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Spectroscopic study of structure and intermolecular interactions of diphenylformamidine and diphenylacetamidine in solution
Journal of Molecular Structure, 263 (1991) 37-44 Elsevier Science Publishers B.V., Amsterdam
37
Spectroscopic study of structure and intermolecular interactions of diphenylformamidine and diphenylacetamidine in solution S.F. Bureiko and I.V. Chernyshova Znstitute of Physics, Leningrad University, Petroduorets, 198904 Leningrad (USSR) (Received 7 May 1991)
Abstract The conformational structures of diphenylformamidine (DPFA) and diphenylacetamidine (DPAA) have been studied by measuring the IR and NMR spectra in solution. The structures of self-associates and hydrogen-bonded complexes with acetic acid were determined. The formation of cyclic dimers of DPFA and open associates of DPAA was proved. Complexes of both amidines with acetic acid were found to be cyclic. Spectral and thermodynamic parameters of cyclic association were obtained.
INTRODUCTION
Compounds containing the amidino-group (amidines) are known to be biologically active. Their pharmaceutical and chemical properties are expected to be strongly dependent on their conformational structure [ 11. The problem of the conformational analysis of amidines has been the subject of a good many papers (see for example ref. 1). However, the data do not seem to be unequivocal due to the contradictory character of the interpretation of IR and NMR spectra. Prevorhek [2,3] has studied the structure of some N,N’-diary1 substituted amidines in CHCl, solution by IR spectroscopy. The data obtained were interpreted in terms of a trans configuration of molecules that cannot form Hbonded dimeric cycles. Disubstituted amidines are of primary interest from the point of view of investigation of cyclic H-bonded complexes. It is the cyclic complexes that play the key role in the hydrogen exchange reactions [ 41. The bifunctional molecules with a proton donor NH-group and proton acceptor nitrogen atom may be considered, in a certain sense, to be analogous to such compounds as carboxylic acids, 3,&dimethylpyrazole and diphenyltriazene which form cyclic Hbonded complexes [ 4,5]. Cyclic dimeric association of diphenylformamidine (DPFA) was assumed in the low temperature study [6] of tetrahydrofuran
0022-2860/91/$03.50
0 1991 Elsevier Science Publishers B.V. All rights reserved.
38
and benzene solutions of DPFA. The same conclusion was made in ref. 7 for crystalline diphenylacetamidine (DPAA) . In the present paper we have investigated the conformational structure of DPFA and DPAA in solution and the structure of their self-associates and complexes with acetic acid. We have obtained spectral and thermodynamic characteristics of cyclic associations which involve the amidines.
EXPERIMENTAL
DPFA and DPAA were synthesized as described in refs. 8 and 9. The purity of the compounds obtained was checked by melting point coincidence, correspondence of A,,, in the UV spectrum [lo] and characteristic IR band frequencies known from refs. 2, 3 and 7. The solvents used (Ccl,, CHC&) were purified and dried. The other compounds were high purity commercial products. The deuterated amidines were prepared by recrystallization from ethanold just before use and were dried under vacuum. The IR spectra were recorded on Hitachi 270-30 and UR-20 spectrometers at temperatures ranging from 174-342 K, measured with an accuracy of 1 K. The cell length was varied from 0.1-20 mm. The concentration range was lo-*1M. The NMR spectrum at room temperature was recorded on a Jeol C-60 HL spectrometer in deuterochloroform ( > 99.8 at.% D), using tetramethylsilane as internal standard.
