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J. Phys. Chem. 1996, 100, 17762-17765
Solvatochromism in N-(2-Hydroxybenzylidene)aniline, N-(2-Hydroxybenzylidene)benzylamine, and N-(2-Hydroxybenzylidene)-2-phenylethylamine Dina Gegiou, Eugenia Lambi, and Eugene Hadjoudis* Institute of Physical Chemistry, NCSR “Democritos”, 153 10 Aghia ParaskeVi Attikis, Greece ReceiVed: April 17, 1996; In Final Form: August 21, 1996X
Solvatochromism is found stronger in N-(2-hydroxybenzylidene)benzylamine (II) and N-(2-hydroxybenzylidene)-2-phenylethylamine (III) than in N-(2-hydroxybenzylidene)aniline (I) because of the increased basicity of the imine nitrogen in the ground state. Electronic absorption and FT-IR spectroscopic studies have provided evidence for an enol-keto tautomerism in all three compounds investigated in methanol, while both keto and protonated tautomers have been observed in their solutions in 2,2,2-trifluoroethanol and in 1,1,1,3,3,3hexafluoro-2-propanol. The increase of the CdN stretching frequency upon protonation is found to be 2118 cm-1 in the compounds investigated in acidic solvents. The higher frequency of the quadrant doublet in the FT-IR spectrum of I is associated with the aromatic stretching vibrations of the aniline ring.
Introduction The electronic absorption spectra of aromatic Schiff bases undergo strong changes when solvents are changed from nonpolar to polar hydrogen-bonding ones. A new absorption band develops around 400 nm in polar hydrogen-bonding solvents.1-3 This solvatochromism has been attributed to the enol-keto enamine tautomerism of N-(2-hydroxybenzylidene)aniline (I).4 In contrast, other researchers have concluded that while in solvents with a proton-donating ability lesser than or equal to 2,2,2-trifluoroethanol (TFE) the CdN bond remains intact, in solvents with a proton-donating ability equal to or greater than 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) actual protonation of I occurs in the ground state.5 Recently, it has been suggested that compound I in TFE and HFP undergoes a structural change to a zwitterionic form as evidenced by Raman and NMR spectroscopy with the help of isotopic 15N and deuterium substitution.6 Furthermore, the 15N and 18O shifts of the 1635 cm-1 infrared absorption of I in HFP have been considered as evidence for the CdNH+ stretching frequency contributed from the C-O- stretch as well.7 The phenomenon of solvatochromism appears stronger in N-(2-hydroxybenzylidene)benzylamine (II) and N-(2-hydroxybenzylidene)-2-phenylethylamine (III) than in I because of the insertion of the methylene group (s) between the imine nitrogen and the aniline ring, which eliminates the resonance of the nitrogen lone pair with the π system of the aniline ring; thus, the basicity of the nitrogen atom increases in the ground state.8 The investigation of the solvatochromism of I, II, and III was performed by electronic absorption and FT-IR spectroscopy. The solvatochromism of II and III was investigated for the first time in this work. Structures of these compounds are shown in Figure 1. Experimental Section Materials and Methods. The compounds were synthesized by direct condensation of the 2-hydroxybenzaldehyde with the appropriate amine in ethanol followed by repeated recrystallization from ethanol. IR spectra, melting points (I 51 °C, II 29 °C, III 42-43 °C) and elemental analysis were utilized to establish the purity of the compounds.8 The solvents used X
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01115-X CCC: $12.00
Figure 1. Structures of (a) N-(2-hydroxybenzylidene)aniline (I), N-(2hydroxybenzylidene)benzylamine (II), and N-(2-hydroxybenzylidene)2-phenylethylamine (III), (b) quinoid forms, and (c) protonated forms.
