The 15N and 13C solid state NMR study of intramolecular hydrogen bond in some Schiff's bases

Solid State Nuclear Magnetic Resonance 16 Ž2000. 285–289 www.elsevier.nlrlocatersolmag

The 15 N and 13 C solid state NMR study of intramolecular hydrogen bond in some Schiff’s bases b B. Kamienski , Z. Rozwadowski b, ´ a,) , W. Schilf a, T. Dziembowska – b A. Szady-Chełmieniecka a

Institute of Organic Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44, Warsaw 01-224, Poland b Institute of Chemistry, Technical UniÕersity of Szczecin, Al. Piastow ´ 42, Szczecin 71-065, Poland Received 18 March 2000; accepted 21 March 2000

Abstract A series of 11 Schiff’s bases derived from substituted salicylaldehyde and aliphatic amines has been studied in the solid state by 15 N and 13 C cross-polarization magic angle spinning ŽCPMAS. nuclear magnetic resonance ŽNMR.. 15 N CPMAS is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers. In the solid state, three of the compounds examined were shown by 15 N NMR to exist as OH tautomeric forms, and the remaining eight as the corresponding NH forms. This was confirmed by 13C CPMAS. The results reported were compared with those obtained in CDCl 3 solutions. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid state NMR;

15

N

13

C CPMAS; Intramolecular hydrogen bond; Schiff’s bases

1. Introduction The problem of the location of hydroxyl group protons in N-salicylidene-amines responsible for their photo- and thermochromic properties has been studied widely w1x. It has already been shown that the electronic structure of these compounds as well as the molecular interactions in the crystal are impor-

) Corresponding author. Tel.: q48-22-632-09-04; fax: q48-22632-66-81. E-mail address: [email protected] ŽB. Kamienski ´ ..

tant factors influencing the proton position. X-ray studies show that most Schiff’s bases exist in the OH tautomeric form and only a few in the NH form w1–6x. In some cases, co-existence of both tautomeric forms has been found w1–3,6x. Proton transfer equilibrium in N-salicylidene-alkylamines in solution has been studied by means of 1 H, 13 C, and 15 N nuclear magnetic resonance ŽNMR. w5–10x. Now, we have undertaken a study of N-Ž R-salicylidene.-alkylamines ŽFig. 1. in the solid state using 15 N and 13 C cross-polarization magic angle spinning ŽCPMAS. NMR. Solid state NMR provides information concerning the structure of compounds, which is complementary to that obtained from X-ray. Up to date,

0926-2040r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 2 0 4 0 Ž 0 0 . 0 0 0 8 0 - 1

286

B. Kamienski ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 285–289

acquisition time 20 ms, contact time 2 ms, spin rate 12 kHz. Additionally, in order to distinguish protonated and unprotonated carbon atoms, short contact cross-polarization ŽSCP. spectra w14x were recorded. In these measurements, 40 ms contact time was applied and only the protonated carbon atoms were observed. The 13 C spectra were referenced to the methylene carbon of solid glycine and then the chemical shifts were recalculated to TMS Ž dglycine s 43.3 ppm.. All CPMAS measurements were run at 297 K.

3. Results and discussion Fig. 1. The tautomeric equilibrium of studied Schiff’s bases.

only a few Schiff’s bases have been studied by solid state NMR w3,4,10–13x.

2. Experimental The N-Ž R-salicylidene.-amines 1–11 studied have been obtained previously w8–10x. The solid state spectra have been recorded on a Bruker DRX 500 spectrometer with a Bruker 4 mm CPMAS probehead. The natural abundance 15 N spectra was measured under the following conditions: spectral width 28 kHz, acquisition time 40 ms, relaxation delay 10–120 s depending on relaxation properties of the compounds, spin rate 6–12 kHz, contact time for spin-lock 5 ms. Originally, the 15 N CPMAS spectra were referred to solid glycine and then the chemical shifts were recalculated to the nitromethane scale Ž dglycine s y347.6 ppm.. Typical parameters for carbon CPMAS spectra were: spectral width 31 kHz,

Notes to Table 1: a R s CH 3 . b Ref. w10x, 302 K. c Ref. w10x, 205.5 K. d Broad signals in the range 111–193 ppm. e R s CH. f R s C. g R s CH 2 . h Ref. w10x, 270 K.

