Struct Chem (2008) 19:975–982 DOI 10.1007/s11224-008-9384-x

ORIGINAL RESEARCH

Ethyl esters of coumarin-3-phosphonic acid and 1,2benzoxaphosphorine-3-carboxylic acid: crystal structure, spectroscopic and theoretical elucidation B. B. Koleva Æ R. D. Nikolova Æ S. Zareva Æ T. Kolev Æ A. G. Bojilova Æ H. Mayer-Figge Æ W. S. Sheldrick

Received: 29 August 2008 / Accepted: 17 October 2008 / Published online: 6 November 2008 Ó Springer Science+Business Media, LLC 2008

Abstract IR-spectroscopic characterization of the coumarin-3-phosphonic acid and 1,2-benzoxaphosphorine3-carboxylic acid ethyl esters has been carried out by means of linear-polarized IR (IR-LD) spectroscopy of oriented colloid suspensions in a nematic host. Quantum chemical DFT calculations at the B3LYP level of theory and 6-311??G** basis set were performed. The electronic structure and vibrational properties of both compounds are discussed. The spectroscopic data for 2-benzoxaphosphorine-3-carboxylic acid ethyl ester are in accordance with the crystal structure determined by single crystal X-ray diffraction. The compound C13H15O5P crystallizes in the noncentrosymmetric space group P212121, and its structure consists of a 3D network formed by short contacts of the ˚. type P=O


HC(Ar) with distances of 3.420 and 2.467 A The geometry of the PO3C fragment exhibits a pseudo Td symmetry.

B. B. Koleva (&)
H. Mayer-Figge
W. S. Sheldrick Lehrstuhl fu¨r Analytische Chemie, Ruhr-Universita¨t Bochum, Universita¨tsstraße 150, 44780 Bochum, Germany e-mail: [email protected] R. D. Nikolova
A. G. Bojilova Department of Organic Chemistry, University of Sofia ‘‘St. Kl. Ohridsky’’, 1164 Sofia, Bulgaria S. Zareva Department of Analytical Chemistry, University of Sofia ‘‘St. Kl. Ohridsky’’, Sofia 1164, Bulgaria T. Kolev Institut fu¨r Umweltforschung, Universita¨t Dortmund, Otto-Hahn-Strasse 6, 44221 Dortmund, Germany

Keywords Coumarin
Phosphonates
Benzoxaphosphorines
IR-LD analysis
Solid-state DFT calculations

Introduction Interest in coumarin derivatives is based on their wide biological activity. A spasmolytic effect and antiarhythmic, cardiothonic, antiviral and anticancer properties have been observed [1–3]. Coumarins containing phosphonic acid represent a novel class of compounds with remarkable cytotoxicity activity towards selected tumour cell lines [1–3]. Derivatives with the phosphorus atom at position 2 of the c-pyrone ring are known as efficient antibacterial agents [4–7]. Our systematic investigations of derivatives of coumarin-3-phosphonic acid have been mainly concerned with these known facts [8–12]. In contrast to the large number of synthetic papers [8–12], very few reports of the optical and magnetic properties of this class of coumarin derivatives have appeared [13, 14]. In our previous articles, the correlation between the structure and spectroscopic properties of coumarin-3-phosphonic acid and its esters has been elucidated [15, 16]. We now report, the IR spectroscopic elucidation of the ethyl esters of coumarin-3-phosphonic acid (1) and 2-ethoxy-2-oxo-2H1,2-benzoxaphosphorine-3-carboxylic acid (2), as depicted in Scheme 1. Linear-polarized IR (IR-LD) spectroscopy of oriented colloid suspensions in a nematic liquid crystal was successfully applied for IR-spectroscopic band assignment and structural elucidation of the embedded compounds. Quantum chemical density functional theory (DFT) calculations at the B3LYP level of theory and 6-31??G** basis set were performed with a view to calculating the electronic structure and vibrational properties. The

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O P OCH2CH3 OCH2CH3 O

O

O OCH2CH3 P O O OCH2CH3

Scheme 1 Chemical diagram of (1) and (2)

structure of (2) was obtained by means of single crystal X-ray diffraction.

