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The NMR study of some macrocyclic and macrobicyclic Schiff bases in solution and solid state
Journal of Molecular Structure 615 (2002) 141–146 www.elsevier.com/locate/molstruc
The NMR study of some macrocyclic and macrobicyclic Schiff bases in solution and solid state W. Schilfa, B. Kamien´skia, B. Kołodziejb, E. Grechb,*, Z. Rozwadowskib, T. Dziembowskab a Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Chemistry and Environmental Protection, Technical University of Szczecin, Al. Piasto´w 42, 71-065 Szczecin, Poland
b
Received 8 October 2001; revised 7 January 2002; accepted 17 January 2002
Abstract One macrocyclic and two macrobicyclic Schiff bases have been synthesized and investigated by 1H, 13C and 15N NMR spectroscopy in solid state and CDCl3 solution at different temperatures. The results obtained have shown that all studied Schiff bases in CDCl3 solution at ambient temperature exist in a fast dynamical equilibrium that leads to the averaged symmetrical structure. The presence of flexible aliphatic chains and possibility of formation of different intramolecular hydrogen bonds in II and III leads to several different conformations in CDCl3 solution at low temperatures. An asymmetry of the Schiff bases molecules in solid state has been established. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Macrocyclic Schiff base; Macrobicyclic Schiff bases; Hydrogen bond; 15N and 1H; 13C NMR; CPMAS
1. Introduction The coordination chemistry of macrocyclic and macrobicyclic ligands is of current interest in chemistry as well as in biochemistry and technology [1 – 4]. Considerable attention has been devoted to the preparation and properties of these Schiff bases showing the ability to form mononuclear and polynuclear complexes with various cations [1 – 8]. The formation of macrocyclic and macrobicyclic ligands offers also a possibility of investigation of the structural and physico-chemical host –guest relationships and specific molecular recognition. Since the first paper of Pinkington and Robson in 1970 [5], * Corresponding author. E-mail address: [email protected] (E. Grech).
numerous macrocyclic and macrobicyclic Schiff bases with one of two compartments have been synthesised [1 – 10]. We have prepared (see Fig. 1) one macrocyclic and two macrobicyclic Schiff bases by condensation of isophthaldehyde with tris-(2-aminoethyl)amine (I), 5methyl-2-hydroxy-isophthaldehyde with 1,4-diaminobutane (II) and 5-methyl-2-hydroxy-isophthaldehyde with tris-(2-aminoethyl)amine (tren) (III). The macrocyclic Schiff bases I [9,10] and III [7] have been synthesized and characterized by 1H NMR spectroscopy previously. Avecilla et al. [7] have shown that the macrobicyclic Schiff base III crystallizes with the two molecules of H2O and 0.5 molecule of CH3CN in the unit cell. The temperature measurements of 1H NMR have shown the existence of dynamic equilibrium for this compound in CDCl3 [7].
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 1 8 - 1
142
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
Fig. 1. Studied macrocyclic and macrobicyclic Schiff bases I–III.
Table 1 1 H, 13C, 15N chemical shifts assignment for compounds I–III in chloroform solution and in solid state Composition
Phase
Solution
I
T (K)
Position 7B
7N
1
2
8.22
–
7.62
127.7
–
160.7
3.8 3.3 60.0
2.95 2.7 55.3
8.18
–
7.56
2.91 2.73 55.6
1
2
–
–
–
–
–
–
–
–
8.22
136.9 250.99 –
132.4 – 5.20
136.9
127.7
223
C N H
–
8.18
C N C
136.6 254.0 137.4 136.7 249.7
132.3 2351.7 132.9
136.6
127.4
129.6
127.4
–
161.0
3.85 3.25 60.2
137.4 136.7
126.8a
128.4 129.8a
126.8a
–
160.0
56.4
60.6 59.3
–
–
14.0 159.6 – 14.25 –14.8 159.7 258.9
– 121.3
7.44 132.4
– 127.3
7.44 132.4
8.5 161.1
8.5 161.1
3.67 60.2
1.78 28.7
3.67 60.2
1.78 28.7
2.29 20.3
– 123.2
7.10–7.75 130.2
– 127.4
7.10–7.75 134.3
8.40–8.20 163.9
8.80–8.65 157.2
3.70 59.0
1.75 28.9
3.70 62.0
1.75 28.9
2.30 20.6
158.6 251.1
123.6
130.0
126.9
133.5
164.7
155.7
60.9
29.2
60.9
29.2
20.4
14.0 14.0 14.2 15.5
– –
7.1 7.83 7.35 7.17 129.4 128.4 127.8 129.8
– –
7.1 7.19 6.59 6.55 134.1 132.3 132.8 133.4
8.5 8.44 7.99 7.88 165.0 163.4
8.5 9.19 8.91
3.63 2.9 4.07–2.61
3.63
2.9
158.8 158.6
,20
2.21 2.26 2.18 2.10 20.74
165.7
156.7
56.7
303
303 223
H C N H C N C N
