A. Synthesis and methods

The acetylation of 4-hydroxycoumarin to give 3-acetyl-4-hydroxycoumarin 1 was carried out by the method of Dholakia et al. (Reference Dholakia, Parekh and Trivedi1968) by using glacial acetic acid in the presence of POCl₃. The chalcone was synthesized using the Claisen–Schmidt (Claisen and Claparède, Reference Claisen and Claparède1881; Schmidt, Reference Schmidt1881) reaction by the condensation of 3-acetyl-4-hydroxycoumarin 1 and 4-hydroxy-3-methoxy benzaldehyde (vanillin) 2 in chloroform with an appropriate quantity of piperidine (mild organic base). This led to vanillin coumarin chalcone 3 (4-hydroxy-3-[(E)-4-hydroxy-3-methoxyphenylprop-2-enoyl]-2H-chromen-2-one) demonstrated in the scheme (see Scheme 1). The mixture was refluxed in a water bath. After cooling at room temperature, an orange solid was obtained in good yield (70%), filtered, washed with ethanol, and dried in air. The melting point 488 K was measured by the Kofler–Bench apparatus. All chemicals and solvents were of analytical grade and were used without further purifications. The 4-hydroxy-3-methoxy benzaldehyde was purchased from Sigma-Aldrich Company and the 4-hydroxycoumarin from Acros Organics.

B. Characterization

The synthesized compound was recrystallized by slow evaporation of acetone at room temperature for several days. The IR spectrum was recorded in the wave number range 400–4000 cm⁻¹ by the KBr pellet technique on a Bruker Tensor 27 FTIR spectrometer. ¹H NMR spectrum was recorded on a Bruker AV-300 NMR spectrometer in CDCl₃ and TMS as an internal standard. The chemical shifts are expressed in δ-units. UV–vis electronic absorption spectra were recorded on a Hewlett Packard Model 8453.

The IR spectrum (see Figure 1) of the title compound shows a broad absorption band at 3450 cm⁻¹ which indicates OH stretching vibration that may be attributed to coumarin hydroxyl. It can also be attributed to water molecules from air or moist KBr. The shoulder at 3250 cm⁻¹ could be assigned to the stretching vibration of phenyl hydroxyl groups. The spectrum also shows an intense stretching band at 1743–1679 cm⁻¹ for the typical lactone carbonyl group. The band at 1613 cm⁻¹ is assigned to the C = O stretching for the ketone group corresponding to the prop-2-en-1-one bond in the structure of the synthesized chalcone. This band becomes less intense compared to the analogous band in the spectra of the starting compound 3-acetyl-4-hydroxycoumarin (Manaev et al., Reference Manaev, Chibisova, Lyssenko, Antipin and Traven2006a). All of absorption bands are comparable to those in related structures (Hamdi et al., Reference Hamdi, Saoud, Romerosa and Ben Hassen2008, Reference Hamdi, Bouabdallah, Romerosa and Ben Hassen2010; Naseema et al., Reference Naseema, Sujith, Manjunatha, Kalluraya, Umesh and Rao2010; Naruka and Mahajan, Reference Naruka and Mahajan2011). In addition to the stretching vibration of C = C and methoxy group O–CH₃ in the chalcone unit, corresponding to the bands at 1590 and 1495 cm⁻¹, respectively, the C–O–C band is observed at 1127 cm⁻¹.

Figure 1.

(Colour online) Infrared spectra of the novel coumarin chalcone.

NMR spectrum was recorded for diluted solutions in CDCl₃ at 300 MHz for ¹H and at 75 MHz for ¹³C. Multiplicities of proton resonance in the ¹H NMR spectrum are designated as singlet (s), doublet (d), triplet (t), quartet (q), and multiplet (m): d (ppm) 6.98–8.36 (m, 9H, H_Ar + H–C=), 6.05, 4.01 (2s, 2H, 2OH), 5.27 (s, 3H, OCH₃). ¹³C NMR (ppm): 192 (C(O)–C = ; enol); 181.83 (C(O) = C) 149.48 (C(O)–O); 148.10 (C = C–Ar) 110.15 (C(O) = C), 114–125, (C_Ar); 124.33 (C(O)–C=); 56.14 (C(O)–CH₃).

Typical UV–vis absorption spectra of the title compound was measured at room temperature in different solvents (see Figure 2), such as acetone, dichloromethane (DCM), tetrahydrofurane (THF), dimethylsulphoxide (DMSO), and methanol (MeOH), which differ considerably in polarity and also in ability to form hydrogen bonds. The measurements were taken at low concentration (5 × 10⁻⁵ M) to reduce the effect of self-absorption and aggregation in absorption.

Figure 2.

(Colour online) UV–vis absorption spectra of title compound taken in different solvents at the concentration of 5 × 10⁻⁵ mol l⁻¹.

