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Specific ion-molecule interactions of azines in nitrobenzene. s-Triazine, pyrazine and pyridazine complexing abilities
Journal of
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 354 (1995) 141-146
Specific ion-molecule interactions of azines in nitrobenzene. s-Triazine, pyrazine and pyridazine complexing abilities Okuma E. Kasende*, Fraqois
M. Kabue, M. Muzomwe
FacultP des Sciences, UniversitP de Kinshasa, B.P. 190, Kinshasa XI, Zaire
First received 4 July 1994; in final form 9 January 1995
Abstract The conductances of solutions of pyridinium ions with azines were measured in order to study their ionic association, and to investigate the effect of substitution of nitrogen for carbon atoms in the pyridine ring on the azine complexing ability. A strong link was noted between the decrease in basicity and the increase in molecular symmetry of the azines with increasing number of nitrogen atoms in the pyridine ring.
1. Introduction
Azines are nitrogen-containing heterocycles widely encountered as building blocks of biologically important molecules. They are also synthetically prepared for many purposes, e.g. as components of pharmaceuticals [ 11.The ring nitrogens in these molecules have differing degrees of basicity, often discussed in terms of proton affinity. Although the proton-acceptor properties of pyridine have been extensively studied, to the best of our knowledge thermodynamic data on the ionpair association of diazinium salts in non-aqueous solutions have never been published. In addition, a review of the existing literature reveals that no thermodynamic data are available on specific interactions of s-triazine with any proton donors, probably because of its low basicity. In fact, an early study of the proton affinity values of nitrogen heterocyclic bases by Meot-Mer [2] gave a
* Corresponding author. 0022-2860/95/$09.50
0
value of 302.0 kcal mol-’ for s-triazine, which is lower than that of ammonia, and thus lower than for any other sp* or sp3 nitrogen base. The present paper reports the results of our investigations on the conductances of solutions of pyridinium picrate with s-triazine, pyrazine, pyridazine and pyridine in nitrobenzene, at 303 and 323 K.
pyridinc
Although a study of the conductance of pyridinium picrate in nitrobenzene at 298 K has already been published [3], we took the measurements again at 303 and 323 K in order to determine the complexation enthalpy values and to compare them with pyridazine, pyrazine and pyrimidine data. Because of its dielectric constant (34.8 D at 298 IS), nitrobenzene is a very convenient solvent for determining the dissociation constants of
1995 Elsevier Science B.V. All rights reserved
SSDZ 0022-2860(95)08845-8
PyTi&&
O.E. Kasende et al.lJournal of Molecular Structure 354 (1995) 141-146
142
dissolved hydrogen-bonded ion pairs [4]. In this range of dielectric constants, the undissociated hydrogen-bonded ion pairs remain significant at the formal concentrations of the ionophore which are adequate for the measurements. Hence it is possible to estimate the value of the complexation constants from measured conductance data. Our objective was to study the effect of the substitution of nitrogen for carbon atoms in the pyridine ring on the azine basicity. From this point of view, a comparison between the basicity of an azine and its ring pyridinic symmetry will be of particular interest, as it has been demonstrated [2] that the inclusion of a second or third nitrogen atom within the aromatic ring, i.e. going from pyridine to di- or triazine, causes a reduction in the gas-phase basicity. If azine (A) is added to a solution of pyridinium picrate (PyHPic) in nitrobenzene, one can expect that the dissociation equilibrium into free ions PyHPic % PyH+ + Pitwill be disturbed by new equilibria complexes of A with the ions:
(1) involving
PyH+ + A 3 PyH+ . . . A
(2)
Picc + A s
(3)
Pit- . . . A
A study of the perturbation of the dissociation constant of the salt, Kd, caused by the addition of increasing amounts of azine provides information about complexes formed between the ions and the azine, and allows the determination of approximate values of the complexation constants (kt, kr). The value of k-’ must be very low and is not discussed in the text.
(Fulka puriss) was distilled from activated alumina under reduced pressure. Its residual conductivity was less than 2 x lo-’ 0-l cm-‘. The s-triazine, pyrazine, pyridine and pyridazine were Janssens Chimica products. 2.2. Methods Electrical conductances were measured with a Copenhagen Radiometer CDM 2e, using Copenhagen Radiometer CDC 104 cells. The cells were calibrated by determining the conductance of KC1 solutions in water at 298 K in the concentration range 0.001-0.01 N, and using the method developed by Lin et al. [5]. A Julabo thermostat U13 was used to keep the temperature of the solutions at 303 f 1 K and 323 f 1 K in the respective experiments. The dissociation constants (Kd) were computed from the experimental conductivities using the method of Fuoss and Kraus [6]. This method also allows the determination of the limiting molar conductivities with a precision of about 3%. We computed this A, value using Kohlrausch’s law. The dielectric constant of the solution is not expected to change on addition of azine because specific solvent effects are very weak [7]. Thus, no corrections to Kd were made for the system PyHPic + azine. The complexation constants were determined using the method proposed by Macau et al. [8] based on the variation in the dissociation constant Kd of the ionophore with the concentration L of the added ligand. In general, the ratio of the dissociation constant of the salt in the presence of the ligand to that in the absence of the ligand follows Eq. (4), which is a generalization of the relationship proposed by Ralph and Gilkerson [9]: R E K,,/K;
2. Materials and methods 2. I. Materials
Pyridinium picrate was prepared by adding pyridine to a solution of picric acid in ethanol. The resulting precipitates were recrystallized from ethanol-acetone mixtures and dried. Nitrobenzene
= (1 + k;L + k;k2+L2 + . ..)
