High-Spin Mn₄ and Mn₁₀ Molecules: Large Spin Changes with Structure in Mixed-Valence Mn^II₄Mn^III₆ Clusters with Azide and Alkoxide-Based Ligands

The use has been explored of both azide (N3−) and alkoxide-containing groups such as the anions of 2-(hydroxymethyl)pyridine (hmpH), 2,6-pyridinedimethanol (pdmH2), 1,1,1-tris(hydroxymethyl)ethane (thmeH3) and triethanolamine (teaH3) in Mn cluster chemistry. The 1:1:1:1 reactions of hmpH, NaN3 and NEt3 with Mn(ClO4)2·6H2O or Mn(NO3)2·H2O in MeCN/MeOH afford [MnII4MnIII6O4(N3)4(hmp)12](X)2 [X = ClO4− (1), N3− (2)]. The [Mn10(μ4-O)4(μ3-N3)4]14+ core of the cation has a tetra-face-capped octahedral topology, with a central MnIII6 octahedron, whose eight faces are bridged by four μ3-N3− and four μ4-O2- ions, the latter also bridging to four extrinsic MnII atoms. The core has Td symmetry, but the complete [MnII4MnIII6O4(N3)4(hmp)12]2+ cation has rare T symmetry, which is crystallographically imposed. A similar reaction of Mn(ClO4)2·6H2O with one equiv each of NaN3, thmeH3, pdmH2, and NEt3 in MeCN/MeOH led to [MnII4MnIII6O2(N3)6(pdmH)4(thme)4] (3). Complex 3 is at the same oxidation level as 1/2 but its core is structurally different, consisting of two edge-fused [MnII2MnIII4(μ4-O)]14+ octahedra. Replacement of thmeH3 with teaH3 in this reaction gave instead [MnII2MnIII2(N3)4(pdmH)2(teaH)2] (4), containing a planar Mn4 rhombus. Variable-temperature, solid-state dc and ac magnetization studies were carried out on 1−4 in the 5.0–300 K range. Complexes 1 and 2 are completely ferromagnetically coupled with a resulting S = 22 ground state, one of the highest yet reported. Fits of dc magnetization vs field (H) and temperature (T) data by matrix diagonalization gave S = 22, g = 2.00, and D ≈ 0.0 cm−1 (D is the axial zero-field splitting parameter). In contrast, the data for 3 revealed dominant antiferromagnetic interactions and a resulting S = 0 ground state. Complex 4 contains weakly ferromagnetically coupled Mn atoms, leading to an S = 9 ground-state and low-lying excited states, and exhibits out-of-phase ac susceptibility signals characteristic of a single-molecule magnet. Theoretical values of the exchange constants in 1 obtained with density functional theory and ZILSH calculations were in good agreement with experimental values. The combined work demonstrates the synthetic usefulness of alcohol-based chelates and azido ligands when used together, and the synthesis in the present work of two “isomeric” MnIII6MnII4 cores that differ in spin by a remarkable 22 units.