RESULTS AND DISCUSSION
The infrared spectra of DPAA (Fig. 1) in dilute CDC& solutions show two bands in the vNH region near 3384 and 3446 cm-‘. It is significant that the low frequency band has an asymmetric shape and its half-width is w 25 cm-l while that of the high frequency band is M 17 cm-l. This feature has not been reported in earlier papers [ 2,3,7]. The 3384 cm-’ band is obviously a complicated band, as is clear from the spectra in Ccl, (Fig. 1) . The similar structures for the low frequency band of the ND-stretching vibration and for one of the first vNH overtones led us to exclude Fermi resonance as a probable reason for the phenomenon. The observed doublet structure of the low frequency vNH band (3384 and 3398 cm- ’ ) in Ccl, seems to be consistent with the presence of a third conformer. This hypothesis was supported by factor analysis [ 111 of the doublet shape. N,N’ -disubstituted amidines have four possible configurations:
39
Ph
Ph
\N R-L’ \
‘N R-i’
N-H
Ph' (E,E)
\
N-Ph
N-Ph
N-Ph
H' (E,Z)
R-V \
N-H / Ph
IZ,E)
R-i’ \
HiN-ph (Z,Z)
Owing to steric hindrance [l-3] the fraction of ZZ-form is very small. If we assign the 3453 cm-’ band (in Ccl,) to the EZ-isomer and the 3384 cm-l band to the Z&isomer, as in refs. 2 and 3, it is likely that the intermediate band (3398 cm- ’ ) originates from the E&isomer. As a matter of fact the EEisomer is a product of rotational isomerism of the EZ-structure with respect to the single C-N bond, just as the EZ-isomer yields the ZIGisomer by combination of two rotations with respect to the C=N and C-N bonds. Consequently, one can expect larger differences in frequencies for corresponding vNH bands in the latter case. This hypothesis agrees with the decreasing frequency of NH stretching vibrations in the series of EZ, EE and ZE conformers due to dipole-dipole interaction of the NH-group with the lone pair (EE) and the phenyl ring (ZE) . The increase in DPAA concentration is accompanied by the appearance of broad absorption at 3300 cm-’ and lower (Fig. 1). Assignment of the new bands to H-bonded NH stretching vibrations is confirmed by the spectra of deutero-substituted DPAA. Similar concentration studies have been carried out for DPFA (Fig. 2). Surprisingly, we see the opposite distribution of intensities in the NH stretching region. Furthermore, a smaller frequency difference for two vNH bands ( z 40 cm-’ >in comparison with DPAA ( z 68 cm-‘) is observed. In the case of DPFA,
Fig. 1. IR spectra of DPAA solutions: (1) 0.003 M (d=5 mm); (2) 0.05 M (dz0.547 0.4 M (d=0.12 mm); (4) 1 M (d=0.068 mm); (2-4) in CDC&, (1) in Ccl,.
mm); (3)
0.3
B.1
0.4 9400
3000
2600 Cm
-1
Fig. 2. The spectra of DPFA in CDCl, solution (d=l.Ol 0.1 M.
mm): (1) 0.0125 M; (2) 0.025 M; (3)
both components have the same half-widths. In the ‘H NMR spectra of DPFA we have observed two NH proton signals. This means that this compound forms two isomers in solution. The monomolecular equilibrium constant estimated from the NMR spectrum in CDC& is about 5.8. From the comparison of frequencies of the NH stretching bands of DPAA and DPFA we attributed the band at 3405 cm-’ to the E&isomer and the weak band at 3445 cm-’ to the E.&isomer. It is important to note that in the case of DPFA the equilibrium is shifted in favour of the ES-isomer which can form cyclic associates. We also examined the DPFA spectra in the 1100-1700 cm-’ region. It is possible to see that the “amidine II” band near 1525 cm-l is missing. This fact agrees with the predominance of the E&isomer [l-3]. With the increase in concentration of DPFA, the associated NH stretching band with its centre of gravity at 3005 cm-’ (Fig. 2) appeared. A similar complicated band of a self-associate was observed for pyrazole [12]. The band shape proved to be sensitive to deutero-substitution and to be independent of temperature and concentration. Therefore, it cannot be explained as a result of the formation of complexes with different structures but may be interpreted in terms of Fermi resonance. In order to clarify the structure of self-associated species we have used the following numerical analysis of extinction coefficients in the maxima of associated vNH stretching bands, D,,,, vs. that of free vNH, D,. This method is analogous to that presented in ref. 13. A priori one can imagine two situations. (A) Cyclic self-association dominates, then
41 where e1 and E,,, C, and C,. are the molar extinction coefficients in the maxima (E= D/C-d) and the concentrations of associated and free vNH stretching bands, respectively, and d is the cell length. If we further denote the order of self-association as n (for this procedure any integer n is taken), then
Consequently the ratio D,,/D;does not depend on D,. (B ) The self-associate has an open structure. In this case terminal NHgroups contribute to the intensity of free vNH stretching vibration bands and the ratio D,,,/D y is dependent on concentration:
