- Email: [email protected]
Charge-transfer molecular complexes of 2-amino-1,3,4-thiadiazoles
Spectroch~mrca Acra, Vol. 44A, No. Prmted in Great Britain.
I I, pp.1185-I
188, 1988
6
0584-8539/88 $3.00+0.00 1988 Pergamon Press plc
Charge-transfer molecular complexes of 2-amino-1,3,4-thiadiazoles M. R. MAHMOUD,
Chemistry (Received
Department, 11 January
M. M. A. HAMED
Faculty
and H. M. A.
of Science, Assiut University,
1988; infinalform
14 April
SALMAN
Assiut, Egypt
1988; accepted 20 April
1988)
Abstract-Charge-transfer molecular complexes of 2-amino-5-X-1,3,4-thiadiazole (D) (X = H, 1; = CH,, II; =phenyl, III) with some n-electron acceptors (A) have been studied in methanol. It is concluded that these complexes are predominantly of the K--K type. Solid 1: 1 CT complexes of the donors I-III with s-acceptors DDQ and TCNE have been synthesized and characterized.
DDQ and TCNE (3.0 mmol) in dichloromethane and refluxed for l-l.5 h. Then the solution was evaporated to a
INTRODIJCTION
small volume where on cooling the solid CT complexes were separated as fine crystals. The precipitates were collected and crystallized from absolute ethanol and dried. The analytical data of the prepared complexes (C, N, S and Cl) along with some of their physical properties, viz. colour and melting points, are given in Table 1. Several attempts were made to prepare the CT complexes of the donors I-III with CHL but all failed.
Charge-transfer complexes of five-membered heteroaromatics with one sulphur, oxygen or nitrogen atom, viz. thiophene, furan and pyrrole, as well as their derivatives, were extensively investigated [l-9]. It was confirmed that these hetero compounds act as xdonors. On the other hand, very little information is available in the literature concerning CT complexes of five-membered compounds with two or more heteroatoms. 1,3,4-thiadiazoles (five-membered with three heteroatoms N, N and S) are very interesting compounds due to their important applications in many pharmaceutical, biological and analytical fields [IO]. Accordingly the present article represents a systematic study on the CT complexes of 2-amino-1,3,4thiadiazole derivatives with some n-acceptors. The study involves investigation of the electronic spectral characteristics of the formed CT complexes and determination of their stability. Furthermore, synthesis and characterization of the solid CT complexes have been investigated.
RESULTS AND DISCUSSION
Spectral characteristics
EXPERIMENTAL
The electron donors (2-amino-5-X-1,3,4_thiadiazoles) (X = H, I; = CH,, II; = phenyl, III) were applied in the present study. Compound I was supplied by Prolabo reagents, while compounds II and III were synthesized according to the procedure described in the literature [Ill. The electron acceptors 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), tetracyanoethylene (TCNE) and chloranil (CHL) (Aldrich reagents grade products) were recrystallized from dry methylene chloride, chlorobenzene and dry benzene respectively. The solvents methanol and methylene chloride were of spectral grade products (B.D.H.). Due to the sparingly soluble character of the donors (I-111) in chloroform, methylene chloride and ethylene chloride, all spectral measurements were carried out in methanol. Stock solutions of the donors or acceptors were prepared in methanol where they were freshly prepared prior to use. The electronic spectra of the studied CT complex methanolic solutions were recorded with a Cecil CE 599 Spectrophotometer using 1 cm matched silica cells. Temperature control was achieved using an ultrathermostat of accuracy f 0.05”C. Infrared spectra measurements were performed in KBr using a Perkin-Elmer 599 B i.r. Spectrophotometer. Synthesis
nf the
solid CT complexes
A saturated solution of each of the donors I-111 (3.2 mmol) in dichloromethane was mixed with a saturated solution of
of the CT complexes formed
Generally the spectra recorded for the methanolic CT complex solutions of compounds IL111 (D) with DDQ (A) are time dependent where some bands disappear and new bands appear (cf. Fig. 1). This can be interpreted on the principle that the formed CT complexes are strong, i.e. the ground CT state has predominantly a dative structure D+-A-. Since methanol is also quite polar and the 2-amino-1,3,4thiadiazoles exist mainly in the amino form [ 123, one should expect that the very polar ground CT state would be highly stabilized by such solvent molecules through dipole-dipole or dipole-induced dipole and H-bonding interactions. Accordingly, dissociation of the CT complex to D+ and A- radicals can be found to take place in the ground state. However, the appearance of isosbestic points in the spectra recorded at later times (> 15 min, Fig. 1) suggest that the reaction has to proceed first to form D+-A-, then an equilibrium dissociation to the radicals D+ and A- is established. This behaviour can be represented by the following equation: D+A-+D+-A-=D+
+A-.
