Molecular Magnetism of M6 Hexagon Ring in D3d Symmetric [(MCl)6(XW9O33)2]12– (M = CuII and MnII, X = SbIII and AsIII)

Ferromagnetic [n-BuNH3]12[(CuCl)6(SbW9O33)2]·6H2O (1) and antiferromagnetic [n-BuNH3]12[(MnCl)6(AsW9O33)2]·6H2O (4) have been synthesized and structurally and magnetically characterized. Two complexes are structural analogues of [n-BuNH3]12[(CuCl)6(AsW9O33)2]·6H2O (2) and [n-BuNH3]12[(MnCl)6(SbW9O33)2]·6H2O (3) with their ferromagnetic interactions, first reported by us in 2006. When variable temperature (T) direct current (dc) magnetic susceptibility (χM) data are analyzed with the isotropic exchange Hamiltonian for the magnetic exchange interactions, χMT vs T curves fitted by a full matrix diagonalization (for 1) and by the Kambe vector coupling method/Van Vleck’s approximation (for 4) yield J = +29.5 and −0.09 cm–1 and g = 2.3 and 1.9, respectively. These J values were significantly distinguished from +61.0 and +0.14 cm–1 for 2 and 3, respectively. The magnetization under the pulsed field (up to 103 T/s) at 0.5 K exhibits hysteresis loops in the adiabatic process, and the differential magnetization (dM/dB) plots against the pulsed field display peaks characteristic of resonant quantum tunneling of magnetization (QTM) at Zeeman crossed fields, indicating single-molecule magnets for 1–3. High-frequency ESR (HFESR) spectroscopy on polycrystalline samples provides g∥ = 2.30, g⊥ = 2.19, and D = −0.147 cm–1 for 1 (S = 3 ground state), g∥ = 2.29, g⊥ = 2.20, and D = −0.145 cm–1 for 2 (S = 3), and g∥ = 2.03 and D = −0.007 cm–1 for 3 (S = 15). An attempt to rationalize the magnetostructural correlation among 1–4, the structurally and magnetically modified D3d-symmetric M (=CuII and MnII)6 hexagons sandwiched by two diamagnetic α-B-[XW9O33]9– (X = SbIII and AsIII) ligands through M-(μ3-O)-W linkages, is made. The strongest ferromagnetic coupling for the Cu6 hexagon of 2, the structure of which approximately provides the Cu6(μ3-O)12 cylindrical geometry, is demonstrated by the polarization mechanism based on the point-dipole approximation, which provides a decrease of the ferromagnetic interaction due to the out-of-cylinder deviation of the Cu atoms for 1. The different nature of the magnetic exchange interaction in 3 and 4 is understood by the combined effect of the out-of plane deviation (the largest for 4) of the Mn atoms from the Mn­(μ3-O)2Mn least-squares plane and the antiferromagnetic contribution arising from the large Mn–O–Mn bond angle. The primary contribution to D is discussed in terms of the magnetic dipole–dipole interaction between the electrons located on the magnetic sites in the M6 hexagon.