x (1 + k;L + k;k;L2 (1+KlL+K,K2L2+...)
where k:, constants molecules K,, K2,...
+ . ..)/ (4)
k2f,... and k,, k, ,... are the equilibrium for the successive additions of the ligand to the cation and anion, respectively, and are the complexation constants for the
143
O.E. Kasende et al./Journal of Molecular Structure 354 (1995) 141-146
ligand with the ion pair. If only complexes of 1:1 stoichiometry are formed between the cation or the anion and the ligand, the ratio R can be written as R-Kd/K;=l+k,L
k, values determined at 303 and 323 K by the Van?
Hoff equation.
3. Results and discussion
where kl is kl or kr. Straight regression lines have been obtained for the plot of R against L in the case of similar studies of 1:1 complexes [3,10,11]. The enthalpies (-AH) of complex formation between ion and ligand were calculated from the
The molar conductances (A), expressed in R-’ cm2 mall’, are given in Table 1 for pyridinium picrate, where C is the concentration of the salt and L is the concentration of the azine (ligand) in
Table 1 Conductances
A (n-’ cm* mol-‘)
103c
Pyridine
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.018 12.994 14.135 16.144 11.724 19.950 21.210
14.831 15.750 17.719 19.152 21.210 22.470
L = 0.015 10.897 12.277 13.912 15.750 18.060 19.530
12.994 14.377 16.144 18.144 19.740 20.751
L = 0.022 10.500 11.592 13.518 15.288 17.640 18.900
12.404 13.150 14.845 16.339 18.587 19.652
L = 1.00 13.519 15.750 17.325 18.858 21.355 22.470
15.947 18.738 20.738 22.260 23.310 23.625
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.045 16.538 16.638 19.556 21.000 23.000 24.150
18.047 18.981 20.738 22.890 24.255 25.410
L = 0.027 12.206 13.408 15.408 17.262 19.635 21.000
14.831 17.365 19.163 20.958 21.300 22.630
L = 0.057 12.403 14.538 16.144 11.976 20.580 21.840
14.100 14.912 16.733 18.310 20.621 22.684
L = 1.70 16.144 17.608 20.738 22.890 23.360 24.560
18.047 20.597 24.150 24.450 25.335 25.935
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.071 20.080 20.838 22.969 24.150 26.250 27.510
21.197 22.212 24.019 25.200 27.195 28.350
L = 0.078 14.109 15.508 17.719 19.446 20.580 22.050
16.931 19.869 21.000 23.520 24.460 25.200
L = 0.075 13.519 15.023 16.931 19.658 21.210 22.680
15.588 16.466 18.425 20.106 22.540 23.646
L = 1.90 18.244 20.152 22.575 24.780 26.350 27.300
21.738 24.038 27.038 29.400 29.600 30.450
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.095 22.969 24.069 25.988 27.090 29.190 30.450
24.28 1 25.200 27.038 28.350 29.925 31.290
L = 0.096 15.028 17.123 18.769 20.622 20.980 24.780
16.93 1 18.496 21.000 22.890 24.360 26.310
L = 0.095 15.422 15.023 19.163 19.658 23.520 24.570
17.130 18.073 20.162 21.940 24.480 25.620
L = 3.00 19.556 22.212 24.413 26.082 27.300 27.720
23.360 26.573 29.196 30.710 30.912 31.070
8.00 6.50 4.00 2.50 1.oo 0.50
L = 0.122 26.250 27.138 28.875 30.030 32.130 33.600
27.169 28.108 29.663 31.290 32.655 33.810
L = 0.114 16.538 17.769 20.344 21.050 22.050 26.040
18.996 20.596 22.181 24.150 25.935 27.200
L = 0.125 15.422 16.638 19.163 20.622 23.520 24.570
18.186 19.154 21.281 23.066 25.574 26.680
L = 3.2 19.556 22.615 25.200 26.460 27.300 27.720
23.494 27.058 30.056 3 1.080 31.290 3 1.790