We find that the spectra for all solutions are broad and their peak positions are sensitive to the solvent polarity. Moreover, the compound displays in different solvents intense and wide absorption bands. An electron-withdrawing substituent such as hydroxyl and methoxy group at the phenyl group in the positions 4 and 3, respectively, of the benzaldehyde coumarine chalcone (Manaev et al., Reference Manaev, Chibisova and Traven2006b; Ghouili et al., Reference Ghouili, Dusek, Petricek, Ben Ayed and Ben Hassen2014) induces a remarkable red shift of the absorption maximum. Two absorption regions are recognized, a broad absorption band with low intensity in the 320–360 nm region and an absorption band with medium intensity covers the 360–450 nm region. The long-wavelength band undergoes a marked red shift at about 21 nm on going from DCM to DMSO increasing the solvent polarity, whereas the position of the second band shows no solvent dependence. Furthermore, solvent polarity has a pronounced effect on the intensities of these two absorption bands.

It was observed that the absorption spectra of the title compound in THF and DMSO were red shifted compared with that in DCM, MeOH, and acetone. Moreover, the weak peak has dramatically no solvatochromic shift than the absorption maximum in more polar solvents. The weak peak is centered at 350 nm in all solvents due to π–π* transitions characteristic for the basic coumarin skeleton (Sun and Cui, Reference Sun and Cui2008), while absorption maximum is at 409 and 430 nm in DCM and DMSO, respectively. The absorption bands in the region 405–450 nm exhibit an absorption maximum at 430 nm, which can be attributed to π–π transition involving the whole electronic system of the compound with a considerable charge-transfer character originating mainly from the phenyl ring and pointing toward the coumarin system, which is characterized by a high electron-accepting character (Donovalova et al., Reference Donovalova, Cigan, Stankovicova, Gaspar, Danko, Gaplovsky and Hrdlovic2012).

C. Crystal structure determination and refinement

For the powder diffraction experiment the sample was ground and placed into the 0.3 mm borosilicate glass capillary. After that it was measured in transmission mode on the PANalytical Empyrean powder diffractometer from 4° to 80° 2Θ with CuKα _1,2 radiation (λ = 1.541 84 Å, focusing mirror, PIXCel^3D detector, step size was 0.013° 2Θ, irradiated length of the capillary was 20 mm).

The indexation in Crysfire2004 (Shirley, Reference Shirley2000) and subsequent Le Bail fitting in JANA2006 (Petricek et al., Reference Petricek, Dusek and Palatinus2006) software found the monoclinic unit-cell a = 14.3181(4) Å, b = 8.040 71(9)  Å, c = 13.5524(3)  Å, β = 100.356 (2)°, and V = 1534.84(6) Å³. After that we used the direct space method called parallel tempering implemented in the program FOX (Favre-Nicolin and Černý, Reference Favre-Nicolin and Černý2002) to determine the crystal structure from powder diffraction data. The starting model of the asymmetric part of the unit cell was built from an existing crystal structure with CSD ref. code EVOPAH (Ghouili and Ben Hassen, Reference Ghouili and Ben Hassen2011) where only the missing parts were placed in the theoretical positions using the usual bond distances and bond angles.

The suggested crystal structure by FOX was refined in JANA2006 software. The final refinement allowed removing the rigid body model and allowed refining positions and one isotropic ADP parameter of all non-hydrogen atoms (hydrogen atoms were kept in their theoretical positions) together with profile parameters, zero-shift, background, and unit-cell parameters. The model had to be restrained with all bonds and bond-angles soft restraints (tolerance was set to 0.001 for bonds and 0.01 for angles) to get the result with reasonable geometry. We used the Mogul (Bruno et. al, Reference Bruno, Cole, Kessler, Luo, Motherwell, Purkis, Smith, Taylor, Cooper, Harris and Orpen2004) software to get the most realistic values of the bonds and the bond angles that are not participating in aromatic rings. For aromatic rings we used bond and bond angles of two existing crystal structures with CSD ref. codes EVOPAH and AVABEE (Arrieta and Mostad, Reference Arrieta and Mostad2004). Hydrogen atoms of two hydroxyl groups were placed in the direction of possible hydrogen bonds. The final agreement profile factors were: R _p = 2.40%, R _wp = 3.38%, GOF = 3.47 with details of measurement and refinement given in Table I and with the final plot shown in Figure 3. The supplementary crystallographic data are available free of charge from the Cambridge Crystallographic Data Centre 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033 (http://www.ccdc.cam.ac.uk, CCDC 959959).

Figure 3.

(Colour online) Final Rietveld plot showing the measured data (red thin-cross), calculated data (black line), and difference curve (blue line). Calculated Bragg positions are shown by vertical bars.

Table I.

Refined structural parameters of 4-hydroxy-3-methoxyphenyl-4-hydroxycoumarin.