Journal of Molecular Structure 979 (2010) 115–121

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Journal of Molecular Structure

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DFT, IR, Raman and NMR study of the coordination abilityof coumarin-3-carboxylic acid to Pr(III)

Ivelina Georgieva a, Irena Kostova b,*, Natasha Trendafilova a, Vinod K. Rastogi c, Wolfgang Kiefer d

a Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgariab Department of Chemistry, Faculty of Pharmacy, Medical University, 2 Dunav St., Sofia 1000, Bulgariac Department of Physics, CCS University, Meerut 250 004, Indiad Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

a r t i c l e i n f o

Article history:Received 13 March 2010Received in revised form 26 May 2010Accepted 9 June 2010Available online 15 June 2010

Keywords:CoumarinsPr(III)DFTFT-IRRamanNMR

0022-2860/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.molstruc.2010.06.013

* Corresponding author. Tel.: +359 2 92 36 569; faxE-mail address: irenakostova@yahoo.com (I. Kosto

a b s t r a c t

A new complex of coumarin-3-carboxylic acid (HCCA) with Pr(III) is synthesized and its structure andmolecular properties are investigated by elemental analysis, IR, Raman, NMR measurements and quan-tum chemical calculations. The elemental analysis suggested the general formula Pr(CCA)2(NO3)(H2O).The HCCA ligand possesses two donor groups (deprotonated carboxylic and carbonylic) and can act asa bidentate ligand in two different binding modes. According to the complex general formula, two modelPr(III) structures accounting for the two binding modes of the ligand are modeled in gas phase and in sol-vent environment (DMSO). Geometrical parameters, vibrational frequencies, IR intensities and Ramanactivities as well as 1H and 13C NMR chemical shifts of HCCA and the two model Pr(III) structures are cal-culated with DFT method at B3LYP/SVP and B3LYP/6-31+G(d,p) levels. The comparative vibrational andNMR analyses, based on both experimental and theoretical data of the ligand and the two model Pr(III)structures predict a bidentate binding of the HCCA ligands to Pr(III) through the deprotonated carboxylicoxygen and the carbonylic oxygen. Vibrational, 1H and 13C NMR data able to distinguish the bidentateligand binding modes are established.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The use of metal complexes as therapeutic agents for treatmentof different diseases has been extensively studied. As theygenerally have different mechanisms of activity from the organiccompounds, the development of metal complexes provides analternative route to novel drugs.

Coumarin derivatives are well known compounds found to bepresent in different food sources such as fruits, herbs and vegeta-bles [1]. They are of great interest owing to their important rolein the fields of biology, medicine, industry, botany and chemistry[2–16]. Due to their versatility the coumarin derivatives have beenused in the pharmaceutical industry as antibiotics, antiviral, anti-microbial and anticoagulants agents and as pH indicators in biolog-ical systems and medical sciences. Coumarin derivatives have alsobeen used as sensitizers in phototherapy as well as in the chemicalindustry as optical brightener and laser dyes [17]. The investiga-tion of the binding properties of coumarin derivatives to differentmetal ions could help in understanding the factors controlling theirbiological activity.

ll rights reserved.

: +359 2 987 987 4.va).

Further it is evident from the literature [2–5] that transitionmetal and rare earth complexes of hydroxycoumarin derivativesare also subjects of increasing interest in bioinorganic and coordi-nation chemistry [7,8,13]. Lanthanides(III) complexes show antitu-mor activity [14]. Some biologically active lanthanide complexesof coumarins like 4-hydroxy-3-(3-oxo-1-phenylbutyl)-2H-1-benzopyran-2-one and its derivatives [18], 3,30-benzylidenebis[4-hydroxycoumarin] and its derivatives [19–21], and coumarin-3-carboxylic acid have been reported from our laboratory. Thesestudies highlighted the potential of the developing novellanthanide complexes as anti-cancer therapeutics. However, moreanalogs are required to obtain better activity profiles for this classof compounds.

