Disorder and polymorphism in Cu(ii)-polyoxometalate complexes: [Cu1.5(H2O)7.5PW12O40]·4.75H2O, cis- & trans-[Cu2(H2O)10SiW12O40]·6H2O

Abstract

Three highly crystalline Cu(II)-polyoxometalate (POM) complexes, [Cu_1.5(H₂O)_7.5PW₁₂O₄₀]·4.75H₂O and cis- and trans-[Cu₂(H₂O)₁₀SiW₁₂O₄₀]·6H₂O, were successfully synthesized and characterized by single crystal and powder X-ray diffraction as well as thermogravimetric analysis. All complexes feature octahedrally coordinated, Jahn–Teller-distorted Cu^II centres, which are coordinated by one terminal oxygen atom of the POM anion and five water molecules. The crystal structure of [Cu_1.5(H₂O)_7.5PW₁₂O₄₀]·4.75H₂O, reveals four-fold disorder at one of the two Cu-sites, whereas [Cu₂(H₂O)₁₀SiW₁₂O₄₀]·6H₂O crystallizes in two fully-ordered polymorphic forms: a monoclinic structure containing an angled cis configuration of the Cu-POM-Cu molecule and a triclinic structure containing a linear trans configuration. Density functional theory (DFT) calculations show that the cis polymorph of [Cu₂(H₂O)₁₀SiW₁₂O₄₀]·6H₂O is more stable than the trans form by about 30 kJ mol⁻¹.