– 121.3 – – 118.6 294.1 290.1 118.9 283.8
H H
– –
C N
b
119.0 286.3
7.56
6
128.7 7.62
2351.0
b
158.6 252.4
b
124.0
b
126.4
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
a b
5
–
C
Solid
4
5.38
Solid Solution
3
00
–
223
III
2 (II)
00
H
N Solution
1 (I)
0
303
Solid
II
Others 0
20.3 2351.3
Unresolved pattern of three lines, assignment can be changed. Due to dynamic character of the spectrum and low solubility those signals were not found. 143
144
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
The Cu2þ complex of compound II has been studied recently [8]. The aim of this work has been the investigation of the structure of macrocyclic and macrobicyclic Schiff bases (Fig. 1) with free imine groups (I) and with these groups engaged into two and three intramolecular hydrogen bonds (II and III) in CDCl3 solution and solid state, by means of 1H, 13C and 15N spectroscopies. The 15N NMR spectroscopy has been shown to be a very efficient tool in studying OH· · ·N hydrogen bond in Schiff bases [11 –15].
2. Experimental
to 120 s depending on the relaxation properties of compounds, spin rate from 6 up to 12 kHz, contact time for spin-lock 5 ms. Originally 15N CPMAS spectra were referred to the spectrum of glycine and then the chemical shifts were converted to nitromethane scale ðdglycine ¼ 2347:6 ppmÞ: Typical parameters of the 13C CPMAS spectra recording were: spectral width 31 kHz, acquisition time 20 ms, contact time 2 ms and spin rate 12 kHz. Additionally, to distinguish between protonated and unprotonated carbon atoms, the experiments with very short contact time (SCT) of ca. 40 ms were performed. The carbon spectra were also referred to the signal of methylene 13 C atom of glycine and then the chemical shifts were converted to the TMS scale ðdglycine ¼ 243:3 ppmÞ:
2.1. Synthesis Compound I. Tris-(2-aminoethyl)amine (tren) (0.50 mmol) and isophthaldehyde (0.75 mmol) in acetonitrile (20 cm3) were stirred at room temperature for half an hour. The white precipitate was filtered off, washed with acetonitrile and water, and then dried in vacuum. The elemental analysis has shown that the compound crystallizes with one molecule of H2O. Compound II. 2-Hydroxy-5-methyl-isophthaldehyde (1 mmol) in absolute ethanol (25 cm3) was added dropwise (in 30 min) to the 1,4-diaminobutane (1 mmol) in absolute ethanol (15 cm3). Then the mixture was refluxed for 2 h. After evaporation of the solvent under reduced pressure, the product was washed several times with ether and dried in vacuum. Compound III. It has been obtained according to a known method [7]. 2.2. NMR measurements The NMR spectra were recorded on Bruker DRX 500 spectrometer. For the liquid state experiments about 10 mg samples of compounds I– III in CDCl3 solution were used. To record carbon and nitrogen signals the two-dimensional GHSQC and GHMBC methods were applied. The internal TMS and external nitromethane were used as the standards for 1H, 13C and 15N measurements, respectively. The solid-state spectra were made by CPMAS method using 4 mm Bruker probe-head. Typical parameters of the 15N spectra recording were: spectral width 28 kHz; acquisition time 40 ms; relaxation delay from 10 up
3. Results and discussion The results of NMR studies in CDCl3 and solid state for compounds I–III are shown in Table 1. The 1H, 13C and 15N NMR spectra of the compound I have shown that fast conformational changes take place in CDCl3 solution at room temperature. They were manifested by very broad signals of H-10 and H-20 protons and by the absence of the signal of the amino nitrogen atom in the 15 N NMR spectrum. A fair degree of flexibility in this compound has been also suggested by MacDowell and Nelson [9]. In low temperatures the signals of these protons became sharp (Table 1). In the 15N NMR spectrum at a low temperature, two N signals were observed at frequencies 2351.7 and 254 ppm, typical of amine and imine group, respectively. The signals of carbon and nitrogen atoms in the solid state were very close to those observed in CDCl3 solution, which suggested a similar structure in both the phases. In Schiff base II, there are the proton donor OH groups and two proton acceptor groups in the ortho positions. As follows from the NMR spectra of compound II in CDCl3 solution at room temperature, also in this compound the fast conformational changes have led to an averaged symmetric structure. The very broad signal of the OH group observed at about 14 ppm suggested the rotation of the OH group as in di-anils of 5-methyl-2-hydroxy-isophthaldehyde [15]. The signals assigned to the atoms at the positions 4 and 6, 7B and 7N, 10 and 100 , 20 and 200 suggested their equivalence. In the 15 N NMR spectrum we were not able to obtain any
