Synthesis, characterization and thermochemical properties of N-acyl-N′,N′-diethylthioureas

Article abstract of DOI:10.1039/b104709b

Three N-acyl-N′,N′-diethylthioureas, RCONHCSNEt₂, R = ⁱPr,ⁱBu,^tBu, have been prepared and characterised. The standard (p^o = 0.1 MPa) molar enthalpies of combustion in oxygen of the three crystalline compounds, at T = 298.15 K, have been measured by rotating bomb-combustion calorimetry, and the standard molar enthalpies of sublimation of the compounds by microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in their crystalline and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are discussed comparatively. Acid constants and some complex stabilities have been measured pH-potentiometrically in dioxane-water mixture. The crystal structure of N,N-diethyl-N′-isovaleroyl-thiourea is presented and shows a delocalization of the π electrons of the C=S group over the carbon-amine nitrogen bond, CS-NEt₂, stabilising the molecule, in accordance with the thermochemical results.

Full text of DOI:10.1039/b104709b

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Synthesis, characterization and thermochemical properties
of N-acyl-NЈ,NЈ-diethylthioureas
Manuel A. V. Ribeiro da Silva,*a Maria D. M. C. Ribeiro da Silva,a Luís C. M. da Silva,a
Frank Dietze,b Eberhard Hoyer,b Lothar Beyer,b Bernd Schröder,b Ana M. Damasc
and Joel F. Liebman*d
2
a Centro de Investigação em Química, Departamento de Química, Faculdade de Ciências,
Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal
b Institut für Anorganische Chemie, Fakultät für Chemie und Mineralogie, Universität Leipzig,
Johannisallee 29, Leipzig 04103, Bundesrepublik Deutschland
c Instituto de Ciências Biomédicas Abel Salazar, Universidade do Porto,
Largo do Prof. Abel Salazar, 2, P-4099-003 Porto, Portugal and Instituto de Biologia
Molecular e Celular, Universidade do Porto, Rua do Campo Alegre, 823, P-4150-180 Porto,
Portugal
d Department of Chemistry and Biochemistry, University of Maryland, Baltimore County,
Baltimore, Maryland 21250, USA
Received (in Cambridge, UK) 30th May 2001, Accepted 6th September 2001
First published as an Advance Article on the web 5th October 2001
Three N-acyl-NЈ,NЈ-diethylthioureas, RCONHCSNEt2, R = iPr, iBu, tBu, have been prepared and characterised. The
standard (po = 0.1 MPa) molar enthalpies of combustion in oxygen of the three crystalline compounds, at T = 298.15
K, have been measured by rotating bomb-combustion calorimetry, and the standard molar enthalpies of sublimation
of the compounds by microcalorimetry. These values were used to derive the standard molar enthalpies of formation
of the compounds in their crystalline and gaseous phases, respectively. The derived standard molar enthalpies of
formation in the gaseous state are discussed comparatively. Acid constants and some complex stabilities have been
measured pH-potentiometrically in dioxane–water mixture. The crystal structure of N,N-diethyl-NЈ-isovaleroyl-
thiourea is presented and shows a delocalization of the π electrons of the C᎐S group over the carbon–amine nitrogen
᎐
bond, CS–NEt2, stabilising the molecule, in accordance with the thermochemical results.
Introduction
Acylchalcogenoureas are known for their use as metal complex-
ing agents and they have been used for the separation of metals
by selective sorption in chromatographic columns.1–3 Their
physico-chemical characteristics are of great interest since the
Fig. 1 Molecular formulae of acylated thioureas.
thiourea, tBuCONHCSNEt2, HPVET, whose formulae are
represented in Fig. 1.
Knowledge of the standard molar enthalpies of formation of
knowledge of them allows one to choose the best working
conditions in different fields. The ligand properties of these
compounds depend greatly on the number and nature of the
substituents at the terminal nitrogen atoms as well as on the
these acylated thioureas, both in the condensed and in the
acyl groups as, for example, acid constants of the molecules
gaseous phases, is necessary not only for the thermochemical
study of their metal complexes, but also to fill a gap in the
available thermochemical data for this class of compounds,
thus providing some reliable data for the estimation of thermo-
in solution and stability constants of the related metal
complexes.1,4–7
The literature reports some solution calorimetric studies
concerned with the acid strengths of ligands and the stability
chemical data for other compounds with similar structure. This
constants of the corresponding metal complexes8,9 for some of
is of particular importance in the study of the metal–ligand
this class of compounds, although there is only one paper10
binding in these systems, which is very important in metal
reporting values for the thermochemical properties of two N,N-
extraction processes.
dialkyl-NЈ-benzoylureas, PhCONHCONR2 (R = Et, iBu), both
in condensed and gaseous phases. In this paper it is shown that
the difference of the standard molar enthalpies of formation of
the two compounds, which differ only by the alkyl substituents
on the nitrogen atom, is the same as can be estimated applying
the group energy contributions.