Experimental Materials and methods Conventional and polarized IR-spectra were measured on a Bomem Michelson 100 FTIR-spectrometer (4000– 400 cm-1, 2 cm-1 resolution, 200 scans) equipped with a Perkin-Elmer wire-grad polarizer. Non-polarized solidstate IR-spectra were recorded using the nujol mull technique. The oriented samples were obtained as a colloid suspension in a nematic liquid crystal ZLI 1695. The theoretical approach, experimental technique for preparing the samples, procedures for polarized IR-spectra interpretation and the validation of this new linear-dichroic infrared orientation solid-state method for accuracy and precision have been presented. The influence of the liquid crystal medium on peak positions and integral absorbances of the guest molecule bands, the rheological model, the nature and balance of the forces in the nematic liquid crystal suspension system, and the morphology of the suspended particles have also been discussed [17–20]. IR-spectra in chloroform solution were recorded using 0.044 cm KBr cells. Quantum chemical calculations were performed with the GAUSSIAN 98 and Dalton 2.0 program packages [21, 22]. The output files were visualized by means of the ChemCraft program [23]. The geometries of (1)–(3) were optimized at DFT using the 6-311??G** basis set. The DFT method employed is B3LYP, which combines Backe’s three-parameter non-local exchange functional with the correlation functional of Lee, Yang and Parr. Molecular geometries of the studied species were fully optimized by the force gradient method using Bernys’ algorithm. For every structure, the stationary points found on the molecule potential energy hypersurfaces were characterized using standard analytical harmonic vibrational analysis. The absence of the imaginary frequencies, as well as of negative eigenvalues of the second-derivative matrix, confirmed that the stationary points correspond to minima of the potential energy hypersurfaces. The calculated vibrational frequencies and infrared intensities were checked to establish which kind of performed calculations

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agreed best with the experimental data. The DFT method provides accurate vibrational data, as far as the calculated standard deviations of less then 11 cm-1 are concerned, which correspond to groups, not participating in significant intra- or intermolecular interactions. A modification of the results using the empirical scaling factor 0.9614 was performed to achieve better correspondence between the experimental and theoretical values. The X-ray diffraction intensities were measured in the x scan mode on a Siemens P4 diffractometer equipped with ˚ hmax = 25°). The single Mo-Ka radiation (k = 0.71073 A crystal X-ray diffraction data and the structure was solved by direct methods and refined against F2 [24, 25]. An ORTEP plot illustrates the structure at the 50% probability level. Relevant crystallographic structure data and refinement details are presented in Table 2, selected bond distances and angles in Table 3. The hydrogen atoms were constrained to calculated positions and refined using riding models in all cases.

Scheme 2 Most stable conformers of (1) and (2)

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Scheme 3 Visualization of the selected transition moments of (1) and (2)

Compounds (1) and (2) were synthesized according to the procedure given in the literature [8]. To a solution of triethylphosphonoacetate (20 mmol) and salicylaldehyde (20 mmol) in dry toluene (60 mL), piperidine was dropped. The solution was refluxed under a Dean-Stark trap for 4 h

and the reaction mixture was worked up as previously described [8]. The products were recrystallized from n-hexane and melting points were obtained in agreement with the expected values: 71–73 °C for the ethyl ester of (1) and 65–67 °C for (2).

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Results and discussion Molecular geometry Theoretical characterization of (1) and (2) was carried out using the tool described in [15, 16]. The conformational analysis was carried out by variation of the dihedral angles. The most stable conformers of (1) and (2) (Scheme 2), correspond to Erel values of 0.2 kJ/mol (1) and 1.4 kJ/mol (2), respectively. The aromatic fragment in coumarin is effectively flat in (1) with a maximal deviation from total planarity of less then 0.1°. In (2), the plane of the P(=O)O2group is tilted towards the coumarin plane at an angle of 1.5(3)°. Similar to other derivatives [15, 16], the latter result indicates a co-linear disposition of the transition moments for the out-of-plane modes (a00 ) (Scheme 3). The calculated geometrical parameters, bond lengths and angles correlated well with experimental values for other coumarins, as previously determined by single crystal X-ray diffraction [26–30] and for the reported crystal structure of ˚ and 3.2(1)° are 2. Differences of less than 0.1002 A observed. The phosphonic acid fragment in (1) exhibits a distorted Td geometry and O–P–O angles within the range 101.1(2)–117.1(5)°. In (2), the PCO2 fragment also displays a distorted Td geometry with angles between 99.1(4)° and 116.4(2)°. The C=O and P=O groups are disposed in