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
signals, probably due to the dynamics of the system. At 223 K in the high frequency region at about 14 ppm, seven very close lines were observed (Table 1). The signals assigned to the protons of the methine groups 7B and 7N were split into three and four lines, respectively. In the region characteristic of aromatic protons (7.10– 7.75 ppm) five signals (singlets) were found. They were assigned to the protons in position 4 and 6 in the aromatic rings of different forms of compound II being in equilibrium. Since these protons are not coupled, no detailed assignment could be made. Also the carbon atoms C-7B and C-7N as well as C-10 and C-100 became non-equivalent. In the region corresponding to the signals of aromatic ring carbons, six lines, characteristic of unsymmetrical-substituted benzene ring, were found. The most informative results were obtained from 1 H– 15N correlation spectrum. The signals located between 294.1 and 290.1 ppm correlate with those assigned to the protons of the OH groups, while those at 258.9 ppm with the signals of proton H0 offree methine, in 8.80–8.65 ppm region. The 15N signals of higher frequency was assigned to the N atom engaged in the intramolecular hydrogen bond, the latter to the N atom of free imine group. In di-anils of 5-methyl-2-hydroxyisophthaldehyde in CDCl3 the free rotation was inhibited at 223 K (in NMR scale), and respective signals of hydrogen bonded and free N atoms were observed at 291.5 and 267.5 ppm, respectively [15]. The 15N NMR chemical shift difference between bonded and non-bonded imino groups indicates the presence of the relatively strong hydrogen bonds similar to those found for other Schiff bases [11–15]. A very characteristic feature of the macrocyclic Schiff base II in solution is the coexistence of several conformations with different intramolecular hydrogen bonds. In solid state, in contrast to the low-temperature CDCl3 solution spectra, only one set of signals assigned to carbon and nitrogen atoms was found. This indicates that only one conformation exists in the solid state. The chemical shift of the signal attributed to the hydrogen-bonded N atom is shifted to higher frequencies in comparison to that found for solution in CDCl3 at a low temperature. This indicates that in the solid state, the hydrogen bond is slightly weaker than in solution. This conclusion has been confirmed by the shift of 13C chemical shifts of the signal of the atom in position 2 to higher frequencies by about 1 ppm. The most interesting of the compounds studied is
145
the macrobicyclic Schiff base III. Due to presence of the OH groups in ortho positions to imine groups, formation of a few intramolecular hydrogen bonds is possible. Some difficulties in the assignments of the NMR spectra arise from the low solubility of this compound in CDCl3 and CD2Cl2. At room temperature, the proton spectrum shows the dynamically averaged symmetric structure, according to Avecilla et al. [7]. In the low-field region only one signal dOH at 14.0 ppm was present. In the aromatic area two very broad signals were observed at 7.1 and 8.5 ppm assigned to the protons of the aromatic ring and of imine group, respectively. In the low-frequency region, two broad lines assigned to the CH2 protons and one sharp resonance signal of the CH3 group protons were observed (Table 1). Due to the dynamic character and low solubility we could not measure 13C spectrum of this compound at room temperature. At 223 K, three well resolved relatively sharp resonance signals of the OH group were observed at 14.0, 14.2 and 15.5 ppm, respectively. The values of dOH indicate that all OH groups are engaged in the three different intramolecular hydrogen bonds. It is worth noting, that Avecilla et al. [7] observed only two signals in this region at 203 