The values of the standard (po = 0.1 MPa) molar enthalpies
of formation, in the crystalline state, of these three compounds
were derived from the respective enthalpies of combustion
in oxygen, at T = 298.15 K, measured by rotating bomb
calorimetry. The enthalpies of sublimation, at T = 298.15 K,
were measured by Calvet microcalorimetry.
As part of a broad thermochemical study on acylchalco-
genourea derivatives involving unsaturated bidentate 1-acyl-
3,3-dialkylthiourea ligands, this paper reports synthesis,
characterization and thermochemical work on the following
three compounds: N,N-diethyl-NЈ-isobutanoylthiourea, iPr-
CONHCSNEt2, HIPET, N,N-diethyl-NЈ-isovaleroylthiourea,
iBuCONHCSNEt2, HIBET, and N,N-diethyl-NЈ-pivaloyl-
Experimental
Materials
The syntheses of the three RCONHCSNRЈ2 compounds, which
have not been described before, were performed according
2174
J. Chem. Soc., Perkin Trans. 2, 2001, 2174–2178
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104709b

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Table 1 Elemental analytical data and values for melting points
Found (%)
Calculated (%)
Mp/ЊC
Compound
C
H
N
S
C
H
N
S
HIPET
HIBET
HPVET
53.70
55.74
55.42
9.24
9.42
9.22
13.87
13.01
12.55
16.15
14.70
15.00
53.43
55.52
55.52
8.97
9.32
9.32
13.85
12.95
12.95
15.85
14.82
14.82
65–66
86–88
88–90
i
i
t
Table 2 1H and 13C NMR spectra of PrCONHCSNEt2, HIPET, BuCONHCSNEt2, HIBET, and BuCONHCSNEt2, HPVET (Varian Gemini
2000, CDCl3, internal TMS, T = 293 K)
HIPET
HIBET
HPVET
1H NMR (200 MHz): δ 1.11 (d, J = 6.8 Hz, 6H), 1.23 (m, 6H), 2.50–2.60 (m, 1H), 3.43 (m, 2H), 3.91 (m, 2H), 8.70 (s, 1H); 13C NMR
(50 MHz): δ 11.9, 13.5, 19.3, 36.0, 48.1, 174.4, 180.1
1H NMR (200 MHz): δ 0.94 (d, J = 6.6 Hz, 6H), 1.25 (m, 6H), 2.14 (m, 1H), 2.18 (m, 2H), 3.50 (m, 2H), 3.91 (m, 2H), 8.61 (s, 1H);
13C NMR (50 MHz): δ 12.1, 13.5, 22.8, 26.5, 46.3, 48.1, 170.1, 179.7
1H NMR (200 MHz): δ 1.23 (s, 9H), 1.28 (m, 6H), 3.47 (m, 2H), 3.92 (m, 2H), 7.26 (s, 1H); 13C NMR (50 MHz): δ 12.0, 13.6, 27.6,
40.2, 48.3, 175.6, 180.4
to the general procedure described by Douglass et al.11 for
other acylthioureas. N,N-Diethyl-NЈ-isobutanoylthiourea,
conditions. The calibration results were corrected to give the
energy equivalent ε(calor) corresponding to the average mass of
4059.0 g water added to the calorimeter. From eight calibration
experiments, ε(calor) = (20690.2 6.6) J KϪ1 for the study of
the compounds iPrCONHCSNEt2 and iBuCONHCSNEt2,
iPrCONHCSNEt2,
iBuCONHCSNEt2
N,N-diethyl-NЈ-isovaleroylthiourea,
N,N-diethyl-NЈ-pivaloylthiourea,
and
tBuCONHCSNEt2 were prepared by adding a solution of
anhydrous acetone containing potassium thiocyanate to the
appropriate acyl chloride, RCOCl (2-methylpropanoyl chloride,
isovaleroyl chloride or pivaloyl chloride, respectively). After
stirring and warming the mixtures, a cold solution of
dialkylamine, NHRЈ2 (diethylamine) in acetone was added. The
final mixtures were poured in a bath at 0 ЊC and the white
crystals of the acylated thioureas were formed, then filtered
and purified by recrystallization from ethanol–water. The
compounds are stable at room temperature and they have
been characterised by their melting points (Table 1), elemental
analysis, IR spectroscopy and potentiometric pKa measure-
ments. The microanalytical results (obtained in Microanalytical
Services of the University of Manchester) are given in Table 1.
