Thermodynamics of donor-acceptor interaction of tetraphenylporphyrinatozinc with amides and dimethyl sulfoxide

Article abstract of DOI:10.1023/A:1026373406523

Thermodynamic parameters of complex formation of alkylamides and dimethyl sulfoxide with tetraphenylporphyrinatozinc in benzene and carbon tetrachloride were determined by calorimetric titration. The composition of the complexes was also determined. Corre

Full text of DOI:10.1023/A:1026373406523

Russian Journal of General Chemistry, Vol. 73, No. 6, 2003, pp. 968 972. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 6, 2003,
pp. 1022 1027.
Original Russian Text Copyright
2003 by Lebedeva, V’yugin, Mikhailovskii.
Thermodynamics of Donor Acceptor Interaction
of Tetraphenylporphyrinatozinc with Amides
and Dimethyl Sulfoxide
N. Sh. Lebedeva, A. I. V’yugin, and K. V. Mikhailovskii
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received June 28, 2001
Abstract Thermodynamic parameters of complex formation of alkylamides and dimethyl sulfoxide with
tetraphenylporphyrinatozinc in benzene and carbon tetrachloride were determined by calorimetric titration.
The composition of the complexes was also determined. Correlations were found between thermodynamic
stability parameters of the complexes and physical parameters of the ligands. The effect of the ligand and
solvent nature on the thermodynamics of complex formation was analyzed.
Specific structure of the amide group and its acid
base properties largely determine structural diversity
of biologically important protein derivatives [1].
Physical properties of amide groups have been studied
to a sufficient extent, especially in the recent years
(for reviews, see [2, 3]). Stereochemistry of amides
was described in detail. As noted in [2], the structures
of amide group in the gas phase and in solution have
some specific features which should be taken into
account while interpreting the results of studies of
compounds possessing such groups. First, the
C(O) N bond in amides has a partially double
character as a result of displacement of the unshared
electron pair of the nitrogen atom toward electrons
of the C O bond. Second, amides in solution can
exist as cis (A) and trans isomers (B) [4].
are protonated at the oxygen atom. An analogous
conclusion was drawn [3, 7] while studying amide
hydrochlorides. However, according to the data of [8],
protonation of the nitrogen atom predominates in
dilute aqueous solutions of acids.
Only a few published data are available on reac-
tions of amides with metals. Saito et al. [7] described
the IR spectra of tertiary amine complexes with various
metals, including alkali metals. The authors presumed
that the observed reduction of the
frequency
relative to free amides indicates coorCdinOation at the
oxygen atom. According to [3], Li(I) [9], Zn(II) [7],
Ni(II), Cr(III), and Co(II) salts coordinate dimethyl-
acetamide, formamide, and N-methylacetamide at the
oxygen atom.
The results of spectral studies showed that most of
the above complexes undergo decomposition by the
action of water. However, some complexes, e.g.,
R2
R3
R3
R2
R1
O
R1
O
C N
C N
(dimethylformamido)(pentaammine)cobalt(III),
are
A
B
sufficiently stable, and hydrolysis of the amide ligand
occurs by the action of water in alkaline medium.
This reaction is essentially similar to enzymatic
hydrolysis of peptides [1]. The lack of information on
thermodynamic parameters characterizing reactions of
amides with metals or their salts leaves no chance
to understand the mechanism of a number of bio-
chemical and chemical processes. Despite an obvious
significance, there are almost no data on the thermo-
dynamics of amide reactions with inner salts, in
particular with porphyrin metal complexes. Owing to
the low solubility of the latter and their chromophoric
properties, they are studied mainly by spectral
methods (NMR, IR, and electronic absorption spectros-
In contrast to physical properties of amides, their
acid base properties have been studied to a consider-
ably lesser extent. Despite the wide use of amides in
practice as plasticizers for paper, artificial leather,
and poly(vinyl chloride), as raw materials in the
synthesis of polymers, as extractants for some radio-
active materials, and as electron-donor solvents, up
to now the problem concerning the basic center in
amides has not been solved unambiguously [5].
Analysis of the dependence of ionization potentials on
the oxygen and nitrogen atoms upon proton affinity
of the base showed [6] that amides in the gas phase
1070-3632/03/7306-0968$25.00 2003 MAIK Nauka/Interperiodica