(1) (2)

3000

2500

2000

1500

1000

500

Absorbance/Wavenumber (cm-1) Fig. 1 Calculated (non-scaled) IR-spectra of (1) and (2)

manner leading to angle of 90.1(2)° between the transition moments of the mC=O and mP=O stretching vibrations (Scheme 3). The calculated IR-spectra of (1) and (2) are depicted in Fig. 1. The presence of the aromatic benzene ring in both molecules leads to the observation of the typical a0 and a00 modes for 1,2-disubstituted benzenes within the 1620– 1450 cm-1 and 900–650 cm-1 ranges (see Scheme 3). The

Fig. 2 Non-polarized IR (1) and difference IR-LD (2) spectra of (1) and (2)

(2)

(2)

(1)

(2)

(1)

(1)

1800

1600

1400

1200

1000

Absorbance / Wavenumber (cm-1)

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800

600

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Table 1 Characteristic IR bands of (1) and (2) in the solid state Assignment

1704

(4)

(2)

(3)

(2)

m (cm-1) (1)

(2)

mC=O

1743/1723

1720/1704

i.p. (a0 )*

1610, 1482, 933

1604, 1484, 937

mP=O

1249

1259

mas P–O msP–O

1165, 1139 1054, 1022

1209 1068

o.p. (a00 )a

761

769

cC=O

569

784

dC=O

520

678

a

1720

The non-polarized IR and difference IR-LD spectra of (1) and (2) are depicted in Fig. 2. The corresponding assignments listed in Table 1 were obtained by reducingdifference IR-LD spectral analysis (see below), preliminary deconvolution, curve fitting and second derivative analysis of the IR-spectroscopic patterns of both (1) and (2), respectively. A comparison between the studied compounds and the data of previous reports on different substituted esters of coumarin-3-phosphonic acid has been carried out (see Table 1). A series of the low-intensive bands corresponding to asymmetric and symmetric stretching vibrations of the –CH2 and –CH3 groups as well as the in-plane (a0 ) stretching CH vibrations of the aromatic fragment is observed within the 3100–2800 cm-1 IR region in both molecules. The IR-band of the mC=O stretching vibration in (1) is shifted to a higher frequency than the corresponding maximum in (2), in accordance with theoretical analysis discussed above. The observation of pairs of bands can be explained as a result of the crystal field splitting effect. Moreover, the IR-spectra in solution

1727

Conventional and linear-polarized IR-spectroscopic data

are characterized with only one band at 1746 cm-1 (1) and 1725 cm-1 (2). The corresponding maxima differ by 23 cm-1. The mC=C stretching modes are observed as low-intensive bands at 1631 cm-1 (1) and 1622 cm-1 (2), respectively. The theoretical values, using the scaling factor 0.9614, of mC=C (Scheme 3) are 1630 cm-1 in the first and 1625 cm-1 in the second compound (Scheme 3). Looking at the differences between the theoretical and experimental values for each of the compounds, we can observe that in (1) a difference of 9 cm-1 is obtained, while in (2) the value is only 2 cm-1. The data are compared in addition with those in chloroform solution, where in first case a band at 1634 cm-1 is observed, while in second compound to mC=O can be assigned a maximums at 1627 cm-1. These data. This result can be explained by the fact that in (1), the flat geometry of the coumarin fragment leads to a conjugation between the C=C and C=O bonds and low-frequency shifting of the corresponding IR band of mC=C. In (2), the C=C and P=O bonds are inclined at an angle of 134.8(6)°, so that conjugation is no longer possible

1743

carboxyl ester fragment is characterized by its typical mC=O, mC–O(R), dC=O and cC=O frequencies. As a rule, the mC=O stretching mode in the discussed fragments is observed at lower frequencies than in the coumarins, where the strain of the cyclic system leads to a high frequency shift of the corresponding IR band. For this reason, the mC=O bands are predicted at 1750 cm-1 (1) and 1735 cm-1 (2), respectively. The presence of the ring system also affects s the peak positions of the mP=O, mas PO2 and mPO2 stretching vibrations. In (2), the predicted values are shifted to higher frequencies at values within 30–10 cm-1. The corresponding values of (1) are 1265, 1143 and 1055 cm-1, respectively.

