Structure and Magnetism of the Tetra-Copper(II)-Substituted Heteropolyanion [Cu4K2(H2O)8(α-AsW9O33)2]8-

• Published in: Inorganic chemistry 2004-01, Vol.43 (1), p.144-154
• Main Authors: Kortz, Ulrich, Nellutla, Saritha, Stowe, Ashley C, Dalal, Naresh S, van Tol, Johan, Bassil, Bassem S
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic034697b

The novel heteropolyanion [Cu4K2(H2O)8(α-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(α-AsW9O33)2]·5.5H2O} n (K 7 Na-1), which crystallizes in the tetragonal system, space group P4̄21 m, with a = 16.705(4) Å, b = 16.705(4) Å, c = 13.956(5) Å, and Z = 2. Interaction of the lacunary [α-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 α-AsW9O33 units joined by a cyclic arrangement of four Cu2+ and two K+ ions, resulting in a structure with C 2 v 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 μ3-oxo bridges. EPR studies on K 7 Na-1 and also on Na9[Cu3Na3(H2O)9(α-AsW9O33)2]·26H2O (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 K 7 Na-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 K 7 Na-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 K 7 Na-1.