K. In the aromatic region of the spectrum, 10 signals integrated as one proton and one signal integrated as two protons were seen. In the high field part of the spectrum, a very complicated, practically unresolved pattern of signals assigned to CH2 protons and two (integration three and six protons) signals assigned to CH3 group protons were found. The two-dimensional GHSQC proton– carbon spectrum allowed us to assign all proton signals to three different parts of the molecule (Table 1). On the basis of these data and results obtained for II, the assignment of the carbon atoms of III has been made (Table 1). In the aliphatic part of the GHSQC spectrum, we found about 20 carbon signals for different CH2 groups indicating strong differentiation of aliphatic chains. For three CH3 groups only one signal at 20.74 ppm was observed. Unfortunately, since the protons at positions 7, 4 and 6 are not scalarcoupled, we were not able to assign exactly the signals to each of three aromatic systems present in molecule III. The change of solvent from CDCl3 to CD2Cl2 and a lowering of temperature to 190 K did not change the spectral picture of III. We can only assume that in this compound each aromatic ring can be distinguished
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W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
from the other. Different frequencies of the protons of the tree OH groups suggest different strengths of the intramolecular hydrogen bond, but the effects resulting from different conformation of the three fragments of the molecule also have to be taken into consideration. The deshielding of the protons of the free imine group in comparison to these engaged in the intramolecular hydrogen bond may suggest, similarly as it was for di-anils of 2-hydroxy-5-methyl-isophthaldehyde [15], the occurrence of CH· · ·O interactions. It should be noted that the exceptionally high frequency 9.19 ppm has been found for only one imine group. The macrocyclic compound III has also been studied in solid state. The 15N CPMAS spectrum of III has three very broad signals at 2 52.4 and 2 86.3 ppm assigned to non-hydrogen-bonded and hydrogen-bonded imino groups, respectively. The third, upfield signal d ¼ 2351:3 ppm comes from amino nitrogen atoms. The line widths of the signals of imino groups, close to 1 kHz indicate that the dynamics of the system is also present in solid state. This conclusion is supported by 13C CPMAS spectra. In the most diagnostic downfield region of 13C solidstate spectrum, three broad signals were observed. These at 165.7 and 156.7 ppm were assigned to the imine C – H atoms 7B and 7N, respectively. The signal at 158.6 ppm was ascribed to C-2 atom. The chemical shift of the C-2 is similar to that observed for compound II and other Schiff bases [11 – 15]. A comparison of 13C CPMAS spectrum with SCT experiment and also with the data for compound II, allowed us to assign the remaining carbon atom signals (Table 1).
4. Conclusion In the present study, we found out that at room temperatures the flexible molecule of the macrocyclic Schiff bases I exist in a dynamic equilibrium in CDCl3 solution. In the macrobicyclic Schiff bases II and III, in the same solvent at ambient temperature, fast conformational changes take place, connected with the presence of several OH groups, as well as the presence of flexible aliphatic chains. An interconversion of the conformers by the rotation of the OH groups associated with the break up and formation of hydrogen bond with the N atom of the imine groups
in ortho position is possible in these conditions. At low temperature and in the solid state the Schiff bases II and III with different intramolecular hydrogen bonds lead to an asymmetric structures. To investigate the intramolecular hydrogen bond OH· · ·N and conformation of Schiff bases, 15N NMR technique, in solution and particularly in the solid state, seems to be the most suitable. However, the 1H and 13C NMR are also very useful techniques, which in some cases provide the key information, particularly concerning the conformations of the molecule studied.
Acknowledgments We gratefully acknowledge partial financial support of this work by a grant number 3TO9A 096 15 from the Polish State Committee for Scientific Research.
