Spin Maximization from <i>S</i> = 11 to <i>S</i> = 16 in Mn₇ Disk-Like Clusters: Spin Frustration Effects and Their Computational Rationalization

The use has been explored in Mn cluster chemistry of N3− or Cl− in combination with N-methyldiethanolamine (mdaH2) or triethanolamine (teaH3). The reactions of Mn(ClO4)2·6H2O, NEt3, NaN3, and either mdaH2 or teaH3 (1:2:1:2) in DMF/MeOH afford {[Na(MeOH)3][Mn7(N3)6(mda)6]}n (1) and {Na[Mn7(N3)6(teaH)6]}n (2), respectively, whereas the 2:1:1 reaction of MnCl2·4H2O, mdaH2, and NEt3 in MeCN gives (NHEt3)[Mn7Cl6(mda)6] (3). Similar reactions using NBun4N3 in place of NaN3 gave (NHEt3)[Mn7(N3)6(mda)6] (4) and (NHEt3)[Mn7(N3)6(teaH)6] (5). The Mn7 anions consist of a Mn6 hexagon of alternating MnII and MnIII ions surrounding a central MnII ion. The remaining ligation is by six bridging and chelating mda2− or teaH2− groups, and either six terminal N3− (1, 2, 4, 5) or Cl− (3) ions. Each bridging mda2− or teaH2− ligand contains both μ- and μ3-O atoms, resulting in a similar, near-planar [Mn7(μ3-OR)6(μ-OR)6]5+ core for all three complexes. The Mn7 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 Mn7 anion of 3 forms a strong hydrogen-bond between the NHEt3+ 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 Mn2 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.