For this reason the spectra of the methanolic CT complex solutions of compounds I and II with DDQ were recorded rapidly after mixing the donor and acceptor solutions. The same concentration of DDQ in methanol as in the test solution was used as a blank to eliminate the possible overlap that may arise between the CT complex bands and those of the acceptor DDQ. Generally the spectra recorded are characterized by two CT bands at 345 and 460 nm, where the latter appeared in structural features. This is probably due to the overlapping of the absorption bands of the
1185
1186
M. R. MAHMOUDet al. Table 1. Microanalysis data of the CT molecular complexes of substituted 2-amino-1,3&thiadiazole (I-III) with n-acceptors DDQ and TCNE % Calculated (found) Complex
Colour
M.p. (“C)
C
N
I-DDQ
Dark brown
95
21.42 (21.32)
II-DDQ
Brown
36.56 (37.10) 38.57 (39.20)
III-DDQ
Red
I-TCNE
Brown
205 d
47.50 (46.80) 41.90 (41.20)
17.40 (17.12) 42.71 (42.52)
II-TCNE
Brown
230 d (Z) 55.09 (54.80)
40.30 (40.13) 32.23 (32.10)
III-TCNE
140 85
Red
245 d
20.54 (20.42)
s
Cl
9.75 (9.46) 9.37 (9.10)
21.63 (21.14)
7.93 (8.15) 14.00 (14.60) 13.18 (12.80) 10.50 (10.57)
17.56 (17.60) -
20.75 (20.24)
1
d, decomposed.
2.c
I.8
E 2 1.2 8 9
0.E
0.4
400
5ixl Xfnm)
Fig. 1. Effect of time on the electronic spectra of the CT complex of donor I with DDQ in methanol at 25°C. [I] =0.08 moldmW3; [DDQ] = 1 x 10-3moldm-3. complex and those of the radicals D+ and A-. On the other hand, the spectra recorded for the methanolic solutions of the CT complexes of the donor I with TCNE and CHL exhibit two structureless hands, suiting that these CT complexes behave mainly in the undissociated state, i.e. not very strong. This is expected since the electron affinity of each of the n-acceptors TCNE and CHL is lower than that of non-dissociated
DDQ (1.68, 1.37 and 1.91 eV, respectively) [13, 141. It is worth reporting that spectral measurements for the methanolic CT complex solutions of I with TCNE or CHL were recorded after 30 and 50 min respectively from mixing the donor and acceptor solutions where the absorbance intensities of the CT bands reached more or less constant value. This time dependence of absorbance intensities of CT bands can be ascribed to the transformation of the formed CT
Charge-transfer complexes of 2-amino-1,3,4-thiadiazoles complex from an outer sphere type (D-A) to an inner sphere type (D+-A-), i.e. increasing stability. Unfortunately, the lack of photoelectron (p.e.) data concerning the studied 2-amino-1,3,4-thiadiazoles in the literature makes a comparison of CT/p.e. data impossible. Thus it is not possible to identify the nature of the donor highest filled molecular orbitals (n and/or rr) that interact with the acceptor. The CT absorption data for the complexes of the studied donors (I, II) with the three used n-acceptors are reported in Table 2. No spectral data have been reported for the CT complex of the donor III with DDQ due to its rapid dissociation to the radicals D+ and A-, i.e. it is a very strong complex. The electron affinities of DDQ and TCNE relative to CHL (e.a. = 1.37 eV) [13] have been calculated from the high CT energies making use of the equation reported previously [ 13,15,16]. The values obtained (1.82 and 1.68 eV, respectively) are in good agreement with those previously reported [13, 141. The ionization potentials of the highest two filled molecular orbitals on the donors I and II have been estimated making use of the empirical equations reported by ALOISI and PIGNATARO [17]. The obtained i.p. values for donor I with the three n-acceptors employed are nearly the same (cf. Table 2), therefore one can deduce that the same orbitals of donor I interact with the three n-acceptors. Determination
