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Spectrochimica Acta Part A 71 (2008) 1466–1473

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

T-IR and theoretical study of 3,5-dimethyl-1H-pyrazole-1-carboxamidine (L)nd the complexes CoL2(H2O)2(NO3)2, NiL2(H2O)2(NO3)2

eter Poganya, Attila Kovacsb,∗, Katalin Meszaros Szecsenyi c, Vukadin M. Leovacc

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Budapest, Szt. Gellert ter 4, HungaryResearch Group for Materials Structure and Modeling of the Hungarian Academy of Sciences, Budapest University of Technology and Economics,-1111 Budapest, Szt. Gellert ter 4, HungaryDepartment of Chemistry, Faculty of Sciences, University of Novi Sad, 21000 Novi Sad, Trg D. Obradovica 3, Serbia

r t i c l e i n f o

rticle history:eceived 13 February 2008ccepted 5 May 2008

a b s t r a c t

In the paper a joint experimental and theoretical study of 3,5-dimethyl-1H-pyrazole-1-carboxamidine(L) as well as its complexes CoL2(H2O)2(NO3)2 and NiL2(H2O)2(NO3)2 is reported. On the basis of FT-IRexperiments and a DFT-derived scaled quantum mechanical force field the normal coordinate analysis of

eywords:,5-Dimethyl-1H-pyrazole-1-arboxamidineransition metal complexesFT computations

L was carried out. The FT-IR spectra of the two complexes were interpreted using the present assignmentof L and computed vibrational data of the complexes. The ionic and charge transfer interactions in thecomplexes were assessed by means of natural bond orbital (NBO) analysis.

© 2008 Elsevier B.V. All rights reserved.

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T-IRcaled quantum mechanical analysis

. Introduction

Pyrazole and its substituted derivatives are known for their richoordination chemistry and numerous applications [1–3]. They findpplication in antipyretics, antirheumatics, herbicides and fungi-ides as well as metal ion extractants [4–6]. There are attemptsowards the screening of drug-like small molecules to identifyovel inhibitors, potential targets for therapeutic interventions,.g. against infectious diseases like anthrax. It was found thatost of the inhibitors identified of anthrax LF belong to phenyl-

uran, phenylpyrazole, and phenylpyrrole carboxylic derivatives7].

The transition metal complexes of pyrazole derivatives attractedome attention in biocoordination chemistry [8,9]. We men-ion particularly the copper complexes, used as models for thective sites in copper proteins (e.g. hemocyanin and tyrosinase)

10]. A Mn complex with hydrotris(3-phenyl,5-methyl-pyrazol-1-l)borate and 3-phenyl,5-methyl-pyrazole ligands was found toimic the native Mn-SOD enzyme [11]. Some of 5-aryl-1-formyl-

,5-dihydro-3-(2-hydroxyphenyl)-1H-pyrazole ligands and their

∗ Corresponding author at: Department of Inorganic and Analytical Chemistry,udapest University of Technology and Economics, H-1111 Budapest, Szt. Gellert ter, Hungary. Tel.: +36 1 463 2278; fax: +36 1 463 3408.

E-mail address: akovacs@mail.bme.hu (A. Kovacs).

mTlotTorc[

386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2008.05.002

omplexes with Cu(II), Co(II), Ni(II), Mn(II) and Zn(II) showedromising fungitoxicity against some phytopathogenic fungi [12].u(II) and Cd(II) complexes with 3-(2-pyridyl)pyrazol-based ligandhowed in vitro cytotoxic specificity and considerable cytotoxicity13]. Additional important research topics of pyrazole complexesre their molecular magnetic properties with emphasis on correla-ion with the structural characteristics [14].

Our research interest covers the structural, vibrational, bonding,hermal and other physico-chemical properties of transition metalomplexes with pyrazole-based ligands [15–26]. In the presentaper, we report the vibrational and characteristic bondingroperties of 3,5-dimethyl-1H-pyrazole-1-carboxamidine (L) andf the diaquabis(3,5-dimethyl-1H-pyrazole-1-carboxamidine-2N,N′)M(II) dinitrate complexes, CoL2(H2O)2(NO3)2 andiL2(H2O)2(NO3)2 (Scheme 1). We recorded the FT-IR spectraf the three compounds. The spectra of L were interpreted byeans of a scaled quantum mechanical (SQM) analysis [27,28].

he harmonic force field was obtained at the B3LYP/6-311++G**evel, which was applied successfully in our previous studiesn 4-acetyl-3-amino-5-methylpyrazole [29], 3,5-dimethyl-1-hiocarboxamidepyrazole [22] and other compounds [30–32].

he FT-IR spectra of the complexes were assigned on the basisf the SQM results on L and B3LYP/6-31G** calculations. Weeport also the most important bonding characteristics of theomplexes obtained by natural bond orbital (NBO) analysis33].

