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, [H(14)Al(B-α-P(2)W(15)O(56))(2)](7−) (1), is described herein. The tetra-n-butylammonium salt of 1, [(n-C(4)H(9))(4)N](7)[H(14)Al(B-α-P(2)W(15)O(56))(2)] (TBA-1) was prepared by passing an aqueous solution of K(6)[B-α-H(3)P(2)W(15)O(59){Al(OH(2))}(3)]⋅14H(2)O through an ion-exchange resin column (H(+)-form), followed by addition of tetra-n-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 C(2h) symmetry. The characterization of TBA-1 was accomplished by elemental analyses, thermogravimetric/differential thermal analyses, Fourier-transform infrared spectroscopy, and solution (31)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).