Coumarin-3-carboxylic acid (HCCA) has previously been used asa ligand in complexation reactions with Cu(II) [22], Sn [23] and lan-thanide cations (Dy(III), Er(III), Eu(III), Gd(III), Tb(III), Sm(III))[24,25]. In a series of papers we have investigated the molecularstructure, energetic and spectroscopic properties of La(III) [26],Ce(III) and Nd(III) [27], Sm(III), Eu(III) and Tb(III) [28] complexesof coumarin-3-carboxylic acid. In the previous studies we pre-dicted the ligand binding mode in the Ln complexes on the basisof the energy calculations of model species M:L = 1:1 andM:L = 1:2 ([Ln(CCA)]2+ or [Ln(CCA)2]+). In this study we presentmore exhaustive modeling of the two neutral complexes account-

116 I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121

ing for the two possible ligand binding modes. We report synthesis,analytical and IR, Raman and 1H, 13C NMR spectroscopic data of anew complex of coumarin-3-carboxylic acid with Pr(III). DFT mod-eling of the neutral complex of the formula Pr(CCA)2(NO3)(H2O)predicted from elemental analysis is performed. Two complex iso-mers with the two possible bidentate HCCA binding modes areconsidered and their relative stability is estimated by energy calcu-lations. Vibrational (IR and Raman) and NMR spectra are calculatedfor the two Pr(III) isomers and on the basis of a comparison withthe experiment the metal coordination polyhedron of the Pr(III)complex is predicted. The new Pr(III) compound completes the ser-ies of lanthanide complexes studied (La, Ce, Pr, Nd) and allows acomparative analysis of geometrical and spectroscopic data.

2. Experimental

2.1. Synthesis of the coordination complex

The compounds used for preparing the solutions are Merckproducts, p.a. grade:Pr(NO3)3�6H2O. Coumarin-3-carboxylic acid(Fig. 1a) is used for the preparation of the metal complex as a li-gand. The complex is synthesized by reaction of praseodymium(III)salt and the ligand, in amounts equal to metal: ligand molar ratioof 1:2. The synthesis of the complex is made in different ratio(1:1, 1:2, 1:3) but in all the cases the product is with the composi-tion 1:2. The complex is prepared by adding the ethanol solution ofPr(III) salt to ethanol solution of the ligand. After stirring with anelectromagnetic stirrer for 1 h at room temperature, a pale yellowsuspension is obtained. The pale yellow solid is filtered, washedthoroughly with ethanol and water and dried in a desiccator toconstant weight. Yellow crystals are obtained. The complex is spar-ingly soluble in common organic solvents but soluble to a largerextent in DMSO. The elemental analyses show that, the complexhas M:L = 1:2 stoichiometry.

2.2. Device descriptions

The elemental analyses for carbon, hydrogen, nitrogen, praseo-dymium and water are performed according to standard microan-alytical procedures.

The FT-IR spectra of the powder samples are recorded in KBr(4000–400 cm�1) using a IFS25 Bruker spectrometer (resolution1 cm�1).

An integrated FRA-106 S Raman module is employed for record-ing the FT-Raman spectra, using a Nd:YAG laser operating at1064 nm line for excitation. The laser power is 300 mW and 50scans are collected for each spectrum. The detection of the Ramansignal is carried out with nitrogen cooled Ge detector. The spectralresolution is 2 cm�1.

(a)

Fig. 1. B3LYP/B1 optimized geometrie

1H NMR spectra are recorded at room temperature on Bruker250 WM (250 MHz) spectrometer in DMSO-d6. Chemical shiftsare given in ppm, downfield from TMS.

13C NMR spectra are recorded at ambient temperature on Bru-ker 250 WM (62.9 MHz) spectrometer in DMSO-d6. Chemical shiftsare given in ppm, downfield from TMS.