(spectrograde) were supplied by Merck; they were dried over sodium sulfate before use. Spectral Measurements. The absorption spectra of fresh solutions were obtained by means of Cary 17, Perkin-Elmer Lamda 16 and Perkin-Elmer 2000 FT-IR spectrometers. Results Electronic Absorption Spectra. Gradual intensification of solvatochromism is observed in the spectra of I, II, and III in methanol (MeOH) as the methylene groups are inserted between the imine nitrogen and the adjacent phenyl ring. The spectra in MeOH are compared in Figure 2 and those of I in CCl4, MeOH, TFE, HFP, and TFE + HCl (gas) are shown in Figure 3 and of II and III in the same solvents in Figure 4. The solvatochromic band around 400 nm, absent in CCl4, is intensified in acidic TFE compared with that in MeOH and disappears in TFE + HCl (gas) when the hydrochloride is formed absorbing in the region 300-350 nm. The spectrum of I at longer wavelengths in these solvents consists of one absorption band which is gradually blue-shifted in going from MeOH to HFP; a similar shift is also observed in the spectra of II and III from MeOH to TFE. However, the solvatochromic absorption of II and III in HFP is a composite of two absorption bands. Well-defined isosbestic points (at 304 and 350 nm) demonstrate that a single ground state equilibrium is involved in solutions of III in the hexane-ethanol system (Figure 5) as has been shown for solutions of N-(2-hydroxybenzylidene)-2-aminophenol in cy© 1996 American Chemical Society
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J. Phys. Chem., Vol. 100, No. 45, 1996 17763
Figure 2. Electronic absorption spectra, in 10-4 M MeOH of N-(2hydroxybenzylidene) aniline (I), N-(2-hydroxybenzylidene)benzylamine (II), and N-(2-hydroxybenzylidene)-2-phenylethylamine (III).
Figure 5. Electronic absorption spectra of N-(2-hydroxybenzylidene)2-phenylethylamine (III), 2.4 × 10-4 M, in hexane (s), hexane-ethanol 95:5 (‚‚‚), 90:10 (- - -), 50:50 (-‚-), and ethanol (-‚‚-).
Figure 3. Electronic absorption spectra of N-(2-hydroxybenzylidene)aniline (I), 10-4 M, in CCl4 (s), TFE (- - -), HFP (-‚-), TFE saturated with HCl gas (-‚‚-), and 10-4 M, 5 × 10-3 M, in MeOH (‚‚‚).
Figure 6. FT-IR absorption spectra of N-(2-hydroxybenzylidene)aniline (I), 0.1 M: top to bottom in CHCl3, TFE, and HFP.
Figure 4. Electronic absorption spectra of N-(2-hydroxybenzylidene)benzylamine (II) and N-(2-hydroxybenzylidene)-2-phenylethylamine (III), 10-4 M, in CCl4 (s), MeOH (‚‚‚), TFE (- - -), HFP (-‚-), and TFE saturated with HCl gas (-‚‚-).
clohexane-ethanol,2 while no isosbestic points are observed in the spectra of I in the CHCl3-TFE system. FT-IR Spectra. The FT-IR spectra of fresh solutions of I, II, and III in CHCl3, TFE, and HFP, and of III in MeOH, were obtained in the region 1700-1500 cm-1 in order to study the solvent dependence of the stretching frequency of the CdN bond in an attempt to shed some light on the equilibria involved. FT-
IR spectra of 2-hydroxybenzaldehyde in the same solvents were also obtained for use in determining whether any hydrolysis of the imines occurs in the solutions investigated, although the solvents used were dried over sodium sulfate immediately prior to use. The FT-IR spectrum of I in CHCl3 exhibits three absorption bands in the region 1700-1500 cm-1, at 1620, 1594, and 1575 cm-1 (Figure 6). The spectra of II and III in CHCl3 in the same region exhibit only two bands, at 1634, 1582 and 1635, 1583 cm-1, respectively (Figure 7). Thus, insertion of the methylene groups results in the disappearance of the 1594 cm-1 absorption band and in a shift of the remaining bands to higher energies. In TFE, the three absorption bands of I in CHCl3 are shifted to higher frequencies, 1623, 1596, and 1578 cm-1, and a new band appears at 1539 cm-1 with a shoulder at ca. 1534 cm-1, which is reported for the first time in this solvent. In addition, significant broadening of the 1623 cm-1 band and a shoulder at ca. 1635 cm-1 is observed for the first time and the absorption band at 1578 cm-1 decreases ca. 50% (Figure 6). The changes in the FT-IR spectrum of III in MeOH are similar to those in the spectrum of I in TFE; a shoulder is revealed at ca. 1650 cm-1 with a parallel broadening of the main band at 1636 cm-1, followed by a significant decrease in intensity of the absorption at 1584 cm-1 and the appearance of a new band at 1538 cm-1 with a shoulder at ca. 1527 cm-1 (Figure 7). The absorption spectrum of II in TFE exhibits more profound changes than that of I in TFE; the high-frequency band appears to be composed of three overlapping bands at ca. 1637, 1651, and ca. 1658 cm-1, and the 1582 cm-1 band is missing; the
17764 J. Phys. Chem., Vol. 100, No. 45, 1996
Figure 7. FT-IR absorption spectra of N-(2-hydroxybenzylidene)benzylamine (II) and N-(2-hydroxybenzylidene)-2-phenylethylamine (III), 0.1 M: top to bottom in CHCl3, TFE, and HFP (II), and CHCl3, MeOH, TFE, and HFP (III).