offormation
constant
(Kc,) of the com-
plexes
The formation constant values of the complexes of donor I with the three n-acceptors in the temperature range 10_25”C, as well as that of the CT complex of donor II with DDQ at 10°C have been determined spectrophotometrically making use of both BENESIFHILDEBRAND and SCOTT equations [18,19] Table 2. Spectral characteristics,
K,,(dm’ E CT
Complex
(nm)
(eV)
i.p. (eV)
I-DDQ
460
2.684
9.06
345
3.579
10.17
-
460
2.684
9.06
35.00 1 2.36
345
3.579
10.17
396
3.118
9.37
IITCNE
IICHL
332
3.719
10.18
355 sh
3.478
9.30
306 sh. shoulder
4.030
It is evident that the numerical
10.00
mol - ’ ),
’C
value of the CT
values as well as thermodynamic
&CT
I5
20
25
30.70 f
27.50 *
24.50 + 1.30
21.00 + 0.54
1.02
K,,
complex II-DDQ is higher than that of I-DDQ. This reveals that the electron donating properties of II (X = CH,) are higher than that of I. On the other hand it was indicated that the CT complex III (X =phenyl)-DDQ behaves as in the dissociated state, therefore one can deduce that the III-DDQ CT complex is stronger than the other I, II-DDQ CT complexes, i.e. the electron donating property of III is very pronounced. If the n-contribution has a significant role, the basic properties of the 5-phenyl derivative (III) should not differ from those of the 5-H derivative (I) and at the same time it is expected to be lower than that of the 5methyl derivative (II). Bearing in mind these considerations, as well as the fact that the 1,3,4-thiadiazole is a planar compound [20], one can conclude that the CT complexes of the studied 2-amino-1,3,4-thiadiazoles with the n-acceptor are predominantly of n-n type. Accordingly, the very strong character of the CT complex of the 5-phenyl derivative (III) could be explained by the fact that the n-conjugation in this compound is pronounced since it assumed a planar configuration. In this respect it is worth mentioning that previous NMR studies on the 5-substituents of 1,3,4-thiadiazoles in various solvents suggested the tendency of these compounds to form n-molecules with benzene [20]. Moreover, examination of the Kc, values of the CT complex of I with the three n-acceptors used reveals that its stability decreases on goin,g from DDQ- TCNE -tCHL. This is consistent with the
IO
1.24
II-DDQ
under the condition of D” > A”[D” = 0.01-0.14 mol dm-3 and A”= 1 x 10m3 moldme (DDQ), 8 5 x 10m4 mol drnm3 x 1O-3 mol dme3 (TCNE), (CHL)]. The mean Kc, and OCRvalues obtained are given in Table 2.
ionization potential (ip.) and formation constant parameters of the different CT complexes
4nar
1187
(dm3mol-‘cm-i) 10°C 2115
-AH” (kJ mol-i)
(b/a)’
17.96 + 0.35 -
0.069
2019
12.84 * 1.17
8.30 :56
0.99
1.03 & 0.17
0.73 5 0.04
0.64 & 0.08
6.20 II
4.26 + 0.22
112
49.03 Zh 0.98
0.16
0.53 f 0.04 -
30820
22.30
0.066
I
0%
M. R. MAHMOUDet al.
1188
decrease in electron affinity of the n-acceptor same direction.