2.3. Computational procedure

Geometry optimization of model Pr(CCA)2(NO3)(H2O) struc-tures and consequent calculations of vibrational (IR and Raman),1H and 13C NMR spectra are carried out with DFT method usingGaussian 03 program [29]. Full geometry optimizations are carriedout without symmetry constraints. The data obtained for the mod-el structures are compared with the corresponding results for theisolated neutral ligand (HCCA) obtained at the same level of theory.The main group elements in the Pr(III) complex are calculated withSVP basis set (split valence plus polarization basis set) developedby the group of Ahlrichs [30,31]: for hydrogen atoms (4s1p)/[2s1p]), for carbon, oxygen and nitrogen atoms (7s4p1d)/[3s2p1d]. Diffuse functions are further added to the standard SVPbasis set (one s and one p set) for the O atoms as well as for theC1, C2 and C3 atoms, included in the chelate ring, Fig. 1. The finalbasis set is (8s5p1d)/[4s3p1d] for O and C1, C2 and C3, atoms de-noted as B1. The exponents of the diffuse functions are obtainedby dividing the respective smallest exponent of the SVP basis setby a factor of three. The computations are performed with theLee, Yang and Parr correlation functional (LYP) [32] combined withthe Becke’s non-local three-parameter hybrid exchange functional,(B3) [33]. The adequacy of B3LYP method for prediction of confor-mational behavior, geometrical parameters and vibrational spec-trum of HCCA as well as of structural parameters of lanthanidecomplexes was already proved in other previous studies[26,27,34]. The Pr(III) ion is calculated with the relativistic effectivecore potential (RECP) optimized by the Stuttgart–Dresden group[35]. The large core RECP is used in combination with its optimizedvalence basis set (7s6p5d)/[5s4p3d]. The large core ECP treats[Kr]4d104f2 (Pr) as a fixed core, whereas 5s25p66s25d16p0 shellsare taken into account explicitly (11 valence electrons). It wasfound that in 4f-element complexes (without f-band splitting)the partially filled 4f orbitals are unimportant for the chemicalbehavior of the lanthanide-ligand interactions [35,36]. The metal4f electrons are contracted into the core and do not contribute tothe valence region. The approximate assignment of the calculatedfrequencies to the molecular vibrational modes is performed usingthe atom movements in Cartesian coordinate calculated with theGaussian program. The calculated atom movements are furtherused as an input of the ChemCraft program which animates themand gives the possibility for a visual inspection of each molecularmode [37]. Due to the lack of symmetry the normal vibrational

(b)

s of: (a) HCCA and (b) CCA [26].

I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121 117

modes are significantly mixed. Therefore, determination of the rel-ative weight contribution of each vibrational mode to the observedfrequencies is performed. The projection of internal coordinatesonto each normal modes in terms of percentage relative weightsis computed as implemented in Gaussian 03 [29].

The calculations of the model Pr(III) complexes of HCCA and thecorresponding 1H and 13C NMR chemical shifts are performed inDMSO at B3LYP/6-31+G(d,p) level. The optimized structures inDMSO are used to simulate the solvent effect by means of thePolarizable Continuum Model (PCM) using the integral equationformalism variant (IEFPCM) [38]. The absolute isotropic magneticshielding constants (ri) are used to obtain the chemical shifts(di = rTMS � ri) for C and H atoms by referring to the standard com-pounds, tetramethylsilane (TMS). The TMS is also calculated inDMSO at B3LYP/6-31+G(d,p) level of theory.

3. Results and discussion

3.1. Analytical and physicochemical data

The new complex is characterized by elemental analysis, 1HNMR, 13C NMR, IR and Raman spectroscopies. The elemental anal-

(a)

(b)

Fig. 2. B3LYP/B1 optimized geometries of: (a) Pr(CCA1)2

ysis data of the Pr(III) complex obtained are in agreement with theformula, Pr(CCA)2(NO3)(H2O), where CCA ¼ C10H5O�4 . The elemen-tal analysis data obtained serve as a basis for determination ofthe empirical complex formula:

Elemental analysis ðcalculated=foundÞ : PrðCCAÞ2ðNO3ÞðH2OÞ :

%C ¼ 40:07=40:37; %H ¼ 2:02=1:65; %N ¼ 2:34=2:44

where CCA ¼ C10H5O�4 :

The experimental data obtained from FTIR, Raman, 1H NMR and13C NMR measurements are used further for elucidation of thePr(III) complex structure.

