Styrylpyrylium Salts: ¹H and ¹³C NMR High-Resolution Spectroscopy (1D and 2D)

Abstract

¹H and ¹³C NMR high-resolution spectroscopy (1D and 2D) (¹H, ¹H-COSY, HSQC, HMBC) for four styrylpyrylium perchlorates were carried out and signal attributions are reported. Chemical shifts observed on ¹³C NMR spectra for the styrylpyrylium salts were compared with net atomic charge for carbon obtained by AM1 semiempirical calculations. The position of the styryl group present low effect on chemical shifts for carbon atoms, while the presence of methyl group led to the unshielding of the substituted carbon.

1. Introduction

NMR spectroscopy reports for pyrylium salts from few authors are available [1, 2]. This investigation presents some complexity in the attribution of the chemical shifts to atoms. The data on pyrylium salts NMR spectroscopy with substitution effects analysis are useful to understand various properties well known for pyrylium cations [3–6]. Particularly emission properties (fluorescence and phosphorescence) of styrylpyrylium salts are reported in relation with proton, chemical shifts in ¹H NMR of the pyrylium ring [7, 8]. However carbon chemical shift data relative to pyrylium ring, proton and carbon chemical shift for styryl group remain unknown, as for us.

In this study, we report high-resolution 1D and 2D ¹H and ¹³C NMR analysis results for four styrylpyrylium salts (Figure 1).

Figure 1 

Structures of studied styrylpyrylium salts.

¹³C chemical shifts of the pyrylium ring, phenyl, and styryl groups are presented with correlation with Austin Model 1(AM1) theoretical calculations. The substitution effects of methyl, phenyl, and styryl groups and their positions on the pyrylium ring were discussed as regards atomic chemical shifts.

2. Results and Discussion

Spectra are recorded with compounds dissolved in d₆-DMSO. Proton NMR chemical shifts recording for studied styrylpyrylium salts were reported in Table 1 and those for carbon 13 in Table 2.

Table 1 

¹H NMR (600 MHz) data: chemical shifts 𝛿 in ppm as unit of measurement; multiplicity and constant coupling J given in Hz for four styrylpyrylium salts 1, 2, 3 and 4.

Table 2 

¹³C-NMR data, 9.40 T (100.6 MHz) or 14.09 T (150.9 MHz) of compounds 1, 2, 3 and 4: Chemical shifts 𝛿 (in ppm as unit of measurement) and net atomic charge (q) of styrylpyrylium salts carbons.

On all spectra, we observed general pyrylium salts characteristics and also specific data due to styryl group with its extracyclic double bond. Data are comparable to those of previous work at low resolution obtained by A. R. Katritzky and coll [1]. Here the study at high resolution gives high precision on chemical shifts for proton and carbon and reveals correlation between atoms.

2.1. ¹H NMR Spectra Data Analysis

H_𝑥 or C_𝑥: x is the position number of carbon or hydrogen beard by carbon x.

All the protons of the phenyl groups resonate between 𝛿_H 7.50 and 𝛿_H 8.45. Pyrylium ring proton signals appear as singlets in the range 8.50–9.20. The signals were assigned and compared with data encountered in literature [8–10].

The two protons H₃ and H₅ of compound 1 are isochrones because of the symmetry of this molecule and appear at 𝛿_H 8.92, whereas in compound 3, because of the asymmetry of the molecule, they are 𝛿_H 9.02 for H₃ and H₅ shifted towards low frequency with 𝛿_H 8.80 (Figure 2).

Figure 2 

1H NMR spectra of compound 3.

The high chemical shift value corresponding to H₃ may be explained by the fact that it is in a paramagnetic anisotropy field of both phenyl rings, those in positions 2 and 4, while H₅ is impacted by only one phenyl group anisotropy, the one in position 4 [11].

For NMR data analysis, comparison with triphenyl pyrylium: 2,4,6-triphenylpyrylium tetrafluoroborate encountered in the literature [12] shows the signal of the pyrylium ring protons at 𝛿_H 8.50. These protons are more shielded than the styrylpyrylium perchlorate due certainly to the mesomeric effects.

The vinylic protons H₇ and H₈ constitute an AX system. Proton H₈ for all the compounds appears like a doublet between 𝛿_H 8.37 and 𝛿_H 8.72.

The scalar coupling value between the two vinylic protons of each molecule varied between 15.90 Hz and 16.20 Hz. This value is an indication for the (E) stereochemistry of the double bond C=C of styryl group [13].

In the case of compound 3, H₇ signal appears on the spectrum 2D (HMBC) like doublet at 𝛿_H 7.81 (Figure 3).

Figure 3 

HMBC spectrum for compound 3.

The allocation of the proton H₇ was confirmed by using proton-proton correlation spectra (COSY). The results show that this proton resonates at 𝛿_H 7.80, in a broad peak for 2 and 4. On the 1D ¹H NMR spectrum of 1, proton H₇ resonates in a good resolution as doublet at 𝛿_H 7.70 (Figure 4).

