Ferromagnetic exchange interactions for Cu ₆ ¹²⁺ and Mn ₆ ¹²⁺ hexagons sandwiched by two B-α-[XW ₉O ₃₃] ^9- (X = As ^III and Sb ^III) ligands in D _3d-symmetric polyoxotungstates

Two novel Cu ₆ and Mn ₆ hexagon sandwiched polyoxometalates, [(CuCl) ₆(AsW ₉O ₃₃) ₂] ^12- (1a) and [(MnCl) ₆(SbW ₉O ₃₃) ₂] ^12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D _3d symmetric and were isolated as [n-BuNH ₃] ⁺ salts from aqueous solutions: (n-BuNH3)12[(CuCl) ₆-(AsW ₉O ₃₃) ₂]·6H ₂O (1), rhombohedral, R3, a = 20.33(1) Å, c = 26.35(2) Å, Z = 3 and (n-BuNH ₃) ₁₂[(MnCl) ₆-(SbW ₉O ₃₃) ₂]·6H ₂O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu⋯Cu (Mn⋯Mn) distance of 2.913(2) Å (3.248(1) Å) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2)° (100.4(2)°). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm ^-1) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm ^-1) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu ₆ ¹²⁺ and Mn ₆ ¹²⁺ hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu ₆ hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between S _z = O and S _z = ±1 of D = -0.182 K to be obtained.