D 888 -= W
1 D1 + const.
Therefore the character of the relationship between D,,,/D~ and D, may indicate either a cyclic or an open structure of the self-associates. Any deviation of the D,,,/D; vs. D,plot from a constant value reveals the occurrence of open association. An apparent lack (within 5% accuracy) of any dependence of D,,/D 7 against d,was found for DPFA, while for DPAA the value strongly depends on the concentration. Thus, DPFA self-associates are mainly cyclic. At the same time the self-associates of DPAA in solution seem to be open. These conclusions agree with the preceding assumptions about the structure of the monomers. The cyclic structure of DPFA self-associates is confirmed by the temperature behaviour of the spectrum in CH,Cl,. The increase in the intensity of the associated band is accompanied by a simultaneous decrease, and eventually the disappearance, of the free vNH band as the temperature fell from 293 K to 174 K. If a significant quantity of open complexes was present the free YNH band should not vanish. If logarithms are taken, eqn. (1) becomes ln L
= nln
D, + const.
and the number of molecules, n, forming the cycle can be determined graphically or numerically. For DPFA, n is estimated as 2 to an accuracy of 5%. Consequently, DPFA self-associates are mainly cyclic dimers: Ph \ /Ph N...H-N R-c// ' C-R \ N-H...N/' \ Px Ph The next step consists of the study of the interaction of amidine molecules
42
with acetic acid. The addition of an equimolar quantity of acid into a solution of DPFA or DPAA (not containing self-associates) leads to the spectral features (Fig. 3) typical for the interaction of 3,Glimethylpyrazole or diphenyltriazene with acids [ 451. Firstly, it consists of a wide intense absorption with maxima at 3000,250O and 1900 cm-l (for DPFA) or at 2550,190O and 1450 cm-’ (for DPAA), caused by a strong OH. **N H-bond (so-called “A, B, C” bands according to Had% [ 141) . Similar bands were observed in the spectra of amidines with the strong base-pyridine. We have assigned them to associated vOH vibrations in amidine-carboxylic acid complexes. Secondly, the band at 3210 cm-’ (DPFA) or 3270 cm-l (DPAA) is observed. The free vNH band of amidines does not undergo a weak (about 5-10 cm-‘) shift to lower frequencies. This shift should take place due to electron density redistribution if the complex formed contains only one H-bond of the OH* **N type [ 51. Following earlier studies [4,5] it seems natural to assign the 3210 and 3270 cm-’ bands to the associated vNH. *~0 vibration in the cyclic complex: Ph \ ,y.. .H-0, R-i:
/ fCH
\
N-H..
Ph’
The cyclic structure of the complex DPAA-acetic acid is further confirmed by spectra in the 1100-1800 cm-’ region. The interaction with acetic acid leads to a large high-frequency shift of the 6 NH-band ( > 40 cm-l). In the case of self-association, the same value is about 18 cm-l. This curious fact is probably
Fig. 3. The spectra of the amidine-acetic acid systems in Ccl, solution: (1) CDPAA=Caeid=0.1 M (d=0.212 mm); (2) CDP,=Caeid=0.002 M (d=lO mm).
43 TABLE 1 Thermodynamic characteristics of cyclic complexes involving amidines in CDCl, solution Complex
K295x (1 mol-‘)
Self-associate of DPFA
20f4
DPFA-acetic acid
11ooIf: 100
DPAA-acetic acid
500 * 50
-AH (kcal mol- ’ )
AS (cal mol-’ K-l)
5.0fO.l
llfl
8.5kO.5
13*1
11 *1
15f3
due to the formation of a cyclic associate with significantly different H-bonds (the first one is much stronger than the second) when the strong bond strengthens the weak one. This effect was observed earlier [ 4,5] and was confirmed by the position of the associated vNH band following interaction with different proton acceptors. Unfortunately, we can say nothing about the behaviour of other amidino-group vibrations due to strong overlapping of acid and amidine bands in this spectral region. The spectra of both DPFA and DPAA with acetic acid are characterized also by a large shift in the associated vC=O ( z 60 cm-‘) band. Under interaction with the acid, the frequency of vC=O is more than 20 cm-’ lower in comparison with the open complex of acetic acid which has one strong H-bond OH* **N (for example, in the complex with pyridine or tributylamine) . All this confirms the idea of cyclic complex formation by interaction between amidine and acid molecules. Therefore, the N,N’-diphenylamidines studied have different conformational structures and different self-associated species in solutions. However, when they interact with acetic acid the amidines form cyclic complexes only. In the case of DPAA, the influence of the acid induces rotational isomerism with respect to the C-N bond. The possibility of analogous processes was shown in ref. 15. The conclusions about the cyclic structures of DPFA self-associates and of the complexes of both amidines with acetic acid are in agreement with the absence of a concentration dependence for the equilibrium constant Kin chloroform solutions. The LIHand dS values for the formation of the cyclic complexes were calculated from the temperature changes of the equilibrium constants and are reported in Table 1.