Figure 4 

Partial view of 1H NMR spectra of compound 1 (600 MHz).

When the styryl group is attached in position 4 of the pyrylium ring, the proton H₈ resonates at 𝛿_H 8.70 as for 1 and at 𝛿_H 8.72 as for 2; when it moves to position 6, H₈ undergoes a slight shielding and resonates at 𝛿_H 8.48 as for 3 and 𝛿_H 8.37 as for 4.

Protons H₁₄ and H₁₈ of 1 are isochrones because of the symmetry of the molecule and resonate like doublet at 𝛿_H 8.44.

Proton-proton correlation allowed allocation of signal in the form of triplet towards 𝛿_H 7.79 to isochrones protons H₁₅ and H_19.

In compound 3, proton H₁₄ is more unshielded than proton H₈ from styryl group (Figure 1 and Table 1). However, on the ¹H NMR spectra of the other compounds, H₈ is more unshielded than all the other aromatic protons.

2.2. ¹³C NMR Spectra Data Analysis

Carbon chemical shifts attribution for pyrylium cation-carbons were made by correlation spectra (HSQC, HMBC) and by DEPT. Data are given in Table 2.

Carbons C₂, C₄, and C₆ of pyrylium ring are unshielded than phenyl carbon and those of styryl group while carbons C₅ and C₃ are the most shielded as it is shown on Figure 5.

Figure 5 

13C NMR spectra of compound 1.

Indeed, the pyrylium cation is a hybrid of resonance between an oxonium form and three carboniums forms (Figure 6).

Figure 6 

Pyrylium cation resonance structures.

So, C₂, C₄, and C₆ exhibit a positive charge in the main resonance structures. The NMR chemical shifts (calculated by the GIAO method) [14] obtained for carbon atoms of the pyrylium cation are in good agreement with these results.

HMBC correlation spectrum based on the ¹H NMR data observed with compound 3 shows that proton H₈ is coupled with carbon C₆, C₄, and C₉ but not with C₇. Also, H₇ is coupled with C₆ and C₅ but not with C₈ (Figure 7).

Figure 7 

Pertinent HMBC correlations observed with 3.

Carbon C₇ resonates between 𝛿_C 118 and 𝛿_C 124 while C₈ resonates between 𝛿_C 145 and 𝛿_C 149 for all the compounds. This difference is due to the mesomeric forms of the molecule given positive load on carbon 8 (Figure 8).

Figure 8 

Mesomeric forms of 1.

This mesomeric form is the same for all the compounds. The net atomic charge for carbon atoms obtained by AM1 semiempirical calculation corroborates with allocations made (Table 2). Indeed, the highest is the net atomic charge, and the more unshielded carbon is observed [15].

3. Substitution Effect Analysis

Comparing the pyrylium cation structure and the obtained ¹³C NMR spectra, we were interested by methyl group position in pyrylium ring, effect on the chemical shifts of styryl group and pyrylium ring carbons.

Disturbance made by the electron donating methyl group led to the unshielding of substituted carbon with a gap of about 𝛿_C 14, while affecting slightly chemical shifts of the other carbons (Table 3). The effect was noticed with isothiazol, where the presence of methyl group causes the unshielding of substituted carbon by 𝛿_C 10 to 𝛿_C 15 [16].

Table 3 

Gap of chemical shifts of styryl group and pyrylium ring carbons for compounds 1, 2, 3 and 4.

Pyrylium ring ¹³C NMR of the 2,4,6-triphenylpyrylium is given in the literature [17]. When one phenyl group is substituted by a styryl group (formation of 1 or 3), we observe a shielding of about 𝛿_C 4 of carbons C₂ and C₆, and about 𝛿_C 6 of C₄. This shielding could be explained by the paramagnetic anisotropy, which is more intense around 2,4,6-triphenylpyrylium which has 3 phenyls groups directly attached to the pyrylium ring.

When styryl group is in position 4, the methyl carbon resonates at 𝛿_C 15.10; when the styryl group is bearing in position 6, the carbon resonates at 𝛿_C 17.63. This slight unshielding is certainly due to a magnetic anisotropy effect [11].

4. Conclusion

Protons and carbons chemical shifts for four styrylpyrylium perchlorates were allocated notably thanks to correlation spectra and to DEPT. Carbon 13 chemical shift assignments have been confirmed by net atomic charge for carbon obtained by AM1 semiempirical calculation. The scalar coupling value indicates an (E) stereochemistry around the double bond C=C of the styryl group.

The effects of the styryl group position in the pyrylium ring and the substitution effects (methyl and phenyl groups) will be compared to their reduction potential obtained by cyclic voltammetry.

5. Experimental Part

Styrylpyrylium perchlorates are prepared as described in literature by Simalty et al. [18] or with a procedure via 𝛿-diketone for compounds 1 and 2 [19].