The 1H NMR and 13C NMR spectra confirm the expected
structures of the compounds, showing also a split signal of
the –CH2– groups in the –CS–(NEt2)– moiety based on the
hindered rotation around the (CS)–N bond (see Table 2).
or ε(calor) = (20675.2
3.0) J KϪ1 for the study of
tBuCONHCSNEt2, the uncertainty quoted being the standard
deviation of the mean. The electrical energy for ignition was
determined from the change in potential difference across a
capacitor when discharged through the platinum ignition wire.
For the cotton-thread fuse, empirical formula CH1.686O0.843, the
massic energy of combustion is ∆cuo = Ϫ16250 J gϪ1 16
,
a value
which has been confirmed in our laboratory. The corrections for
nitric acid formation were based on Ϫ59.7 kJ molϪ1 17
, for the
molar energy of formation of 0.1 mol dmϪ3 HNO3 (aq) from
N2, O2 and H2O (l).
The procedure described by Waddington et al.18 for com-
bustion calorimetry of organosulfur compounds was followed.
The RCONHCSNRЈ2 compounds were burned in pellet form
in oxygen at p = 3.04 MPa with 10 cm3 of water placed in the
bomb. After each combustion experiment the amount of nitric
acid was determined using Devarda’s alloy; the amount of
nitrous acid was found to be thermally insignificant. At T =
298.15 K, (∂u/∂p)T for these solids was assumed to be Ϫ0.2 J gϪ1
MPaϪ1, a typical value for organic solids; the massic energy of
combustion Ϫ∆cuo was calculated by the procedure given by
Hubbard et al.19
Crystal structure determination by X-ray analysis has been
performed for N,N-diethyl-NЈ-isovaleroylthiourea, for which
the crystals were grown from a solution of ethanol–water (1 : 1).
Combustion calorimetry
The combustion experiments for the three compounds were
performed using a rotating bomb calorimeter originally from
the National Physical Laboratory, Teddington, UK.12 The
apparatus, after having been installed in the University of
Manchester, UK,13 was transferred to Porto University, where
some changes in the auxiliary equipment were made as
previously described.14 The rotating bomb calorimeter was used
with a platinum-lined bomb of internal volume 0.337 dm3.
Water was added to the calorimeter from a weighted perspex
vessel and for each experiment a correction to the energy
equivalent was made for the deviation in the mass of water from
4059.0 g. The ignition temperature for each experiment was
chosen so that the final temperature was close to T = 298.15 K.
Rotation of the bomb was started when the temperature rise in
the main period reached about 60% of its final value and was
then continued throughout the rest of the experiment; so,
according to the procedure of Good et al.,15 the frictional work
of bomb rotation was automatically included in the corrections
for the heat exchange with the surrounding thermostat and the
work of stirring.