in the
Thermodynamic parameters of the CT complexes
The enthalpy changes (AH) connected with the complex formation for the.CT complexes of I with the three n-acceptors studied were determined from the formation constant values at a series of temperatures (Table 2) making use of Van? Hoff equation plots. The data obtained are given in Table 2. According to MULLIKEN [21,22], the ratio between the coefficient of the dative bond to the non-bond wave functions (b/a) was calculated. The obtained ratio (Table 2) is compared to those of many strong CT complexes [9]. Characterization of the solid complexes
The results of chemical analysis of the synthesized solid CT complexes (Table 1) indicate the formation of a 1: 1 CT complex. A comparison of the important i.r. spectral bands of the free donors and acceptors with those of the complexes gives some information on the nature of the CT complexes formed. Generally the VNH of the donors I-III, vcN of the acceptors DDQ, TCNE and vcc, of the DDQ are shifted to lower frequencies on complex formation. This behaviour is in accordance with charge migration from the donor to acceptor. The extent of shift in vccl on complexation with the donors I-III (appearing at 772, 768 and 762 cm- ’ respectively) copes with the determined stability of the formed complex. REFERENCES [l] R. P. LANG, J. Am. them. Sot. 84,4438 (1962). [2] R. ZAHRADNIK and C. PARKANYI,Collect. Czech. Chem.
Commun. 30, 195 (1965).
c31 A. R. COOPER, C. W. CROWNE and P. G. FARRELL, Trans. Faraday Sot. 62, 18 (1966). c41 Z. YOSHIDA and T. KOBAYASHI,Tetrahedron 26, 261 (1970). PI A. R. COOPER, C. W. CROWNE and P. G. FARRELL, Trans. Faraday Sot. 62, 13 (1966). R. FOSTERand P. HANSON,Tetrahedron 21,255 (1965). ;t; R. FOSTER and P. HANSON, Trans. Faraday Sot. 60, 2189 (1964). PI E. I. GINNS and R. L. STRONG, J. phys. Chem. 71,3059 (1967). c91 R. ABU-EITTAHand M. M. HAMED, Can. J. Chem. 57, WI
[l l] [ 121
[13] [14] [15] [16] [173 [18] [ 191 [20]
[21]
[223
2337 (1979). G. KORNIS, in Comprehensive Heterocyclic Chemistry, Vol. 6, Part 4B, pp. 575-577 (edited by A. KATRITZKY, C. W. REESand K. T. Ports). Pergamon Press, Oxford ,.^_ (1Y84). V. R. RAO and V. R. SRINIVASAN, Indian J. Chem. 8,509 (1969). G. KORNIS, in ComprehensiveHeterocyclic Chemistry, Vol. 6, Part 4B, p. 557 (edited by A. KATRITZKY,C. W. REESand K. T. POTTS).Pergamon Press, Oxford (1984). G. BRIEGLEB,Angew. Chem. 76, 326 (1974); Angew. Chem. Int. Ed. Engl. 3, 617 (1964). R. D. SRIVASTAVAand G. PRASAD, Bull. Chem. Sot. Japan 43, 1611 (1970). J. JORTNERand U. SOKOLOV, Nature 1!30,1003 (1961). M. BATLEYand L. E. LYONES, Nature I%, 573 (1962). G. G. ALOISI and S. PIGNATARO,J. them. Sot. Faraday Trans. I 69, 534 (1973). H. A. BENESIand J. H. HILDEBRAND,J. Am. them. Sot. 71, 2703 (1949). R. L. SCOT, Reck Two. Chim. 75, 787 (1956). G. KORNIS, in Comprehensiue Heterocyclic Chemistry, Vol. 6, Part 4B, pp. 547, 549 (edited by A. KATRITZKY, C. W. REESand K. T. POTTS).Pergamon Press, Oxford (1984). R. S. MULLIKEN,J. Am. them. Sot. 74, 811 (1952); J. phys. Gem. 56, 801 (1952); Reel. Trau. Chim. 75, 845 (19561: J. Gem. Phvs. 61. 20 119641. k. S. ‘MULLIKEN aid W.’ P. PER&N, Ann. Rev. phys. Chem. 13,107 (1962); J. Am. them. Sot. 91,3409 (1969).