The NMR spectra were recorded at 298 K in DMSO-d6 on a Bruker Avance spectrometer operating at 7.05 T (300 MHz for ¹H and 75.4 MHz for ¹³C) or Varian VNMRS spectrometers operating at 9.40 T (400 MHz for ¹H and 100.6 MHz for ¹³C) and 14.09 T (600 MHz for ¹H and 150.9 MHz for ¹³C).

The chemical shift scales were calibrated using the signal of the solvent (2.50 ppm for ¹H and 39.5 ppm for ¹³C) [20] or the signal of internal TMS (0.00 ppm).

Mass spectra are recorded using ES ionization with a Waters QTOF2 spectrometer.

Calculations were carried out using the CS MOPAC programs version 5.0. All the structures were completely optimized by the AM1 method [21].

1
2,6-diphenyl-4-styrylpyrylium perchlorate [18, 19].
Red compound; yield: 31%; Mp: 274^∘C.
IR (cm^-1): 1638.35; 1603.67; 1593.16; 1576.99; 1517.97; 1495.32; 1469.78; 1191.17; 1083.42; 984.77; 776.86; 714.24; 683.39; 646.12; 621.12.
MS ES: m/z (%): 337(5); 336(35); 335 [M⁺] (100); 254(11); 253(55); 231(11); 178(21); 157(22); 129(28); 100(5).
¹H-NMR; 𝛿 (ppm): 8.94 (s, 2H, pyr H₃ and H₅); 8.70 (d, 1H, =C₈H–); 7.50–8.50 (15H, Ar and 1H, d, –C₇H= at 7,70).
NMR ¹³C: 168.88 (C₂, C₆); 114.90 (C₃, C₅); 163.23 (C₄); 123.77 (C₇); 148.67 (C₈).

2
3-methyl-2,6-diphenyl-4-styrylpyrylium perchlorate [18, 19]
Red compound; yield: 20%; Mp: 287^∘C.
IR (cm^-1): 1626.16; 1601.89; 1590.82; 1574.97; 1508.48; 1080.41; 978.00; 738.37; 728.39; 698.87; 623.78.
MS ES: m/z (%): 350(25); 349 [M⁺] (100); 335(10); 267(11); 254(15); 253(100); 244(5); 159(5); 157(58); 120(8).
¹H-NMR; 𝛿 (ppm): 9.17 (s, 1H, pyr H₅); 8.70 (d, 1H, =C₈H–); 7.7–8(m, 16H, Ar and –CH₇=); 2.67 (s, 3H, –CH₃).
NMR ¹³C: 168.97 (C₆); 167.19 (C₂); 163.61 (C₄); 149.22 (C₈); 127.52; 121.18; 112.75.

3
2,4-diphenyl-6-styrylpyrylium perchlorate [18].
Red compound; yield: 22%; Mp: 213^∘C.
IR (cm^-1): 1629.89; 1592.25; 1504.82; 1492.34; 1467.63; 1215.65; 1092.70; 975.90; 766.67; 679.75; 622.87.
MS ES: m/z (%): 337 (5); 336 (30); 335 [M⁺](100)
¹H-NMR; 𝛿 (ppm): 9.02 (s, 1H, pyr H₅); 8.8(s, 1H, pyr H₃); 8.5(d, 1H, =C₈H–); 7–8(m, 16H, Ar and –C₇H=)
NMR ¹³C: NMR ¹³C: 170.15 (C₆); 169.12 (C₂); 163.69 (C₄); 145.80 (C₈); 134.96; 134.87; 134.38; 132.53; 132.17; 129.96; 129.76; 129.53; 129.47; 129.25; 129.15; 128.71; 118.85 (C₇); 116.93 (C₅); 114.28 (C₃).

4
3-methyl-2,4-biphenyl-6-styrylpyrylium perchlorate [18]
Red compound; yield: 23%; Mp: 208^∘C.
IR (cm^-1): 1626.30; 1606.20; 1594.25; 1504.85; 1091.11; 972.04; 764.06; 698.70; 624.03.
MS ES: m/z (%): 350 (12); 349 [M⁺] (45); 244 (15); 243 (100); 179 (12); 101 (6).
¹H-NMR; 𝛿 (ppm): 8.52 (s, 1H, pyr H₅) 8.4 (d, 1H, =C₈H–); 7.5–8 (m, 16H, Ar and –C₇H=); 1.9 (s, 3H, –CH₃)
NMR ¹³C: 169.60 (C₆); 169.46 (C₂); 167.94 (C₄); 146.40 (C₈); 135.14; 134.26; 133.04; 132.23; 131.88; 130.65; 130.24; 129.44; 129.33; 129.26; 129.24; 129.13; 128.39; 121.01 (C₇); 118.34 (C₅); 17.63 (CH₃).

Acknowledgments

The authors gratefully acknowledge Professor Michel Luhmer and Mrs. Rita D’Orazio (CIREM, Université Libre de Bruxelles, Belgium) for the NMR measurements.