Yamase Toshihiro, Abe Hiroko, Ishikawa Eri, Nojiri Hiroyuki, Ohshima Yuhgo
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-21 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
Inorg Chem. 2009 Jan 5;48(1):138-48. doi: 10.1021/ic8013267.
A sandwich-type polyoxometalate, Cu(4)(GeW(9)O(34))(2) (1a), in which two B-alpha-GeW(9)O(34) ligands sandwich a rhomblike Cu(4)(8+) tetragon through alpha-Kappaeggin linkage, is first isolated as a n-BuNH(3) salt, n-BuNH(3)[Cu(4)(GeW(9)O(34))(2)].14H(2)O (1). A Cu(4)O(14) cluster for the rhomblike Cu(4)(8+) tetragon in 1a with C(2h) local symmetry consists of two Jahn-Teller (JT) distorted CuO(6) octahedra (at internal sites) with a short diagonal Cu(int)...Cu(int) distance of 3.10-3.11 A and two CuO(5) square pyramids (at external site) with a long diagonal Cu(ext)...Cu(ext) distance of 5.34-5.35 A, the feature of which is different from Cu(4)(H(2)O)(2)(GeW(9)O(34))(2) (2a), comprising the four JT-distorted CuO(6) octahedral Cu(4)(8+) tetragons through beta-Keggin linkage: the axial Cu(ext)-O bond distance (2.27-2.29 A) for 1a is shorter than the corresponding JT-axial distance (2.36 A) 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 Cu(4)(8+) 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 Cu(ext)..Cu(int) sides, J' = -99.1 cm(-1) for the Cu(int)...Cu(int) diagonal, and J'' = +0.04 cm(-1) for the Cu(ext)...Cu(ext) diagonal of the Cu(4)(8+) rhombus. The S = 1 ground state of 1 displays g(||) = 2.42, g( perpendicular)= 2.07, D = -1.44 x 10(-2) cm(-1), and |A(Cu||)| = 46.5 x 10(-4) cm(-1). An observation of the asymmetric magnetization between a positive and a negative pulsed field (up to 10(3) T/s) at 0.5 K on the hysteresis loop indicates the quantum tunneling at zero field. The magnetic exchange interactions of four unpaired d(x(2)-y(2))-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 Cu(ext) centers.
一种三明治型多金属氧酸盐[Cu₄(GeW₉O₃₄)₂]¹²⁻（1a），其中两个B-α-[GeW₉O₃₄]¹²⁻配体通过α-凯琴连接将一个菱形Cu₄⁸⁺四边形夹在中间，首次被分离为[n-BuNH₃]⁺盐，即[n-BuNH₃]₁₂[Cu₄(GeW₉O₃₄)₂]·14H₂O（1）。1a中具有C₂h局部对称性的菱形Cu₄⁸⁺四边形的Cu₄O₁₄簇由两个 Jahn-Teller（JT）畸变的CuO₆八面体（位于内部位置）组成，其短对角线Cu(int)…Cu(int)距离为3.10 - 3.11 Å，还有两个CuO₅四方锥（位于外部位置），其长对角线Cu(ext)…Cu(ext)距离为5.34 - 5.35 Å，其特征与通过β-凯琴连接包含四个JT畸变的CuO₆八面体Cu₄⁸⁺四边形的[Cu₄(H₂O)₂(GeW₉O₃₄)₂]¹²⁻（2a）不同：1a的轴向Cu(ext)-O键距离（2.27 - 2.29 Å）比2a相应的JT轴向距离（2.36 Å）短。对1进行了磁化率、磁化强度和电子自旋共振光谱测量，以便与2a相比更好地理解Cu₄⁸⁺四边形的分子磁性。基于包含三个交换参数（J、J'和J''）的各向同性海森堡自旋哈密顿量对磁行为进行分析，得出Cu(ext)…Cu(int)边的J = -24.1 cm⁻¹，Cu(int)…Cu(int)对角线的J' = -99.1 cm⁻¹，以及Cu(ext)…Cu(ext)对角线的J'' = +0.04 cm⁻¹。1的S = 1基态显示g(||) = 2.42，g(⊥) = 2.07，D = -1.44×10⁻² cm⁻¹，且|A(Cu||)| = 46.5×10⁻⁴ cm⁻¹。在0.5 K下对滞后回线上正负脉冲场（高达10³ T/s）之间不对称磁化的观察表明在零场下存在量子隧穿。根据点偶极近似讨论了四个未成对d(x² - y²)电子自旋的磁交换相互作用，并且暗示对D的主要贡献来自Cu(ext)中心的两个自旋之间的磁偶极 - 偶极相互作用。