I I, pp.1185-I
188, 1988
6
0584-8539/88 $3.00+0.00 1988 Pergamon Press plc
Charge-transfer molecular complexes of 2-amino-1,3,4-thiadiazoles M. R. MAHMOUD,
Chemistry (Received
Department, 11 January
M. M. A. HAMED
Faculty
and H. M. A.
of Science, Assiut University,
1988; infinalform
14 April
SALMAN
Assiut, Egypt
1988; accepted 20 April
1988)
Abstract-Charge-transfer molecular complexes of 2-amino-5-X-1,3,4-thiadiazole (D) (X = H, 1; = CH,, II; =phenyl, III) with some n-electron acceptors (A) have been studied in methanol. It is concluded that these complexes are predominantly of the K--K type. Solid 1: 1 CT complexes of the donors I-III with s-acceptors DDQ and TCNE have been synthesized and characterized.
DDQ and TCNE (3.0 mmol) in dichloromethane and refluxed for l-l.5 h. Then the solution was evaporated to a
INTRODIJCTION
small volume where on cooling the solid CT complexes were separated as fine crystals. The precipitates were collected and crystallized from absolute ethanol and dried. The analytical data of the prepared complexes (C, N, S and Cl) along with some of their physical properties, viz. colour and melting points, are given in Table 1. Several attempts were made to prepare the CT complexes of the donors I-III with CHL but all failed.
Charge-transfer complexes of five-membered heteroaromatics with one sulphur, oxygen or nitrogen atom, viz. thiophene, furan and pyrrole, as well as their derivatives, were extensively investigated [l-9]. It was confirmed that these hetero compounds act as xdonors. On the other hand, very little information is available in the literature concerning CT complexes of five-membered compounds with two or more heteroatoms. 1,3,4-thiadiazoles (five-membered with three heteroatoms N, N and S) are very interesting compounds due to their important applications in many pharmaceutical, biological and analytical fields [IO]. Accordingly the present article represents a systematic study on the CT complexes of 2-amino-1,3,4thiadiazole derivatives with some n-acceptors. The study involves investigation of the electronic spectral characteristics of the formed CT complexes and determination of their stability. Furthermore, synthesis and characterization of the solid CT complexes have been investigated.
RESULTS AND DISCUSSION
Spectral characteristics
EXPERIMENTAL
The electron donors (2-amino-5-X-1,3,4_thiadiazoles) (X = H, I; = CH,, II; = phenyl, III) were applied in the present study. Compound I was supplied by Prolabo reagents, while compounds II and III were synthesized according to the procedure described in the literature [Ill. The electron acceptors 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), tetracyanoethylene (TCNE) and chloranil (CHL) (Aldrich reagents grade products) were recrystallized from dry methylene chloride, chlorobenzene and dry benzene respectively. The solvents methanol and methylene chloride were of spectral grade products (B.D.H.). Due to the sparingly soluble character of the donors (I-111) in chloroform, methylene chloride and ethylene chloride, all spectral measurements were carried out in methanol. Stock solutions of the donors or acceptors were prepared in methanol where they were freshly prepared prior to use. The electronic spectra of the studied CT complex methanolic solutions were recorded with a Cecil CE 599 Spectrophotometer using 1 cm matched silica cells. Temperature control was achieved using an ultrathermostat of accuracy f 0.05”C. Infrared spectra measurements were performed in KBr using a Perkin-Elmer 599 B i.r. Spectrophotometer. Synthesis
nf the
solid CT complexes
A saturated solution of each of the donors I-111 (3.2 mmol) in dichloromethane was mixed with a saturated solution of
of the CT complexes formed
Generally the spectra recorded for the methanolic CT complex solutions of compounds IL111 (D) with DDQ (A) are time dependent where some bands disappear and new bands appear (cf. Fig. 1). This can be interpreted on the principle that the formed CT complexes are strong, i.e. the ground CT state has predominantly a dative structure D+-A-. Since methanol is also quite polar and the 2-amino-1,3,4thiadiazoles exist mainly in the amino form [ 123, one should expect that the very polar ground CT state would be highly stabilized by such solvent molecules through dipole-dipole or dipole-induced dipole and H-bonding interactions. Accordingly, dissociation of the CT complex to D+ and A- radicals can be found to take place in the ground state. However, the appearance of isosbestic points in the spectra recorded at later times (> 15 min, Fig. 1) suggest that the reaction has to proceed first to form D+-A-, then an equilibrium dissociation to the radicals D+ and A- is established. This behaviour can be represented by the following equation: D+A-+D+-A-=D+
+A-.