The a0 and a00 modes are assigned according to the Cs symmetry of the molecules

(1)

(1) 1750

1700

1650

1600

1550

Absorbance / Wavenumber (cm-1)

Fig. 3 Non-polarized IR-spectra of (1) (1) and (2) (3); difference IRLD spectra of (1) and (2) after elimination of the bands at 1743 cm-1 (2) and 1720 cm-1 (4), respectively

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Struct Chem (2008) 19:975–982

Table 2 Crystal data, intensity collection and refinement conditions for (2)

˚ ) and angles (8) for (2) Table 3 Bond lengths (A P1–O3 1.457(3)

C6–C7 1.389(5)

Empirical formula

C13H15O5P

P1–O1 1.556(3)

C6–C8 1.402(5)

Formula weight

279.28

P1–O4 1.596(3)

C7–C15 1.399(6)

Temperature

293(2) K

P1–C2 1.775(4)

C8–C11 1.384(5)

Wavelength

˚ 0.71073 A

O11–C9 1.332(5)

C9–O12 1.196(5)

Crystal system, space group

Orthorhombic, P212121 ˚ a = 7.2622(19) A

O11–C14 1.458(4)

O1–C17 1.420(7)

C2–C5 1.336(5)

C11–C13 1.374(6)

˚ b = 11.199(3) A ˚ c = 17.068(3) A

C2–C9 1.487(5)

C14–C16 1.480(8)

O4–C8 1.383(5)

C17–C18 1.430(8)

C5–C6 1.447(4)

Volume

a = b = c = 90.0° ˚3 1388.1(5) A

O3–P1–O1 110.28(19)

C8–C6–C5 120.9(3)

Z Calculated density

4 1.336 Mg/m3

O3–P1–O4 113.59(19)

C6–C7–C15 120.8(4)

O1–P1–O4 103.50(18)

C11–C8–O4 117.5(3)

Absorption coefficient

0.209 mm-1

O3–P1–C2 114.57(19)

C11–C8–C6 121.9(4)

F(000)

604

Crystal size

0.59 9 0.51 9 0.39 mm3

O1–P1–C2 111.83(19) O4–P1–C2 102.34(16)

O4–C8–C6 120.6(3) O12–C9–O11 124.3(4)

h Range for data collection

3.00–30.008

C9–O11–C14 117.3(3)

O12–C9–C2 123.3(4)

Limiting indices

-1 B h B 10, -1 B k B 15, 24 B l B 1

C5–C2–C9 122.6(3)

O11–C9–C2 112.5(3)

Reflections collected/unique

2792/1970 [R(int) = 0.0248]

C5–C2–P1 120.6(3)

C17–O1–P1 123.1(4)

Refinement method

Full-matrix least-squares on F2

C9–C2–P1 116.5(3)

C13–C11–C8 118.9(4)

Data/restraints/parameters

1970/0/175

C8–O4–P1 127.1(2)

C15–C13–C11 121.1(4)

Goodness-of-fit on F2

1.007

C2–C5–C6 124.7(3)

O11–C14–C16 107.3(4)

Final R indices [I [ 2r(I)] R indices (all data)

R1 = 0.0582, wR2 = 0.1475 R1 = 0.1724, wR2 = 0.0875

C7–C6–C8 117.3(3)

C13–C15–C7 119.9(4)

C7–C6–C5 121.8(3)

O1–C17–C18 110.4(6)

