Tetrahedral vanadium oxide compounds

Vanadium(II) compounds are usually prepared by reduction of acidic solutions of higher oxidation states. The usual geometry is octahedral and the electronic spectra often show three d-d spin-allowed bands. These spectra, as well as those of tetrahedral and linear complexes, have been reviewed.58 For this cr configuration, a spin-only magnetic moment of 3.87 BM is expected. Slow substitution can be predicted for vanadium(II) complexes but such reactions are not as slow as those for the isoelectronic Crm ions. 

The Raman spectra oT model vanadium(V) oxide compounds are very sensitive to vanadium oxygen coordination. A distinct trend in the Raman spectra oT the reference tetrahedral and octahedral vanadium(V) oxide compounds is observed. Vanadium oxide is Tound to interact dit-ferently with the ditterent alumina supports. On the 7-AI2O3 support, the surtace vanadium oxide overlayer consists oT polymeric tetrahedra and distorted... 

Vanadium compounds exist in a variety of formal oxidation states from -3 to +5. Their coordination number varies from 3 to 8. The stereochemistry depends on their oxidation state V(II) 6-coordinate octahedral, V(III) 3-7-coordinate (V[CH(SiMe3)2]3 3-coordinate planar), V(rV) 4-6 or 8-coordinate (V(CH2SiMe3)4 4-coordinate tetrahedral, VO(acac)2 5-coordinate tetragonal), V(V) 4-7-coordinate (VOCI3 4-coordinate tetrahedral). Standard electrode potential in acidic aqueous solution is as follows V + -F 2e- = V-1.19V, V + -F e =... 

The vanadium silicalites (with MFI and MEL stmcture) are active oxidation catalyst in gas and liquid phase reactions [180]. As for the titanium silicalites, only the ftamework associated vandium exhibits redox properties [181]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [182]. The catalytically active vanadium species is speculated to be located in non-tetrahedral positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed stmcture of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with SiA ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with V MAS-NMR measurements) that the V sites are isolated in the lattice. 

Zeotype materials containing metal cations, for example ions of titanium, vanadium, chromium, iron, or tin, in the tetrahedral positions of their frameworks have been explored as solid Lewis acid catalysts (Ig). Such materials have been shown to be active in the Meerwein—Pormdorfr-Verley reduction of carbonyl compounds (151,233), and the BV oxidation (also called BV rearrangement) (234). 

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