REFERENCES 1 2
S. Patai, (Ed.), The Chemistry of Amidines and Imidates, Wiley, London, 1975. D. Prevo&k, J. Phys. Chem., 66 (1962) 769.
44 3 4 5 6 7 8 9 10 11 12 13 14 15
D. Prevor%k, Bull. Sot. Chim. Fr., (1958) 788. S.F. Bureiko and I.V. Chernyshova, Ann. Acad. Sci. Fenn., Ser. A II, 227 (1990) 205. S.F. Bureiko, I.V. Chernyshova and N.S. Golubev, Khim. Fizz., (Sov. J. Chem. Phys.), 6 (1987) 176. N.E. White and M. Kilpatrick, J. Phys. Chem., 59 (1955) 1044. P. Sohar, Acta Chim. Acad. Sci. Hung., 54 (1967) 91. R.L. Shriner and F.W. Neumann, Chem. Rev., 35 (1944) 351. K. Caisen, Ann. Chem., 287 (1895) 360. W. Bradley and I. Wright, J. Chem. Sot., (1956) 640. E.R. Malinowski and D.G. Howery, Factor Analysis in Chemistry, Wiley, New York, 1980. D.M. Anderson, J.L. Duncan and F.J.C. Rossotti, J. Chem. Sot., (1961) 140. G.C. Pimentel and A.L. McClellan, The Hydrogen Bond, Freeman, San Francisco, 1960. D. Had&i and S. Bratos, in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond, Vol. 2, North-Holland, Amsterdam, 1976, p. 565. I.D. Cunningham and A.F. Herarty, J. Chem. Sot. Perkin. Trans. II, (1986) 537.
37
Spectroscopic study of structure and intermolecular interactions of diphenylformamidine and diphenylacetamidine in solution S.F. Bureiko and I.V. Chernyshova Znstitute of Physics, Leningrad University, Petroduorets, 198904 Leningrad (USSR) (Received 7 May 1991)
Abstract The conformational structures of diphenylformamidine (DPFA) and diphenylacetamidine (DPAA) have been studied by measuring the IR and NMR spectra in solution. The structures of self-associates and hydrogen-bonded complexes with acetic acid were determined. The formation of cyclic dimers of DPFA and open associates of DPAA was proved. Complexes of both amidines with acetic acid were found to be cyclic. Spectral and thermodynamic parameters of cyclic association were obtained.
INTRODUCTION
Compounds containing the amidino-group (amidines) are known to be biologically active. Their pharmaceutical and chemical properties are expected to be strongly dependent on their conformational structure [ 11. The problem of the conformational analysis of amidines has been the subject of a good many papers (see for example ref. 1). However, the data do not seem to be unequivocal due to the contradictory character of the interpretation of IR and NMR spectra. Prevorhek [2,3] has studied the structure of some N,N’-diary1 substituted amidines in CHCl, solution by IR spectroscopy. The data obtained were interpreted in terms of a trans configuration of molecules that cannot form Hbonded dimeric cycles. Disubstituted amidines are of primary interest from the point of view of investigation of cyclic H-bonded complexes. It is the cyclic complexes that play the key role in the hydrogen exchange reactions [ 41. The bifunctional molecules with a proton donor NH-group and proton acceptor nitrogen atom may be considered, in a certain sense, to be analogous to such compounds as carboxylic acids, 3,&dimethylpyrazole and diphenyltriazene which form cyclic Hbonded complexes [ 4,5]. Cyclic dimeric association of diphenylformamidine (DPFA) was assumed in the low temperature study [6] of tetrahydrofuran
0022-2860/91/$03.50
0 1991 Elsevier Science Publishers B.V. All rights reserved.
