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Thermochemistry of dehydration and dehydrohalogenation of some compounds containing the H5O2+ ion
INORG.
NUCL.
CHEM.
LETTERS
Vol.
7,
pp.
355-358,
1971.
Pergamon Press.
Printed in Great Britain.
THERMOCHEMISTRY OF DEHYDRATIONAND DEHYDROHALOGENATION OF SOMECOMPOUNDSCONTAINING THE Hs02+ ION H. E. LeMay, Jr. and P. So Nolan Department of Chemistry, University of Nevada Reno, Nevada 89507 (Received 16 N o v e m ~ r 1970)
A number of recent papers have dealt with the stoichiometry and kinetics of dehydration-dehydrohalogenation reactions of compounds containing the H502+ ion (I-4).
Part of this former work is related to studies of the isomerization of
trans-[Co(pn)2Cl2](HsO~)Cl2 and companion studies of its ethylenediamine analogues which do not isomerize in the solid phase. Differences between the propylenediamine and ethylenediamine complexes have been sought in order to provide insight into the isomerization mechanism.
The present paper reports some thermochemical
studies of these compounds which support previous studies and provide additional information about the isomerization process. Experimental Preparations of the compounds studied have been described elsewhere (see Table l ) , and each compound was analyzed as in earlier studies (2).
A Perkin-
Elmer Differential Scanning Calorimeter (DSC-IB) was employed to measure enthalpy changes. Samples weighing 7-16 mg were heated in aluminum pans with perforated aluminum covers under a N2 flow of ca.
25 ml min"I.
The instrument
was calibrated against In, and heating rates of 5 or lO° min"I were used.
In
addition, studies under reduced pressure (15 torr) and at a heating rate of 1.25° min- l were conducted on trans-[Co(en)2Br~](HsO2)Br2. Results and Discussion The enthalpies associated with the dehydration-dehydrohalogenation reaction 355
356
DEHYDROHALOOENATION OF SOME COMPOUNDS CONTAINING THE H502+ ION
Vol. 7, No. 4
are summarized in Table. l together with the average temperatures of the major DSC peak maxima. TABLE l
Enthalpy Changes and DSC Peak Maxima Compound
AH~ kcal mole"I
Tp~ °Ka
trans-[Co(en)zCl2](HsO2)Clz(5)
36.3 ± 0.9 b
362 ± I b
trans-[Co(en)2Br2](HsO2)Br2(4)
35.1 ~ 1.6
382 ± 3
trans-[Rh(en)2Cl2](Hs02)Cl2(6)
37.0 ± 0.6
369 ± l
trans-[Co(pn)2Cl2](HsO~)Cl2(7)
40.3 ± 0.6
403 ± 2
trans-[Co(pn)2Cl2](HsO2)Br2(8)
35.6 ± 0.4
407 ± 5
trans-[Co(pn)=Br2](HsO2)Br=(8)
38.9 ± 1.5
415 ± l
[As(CGHs)~](Hs02)CI2
21.2 ± 0.3
354 ± l
(9)
a.
Heating rates of 5° min"~
b.
Error limits are average deviations from the mean for 2-5 determinations.
Except for [As(C~Hs)~](HsO~)Cl2and trans-[Co(pn)2Br2](HsO2)B&, single DSC endotherms were observed. These two compounds exhibited weak shoulders on the high temperature side of the endotherm. The shoulder was far more prominent for the arsenic compound, but in neither case could separate enthalpies be evaluated. A two-step dehydration-dehydrochlorination
has been reported for the arsenic
compound based on TGA studies (3) and has been suggested for other compounds containing the Hs02+ ion ( I ) .
However, TGA.studies of trans-[Co(pn)2Br2](HsO2)Br2
indicate mass loss in a single step. Except for [As(C6Hs)~](Hs02)Cl2, trans-[Co(pn)2Cl2](Hs02)Cl2 and trans[Co(pn)2Br2](HsO2)Br2, the enthalpies shown in Table l fall in the area of 36 ± l kcal mole").
This compares with an activation energy of 37 ± 2 kcal mole-~
for dehydration-dehydrobromination of trans-[Co(en)2Br~](HsO2)Br2 (4).