of pyridinium
picrate
in nitrobenzene
Pyridazine
in the presence
of azine at 303 and 323 K
Pyrazine
s-Triazine
O.E. Kasende et aLlJournal of Molecular Structure 354 (1995) 141-146
144
mol dmp3 . The dissociation constants (&) and the values of the ratio R are reported in Table 2 for the systems PyHPic + s-triazine, PyHPic + pyrazine, PyHPic + pyridazine and PyHPic + pyridine in nitrobenzene. Fig. 1 shows the variation in R with the concentration of added ligand. Table 3 lists the values of the formation constants (ki) determined at 303 and 323 K, and the enthalpies (-AH) of complex formation for the interaction between the pyridinium ion and azine molecule in nitrobenze. Table 3 also contains the nitrogen ring ionization energy (Is,min), pK, and proton affinity values of the azines. The conductivity of electrolyte solutions is
81_
6
II: 4
2
0
‘,,...‘,,,I.,,,,,.‘,I.‘,,,‘,,,I..,,,”’,
0
Table 2 Dissociation constants Kd (dm3 mol-‘) and values of the ratio R at 303 and 323 K for pyridinium picrate in nitrobenzene at various concentrations C (mol dmm3) of the corresponding azine c
303 K 104Kd
Pyridine 0
0.018 0.045 0.071 0.095 0.122
3.519
6.136 10.060 13.820 17.897 21.371
R
104&
R
1 1.715 2.811 3.861 4.834 5.971
6.216 8.564 11.856 16.248 20.181 23.761
1 1.348 1.907 2.614 3.241 3.823
6.216
1 1.133 1.240 1.693 1.853 2.012
3.579
1
0.015 0.027 0.078 0.096 0.114
4.319 4.980 7.675 8.647 9.680
1.208 1.391 2.144 2.416 2.705
3.579
0.022 0.057 0.075 0.095 0.125
4.085 4.937 5.400 5.920 6.614
s-Triazine 0
1.00 1.70 1.90 3.00 3.20
3.579
6.437 8.363 8.893 12.245 12.846
0.08
0.12
0.16
3
4
L [mol/dm3]
(4 4 ,
323 K
Pyridazine 0
Pyrazine 0
0.04
7.043 7.708 10.524 11.518 12.507
1 1.141 1.379 1.510 1.654 1.848
6.216 6.819 7.770 8.261 8.808 9.628
1 1.097 1.250 1.329 1.417 1.549
1 1.8 2.337 2.485 3.421 3.489
6.21 10.409 12.443 12.473 16.390 16.800
1 1.675 2.007 2.001 2.637 2.703
0
(b)
1
2
L [mol/dm3]
Fig. 1. Variation in R as a function of the concentration of added ligand L (mol dm-3) in nitrobenzene at 303 K: (a) (0) PyHPic + pyridine, (0) PyHPic + pyridazine, (V) PyHPit + pyrazine; (b) PyHPic + s-triazine.
controlled by ion-ion and ion-solvent molecule interactions in perturbed equilibria, which are so complex that a statistical-mechanical treatment of electrolyte conductivity is possible only for low electrolyte concentrations [ 121. In this low concentration range, it is known that the equivalent conductivity increases with dilution. The results in Table 1 illustrate this phenomenon. Another point worthy of comment concerning Table 1 is
O.E. Kasende et aLlJournal of Molecular
145
Structure 354 (1995) 141-146
Table 3 Nitrogen ring ionization energy (Zs,min),pK, and proton affinity (PA) values of azines, and thermodynamic data for the interaction between the pyridinium ion and azine in nitrobenzene Azine
pKab
Pyridine Pyridazine Pyrazine s-Triazine
12.45 12.85 13.11 13.59
5.17 2.30 0.67 -2.3 Id
-AH
PA” (kJ mol-t)
k:03K (dm3 mol-‘)
k:23K (dm3 mol-‘)
(J mol-‘)
929 912 882 851
41 15 7 0.8
23 8.8 4.6 0.55
23.5 21.7 17.1 14.5
a Ref. [15]. b Ref. [18]. ‘Ref. [2]; original values in kcal mol-‘. d Estimated value in Ref. [15].
