Synthesis, X-ray Crystal Structure, and Photochromism of a Sandwich-Type Mono-Aluminum Complex Composed of Two Tri-Lacunary α-Dawson-Type Polyoxotungstates.

The synthesis and molecular structure of a dimeric, mono-aluminum complex composed of two tri-lacunary α-Dawson polyoxometalates, [HAl(B-α-PWO)] (1), is described herein. The tetra--butylammonium salt of 1, [(-CH)N][HAl(B-α-PWO)] (TBA-1) was prepared by passing an aqueous solution of K[B-α-HPWO{Al(OH)}]⋅14HO through an ion-exchange resin column (H-form), followed by addition of tetra--butylammonium bromide. Analytically pure and colorless crystals of TBA-1 were obtained via vapor diffusion from acetonitrile/methanol at ~25 °C. Single-crystal X-ray structure analysis revealed that a six-coordinate aluminum ion was sandwiched between two tri-lacunary α-Dawson-type units, resulting in an overall symmetry. The characterization of TBA-1 was accomplished by elemental analyses, thermogravimetric/differential thermal analyses, Fourier-transform infrared spectroscopy, and solution P nuclear magnetic resonance spectroscopy. The photochromic properties of TBA-1 were also characterized in methanol under light irradiation (λ = 365 nm and ≥400 nm).