Crystal structure and spectroscopic study of polymolybdates. The crystal structure and bonding of hexakis(n-pentylammonium) heptamolybdate(VI) trihydrate

Abstract

Three new heptamolybdates ofn-alkylammonium have been synthesized, with the general formula: (BH)₆[Mo₇O₂₄]·3H₂O, where B=n-butyl-,n-pentyl-, andn-hexylamine (hereafter abbreviated as BUTMO, PENTMO, and HEXMO, respectively). Crystal structure analyses of these compounds show that all of them are isostructural and crystallize in the monoclinic system, the space group beingP2₁/n,Z=4. Crystal data for PENTMO are: (C₅H₁₄N)₆[Mo₇O₂₄]·3H₂O,a=17.085(2),b=31.344(4),c=11.487(12) Å,β=93.04(3)°,V=6143(6) ų,F(000)=3288,D _x =1.77, D₀=1.78(1) Mg·m⁻³,R=0.059, andR _w =0.066 for 5526 observed reflections. Only slight differences in cell dimensions have been observed for these compounds, except for the parameterb which is significantly different in all of them. Thermogravimetric studies show that the compounds contain three water molecules. IR spectra indicate that the organic bases are protonated and the polyanion presents the well-known infrared spectrum for heptamolybdates in the solid state. The structure solution confirms that the compound PENTMO contains discrete [Mo₇O₂₄]⁶⁻ anions, (C₅H₁₄N)⁺ cations, and water molecules, connected through hydrogen bonds; the distinguishing features of the compound PENTMO are its extensive hydrogen bonding. The cations and the water molecules are positioned so as to be able to form hydrogen bonds with either molybdate oxygen atoms or water oxygen atoms. The proposed strong hydrogen bonding interactions appear to stabilize the structure. Some of these hydrogen bonds can play an important role in the possible photochromism of this compound.