Thus the
activation energies for dehydration-dehydrohalogenation of compounds of the type
Vol. 7, No. 4
DEHYDROHALOGENATION OF SOME COMPOUNDS CONTAINING THE H502 + ION
trans-[M(AA)2X2](HsO2)X2 appear to be very similar
357
in magnitude to the reaction
enthalpies (this is frequently found for endothermic solid-phase reactions (lO)). In view of this agreement, the dehydration-dehydrochlorination
reaction of
trans-[Co(pn)2Cl~](Hs02)Cl2 was restudied in vacuo by isothermal mass-loss measurements over a temperature range larger than that previously employed (2). This study gave Ea = 36 ± 3 kcal mole-~.
The Arrhenius plots show curvature as
found for trans-[Co(en)2Br2](HsO2)Br2 (4).
Since the curvature is reduced by the
use of smaller samples, i t is attributed to sample self-cooling. The comparison between AH and Ea pointed out above suggests that they both fall in the area of 36 kcal mole-z for the dehydration-dehydrohalogenation of trans-[M(AA)2X2](HsO2)X2-type compounds, regardless of whether isomerization occurs.
This is consistent with dehydration-dehydrohalogenation preceding iso-
merization.
The slightly higher enthalpies measured for trans-[Co(pn)2Cl2](Hs02)Cl2
and trans-[Co(pn)EBr2](HsO2)Br2 can be attributed either to isomerization accompanying dehydration-dehydrohalogenation or to strain energies associated with formation of a microcrystalline product lattice (lO).
Under the conditions of the DSC
experiment these two compounds undergo about 50% isomerization, while trans[Co(pn)2Cl2](HsO2)Br2 undergoes less than I0%.
No isomerization was observed for
the other complexes. Formation of microcrystalline products has been observed for all three propylenediamine complexes (8).
Since the higher AH is observed for
trans-[Co(pn)2Cl2](HsO2)Cl2and trans-[Co(pn)2Br2](HsO2)Br2, but not for trans[Co(pn)2Cl2](HsO2)Br2, i t is most likely caused by an endothermic isomerization process.
In this case the driving force for isomerization must be provided by an
increase in entropy. The DSC data were also used to determine activation energies according to the method of Thomas and Clarke.(ll) together with rate laws reported earlier for these compounds (2, 4).
In several instances Arrhenius plots showed the same
curvature found in isothermal mass-loss studies. the range of 18-30 kcal mole-z.
The activation energies fell in
Since Thomas and Clarke ( l l ) mention observing a
small thermal lag in the DSC instrument, sample self-cooling leading to low activation energies is possible.
358
DEHYDROHALOGENAT1ON OF SOME COMPOUNDS CONTAINING THE H502 + ION
Vol. 7, No. 4
Acknowledgement The authors wish to thank the Research Corporation and the Research Advisory Board of the University of Nevada for financial support of this work. References I.
D. Dollimore, R. D. Gillard, E. D. McKenzie and R. Ugo, J. Inorg. Nucl. Chem., 30, 2755 (1968).
2.
H.E. LeMay, Jr., Inorg. Chem., ~, 2531 (1968).
3.
H.E. LeMay, Jr., Inorg. Nucl. Chem. Lett., 5, 941 (1969).
4.
H.E. LeMay, Jr., J. Phys. Chem., 7__4, 1345 (1970).
5.
J.C. Bailar, Jr., Inorg. Syn., 2, 222 (1946).
6.
R.D. Gillard and G. Wilkinson, J. Chem'. Soc., 1640 (1964).
7.
J.C. Bailar, Jr., C. A. Stiegman, J. H. Balthis, Jr., and E. H. Huffman, J. Am. Chem. Soc., 61, 2402 (1939).
8.
H. E. LeMay, Jr., submitted for publication.
9.
K. M. Harmon and R. R. Lake, Inorg. Chem., ~, 1921 (1968).
lO.
D. A. Young, "Decomposition of Solids," Pergamon Press, New York, N.Y., 1966, Chapter 3.
II.