3.2. Molecular structure of Pr(CCA)2(NO3)(H2O)

The molecular structure of Pr(III) complex of HCCA is modeledin agreement with the empirical formula Pr(CCA)2(NO3)(H2O)based on the elemental analysis. Molecular electrostatic potential(MEP) calculations of HCCA in water solution revealed two reactiveregions and hence two possible bidentate binding modes: (1)through O2 and O3 atoms (CCA1) and (2) through O3 and O4 atoms

(H2O)(NO3) and (b) Pr(CCA2)2(H2O)(NO3) complex.

118 I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121

(CCA2) [26]. For the Pr(III) complex of HCCA we model bothbidentate bindings of HCCA, Pr(CCA1)2(NO3)(H2O) (Fig. 2a) andPr(CCA2)2(NO3)(H2O) (Fig. 2b). Energy calculations of the twomodel complexes are performed in gas phase and in polar solution(DMSO). In both environments, Pr(CCA1)2(NO3)(H2O) complex isthe preferred structure (with 8–11 kcal/mol). Therefore, the biden-tate coordination of HCCA through the deprotonated carboxylicoxygen and the carbonylic oxygen is assumed as the more probableone for the Pr(III) complex of HCCA. Further, IR, Raman, 1H and 13CNMR spectra of the two model binding structures are calculatedand compared with the experimental spectroscopic data. The the-oretical simulation of the spectral properties of the two modelcomplexes helped to specify the vibrational and NMR characteris-tics able to distinguish the CCA1 and CCA2 coordinations in themetal complex. Selected atomic distances for optimizedHCCA and CCA are given in Fig. 1 and these of optimizedPr(CCA1)2(NO3)(H2O) and Pr(CCA2)2(NO3)(H2O) are presented in Fig. 2.

Fig. 3. Experimental IR spectra of HCCA and its Pr(III) complex compared to thePr(CCA2)2(H2O)(NO3) at B3LYP/B1.

3.3. Vibrational analysis

The comparative vibrational analysis based on both theoreticaland experimental data of HCCA ligand and the Pr(III) complex is auseful tool for assessment of the ligand structural changes uponcomplexation as well as for suggesting the complex polyhedronand the ligand binding mode. The experimental IR and Ramanspectra of Pr(III) complex of HCCA and the calculated vibrationalspectra, IR intensities and Raman activities of the two model Pr(III)complexes, Pr(CCA1)2(NO3)(H2O) and Pr(CCA2)2(NO3)(H2O), areshown in Figs. 3 and 4. The deviations of the calculated frequenciesfrom the experimental ones are obtained due to the harmonicapproximation of the calculated frequencies as well as due toneglecting of intermolecular interactions in solid. To improve theagreement with the experimental wavenumbers, scaling factor of0.966 is applied to the calculated frequencies, Table S1 (in Supple-mentary materials), Figs. 3 and 4.

calculated (scaled with factor 0.966) IR spectra of Pr(CCA1)2(H2O)(NO3) and

Fig. 4. Experimental Raman spectrum of Pr(III) complex compared to B3LYP/B1 Raman spectra of model Pr(CCA1)2(H2O)(NO3) and Pr(CCA2)2(H2O)(NO3) complexes.

I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121 119

The comparison of the experimental and calculated vibrational(IR and Raman) spectra (Figs. 3 and 4) of Pr(III) complexes revealsbetter agreement with the Pr(CCA1)2(NO3)(H2O) structure (Fig. 2a).According to the IR calculations of Pr(CCA1)2(NO3)(H2O) (calc.1722, 1698 cm�1) the observed bands at 1748 and 1713 cm�1 areassigned to m(C3O4) vibrations of the free carboxylic groups ofthe two HCCA ligands and they are coupled with m(C1O2) modes,Fig. 3. The intense IR band observed at 1684 cm�1 is due to them(C1O2) vibrations of the coordinated carbonylic groups (calc.1653 cm�1).