absorption at 1647 cm-1 is probably the result of the overlap of the bands at 1637 and 1651 cm-1. Furthermore, two new absorption bands appear at 1613 and 1541 cm-1 with a shoulder at ca. 1536 cm-1. Similar changes are observed in the spectrum of III in TFE; the high-frequency absorption band appears to consist of three overlapping bands at ca. 1639, 1651, and ca. 1658 cm-1; the band at 1538 cm-1 is missing and two new bands appear at 1612 and 1542 cm-1 with a shoulder at ca. 1536 cm-1 (Figure 7). The high-frequency band in the FT-IR spectrum of I in acidic HFP appears to be composed of five overlapping bands at ca. 1612, ca. 1625, 1636, ca. 1644, and ca. 1652 cm-1. In addition, the band at 1578 cm-1 in TFE disappears and the bands at 1596 and 1539 cm-1 in TFE are shifted to 1597 and 1543 cm-1, respectively; the latter band exhibits a shoulder at ca. 1537 cm-1 and its intensity is considerably increased (Figure 6). The highfrequency band of the spectrum of II in HFP is composed of three overlapping bands at ca. 1641, 1651, and ca. 1659 cm-1; the latter is relatively intensified compared to that in TFE. The two new bands are shifted to 1610 and 1547 cm-1 with a shoulder at ca. 1543 cm-1; the intensity of the 1547 cm-1 band remains unchanged. The high-frequency band of the FT-IR spectrum of III in HFP comprises two bands at 1652 and ca. 1659 cm-1, the intensity relationship of which has changed in favor of the band at 1659 cm-1 compared to that in TFE and the two new bands are observed at 1610 and 1547 cm-1 with a shoulder at ca. 1543 cm-1 (Figure 7). Discussion Electronic Absorption Spectra. The electronic absorption spectra of compound III in various solvents, Figure 4, demonstrate that the solvatochromic absorption in HFP is a composite of two bands, the one at longer wavelengths coinciding with the solvatochromic band in MeOH and the other with the absorption of the hydrochloride. The spectrum of III in TFE comprises the same absorption bands but in different intensity ratio. The species absorbing at longer wavelengths is attributed to the quinoid form and the other absorbing at shorter wavelengths to the protonated form (Figure 1). The spectrum of III in MeOH appears not to include absorption of the protonated form. This is also demonstrated by the well-defined isosbestic points of absorption spectra of solutions of III in the system hexane-ethanol (Figure 5). The absorption spectra of compound II are similar to those of III in the same solvents,
Gegiou et al. but the contribution of the quinoid form is relatively larger as expected in this case (Figure 4). The spectrum of I at longer wavelengths in the same solvents consists of one absorption band which gradually shifts to the blue in going from MeOH to HFP (Figure 3). The single solvatochromic band in the spectra of I in these solvents has led to the conclusion that this band in the aforementioned solvents is due to an n-π* transition of the zwitterion of I.6 This assignment is inconsistent with our results which show that in TFE more than one equilibrium occurs since no isosbestic points are observed in the spectra of I in the CHCl3-TFE system in accordance with the results of the spectral studies in HFP.4,5 Our results also demonstrate the presence of two isomers for III in HFP, reflecting the higher basicity of the nitrogen atom among the three compounds investigated. FT-IR Spectra. The IR spectrum of I in nonpolar solvents in the region 1700-1500 cm-1 exhibits three absorption bands, one of which is assigned to the stretching frequency of the CdN bond (1620 cm-1, νCdN) and the other two (1594 and 1575 cm-1) are assigned to quadrant stretching vibrations resolved in substituted benzenes when the substituents are conjugated (Figure 6).9 The lower frequency band of the quadrant doublet has been found to be vibrationally coupled to νCdN through a study of 15N-labeled compounds (νPh+CdN)10 and, recently, to a mode centered on the conjugated HOsCdCsCdN region of the molecule due the sensitivity of the band to both 15N substitution and deuteration.6 More recently, however, the 1575 cm-1 band has not shown a significant 18O shift in a study of I-18O