Enthalpies of sublimation
The enthalpies of sublimation were measured in Porto, using
the vacuum sublimation drop microcalorimetric method:20
samples, about 5 mg of each crystalline compound, contained
in a thin glass capillary tube sealed at one end, were dropped,
at room temperature, into the hot reaction vessel, in a high
temperature Calvet microcalorimeter held at T = 363 K, in the
case of the compounds HIPET and HIBET, and T = 366 K, in
the case of the compound HPVET. The observed enthalpies of
sublimation were corrected to T = 298.15 K, using the value of
T
∆
298.15 KHmo estimated by the Benson’s Group Method,21 using
values from Stull et al.22 The microcalorimeter was calibrated
in situ for these measurements using the reported enthalpy of
sublimation of naphthalene.22 The relative atomic masses used
throughout this paper were those recommended by the IUPAC
Commission in 1995.23
Potentiometric measurements
Benzoic acid (Bureau of Analysed Samples, Thermochemical
Standard, BCS-CRM-190r) was used for the calibration of the
bomb. The specific energy of combustion of this sample of
All measurements (alkalimetric titrations) were carried out
under an argon atmosphere in 1,4-dioxane–water mixture (75%
v/v), T = 298 K, ionic strength = 0.1 mol dmϪ3 [(CH3)4N]NO3,
benzoic acid is Ϫ(26432.3
3.8) J gϪ1, under certificate
(pKH O = 16.74),24 using automated titration equipment (PHM
2
J. Chem. Soc., Perkin Trans. 2, 2001, 2174–2178
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Table 3 Typical combustion experimental results, at T = 298.15 K
Table 4 Individual values of the massic energy of combustion, Ϫ∆cuo,
at T = 298.15 K (po = 0.1 MPa). The mean values are represented by
〈∆cuo〉
HIPET
HIBET
HPVET
HIPET
HIBET
HPVET
m(cpd)/g
0.79291
0.00493
54.16
54.33
Ϫ0.3
23.7293
24.9729
1.20649
25025.2
62.09
1.18
22.28
0.90449
0.00353
54.35
54.79
2.4
23.5772
25.0204
1.41031
29269.2
73.73
1.18
25.32
0.53903
0.00461
33.07
m(fuse)/g
εi/J KϪ1
Ϫ∆cuo/J gϪ1
32 185.8
32 181.2
32 193.2
32 208.8
32 194.4
32 187.0
εf/J KϪ1
32.89
31 356.7
31 353.8
31 354.4
31 344.7
31 371.7
31 346.2
32 160.6
32 164.6
32 147.1
32 153.5
32 147.4
32 144.8
32 146.4
∆m(H2O)/g
(T i/K) Ϫ273.15
(T f/K) Ϫ273.15
∆T ad/K
Ϫ∆U(IBP)/J
Ϫ∆U(HNO3)/J
∆U(ign)/J
Ϫ11.7
24.1886
25.0659
0.84563
17469.0
48.48
1.18
10.10
74.87
32160.6
Ϫ〈∆cuo〉/J gϪ1 a
32 191.7 4.0
∆UΣ/J
31 354.6 3.9
a Mean value and standard deviation of the mean.
32 152.1 2.9
Ϫm∆cuo(fuse)/J
Ϫ∆cuo(cpd)/J gϪ1
80.06
31353.8
57.33
32187.0
well as the standard molar enthalpies of formation, ∆fHmo (cr),
for the three compounds, in the crystalline state at T = 298.15
K. In accordance with the normal thermochemical practice,
the uncertainties assigned to the standard molar enthalpies
of combustion and formation are twice the overall standard
deviation of the mean and include the uncertainties in calibra-
tion28 and in the auxiliary quantities used. To derive ∆fHmo (cr)
from ∆cHmo (cr) the standard molar enthalpies of formation of
H2O (l), of CO2 (g) and H2SO4 in 115 H2O (l), at T = 298.15 K,
84, ABU 80, Radiometer Copenhagen) driven online by an
IBM compatible. As a cell a combination of G202B–K701
electrodes (Radiometer Copenhagen) was used. The cell was
calibrated in terms of Ϫlg [Hϩ] in dioxane–water mixture by
titration of HNO3 with KOH. Carbonate-free KOH solution
was prepared by reaction of KCl with carbonate-free Ag2O and
was checked by pH titration using recrystallised amidosulfonic
acid as a standard substance. The MINIQUAD 75A/B
program25 processed the potentiometric data. The calculated
results were evaluated by means of the given statistical criteria
sigma, σ2, and R-factor.
respectively, Ϫ(285.830 0.042) kJ molϪ1 29
kJ mol
Ϫ1 29 and Ϫ(887.81 0.01) kJ molϪ1 17 were used.
, Ϫ(393.51 0.13)
,
Crystal structure determination of N,N-diethyl-NЈ-isovaleroyl-
thiourea
Sublimation
Measurements of the enthalpies of sublimation, ∆cgrHmo, are
given in Table 6 with uncertainties of twice the standard
deviation of the mean. The derived enthalpies of formation, in
both crystalline and gaseous phases, at T = 298.15 K, for the
compounds studied are also summarised in Table 6.
A crystal with approximate dimensions 0.8 × 0.6 × 0.3 mm3
was used for the X-ray diffraction studies. Crystal data:
C10H20N2OS, M = 216.34, monoclinic, a = 12.402 (8), b = 9.568
(4), c = 13.150 (6) Å and β = 124.59 (6)Њ; U = 1284.6 (11) Å3, T =
293 K, space group P21/c, Z = 4, 12355 reflections measured,
2924 unique and 2419 were observed (I > 2σI) and used in all
calculations. The final wR(F 2) was 0.14 (all data). The calcu-
lated density is Dx = 1.12 g cmϪ3. CCDC reference number
164615. See http://www.rsc.org/suppdata/p2/b1/b104709b/ for
crystallographic files in .cif or other electronic format.