that the addition of larger amounts of azine to nitrobenzene increases the molar conductance and thus the dissociation of PyHPic. As the conductance in a solution is essentially due to the migration of “free” ions, this increase in molar conductances demonstrates that complex formation does occur between azine and the pyridinium ions. The increase in equivalent conductance with dilution in the follows presence the order: s-triazine < pyrazine < pyridazine < pyridine. Fig. 2 depicts the change in the complexation enthalpies as a function of the proton affinities of the ligands. It can be seen that -AH increases with increasing proton affinity, which is also correlated 26
22 -
14 -
with the number of nitrogen atoms in the pyridine ring. In the two diazines, both -AH and the proton affinity increase with increasing distance between the two nitrogen atoms in the ring. It is worth noting that this order follows the basicity (p& and proton affinities) and the molecular symmetry of azines. Although there is no quantitative correlation between the increase of symmetry elements with the decrease in proton affinity, the similarity between the basicity and molecular symmetry trends can be explained by electronic delocalization in the pyridine rings. In fact, one of the most important pieces of information that can be drawn from electronic delocalization is the charge distribution, in particular as expressed by the dipole moment, as it can be measured experimentally and is due to the structural asymmetry of the molecule. In pyridine and pyridazine, the least symmetrical molecules, there is a noteworthy polarity of the molecule (ZJ= 2.25 and 3.37 D for pyridine and pyridazine, respectively [ 13,141) and a significant basicity presumably due to the high electron densities on nitrogen (1.048 and 1.041 for pyridine and pyridazine, respectively [ 141). When the symmetry of the azine increases by inclusion of a second or a third nitrogen atom in the pyridine ring, the electronic delocalization of the pyridine ring also increases. In these symmetric molecules the dipole moment ,LL= 0 for pyrazine and s-triazine [13,14]. The basicity of these azines decreases due to the reduction of the nitrogen electron densities (to 1.012 for pyrazine [14]). In addition, the ring nitrogen ionization energy (Zs,min)has been shown to be correlated with the total nitrogen electron densities [ 151. Considera-
‘111(11111(11111(11~I~,,~,,,~,I~~~’~,,~,I,,,,(~,~”
10 8.4
8.6
8.8
PA/100
9
9.2
9.4
[kJ/mol]
Fig. 2. Complexation enthalpy (-AH) in kJ mol-’ as a function of the proton affinity (PA) of azines in kJ mol-‘: (1) s-triazine, (2) pyrazine, (3) pyridazine, (4) pyridine.
146
O.E. Kasende et aLlJournal of Molecular Structure 354 (1995) 141-146
tion of the magnitude of Is,min(which is interpreted as the minimum energy required to remove an electron from the surface of the molecule at the nitrogen atom [15]), gives information about the trend of the nitrogen electron densities in this series of azines. Thus, the increase in Is,min with decreasing azine basicity shown in Table 3 is consistent with the above comments, and confirms the correlation of basicity with electron densities. Nevertheless, it is interesting to note that pyridazine has a higher dipole moment than pyridine, but a weaker basicity weaker. This is most likely at least partly due to a solvatation effect of the second nitrogen atom [16]. Looking at the proton affinities of the diazine series, it is important to emphasize that the values for pyridazine are relatively higher within this subset of compounds than those of pyrimidine (PA = 215.5kcalmoll’ [2]) and pyridazine, which are in the same symmetry group. According to an ab initio molecular study on the protonation of azines by Mo et al. [17], extra stabilization of the protonated form of pyridazine occurs via the intramolecular hydrogen bond between the acidic hydrogen (bonded to Nl) and the second basic nitrogen N2. Obviously, such a peri-interaction is not possible either in pyrimidine or in pyrazine. This increased stabilization probably explains the higher proton affinity values for pyridazine. Finally, we should stress the importance of additional information provided by the conductance data about the complexing ability of s-triazine. In fact, s-triazine is a very weak base. The complexation enthalpy value of -14.5 kJ mol-’ observed with the pyridinium ion in nitrobenzene is striking proof that the s-triazine complexing ability is very low.
Acknowledgments The authors are grateful to the Fulbright Program for its support of O.E. Kasende at the University of Florida (USA) during the period in
which the final version of this paper was completed. We also wish to acknowledge the cooperation of Professor Th. Zeegers-Huyskens (K.U. Leuven, Belgium) who provided us with samples of s-triazine, pyrazine and pyridazine. Finally, we appreciate the hospitality and support provided by Dr. K. Szczepaniak-Person and Professor Willis B. Person at the University of Florida.
References PI N.L. Allinger,
M.P. Cava, D.C. De Jongh, CR. Johnson, N.A. Lebel and C.L. Stevens, Organic Chemistry, Worth, New York, 1971, Chapter 28. 121M. Meot-Ner, J. Am. Chem. Sot., 100 (1970) 2396. and P. Huyskens, in H. Ratajczak [31 Th. Zeegers-Huyskens and W.J. Orville-Thomas (Eds.), Molecular Interactions, Vol. 2, Wiley, Chichester, 1980, p. 82. and M. De Pauw, J. Phys. Chem., 85 [41 M. Haulait-Pirson (1980) 1381. [51 J.E. Lin, Z. Zwolenik and R.M. Fuoss, J. Am. Chem. Sot., 81 (1959) 1557. [cl R.M. Fuoss and F. Accascina, Electronic Conductance, Wiley-Interscience, New York, 1959. [71 M. Haulait and P.L. Huyskens, J. Sol. Chem., 4 (1975) 853. PI J. Macau, L. Lambert and P. Huyskens, Bull. Sot. Chim. Fr., 7 (1971) 2387. 191E.K. Ralph and W.R. Gilkerson, J. Am. Chem. Sot., 86 (1964) 4763. VOI V. Delcoigne and M. Haulait, J. Solution Chem., 5 (1976) 47. 1111O.E. Kasende, K.K. Ngala and S.Kayembe, J. Mol. Strut., 354 (1995) 147. WI J. Barthel, in P.L. Huyskens, W.A.P. Luck and T. ZeegersHuyskens (Eds.), Intermolecular Forces, An Introduction to Modern Methods and Results, Springer, Berlin, 1991, p. 427. v31 A.L. McClellan, Tables of Experimental Dipole Moments, Freeman, San Francisco, CA, 1963. [I41 J.E. Del Bene, Chemi. Phys., 15 (1976) 463. P51 T. Brinck, J.S. Murray, P. Politzer and R.E. Carter, J. Org. Chem., 56 (1991) 2934. J. Phys. Chem., 88 P61 0. Kasende and Th. Zeegers-Huyskens, (1984) 2132. 11710. Mo, J.L.G. De Paz and M. Yanez, J. Mol. Struct. (Theochem), 150 (1987) 135. Handbook of Heterocyclic Chemistry, [I81 A.R. Katritzky, Pergamon Press, New York, 1985.