Unit cell dimensions

and, as a result, the P=O bond does not affect the peak position of the mC=C vibration. The values of mP=O, mas PO2 and msPO2 in (2) are shifted to higher frequencies than those observed in (1) with a difference between the values of 44 and 10 cm-1. A significant influence is observed for the -1 mas PO2 stretching vibration in (2) at 1209 cm , in comparison with the corresponding value in (1) at 1165 cm-1 (Table 1). The mP=O band in (2) is shifted by 10 cm-1 to a higher frequency than the IR-band in (1), which is observed at 1249 cm-1. These data correlate well with theoretically predicted values and tendencies discussed above. The values obtained for (1) correlate well with those of different substituted esters of coumarin-3-phosphonic acid s [16], where the mP=O, mas PO2 and mPO2 stretching frequencies are observed at 1255, 1160 and 1020 cm-1, respectively [16]. The most intensive a00 mode of the benzene ring in (1) and (2) is observed at about 765 cm-1. The application of the reducing difference procedure for the interpretation of the polarized IR-LD spectra leads to the following results: (1) the elimination of the bands at about 1610 cm-1 leads to disappearance of the maxima at about 1480 and 930 cm-1 due to their possessing same symmetry class; (2) the disappearance of the bands at 1743 and 1727 cm-1 at equal dichroic ratio is possible if mC=O and mC=C are oriented in a co-linear manner, which is the

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Fig. 4 The molecular structure of (2), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level

case in the flat and conjugated C=C–C=O fragment (Scheme 2, Fig. 3(2)). In contrast, the discussed bands in (2) are eliminated at a different dichroic ratio, which is in accordance with the structure obtained in Scheme 2 (Fig. 3(4)); (iii) The a00 , and cC=O IR-bands at 761 and 569 cm-1 are eliminated at an equal dichroic ratio, which are also in accordance with the geometry obtained in

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Fig. 5 Short contacts and the PO3C fragment

Scheme 2. The elimination of the bands at 769 cm-1 (a00 ) and 784 cm-1 (cC=O) is observed at different dichroic ratios, which also confirms that the C=O fragment in (2) is tilted at an angle of 123.4(4)° towards the plane of the benzene (Scheme 2).

˚ . The geometry of the PO3C fragment can be 2.467 A described as distorted tetrahedral.

Crystal structure of (2)

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 693517. Copies of this information may be obtained from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: ?44 1223 336 033; e-mail: [email protected] or http://www.ccdc.cam. ac.uk).

Relevant crystal data and refinement details for (2) are presented in Table 2. The bond lengths and angles are summarized in Table 3. Figure 4 depicts the molecular structure of (2) with the thermal ellipsoids drawn at the 50% probability level. A 3D network is formed by short contacts of the type P=O


HC(Ar) with bond lengths of ˚ . The geometry of the PO3C fragment 3.420 and 2.467 A can be described as distorted tetrahedral with corresponding angles in the range 102.3(8)–114.5(7)° (Fig. 5). These results confirm the theoretical and spectroscopic elucidation that predicted angles within the range 99.1(4)– 116.4(4)°. The C=O and P=O groups are disposed in a manner leading to an angle of 90.5(8)° between the transition moments of the mC=O and mP=O stretching vibrations (theoretical value of 90.1(2)°).

Conclusions Two ethyl esters of coumarin-3-phosphonic acid and 1,2benzoxaphosphorine-3-carboxylic acid have been characterized structurally, spectroscopically and theoretically by means of IR-LD spectroscopy of oriented colloid suspensions in a nematic host and quantum chemical DFT calculations at the B3LYP level of theory with a 6-311??G** basis set. The electronic structure and vibrational properties of both compounds are discussed. The spectroscopic data for the 2-benzoxaphosphorine3-carboxylic acid ethyl ester are in accordance with the crystal structure of the compound. C13H15O5P (2) crystallizes in the non-centrosymmetric space group P212121. The structure consists of a 3D network formed by short contacts of the type P=O


HC(Ar) with bond lengths of 3.420 and

Supporting information

Acknowledgements B.K. wishes to thank the Alexander von Humboldt Foundation for the Fellowship and T.K. the DAAD for a grant within the priority program ‘‘Stability Pact South-Eastern Europe’’ and the Alexander von Humboldt Foundation. R.N. is grateful for the financial support from the Rila program of the Bulgarian National Research Fund.

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