Equilibrium among potassium polytellurides in N,N-dimethylformamide solution

Article abstract of DOI:10.1016/j.molstruc.2012.04.038

Reactions between elemental potassium and tellurium in N,N- dimethylformamide (DMF) are monitored using UV-visible spectroscopy and compared with those in liquid ammonia solution. In liquid ammonia, the elements react together, via a step-wise sequence, to form polytellurides, each of which is characterized by a distinctive color, the highest being potassium tritelluride. However, when the elements are combined in DMF, these distinctive color changes are not observed - the solution develops an initial plum color, which gradually darkens to purple as the reaction progresses. UV-visible and Raman spectroscopic studies indicate that equilibrium exists among the mono-, di-, and tritelluride in DMF. This equilibrium is not seen in liquid ammonia solution due to the insolubility of potassium monotelluride in that solvent. Spectral data also indicate that potassium tetratelluride is formed in DMF solution.

Full text of DOI:10.1016/j.molstruc.2012.04.038

Journal of Molecular Structure 1022 (2012) 68–71
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Equilibrium among potassium polytellurides in N,N-dimethylformamide solution
⇑
Jason L. McAfee, Jeremy R. Andreatta, Richard S. Sevcik, Linda D. Schultz
Department of Chemistry, Geosciences, and Environmental Science, Tarleton State University, Stephenville, TX 76402, United States
h i g h l i g h t s
"
"
"
"
Potassium polytellurides form when the elements directly combine in DMF solution.
Te2ꢀ þ Te32ꢀ ꢀ 2Te22ꢀ occurs in DMF solution.
Equilibria do not occur in liquid ammonia solution, because Te2ꢀ is not soluble.
Potassium tetratelluride is formed in DMF solution.
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactions between elemental potassium and tellurium in N,N-dimethylformamide (DMF) are monitored
using UV–visible spectroscopy and compared with those in liquid ammonia solution. In liquid ammonia,
the elements react together, via a step-wise sequence, to form polytellurides, each of which is character-
ized by a distinctive color, the highest being potassium tritelluride. However, when the elements are
combined in DMF, these distinctive color changes are not observed – the solution develops an initial plum
color, which gradually darkens to purple as the reaction progresses. UV–visible and Raman spectroscopic
studies indicate that equilibrium exists among the mono-, di-, and tritelluride in DMF. This equilibrium is
not seen in liquid ammonia solution due to the insolubility of potassium monotelluride in that solvent.
Spectral data also indicate that potassium tetratelluride is formed in DMF solution.
Received 8 March 2012
Received in revised form 15 April 2012
Accepted 15 April 2012
Available online 3 May 2012
Keywords:
Potassium polytellurides
Alkali metal tellurides
Potassium telluride spectra
Potassium polytelluride equilibria
Potassium tellurides in
N,N-dimethylformamide
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
species in solution is often not known with certainty, although
they are generally considered to be equilibrium mixtures [11].
The reaction of elemental tellurium with alkali metals in liquid
ammonia to form polytellurides has been known since the early
1900s, although the air-sensitivity of the products recovered from
these solutions rendered them of limited interest [1]. This situation
changed dramatically with the discovery that polytellurides could
be used as versatile synthetic reagents to form a wide variety of
exciting organometallic materials. Work in this area has been well
summarized in a series of review articles which appeared in the
1990s [2–6], and since 2000 [7–9]. A major factor in this dynamic
growth was the discovery that these materials were stable and sol-
uble in polar solvents, such as N,N-dimethylformamide (DMF) [10].
However, although a vast number and variety of compounds have
been synthesized utilizing polytellurides and characterized by
X-ray crystallography and other techniques, the reaction mecha-
nisms are not known and the actual identity of the polytelluride
1.1. Alkali metal polytellurides in liquid ammonia
One classic procedure for alkali metal polytelluride synthesis is
direct reaction of the elements in liquid ammonia solution. This re-
sults in initial formation of an alkali metal cation and a solvated
electron, which is characterized by a deep blue color. The solvated
electron then reduces tellurium in a step-wise manner to form the
polytelluride of appropriate stoichiometry. The anions formed can
be differentiated by their distinctive colors as follows:
(1) 2 M ? 2 M+ + 2eꢀ (solvated electron, blue, forms rapidly)
(2) Te + 2eꢀ ? Te2ꢀ (white precipitate)
(3) Te2ꢀ þ Te ! Te22ꢀ (blue/violet solution)
(4) Te22ꢀ þ Te ! Te23ꢀ (red solution)
These anions have been characterized in liquid ammonia and
ethylenediamine solution by 123Te and 125Te NMR [12]. However,
since the solutions are so intensely colored, UV–visible spectroscopy
⇑
Corresponding author. Tel.: +1 3256420490; fax: +1 2549689953.
E-mail address: schultz@tarleton.edu (L.D. Schultz).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.04.038