THERMODYNAMICS OF DONOR ACCEPTOR INTERACTION
969
Table 1. Thermodynamic parameters of complex formation between (tetraphenylporphyrinato)zinc(II) and ligands I VI
in benzene and carbon tetrachloride at 298.15 K
C6H6
CCl4
Ligand
no.
H0,
S0
H0,
kJ mol
S0,
Ks
Ks
1
1
1
1
kJ mol
J mol 1 K
J mol 1 K
a
I
II
III
370 21
838 121
211 14
3 7
759 205
526 103
1275 247
685 64
1151 366
1.58 0.42
2.20 0.34
44 12
49 17
44 16
8 6
36 9
49 12
71 24
37 6
2 5
1978 478
1638 318
1232 59
2650 345
890 77
4810 308
1200 264
2477 265
1310 240
1.28 0.36
5.38 0.54
1.65 0.07
9.42 0.23
1.32 0.07
15.15 0.61
2.56 0.58
9.23 0.44
2.14 0.09
59 14
43 14
54 16
34 9
52 18
20 6
50 19
34 7
52 13
26.41 0.56
0.197 0.074
27.27 0.32
0.884 0.040
39.01 0.78
5.04 0.12
IV
V
VI
16.95 0.78
a
Amide I is insoluble in carbon tetrachloride.
copy). However, the application of these methods to
systems containing amides is often impossible, for
the measured parameters are determined not only by
the degree of amide metal complex interaction but
also by experimental conditions, specifically by
amide concentration and formation of intermolecular
associates.
dimethyl sulfoxide (VI) (ligand L) with tetraphenyl-
porphyrinatozinc(II) (TPPZn) in benzene and carbon
tetrachloride by calorimetric titration.
H3C
H3C
N
R
O
H3C
O
CH3
CH3
H
R
H3C
H3C
H3C
H3C
N P=O
N
C N
C N
Amides are known [2] to form three types of
associates: hydrogen-bonded dimers C and polymers
D and dipole dipole dimers E [2]. In addition, amides
are capable of forming associates with aromatic sol-
vent molecules. In the course of NMR studies, the
existence and rotation period of stable and metastable
isomers A and B must be taken into account. All these
factors strongly complicate the spectra of amides
and make their interpretation difficult.
I, II
III, IV
V
I, III, R = H; II, R = CH3; IV, R = C2H5.
All the above ligands with TPPZn form thermo-
dynamically stable 1:1 or 1:2 complexes (Table 1).
The complex formation is accompanied by heat evolu-
tion. The exothermic effect decreases in going from
tertiary to secondary and then to primary amides
(Table 1). Assuming that the coordination involves
the amide nitrogen atom, the greater exothermic effect
observed for ligands III V may be explained by the
inductive effect of the alkyl groups on the nitrogen,
which increases the basicity of the nitrogen atom. In
this case, steric hindrance and strain due to compres-
sion of substituents increase in going from secondary
to tertiary amides, which should lead to reduced
stability constants of the complexes TPPZn nL (L =
III V), as compared to TPPZn L (L = I, II). No such
reduction was observed for the systems under study
(Table 1). Therefore, it was reasonable to presume
that amides are coordinated to TPPZn through the
oxygen atom and that the lower exothermic effect in
the reactions with primary and secondary amides is
explained by intermolecular association like C and D.
R2
R1
R1
H
C N
C N
R2
O
O
H
H
O
R1
O
H
C N
N C
R2
R2
R1
C
D
R1
O
N+
R3
R2
+
C
N
O
R2
R3
C
R1
E
The goal of the present work was to study reactions
of primary, secondary, and tertiary amides I V and
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 6 2003

