Arsenic(III)-Capped 12-Tungsto-2-Arsenates(III) [M₂(As^IIIW₆O₂₅)₂(As^IIIOH)_<i>x</i>]^<i>n</i>− (M = Cr^III, Fe^III, Sc^III, In^III, Ti^IV, Mn^II) and Their\nMagnetic Properties

Abstract

Six\narsenic­(III)-capped 12-tungsto-2-arsenates­(III) of the type\n[M₂­(As^III­W₆­O₂₅)₂­(As^III­OH)_<i>x</i>]^<i>n</i>− (M = Cr^III, <b>1</b>; Fe^III, <b>2</b>; Sc^III, <b>3</b>; In^III, <b>4</b>; Ti^IV, <b>5</b>; Mn^II, <b>6</b>) have been\nsynthesized in aqueous medium by direct reaction of the elements using\na one-pot strategy and structurally characterized by FT-IR spectroscopy,\nsingle-crystal XRD, and elemental analysis. Polyanions <b>1</b>–<b>6</b> are comprised of two octahedrally coordinated\nguest metal ions M sandwiched between two {AsW₆} units,\nresulting in a structure with <i>C</i>_2<i>h</i> point-group symmetry. Polyanions <b>1</b>–<b>5</b> contain tri- and tetravalent metal ion guests M (M = Cr^III, Fe^III, Sc^III, In^III,\nand Ti^IV, respectively), and they have one {As^IIIOH} group grafted on each {AsW₆} unit, whereas the divalent\nMn^II-containing derivative <b>6</b> has two such\n{As^IIIOH} groups grafted on each {AsW₆} unit.\nMagnetic studies on polyanions <b>3</b>–<b>5</b> over the temperature range 1.8–295 K and magnetic fields\nof 0–7 T confirmed that they are diamagnetic. On the other\nhand, polyanions <b>1</b>, <b>2</b>, and <b>6</b> are strongly magnetic and follow the Curie–Weiss law above\n30 K. The susceptibility plots of <b>1</b> and <b>6</b> exhibit broad peaks suggesting short-range antiferromagnetic ordering,\nwhile the very weak antiferromagnetic ordering of <b>2</b> is\novershadowed by traces of a paramagnetic impurity. The magnetization\ndata of <b>1</b>, <b>2</b>, and <b>6</b> at 1.8\nK over 0–7 T were analyzed by using the Heisenberg exchange\nprocedure. Small (negative) values of the obtained <i>J</i> values help in understanding the absence of long-range antiferromagnetic\nordering.