in acetonitrile-d3.7 The vibrational assignments of the FTIR bands are presented in Table 1. The FT-IR spectra of II and III in CHCl3 in the region 15001700 cm-1 exhibit only two absorption bands at 1634, 1582 and 1635, 1583 cm-1, respectively (Figure 7). The insertion of the methylene group(s) results in a shift of the νCdN and νPh+CdN to higher energies and in the disappearance of the higher frequency of the quadrant stretching vibrations. Therefore, the higher frequency of the quadrant doublet is associated with the aromatic stretching vibrations of the aniline moiety (νPh(NH2)), the conjugation of which with the CdN bond is eliminated, and the other at lower frequency with that of the salicylaldehyde moiety. The FT-IR spectrum of I in TFE exhibits two characteristic features: a new absorption band at 1539 cm-1 and a shoulder at ca. 1635 cm-1 on the νCdN band. The new band shifts to 1543 cm-1 and intensifies ca. 5-fold in HFP and does not show any 15N or 18O effects.6,7 Thus, it is assigned to a CdC stretching vibration (νCdC). The band appears also in the spectrum of III in MeOH discussed below, where isosbestic points show that only one equilibrium takes place. Ledbetter has also observed an absorption band at 1544 cm-1 in the spectrum of I in HFP and has assigned it to the νCdC of the bond between the azomethine group and the salicylaldehyde moiety.4 These new bands in the region 1539-1547 cm-1 appear to accompany the absorption of the quinoid tautomers of the three compounds investigated (Table 1). The appearance of the shoulder on the νCdN band is accompanied by a significant decrease in intensity of the νPh+CdN band at 1578 cm-1, as the double bond character of the CdN bond is reduced from that of the bond in aprotic solvents, and by a broadening of the νCdN absorption band; it is assigned to the carbonyl stretching frequency (νCdO) in accordance with the assignments for I in HFP4 and with our results for III in MeOH (see below). Our results show the existence of more than one equilibrium in the solution of I in TFE. However, no new absorption band which may be attributed to the protonated CdN bond (νCdNH+)
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J. Phys. Chem., Vol. 100, No. 45, 1996 17765
TABLE 1: Vibrational Assignmentsa of the FT-IR Bands (cm-1) compd
solventb
I II III I III II III I II III
CHCl3
a
TFE MeOH TFE HFP
νCdNH+
(1658) (1658) (1644) (1659) (1659)
νCdO
(1635) (1650) 1651 1651 1636 1651 1652
νCdN 1620 1634 1635 1623 1636 (1637) (1639) (1625) (1641)
νPh(OH)
νPh(NH2)
νPh+CdN
1594
1575 1582 1583 1578 1584
1596 (1614) 1613 1612 (1612) 1610 1610
1597
νCdC
1539 1540 1541 1542 1543 1547 1547
(1534) (1533) (1536) (1536) (1537) (1543) (1543)
Shoulders are presented in parentheses. b See text for abbreviations.
stretching vibration is detected; this may be due to the small amount of the protonated species present in the solution and consequently to a too weak νCdNH+ absorption band. The νPh+CdN band is missing from the spectrum of I in HFP and instead another new band appears as a shoulder at ca. 1612 cm-1 attributed to the phenyl vibration of the salicylaldehyde moiety (νPh(OH)). The νPh+CdN and the νPh(OH) bands appear also to be mutually exclusive in the spectra of II and III in TFE and HFP (Table 1). The main absorption band of I in HFP at 1636 cm-1 is a composite of five bands: the νPh(OH) at ca. 1612 cm-1, the νCdN at ca. 1625 cm-1, the νCdO at 1636 cm-1, and two bands at 1644 and 1650 cm-1; the former is attributed to the νCdNH+ on the following grounds: the absorption band at 1636 cm-1 undergoes 15N and 18O shifts of 11 and 4 cm-1, respectively. The sensitivity of the band to 15N requires strong participation of the νCdNH+ absorption since the νCdN absorption is reduced (νPh+CdN is missing). Thus, the increase of the CdN stretching frequency upon protonation is 19 cm-1 in good agreement with the cases of II and III in acidic solvents (Table 1) where the phenomena are stronger and the situation is more clear. Aton et al. have proposed that upon protonation the CdNH+ stretching and the CdN+sH deformation are coupled and therefore the observed CdNH+ stretching occurs at higher