References [1] P. Guerriero, S. Tamburini, P.A. Vigato, Coord. Chem. Rev. 139 (1995) 17. [2] V. Alexander, Chem. Rev. 95 (1995) 273. [3] D.E. Fenton, H. Okawa, Chem. Ber. 130 (1997) 433. [4] B. Linton, A.D. Hamilton, Chem. Rev. 97 (1997) 1669. [5] N.M. Pinkington, R.R. Robson, Aust. J. Chem. 23 (1970) 515. [6] A. Aguiari, N. Brianese, S. Tamburini, P.A. Vigato, Inorg. Chem. Acta 235 (1995) 233. [7] F. Avecilla, R. Bastida, A. de Blas, D.E. Fenton, A. Macias, A. Rodriguez, T. Rodriguez-Blas, S. Garcia-Granda, R. CorzoSuarez, J. Chem. Soc., Dalton Trans. (1997) 409. [8] T. Dziembowska, N. Guskos, J. Typek, H. Szymczak, V. Likodimos, S. Glenis, C.L. Lin, M. Wabia, E. Jagodzin´ska, E. Fabrycy, Mat. Res. Bull. 34 (1999) 943. [9] D. MacDowell, J. Nelson, Tetrahedron Lett. 29 (1988) 385. [10] C.J. Harding, Q. Lu, J.F. Malone, D.J. Marrs, N. Martin, V. McKee, J. Nelson, J. Chem. Soc., Dalton Trans. 10 (1995) 1739. [11] B. Kamien´ski, W. Schilf, T. Dziembowska, Z. Rozwadowski, A. Szady-Chelmieniecka, Solid State NMR 16 (2000) 285. [12] W. Schilf, B. Kamien´ski, T. Dziembowska, Z. Rozwadowski, A. Szady-Chelmieniecka, J. Mol. Struct. 552 (2000) 33. [13] A. Szady-Chelmieniecka, E. Grech, Z. Rozwadowski, T. Dziembowska, W. Schilf, B. Kamien´ski, J. Mol. Struct. 565– 566 (2001) 125. [14] W. Schilf, B. Kamien´ski, A. Szady-Chelmieniecka, E. Grech, Solid State NMR 18 (2000) 97. [15] T. Dziembowska, Z. Rozwadowski, J. Sitkowski, L. Stefaniak, B. Brzezin´ski, J. Mol. Struct. 407 (1997) 131.
The NMR study of some macrocyclic and macrobicyclic Schiff bases in solution and solid state W. Schilfa, B. Kamien´skia, B. Kołodziejb, E. Grechb,*, Z. Rozwadowskib, T. Dziembowskab a Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Chemistry and Environmental Protection, Technical University of Szczecin, Al. Piasto´w 42, 71-065 Szczecin, Poland
b
Received 8 October 2001; revised 7 January 2002; accepted 17 January 2002
Abstract One macrocyclic and two macrobicyclic Schiff bases have been synthesized and investigated by 1H, 13C and 15N NMR spectroscopy in solid state and CDCl3 solution at different temperatures. The results obtained have shown that all studied Schiff bases in CDCl3 solution at ambient temperature exist in a fast dynamical equilibrium that leads to the averaged symmetrical structure. The presence of flexible aliphatic chains and possibility of formation of different intramolecular hydrogen bonds in II and III leads to several different conformations in CDCl3 solution at low temperatures. An asymmetry of the Schiff bases molecules in solid state has been established. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Macrocyclic Schiff base; Macrobicyclic Schiff bases; Hydrogen bond; 15N and 1H; 13C NMR; CPMAS
1. Introduction The coordination chemistry of macrocyclic and macrobicyclic ligands is of current interest in chemistry as well as in biochemistry and technology [1 – 4]. Considerable attention has been devoted to the preparation and properties of these Schiff bases showing the ability to form mononuclear and polynuclear complexes with various cations [1 – 8]. The formation of macrocyclic and macrobicyclic ligands offers also a possibility of investigation of the structural and physico-chemical host –guest relationships and specific molecular recognition. Since the first paper of Pinkington and Robson in 1970 [5], * Corresponding author. E-mail address: [email protected] (E. Grech).
numerous macrocyclic and macrobicyclic Schiff bases with one of two compartments have been synthesised [1 – 10]. We have prepared (see Fig. 1) one macrocyclic and two macrobicyclic Schiff bases by condensation of isophthaldehyde with tris-(2-aminoethyl)amine (I), 5methyl-2-hydroxy-isophthaldehyde with 1,4-diaminobutane (II) and 5-methyl-2-hydroxy-isophthaldehyde with tris-(2-aminoethyl)amine (tren) (III). The macrocyclic Schiff bases I [9,10] and III [7] have been synthesized and characterized by 1H NMR spectroscopy previously. Avecilla et al. [7] have shown that the macrobicyclic Schiff base III crystallizes with the two molecules of H2O and 0.5 molecule of CH3CN in the unit cell. The temperature measurements of 1H NMR have shown the existence of dynamic equilibrium for this compound in CDCl3 [7].
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 1 8 - 1
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W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
Fig. 1. Studied macrocyclic and macrobicyclic Schiff bases I–III.