The structure was solved using SHELXS9726 and refined
with SHELXL97.27
pKa and lg ꢀn measurements
The three compounds behave as weak acids in solution. Their
acid constants (see Table 7) are within the expected range of
this class of compounds (e.g., 1,1-diethyl-3-benzoylthiourea,
pKa = 10.87),1 but they are less acidic because of the lack of the
aromatic substituent and its delocalizing effect on the anionic
charge. The small trend in pKa values 11.93–12.06–12.12
(sequence HIBET–HIPET–HPVET) reflects the increasing
shielding influence of the acyl part of the molecules against
solvation of the polar molecule anion by the polar water and
the polar parts of the cosolvent dioxane which would in turn
also stabilise the anionic charge and thereby heighten the
acidity.
The same is true for the stability constants of nickel
complexes (1,1-diethyl-3-benzoylthiourea, lg β2 = 15.32).1 The
stability constants of the copper complexes were found to be
too high to be measured pH-potentiometrically (lg β2 > 22)
which is typical of the whole class of acyl-thioureas. Zinc
complexes of the ligands investigated undergo hydrolysis dur-
ing the titration experiments, showing the hydroxide ions to be
stronger ligands than the organic anions. On the other hand,
benzoyl substituted thioureas do form zinc complexes with
stabilities similar to those of nickel. The stability constants (lg
β2) of the cadmium and lead complexes of HPVET that we
measured as a probe are higher than those of 1,1-diethyl-3-
benzoylthiourea by about half a logarithmic unit.
Results and discussion
Combustion calorimetry
Detailed results for a typical combustion experiment on each
compound are given in Table 3; ∆m(H2O) is the deviation of the
mass of water added to the calorimeter from the mass assigned
for ε(calor) (4059.0 g); ∆UΣ is the correction to the standard
state; the remaining terms are as previously described.19 The
internal energy for the isothermal bomb process, ∆U(IBP) was
calculated according to eqn. (1),where ∆T ad is the calorimeter
∆U(IBP) = Ϫ{ε(calor) ϩ cp(H2O, l) ∆m(H2O)} ∆T ad ϩ
εi(T i Ϫ 298.15) ϩ εf(298.15 Ϫ T i Ϫ ∆T ad) ϩ ∆U(ign) (1)
temperature change corrected for heat exchange and the work
of stirring, and the remaining terms are as previously
described.19 The individual values of Ϫ∆cuo together with the
mean and its standard deviation are given in Table 4. Table 5
lists the derived standard molar energies and enthalpies of
combustion, ∆cUmo (cr) and ∆cHmo (cr) relate to the reaction (2) as
Crystallography
The three-dimensional structure of compound HIBET was
determined and is presented in Fig. 2. Refinement of positional
and anisotropic thermal parameters for all non-hydrogen atoms
converged to R[F 2 > 2σ(F 2)] = 0.06 and wR = 0.14. All hydrogen
CaHbN2SO (cr) ϩ
(a ϩ b/4 ϩ 1) O2 (g) ϩ (116 Ϫ b/2) H2O (l) =
a CO2 (g) ϩ N2 (g) ϩ H2SO4ؒ115 H2O (l) (2)
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J. Chem. Soc., Perkin Trans. 2, 2001, 2174–2178

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Table 5 Derived standard molar energies of combustion, standard molar enthalpies of combustion and standard molar enthalpies of formation for
iPrCONHCSNEt2, HIPET, iBuCONHCSNEt2, HIBET, and tBuCONHCSNEt2, HPVET, at T = 298.15 K
Ϫ∆cUmo (cr)/kJ molϪ1
Ϫ∆cHmo (cr)/kJ molϪ1
∆fHmo (cr)/kJ molϪ1
HIPET
HIBET
HPVET
6343.7 2.7
6964.6 3.0
6956.0 1.9
6354.8 2.7
6977.0 3.0
6968.4 1.9
Ϫ361.2 3.0
Ϫ418.4 3.3
Ϫ427.0 2.3
Table 6 Derived standard molar enthalpies of formation and sublim-
Table 8 Selected bond lengths (Å) and angles (Њ) for N,N-diethyl-NЈ-
i
i