J.L. McAfee et al. / Journal of Molecular Structure 1022 (2012) 68–71
69
is also an excellent tool for monitoring the reaction process. This
technique has been successfully applied to sodium polyselenides
[13] and ammonium polysulfides [14] in liquid ammonia solution.
In this laboratory, it has also been used to investigate polytellurides
of sodium [15], potassium [16], and rubidium (unpublished results)
in liquid ammonia solution.
The data obtained were then hand entered into MicrosoftÓ Excel
spreadsheets and imported into PeakFitÒ for peak resolution
analysis.
All reactions in liquid ammonia were carried out on an all-glass
vacuum line. The liquid ammonia was initially condensed over so-
dium to remove any impurities and then distilled by temperature
differential into a reaction flask on the vacuum line.
Solutions with K:Te ratios of 2:1, 2:2, 2:3, and 2:4 of approxi-
mately 0.01 M were prepared in DMF and sealed in glass NMR
tubes with a torch on a vacuum line, as was a sample of pure
DMF to be used as a background.
A solution of K2Te3 in liquid ammonia was prepared and sealed
in an NMR tube on the vacuum line, as was a sample of liquid
ammonia to be used as a background.
Raman spectra were obtained on a Nicolet Magna 550 FT-IR/Ra-
man with a resolution of 4 cmꢀ1 and were analyzed using Thermo-
Nicolet OMNIC 6.2 software.
1.2. Alkali metal polytellurides in DMF
Alkali metal polytellurides synthesized in liquid ammonia solu-
tion have been successfully utilized as reactants in DMF solution
for many interesting syntheses – including some in which product
materials containing tetratelluride bidantate ligands have been
formed [17–19]. However, although the polytellurides of sodium
and potassium are well-defined entities in liquid ammonia solu-
tion, when these materials were synthesized in liquid ammonia
and transferred to DMF solution, a color change was observed.
Rather than the characteristic compound colors observed in liquid
ammonia, a deep permanganate purple solution was noted, which
was independent of alkali metal/tellurium stoichiometry and alkali
metal identity. Therefore, a study of potassium polytelluride an-
ions in DMF solution was initiated in this laboratory.
3. Results
3.1. UV–visible spectra
Although liquid ammonia and N,N-dimethylformamide have
very different chemical characteristics, since the former is a protic
polar solvent and the latter an aprotic polar solvent, one physical
property which is very similar is their dielectric constants; 26.6
versus 26.7 for DMF and liquid ammonia, respectively [20]. In
DMF neither element is soluble nor is the solvated electron of
the liquid ammonia reaction formed; thus no reaction is expected
from the direct combination of these two elements. However, di-
rect combination in DMF does result in formation of deep purple
solutions, which appear to be identical to those formed when alkali
metal polytellurides synthesized in liquid ammonia are transferred
to DMF solution [17].
When potassium and tellurium were placed in DMF, no color
was evident for approximately 30 min. Then a pale pink/amber col-
or was observed, which progressed to a plum color, then deepened
to a dark permanganate color as higher polytellurides were
formed. No unreacted potassium was observed in the solutions
after approximately 5 h, but solution color continued to darken
for some time – especially with higher potassium/tellurium ratios.
As shown in Fig. 1, the actual reaction sequence in DMF be-
comes more evident when spectra are obtained during the reaction
sequence. This figure shows spectra for a K:Te reaction ratio of 2:4
at 30 min intervals for the initial 4 h of reaction.
The reaction sequence in DMF is also visually different from
that in liquid ammonia. Instead of the dramatic sequence of differ-
ently colored solutions observed in liquid ammonia, alkali metal
and tellurium in DMF do not appear to react at all initially. After
about 30 min, at room temperature, a faint pink color does appear,
which gradually deepens to a final dark purple as the reaction pro-
gresses over a period of several hours.
At 30 min, no color is evident. However, two overlapping
absorption peaks are noted – one at about 290 nm and one at about
330 nm. These are attributed to the formation of a monotelluride
species, which one would expect to be colorless, and would be con-
sistent with the reaction sequence in liquid ammonia.
At 60 min the solution is pale pink/amber, the peaks previously
observed have increased in intensity, and a peak has appeared at
about 400 nm. This is attributed to the formation of a ditelluride.
At 90 min the solution is darker and beginning to acquire a pur-
ple tint, the previously mentioned peaks have increased in inten-
sity, and a peak at 530 nm has appeared.
2. Experimental
2.1. Chemicals
All chemicals were reagent grade from Aldrich Chemical Com-
pany and were used without further purification.
2.2. Procedures
To elucidate the reaction sequence, the progress of the reaction
between potassium and tellurium was followed using UV–visible
spectroscopy for a series of varying alkali metal/tellurium ratios
in the millimolar concentration range. All mass determinations
were performed on an analytical balance to the nearest 0.1 mg un-
der ultrapure argon in a Vacuum Atmospheres Dry Box. DMF was
transferred under ultrapure argon using standard Schlenk tech-
niques for air-sensitive materials. Reactions were run at room tem-
perature, under an argon atmosphere, and solution temperatures
were monitored with
a mercury thermometer and were
22( 1) °C. Samples were withdrawn at timed intervals with a glass
syringe, transferred into a fused silica cuvet, and capped. The sam-
ple was then immediately scanned using a Perkin-Elmer Model
320 UV–visible spectrophotometer with a Model 3600 data station.
Fig. 1. Reaction of tellurium with potassium in DMF: reaction mixture absorbance
spectra taken every 30 min over 4 h. The 240 min spectrum was off scale at 530 nm,
so the spectrum shown is diluted 1:1 with DMF.