P. Pogany et al. / Spectrochimica Acta Part A 71 (2008) 1466–1473 1467

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. Experimental

The sample of 3,5-dimethyl-1H-pyrazole-1-carboxamidine wasurchased from Fluka (purity >98%) while the synthesis of the twoomplexes was carried out as described in Ref. [25].

The infrared spectra were recorded on a PerkinElmer System000 FT-IR spectrometer equipped with an MCT detector for theid-IR range and with a DTGS detector for the far-IR range. The

pectra were measured using KBr (mid-IR) and polyethylene (far-R) pellets. The number of scans accumulated in the mid-IR andar-IR range were 16 and 128, respectively. In all the measurementsresolution of 4 cm−1 was applied.

. Computational details

The geometry optimization and computation of the har-onic vibrational force field of L were carried out using the

ecke3–Lee–Yang–Parr (B3LYP) exchange-correlation functional34,35] in conjunction with a 6-311++G** basis set. Note thathis theoretical level performed very well in our recent vibra-ional analyses of 4-acetyl-3(5)-amino-5(3)-methylpyrazole [29]nd 3,5-dimethyl-1-thiocarboxamide pyrazole [22]. The geome-ries and vibrational frequencies of the larger complexes wereomputed at the B3LYP/6-31G** level using relativistic effectiveore potentials for the metals [36,37]. We started our geometryptimizations from the Cartesian coordinates obtained in the X-ay diffraction studies [38,39]. The open-shell electronic structuresf the CoL2(H2O)2(NO3)2 and NiL2(H2O)2(NO3)2 complexes werenvestigated carefully with multiplicities of 4 and 6 for the Co and

ultiplicities of 3 and 5 and for the Ni complex. In both cases, theower multiplicity states proved to be the electronic ground states

ith negligible spin contamination. All the quantum chemical com-

utations were performed with the GAUSSIAN03 program [40].

Our vibrational analysis of L was based on the computed3-LYP/6-311++G** force field. The deficiencies of the computedarmonic force field were corrected by selective scale factorsccording to the SQM method of Pulay et al. [27,41]. Generally, nat-

niaft

. .

ral internal coordinates [42] were used in the SQM treatment.xceptions were the stretching vibrations of the NH2 and CH3roups and NO3

− ion. Instead of the primitive internal coordinatesefined in the natural internal coordinate scheme [42] we preferredhe local symmetry stretching coordinates providing a better repre-entation of these group vibrations. For the coordinates includinghe NH· · ·O hydrogen bonds primitive internal coordinates weresed. The scaling procedure was performed by using the programs

NTC [42], TRA3 [43] and SCALE3 [44,45]. The scale factors wereransferred from Ref. [22]. The fundamentals were characterizedy their total energy distribution (TED) [46,47].

. Results and discussion

.1. Vibrational analysis of,5-dimethyl-1H-pyrazole-1-carboxamidine nitrate (L·HNO3)

The 3,5-dimethyl-1H-pyrazole-1-carboxamidine (L) ligand wasvailable as the nitrate salt hence, we performed computationsn this form in order to obtain the harmonic force field for ouribrational analysis. We started the geometry optimization fromhe Cartesian coordinates obtained by X-ray diffraction [38], andllowed to converge to the global minimum on the completelyelaxed potential energy surface. The analysis of the obtainedeometry showed much stronger LH+· · ·NO3

− interactions thanound in the crystal: the NH· · ·ON hydrogen bonds were com-uted to be 1.6 A in agreement with the ionic character of thewo species, while in the crystal 1.9–2.0 A distances were observed38]. The reason of this difference is that in the crystal a three-imensional hydrogen-bonding network is established, where aitrate ion is connected to four L molecules [38]. On the other hand,

n the computed LH+NO3− model the NO3

− ion has only one part-

er. Obviously, with only one hydrogen bond donor much stronger

nteraction is formed than with four donors pulling the NO3− ion to

n intermediate position between the four LH+ moieties. This dif-erence between the computed model and the crystal affects alsohe interpretation of the IR spectrum: both the computed NH and

1468 P. Pogany et al. / Spectrochimica Acta Part A 71 (2008) 1466–1473

ional analysis of the IR spectrum of solid L·HNO3.