ation of PrCONHCSNEt2, HIPET, BuCONHCSNEt2, HIBET, and
isovaleroylthiourea
tBuCONHCSNEt2, HPVET, at T = 298.15 K
Bond
Length
Bond
Angle
∆fHmo (cr)/kJ molϪ1
∆cgrHmo/kJ molϪ1
∆fHmo (g)/kJ molϪ1
S1–C1
C1–N1
N1–C11Ј
C11Ј–O11Ј
C1–N2
C11Ј–C1Ј
N2–C2
1.672(2)
1.433(2)
1.359(2)
1.228(2)
1.323(2)
1.510(3)
1.480(2)
1.476(3)
C1–N1–C11Ј
N1–C11Ј–O11Ј
S1–C1–N1
N2–C1–S1
N2–C1–N1
C2–N2–C4
C2–N2–C1
C1–N2–C4
121.5(1)
121.6(2)
118.3(1)
126.0(2)
115.7(2)
115.6(2)
124.5(2)
119.9(2)
HIPET
HIBET
HPVET
Ϫ361.2 3.0
Ϫ418.4 3.3
Ϫ427.0 2.3
120.8 2.5
121.5 3.2
114.9 2.7
Ϫ240.4 3.9
Ϫ296.9 4.6
Ϫ312.1 3.6
Table 7 pKa and lg βn values, at T = 298.15 K; standard deviations
0.04 (pKa), 0.05 (lg βn)
N2–C4
pKa
Ni
Cd
Pb
by van der Waals forces and an intermolecular N–H ؒ ؒ ؒ O
hydrogen bond (N1H1 ؒ ؒ ؒ O11Јi is 2.06 Å)
A perspective view of the molecule showing the atomic
numbering scheme was obtained using XPMA32 and ORTEP33
and is given in Fig. 2.
The three compounds RCONHCSNEt2, with R = iPr, iBu,
tBu studied in this work differ only in the alkyl substituent of
the acyl group, which enables us to compare the effect of the
substituent R on the molecule, independently of the nature of
the group RЈ = Et.
HIPET
HIBET
HPVET
12.06
11.93
12.12
—
Not measured
Not measured
Not measured
Not measured
lg β2 = 15.47
—
lg β2 = 14.51
—
lg β2 = 16.48
lg β1 = 7.31
lg β2 = 14.78
lg β1 = 8.14
lg β2 = 15.72
The crystal structure of N,N-diethyl-NЈ-isovaleroylthiourea
reveals a close agreement, in terms of bond lengths and angles,
with the structures reported for other acylated thioureas,
namely 1,1-diethyl-3-(4-methylbenzoyl)thiourea30 and 1,1-
diethyl-3-(2-chlorobenzoyl)thiourea.31 As expected, C11Ј of the
acyl group is planar. However, while acylureas have planar
C(O)NC(O)N groups, this acylthiourea has a twisted conform-
ation, the angle defined by the planar C(O)N and NC(S)N
being 69.58(0.09)Њ.
The increment in ∆fHmo (g) is sensitive to the nature of the
group R, although it cannot be predicted directly by a bond
energy scheme introducing the energetic parameter from an
identical substitution on an amine derivative. Considering the
values available in the literature for ∆fHmo (g) of 2-methyl-
propanamide, iPrCONH2, and 2,2-dimethylpropanamide,
tBuCONH2, respectively Ϫ(282.6 0.9) kJ molϪ1 34 and Ϫ(313.1
Fig. 2 The molecular structure of N,N-diethyl-NЈ-isovaleroylthiourea
showing the atom-labelling scheme. H atoms are depicted as spheres
of arbitrary radii. Displacement ellipsoids are shown at the 50%
probability level.
1.4) kJ mol ,
Ϫ1 34 the derived increment for changing iPr to tBu
atoms were found in the difference Fourier map and sub-
sequently subjected to isotropic refinement. Tables containing
the final fractional coordinates, temperature parameters, bond
distances and bond angles have been deposited with the
Cambridge Crystallographic Data Centre, Cambridge, England
(deposition number 164615).
is Ϫ(30.5 1.7) kJ molϪ1. For the corresponding acylthiourea
derivatives, iPrCONHCSNEt2 and tBuCONHCSNEt2, the
increment is Ϫ(71.7
5.3) kJ molϪ1, which suggests an
additional stabilisation on the molecules under study. It is
apparent that the energetic effect of the stabilisation between
the π electrons of the thiocarbonyl bond is very significant,
The nitrogen (N1) to which the acyl group is attached is
trigonal, exhibiting a C1–N1–C11Ј angle of 121.5Њ. The N1–C
bond lengths are different (1.433 and 1.359 Å), the acyl-
substituted N1–C11Ј distance being shorter. The N2–C bond
lengths are also of unequal size (1.480, 1.476 and 1.323 Å),
where the thiocarbonyl-substituted C1–N2 bond is shorter.
