Spin-frustrated (VO)₃⁶⁺-triangle-sandwiching octadecatungstates as a new class of molecular magnets

Spin-frustrated polyoxometalates, K₁₁H[(VO)₃(SbW ₉O₃₃)₂]·27H₂O (1) and K ₁₂[(VO)₃(BiW₉O₃₃) ₂]·29H₂O (2), containing approximately equilateral and isosceles (VO)₃⁶⁺-triangles (V^IV⋯ V^IV separation of 5.4-5.5 Å) sandwiched by two diamagnetic α-B nonatungstate ligands ([SbW₉O₃₃]^9- and [BiW₉O₃₃]^9-) with approximate D _3h symmetry, are found to show magnetization jumps with distinct hysteresis for the S = 1/2 ↔ S = 3/2 level crossing under fast sweeping pulsed magnetic fields (∼10³ T/s) at T ≤ 0.5 K. This unusual phenomenon is attributed to the theoretical prediction of half step magnetization, which is expected for an antiferromagnetic spin triangle with antisymmetrical Dzyaloshinky-Moriya interaction. The degeneracy of the S = 1/2 states for 1 is removed by slightly lower symmetry effects of triangular structure for 2. The calorimetry of 1 and 2 shows the heat capacity anomaly at 2 ≤ T ≤ 20 K which is associated with a thermal excitation from the S = 1/2 ground states to the S = 3/2 state at zero field. Zero-field splitting energies (5-7 K) between S = 1/2 and S = 3/2 states for 1 and 2, readily estimated by the level-crossing field for the magnetization, allow us to measure the hyperfine-structural 22 lines due to three equivalent I = 7/2 ⁵¹V nuclei, the fine-structural triplet line of the S = 3/2 excited state, and the g anisotropy on the high-frequency ESR spectra. The spin-frustrated (VO) ₃⁶⁺-triangle for 1 and 2 is a good model of the magnetization between pure quantum states S = 1/2 and 3/2 and provides a new class of single-molecule magnets.