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Title: Spin maximization from S = 11 to S = 16 in Mn(7) disk-like clusters: spin frustration effects and their computational rationalization.
Author: Stamatatos TC, Foguet-Albiol D, Poole KM, Wernsdorfer W, Abboud KA, O'Brien TA, Christou G.
Journal: Inorg Chem; 2009 Oct 19; 48(20):9831-45. PubMed ID: 19764790.
Abstract:
The use has been explored in Mn cluster chemistry of N(3)(-) or Cl(-) in combination with N-methyldiethanolamine (mdaH(2)) or triethanolamine (teaH(3)). The reactions of Mn(ClO(4))(2).6H(2)O, NEt(3), NaN(3), and either mdaH(2) or teaH(3) (1:2:1:2) in DMF/MeOH afford {[Na(MeOH)(3)][Mn(7)(N(3))(6)(mda)(6)]}(n) (1) and {Na[Mn(7)(N(3))(6)(teaH)(6)]}(n) (2), respectively, whereas the 2:1:1 reaction of MnCl(2).4H(2)O, mdaH(2), and NEt(3) in MeCN gives (NHEt(3))[Mn(7)Cl(6)(mda)(6)] (3). Similar reactions using NBu(n)(4)N(3) in place of NaN(3) gave (NHEt(3))[Mn(7)(N(3))(6)(mda)(6)] (4) and (NHEt(3))[Mn(7)(N(3))(6)(teaH)(6)] (5). The Mn(7) anions consist of a Mn(6) hexagon of alternating Mn(II) and Mn(III) ions surrounding a central Mn(II) ion. The remaining ligation is by six bridging and chelating mda(2-) or teaH(2-) groups, and either six terminal N(3)(-) (1, 2, 4, 5) or Cl(-) (3) ions. Each bridging mda(2-) or teaH(2-) ligand contains both mu- and mu(3)-O atoms, resulting in a similar, near-planar [Mn(7)(mu(3)-OR)(6)(mu-OR)(6)](5+) core for all three complexes. The Mn(7) anions of 1 and 2 are connected via Na(+) cations to yield one-dimensional zigzag chains and three-dimensional windmill-like "hexagons-of-hexagons", respectively. In contrast, the Mn(7) anion of 3 forms a strong hydrogen-bond between the NHEt(3)(+) cation and a terminal Cl(-) ion giving a discrete ion-pair. Variable-temperature, solid-state direct current (dc) and alternating current (ac) magnetization studies were carried out in the 5.0-300 K range. Fits of dc magnetization versus field (H) and temperature (T) data by matrix diagonalization gave S = 11, g = 1.95, D = -0.15 cm(-1) for 1, S = 16, g = 1.95, D = -0.02 cm(-1) for 2, and S = 11, g = 1.92, D = -0.13 cm(-1) for 3 (D is the axial zero-field splitting parameter). Complexes 4 and 5 were also found to possess S = 11 and S = 16 ground states, respectively. The different ground states of 1 and 2 were rationalized on the basis of the sign and magnitude of the various Mn(2) exchange parameters obtained from density functional theory (DFT) calculations. This analysis confirmed the presence of spin frustration effects, with the ground states being determined by the relative magnitude of the two weakest interactions. The combined results demonstrate the usefulness of N-based dipodal and tripodal alkoxide-based chelates as a route to structurally and magnetically interesting Mn clusters.

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