These bonds are essentially the same as in compounds
1,1-diethyl-3-(4-methylbenzoyl)thiourea30 and 1,1-diethyl-3-(2-
chlorobenzoyl)thiourea.31
The O11Ј–C11Ј–N1–C1–S1 group is not planar, the dihedral
angle between planes C1Ј–C11Ј–O11Ј–N1 and N1–C1–S1–N2
being 69.6 (1)Њ. The angle between the planes N1–C1–S1–N2
and C1–N2–C2–C4 is 4.5(1)Њ.
particularly due to the fact of the C᎐S group being adjacent to
two nitrogen atoms. From the crystal structure of N,N-diethyl-
NЈ-isovaleroylthiourea, we found that the bond lengths between
᎐
the nitrogen atoms and the C᎐S group are different, the N2–CS
᎐
being the shortest and with a value close to a double bond or to
the typical value for the group CO–NH in a peptide. This result
is consistent with a delocalization of the C᎐S bond over the
᎐
S᎐C–N group.
᎐
The experimental values of ∆fHmo (g) available for this class
of 1-acyl-3,3-diethylthiourea are insufficient to be a basis for
a bond-energy scheme to estimate the values of other ∆fHmo
of the other acyldialkylthioureas, particularly due to the lack
of literature data on other similar compounds. This is also
the situation for the thermochemistry of acyldialkylureas,
for which only two compounds have been published.10 In a
Selected bond lengths and angles are listed in Table 8.
The molecules in the crystal structure are held together
J. Chem. Soc., Perkin Trans. 2, 2001, 2174–2178
2177

View Article Online
9 F. Dietze, J. Lerchner, S. Schmidt, L. Beyer and R. Köhler, Z. Anorg.
Allg. Chem., 1991, 600, 37.
10 M. A. V. Ribeiro da Silva, M. D. M. C. Ribeiro da Silva, L. C. M.
Silva, F. Dietze and E. Hoyer, J. Chem. Thermodyn., 2000, 32,
1113.
11 I. B. Douglass and F. B. Dains, J. Am. Chem. Soc., 1934, 56, 719.
12 J. D. Cox, H. A. Gundry and A. Head, Trans. Faraday Soc., 1964,
60, 653.
13 M. D. M. C. Ribeiro da Silva, P. Souza and G. Pilcher, J. Chem.
Thermodyn., 1989, 21, 173.
14 M. A. V. Ribeiro da Silva, J. M. Gonçalves and G. Pilcher, J. Chem.
Thermodyn., 1997, 29, 253.
15 W. D. Good, W. D. Scott and G. Waddington, J. Phys. Chem., 1960,
60, 1080.
16 J. Coops, R. S. Jessup and K. Van Nes, in Experimental
Thermochemistry, ed. F. D. Rossini, Interscience, New York, 1956,
Vol. 1, Ch. 3.
17 The NBS Tables of Chemical Thermodynamics Properties, J. Phys.
Chem. Ref. Data, 1982,; II, Supplement no. 2.
previous work, we reported the enthalpies of formation, in the
gaseous state, for the compounds PhCONHCON(iBu)2 and
PhCONHCSN(iBu)2, respectively as Ϫ(445.3 7.5) kJ molϪ1 10
and Ϫ(164.8
the increment for changing C᎐O to C᎐S, for the pair
4.9) kJ molϪ1 35
, which enabled us to derive
᎐
᎐
PhCONHCON(iBu)2–PhCONHCSN(iBu)2, as (280.5 8.9) kJ
molϪ1. This increment is consistent, although larger, with those
of identical changes in the pairs acetamide36–thioacetamide37
(251.0 1.4) kJ molϪ1, benzamide38–thiobenzamide13 (248.7
3.0) kJ molϪ1 and urea39–thiourea40 (258.4 2.0) kJ molϪ1. We
understand the additional 280–258 = ca. 22 kJ molϪ1 stabilis-
ation of the acylurea over the acylthiourea arising from the
destabilisation of the acylthiourea. While the acylurea has
a totally planar –C(O)–NH–C(O)–N< skeleton, our crystallo-
graphic study shows that the corresponding acylthiourea
–C(O)–NH–C(S)–N< skeleton is significantly twisted. This
twisting results in poorer π overlap and so in less resonance
stabilisation for the acylthiourea. At the current time there are
no thermochemical data for planar acylthioureas and corre-
sponding acylureas and so this interpretation remains
somewhat speculative.
