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.