70
J.L. McAfee et al. / Journal of Molecular Structure 1022 (2012) 68–71
Fig. 2. Reaction of tellurium with potassium in DMF after 2 days.
Fig. 3. 30 min interval spectral peak intensities: K:Te molar reactant ratio 2:4.
At 120 min the solution is a pale plum color and all of the pre-
4. Discussion
viously observed peaks have increased in intensity. The 400 nm
peak is more intense than the 530 nm peak, but the relative inten-
sity of the 530 nm peak is slightly greater. It should be noted that
unreacted potassium and tellurium are still visible in the reaction
flask.
At 180 min, the color of the solution resembles dilute potassium
permanganate, all peak intensities are increased, and relative peak
intensities are not greatly changed. However, by 240 min the solu-
tion required dilution by approximately 50% to obtain a spectrum
due to the color intensity, and the observed intensity at 530 nm is
greater than that at 400 nm.
An identical mixture was prepared and allowed to react for
2 days before samples were withdrawn for analysis. The solution
color was a deep cranberry red color. The intensity was too great
for the spectrophotometer to read (A > 3.0), so the solution was di-
luted in the cuvet approximately 1:1 with DMF to obtain the spec-
trum, which is shown in Fig. 2. The highest telluride absorbance
peak appeared at ꢁ550 nm, and the next highest peak was at
ꢁ400 nm. This is consistent with the previous spectrum reported
for the 240 min reaction mixture. Attempts to obtain spectra at
longer reaction times were unsuccessful due to sample
decomposition.
We believe an initial reaction occurs between potassium and
tellurium when physical contact is made between the elements
in DMF, not unlike in a thermal technique. This results in formation
of the Te2ꢀ ion, which is colorless and absorbs at about 290 nm.
This species is soluble in DMF and acts as a reducing agent toward
the excess elemental tellurium present, forming polytelluride ions
as shown in the following sequence:
(1) Te + 2 M ? 2 M+ + Te2ꢀ
(2) Te2ꢀ þ Te ! Te22ꢀ
(3) Te22ꢀ þ Te ! Te23ꢀ
(4) Te23ꢀ þ Te ! Te24ꢀ
The following equilibrium also occurs in DMF:
Te2ꢀ þ Te32ꢀ ꢀ 2Te22ꢀ
This equilibrium is analogous to that reported by Sharp and
Koehler [13] for sodium polyselenides in liquid ammonia and Kolis
[17] for alkali metal polytellurides in other solvents, but is not ob-
served with polytellurides in liquid ammonia because of the insol-
ubility of the Teꢀ2 ion in that solvent [15,16]. The appearance of the
162 cmꢀ1 peak observed in the Raman spectrum of K2Te3 in liquid
ammonia solution and in all of the DMF solutions except the theo-
retical monotelluride supports this conclusion.
3.2. Resolved spectra
All spectra consisted of a series of overlapping peaks and were
resolved using PeakFitÒ. Optimal resolution was obtained with a
5 peak fit. Major peaks were located at about 293–315 nm (vari-
able because several spectra went off scale close to this peak),
376 nm, 403 nm, and 530 nm. Fig. 3 shows a plot of peak intensi-
ties versus time.
This equilibrium would be characterized by the equilibrium
constant expression
2
½Te22ꢀꢂ
K ¼
½Te2ꢀꢂ½Te23ꢀꢂ
The peak at about 293 nm appears first, and is attributed to the
formation of a monotelluride. The peaks at 403 nm and 373 nm
both increase continuously with time and are assigned to the ditel-
luride and tritelluride respectively. The peak at 530 nm is the last
to appear and steadily increases, with its intensity passing that of
both the 403 nm and 376 nm peaks, and is, therefore, assigned to
a tetratelluride.
Since Beer’s Law states that absorbancex =
above equilibrium constant can be rewritten as
ex[X], or Ax = ex[X], the
A2ditelluride
Amonotelluride ꢃ Atritelluride
ꢃ ꢀmonotelluride
ꢃ
ꢀtritelluride
K ¼
ꢀ2ditelluride
Although molar absorptivities cannot be measured in this
experiment due to the existence of multiple species in solution,
these values are considered constants, and hence the second com-
ponent of the above equation is also a constant. Therefore, if the
peak assignments are correct and the postulated equilibrium does
exist, the first component of the above equation (K0) will also be a
constant and can be written as follows.
3.3. Raman spectra
The Raman spectrum of K2Te3 in liquid ammonia shows a prom-
inent peak at about 163 cmꢀ1. All of the polytelluride spectra in
DMF solution also show a similar peak at about 160–163 cmꢀ1
(Spectrosc. Lett., accepted), but this peak is absent in the Raman
spectrum of the 2:1 ratio of K:Te (monotelluride).
A2ditelluride
Amonotelluride ꢃ Atritelluride
K0 ¼