38
and benzene solutions of DPFA. The same conclusion was made in ref. 7 for crystalline diphenylacetamidine (DPAA) . In the present paper we have investigated the conformational structure of DPFA and DPAA in solution and the structure of their self-associates and complexes with acetic acid. We have obtained spectral and thermodynamic characteristics of cyclic associations which involve the amidines.
EXPERIMENTAL
DPFA and DPAA were synthesized as described in refs. 8 and 9. The purity of the compounds obtained was checked by melting point coincidence, correspondence of A,,, in the UV spectrum [lo] and characteristic IR band frequencies known from refs. 2, 3 and 7. The solvents used (Ccl,, CHC&) were purified and dried. The other compounds were high purity commercial products. The deuterated amidines were prepared by recrystallization from ethanold just before use and were dried under vacuum. The IR spectra were recorded on Hitachi 270-30 and UR-20 spectrometers at temperatures ranging from 174-342 K, measured with an accuracy of 1 K. The cell length was varied from 0.1-20 mm. The concentration range was lo-*1M. The NMR spectrum at room temperature was recorded on a Jeol C-60 HL spectrometer in deuterochloroform ( > 99.8 at.% D), using tetramethylsilane as internal standard.
RESULTS AND DISCUSSION
The infrared spectra of DPAA (Fig. 1) in dilute CDC& solutions show two bands in the vNH region near 3384 and 3446 cm-‘. It is significant that the low frequency band has an asymmetric shape and its half-width is w 25 cm-l while that of the high frequency band is M 17 cm-l. This feature has not been reported in earlier papers [ 2,3,7]. The 3384 cm-’ band is obviously a complicated band, as is clear from the spectra in Ccl, (Fig. 1) . The similar structures for the low frequency band of the ND-stretching vibration and for one of the first vNH overtones led us to exclude Fermi resonance as a probable reason for the phenomenon. The observed doublet structure of the low frequency vNH band (3384 and 3398 cm- ’ ) in Ccl, seems to be consistent with the presence of a third conformer. This hypothesis was supported by factor analysis [ 111 of the doublet shape. N,N’ -disubstituted amidines have four possible configurations:
39
Ph
Ph
\N R-L’ \
‘N R-i’
N-H
Ph' (E,E)
\
N-Ph
N-Ph
N-Ph
H' (E,Z)
R-V \
N-H / Ph
IZ,E)
R-i’ \
HiN-ph (Z,Z)
Owing to steric hindrance [l-3] the fraction of ZZ-form is very small. If we assign the 3453 cm-’ band (in Ccl,) to the EZ-isomer and the 3384 cm-l band to the Z&isomer, as in refs. 2 and 3, it is likely that the intermediate band (3398 cm- ’ ) originates from the E&isomer. As a matter of fact the EEisomer is a product of rotational isomerism of the EZ-structure with respect to the single C-N bond, just as the EZ-isomer yields the ZIGisomer by combination of two rotations with respect to the C=N and C-N bonds. Consequently, one can expect larger differences in frequencies for corresponding vNH bands in the latter case. This hypothesis agrees with the decreasing frequency of NH stretching vibrations in the series of EZ, EE and ZE conformers due to dipole-dipole interaction of the NH-group with the lone pair (EE) and the phenyl ring (ZE) . The increase in DPAA concentration is accompanied by the appearance of broad absorption at 3300 cm-’ and lower (Fig. 1). Assignment of the new bands to H-bonded NH stretching vibrations is confirmed by the spectra of deutero-substituted DPAA. Similar concentration studies have been carried out for DPFA (Fig. 2). Surprisingly, we see the opposite distribution of intensities in the NH stretching region. Furthermore, a smaller frequency difference for two vNH bands ( z 40 cm-’ >in comparison with DPAA ( z 68 cm-‘) is observed. In the case of DPFA,
Fig. 1. IR spectra of DPAA solutions: (1) 0.003 M (d=5 mm); (2) 0.05 M (dz0.547 0.4 M (d=0.12 mm); (4) 1 M (d=0.068 mm); (2-4) in CDC&, (1) in Ccl,.
mm); (3)
0.3
B.1
0.4 9400
3000
2600 Cm
-1
Fig. 2. The spectra of DPFA in CDCl, solution (d=l.Ol 0.1 M.
