Preparation and aqueous solution properties of the heterometallic cuboidal complex [W3CuS4(H2O)10]5+

Abstract

The first heterometallic derivative of the trinuclear W^IV₃ incomplete cuboidal cluster [W₃S₄(H₂O)₉]⁴⁺ has been prepared by reaction with (i) Cu metal, and (ii) Cu⁺ or CuCl(solid). In both cases the product is the cuboidal [W₃CuS₄(H₂O)₁₀]⁵⁺ ion. The corresponding reactions of [Mo₃S₄(H₂O)₉]⁴⁺ yield [Mo₃Cu-S₄(H₂O)₁₀]⁴⁺ and [Mo₃CuS₄(H₂O)₁₀]⁵⁺ respectively. The observation that the reaction of [W₃S₄(H₂O)₉]⁴⁺ with copper yields the 5+ and not the 4+ product suggests that the 4+ cluster is less stable in the tungsten case. Rate constants (25 °C) for the reaction of Cu⁺ with [W₃S₄(H₂O)₉]⁴⁺(1.82 × 10³ M^–1 s^–1) and [Mo₃S₄(H₂O)₉]⁴⁺(980 M^–1 s^–1) suggest a similar addition process. The 1:1 stoichiometries for the oxidation of [W₃CuS₄(H₂O)₉]⁵⁺ with [Co(dipic)₂]^–(dipic = pyridine-2,6-dicarboxylate) and [Fe(H₂O)₆]³⁺ giving [W₃S₄(H₂O)₉]⁴⁺ and Cu²⁺ as products, further support the 5+ overall charge assignment. Rate constants have been determined and mechanisms assigned to these reactions. At 25°C, I= 2.00 M (LiClO₄), the [Co(dipic)₂]^– reaction is outer-sphere with a rate constant of 17.7 × 10³ M^–1 s^–1, while outer-sphere [Fe(H₂O)₆]³⁺(2.68 × 10² M^–1 s^–1) and inner-sphere [Fe(H₂O)₅(OH)]²⁺(5.0 × 10⁵ M^–1 s^–1) reactions are observed for the Fe^III oxidant. Rate constants are of the same order of magnitude to those for the corresponding reactions of [Mo₃CuS₄(H₂O)₁₀]⁵⁺, suggesting that a common electron-transfer process involving the copper centre may be relevant in both cases.