MOLECULAR STRUCTURE ELSEVIER
Journal of Molecular Structure 354 (1995) 141-146
Specific ion-molecule interactions of azines in nitrobenzene. s-Triazine, pyrazine and pyridazine complexing abilities Okuma E. Kasende*, Fraqois
M. Kabue, M. Muzomwe
FacultP des Sciences, UniversitP de Kinshasa, B.P. 190, Kinshasa XI, Zaire
First received 4 July 1994; in final form 9 January 1995
Abstract The conductances of solutions of pyridinium ions with azines were measured in order to study their ionic association, and to investigate the effect of substitution of nitrogen for carbon atoms in the pyridine ring on the azine complexing ability. A strong link was noted between the decrease in basicity and the increase in molecular symmetry of the azines with increasing number of nitrogen atoms in the pyridine ring.
1. Introduction
Azines are nitrogen-containing heterocycles widely encountered as building blocks of biologically important molecules. They are also synthetically prepared for many purposes, e.g. as components of pharmaceuticals [ 11.The ring nitrogens in these molecules have differing degrees of basicity, often discussed in terms of proton affinity. Although the proton-acceptor properties of pyridine have been extensively studied, to the best of our knowledge thermodynamic data on the ionpair association of diazinium salts in non-aqueous solutions have never been published. In addition, a review of the existing literature reveals that no thermodynamic data are available on specific interactions of s-triazine with any proton donors, probably because of its low basicity. In fact, an early study of the proton affinity values of nitrogen heterocyclic bases by Meot-Mer [2] gave a
* Corresponding author. 0022-2860/95/$09.50
0
value of 302.0 kcal mol-’ for s-triazine, which is lower than that of ammonia, and thus lower than for any other sp* or sp3 nitrogen base. The present paper reports the results of our investigations on the conductances of solutions of pyridinium picrate with s-triazine, pyrazine, pyridazine and pyridine in nitrobenzene, at 303 and 323 K.
pyridinc
Although a study of the conductance of pyridinium picrate in nitrobenzene at 298 K has already been published [3], we took the measurements again at 303 and 323 K in order to determine the complexation enthalpy values and to compare them with pyridazine, pyrazine and pyrimidine data. Because of its dielectric constant (34.8 D at 298 IS), nitrobenzene is a very convenient solvent for determining the dissociation constants of
1995 Elsevier Science B.V. All rights reserved
SSDZ 0022-2860(95)08845-8
PyTi&&
O.E. Kasende et al.lJournal of Molecular Structure 354 (1995) 141-146
142
dissolved hydrogen-bonded ion pairs [4]. In this range of dielectric constants, the undissociated hydrogen-bonded ion pairs remain significant at the formal concentrations of the ionophore which are adequate for the measurements. Hence it is possible to estimate the value of the complexation constants from measured conductance data. Our objective was to study the effect of the substitution of nitrogen for carbon atoms in the pyridine ring on the azine basicity. From this point of view, a comparison between the basicity of an azine and its ring pyridinic symmetry will be of particular interest, as it has been demonstrated [2] that the inclusion of a second or third nitrogen atom within the aromatic ring, i.e. going from pyridine to di- or triazine, causes a reduction in the gas-phase basicity. If azine (A) is added to a solution of pyridinium picrate (PyHPic) in nitrobenzene, one can expect that the dissociation equilibrium into free ions PyHPic % PyH+ + Pitwill be disturbed by new equilibria complexes of A with the ions:
(1) involving
PyH+ + A 3 PyH+ . . . A
(2)
Picc + A s
(3)
Pit- . . . A
A study of the perturbation of the dissociation constant of the salt, Kd, caused by the addition of increasing amounts of azine provides information about complexes formed between the ions and the azine, and allows the determination of approximate values of the complexation constants (kt, kr). The value of k-’ must be very low and is not discussed in the text.
