Structure and magnetism of [n-BuNH3]12[Cu 4(GeW9O34)2]-14H2O sandwiching a rhomblike Cu48+ tetragon through α-keggin linkage

Abstract

A sandwich-type polyoxometalate, [Cu4(GeW9O 34]12- (1a), in which two B-α-[GeW90 34]12- ligands sandwich a rhomblike Cu48+ tetragon through α-Keggin linkage, is first isolated as a [n-BuNH3]+ salt, [n-BuNH3]12- [Cu4(GeW9O34)2]-14H2O (1). A Cu4O14 cluster for the rhomblike Cu 48+ tetragon in 1a with C2h local symmetry consists of two Jahn-Teller (JT) distorted Cu06 octahedra (at internal sites) with a short diagonal Cuint ⋯ Cuint distance of 3.10-3.11 Å and two Cu05 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 [Cu 4(H2O)2(GeW9O34) 2]12- (2a), comprising the four JT-distorted Cu0 6 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″), Qives J = -24.1 cm-1 for the Cu ext · · Cuint sides, J′ = -99.1 cm-1 for the Cuint ⋯ Cuint diagonal, and J″ = +0.04 cm-1 for the Cuext ⋯ Cu ext diagonal of the Cu48+ rhombus. The S = 1 ground state of 1 displays gII = 2.42, g⊥= 2.07, D = -1.44 × 10-2 cm-1, and |ACull| = 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. © 2009 American Chemical Society.