For this reason the spectra of the methanolic CT complex solutions of compounds I and II with DDQ were recorded rapidly after mixing the donor and acceptor solutions. The same concentration of DDQ in methanol as in the test solution was used as a blank to eliminate the possible overlap that may arise between the CT complex bands and those of the acceptor DDQ. Generally the spectra recorded are characterized by two CT bands at 345 and 460 nm, where the latter appeared in structural features. This is probably due to the overlapping of the absorption bands of the
1185
1186
M. R. MAHMOUDet al. Table 1. Microanalysis data of the CT molecular complexes of substituted 2-amino-1,3&thiadiazole (I-III) with n-acceptors DDQ and TCNE % Calculated (found) Complex
Colour
M.p. (“C)
C
N
I-DDQ
Dark brown
95
21.42 (21.32)
II-DDQ
Brown
36.56 (37.10) 38.57 (39.20)
III-DDQ
Red
I-TCNE
Brown
205 d
47.50 (46.80) 41.90 (41.20)
17.40 (17.12) 42.71 (42.52)
II-TCNE
Brown
230 d (Z) 55.09 (54.80)
40.30 (40.13) 32.23 (32.10)
III-TCNE
140 85
Red
245 d
20.54 (20.42)
s
Cl
9.75 (9.46) 9.37 (9.10)
21.63 (21.14)
7.93 (8.15) 14.00 (14.60) 13.18 (12.80) 10.50 (10.57)
17.56 (17.60) -
20.75 (20.24)
1
d, decomposed.
2.c
I.8
E 2 1.2 8 9
0.E
0.4
400
5ixl Xfnm)
Fig. 1. Effect of time on the electronic spectra of the CT complex of donor I with DDQ in methanol at 25°C. [I] =0.08 moldmW3; [DDQ] = 1 x 10-3moldm-3. complex and those of the radicals D+ and A-. On the other hand, the spectra recorded for the methanolic solutions of the CT complexes of the donor I with TCNE and CHL exhibit two structureless hands, suiting that these CT complexes behave mainly in the undissociated state, i.e. not very strong. This is expected since the electron affinity of each of the n-acceptors TCNE and CHL is lower than that of non-dissociated
DDQ (1.68, 1.37 and 1.91 eV, respectively) [13, 141. It is worth reporting that spectral measurements for the methanolic CT complex solutions of I with TCNE or CHL were recorded after 30 and 50 min respectively from mixing the donor and acceptor solutions where the absorbance intensities of the CT bands reached more or less constant value. This time dependence of absorbance intensities of CT bands can be ascribed to the transformation of the formed CT
Charge-transfer complexes of 2-amino-1,3,4-thiadiazoles complex from an outer sphere type (D-A) to an inner sphere type (D+-A-), i.e. increasing stability. Unfortunately, the lack of photoelectron (p.e.) data concerning the studied 2-amino-1,3,4-thiadiazoles in the literature makes a comparison of CT/p.e. data impossible. Thus it is not possible to identify the nature of the donor highest filled molecular orbitals (n and/or rr) that interact with the acceptor. The CT absorption data for the complexes of the studied donors (I, II) with the three used n-acceptors are reported in Table 2. No spectral data have been reported for the CT complex of the donor III with DDQ due to its rapid dissociation to the radicals D+ and A-, i.e. it is a very strong complex. The electron affinities of DDQ and TCNE relative to CHL (e.a. = 1.37 eV) [13] have been calculated from the high CT energies making use of the equation reported previously [ 13,15,16]. The values obtained (1.82 and 1.68 eV, respectively) are in good agreement with those previously reported [13, 141. The ionization potentials of the highest two filled molecular orbitals on the donors I and II have been estimated making use of the empirical equations reported by ALOISI and PIGNATARO [17]. The obtained i.p. values for donor I with the three n-acceptors employed are nearly the same (cf. Table 2), therefore one can deduce that the same orbitals of donor I interact with the three n-acceptors. Determination