(Fulka puriss) was distilled from activated alumina under reduced pressure. Its residual conductivity was less than 2 x lo-’ 0-l cm-‘. The s-triazine, pyrazine, pyridine and pyridazine were Janssens Chimica products. 2.2. Methods Electrical conductances were measured with a Copenhagen Radiometer CDM 2e, using Copenhagen Radiometer CDC 104 cells. The cells were calibrated by determining the conductance of KC1 solutions in water at 298 K in the concentration range 0.001-0.01 N, and using the method developed by Lin et al. [5]. A Julabo thermostat U13 was used to keep the temperature of the solutions at 303 f 1 K and 323 f 1 K in the respective experiments. The dissociation constants (Kd) were computed from the experimental conductivities using the method of Fuoss and Kraus [6]. This method also allows the determination of the limiting molar conductivities with a precision of about 3%. We computed this A, value using Kohlrausch’s law. The dielectric constant of the solution is not expected to change on addition of azine because specific solvent effects are very weak [7]. Thus, no corrections to Kd were made for the system PyHPic + azine. The complexation constants were determined using the method proposed by Macau et al. [8] based on the variation in the dissociation constant Kd of the ionophore with the concentration L of the added ligand. In general, the ratio of the dissociation constant of the salt in the presence of the ligand to that in the absence of the ligand follows Eq. (4), which is a generalization of the relationship proposed by Ralph and Gilkerson [9]: R E K,,/K;
2. Materials and methods 2. I. Materials
Pyridinium picrate was prepared by adding pyridine to a solution of picric acid in ethanol. The resulting precipitates were recrystallized from ethanol-acetone mixtures and dried. Nitrobenzene
= (1 + k;L + k;k2+L2 + . ..)
x (1 + k;L + k;k;L2 (1+KlL+K,K2L2+...)
where k:, constants molecules K,, K2,...
+ . ..)/ (4)
k2f,... and k,, k, ,... are the equilibrium for the successive additions of the ligand to the cation and anion, respectively, and are the complexation constants for the
143
O.E. Kasende et al./Journal of Molecular Structure 354 (1995) 141-146
ligand with the ion pair. If only complexes of 1:1 stoichiometry are formed between the cation or the anion and the ligand, the ratio R can be written as R-Kd/K;=l+k,L
k, values determined at 303 and 323 K by the Van?
Hoff equation.
3. Results and discussion
where kl is kl or kr. Straight regression lines have been obtained for the plot of R against L in the case of similar studies of 1:1 complexes [3,10,11]. The enthalpies (-AH) of complex formation between ion and ligand were calculated from the
The molar conductances (A), expressed in R-’ cm2 mall’, are given in Table 1 for pyridinium picrate, where C is the concentration of the salt and L is the concentration of the azine (ligand) in
Table 1 Conductances
A (n-’ cm* mol-‘)
103c
Pyridine
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.018 12.994 14.135 16.144 11.724 19.950 21.210
14.831 15.750 17.719 19.152 21.210 22.470
L = 0.015 10.897 12.277 13.912 15.750 18.060 19.530
12.994 14.377 16.144 18.144 19.740 20.751
L = 0.022 10.500 11.592 13.518 15.288 17.640 18.900
12.404 13.150 14.845 16.339 18.587 19.652
L = 1.00 13.519 15.750 17.325 18.858 21.355 22.470
15.947 18.738 20.738 22.260 23.310 23.625
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.045 16.538 16.638 19.556 21.000 23.000 24.150
18.047 18.981 20.738 22.890 24.255 25.410
L = 0.027 12.206 13.408 15.408 17.262 19.635 21.000
14.831 17.365 19.163 20.958 21.300 22.630
L = 0.057 12.403 14.538 16.144 11.976 20.580 21.840
14.100 14.912 16.733 18.310 20.621 22.684
L = 1.70 16.144 17.608 20.738 22.890 23.360 24.560
18.047 20.597 24.150 24.450 25.335 25.935
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.071 20.080 20.838 22.969 24.150 26.250 27.510
21.197 22.212 24.019 25.200 27.195 28.350
L = 0.078 14.109 15.508 17.719 19.446 20.580 22.050
16.931 19.869 21.000 23.520 24.460 25.200
L = 0.075 13.519 15.023 16.931 19.658 21.210 22.680
