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 alpha-Dawson polyoxometalates, [H14Al(B-alpha-P2W15O56)(2)](7-) (1), is described herein. The tetra-n-butylammonium salt of 1, [(n-C4H9)(4)N](7)[H14Al(B-alpha-P2W15O56)(2)] (TBA-1) was prepared by passing an aqueous solution of K-6[B-alpha-H3P2W15O59{Al(OH2)}(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 degrees C. Single-crystal X-ray structure analysis revealed that a six-coordinate aluminum ion was sandwiched between two tri-lacunary alpha-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 P-31 nuclear magnetic resonance spectroscopy. The photochromic properties of TBA-1 were also characterized in methanol under light irradiation (lambda = 365 nm and >= 400 nm).