J. M. Thomas and T. A. Clarke, J. Chem. Soc. (A), 457 (1968).
NUCL.
CHEM.
LETTERS
Vol.
7,
pp.
355-358,
1971.
Pergamon Press.
Printed in Great Britain.
THERMOCHEMISTRY OF DEHYDRATIONAND DEHYDROHALOGENATION OF SOMECOMPOUNDSCONTAINING THE Hs02+ ION H. E. LeMay, Jr. and P. So Nolan Department of Chemistry, University of Nevada Reno, Nevada 89507 (Received 16 N o v e m ~ r 1970)
A number of recent papers have dealt with the stoichiometry and kinetics of dehydration-dehydrohalogenation reactions of compounds containing the H502+ ion (I-4).
Part of this former work is related to studies of the isomerization of
trans-[Co(pn)2Cl2](HsO~)Cl2 and companion studies of its ethylenediamine analogues which do not isomerize in the solid phase. Differences between the propylenediamine and ethylenediamine complexes have been sought in order to provide insight into the isomerization mechanism.
The present paper reports some thermochemical
studies of these compounds which support previous studies and provide additional information about the isomerization process. Experimental Preparations of the compounds studied have been described elsewhere (see Table l ) , and each compound was analyzed as in earlier studies (2).
A Perkin-
Elmer Differential Scanning Calorimeter (DSC-IB) was employed to measure enthalpy changes. Samples weighing 7-16 mg were heated in aluminum pans with perforated aluminum covers under a N2 flow of ca.
25 ml min"I.
The instrument
was calibrated against In, and heating rates of 5 or lO° min"I were used.
In
addition, studies under reduced pressure (15 torr) and at a heating rate of 1.25° min- l were conducted on trans-[Co(en)2Br~](HsO2)Br2. Results and Discussion The enthalpies associated with the dehydration-dehydrohalogenation reaction 355
356
DEHYDROHALOOENATION OF SOME COMPOUNDS CONTAINING THE H502+ ION
Vol. 7, No. 4
are summarized in Table. l together with the average temperatures of the major DSC peak maxima. TABLE l
Enthalpy Changes and DSC Peak Maxima Compound
AH~ kcal mole"I
Tp~ °Ka
trans-[Co(en)zCl2](HsO2)Clz(5)
36.3 ± 0.9 b
362 ± I b
trans-[Co(en)2Br2](HsO2)Br2(4)
35.1 ~ 1.6
382 ± 3
trans-[Rh(en)2Cl2](Hs02)Cl2(6)
37.0 ± 0.6
369 ± l
trans-[Co(pn)2Cl2](HsO~)Cl2(7)
40.3 ± 0.6
403 ± 2
trans-[Co(pn)2Cl2](HsO2)Br2(8)
35.6 ± 0.4
407 ± 5
trans-[Co(pn)=Br2](HsO2)Br=(8)
38.9 ± 1.5
415 ± l
[As(CGHs)~](Hs02)CI2
21.2 ± 0.3
354 ± l
(9)
a.
Heating rates of 5° min"~
b.
Error limits are average deviations from the mean for 2-5 determinations.
Except for [As(C~Hs)~](HsO~)Cl2and trans-[Co(pn)2Br2](HsO2)B&, single DSC endotherms were observed. These two compounds exhibited weak shoulders on the high temperature side of the endotherm. The shoulder was far more prominent for the arsenic compound, but in neither case could separate enthalpies be evaluated. A two-step dehydration-dehydrochlorination
has been reported for the arsenic
compound based on TGA studies (3) and has been suggested for other compounds containing the Hs02+ ion ( I ) .
However, TGA.studies of trans-[Co(pn)2Br2](HsO2)Br2
indicate mass loss in a single step. Except for [As(C6Hs)~](Hs02)Cl2, trans-[Co(pn)2Cl2](Hs02)Cl2 and trans[Co(pn)2Br2](HsO2)Br2, the enthalpies shown in Table l fall in the area of 36 ± l kcal mole").
This compares with an activation energy of 37 ± 2 kcal mole-~
for dehydration-dehydrobromination of trans-[Co(en)2Br~](HsO2)Br2 (4).