Table 1 1 H, 13C, 15N chemical shifts assignment for compounds I–III in chloroform solution and in solid state Composition
Phase
Solution
I
T (K)
Position 7B
7N
1
2
8.22
–
7.62
127.7
–
160.7
3.8 3.3 60.0
2.95 2.7 55.3
8.18
–
7.56
2.91 2.73 55.6
1
2
–
–
–
–
–
–
–
–
8.22
136.9 250.99 –
132.4 – 5.20
136.9
127.7
223
C N H
–
8.18
C N C
136.6 254.0 137.4 136.7 249.7
132.3 2351.7 132.9
136.6
127.4
129.6
127.4
–
161.0
3.85 3.25 60.2
137.4 136.7
126.8a
128.4 129.8a
126.8a
–
160.0
56.4
60.6 59.3
–
–
14.0 159.6 – 14.25 –14.8 159.7 258.9
– 121.3
7.44 132.4
– 127.3
7.44 132.4
8.5 161.1
8.5 161.1
3.67 60.2
1.78 28.7
3.67 60.2
1.78 28.7
2.29 20.3
– 123.2
7.10–7.75 130.2
– 127.4
7.10–7.75 134.3
8.40–8.20 163.9
8.80–8.65 157.2
3.70 59.0
1.75 28.9
3.70 62.0
1.75 28.9
2.30 20.6
158.6 251.1
123.6
130.0
126.9
133.5
164.7
155.7
60.9
29.2
60.9
29.2
20.4
14.0 14.0 14.2 15.5
– –
7.1 7.83 7.35 7.17 129.4 128.4 127.8 129.8
– –
7.1 7.19 6.59 6.55 134.1 132.3 132.8 133.4
8.5 8.44 7.99 7.88 165.0 163.4
8.5 9.19 8.91
3.63 2.9 4.07–2.61
3.63
2.9
158.8 158.6
,20
2.21 2.26 2.18 2.10 20.74
165.7
156.7
56.7
303
303 223
H C N H C N C N
– 121.3 – – 118.6 294.1 290.1 118.9 283.8
H H
– –
C N
b
119.0 286.3
7.56
6
128.7 7.62
2351.0
b
158.6 252.4
b
124.0
b
126.4
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
a b
5
–
C
Solid
4
5.38
Solid Solution
3
00
–
223
III
2 (II)
00
H
N Solution
1 (I)
0
303
Solid
II
Others 0
20.3 2351.3
Unresolved pattern of three lines, assignment can be changed. Due to dynamic character of the spectrum and low solubility those signals were not found. 143
144
W. Schilf et al. / Journal of Molecular Structure 615 (2002) 141–146
The Cu2þ complex of compound II has been studied recently [8]. The aim of this work has been the investigation of the structure of macrocyclic and macrobicyclic Schiff bases (Fig. 1) with free imine groups (I) and with these groups engaged into two and three intramolecular hydrogen bonds (II and III) in CDCl3 solution and solid state, by means of 1H, 13C and 15N spectroscopies. The 15N NMR spectroscopy has been shown to be a very efficient tool in studying OH· · ·N hydrogen bond in Schiff bases [11 –15].
2. Experimental
to 120 s depending on the relaxation properties of compounds, spin rate from 6 up to 12 kHz, contact time for spin-lock 5 ms. Originally 15N CPMAS spectra were referred to the spectrum of glycine and then the chemical shifts were converted to nitromethane scale ðdglycine ¼ 2347:6 ppmÞ: Typical parameters of the 13C CPMAS spectra recording were: spectral width 31 kHz, acquisition time 20 ms, contact time 2 ms and spin rate 12 kHz. Additionally, to distinguish between protonated and unprotonated carbon atoms, the experiments with very short contact time (SCT) of ca. 40 ms were performed. The carbon spectra were also referred to the signal of methylene 13 C atom of glycine and then the chemical shifts were converted to the TMS scale ðdglycine ¼ 243:3 ppmÞ:
2.1. Synthesis Compound I. Tris-(2-aminoethyl)amine (tren) (0.50 mmol) and isophthaldehyde (0.75 mmol) in acetonitrile (20 cm3) were stirred at room temperature for half an hour. The white precipitate was filtered off, washed with acetonitrile and water, and then dried in vacuum. The elemental analysis has shown that the compound crystallizes with one molecule of H2O. Compound II. 2-Hydroxy-5-methyl-isophthaldehyde (1 mmol) in absolute ethanol (25 cm3) was added dropwise (in 30 min) to the 1,4-diaminobutane (1 mmol) in absolute ethanol (15 cm3). Then the mixture was refluxed for 2 h. After evaporation of the solvent under reduced pressure, the product was washed several times with ether and dried in vacuum. Compound III. It has been obtained according to a known method [7]. 2.2. NMR measurements The NMR spectra were recorded on Bruker DRX 500 spectrometer. For the liquid state experiments about 10 mg samples of compounds I– III in CDCl3 solution were used. To record carbon and nitrogen signals the two-dimensional GHSQC and GHMBC methods were applied. The internal TMS and external nitromethane were used as the standards for 1H, 13C and 15N measurements, respectively. The solid-state spectra were made by CPMAS method using 4 mm Bruker probe-head. Typical parameters of the 15N spectra recording were: spectral width 28 kHz; acquisition time 40 ms; relaxation delay from 10 up
