●Õ
66
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
Thermodynamics of [Cr{©6-(t-BuPh)}2]●+C60
of the overall heat capacity of the calorimetric ampoule with
the substance under temperature change from 6 to 360 K. The
experimental Cop values were smoothed by means of a computer
program in the form of degree and semi-logarithmic polynomials.
As an example, the polynomials with the corresponding coef-
ficients for ranges from 6 to 18 K and from 208 to 360 K are cited
below. For fulleride in the interval between 6 and 18 K, the
equation Cop,m (T) = 7.428¢10 Õ 1.413¢103¢(T/30) + 1.041¢104¢
(T/30)2 Õ 3.536¢104¢(T/30)3 + 6.33¢104¢(T/30)4 Õ 5.729¢104¢
(T/30)5 + 2.066¢104¢(T/30)6 as well as the equation Cop,m (T) =
1.43317421¢106 Õ 3.12766475¢106¢ln(T/30) + 2.71804584¢106¢
(ln(T/30))2 Õ 1.1751988¢106¢(ln(T/30))3 + 2.52755168¢105¢(ln(T/
30))4 Õ 2.16201458¢104¢(ln(T/30))5 in the range from 208 to
360 K were used. In the above equations the Cop,m is given in
complex under study, C60 fullerite, neutral dimer (C60)2 as well
as some previously studied fullerides.
Experimental
The tested sample of bis(©6-t-butylphenyl)chromium fulleride
●Õ
[Cr{©6-(t-BuPh)}2]●+C60 was synthesized by the method de-
scribed in Ref. 16. The solution of [Cr(©6-(t-BuPh))2]0 in toluene
was added to the saturated solution of C60 in toluene at room
temperature. The resulting sediment was decanted and then
washed with toluene and dried in vacuum. The elemental analysis
yielded the Cr composition at 4.98% which compares with 5.00%
calculated for C80H28Cr. The t-butylbenzene and equimolar
mixture of fullerene with chromium was quantitatively formed at
thermo-decomposition. The ion structure of fulleride was con-
firmed by results of EPR and ESR spectra. The EPR spectra were
recorded on a Bruker EPX radiospectrometer and electron spectra
on a Perkin-Elmer Lambda25 spectrometer. The EPR spectrum of
bis(©6-t-butylphenyl)chromium fulleride in tetrahydrofuran (THF)
at 293 K shows a line with typical for cation [Cr(©6-arene)2]●+
hyperfine structure, g = 1.986, aH = 3.5 G, aCr = 18.1 G. Thus,
bis(©6-t-butylphenyl)chromium is the cation-radical in fulleride.
The visible spectroscopy of bis(©6-t-butylphenyl)chromium full-
eride in THF has shown presence of an absorption band which
J KÕ1 molÕ1
.
The mean-square deviation of experimental Cop,m points from
the corresponding averaging Cop,m = f(T) curve did not exceed
«0.6% in the range 6Í20 K, «0.08% between 20 and 170 K,
and «0.05% from 210 to 360 K. The molar mass of the object
under study was calculated from the IUPAC table of atomic
weights.19
Results and Discussion
●Õ
is characteristic for C60 at - = 1081 nm. Thus, bis(©6-t-butyl-
Heat Capacity and Thermodynamic Characteristics
phenyl)chromium fulleride is the ion-radical salt: [Cr{©6-(t-
of Transformation.
The experimental values of molar
●Õ
BuPh)}2]●+C60
.
heat capacity of [Cr{©6-(t-BuPh)}2]●+C60 in the range 6Í
360 K and the averaging Cop,m = f(T) curve are presented in
Figure 1. It can be seen that heat capacity of fulleride gradually
increases with rising temperature until T = 170 K. In the
temperature interval from 170 to 210 K, endothermic trans-
formation occurs that manifest itself as a positive deviation
from the normal trend of the temperature dependence of Cpo,m
(Figure 1).
●Õ
The tested sample is relatively unstable in air and therefore all
operations related to its preparation for calorimetric measurements
were conducted in a special box in a flow of high-purity argon.
To study the temperature dependence of the heat capacity and
●Õ
temperatures of transformations of [Cr{©6-(t-BuPh)}2]●+C60
sample in the range from T = 6 to 360 K, an automatic thermo-
physical device, a BCT-3 low-temperature adiabatic vacuum
calorimeter was employed. The calorimeter was manufactured
at “Termis” JSC at the All-Russia Metrology Research Institute,
Moscow region, Russia, its design and the operation procedure
were described earlier.17 The reliability of the calorimeter
operation was checked by measuring Cop,m of standard samples
of highly purified copper, standard synthetic corundum and K-2
benzoic acid18 prepared at the Institute of Metrology of the State
Standard Committee of the Russian Federation. The amounts of the
indicated samples were 2.0000, 1.5000, and 0.7682 g respectively.
It was established that the apparatus and the measurement
technique enable determination of the heat capacity Cop,m of
substances with the error not exceeding «2% near 15 K, «0.5%
between 15 and 40 K, and «0.2% in the range from 40 to 360 K
and measurement of the phase-transition temperatures within about
«0.01 K and to measure the enthalpies of transformations within
«0.2%.
A sample of bis(©6-t-butylphenyl)chromium fulleride with a
mass 0.1240 g was placed in a thin-walled (1.5 cm3) cylindrical
titanium ampoule. After pumping, the ampoule was filled with
high-purity helium as a heat-exchange gas to a pressure of 6 kPa (at
room temperature) and selected. The measurements of Cop,m were
made in the temperature range from 6 to 360 K. 169 experimental
values of Cop,m were obtained in three series reflecting the sequence
of experiments. The first cycle of measurements was completed in
the temperature range from 6 to 90 K. The second series of Cpo,m
measurements was carried out between T = 80 and 360 K. The
sample was cooled down to T = 160 K and a repeated cycle of
Cop,m measurements (third series) was conducted to T = 250 K.
The heat capacity of the sample was between 15% and 30%
The heat capacity grows relatively rapidly from 508.25
J KÕ1 molÕ1 at T = 170 K until the p. C (Figure 1) and then it
decreases down to 651.5 J KÕ1 molÕ1 at T = 210 K. The above
temperature region for the relation Cop,m = f(T) is described by
the BCD curve. The transition is reversible. It was reproduced
on repeated cooling and heating (as it described above). The
Cpo /(J K-1 mol-1)
C
1600
E
1200
D
800
400
B
A
0
100
200
300
400
T / K
Figure 1. The temperature dependence of heat capacity
of the bis(©6-t-butylphenyl)chromium fulleride [Cr{©6-
(t-BuPh)}2]●+C60●Õ: AB: dimeric form, ED: monomeric
form, BCD: apparent heat capacity in the transformation
interval.