mm): (1) 0.0125 M; (2) 0.025 M; (3)
both components have the same half-widths. In the ‘H NMR spectra of DPFA we have observed two NH proton signals. This means that this compound forms two isomers in solution. The monomolecular equilibrium constant estimated from the NMR spectrum in CDC& is about 5.8. From the comparison of frequencies of the NH stretching bands of DPAA and DPFA we attributed the band at 3405 cm-’ to the E&isomer and the weak band at 3445 cm-’ to the E.&isomer. It is important to note that in the case of DPFA the equilibrium is shifted in favour of the ES-isomer which can form cyclic associates. We also examined the DPFA spectra in the 1100-1700 cm-’ region. It is possible to see that the “amidine II” band near 1525 cm-l is missing. This fact agrees with the predominance of the E&isomer [l-3]. With the increase in concentration of DPFA, the associated NH stretching band with its centre of gravity at 3005 cm-’ (Fig. 2) appeared. A similar complicated band of a self-associate was observed for pyrazole [12]. The band shape proved to be sensitive to deutero-substitution and to be independent of temperature and concentration. Therefore, it cannot be explained as a result of the formation of complexes with different structures but may be interpreted in terms of Fermi resonance. In order to clarify the structure of self-associated species we have used the following numerical analysis of extinction coefficients in the maxima of associated vNH stretching bands, D,,,, vs. that of free vNH, D,. This method is analogous to that presented in ref. 13. A priori one can imagine two situations. (A) Cyclic self-association dominates, then
41 where e1 and E,,, C, and C,. are the molar extinction coefficients in the maxima (E= D/C-d) and the concentrations of associated and free vNH stretching bands, respectively, and d is the cell length. If we further denote the order of self-association as n (for this procedure any integer n is taken), then
Consequently the ratio D,,/D;does not depend on D,. (B ) The self-associate has an open structure. In this case terminal NHgroups contribute to the intensity of free vNH stretching vibration bands and the ratio D,,,/D y is dependent on concentration:
D 888 -= W
1 D1 + const.
Therefore the character of the relationship between D,,,/D~ and D, may indicate either a cyclic or an open structure of the self-associates. Any deviation of the D,,,/D; vs. D,plot from a constant value reveals the occurrence of open association. An apparent lack (within 5% accuracy) of any dependence of D,,/D 7 against d,was found for DPFA, while for DPAA the value strongly depends on the concentration. Thus, DPFA self-associates are mainly cyclic. At the same time the self-associates of DPAA in solution seem to be open. These conclusions agree with the preceding assumptions about the structure of the monomers. The cyclic structure of DPFA self-associates is confirmed by the temperature behaviour of the spectrum in CH,Cl,. The increase in the intensity of the associated band is accompanied by a simultaneous decrease, and eventually the disappearance, of the free vNH band as the temperature fell from 293 K to 174 K. If a significant quantity of open complexes was present the free YNH band should not vanish. If logarithms are taken, eqn. (1) becomes ln L
= nln
D, + const.
and the number of molecules, n, forming the cycle can be determined graphically or numerically. For DPFA, n is estimated as 2 to an accuracy of 5%. Consequently, DPFA self-associates are mainly cyclic dimers: Ph \ /Ph N...H-N R-c// ' C-R \ N-H...N/' \ Px Ph The next step consists of the study of the interaction of amidine molecules
42
with acetic acid. The addition of an equimolar quantity of acid into a solution of DPFA or DPAA (not containing self-associates) leads to the spectral features (Fig. 3) typical for the interaction of 3,Glimethylpyrazole or diphenyltriazene with acids [ 451. Firstly, it consists of a wide intense absorption with maxima at 3000,250O and 1900 cm-l (for DPFA) or at 2550,190O and 1450 cm-’ (for DPAA), caused by a strong OH. **N H-bond (so-called “A, B, C” bands according to Had% [ 141) . Similar bands were observed in the spectra of amidines with the strong base-pyridine. We have assigned them to associated vOH vibrations in amidine-carboxylic acid complexes. Secondly, the band at 3210 cm-’ (DPFA) or 3270 cm-l (DPAA) is observed. The free vNH band of amidines does not undergo a weak (about 5-10 cm-‘) shift to lower frequencies. This shift should take place due to electron density redistribution if the complex formed contains only one H-bond of the OH* **N type [ 51. Following earlier studies [4,5] it seems natural to assign the 3210 and 3270 cm-’ bands to the associated vNH. *~0 vibration in the cyclic complex: Ph \ ,y.. .H-0, R-i:
/ fCH
\
N-H..