The structure of the compounds under study is shown in Fig. 1. The results of 15 N and 13 C CPMAS NMR measurements are collected in Table 1. For comparison, the relevant data on 15 N NMR Žat 302 and 205.5 K. w10x and the 13 C NMR Žat 302 K. for solutions in CDCl 3 are also included. The 13 C chemical shifts in CDCl 3 solutions are in accordance with those obtained previously w9x. The 13 C chemical shifts assignment in the solid state has been done by analogy with those in solutions, and by using the SCP CPMAS sub-spectra. The chemical shift of the imino nitrogen of the Schiff’s bases is sensitive to protonation effects as well as to substituent effects and hydrogen bond formation w7,10,15,16x. In agreement with the foregoing, we have found that the solid state 15 N signals of Schiff’s bases studied appear within wide range of chemical shifts, from y85 to y239.4 ppm. The values between y85 ppm and about y110 ppm are typical of Schiff’s bases with the OH . . . N intramolecular hydrogen bond present in the OH tautomeric form w7,10,16x. The 15 N signals of compounds 1, 2, and 3 are within this

B. Kamienski ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 285–289

range, which indicates that they exist as OH tautomers in the solid state. Compounds 4–11 give their 15 N CPMAS signals within the range y192.2 to y239.4 ppm. This indicates the presence of the NH Table 1 The 15 N and Number 1

13

State

N5C

Solid

y85.1 y86.3 y90.6 y90.3 b y91.65c y100.3

Solid Solution

3

Solid Solution

4

Solid Solution

5

Solid Solution

6

Solid

Solution 7

Solid

Solution 8

Solid Solution

9

Solid Solution

10

Solid Solution

11

tautomeric forms in the solid state. The nitrogen chemical shifts of OH and NH tautomers, as well as the large difference between them, in excess of 100 ppm, make 15 N NMR an excellent method to study

C chemical shifts Ž d . wppmx for compounds 1–11 in solid state and in CDCl 3 solution

Solution 2

287

Solid Solution

NO 2

C-1

C-2

C-3

C-4

C-5

C-6

C-7

OCH 3

N-R

118.6

156.2

119.5

119.5

151.0

111.7 110.4

167.3 166.2

54.8

46.1a

118.57

155.19

117.61

118.86

151.9

114.74

165.8

55.92

46.13 a

119.4

159.8

118.6

130.2

129.9

168.9

46.5a

119.5

159.75

118.48

131.75

120.8 123.9 122.87

130.04

164.96

45.86 a

119.2 118.56

150.9 152.14

147.1 148.52

112.9 113.79

118.0 117.61

123.8 122.66

169.6 166.21

115.7 113.4 119.15

169.6

126.1

134.6

126.1

170.6

38.3 a

157.71

123.14

132.14

121.95

130.6 130.9 128.68

164.68

44.65a

116.1

170.5

d

d

133.6

170.5

38.6 a

112.96

159.34

119.53

139.3 138.0 137.68

108.66

132.45

164.5

44.37 a

109.7

180.6 179.5

101.2

167.4 166.7

104.2 108.0

135.9 136.6

163.3 163.0

53.0 55.6

37.3 a

112.28

166.42

101.34

163.65

106.22

132.32

164.94

55.26

43.97 a

112.9

180.1

123.4

130.1

132.3

133.8

163.4

116.08

171.0

119.72

128.34

138.15

128.66

161.51

112.8

180.6

123.1

131.2

132.8

134.9

165.2

114.9

174.0

121.07

128.88

137.12

129.77

159.87

113.8

180.0

122.8

129.5

131.1

135.8

166.0

116.28

170.60

119.58

128.17

138.19

128.34

163.42

113.3

173.3

152.2

104.4

130.7

128.3

166.8

y176.1b y206.8 c y239.4

112.14

169.64

151.52

106.58

135.50

122.65

164.23

102.2

180.0

97.5

168.5

86.6

161.4

157.9

y224.0 h y244.2 c

102.26

176.91

95.65

162.27

87.5

160.78

159.35

y90.7 b y93.4 c y97.6 y100.3 b y109.3 c y214.4 y106.5 b y134.3 c y213.6 y110.4 b y145.5c y234.3 y235.3 y237.3 y130.5 b y186.5c y192.2

y102.7 b y159.3 c y184.8 y122.0 b y170.8 c y200.9 y110.0 b y168.2 c y207.9

y12.8

y12.4

y13.3

y12.8

53.4 56.05

55.4

55.01 54.7 55.36

43.8 a 45.51a

22.4 a 19.5a 57.2 e 23.53 a 57.91e 29.3 a 57.1f 29.3 a 57.14 f 16.6 a 50.0 g 15.66 a 51.68 g 16.7 a 47.2 g 15.27 a 48.11g 38.5a 39.03 a

