A novel optical complex between an organic substrate and a polyoxometalate. Crystal and molecular structure of alpha-H4SiW12O40 center dot 4HMPA center dot 2H(2)O (HMPA equals hexamethylphosphoramide)

A solvated complex of alpha-H4SiW12O40. 4HMPA . 2H(2)O composed the heteropolytungstate alpha-H4SiW12O40 and the organic substrate hexamethylphosphoramide (HMPA) has been synthesised, purified, and characterized. The electronic spectra (lambda = 220-500 nm) as well as the H-1 NMR spectra for the title compound dissolved in CD3CN establish that this complex dissociates into free SiW12O404- and HMPA moieties in solution unless the organic substrate HMPA is present in very high concentrations. The solid reflectance electronic spectra and LR spectra indicate that there is interaction between the alpha-H4SiW12O40 and the organic substrate. The complex has no photosensitivity under irradiation of sunlight, but under the near-UV light result in a charge transfer by oxidation of the HMPA and the reduction of the polyoxometalate. Light yellow polyhedrons of the title compound crystallize from the aqueous solvent of acetonitrile and aqueous solution as the formula of alpha-H4SiW12O40. 4HMPA . 2H(2)O in the monoclinic, space group P2(1). The unit cell has a = 12.791(3) Angstrom, b = 22.103(6) Angstrom, c = 15.532(4) Angstrom, beta = 102.860(10)degrees, and Z = 2. From the bond-valence parameters, it was found that the four hydrogen atoms of the polyoxometalate were combined with the N atoms of the four HMPA respectively. The title compound shows a certain second-order and third-order nonlinear optical response of I-2 omega = 0.71(KDP)(2 omega) and ((3)(chi)) = 2.63 x 10(-11) esu, respectively.