Thermochemical properties of palladium(II) chelates involving dialkyldithiocarbamates

Article abstract of DOI:10.1023/A:1013912207743

Thermochemical properties of palladium(II) chelates involving diakyldithiocarbamates were studied using reaction-solution calorimetry. The homolytic and heterolytic mean palladium-sulphur bond-dissociation enthalpies were calculated using the standard mol

Full text of DOI:10.1023/A:1013912207743

Journal of Thermal Analysis and Calorimetry, Vol. 67 (2002) 413–417
THERMOCHEMICAL PROPERTIES
OF PALLADIUM(II) CHELATES INVOLVING
DIALKYLDITHIOCARBAMATES
J. R. Botelho1*, A. G. Souza1, L. M. Nunes1, A. P. Chagas2,
I. M. Garcia dos Santos1, M. M. da Conceição1 and P. O. Dunstan2
1Departamento de Química, CCEN, Universidade Federal da Paraíba, 58059-900, João Pessoa,
Paraíba, Brazil
2Instituto de Química, Universidade Estadual de Campinas, 13083-970, Campinas, São Paulo, Brazil
Abstract
The standard molar enthalpies of formation of crystalline dialkyldithiocarbamates chelates,
[Pd(S2CNR2)2], with R=C2H5, n-C3H7, n-C4H9 and i-C4H9, were determined through reaction-solution
calorimetry in acetone, at 298.15 K. From the standard molar enthalpies of formation of the gaseous
chelates, the homolytic (172.4±3.8, 182.5±3.2, 150.9±3.1 and 162.6±3.1 kJ mol–1) and heterolytic
(745.0±3.8, 803.7±3.3, 834.3±3.1 and 735.2±3.0 kJ mol–1) mean palladium-sulphur bond-dissociation
enthalpies were calculated.
Keywords: calorimetry, dialkyldithiocarbamates, enthalpies, palladium
Introduction
The thermochemical data found in literature concerning dithiocarbamates are
essentially originating from application of several techniques such as: thermogravi-
metry, differential thermal analysis, differential scanning calorimetry, chromatogra-
phy in gaseous phase, mass spectrometry and, in minor extension from solution calo-
rimetry and thermometric titration [1–8].
This publication focuses on the thermochemical data for Pd(II) chelates of
dialkyldithiocarbamates. These results are useful for helping to understand the effect
of the length of the alkyl chain bonded to the nitrogen atom of the ligand, over the
thermochemical parameters of the chelates.
Experimental
The salts dialkyl ammonium chloride, R2NH2Cl, and dialkylammonium dialkyl-
dithiocarbamate, R2NH2S2CNR2, were synthesized with R=C2H5, n-C3H7, n-C4H9 and
*
Author for correspondence: E-mail: regisbotelho@uol.com.br
1418–2874/2002/ $ 5.00
Akadémiai Kiadó, Budapest
© 2002 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

414 BOTELHO et al.: PALLADIUM(II) CHELATES INVOLVING DIALKYLDITHIOCARBAMATES
i-C4H9. Their chemical analysis and melting points were in complete agreement with
their postulated formulation and reported melting points [5–8]. The palladium(II)
dialkyldithiocarbamates complexes, [Pd(S2CNR2)2], with R=C2H5, n-C3H7, n-C4H9,
and i-C4H9 were synthesized from the direct reaction of PdCl2 with the corresponding
amine and carbon disulphide using 1,2-dichloroethane as the solvent.
The microanalysis (C, H, N) of the chelates were carried out using a Perkin
Elmer elemental analyser. Infrared spectra were recorded with the compounds in KBr
pellets on a BOMEM, model MB-102 spectrophotometer. The melting point temper-
atures for all compounds were determined by means of a MICROQUÍMICA, model
MQAPF-301 apparatus. Thermogravimetric curves were obtained using a Shimadzu
model TGA-50 thermobalance, at a heating rate of 10°C min–1 in a dynamic atmo-
sphere of dry nitrogen flux of 50 mL min–1 and the mass spectra of the ligands and
their Pd(II) chelates, were recorded on a Hewlett–Packard model 5988A spectrome-
ter with an ionization energy of 70 eV, at 523 K.
All solution calorimetric determinations were carried out in LKB 8700-1 iso-
peribolic precision calorimeter system as described before [9].
The melting enthalpies and the heat capacities of the chelates were determined
in a Shimadzu model DSC-50 differential scanning calorimeter, at a heating rate of
5 K min–1 purged with dry nitrogen [9, 10].
Results and discussion
The CHN elemental analysis of the complexes, [Pd(S2CNR2)2], with R=C2H5, n-C3H7,
n-C4H9, and i-C4H9, are in good agreement with the expected values for the postulated
formulation of these compounds. The melting points observed for them are consistent
with their high purities. The mode of coordination of the dialkyldithocarbamates and
the defined melting points suggest the existence of very stable compounds. However,
on cooling the molten samples and heating them again, the previous melting-point in-
tervals were not reproduced.
The band attributed to νCN in the IR spectra of the ligands, splits into two after
coordination, one of which is shifted to higher frequencies relative to the uncoordi-
nated ligands. For all complexes this band frequency is higher than that of the respec-
tive ligand. On the other hand, a decrease in the band attributed to CS is observed in
the chelates relative to the uncoordinated ligands, and a new band at 1000 cm–1 ap-
pears, indicating that the ligands are bonded in a bidentate way to the metallic ion
[11]. The mass spectra of the complexes revealed the presence of the parent ions with
the following stability order: Pd(S2CNPr2n)2>Pd(S2CNBu2n)2>Pd(S2CNEt2)2>
Pd(S2CNBui2 )2).
The standard molar enthalpies of reaction, ∆ r Hmo , were obtained at 298.15 K,
from the reaction-solution enthalpies, ∆ iHmo , of each reagent and product dissolved
sequentially in 1,2-dichloroethane (Table 1), where a strict control of stoichiometry
was maintained, for the following general reaction [5–8]:
PdCl2 (cr)+2NH2R2S2CNR2 (cr)→Pd(S2CNR2 )2 (cr)+2NH2R2Cl(cr); ∆ r Hmo (1)
J. Therm. Anal. Cal., 67, 2002

