Coordination geometry of Zn2+ on hexagonal turbostratic birnessites with different Mn average oxidation states and its stability under acid dissolution.

Hexagonal turbostratic birnessite, with the characteristics of high contents of vacancies, varying amounts of structural and adsorbed Mn3+, and small particle size, undergoes strong adsorption reactions with trace metal (TM) contaminants. While the interactions of TM, i.e., Zn2+, with birnessite are well understood, the effect of birnessite structural characteristics on the coordination and stability of Zn2+ on the mineral surfaces under proton attack is as yet unclear. In the present study, the effects of a series of synthesized hexagonal turbostratic birnessites with different Mn average oxide states (AOSs) on the coordination geometry of adsorbed Zn2+ and its stability under acidic conditions were investigated. With decreasing Mn AOS, birnessite exhibits smaller particle sizes and thus larger specific surface area, higher amounts of layer Mn3+ and thus longer distances for the first MnO and MnMn shells, but a low quantity of available vacancies and thus low adsorption capacity for Zn2+. Zn K-edge EXAFS spectroscopy demonstrates that birnessite with low Mn AOS has smaller adsorption capacity but more tetrahedral Zn (IVZn) complexes on vacancies than octahedral (VIZn) complexes, and Zn2+ is more unstable under acidic conditions than that adsorbed on birnessite with high Mn AOS. High Zn2+ loading favors the formation of VIZn complexes over IVZn complexes, and the release of Zn2+ is faster than at low loading. These results will deepen our understanding of the interaction mechanisms of various TMs with natural birnessites, and the stability and thus the potential toxicity of heavy metal pollutants sequestered by engineered nano-sized metal oxide materials.

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(O(2)CBu(t))(2) and NBu(n)(4)MnO(4) with an excess of pivalic acid in the presence of Mn(ClO(4))(2) and NBu(n)(4)Cl in hot MeCN led to the isolation of [Mn(8)O(6)(OH)(O(2)CBu(t))(9)Cl(3)(Bu(t)CO(2)H)(0.5)(MeCN)(0.5)] (1). In contrast, the reaction of Mn(NO(3))(2) and NBu(n)(4)MnO(4) in hot MeCN with an excess of pivalic acid gave a different octanuclear complex, [Mn(8)O(9)(O(2)CBu(t))(12)] (2). The latter reaction but with Mn(O(2)CBu(t))(2) in place of Mn(NO(3))(2), and in a MeCN/THF solvent medium, gave [Mn(9)O(7)(O(2)CBu(t))(13)(THF)(2)] (3). Complexes 1-3 possess rare or unprecedented Mn(x) topologies: 1 possesses a [Mn(III)(7)Mn(IV)(µ(3)-O)(4)(µ(4)-O)(2)(µ(3)-OH)(µ(4)-Cl)(µ(2)-Cl)](8+) core consisting of two body-fused Mn(4) butterfly units attached to the remaining Mn atoms via bridging O(2-), OH(-), and Cl(-) ions. In contrast, 2 possesses a [Mn(6)(IV)Mn(2)(III)(µ(3)-O)(6)(µ-O)(3)](12+) core consisting of two [Mn(3)O(4)] incomplete cubanes linked by their O(2-) ions to two Mn(III) atoms. The cores of 1 and 2 are unprecedented in Mn chemistry. The [Mn(III)(9)(µ(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)/(4)MeCN, 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 Mn(x) clusters, and the sensitivity of the product identity to minor changes in the reaction conditions.