Ns

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Table 1Scale factors applied for the SQM analysis of LH+· · ·(NO3

−)2

Internal coordinatea Symbol Scale factorsb

L and NO3−

Heavy atom stretch �XY 0.945NH2, CH, CH3 stretch �NH2, �CH, �CH3 0.922Heavy atom bend �XYZ 0.974NH2 CH, CH3 deformation �NH2, �CH, �CH3 0.963Out-of-plane modes �XY, �CH 0.976Torsional modes �XY, �NH, �CH3 0.960

Hydrogen bondH· · ·O stretch �H· · ·O 0.922NH· · ·O, H· · ·ON bend �NH· · ·O, �H· · ·ON 0.963NO3

− out-of-plane of L �H···O 0.976NH· · ·ON torsion �H· · ·O 0.960H· · ·ONO2 torsion �ON 0.960

i

achbattmtta

Fig. 1. Model structures tested for the vibrat

O3 IR characteristics may deviate considerably from the measuredpectrum, making the interpretation difficult.

To model the situation in the crystal more efficiently, we fixedhe H· · ·O distances at the average value in the crystal, 1.9 A (Fig. 1a).he computed IR spectrum contained an imaginary frequency dueo the applied constrains. In addition, it still showed considerableiscrepancies for the strong bands of the NH vibrations, as a con-equence of the two non-hydrogen-bonded NH hydrogens in theodel while all NH hydrogens are involved in hydrogen bond-

ng in the crystal. In order to account for this source of error, wentroduced a second NO3

− ion into the model binding in this wayll the four NH hydrogens of LH+. The various possibilities testedwith constrained 1.9 A H· · ·O distances) are shown in Fig. 1b–f.our from the five cases showed imaginary frequencies after theeometry optimizations indicating unreasonable force fields. Thenly model lacking imaginary frequencies was the one shown inig. 1e. Its computed IR spectrum resembled the measured spec-rum reasonably. Hence, we used this harmonic force field as inputn our SQM analysis. Note that even in structure 1e we were not ableo achieve a perfect model of the hydrogen bonding in the crystal,s the relaxed NH· · ·O distances were converged to weak hydrogenonds with H· · ·O distances of 2.458 A. In addition, one NO3

− anionas five hydrogen bonds with surrounding NH2

+ groups (of threeH+ molecules) in the crystal in contrast to the two interactionsn the model structure. As the most striking difference, one of theO3

− oxygens is not involved in hydrogen bonding in structure 1e.ll these are the main reasons for some remained discrepanciesetween the experimental and computed IR spectra (vide infra).

The initial scale factors were transferred from the recent SQMtudy of 3,5-dimethyl-1-thiocarboxamide pyrazole [22]. A com-

arison of the experimental and scaled frequencies proved thathe scale factors from Ref. [22] are in general well transferableo LH+· · ·(NO3

−)2. Considerable deviations were observed only forfew NH2 modes as a result of the differences in the hydrogen

onding interactions. However, these deviations were not system-

w

tid

a X, Y, Z mean heavy atoms (C, N, and/or O). The symbols �, �, �, �, � mean stretch-ng, bending, deformation, out-of-plane and torsional vibrations, respectively.

b From Ref. [22].

tic due to the asymmetric hydrogen bonding arrangement in theomputed model structure 1e (different character of the two NH2ydrogens and only two nitro oxygens are involved in hydrogenonding, vide supra). In addition, most vibrations of these groupsre strongly mixed with each other making a straightforward selec-ive scaling impossible. Therefore, we omitted a re-optimization ofhese scale factors and used the ones from Ref. [22] without any

odification (Table 1). Similarly, the transferred general scale fac-ors were used for the vibrations involving the hydrogen bond. Forhe few observed bands below 150 cm−1 involving these vibrations,good agreement was found between the experimental and scaled

avenumbers.