BOTELHO et al.: PALLADIUM(II) CHELATES INVOLVING DIALKYLDITHIOCARBAMATES 415
Table 1 Standard molar enthalpies (kJ mol–1) of solution-reaction, at 298.15 K
∆iHmo
Reactant
Solvent
Solution
A1
A2
A3
A4
B1
2Et2NH2S2CNEtr(cr)
PdCl2 (cr)
1,2-dichloroethane
67.66±1.23
34.79±2.18
12.04±0.46
1.99±0.01
A1
Pd(S2CNEt2 )2 (cr)
2Et2NH2Cl(cr)
2Pr2nNH2S2CNPr2n (cr)
PdCl2 (cr)
1,2-dichloroethane
A3
1,2-dichloroethane
42.81±0.95
21.73±1.97
41.03±1.86
18.01±1.05
34.54±0.42
18.34±2.01
46.20±0.68
27.24±0.51
36.76±0.77
19.98±1.98
21.58±1.08
28.50±0.58
B1
B2
Pd(S2CNPr2n )2 (cr)
2Prn2NH2Cl(cr)
2Bun2NH2S2CNBun2 (cr)
PdCl2 (cr)
Pd(S2CNBun2 )2 (cr)
2Bun2NH2Cl(cr)
2Bui2NH2S2CNBui2 (cr)
PdCl2 (cr)
1,2-dichloroethane
B3
B2
B4
1,2-dichloroethane
D1
D2
D3
D4
E1
D1
1,2-dichloroethane
D3
1,2-dichloroethane
E1
1,2-dichloroethane
E3
E2
Pd(S2CNBui2 )2 (cr)
2Bui2NH2Cl(cr)
E3
E4
Table 2 Standard molar enthalpies (kJ mol–1), heat capacities (kJ mol–1 K–1) and temperatures
(K) for the chelates
Pd(S2CNEt2 )2
Pd(S2CNPr2n )2
Pd(S2CNBun2 )2
Pd(S2CNBui2 )2
Parameters
∆rHmo
∆f Hmo (cr)
88.42±2.54
110.78±6.42
36.68±3.52
58.96±2.64
503
5.50±3.06
–120.60±5.44
30.32±0.88
63.79±3.96
416
–20.56±2.22
–237.72±5.03
27.18±3.52
64.41±0.44
375
6.66±2.45
–197.50±4.93
25.49± 3.96
64.08±3.52
465
l
∆crHmo
∆glHmo
Tfus
Tvap
582
594
587
597
Cp,m (cr)
Cp,m (l)
Cp,m (g)
∆gcrHmo
∆f Hmo (g)
∆hoHmo
0.271
0.573
0.511
0.799
0.124
0.649
0.700
0.991
0.533
0.692
0.833
0.848
9.65±4.40
120.43±7.60
–689.57±15.08
72.41±4.06
–48.19±6.49
–729.99±12.98
38.44±3.55
–139.30±7.58
–603.70±12.42
100.13±5.30
–97.37±5.03
–650.57±12.13
∆heHmo
–2979.97±15.16 –3214.79±13.07 –3337.30±12.51 –2940.97±12.22
<D>(Pd–S)
<D′>(Pd–S)
172.4±3.8
745.0±3.8
182.5±3.2
803.7±3.3
150.9±3.1
834.3±3.1
162.6±3.0
735.2±3.0
J. Therm. Anal. Cal., 67, 2002