The Pr(CCA2)2(NO3)(H2O) model calculations reveal differentvibrational behavior: two bands: one at 1794 cm�1 assigned tom(C1O2) vibration of the free carbonylic group and second one at1703 cm�1 attributed to m(C1O2) vibration of the H-bonded C1O2group, Fig. 2b. Thus, the vibration of the free carbonylic groupshould appear at higher frequencies than the experiment shows.At the same time the H-bonding of the carbonylic group lowersthe m(C1O2) frequency about 90 cm�1. The comparison of the

vibrational spectra of two model complex structures shows thatthe vibrational features of the Pr(CCA1)2(NO3)(H2O) structure inthis region better approach the experimental data.

The experimental and calculated Raman spectra of Pr(III) com-plexes in Fig. 4 are also examined. In the 1800–1620 cm�1 Ramanregion, the bands assigned to C@O vibrations show low Ramanactivities. No intensity differences are calculated and observedfor the m(CC) bands in the region from 1500 up to1800 cm�1. Thisfinding could be used to distinguish m(CO) and m(CC) bands whichappear close to each other or could be overlapped. As it isseen from Fig. 4, the calculated Raman spectrum ofPr(CCA1)2(NO3)(H2O) better consists with the experimental one.The Raman spectrum of Pr(CCA1)2(NO3)(H2O) well reproducesthe observed three low intense bands: at 1738 (calc. 1722),1705 cm�1 (calc. 1698 cm�1) assigned to the stretching vibration,m(C3O4), of the free carboxylic group and at 1678 cm�1 (calc.1653 cm�1) attributed to the stretching vibration m(C1O2) of thecoordinated carbonylic group as well as the band at 473 cm�1 (calc.

Table 1Experimental and calculated 1H and 13C NMR chemical shifts in DMSO (at B3LYP/6-31+G(d,p)) for HCCA ligand and its Pr(III) complex.

Atom HCCA Pr(CCA)2(NO3)(H2O)

Exp Calc. Exp Calc.

PrCCA1 PrCCA2

H(C4) 8.68 9.04 8.93 9.53 8.75H(C7) 7.83 8.22 7.68d 8.32 8.15H(C5) 7.67 8.01 7.64d 8.18 7.98H(C8) 7.38 7.79 7.56d 7.87 7.80H(C6) 7.32 7.73 7.61d 7.93 7.81H(O3) 13.2 13.0 – – –H(O)w 6.72 3.61 3.62H(O)w 6.69 5.62 (HB) 8.92C1 164.1 162.6 177.2 161.6 156.7C3 156.9 161.9 154.5 160.1 174.2C9 154.6 153.5 149.0 151.4 153.1C4 148.5 151.5 150.5 152.4 146.7C7 134.4 134.8 133.3 134.0 132.5C5 130.3 130.1 130.6 129.6 128.9C6 124.9 123.7 125.3 124.5 122.9C10 118.4 119.5 120.5 119.3 119.1C2 116.2 112.4 118.0 115.6 119.5C8 117.4 114.6 115.8 115.0 114.4

120 I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121

474 cm�1) assigned to the m(PrO2) vibration. The last band does notappear in the calculated Raman spectrum of the Pr(CCA2)2(NO3)-(H2O). In the Raman spectrum of Pr(CCA2)2(NO3)(H2O), the calcu-lated two low intense bands at 1794, 1703 cm�1 (assigned tom(C1O2) vibrations of the uncoordinate carbonylic group) and theintense band at 1406 cm�1 (assigned to symmetric m(COO) vibra-tions) are missing in the experimental spectrum. Hence, the calcu-lated IR and Raman spectra of the model Pr(III) complexes and thecomparison with the experiment confirm the Pr(CCA1)2(NO3)(H2O)structure, where HCCA is bidentate bonded to Pr(III) through thedeprotonated carboxylic oxygen (O3) and the carbonylic (O2) oxygen.

