Synthesis, molecular structure, and stability of a zirconocene derivative with α-Keggin mono-aluminum-substituted polyoxotungstate

Abstract

The synthesis and crystal structure of a zirconocene derivative with α-Keggin mono-aluminum-substituted polyoxotungstate, [α-PW₁₁Al(OH)O₃₉ZrCp₂]₂⁶⁻ (Cp = C₅H₅⁻) (1), which was obtained by the reaction of α-Keggin mono-aluminum-substituted polyoxotungstate with a biscyclopentadienylzirconium(IV) complex, is described herein. Analytically pure, homogeneous, yellow crystals of the tetra-n-butylammonium salt of polyoxoanion 1, [(n-C₄H₉)₄N]₆[α-PW₁₁Al(OH)O₃₉ZrCp₂]₂ (TBA-1), were obtained from the ca. 1 : 1 reaction of [(n-C₄H₉)₄N]₄[α-PW₁₁{Al(OH₂)}O₃₉] with Cp₂Zr(OTf)₂·THF (OTf = O₃SCF₃⁻) in acetonitrile solution under an argon atmosphere, followed by precipitation from water and crystallization from acetonitrile. TBA-1 was characterized based on X-ray structure analysis, elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR), and solution (³¹P, ²⁷Al, ¹⁹F, ¹H, and ¹³C) nuclear magnetic resonance (NMR) spectroscopy. Single-crystal X-ray structure analysis revealed that the two {PW₁₁AlO₄₀} units are bridged by two “bent sandwich” Cp₂Zr²⁺ fragments with C₂ symmetry. Each zirconium center was bound to a terminal oxygen atom of the aluminum and tungsten sites and an edge-sharing oxygen atom at the Al–O–W linkage. Further, the stability towards water was investigated by NMR (³¹P, ¹H, and ¹³C) and FTIR spectroscopy. The η⁵-cyclopentadienylzirconium fragments were not eliminated from the surface of [α-PW₁₁{Al(OH₂)}O₃₉]⁴⁻ even after 24 h exposure to 50 equiv. of water.