416 BOTELHO et al.: PALLADIUM(II) CHELATES INVOLVING DIALKYLDITHIOCARBAMATES
The standard molar enthalpy for each reaction was calculated from these tabu-
lated values by applying a convenient thermochemical cycle. The individual
enthalpies values are listed in Table 2.
Table 3 Auxiliary data (kJ mol–1)
∆f Hmo
Species
Ref.
[12]
[13]
[14]
[13]
[8]
PdCl2 (cr)
–198.70±3.08
–249.2±2.2
–359.73±1.22
–353.20±1.30
–389.50±1.00
–461.3±1.6
–452.44±0.39
–463.8±1.4
–461.07±0.50
378.20±0.88
3069.40±1.76
215.9±6.5
Et2NH2S2CNEt2 (cr)
Et2NH2Cl(cr)
Pr2nNH2S2CNPr2n (cr)
Pr2nNH2Cl(cr)
Bun2NH2S2CNBun2 (cr)
Bun2NH2Cl(cr)
Bui2NH2S2CNBui2 (cr)
BuirNH2Cl(cr)
Pd(g)
[13]
[8]
[13]
[8]
[12]
[12]
[9]
Pd2+ (g)
Et2NCS2. (g)
Et2NCS–2 (g)
15.5±6.5
[9]
Pr2nNCS2. (g)
151.8±5.6
[6]
Pr2nNCS–2 (g)
48.6±5.6
[6]
Bun2NCS2. (g)
Bun2NCS–2 (g)
Bui2NCS2. (g)
Bui2NCS–2 (g)
117.9±5.5
[10]
[10]
[5]
82.5±5.5
87.5±7.6
112.9±7.6
[5]
The standard molar enthalpies of formation of the crystalline chelates shown in Ta-
ble 2, were determined from: a) the values of ∆ r Hmo , b) the standard molar enthalpies of
formation of the palladium chloride [12], ligands [13] and dialkyl ammonium chlorides
[14], c) by using expression (2), d) the auxiliary data listed in Table 3.
∆ f Hmo [ln(S2CNR2 )3 ](cr)=∆ r Hmo −3∆ f Hmo (NH2R2Cl)(cr)+∆ f Hmo (lnCl3 )(cr)
+3∆ f Hmo (NH2R2S2CNR2 )(cr)
(2)
The standard molar enthalpies of sublimation, ∆gcr Hmo , were determined by
means of differential scanning calorimetry and estimation methods [5–8,15].
The molar standard enthalpies of vaporization were determined through estima-
tive methods [8]. The molar standard enthalpies of melting and heat capacities for the
solid and liquid phases of the chelates were determined by DSC technique. However,
for the gas phase these quantities were estimated [8]. A complete series of these val-
ues obtained are listed in Table 2, together with the standard molar enthalpies of sub-
J. Therm. Anal. Cal., 67, 2002

BOTELHO et al.: PALLADIUM(II) CHELATES INVOLVING DIALKYLDITHIOCARBAMATES 417
limation. These values were calculated by applying all these data in the expression (3)
and consequently the standard molar enthalpies of formation in the gas phase, listed
in Table 2, were calculated.
Tvap
Tfus
298.15 K
∆gcr Hmo = Cp (cr)dT +∆lcr Hmo + Cp (l)dT +∆gl Hmo + Cp (g)dT
(3)
∫
∫
∫
298.15 K
Tfus
Tvap
The auxiliary data listed in Table 3 enable the calculation of the mean enthalpies
of palladium–sulphur bonds [5–8, 10] for homolytic, <D>(Pd–S)=∆ ho Hmo /4 and
heterolytic, <D′>(Pd–S)=∆ he Hmo /4 processes. These calculated values are listed in
Table 2.
Conclusions
The results presented in Table 2 show that <D>(Pd–S) and <D′>(Pd–S) values fol-
low a different sequence as compared with the expected one. This behaviour demon-
strates clearly that, in the case of alkyl substitution, the possible polar or steric effects
related to it did not affect directly the interaction Pd–S as the final result did not indi-
cate any clear dependence on either of these effects.
* * *
The authors thank CNPq Foundation (Brazil) for the scholarships and financial support for the ac-
complishment of this work.
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