Synthesis of Tetramethylammonium Polyoxovanadates
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Synthesis of Tetramethylammonium Polyoxovanadates
Nathalie Steunou, Laure Bouhedja, Jocelyne Maquet, Jacques Livage Chimie de la Matière Condensée, UMR CNRS 7574 Université Pierre et Marie Curie, Paris, France
ABSTRACT Tetramethylammonium (TMA) polyoxovanadates have been precipitated from aqueous solutions around pH 7. Decavanadate clusters (TMA)4[H2V10O28]•4H2O are formed at room temperature whereas a layered (TMA)[V4O10] mixed valence compound is formed under hydrothermal conditions. 51V NMR spectra recorded on the solution at different temperatures show that upon heating decavanadate clusters are progressively transformed into cyclic [V4O12]4- metavanadates. This suggests that the mixed valence polyoxovanadates are formed via the ring opening polymerization of metavanadate precursors.
INTRODUCTION The synthesis of polyoxovanadates from aqueous solutions under mild conditions leads to a large variety of compounds. The decavanadate cluster [HnV10O28](6-n)- has already been characterised for a long time, both in the solution and in the solid state [1]. New layered polyoxovanadates such as (TMA)[V4O10], (TMA)[V3O7] and (TMA)[V8O20] have been recently synthesized under hydrothermal conditions by Whittingham et al [2]. A large family of hollow clusters encapsulating anionic or neutral species such as (TMA)6[V15O36Cl]•4H2O or (Cs)9[H4V18O42I]•12H2O have been reported [3]. Most of these polyoxovanadates are formed at pH values close to neutrality where VV coordination changes from five to four. Therefore a more detailed analysis has been undertaken in order to get a better understanding of the chemical processes leading from molecular precursors in the solution to solid compounds.
EXPERIMENTAL Synthesis of tetramethylammonium polyoxovanadates The vanadate salts of the organic tetramethylammonium (TMA) cation were synthesized via the dissolution of a V2O5 powder (1g) into an aqueous solution of TMAOH (10%, pH=13, 5 mL). The V2O5/TMAOH molar ratio was close to 1. At room temperature the dissolution of the oxide was rather slow. An orange suspension was obtained after about one day. The vanadium pentoxide in excess was removed by filtration and the resulting clear solution (pH = 6.7) was slowly evaporated at room temperature for one week giving rise to orange crystals of compound 1. Black shiny flat crystals corresponding to compound (2) precipitated from a yellow solution (pH≈7.2) when the mixture was heated under hydrothermal conditions (48 h at 200°C) in a Parr Teflon-lined digestion bomb of 23 mL capacity. GG8.3.1
X-ray structures of [H2V10O28][N(CH3) ]4 •4 H2O (1) and (TMA)[V4O10] (2) The structures of compounds (1) and (2) were determined by single crystal X-ray diffraction. Measurements were collected at room temperature on a CAD4 diffractometer using MoKα radiation (λ = 0.71069 Å) and graphite monochromator. The asymmetric unit of compound (1) is composed of one [H2V10O28]4- anion, four tetramethylammonium cations and four water molecules of crystallization [4]. A polyhedral view of the [H2V10O28]4- anion is r
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