288

B. Kamienski ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 285–289

such tautomerism. The results clearly indicate that in the solid state, the compounds exists as single tautomers because each nitrogen spectrum shows a single signal. Co-existence of two tautomers would reveal two 15 N signals, as was found for a naphthalene-derived Schiff’s base w17x. The 15 N CPMAS NMR signals of compounds 1 and 6 are split into three resonance lines each ŽTable 1.. Similar splitting of the signals is observed in some 13 C NMR spectra ŽTable 1.. These effects are much too small to be attributed to different tautomeric forms. The splitting arises from the presence of polymorphs in the solid samples examined. A comparison of the nitrogen spectra of the solid state with those in CDCl 3 solutions ŽTable 1. at room temperature shows that compounds 1, 2, and 3 exist as OH tautomers both in the solid state and in solution. This conclusion is confirmed by low temperature Ž205.5 K. spectra of solution, which are practically the same as those at 302 K. Differences between the 15 N chemical shifts in the solid and solution spectra at room temperature for compounds 4, 5, and 6 exceed 100 ppm. This indicates that these compounds in solution exist as the OH forms. The nitrogen chemical shifts of these compounds in solution show large upfield shift with decreasing temperature, indicating that the tautomeric equilibrium is moved towards the NH form at lower temperatures. For the remaining compounds, the difference between the 15 N chemical shifts in the solid state and

solution Ž302 K. is smaller because the tautomeric equilibrium in solution is already shifted in the direction of the NH form at room temperature Ž302 K. and is more shifted in the same direction at lower temperatures w9,10x. The 13 C CPMAS spectra of the compounds are quite similar to those in CDCl 3 solutions. It was helpful in spectral assignment. The main difference between the carbon spectra in the solid state and in the solution was observed for the C-2 carbon. This is in agreement with the finding that the C-2 atom of Schiff’s bases is the most sensitive for proton transfer equilibria w8,9x. The 15 N chemical shift can indicate either the position of the proton involved in the potential tautomeric equilibrium or the state of the latter. Since it is demonstrated that in the solid state only one of the two tautomers is present, the difference in the 15 N shifts between the solid state and the solution should reflect the state of the tautomeric equilibrium. The differences were calculated from: D d Ž 15 N . s d Ž 15 N,solid . y d Ž 15 N,solution . The values obtained for 15 N were correlated with the corresponding differences calculated for carbons C-1, C-2, C-7, and the methyl carbon. The use of differences in the chemical shifts Ž D d ., instead of chemical shifts as such Ž d ., largely eliminates substituent effects and leaves the tautomeric equilibrium position as the source of correlation. A fairly good

Fig. 2. The correlation of solid solution chemical shift differences of nitrogen signals D d Ž15 N. with the corresponding differences for carbon C-2 D d Ž13 C.. The correlation equation is D d Ž13 C. s 0.110D d Ž15 N. q 0.1 with the correlation coefficient R s 0.98.

B. Kamienski ´ et al.r Solid State Nuclear Magnetic Resonance 16 (2000) 285–289

correlation with a correlation coefficient R s 0.98 was found for the C-2 carbon ŽFig. 2., a rather poor one Ž R s y0.95. for the carbon of the methyl group, and no correlation Ž R below 0.5. for the C-1 and C-7 carbons. These results show that the chemical shift of C-2 can be diagnostic for the tautomerism of the Schiff’s bases. Attention is drawn to the fact that the chemical shifts of C-2 behave differently with respect to the other carbons. The special NMR behaviour of C-2 can be explained in terms of the relevant bond lengths, as X-ray studies indicate that the C`O bond length is related to the population of the NH tautomer in the crystal w6x. The X-ray data for the compounds under consideration are in agreement with the NMR results. According to X-ray, compound 2 exists in the OH form w5x. The NH structure was proven for compounds 4 and 9 by the X-ray w4,5x. For the latter compound, some amount of the OH tautomer in the solid state has been suggested by 13 C CPMAS NMR measurements w4x. In the 15 N CPMAS spectrum of compound 9, we found only one signal with chemical shift characteristic of the NH tautomer. Thus, the latter is the only species which occurs in the solid. The 13 C CPMAS spectrum of 9 shows also only one signal for each carbon atom concerned. None of the signals shows any traces of distortion that could eventually indicate the simultaneous presence of both tautomers.

4. Conclusion The 15 N CPMAS NMR spectra are shown to constitute an attractive means of insight into the tautomerism in Schiff’s bases. The spectra of the compounds studied show that only one of two possible tautomers is present in the solid. As far as solutions are concerned, there are cases where the same tautomer, either NH or OH, is present in both solution and solid. Others show that the OH form in solution is converted totally into the corresponding NH tautomer in the solid state. However, we have found no example of an analogous transfer of NH into OH. The present work shows that 15 N NMR is far superior over 13 C NMR from the point of view of

289

monitoring the potential tautomerism of the Schiff’s bases concerned. Therefore, one should be cautious with inferences drawn from the 13 C spectra alone.

Acknowledgements We gratefully acknowledge partial support of this work by the Polish State Committee for Scientific Research grant no. 3 TO9A 096 15.

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