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

The comproportionation reaction between Mn-II and Mn-VII 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 Mn-IV. The reaction of Mn(O2CBut)(2) and (NBu4MnO4)-Mn-n with an excess of pivalic acid in the presence of Mn(ClO4)(2) and (NBu4Cl)-Cl-n in hot MeCN led to the isolation of [Mn8O6(OH)(O2CBut)(9)Cl-3((BuCO2H)-C-t)(0.5)(MeCN)(0.5)] (1). In contrast, the reaction of Mn(NO3)(2) and (NBu4MnO4)-Mn-n 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 Mn-x topologies: 1 possesses a [(Mn7MnIV)-Mn-III(mu(3)-O)(4)(mu(4)-O)(2)(mu(3)-OH)(mu(4)-Cl)(mu(2)-Cl)](8+) core consisting of two body-fused Mn-4 butterfly units attached to the remaining Mn atoms via bridging O2-, OH-, and Cl- ions. In contrast, 2 possesses a [(Mn6Mn2III)-Mn-IV(mu(3)-O)(6)(mu-O)(3)](12+) core consisting of two [Mn3O4] incomplete cubanes linked by their O2- ions to two Mn-III atoms. The cores of 1 and 2 are unprecedented in Mn chemistry. The [Mn-9(III)(mu(3)-O)(7)](13+) core of 3 also contains two body-fused Mn-4 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 (chi ''(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 Mn-x clusters, and the sensitivity of the product identity to minor changes in the reaction conditions.