15.588 16.466 18.425 20.106 22.540 23.646
L = 1.90 18.244 20.152 22.575 24.780 26.350 27.300
21.738 24.038 27.038 29.400 29.600 30.450
8.00 6.50 4.00 2.50 1.00 0.50
L = 0.095 22.969 24.069 25.988 27.090 29.190 30.450
24.28 1 25.200 27.038 28.350 29.925 31.290
L = 0.096 15.028 17.123 18.769 20.622 20.980 24.780
16.93 1 18.496 21.000 22.890 24.360 26.310
L = 0.095 15.422 15.023 19.163 19.658 23.520 24.570
17.130 18.073 20.162 21.940 24.480 25.620
L = 3.00 19.556 22.212 24.413 26.082 27.300 27.720
23.360 26.573 29.196 30.710 30.912 31.070
8.00 6.50 4.00 2.50 1.oo 0.50
L = 0.122 26.250 27.138 28.875 30.030 32.130 33.600
27.169 28.108 29.663 31.290 32.655 33.810
L = 0.114 16.538 17.769 20.344 21.050 22.050 26.040
18.996 20.596 22.181 24.150 25.935 27.200
L = 0.125 15.422 16.638 19.163 20.622 23.520 24.570
18.186 19.154 21.281 23.066 25.574 26.680
L = 3.2 19.556 22.615 25.200 26.460 27.300 27.720
23.494 27.058 30.056 3 1.080 31.290 3 1.790
of pyridinium
picrate
in nitrobenzene
Pyridazine
in the presence
of azine at 303 and 323 K
Pyrazine
s-Triazine
O.E. Kasende et aLlJournal of Molecular Structure 354 (1995) 141-146
144
mol dmp3 . The dissociation constants (&) and the values of the ratio R are reported in Table 2 for the systems PyHPic + s-triazine, PyHPic + pyrazine, PyHPic + pyridazine and PyHPic + pyridine in nitrobenzene. Fig. 1 shows the variation in R with the concentration of added ligand. Table 3 lists the values of the formation constants (ki) determined at 303 and 323 K, and the enthalpies (-AH) of complex formation for the interaction between the pyridinium ion and azine molecule in nitrobenze. Table 3 also contains the nitrogen ring ionization energy (Is,min), pK, and proton affinity values of the azines. The conductivity of electrolyte solutions is
81_
6
II: 4
2
0
‘,,...‘,,,I.,,,,,.‘,I.‘,,,‘,,,I..,,,”’,
0
Table 2 Dissociation constants Kd (dm3 mol-‘) and values of the ratio R at 303 and 323 K for pyridinium picrate in nitrobenzene at various concentrations C (mol dmm3) of the corresponding azine c
303 K 104Kd
Pyridine 0
0.018 0.045 0.071 0.095 0.122
3.519
6.136 10.060 13.820 17.897 21.371
R
104&
R
1 1.715 2.811 3.861 4.834 5.971
6.216 8.564 11.856 16.248 20.181 23.761
1 1.348 1.907 2.614 3.241 3.823
6.216
1 1.133 1.240 1.693 1.853 2.012
3.579
1
0.015 0.027 0.078 0.096 0.114
4.319 4.980 7.675 8.647 9.680
1.208 1.391 2.144 2.416 2.705
3.579
0.022 0.057 0.075 0.095 0.125
4.085 4.937 5.400 5.920 6.614
s-Triazine 0
1.00 1.70 1.90 3.00 3.20
3.579
6.437 8.363 8.893 12.245 12.846
0.08
0.12
0.16
3
4
L [mol/dm3]
(4 4 ,
323 K
Pyridazine 0
Pyrazine 0
0.04
7.043 7.708 10.524 11.518 12.507
1 1.141 1.379 1.510 1.654 1.848
6.216 6.819 7.770 8.261 8.808 9.628
1 1.097 1.250 1.329 1.417 1.549
1 1.8 2.337 2.485 3.421 3.489
6.21 10.409 12.443 12.473 16.390 16.800
1 1.675 2.007 2.001 2.637 2.703
0
(b)
1
2
L [mol/dm3]
Fig. 1. Variation in R as a function of the concentration of added ligand L (mol dm-3) in nitrobenzene at 303 K: (a) (0) PyHPic + pyridine, (0) PyHPic + pyridazine, (V) PyHPit + pyrazine; (b) PyHPic + s-triazine.
controlled by ion-ion and ion-solvent molecule interactions in perturbed equilibria, which are so complex that a statistical-mechanical treatment of electrolyte conductivity is possible only for low electrolyte concentrations [ 121. In this low concentration range, it is known that the equivalent conductivity increases with dilution. The results in Table 1 illustrate this phenomenon. Another point worthy of comment concerning Table 1 is
O.E. Kasende et aLlJournal of Molecular
145
Structure 354 (1995) 141-146
Table 3 Nitrogen ring ionization energy (Zs,min),pK, and proton affinity (PA) values of azines, and thermodynamic data for the interaction between the pyridinium ion and azine in nitrobenzene Azine
pKab
Pyridine Pyridazine Pyrazine s-Triazine
12.45 12.85 13.11 13.59
5.17 2.30 0.67 -2.3 Id
-AH
PA” (kJ mol-t)
k:03K (dm3 mol-‘)
k:23K (dm3 mol-‘)
(J mol-‘)
929 912 882 851
41 15 7 0.8
23 8.8 4.6 0.55
23.5 21.7 17.1 14.5
a Ref. [15]. b Ref. [18]. ‘Ref. [2]; original values in kcal mol-‘. d Estimated value in Ref. [15].