18 G. Waddington, S. Sunner and W. N. Hubbard, in Experimental
Thermochemistry, ed. F. D. Rossini, Interscience, New York, 1956,
Vol. 1, Ch.7.
19 W. N. Hubbard, D. W. Scott and G. Waddington, in Experimental
Thermochemistry, ed. F. D. Rossini, Interscience, New York, 1956,
Vol. 1, Ch.5.
20 F. A. Adedeji, D. L. S. Brown, J. A. Connor, M. Leung, M. I.
Paz-Andrade and H. A. Skinner, J. Organomet. Chem., 1975, 97,
221.
Acknowledgements
21 S. W. Benson, Thermochemical Kinetics Methods for the Estimation
of Thermochemical Data Rate Parameters, 2nd edition, Wiley, New
York, 1976.
22 D. R. Stull, E. F. Westrum and G. C. Sinke, The Chemical
Thermodynamics of Organic Compounds, Wiley, New York, 1969.
23 J. Phys. Chem. Ref. Data, 1997, 26, 1239.
24 E. Uhlig and D. Linke, J. Prakt. Chem., 1972, 314, 570.
25 P. Gans, A. Sabatini and A. Vacca, Inorg. Chim. Acta, 1976, 18,
237.
26 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
27 G. M. Sheldrick and T. R. Schneider, Methods Enzymol., 1997, 277,
319.
This work is part of a joint research project between the
University of Porto, Portugal, and the University of Leipzig,
F. R. of Germany, under the auspices of the INIDA program,
between ICCTI (Instituto de Cooperação Ciêntífica e Tecno-
logica Internacional), Lisboa, Portugal, and DAAD (Deutscher
Akademischer Austauschdienst), Bonn, F. R. of Germany, to
whom we express our thanks.
Thanks are also due to FCT (Fundação para a Ciência e
Tecnologia), for financial support to PRAXIS XXI research
projects (project PRAXIS XXI/2/2.1/QUI/17/94 and project
PRAXIS XXI/PCEX/QUI/62/96). One of us (L.C.M.S.) thanks
FCT for the award of a scholarship (PRAXIS XXI/BD/5591/
95). One of us (J.F.L.) thanks the U.S. National Institute of
Standards and Technology for partial support of his thermo-
chemical studies.
28 F. D. Rossini, in Experimental Thermochemistry, ed. F. D. Rossini,
Interscience, New York, 1956, Vol. 1, Ch. 14.
29 J. D. Cox, D. D. Wagman and V. A. Medvedev, CODATA Key Values
for Thermodynamics, Hemisphere, New York, 1989.
30 A. D. Morales, S. Garcia-Granda, V. R. Esteva, A. P. Stevens and
G. A. A. Crespo, Acta Crystallogr., Sect. C, 1997, 53, IUC9700019.
31 R. A. Bailey and K. L. Rothaupt, Inorg. Chim. Acta, 1988, 147, 233.
32 L. Zsolnai, XPMA. Molecular Geometry Program for Silicon
Graphics Computer, University of Heidelberg, Germany, 1996.
33 L. Zsolnai and H. Pritzkow, Program for Silicon Graphics
Computer, University of Heidelberg, Germany, 1995.
34 J. L. Abboud, P. Jimenez, M. V. Roux, C. Turrion and C. Lopez-
Mardomingo, J. Chem. Thermodyn., 1989, 21, 859.
35 M. A. V. Ribeiro da Silva, M. L. C. C. H. Ferrão, R. D. M. A.
Ribeiro, F. Dietze and E. Hoyer, Book of Abstracts of 15th IUPAC
Conference on Chemical Thermodynamics, Porto, Portugal, 1998,
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36 D. S. Barnes and G. Pilcher, J. Chem. Thermodyn., 1975, 7, 377.
37 S. Inagaki, S. Murata and M. Sakiyama, Bull. Chem. Soc. Jpn., 1982,
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38 L. A. T. Gomez and R. Sabbah, Thermochim. Acta, 1982, 58, 311.
39 G. Ya. Kabo, E. A. Miroshnichenko, M. L. Frenkel, A. A. Kozyro,
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