W. Przybylski et al. / Thermochimica Acta 514 (2011) 32–36
35
bipyramid, and differ only in the type of the counter ion and the
type and number of lattice solvent molecules (for X-ray details see
25] and Supplementary data were the CIFs are given). Hydrogen
[
bonds play a very important role in stabilizing crystal structures.
Two types of hydrogen bonds are present in these compounds: (i)
formed between water molecules and (ii) formed between a water
molecule and an oxygen atom of a peroxo group.
The thermal stability of the hydrated complexes with respect to
the loss of water depends on hydrogen bond network composed
mostly of water molecules. During heating, the weakest hydro-
gen bonds are broken first, the crystal structure becomes unstable
and water molecules are released in the first stage (dehydration).
The observed order of the DTG peak positions for Na (52 C) < K
(
62 C) < Rb (79 C) < Li (99 C) salts coincides with the ordering of
the D· · ·A distances (longer distance = weaker hydrogen bond)
of the weakest hydrogen bond in Na (2.823 A˚ ) > K (2.726 A˚ ) > Rb
(
2.711 A˚ ) > Li (2.673 A˚ ) salts. The crystallographic structures suggest
influence of the alkali metal ions on the hydrogen bond network
and additionally on peroxo ligands only in the K and Rb salts. How-
ever, because of complexity of this interaction, no correlation with
respect to thermal stability could be found.
Fig. 6. Exemplary e.s.r. spectrum for the decomposition product of NabpV.
Hydrogen bonds between water molecules and peroxo group
affects simultaneous liberation of water and oxygen (stage two).
Breaking of hydrogen bonds destabilizes the VO(O2)2 fragment,
leading to oxygen liberation accompanied by transfer of two
electrons between peroxo groups, via vanadium. This electron
transfer is also affected by the presence of the alkali metal cations.
In the structure, the alkali metals are placed near (ca. 3 A˚ ) the
vanadyl oxygen atom, although they influence also the hydrogen
bond network and additionally K and Rb have the ability to polarise
the peroxo groups.
The polarisation effect of alkali metal ion can be clearly seen
by comparing DTG peaks of K and Rb salts. Both complexes are
isomorphic, therefore effect of the hydrogen bond network on the
stability should be the same. The observed difference in the peak
position (143 C for K and 107 C for Rb) is caused by their different
polarising powers (which is higher for K).
4
. Conclusions
Three stages were observed during thermal decomposition of
the complexes. In the first stage (dehydration) 2–4 water molecules
are released and the positions of DTG peaks depend on the strength
of the weakest bond in the hydrogen bond network (which is also
influenced by the alkali metal ion). Water release disrupts the
hydrogen bonds which stabilized the peroxo groups. Therefore in
the second stage both water and oxygen molecules are released. In
the case of K and Rb salts the peroxo groups are also stabilized by
the alkali metal ion through polarisation effect. In the third stage
ꢀ
2
,2 -bpy and its decomposition products are released.
The most stable is the Cs[VO(O ) bpy]·H O complex in which
2
2
2
2
the lattice water molecule was substituted by H O . In the first
2
2
stage hydrogen peroxide undergoes decomposition to water and
molecular oxygen. Following stages are similar to hydrated com-
plexes.
The main component of the residue is proper vanadate, MVO3,
with small amounts of paramagnetic vanadium(IV) species, most
likely VO2.
Main solid product of the thermal decomposition is MVO3
(
M = Li, Na, K, Rb, Cs) as evidenced by the presence of bands in the
−1
FT-IR spectra (in KBr, cm ): 962, ∼916 (sh), 841, ∼660 (sh), 481
(
which corresponds well with the literature data [27] for ␣-NaVO
3
−
1
(
cm ): (VO )sym 962, 916, 828; (VOV)asym 651; (VOV)sym 482).
2
Small differences in the positions of the bands between our and
literature data can be explained by small differences in their poly-
morphic forms.
Acknowledgement
However, the colour of the residue after heating to 550 C was
grey or black suggesting formation of small amounts of byproducts.
The analysis of the residue by e.s.r. method gave a spectra (example
is given in Fig. 6) with a single resonance centred around g = 1.96
One of the authors (R.G.) wishes to thank Dr. Robert Grybos for
helpful discussions.
(
Fig. 6) with no hyperfine structure. These spectra are characteris-
Appendix A. Supplementary data
4+
tic for magnetically interacting V centres which are close enough
dimer, cluster) to cause a dipolar broadening of the e.s.r. line [28].
(
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tca.2010.11.029.
The same spectrum we obtained for VO2 (99.9% Aldrich) suggest-
ing that tetravalent vanadium should likely be present as a VO2
in the residue obtained (in our TG experiments) after heating the
peroxocomplexes to 550 C under argon.
Quantitative estimation using vanadyl sulphate as a reference
sample indicated about 8 ± 2% of total vanadium in +4 oxidation
state.
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2
M[VO(O ) bpy] · nH O−→ 2VO + M O + 2.5O + 2nH O
+
2
2
2
2
2
2
2
[
2bpy (or degradation products of bpy)
(4)