Controlled Redox Conversion of New X-ray-Characterized Mono- and Dinuclear Heptacoordinated Mn(II) Complexes into Di-μ-oxo-dimanganese Core Complexes

Two heptacoordinated Mn(II) complexes are isolated and X-ray characterized using the well-known tpen ligand
(tpen = N,N,N‘,N‘-tetrakis(2-pyridylmethyl)-1,2-ethanediamine):  [(tpen)Mn(OH2)](ClO4)2 (1(ClO4)2) and [(tpen)Mn(μ-OAc)Mn(tpen)](ClO4)3·2H2O (2(ClO4)3·2H2O). Crystallographic data for 1(ClO4)2 at 110(2) K (respectively at 293(2) K): monoclinic, space group C2/c, a = 15.049(3) Å (15.096(3) Å), b = 9.932(2) Å (10.105(2) Å), c = 19.246(4)
Å (19.443(4) Å), β = 94.21(3)° (94.50(3)°), Z = 4. Crystallographic data for 2(ClO4)3·0.5(C2H5)2O at 123(2) K:
triclinic, space group P1̄, a = 12.707(3) Å, b = 12.824(3) Å, c = 19.052(4) Å, α = 102.71(3)°, β = 97.83(3)°,
γ = 98.15(3)°, Z = 2. Investigation of the variation upon temperature of the molar magnetic susceptibility of
compound 2(ClO4)3·2H2O reveals a weak antiferromagnetic exchange interaction between the two high-spin Mn(II)
ions (J = −0.65 ± 0.05 cm-1, H = −JŜ1·Ŝ2). EPR spectra are recorded on powder samples and on frozen
acetonitrile solutions, demonstrating the maintenance upon dissolution of the heptacoordination of Mn in complex
1 while complex 2 partially dissociates. Electrochemical responses of complexes 1 and 2 are investigated in
acetonitrile, and bulk electrolyses are performed at oxidative potential in the presence of various amounts of 2,6-lutidine (0−2.65 equiv per Mn ion). The formation from either 1 or 2 of the mixed-valent complex [(tpen)MnIII(μ-O)2MnIV(tpen)]3+ (3) is established from mass spectrometry and EPR and IR spectroscopy measurements. When
reaction is started from 2, formation of [(tpen)MnIV(μ-O)2(μ-OAc)MnIV]3+ (4) is evidenced from cyclic voltammetry,
EPR, and UV−vis data. The Mn vs tpen ratio in the electrogenerated complexes is accurately controlled by the
quantity of additional 2,6-lutidine. The role of tpen as a base is discussed.

本研究采用经典配体tpen（N,N,N',N'-四(2-吡啶甲基)-1,2-乙二胺）分离得到两种七配位二价锰（Mn(II)）配合物，并通过X射线衍射完成表征：[(tpen)Mn(OH₂)](ClO₄)₂（记为1(ClO₄)₂）与[(tpen)Mn(μ-OAc)Mn(tpen)](ClO₄)₃·2H₂O（记为2(ClO₄)₃·2H₂O）。配合物1(ClO₄)₂在110(2) K（对应293(2) K）下的晶体学参数如下：单斜晶系，空间群C2/c，晶胞参数a = 15.049(3) Å（15.096(3) Å），b = 9.932(2) Å（10.105(2) Å），c = 19.246(4) Å（19.443(4) Å），β = 94.21(3)°（94.50(3)°），Z = 4。配合物2(ClO₄)₃·0.5(C₂H₅)₂O在123(2) K下的晶体学参数为：三斜晶系，空间群P1̄，晶胞参数a = 12.707(3) Å，b = 12.824(3) Å，c = 19.052(4) Å，α = 102.71(3)°，β = 97.83(3)°，γ = 98.15(3)°，Z = 2。对配合物2(ClO₄)₃·2H₂O的摩尔磁化率随温度的变化进行研究，结果表明两个高自旋二价锰（Mn(II)）离子之间存在弱反铁磁交换相互作用（J = −0.65 ± 0.05 cm⁻¹，哈密顿量H = −JŜ₁·Ŝ₂）。以粉末样品与冷冻乙腈溶液为测试对象，记录其电子顺磁共振（EPR）谱，结果显示配合物1中Mn的七配位结构在溶解后得以保留，而配合物2则发生部分解离。在乙腈体系中研究了配合物1与2的电化学响应，并在氧化电位下进行宏观电解实验，实验体系中添加了不同当量的2,6-二甲基吡啶（每Mn离子对应0~2.65当量）。通过质谱、电子顺磁共振及红外光谱表征，证实从配合物1或2均可生成混合价配合物[(tpen)Mn(III)(μ-O)₂Mn(IV)(tpen)]³⁺（记为3）。当以配合物2为起始反应物时，通过循环伏安法、电子顺磁共振及紫外-可见光谱数据，证实了配合物[(tpen)Mn(IV)(μ-O)₂(μ-OAc)Mn(IV)(tpen)]³⁺（记为4）的生成。电生成配合物中Mn与tpen的比例可通过添加的2,6-二甲基吡啶的用量精准调控。本研究还探讨了tpen作为碱的作用。