Comproportionation Reactions to Manganese(III/IV) Pivalate Clusters: A New Half-Integer Spin Single-Molecule Magnet

The comproportionation reaction between MnII and MnVII reagents under acidic conditions has been investigated
in the presence of pivalic acid as a route to new high oxidation state
manganese pivalate clusters containing some MnIV. The reaction
of Mn­(O2CBut)2 and
NBun4MnO4 with an
excess of pivalic acid in the presence of Mn­(ClO4)2 and NBun4Cl in hot MeCN led to the
isolation of [Mn8O6(OH)­(O2CBut)9Cl3(ButCO2H)0.5(MeCN)0.5] (1). In contrast, the reaction of Mn­(NO3)2 and NBun4MnO4 in hot MeCN with an excess of pivalic acid gave a different
octanuclear complex, [Mn8O9(O2CBut)12] (2). The latter
reaction but with Mn­(O2CBut)2 in
place of Mn­(NO3)2, and in a MeCN/THF solvent
medium, gave [Mn9O7(O2CBut)13(THF)2] (3).
Complexes 1–3 possess rare or unprecedented
Mnx topologies: 1 possesses
a [MnIII7MnIV(μ3-O)4(μ4-O)2(μ3-OH)­(μ4-Cl)­(μ2-Cl)]8+ core consisting of two body-fused Mn4 butterfly units
attached to the remaining Mn atoms via bridging O2–, OH–, and Cl– ions. In contrast, 2 possesses a [Mn6IVMn2III(μ3-O)6(μ-O)3]12+ core consisting of two [Mn3O4] incomplete cubanes linked by their O2– ions to
two MnIII atoms. The cores of 1 and 2 are unprecedented in Mn chemistry. The [MnIII9(μ3-O)7]13+ core
of 3 also contains two body-fused Mn4 butterfly
units, but they are linked to the remaining Mn atoms in a different
manner than in 1. Solid-state direct current (dc) and/or
alternating current (ac) magnetic susceptibility data established S = 15/2, S = 2,
and S = 1 ground states for 1·MeCN, 2·1/4MeCN, and 3,
respectively. The ac susceptibility data also revealed nonzero, frequency-dependent
out-of-phase (χ″M) signals for 1·MeCN at temperatures below 3 K, suggesting
possible single-molecule magnet behavior, which was confirmed by single-crystal
magnetization vs dc field scans that exhibited hysteresis loops. The
combined work thus demonstrates the continuing potential of comproportionation
reactions for isolating high oxidation state Mnx clusters, and the sensitivity of the product identity to minor
changes in the reaction conditions.

本研究以新戊酸（pivalic acid）为配体，探究了酸性条件下二价锰（MnII）与七价锰（MnVII）试剂间的归中反应（comproportionation reaction），以期合成含四价锰（MnIV）的新型高氧化态锰新戊酸簇合物。以二(新戊酸)锰[Mn(O₂Cᵗᵇᵤ)₂]与四正丁基高锰酸铵（NBun₄MnO₄）为原料，在高氯酸锰[Mn(ClO₄)₂]、四正丁基氯化铵（NBun₄Cl）存在下，于热乙腈（MeCN）中加入过量新戊酸进行反应，成功分离得到配合物[Mn₈O₆(OH)(O₂Cᵗᵇᵤ)₉Cl₃(ᵗᵇᵤCO₂H)₀.₅(MeCN)₀.₅]（记为1）。与之对照，以硝酸锰[Mn(NO₃)₂]与四正丁基高锰酸铵为原料，在热乙腈中加入过量新戊酸反应，得到了另一八核锰配合物[Mn₈O₉(O₂Cᵗᵇᵤ)₁₂]（记为2）。若将原料中的硝酸锰替换为二(新戊酸)锰，并以乙腈/四氢呋喃（MeCN/THF）为混合溶剂，则得到配合物[Mn₉O₇(O₂Cᵗᵇᵤ)₁₃(THF)₂]（记为3）。配合物1~3具备罕见乃至前所未有的Mnₓ簇拓扑结构：1的核心为[MnIII₇MnIV(μ₃-O)₄(μ₄-O)₂(μ₃-OH)(μ₄-Cl)(μ₂-Cl)]⁸⁺，由两个稠合的Mn₄蝶形单元通过桥连O²⁻、OH⁻与Cl⁻离子与剩余锰原子连接而成。与之不同，2的核心为[MnIV₆MnIII₂(μ₃-O)₆(μ-O)₃]¹²⁺，由两个[Mn₃O₄]不完全立方烷通过O²⁻离子连接两个MnIII原子构成。1与2的核心结构在锰化学领域尚无先例。配合物3的[MnIII₉(μ₃-O)₇]¹³⁺核心同样包含两个稠合的Mn₄蝶形单元，但与1的连接方式存在显著差异。通过固态直流（direct current, dc）与/或交流（alternating current, ac）磁化率测试，确定了1·MeCN、2·1/4MeCN与3的基态自旋分别为S=15/2、S=2与S=1。交流磁化率数据还显示，1·MeCN在3 K以下温度下呈现非零且随频率变化的异相磁化率（χ″_M）信号，提示其可能具备单分子磁体（single-molecule magnet）行为，这一推测通过单晶磁化强度随直流场变化的测试得到验证——该测试呈现出典型的磁滞回线。综上，本研究证实了归中反应在分离高氧化态Mnₓ簇合物方面的持续应用潜力，同时也体现了反应条件的细微变化对产物结构的显著调控作用。