J.L. McAfee et al. / Journal of Molecular Structure 1022 (2012) 68–71
71
Table 1
are combined with stirring. They also undergo the following
equilibrium.
Resolved peak intensities and times with resultant values of K0.
Peak location
Telluride
293 nm
Mono-
376 nm
Tri-
403 nm
Di-
530 nm
Tetra
K0
Te2ꢀ þ Te32ꢀ ꢀ 2Te22ꢀ
30 min
60 min
90 min
120 min
150 min
2 days
0.168
0.223
0.231
0.381
1.072
2.231
0.025
0.093
0.158
0.245
0.433
0.688
0.023
0.052
0.069
0.074
0.218
0.399
0.13
0.13
0.13
0.06
0.10
0.10
The presence of this equilibrium results in a mixture of the
mono-, di-, and tritelluride species. This process is not observed
in liquid ammonia solution, because Te2ꢀ is not soluble in liquid
ammonia. Additionally, further reaction in DMF results in forma-
tion of a tetratelluride anion, which is not observed in liquid
ammonia solutions.
0.043
0.149
0.492
1.135
Acknowledgments
The absorbances of the peaks assigned to the mono-, di-, and
tetratelluride species and the resultant values calculated for K0
when these are substituted into the above equation are shown in
Table 1.
The authors wish to acknowledge the assistance of Mr. Peter D.
Poulsen for obtaining portions of this data, Dr. J.D. Lewis of St.
Edward’s University in Austin, TX for use of their FT-IR/Raman,
and of Mr. Steven L. Schultz for converting data into ExcelÒ format.
The financial support of The Robert A. Welch Foundation, Chemis-
try Departmental Grant # AS-0012, is gratefully acknowledged.
The calculated value for K0 remains constant for the initial por-
tion of the reaction. However, when significant absorbance at
530 nm occurs, this value drops markedly, indicating that the
new species being formed is affecting the equilibrium. The increas-
ing absorbance at 530 nm is also accompanied by a marked in-
crease in the 293 nm peak, attributed to the monotelluride, and
would be expected to result in a lower value of K0. K0 stabilizes
again after about 150 min at a slightly lower value.
References
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The delayed appearance of this peak at 530 nm, whose increas-
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376 nm and 403 peaks but is accompanied by a sharp increase in
the relative intensity of the 293 nm peak, is indicative of the for-
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ride observed in liquid ammonia is tritelluride [15,16]. This is
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5. Conclusions
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Potassium polytelluride species in DMF solution do form by a
sequential reaction process at room temperature when the elements