes Derived from a Macrocyclic Ligand

The work in this paper presents syntheses, characterization, magnetic properties (experimental and density functional theoretical), catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) studies of two mixed-valence dinuclear MnIIIMnII complexes, [MnIIIMnIIL­(μ-O2CMe)­(H2O)2]­(ClO4)2·H2O·MeCN (1) and [MnIIIMnIIL­(μ-O2CPh)­(MeOH)­(ClO4)]­(ClO4) (2), and a MnIIIMnIIMnIIMnIII complex, [{MnIIIMnIIL­(μ-O2CEt)­(EtOH)}2(μ-O2CEt)]­(ClO4)3 (3), derived from the Robson-type macrocycle H2L, which is the [2 + 2] condensation product of 2,6-diformyl-4-methylphenol and 2,2-dimethyl-1,3-diaminopropane. In 1 and 2 and in two MnIIIMnII units in 3, the two metal centers are bridged by a bis­(μ-phenoxo)-μ-carboxylate moiety. The two MnII centers of the two MnIIIMnII units in 3 are bridged by a propionate moiety, and therefore this compound is a dimer of two dinuclear units. The coordination geometry of the MnIII and MnII centers are Jahn–Teller distorted octahedral and distorted trigonal prism, respectively. Magnetic studies reveal weak ferro- or antiferromagnetic interactions between the MnIII and MnII centers in 1 (J = +0.08 cm–1), 2 (J = −0.095 cm–1), and 3 (J1 = +0.015 cm–1). A weak antiferromagnetic interaction (J2 = −0.20 cm–1) also exists between the MnII centers in 3. DFT methods properly reproduce the nature of the exchange interactions present in such systems. A magneto-structural correlation based on Mn–O bridging distances has been proposed to explain the different sign of the exchange coupling constants. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH2) as the substrate, catecholase activity of all the three complexes has been checked in MeCN and MeOH, revealing that all three are active catalysts with Kcat values lying in the range 7.5–64.7 h–1. Electrospray ionization mass (ESI-MS positive) spectra of the complexes 1–3 have been recorded in MeCN solutions, and the positive ions have been well characterized. ESI-MS positive spectrum of complex 1 in presence of 3,5-DTBCH2 has also been recorded, and a positive ion, [MnIIIMnIIL­(μ-3,5-DTBC2–)]+, having most probably a bridging catecholate moiety has been identified.

本文工作报道了两种混合价双核MnIIIMnII配合物[MnIIIMnIIL(μ-O₂CMe)(H₂O)₂](ClO₄)₂·H₂O·MeCN（1）与[MnIIIMnIIL(μ-O₂CPh)(MeOH)(ClO₄)](ClO₄)（2），以及一种MnIIIMnIIMnIIMnIII配合物[{MnIIIMnIIL(μ-O₂CEt)(EtOH)}₂(μ-O₂CEt)](ClO₄)₃（3）的合成、表征、磁性质（实验与密度泛函理论（Density Functional Theoretical，DFT））、儿茶酚酶活性及电喷雾电离正离子质谱（Electrospray Ionization Mass Spectroscopic，ESI-MS正离子模式）研究。所用配体H₂L为Robson型大环（Robson-type macrocycle），是2,6-二甲酰基-4-甲基苯酚与2,2-二甲基-1,3-丙二胺发生[2+2]缩合得到的产物。在配合物1、2以及3中的两个MnIIIMnII单元内，两个金属中心通过双(μ-苯氧基)-μ-羧酸酯桥联基团相连。3中两个MnIIIMnII单元的MnII中心之间由丙酸酯桥联，因此该化合物为双核单元的二聚体。MnIII与MnII中心的配位几何构型分别为姜-泰勒（Jahn-Teller）畸变八面体与畸变三角棱柱。磁性质研究显示，在1（J=+0.08 cm⁻¹）、2（J=−0.095 cm⁻¹）与3（J₁=+0.015 cm⁻¹）的MnIII与MnII中心之间存在弱铁磁性或反铁磁性相互作用；3的MnII中心之间还存在弱反铁磁性相互作用（J₂=−0.20 cm⁻¹）。密度泛函理论（DFT）方法可准确复现此类体系中交换相互作用的性质。研究人员提出了基于Mn-O桥联距离的磁构关系，用以解释交换耦合常数符号的差异。以3,5-二叔丁基邻苯二酚（3,5-Di-tert-butyl Catechol，3,5-DTBCH₂）为底物，在乙腈（MeCN）与甲醇（MeOH）中测试了三种配合物的儿茶酚酶活性，结果表明三种配合物均为活性催化剂，其催化周转数（Kcat）介于7.5~64.7 h⁻¹之间。在乙腈溶液中记录了配合物1~3的电喷雾电离正离子质谱（ESI-MS）光谱，并对其正离子峰进行了充分表征。此外，还记录了添加3,5-DTBCH₂后配合物1的ESI-MS正离子光谱，鉴定出一个正离子峰[MnIIIMnIIL(μ-3,5-DTBC²⁻)]⁺，该离子极有可能通过桥联儿茶酚酸根基团连接。