offormation
constant
(Kc,) of the com-
plexes
The formation constant values of the complexes of donor I with the three n-acceptors in the temperature range 10_25”C, as well as that of the CT complex of donor II with DDQ at 10°C have been determined spectrophotometrically making use of both BENESIFHILDEBRAND and SCOTT equations [18,19] Table 2. Spectral characteristics,
K,,(dm’ E CT
Complex
(nm)
(eV)
i.p. (eV)
I-DDQ
460
2.684
9.06
345
3.579
10.17
-
460
2.684
9.06
35.00 1 2.36
345
3.579
10.17
396
3.118
9.37
IITCNE
IICHL
332
3.719
10.18
355 sh
3.478
9.30
306 sh. shoulder
4.030
It is evident that the numerical
10.00
mol - ’ ),
’C
value of the CT
values as well as thermodynamic
&CT
I5
20
25
30.70 f
27.50 *
24.50 + 1.30
21.00 + 0.54
1.02
K,,
complex II-DDQ is higher than that of I-DDQ. This reveals that the electron donating properties of II (X = CH,) are higher than that of I. On the other hand it was indicated that the CT complex III (X =phenyl)-DDQ behaves as in the dissociated state, therefore one can deduce that the III-DDQ CT complex is stronger than the other I, II-DDQ CT complexes, i.e. the electron donating property of III is very pronounced. If the n-contribution has a significant role, the basic properties of the 5-phenyl derivative (III) should not differ from those of the 5-H derivative (I) and at the same time it is expected to be lower than that of the 5methyl derivative (II). Bearing in mind these considerations, as well as the fact that the 1,3,4-thiadiazole is a planar compound [20], one can conclude that the CT complexes of the studied 2-amino-1,3,4-thiadiazoles with the n-acceptor are predominantly of n-n type. Accordingly, the very strong character of the CT complex of the 5-phenyl derivative (III) could be explained by the fact that the n-conjugation in this compound is pronounced since it assumed a planar configuration. In this respect it is worth mentioning that previous NMR studies on the 5-substituents of 1,3,4-thiadiazoles in various solvents suggested the tendency of these compounds to form n-molecules with benzene [20]. Moreover, examination of the Kc, values of the CT complex of I with the three n-acceptors used reveals that its stability decreases on goin,g from DDQ- TCNE -tCHL. This is consistent with the
IO
1.24
II-DDQ
under the condition of D” > A”[D” = 0.01-0.14 mol dm-3 and A”= 1 x 10m3 moldme (DDQ), 8 5 x 10m4 mol drnm3 x 1O-3 mol dme3 (TCNE), (CHL)]. The mean Kc, and OCRvalues obtained are given in Table 2.
ionization potential (ip.) and formation constant parameters of the different CT complexes
4nar
1187
(dm3mol-‘cm-i) 10°C 2115
-AH” (kJ mol-i)
(b/a)’
17.96 + 0.35 -
0.069
2019
12.84 * 1.17
8.30 :56
0.99
1.03 & 0.17
0.73 5 0.04
0.64 & 0.08
6.20 II
4.26 + 0.22
112
49.03 Zh 0.98
0.16
0.53 f 0.04 -
30820
22.30
0.066
I
0%
M. R. MAHMOUDet al.
1188
decrease in electron affinity of the n-acceptor same direction.
