Structure and Magnetism of [n-BuNH3]12[Cu4(GeW9O34)2]·14H2O Sandwiching a Rhomblike Cu48+ Tetragon through α-Keggin Linkage

A sandwich-type polyoxometalate, [Cu4(GeW9O34)2]12− (1a), in which two B-α-[GeW9O34]12− ligands sandwich a rhomblike Cu48+ tetragon through α-Κeggin linkage, is first isolated as a [n-BuNH3]+ salt, [n-BuNH3]12[Cu4(GeW9O34)2]·14H2O (1). A Cu4O14 cluster for the rhomblike Cu48+ tetragon in 1a with C2h local symmetry consists of two Jahn−Teller (JT) distorted CuO6 octahedra (at internal sites) with a short diagonal Cuint···Cuint distance of 3.10−3.11 Å and two CuO5 square pyramids (at external site) with a long diagonal Cuext···Cuext distance of 5.34−5.35 Å, the feature of which is different from [Cu4(H2O)2(GeW9O34)2]12− (2a), comprising the four JT-distorted CuO6 octahedral Cu48+ tetragons through β-Keggin linkage: the axial Cuext−O bond distance (2.27−2.29 Å) for 1a is shorter than the corresponding JT-axial distance (2.36 Å) for 2a. Measurements of magnetic susceptibility, magnetization, and electron spin resonance spectroscopy for 1 are carried out for better understanding of the molecular magnetism of the Cu48+ tetragon in comparison with 2a. The analysis of the magnetic behavior, based on the isotropic Heisenberg spin Hamiltonian comprising three exchange parameters (J, J′, and J′′), gives J = −24.1 cm−1 for the Cuext··Cuint sides, J′ = −99.1 cm−1 for the Cuint···Cuint diagonal, and J′′ = +0.04 cm−1 for the Cuext···Cuext diagonal of the Cu48+ rhombus. The S = 1 ground state of 1 displays g|| = 2.42, g⊥= 2.07, D = −1.44 × 10−2 cm−1, and |ACu||| = 46.5 × 10−4 cm−1. An observation of the asymmetric magnetization between a positive and a negative pulsed field (up to 103 T/s) at 0.5 K on the hysteresis loop indicates the quantum tunneling at zero field. The magnetic exchange interactions of four unpaired dx2−y2-electron spins are discussed in terms of the point-dipole approximation, and the primary contribution to D is implied to come from the magnetic dipole−dipole interaction between two spins at the Cuext centers.