The bands observed in the IR spectrum at 1573 cm�1,1031 cm�1 and 816 cm�1 are assigned to the m(NOfree), m(NObonded)s

and d(ONO) modes, respectively. The observed bands at 3524 and3422 cm�1 are due to asymmetric and symmetric m(OH) modesof coordinated water molecule.

The experimental and calculated IR spectra of HCCA were alreadydiscussed in detail in our previous studies [26,27]. In Fig. 3 the exper-imental IR spectrum of HCCA is compared with the calculated IRspectrum of Pr(CCA1)2(NO3)(H2O). In the IR spectrum of Pr(III) com-plex, m(C3O4) frequency of the uncoordinated C3O4 group slightlydownshifts (1746 cm�1 ? 1713 cm�1 (1748 cm�1 shoulder). Hence,the carboxylic m(C3O4) band shift of�30 cm�1 should not be consid-ered as an evidence for complexation through the carboxylic O4atom. As it is seen in Fig. 3, the carbonylic m(C1O2) is nearly un-changed (at 1684 cm�1) in the Pr(III) complex, although the car-bonylic group coordinates to Pr(III) (Fig. 2a). Our previous studyshowed that KCCA is better reference for comparative vibrationalanalysis with the lanthanide complexes of CCA and it could be morereliably used for prediction of the ligand binding mode in the lantha-nide complexes [27]. It was shown that the bidentate binding ofHCCA through the carbonylic O2 and carboxylic O3 atoms producedupshift of the m(C3O4) band about 70 cm�1 (as compared to KCCA IRspectrum) and downshift of the carbonylic m(C1O2) and carboxylicm(C3O3) bands about 50 and 70 cm�1, respectively.

3.4. 1H NMR and 13C NMR spectra

The 1H NMR and 13C NMR spectra of HCCA and its Pr(III) com-plex are measured in DMSO and they are used to confirm the com-plex formation and structure. The observed NMR spectra areelucidated on the basis of the calculated NMR chemical shifts inDMSO for HCCA, Pr(CCA1)2(NO3)(H2O) and Pr(CCA2)2(NO3)(H2O)complexes. The experimental and calculated 1H NMR and 13CNMR chemical shifts are compared in Table 1 (the atom numberingis in line with Figs. 1 and 2).

In 1H NMR spectra of the ligand and its Pr(III) complex, thepeaks observed in the interval 7.3–9.0 ppm are due to the aromaticH atoms. These peaks do not change significantly going from HCCAto Pr(III) complex with the exception of the peak at 8.68 ppm as-signed to H(C4) atom in the HCCA, which shows upshift to8.96 ppm in the complex spectrum. The calculated 1H chemicalshifts of the ligand and the two model complexes predict thatH(C4) peak goes to low field in Pr(CCA1)2(NO3)(H2O) in agreementwith the experiment, whereas in Pr(CCA2)2(NO3)(H2O) spectrumthis peak shifts to high field. The result obtained certainly indicatesthat the ligand binding mode in Pr(CCA1)2(NO3)(H2O) is throughthe deprotonated carboxylic and the carbonylic oxygens. The rela-tion found can be used to distinguish the two bidentate bindingmodes of CCA in the metal complexes. The ligand 1H NMR spec-trum shows a peak at 13.2 ppm due to the carboxylic proton. Thispeak is absent in the spectrum of the complex because of deproto-nation of the carboxylic group.

The comparison of 13C NMR spectra of HCCA and its Pr(III) com-plex shows largest movement of the carbonylic C1 peak to lowfield, Table 1. The calculated 13C NMR chemical shifts reveal that

the peaks at 156.9 ppm assigned to C3 atom and at 148.5 ppm –to C4 atom have different behavior in the Pr(III) modelcomplexes in dependence on the ligand binding mode. ThePr(CCA1)2(NO3)(H2O) complex formation produces upfield shift ofthe C3 peak and downfield shift of the C4 peak, which is in agree-ment with the experimental chemical shifts. A reverse trend isfound for Pr(CCA2)2(NO3)(H2O): the C3 peak moves to low fieldand C4 peak goes to high field. The relation obtained could helpto distinguish the ligand binding modes.

