Structure and magnetism of the tetra-copper(II)-substituted heteropolyanion [CU4K2(H2O)8(α-AsW9O33)2]8-

The novel heteropolyanion [Cu4K2(H2O)(8)(alpha-AsW9O33)(2)](8-) (1) has been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic studies. Single-crystal X-ray analysis was carried out on {K7Na[Cu4K2(H2O)(6)(alpha-AsW9O33)(2)].5.5H(2)O}(n) (K7Na-1), which crystallizes in the tetragonal system, space group A (4) over bar2(1)m, with a = 16.705(4) Angstrom, b = 16.705(4) Angstrom, c = 13.956(5) Angstrom, and Z = 2. Interaction of the lacunary [alpha-AsW9O33](9-) with Cu2+ ions in neutral, aqueous medium leads to the formation of the dimeric polyoxoanion 1 in high yield. Polyanion 1 consists of two alpha-AsW9O33 units joined by a cyclic arrangement of four Cu2+ and two K+ ions, resulting in a structure with C-2v symmetry. All copper ions have one terminal water molecule, resulting in square-pyramidal coordination geometry. Three of the copper ions are adjacent to each other and connected via two mu(3)-oxo bridges. EPR studies on K7Na-1 and also on Na-9[Cu3Na3(H2O)(9)(alpha-AsW9O33)(2)].26H(2)O (Na-9-2) over 2-300 K yielded g values that are consistent with a square-pyramidal coordination around the copper(II) ions in 1 and 2. No hyperfine structure was observed due to the presence of strong spin exchange, but fine structure was observed for the excited (S-T = 3/2) state of Na-9-2 and the ground state (S-T = 1) of K7Na-1. The zero-field (D) parameters have also been determined for these states, constituting a rare case wherein one observes EPR from both the ground and the excited states. Magnetic susceptibility data show that Na-9-2 has antiferromagnetically coupled Cu2+ ions, with J = -1.36 +/- 0.01 cm(-1), while K7Na-1 has both ferromagnetically and antiferromagnetically coupled Cu2+ ions (J(1) = 2.78 +/- 0.13 cm(-1), J(2) = -1.35 +/- 0.02 cm(-1), and J(3) = -2.24 +/- 0.06 cm(-1)), and the ground-state total spins are S-T = 1/2 in Na-9-2 and S-T = 1 in K7Na-1.