frequency than that of CdN.11 Lo´pez-Garriga et al. have also found by ab initio calculations that the CdN stretching force constant of methylimine increases upon protonation, causing an increase in the νCdNH+ of ca. 30 cm-1.12 The FT-IR spectrum of III in MeOH, where only a single equilibrium exists, exhibits the same characteristics as the spectrum of I in TFE: a new band at 1540 cm-1, assigned to the νCdC stretching vibration, and a shoulder at 1650 cm-1, assigned to the νCdO and accompanied by a broadening of the νCdN band. The νPh+CdN absorption band in the FT-IR spectra of II and III in TFE and HFP is missing and two new bands appear instead in the region 1540-50 cm-1 and around 1610 cm-1; the behavior is similar to that of I in HFP and the new bands are similarly assigned to the νCdC stretching vibration and to the aromatic stretching vibration of the salicylaldehyde moiety, respectively (Table 1). It should be noted that the absorption bands in the region 1540-50 cm-1 in TFE and HFP are composed of several bands and are of equal intensity in both solvents. This might be explained if the quinoid tautomer would be also protonated; such a tautomer has also been proposed for I in HFP.4,7 The main band in the spectra of II and III in TFE is composed of three bands. The shoulders at ca. 1637 and 1639 cm-1 are assigned to the νCdN, the bands at 1651 cm-1 to the νCdO, and the shoulders at 1658 cm-1 to the νCdNH+. The main absorptions of the FT-IR spectra of II and III in HFP consist of three bands and of only two bands,
respectively. The absorption of the νCdN is practically missing in the spectrum of III, and the shoulders at 1659 cm-1 assigned to the νCdNH+ are more intense in relation to the bands at 1651 and 1652 cm-1 assigned to the νCdO. The assignment is confirmed by the electronic absorption spectra which show that both tautomers are present in the solutions of II and III in TFE and HFP but that the proportion of the protonated species is, as expected, higher in HFP. Thus, protonation of the CdN bond leads to a shift of 21-18 cm-1 of its stretching frequency in the cases of II and III in acidic solvents. Conclusions The higher frequency of the quadrant doublet on the FT-IR spectrum of I is associated with the aromatic stretching vibrations of the aniline moiety, since it is missing from the spectra of II and III where conjugation with the CdN bond is reduced or interrupted by the methylene group(s). The electronic and FT-IR spectral studies presented herein show the existence of a single equilibrium in all compounds investigated in methanol, attributed to an enol-keto equilibrium. In acidic TFE and HFP more than one equilibrium takes place involving enol-keto, enol-protonated species, and possibly keto-keto protonated tautomers. Our results do not establish whether the protonated species has a zwitterionic structure or whether it reflects a mere protonation of the imine nitrogen. The increase of the CdN stretching frequency upon protonation is found to be 21-18 cm-1 in all three compounds. Acknowledgment. We thank Dr. K. Staphylakis and Dr. G. Pistolis for their valuable experimental assistance. References and Notes (1) Charette, J.; Faithairst, G.; Teyssie, Ph. Spectrochim. Acta 1964, 20, 597. (2) Ledbetter, Jr. J. W. J. Phys. Chem. 1966, 70, 2245. (3) Hadjoudis, E. In Photochromism, Molecules and Systems; BouasLaurent, H., Du¨rr, H., Eds.; Elsevier: Amsterdam, 1990; p 685. (4) Ledbetter, Jr. J. W. J. Phys. Chem. 1977, 81, 54. (5) Lewis, J. W.; Sandorfy, C. Can. J. Chem. 1982, 60, 1727. (6) Turbeville, W.; Dutta, P. K. J. Phys. Chem. 1990, 94, 4060. (7) Yuzawa, T.; Takahashi, H.; Hamaguchi, H. Chem. Phys. Lett. 1993, 202, 221. (8) Lambi, E.; Gegiou, D.; Hadjoudis, E. J. Photochem. Photobiol. A: Chem. 1995, 86, 241. (9) Katritzky, A. R. Q. ReV. 1959, 13, 353. (10) Percy, G. C.; Thornton, D. A. J. Inorg. Nucl. Chem. 1972, 34, 3357. (11) Aton, B.; Doukas, A. G.; Narva, D.; Callender, R. H.; Dinur, U.; Honig, B. Biophys. J. 1980, 29, 79. (12) Lo´pez-Garriga, J. J.; Hanton, S.; Babcock, G. T.; Harrison, J. F. J. Am. Chem. Soc. 1986, 108, 7251.
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