3.5. Comparative analysis of La(III), Ce(III), Pr(III) and Nd(III)complexes of HCCA

The new Pr(III) compound completes the series of lanthanidecomplexes studied (La, Ce, Pr, Nd) [26,27] and allows a compara-tive analysis of geometrical and spectroscopic data to be per-formed. It was found that the CCA anion coordinates to thelanthanide ions bidentate through the deprotonated carboxylicand carbonylic oxygens and forms complexes with empirical for-mula Ln(CCA)2(NO3)(H2O)2 for La(III) and Ln(CCA)2(NO3)(H2O) forCe(III), Pr(III) and Nd(III). The coordination number of La(III) inthe complex is 7 and it decreases to 6 for Ce(III), Pr(III) and Nd(III)in line with decreasing lanthanide ionic radii. The ionic radii of theLn(III) ions studied exhibit a smooth decrease in the order: La(103.2 pm) > Ce (101.0 pm) > Pr (99.0 pm) > Nd (98.3 pm). As a re-sult of the decreased ionic radii in the order La, Ce, Pr, Nd, the cal-culated Ln-O3 and Ln-O2 bond lengths decrease about 0.030 Å andabout 0.015 Å respectively (from La to Ce, from Ce to Pr and from Prto Nd). In the same order of decreasing ionic radii one may expect asmooth increase in the complex stability due to the greater chargedensity of the smaller ions. The interaction between the Ln(III) andCCA anion ligand is mainly electrostatic [26,28] and the Ln-O3/O2bond length changes slightly affect the C3O3, C1O2 and C3O4 bondlengths. The corresponding m(CO) frequencies in the lanthanidecomplex spectra also slightly change up to 10 cm�1. In general,the vibrational spectra of the lanthanide complexes studied arevery similar and support the identical coordination of HCCA andsimilar complex structures.

4. Conclusion

A new complex of coumarin-3-carboxylic acid with Pr(III)[Pr(CCA)2(NO3)(H2O)] is synthesized and characterized by elemen-

I. Georgieva et al. / Journal of Molecular Structure 979 (2010) 115–121 121

tal analysis, IR, Raman and 1H NMR and 13C NMR spectroscopies.The molecular structure of the Pr(III) complex is elucidated interms of modeling and theoretical spectroscopic studies of twopossible Pr(III) complexes: Pr(CCA1)2(NO3)(H2O) (CCA anion isbound to Pr(III) through the deprotonated carboxylic and carbony-lic oxygens) and Pr(CCA2)2(NO3)(H2O) (CCA anion is boundthrough the carboxylic oxygens). From energetic point ofview more stable, respectively more probable structure isPr(CCA1)2(NO3)(H2O). The calculated IR, Raman and NMR data ofthe Pr(CCA1)2(NO3)(H2O) complex are in better agreement withthe experimental spectra and thus verify the CCA1 binding modecoumarin-3-carboxylic acid. The theoretical vibrational analysisrevealed that the comparison of the experimental IR spectra ofPr(III) complex with the spectrum of the neutral HCCA ligand isnot indicative for prediction of the ligand coordination mode inPr(III) complex (downshift of m(C1O2) was not detected,1685 ? 1684 cm�1). The 1H and 13C NMR spectra of HCCA couldbe used as better reference for comparative analysis of its Ln(III)complexes. The calculated spectroscopic data helped to find reli-able vibrational and NMR characteristics for prediction of the coor-dination mode of coumarin-3-carboxylic acid in lanthanidecomplexes.

Acknowledgements

The authors thank the National Science Fund of Bulgaria for thefinancial support under Grants DO-02-233/2008 and DO-02-82/2008. IK gratefully acknowledges the financial support from theBulgarian Ministry of Education and Science (through the Indo-Bulgarian Project BIn-8/07 under Indo-Bulgarian Intergovernmen-tal Programme of Cooperation in Science & Technology). IK alsothanks Assoc. Prof. S. Cînta Pînzaru for recording the Ramanspectra.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2010.06.013.

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