3. Results and discussion The results of NMR studies in CDCl3 and solid state for compounds I–III are shown in Table 1. The 1H, 13C and 15N NMR spectra of the compound I have shown that fast conformational changes take place in CDCl3 solution at room temperature. They were manifested by very broad signals of H-10 and H-20 protons and by the absence of the signal of the amino nitrogen atom in the 15 N NMR spectrum. A fair degree of flexibility in this compound has been also suggested by MacDowell and Nelson [9]. In low temperatures the signals of these protons became sharp (Table 1). In the 15N NMR spectrum at a low temperature, two N signals were observed at frequencies 2351.7 and 254 ppm, typical of amine and imine group, respectively. The signals of carbon and nitrogen atoms in the solid state were very close to those observed in CDCl3 solution, which suggested a similar structure in both the phases. In Schiff base II, there are the proton donor OH groups and two proton acceptor groups in the ortho positions. As follows from the NMR spectra of compound II in CDCl3 solution at room temperature, also in this compound the fast conformational changes have led to an averaged symmetric structure. The very broad signal of the OH group observed at about 14 ppm suggested the rotation of the OH group as in di-anils of 5-methyl-2-hydroxy-isophthaldehyde [15]. The signals assigned to the atoms at the positions 4 and 6, 7B and 7N, 10 and 100 , 20 and 200 suggested their equivalence. In the 15 N NMR spectrum we were not able to obtain any
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signals, probably due to the dynamics of the system. At 223 K in the high frequency region at about 14 ppm, seven very close lines were observed (Table 1). The signals assigned to the protons of the methine groups 7B and 7N were split into three and four lines, respectively. In the region characteristic of aromatic protons (7.10– 7.75 ppm) five signals (singlets) were found. They were assigned to the protons in position 4 and 6 in the aromatic rings of different forms of compound II being in equilibrium. Since these protons are not coupled, no detailed assignment could be made. Also the carbon atoms C-7B and C-7N as well as C-10 and C-100 became non-equivalent. In the region corresponding to the signals of aromatic ring carbons, six lines, characteristic of unsymmetrical-substituted benzene ring, were found. The most informative results were obtained from 1 H– 15N correlation spectrum. The signals located between 294.1 and 290.1 ppm correlate with those assigned to the protons of the OH groups, while those at 258.9 ppm with the signals of proton H0 offree methine, in 8.80–8.65 ppm region. The 15N signals of higher frequency was assigned to the N atom engaged in the intramolecular hydrogen bond, the latter to the N atom of free imine group. In di-anils of 5-methyl-2-hydroxyisophthaldehyde in CDCl3 the free rotation was inhibited at 223 K (in NMR scale), and respective signals of hydrogen bonded and free N atoms were observed at 291.5 and 267.5 ppm, respectively [15]. The 15N NMR chemical shift difference between bonded and non-bonded imino groups indicates the presence of the relatively strong hydrogen bonds similar to those found for other Schiff bases [11–15]. A very characteristic feature of the macrocyclic Schiff base II in solution is the coexistence of several conformations with different intramolecular hydrogen bonds. In solid state, in contrast to the low-temperature CDCl3 solution spectra, only one set of signals assigned to carbon and nitrogen atoms was found. This indicates that only one conformation exists in the solid state. The chemical shift of the signal attributed to the hydrogen-bonded N atom is shifted to higher frequencies in comparison to that found for solution in CDCl3 at a low temperature. This indicates that in the solid state, the hydrogen bond is slightly weaker than in solution. This conclusion has been confirmed by the shift of 13C chemical shifts of the signal of the atom in position 2 to higher frequencies by about 1 ppm. The most interesting of the compounds studied is
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the macrobicyclic Schiff base III. Due to presence of the OH groups in ortho positions to imine groups, formation of a few intramolecular hydrogen bonds is possible. Some difficulties in the assignments of the NMR spectra arise from the low solubility of this compound in CDCl3 and CD2Cl2. At room temperature, the proton spectrum shows the dynamically averaged symmetric structure, according to Avecilla et al. [7]. In the low-field region only one signal dOH at 14.0 ppm was present. In the aromatic area two very broad signals were observed at 7.1 and 8.5 ppm assigned to the protons of the aromatic ring and of imine group, respectively. In the low-frequency region, two broad lines assigned to the CH2 protons and one sharp resonance signal of the CH3 group protons were observed (Table 1). Due to the dynamic character and low solubility we could not