that the addition of larger amounts of azine to nitrobenzene increases the molar conductance and thus the dissociation of PyHPic. As the conductance in a solution is essentially due to the migration of “free” ions, this increase in molar conductances demonstrates that complex formation does occur between azine and the pyridinium ions. The increase in equivalent conductance with dilution in the follows presence the order: s-triazine < pyrazine < pyridazine < pyridine. Fig. 2 depicts the change in the complexation enthalpies as a function of the proton affinities of the ligands. It can be seen that -AH increases with increasing proton affinity, which is also correlated 26
22 -
14 -
with the number of nitrogen atoms in the pyridine ring. In the two diazines, both -AH and the proton affinity increase with increasing distance between the two nitrogen atoms in the ring. It is worth noting that this order follows the basicity (p& and proton affinities) and the molecular symmetry of azines. Although there is no quantitative correlation between the increase of symmetry elements with the decrease in proton affinity, the similarity between the basicity and molecular symmetry trends can be explained by electronic delocalization in the pyridine rings. In fact, one of the most important pieces of information that can be drawn from electronic delocalization is the charge distribution, in particular as expressed by the dipole moment, as it can be measured experimentally and is due to the structural asymmetry of the molecule. In pyridine and pyridazine, the least symmetrical molecules, there is a noteworthy polarity of the molecule (ZJ= 2.25 and 3.37 D for pyridine and pyridazine, respectively [ 13,141) and a significant basicity presumably due to the high electron densities on nitrogen (1.048 and 1.041 for pyridine and pyridazine, respectively [ 141). When the symmetry of the azine increases by inclusion of a second or a third nitrogen atom in the pyridine ring, the electronic delocalization of the pyridine ring also increases. In these symmetric molecules the dipole moment ,LL= 0 for pyrazine and s-triazine [13,14]. The basicity of these azines decreases due to the reduction of the nitrogen electron densities (to 1.012 for pyrazine [14]). In addition, the ring nitrogen ionization energy (Zs,min)has been shown to be correlated with the total nitrogen electron densities [ 151. Considera-
‘111(11111(11111(11~I~,,~,,,~,I~~~’~,,~,I,,,,(~,~”
10 8.4
8.6
8.8
PA/100
9
9.2
9.4
[kJ/mol]
Fig. 2. Complexation enthalpy (-AH) in kJ mol-’ as a function of the proton affinity (PA) of azines in kJ mol-‘: (1) s-triazine, (2) pyrazine, (3) pyridazine, (4) pyridine.
146
O.E. Kasende et aLlJournal of Molecular Structure 354 (1995) 141-146
tion of the magnitude of Is,min(which is interpreted as the minimum energy required to remove an electron from the surface of the molecule at the nitrogen atom [15]), gives information about the trend of the nitrogen electron densities in this series of azines. Thus, the increase in Is,min with decreasing azine basicity shown in Table 3 is consistent with the above comments, and confirms the correlation of basicity with electron densities. Nevertheless, it is interesting to note that pyridazine has a higher dipole moment than pyridine, but a weaker basicity weaker. This is most likely at least partly due to a solvatation effect of the second nitrogen atom [16]. Looking at the proton affinities of the diazine series, it is important to emphasize that the values for pyridazine are relatively higher within this subset of compounds than those of pyrimidine (PA = 215.5kcalmoll’ [2]) and pyridazine, which are in the same symmetry group. According to an ab initio molecular study on the protonation of azines by Mo et al. [17], extra stabilization of the protonated form of pyridazine occurs via the intramolecular hydrogen bond between the acidic hydrogen (bonded to Nl) and the second basic nitrogen N2. Obviously, such a peri-interaction is not possible either in pyrimidine or in pyrazine. This increased stabilization probably explains the higher proton affinity values for pyridazine. Finally, we should stress the importance of additional information provided by the conductance data about the complexing ability of s-triazine. In fact, s-triazine is a very weak base. The complexation enthalpy value of -14.5 kJ mol-’ observed with the pyridinium ion in nitrobenzene is striking proof that the s-triazine complexing ability is very low.
Acknowledgments The authors are grateful to the Fulbright Program for its support of O.E. Kasende at the University of Florida (USA) during the period in
which the final version of this paper was completed. We also wish to acknowledge the cooperation of Professor Th. Zeegers-Huyskens (K.U. Leuven, Belgium) who provided us with samples of s-triazine, pyrazine and pyridazine. Finally, we appreciate the hospitality and support provided by Dr. K. Szczepaniak-Person and Professor Willis B. Person at the University of Florida.
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