Ph’
The cyclic structure of the complex DPAA-acetic acid is further confirmed by spectra in the 1100-1800 cm-’ region. The interaction with acetic acid leads to a large high-frequency shift of the 6 NH-band ( > 40 cm-l). In the case of self-association, the same value is about 18 cm-l. This curious fact is probably
Fig. 3. The spectra of the amidine-acetic acid systems in Ccl, solution: (1) CDPAA=Caeid=0.1 M (d=0.212 mm); (2) CDP,=Caeid=0.002 M (d=lO mm).
43 TABLE 1 Thermodynamic characteristics of cyclic complexes involving amidines in CDCl, solution Complex
K295x (1 mol-‘)
Self-associate of DPFA
20f4
DPFA-acetic acid
11ooIf: 100
DPAA-acetic acid
500 * 50
-AH (kcal mol- ’ )
AS (cal mol-’ K-l)
5.0fO.l
llfl
8.5kO.5
13*1
11 *1
15f3
due to the formation of a cyclic associate with significantly different H-bonds (the first one is much stronger than the second) when the strong bond strengthens the weak one. This effect was observed earlier [ 4,5] and was confirmed by the position of the associated vNH band following interaction with different proton acceptors. Unfortunately, we can say nothing about the behaviour of other amidino-group vibrations due to strong overlapping of acid and amidine bands in this spectral region. The spectra of both DPFA and DPAA with acetic acid are characterized also by a large shift in the associated vC=O ( z 60 cm-‘) band. Under interaction with the acid, the frequency of vC=O is more than 20 cm-’ lower in comparison with the open complex of acetic acid which has one strong H-bond OH* **N (for example, in the complex with pyridine or tributylamine) . All this confirms the idea of cyclic complex formation by interaction between amidine and acid molecules. Therefore, the N,N’-diphenylamidines studied have different conformational structures and different self-associated species in solutions. However, when they interact with acetic acid the amidines form cyclic complexes only. In the case of DPAA, the influence of the acid induces rotational isomerism with respect to the C-N bond. The possibility of analogous processes was shown in ref. 15. The conclusions about the cyclic structures of DPFA self-associates and of the complexes of both amidines with acetic acid are in agreement with the absence of a concentration dependence for the equilibrium constant Kin chloroform solutions. The LIHand dS values for the formation of the cyclic complexes were calculated from the temperature changes of the equilibrium constants and are reported in Table 1.
REFERENCES 1 2
S. Patai, (Ed.), The Chemistry of Amidines and Imidates, Wiley, London, 1975. D. Prevo&k, J. Phys. Chem., 66 (1962) 769.
44 3 4 5 6 7 8 9 10 11 12 13 14 15
D. Prevor%k, Bull. Sot. Chim. Fr., (1958) 788. S.F. Bureiko and I.V. Chernyshova, Ann. Acad. Sci. Fenn., Ser. A II, 227 (1990) 205. S.F. Bureiko, I.V. Chernyshova and N.S. Golubev, Khim. Fizz., (Sov. J. Chem. Phys.), 6 (1987) 176. N.E. White and M. Kilpatrick, J. Phys. Chem., 59 (1955) 1044. P. Sohar, Acta Chim. Acad. Sci. Hung., 54 (1967) 91. R.L. Shriner and F.W. Neumann, Chem. Rev., 35 (1944) 351. K. Caisen, Ann. Chem., 287 (1895) 360. W. Bradley and I. Wright, J. Chem. Sot., (1956) 640. E.R. Malinowski and D.G. Howery, Factor Analysis in Chemistry, Wiley, New York, 1980. D.M. Anderson, J.L. Duncan and F.J.C. Rossotti, J. Chem. Sot., (1961) 140. G.C. Pimentel and A.L. McClellan, The Hydrogen Bond, Freeman, San Francisco, 1960. D. Had&i and S. Bratos, in P. Schuster, G. Zundel and C. Sandorfy (Eds.), The Hydrogen Bond, Vol. 2, North-Holland, Amsterdam, 1976, p. 565. I.D. Cunningham and A.F. Herarty, J. Chem. Sot. Perkin. Trans. II, (1986) 537.