Thus the
activation energies for dehydration-dehydrohalogenation of compounds of the type
Vol. 7, No. 4
DEHYDROHALOGENATION OF SOME COMPOUNDS CONTAINING THE H502 + ION
trans-[M(AA)2X2](HsO2)X2 appear to be very similar
357
in magnitude to the reaction
enthalpies (this is frequently found for endothermic solid-phase reactions (lO)). In view of this agreement, the dehydration-dehydrochlorination
reaction of
trans-[Co(pn)2Cl~](Hs02)Cl2 was restudied in vacuo by isothermal mass-loss measurements over a temperature range larger than that previously employed (2). This study gave Ea = 36 ± 3 kcal mole-~.
The Arrhenius plots show curvature as
found for trans-[Co(en)2Br2](HsO2)Br2 (4).
Since the curvature is reduced by the
use of smaller samples, i t is attributed to sample self-cooling. The comparison between AH and Ea pointed out above suggests that they both fall in the area of 36 kcal mole-z for the dehydration-dehydrohalogenation of trans-[M(AA)2X2](HsO2)X2-type compounds, regardless of whether isomerization occurs.
This is consistent with dehydration-dehydrohalogenation preceding iso-
merization.
The slightly higher enthalpies measured for trans-[Co(pn)2Cl2](Hs02)Cl2
and trans-[Co(pn)EBr2](HsO2)Br2 can be attributed either to isomerization accompanying dehydration-dehydrohalogenation or to strain energies associated with formation of a microcrystalline product lattice (lO).
Under the conditions of the DSC
experiment these two compounds undergo about 50% isomerization, while trans[Co(pn)2Cl2](HsO2)Br2 undergoes less than I0%.
No isomerization was observed for
the other complexes. Formation of microcrystalline products has been observed for all three propylenediamine complexes (8).
Since the higher AH is observed for
trans-[Co(pn)2Cl2](HsO2)Cl2and trans-[Co(pn)2Br2](HsO2)Br2, but not for trans[Co(pn)2Cl2](HsO2)Br2, i t is most likely caused by an endothermic isomerization process.
In this case the driving force for isomerization must be provided by an
increase in entropy. The DSC data were also used to determine activation energies according to the method of Thomas and Clarke.(ll) together with rate laws reported earlier for these compounds (2, 4).
In several instances Arrhenius plots showed the same
curvature found in isothermal mass-loss studies. the range of 18-30 kcal mole-z.
The activation energies fell in
Since Thomas and Clarke ( l l ) mention observing a
small thermal lag in the DSC instrument, sample self-cooling leading to low activation energies is possible.
358
DEHYDROHALOGENAT1ON OF SOME COMPOUNDS CONTAINING THE H502 + ION
Vol. 7, No. 4
Acknowledgement The authors wish to thank the Research Corporation and the Research Advisory Board of the University of Nevada for financial support of this work. References I.
D. Dollimore, R. D. Gillard, E. D. McKenzie and R. Ugo, J. Inorg. Nucl. Chem., 30, 2755 (1968).
2.
H.E. LeMay, Jr., Inorg. Chem., ~, 2531 (1968).
3.
H.E. LeMay, Jr., Inorg. Nucl. Chem. Lett., 5, 941 (1969).
4.
H.E. LeMay, Jr., J. Phys. Chem., 7__4, 1345 (1970).
5.
J.C. Bailar, Jr., Inorg. Syn., 2, 222 (1946).
6.
R.D. Gillard and G. Wilkinson, J. Chem'. Soc., 1640 (1964).
7.
J.C. Bailar, Jr., C. A. Stiegman, J. H. Balthis, Jr., and E. H. Huffman, J. Am. Chem. Soc., 61, 2402 (1939).
8.
H. E. LeMay, Jr., submitted for publication.
9.
K. M. Harmon and R. R. Lake, Inorg. Chem., ~, 1921 (1968).
lO.
D. A. Young, "Decomposition of Solids," Pergamon Press, New York, N.Y., 1966, Chapter 3.
II.
J. M. Thomas and T. A. Clarke, J. Chem. Soc. (A), 457 (1968).