970
LEBEDEVA et al.
Table 2. Normalized and natural-scale parameters of ligands I IV and standardized Ks values for the reactions of
(tetraphenylporphyrinato)zinc(II) with amides I IV in benzene at 298.15 K
+
pKBH
V
Por
Ks
Ligand
no.
a
b
1
b
5
b
b
cm3 mol
10 , m3
I
II
III
IV
0.73
0.49
1.35
0.33
0.011
0.525
1.395
0.882
36.11
60.01
46.84
59.00
1.272
0.842
0.323
0.753
242.15
394.37
304.89
283.18
0.995
1.372
0.020
0.357
0.578
0.971
1.104
0.710
a
b
Dimensionless quantity.
Normalized values.
Such association is impossible for tertiary amides
which are incapable of forming hydrogen bonds. Thus,
the interaction of ligands I and II with TPPZn
requires an additional energy for cleavage of L L
hydrogen bonds.
cavity turns out to be important for conformationally
rigid macrocycles (such as metal porphyrinates).
It should be noted that the stability of the TPPZn
nL complexes in the examined solvents decreases in
the amide series V > IV II > I > III. It is seen that
the complex TPPZn III is characterized by the lowest
thermodynamic stability. Unlike the other amides,
ligand III contains no hydrocarbon group at the
carbonyl group. Obviously, the observed low Ks value
for the complex TPPZn 2III cannot be explained by
the absence of electron-donor groups in ligand III,
for this fact should necessarily be reflected in the
enthalpy component of the TPPZn amide interaction.
Presumably, the TPPZn nL complexes with ligands
I, II, and IV are additionally stabilized by attractive
Comparison of the thermodynamic parameters
obtained for the systems containing dimethylform-
amide and N,N-dimethylacetamide shows that the
presence of an alkyl group at the amide carbonyl
group favors formation of stronger TPPZn 2L com-
plexes and increases the exothermic effect of the
complex formation. These data also support the
assumption that coordination of TPPZn with amides
involves the oxygen atom of the latter.
interaction between the
system of TPPZn and
Our results indicate a considerable effect of the
medium on the thermodynamics of complex forma-
tion. In all cases, replacement of benzene by carbon
tetrachloride resulted in formation of more thermo-
dynamically stable complexes and less exothermic
process (Table 1). Obviously, the reason is specific
features of solvation of tetraphenylporphyrinato-
zinc(II). As was found previously [10], carbon tetra-
chloride molecules do not solvate TPPZn in a specific
mode, but the cavity in TPPZn appears to be sterically
locked by four molecules of the solvent [10]. Benzene
and TPPZn give rise to a thermally stable 2:1 -com-
plex (2PhH TPPZn) [10].
methyl groups of the amides. An analogous strong
effect of attractive interactions between metal por-
phyrinates and electron-donor ligands on the stability
of complexes derived therefrom was repeatedly noted
in the literature [11].
The existence of correlations between the pro-
perties of complexes and physicochemical parameters
of their components makes it possible to estimate the
effect of those parameters and elucidate driving forces
of complex formation. The use of multiparameter cor-
relations for donor acceptor interaction between
metal porphyrinate and electron-donor ligand requires
that the electron-donor power of the ligand be deter-
mined. Unfortunately, we have found no published
data on gas-phase proton affinities for the ligands
under study (except for compounds III and IV). Other
empirical parameters characterizing the electron-donor
power and including effect of the medium, i.e.,
Gutmann donor numbers (DN) [12] and basicity con-
According to the X-ray diffraction data, aromatic
solvent molecules in -complexes with metal por-
phyrins can be located either above the trans-pyrrole
fragments of the macroring or directly above the
the cavity. The greater exothermic effect of complex
formation in benzene indicates formation of a mixed
complex like TPPZn 2C6H6 nL; otherwise, the
ligand coordination (which is accompanied by decom-
position of the 2PhH TPPZn complex) in benzene
would be characterized by a lesser exothermic effect
than in carbon tetrachloride. Thus, the state of the
+
stants pKBH [13], are also available for a few ligands
(Tables 2, 3). Therefore, in the present study we drew
correlations for two groups of the ligands. Acetamide
is poorly soluble in CCl4, so that we failed to deter-
mine thermodynamic parameters for complex forma-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 6 2003