in the
Thermodynamic parameters of the CT complexes
The enthalpy changes (AH) connected with the complex formation for the.CT complexes of I with the three n-acceptors studied were determined from the formation constant values at a series of temperatures (Table 2) making use of Van? Hoff equation plots. The data obtained are given in Table 2. According to MULLIKEN [21,22], the ratio between the coefficient of the dative bond to the non-bond wave functions (b/a) was calculated. The obtained ratio (Table 2) is compared to those of many strong CT complexes [9]. Characterization of the solid complexes
The results of chemical analysis of the synthesized solid CT complexes (Table 1) indicate the formation of a 1: 1 CT complex. A comparison of the important i.r. spectral bands of the free donors and acceptors with those of the complexes gives some information on the nature of the CT complexes formed. Generally the VNH of the donors I-III, vcN of the acceptors DDQ, TCNE and vcc, of the DDQ are shifted to lower frequencies on complex formation. This behaviour is in accordance with charge migration from the donor to acceptor. The extent of shift in vccl on complexation with the donors I-III (appearing at 772, 768 and 762 cm- ’ respectively) copes with the determined stability of the formed complex. REFERENCES [l] R. P. LANG, J. Am. them. Sot. 84,4438 (1962). [2] R. ZAHRADNIK and C. PARKANYI,Collect. Czech. Chem.
Commun. 30, 195 (1965).
c31 A. R. COOPER, C. W. CROWNE and P. G. FARRELL, Trans. Faraday Sot. 62, 18 (1966). c41 Z. YOSHIDA and T. KOBAYASHI,Tetrahedron 26, 261 (1970). PI A. R. COOPER, C. W. CROWNE and P. G. FARRELL, Trans. Faraday Sot. 62, 13 (1966). R. FOSTERand P. HANSON,Tetrahedron 21,255 (1965). ;t; R. FOSTER and P. HANSON, Trans. Faraday Sot. 60, 2189 (1964). PI E. I. GINNS and R. L. STRONG, J. phys. Chem. 71,3059 (1967). c91 R. ABU-EITTAHand M. M. HAMED, Can. J. Chem. 57, WI
[l l] [ 121
[13] [14] [15] [16] [173 [18] [ 191 [20]
[21]
[223
2337 (1979). G. KORNIS, in Comprehensive Heterocyclic Chemistry, Vol. 6, Part 4B, pp. 575-577 (edited by A. KATRITZKY, C. W. REESand K. T. Ports). Pergamon Press, Oxford ,.^_ (1Y84). V. R. RAO and V. R. SRINIVASAN, Indian J. Chem. 8,509 (1969). G. KORNIS, in ComprehensiveHeterocyclic Chemistry, Vol. 6, Part 4B, p. 557 (edited by A. KATRITZKY,C. W. REESand K. T. POTTS).Pergamon Press, Oxford (1984). G. BRIEGLEB,Angew. Chem. 76, 326 (1974); Angew. Chem. Int. Ed. Engl. 3, 617 (1964). R. D. SRIVASTAVAand G. PRASAD, Bull. Chem. Sot. Japan 43, 1611 (1970). J. JORTNERand U. SOKOLOV, Nature 1!30,1003 (1961). M. BATLEYand L. E. LYONES, Nature I%, 573 (1962). G. G. ALOISI and S. PIGNATARO,J. them. Sot. Faraday Trans. I 69, 534 (1973). H. A. BENESIand J. H. HILDEBRAND,J. Am. them. Sot. 71, 2703 (1949). R. L. SCOT, Reck Two. Chim. 75, 787 (1956). G. KORNIS, in Comprehensiue Heterocyclic Chemistry, Vol. 6, Part 4B, pp. 547, 549 (edited by A. KATRITZKY, C. W. REESand K. T. POTTS).Pergamon Press, Oxford (1984). R. S. MULLIKEN,J. Am. them. Sot. 74, 811 (1952); J. phys. Gem. 56, 801 (1952); Reel. Trau. Chim. 75, 845 (19561: J. Gem. Phvs. 61. 20 119641. k. S. ‘MULLIKEN aid W.’ P. PER&N, Ann. Rev. phys. Chem. 13,107 (1962); J. Am. them. Sot. 91,3409 (1969).