Chemical Composition of an Aqueous Oxalato-/Citrato-VO(2+) Solution as Determinant for XXXX Phase Formation.

@entity1792 @entity26 @entity46715 @entity4962
UNASSIGNED: Aqueous solutions of oxalato- and citrato-VO(2+) complexes are prepared, and their ligand exchange reaction is investigated as a function of the amount of @entity1792 present in the aqueous solution via continuous-wave electron paramagnetic resonance (CW EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopy. With a low amount of @entity1792 , monomeric cis-oxalato-VO(2+) complexes occur with a distorted square-pyramidal geometry. As the amount of @entity1792 increases, oxalate is gradually exchanged for @entity1792 . This leads to (i) an intermediate situation of monomeric @entity4962 complexes with a mix of oxalate/ @entity1792 ligands and (ii) a final situation of both monomeric and dimeric complexes with exclusively citrato ligands. The monomeric citrato-VO(2+) complexes dominate (abundance > 80%) and are characterized by a 6-fold chelation of the vanadium(IV) ion by 4 RCO2(-) ligands at the equatorial positions and a H2O/R-OH ligand at the axial position. The different redox stabilities of these complexes, relative to that of dissolved O2 in the aqueous solution, is analyzed via (51)V NMR. It is shown that the oxidation rate is the highest for the oxalato-VO(2+) complexes. In addition, the stability of the @entity4962 complexes can be drastically improved by evacuation of the dissolved O2 from the solution and subsequent storage in a N2 ambient atmosphere. The @entity46715 phase formation process, starting with the chemical solution deposition of the aqueous solutions and continuing with subsequent processing in an ambient 0.1% O2 atmosphere, differs for the two complexes. The oxalato-VO(2+) complexes turn into the @entity26 -deficient crystalline VO2 B at 400   C, which then turns into crystalline V6O13 at 500   C. In contrast, the citrato-VO(2+) complexes form an amorphous film at 400   C that crystallizes into VO2 M1 and V6O13 at 500   C.