THERMODYNAMICS OF DONOR ACCEPTOR INTERACTION
971
Table 3. Normalized and natural-scale parameters of ligands III VI and standardized Ks values for the reactions of
(tetraphenylporphyrinato)zinc(II) with amides III VI in benzene and carbon tetrachloride at 298.15 K
DN
V
Por (C6H6)
Ks (C6H6)
Por (CCl4)
Ks (CCl4)
Ligand
no.
a
1
a
5
a
a
5
a
a
kJ/mol
cm3 mol
10 , m3
10 , m3
III
IV
V
111.45
116.48
162.57
124.86
0.751
0.534
1.457
0.172
46.84
59.00
112.15
44.84
0.597
0.212
1.470
0.661
304.89
283.18
619.01
311.24
0.467
0.602
1.496
0.427
1.332
0.188
0.890
0.631
298.61
280.14
380.12
371.36
0.671
1.037
0.941
0.768
0.929
0.180
1.418
0.308
VI
a
Normalized values.
tion of acetamide with TPPZn, and the corresponding
multiparameter correlation in carbon tetrachloride
was not drawn.
orientation of fixed dipoles which, in keeping with
published data [4], are oriented toward the oxygen
atom in amide molecules. The significance of Por
together with the insignificant contribution of V may
be regarded as further evidence of the assumption that
amides are coordinated to (tetraphenylporphyrinato)-
zinc(II) through the oxygen atom.
By analysis of the quantities given in Tables 1 and
2 we obtained correlation (1) between the thermo-
dynamic stability of TPPZn L complexes (L = I IV)
in benzene and ligand properties:
The above correlation equations are unlikely to
possess a predictive power, for they have been drawn
for a small number of amides.
4
4
+
Ks = (0.7361 6.4 10 )pKBH + (0.2196 9.1 10 )V
4
+ ( 0.2916 7.8 10 )Por;
4
Sad = 8.6 10 ; R = 1.
(1)
EXPERIMENTAL
Here, Ks is the normalized equilibrium constant;
V is the normalized van der Waals volume, Por is the
normalized orientational polarizability, Sad is the
remainder variance, and R is the normalized electronic
polarizability.
(Tetraphenylporphyrinato)zinc(II) (TPPZn) was
synthesized and purified by the procedures described
in [14]; its purity was checked by the electronic
absorption spectra. The ligands (L): acetamide (I),
N-ethylacetamide (II), dimethylformamide (III),
N,N-dimethylacetamide (IV), hexamethylphosphoric
triamide (V), and dimethyl sulfoxide (VI) were
purified by column chromatography on activated
Al2O3 and subsequent vacuum distillation [15]. Acet-
amide was purified according to the recommendations
given in [15]; its purity was checked by thermo-
gravimetry using an MOM-1000D derivatograph
(Hungary); its melting point coincided with that
given in [5]. Benzene and carbon tetrachloride of
ultrapure grade were additionally purified by drying
over 4 molecular sieves, followed by fractional
distillation. Their purity was checked by chromatog-
raphy (it was 99.98%). The concentration of water
was determined according to Fischer (it was no more
than 0.02%).
Analysis of the quantities given in Tables 1 and 3
allowed us to draw correlations (2) and (3) (for
benzene and carbon tetrachloride, respectively)
between the thermodynamic stability of TPPZn L
complexes (L = III VI) and ligand properties:
3
3
Ks = (3.5820 1.3 10 )DN + ( 0.0463 1.1 10 )V
3
4
+ ( 2.8484 1.1 10 )Por; Sad = 5.0 10 , R = 1; (2)
3
3
Ks = (1.2436 1.3 10 )DN + (0.8569 1.1 10 )V
3
4
+ ( 1.1050 1.6 10 )Por; Sad = 5.0 10 , R = 1. (3)
The values of R and Sad indicate that Eqs. (1) (3)
satisfactorily describe thermodynamic stability of the
corresponding complexes. The significance of each
parameter in the above equations was estimated by
their successive exclusion. The results showed that
the stability of TPPZn L complexes is determined
mainly by the electron-donor power and orientational
polarizability of the amide molecule; the ligand
volume turned out to be less significant. It should
be noted that orientational polarizability reflects
The study was performed using an automatic dif-
ferential titration calorimeter [16]. Its dosing unit
was charged with a solution of appropriate ligand
(c = 0.2 0.3 mol/kg), and the cell was charged with
7
3
a solution of TPPZn (10 10
mol/kg). Molal
concentrations were recalculated into molar concentra-
tions using the solution densities measured at
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 6 2003

972
LEBEDEVA et al.
298.15 K by the volumetric method. The thermo-
dynamic parameters: stability constants Ks (expressed
in the molarity units) and H0 for reactions of TPPZn
with ligands L were calculated from the experimental
titration curves with the aid of KALORY program
4. Planche, L.A. and La Rogers, M.T., J. Am. Chem.
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[17]. In order to exclude possible local minima of the
2
function (Qnexp
Qncalc
)
(where Qnexp is the
6. Brown, R.S., J. Am. Chem. Soc., 1980, vol. 102,
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best agreement between the calculated and experi-
mental unit heat effects. The presence of a single
minimum pointed to its global character and con-
firmed the reliability of the calculated thermodynamic
parameters.
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Multiparameter correlation equations were drawn
as follows. In keeping with the recommendations
given in [18], the ligand volume was estimated as
algebraic sum of the van der Waals volumes of
particular atoms and bonds. As a measure of charge
mobility in the ligand molecules we used orientational
polarizability calculated according to [19].
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following a multilinear regression analysis program.
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the multiple correlation coefficient R and remainder
variance Sad.
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Physicochemical parameters of the ligands and
thermodynamic parameters of complex formation
were standardized according to [20]. Multiparameter
regresions with normalized variables (Tables 2, 3)
make it possible to estimate the significance of
each particular parameter directly from the regression
coefficient [20].
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