measure 13C spectrum of this compound at room temperature. At 223 K, three well resolved relatively sharp resonance signals of the OH group were observed at 14.0, 14.2 and 15.5 ppm, respectively. The values of dOH indicate that all OH groups are engaged in the three different intramolecular hydrogen bonds. It is worth noting, that Avecilla et al. [7] observed only two signals in this region at 203 K. In the aromatic region of the spectrum, 10 signals integrated as one proton and one signal integrated as two protons were seen. In the high field part of the spectrum, a very complicated, practically unresolved pattern of signals assigned to CH2 protons and two (integration three and six protons) signals assigned to CH3 group protons were found. The two-dimensional GHSQC proton– carbon spectrum allowed us to assign all proton signals to three different parts of the molecule (Table 1). On the basis of these data and results obtained for II, the assignment of the carbon atoms of III has been made (Table 1). In the aliphatic part of the GHSQC spectrum, we found about 20 carbon signals for different CH2 groups indicating strong differentiation of aliphatic chains. For three CH3 groups only one signal at 20.74 ppm was observed. Unfortunately, since the protons at positions 7, 4 and 6 are not scalarcoupled, we were not able to assign exactly the signals to each of three aromatic systems present in molecule III. The change of solvent from CDCl3 to CD2Cl2 and a lowering of temperature to 190 K did not change the spectral picture of III. We can only assume that in this compound each aromatic ring can be distinguished
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from the other. Different frequencies of the protons of the tree OH groups suggest different strengths of the intramolecular hydrogen bond, but the effects resulting from different conformation of the three fragments of the molecule also have to be taken into consideration. The deshielding of the protons of the free imine group in comparison to these engaged in the intramolecular hydrogen bond may suggest, similarly as it was for di-anils of 2-hydroxy-5-methyl-isophthaldehyde [15], the occurrence of CH· · ·O interactions. It should be noted that the exceptionally high frequency 9.19 ppm has been found for only one imine group. The macrocyclic compound III has also been studied in solid state. The 15N CPMAS spectrum of III has three very broad signals at 2 52.4 and 2 86.3 ppm assigned to non-hydrogen-bonded and hydrogen-bonded imino groups, respectively. The third, upfield signal d ¼ 2351:3 ppm comes from amino nitrogen atoms. The line widths of the signals of imino groups, close to 1 kHz indicate that the dynamics of the system is also present in solid state. This conclusion is supported by 13C CPMAS spectra. In the most diagnostic downfield region of 13C solidstate spectrum, three broad signals were observed. These at 165.7 and 156.7 ppm were assigned to the imine C – H atoms 7B and 7N, respectively. The signal at 158.6 ppm was ascribed to C-2 atom. The chemical shift of the C-2 is similar to that observed for compound II and other Schiff bases [11 – 15]. A comparison of 13C CPMAS spectrum with SCT experiment and also with the data for compound II, allowed us to assign the remaining carbon atom signals (Table 1).
4. Conclusion In the present study, we found out that at room temperatures the flexible molecule of the macrocyclic Schiff bases I exist in a dynamic equilibrium in CDCl3 solution. In the macrobicyclic Schiff bases II and III, in the same solvent at ambient temperature, fast conformational changes take place, connected with the presence of several OH groups, as well as the presence of flexible aliphatic chains. An interconversion of the conformers by the rotation of the OH groups associated with the break up and formation of hydrogen bond with the N atom of the imine groups
in ortho position is possible in these conditions. At low temperature and in the solid state the Schiff bases II and III with different intramolecular hydrogen bonds lead to an asymmetric structures. To investigate the intramolecular hydrogen bond OH· · ·N and conformation of Schiff bases, 15N NMR technique, in solution and particularly in the solid state, seems to be the most suitable. However, the 1H and 13C NMR are also very useful techniques, which in some cases provide the key information, particularly concerning the conformations of the molecule studied.
Acknowledgments We gratefully acknowledge partial financial support of this work by a grant number 3TO9A 096 15 from the Polish State Committee for Scientific Research.
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