The results of our SQM analysis together with the experimen-al data are compiled in Table 2. The IR spectrum of solid L·HNO3s shown in Fig. 2. Using the scaled force field we achieved a goodescription of the IR spectrum of L·HNO3. The average deviation

P. Pogany et al. / Spectrochimica Acta Part A 71 (2008) 1466–1473 1469

Table 2Assignment of the FT-IR spectrum of solid L·HNO3 on the basis of the SQM resultsa

Experimental Computed

Scaled Unscaled TEDb

3290 s, br 3484 3628(141) 62% �asNH2, 38% �sNH2

3112 vs, br 3065 3191(2271) 62% �sNH2, 38% �asNH2

1693 s 1694 1736(590) 66% �C Nam, 17% �sciNH2

1644 m 1640 1673(205) 76% �sciNH2, 13% �NH· · ·O1592 m 1579 1618(102) 31% �C C, 18% �CNpy, 16% �sciNH2

1563 sh 1565 1599(22) 49% �sciNH2, 15% �C Nam, 10% �NH· · ·O1546 s 1510 1547(224) 21% �Npy–Cam, 15% �C Nam, 15% �asCH3, 12% �CNpy

1479 sh 1479 1510(84) 80% �asCH3

1479 sh 1476 1509(71) 47% �asCH3, 18% �CNpy

1462 m 1457 1486(24) 83% �asCH3

1441 sh 1442 1475(10) 43% �asCH3, 19% �CNpy, 15% �CC1409 w 1411 1444(69) 27% �C C, 23% �sCH3, 16% �asCH3

1384 s 1413 1450(1226) 77% �asNO3, 11% �asNO3

1367 sh 1376 1404(6) 76% �sCH3, 11% �C C1328 s 1318 1349(302) 53% �CNpy, 25% �py1319 s 1294 1327(475) 71% �asNO3, 15% �asNO3

1204 w 1188 1217(16) 29% �NN, 19% �py, 11% �C C, 11% �Npy–Cam

1159 w 1154 1178(5) 58% �CH, 11% �CC1117 w 1109 1135(16) 34% �NN, 17% �CC, 11% �rCH3

1092 w 1074 1099(39) 50% �rNH2, 36% �C Nam

1064 w 1053 1072(4) 78% �rCH3, 11% �asCH3

1047 sh 1043 1062(2) 81% �rCH3, 11% �C–CH3

1040 w 1031 1060(48) 92% �sNO3

1030 w 1031 1052(4) 31% �rCH3, 27% �py, 12% �CNpy

990 w 985 1005(26) 42% �rCH3, 25% �rNH2

971 m 964 986(22) 33% �rCH3, 33% �C C, 12% �py805 w 799 808(28) 100% �CH782 w 771 790(8) 33% �CC, 14% �py, 14% �CNpy

738 m 849 864(54) 100% �NH701 m, br 835 849(185) 65% �NH, 30% �NO3

720 w 714 728(21) 56% �CNam, 25% �NH, 14% �asNO3

686 sh 702 714(9) 74% �asNO3, 15% �asNO3

666 w 658 670(1) 70% �py, 12% �C–CH3, 10% �NH653 w 639 652(32) 19% �NCN, 17% �py, 14% �Npy–Cam, 10% �NH612 w 613 625(14) 58% �py, 12% �C–CH3

587 w 577 590(1) 45% �CC, 35% �py536 s 531 540(16) 22% �NCN, 20% �CCN, 13% �CNN430 s 422 429(24) 45% �NCN, 17% �Npy–Cam, 11% �py399 m 393 399(12) 46% �CCN, 24% �NCN347 m 339 344(12) 62% �C–CH3, 14% �py288 w 283 288(10) 62% �CCN224 m 208 212(4) 39% �CNN, 15% �CCN, 14% �CH3

176 w 187 190(4) 57% �C–CH3, 27% �py163 w 150 153(14) 36% �Npy–Cam, 24% �H· · ·ON, 23% �NH138 m 132 130(32) 60% �H· · ·O, 16% �H· · ·O121 m 127 135(16) 47% �H· · ·ON, 18% �H· · ·O, 13% �Npy–Cam

113 w 106 108(1) 45% �CNam, 30% �H· · ·O, 23% �H· · ·ON106 w 94 96(2) 87% �CH3

88 w 84 87(33) 73% �H· · ·O65 w 68 70(5) 43% �H· · ·O, 27% �H· · ·ON, 10% �H· · ·O

a The wavenumbers are given in cm−1. The abbreviations vs, s, m, w, vw, sh and br mean very strong, strong, medium, weak, very weak, shoulder and broad, respectively.T were

given.a ctivelg

beD1iswa

cpih

4

iygio

he unscaled calculated data (in parentheses the calculated IR intensities, km/mol)b TED = total energy distribution [46,47]; only contributions larger then 10% are

symmetric, stretch, deformation, bend, scissoring, rock, wag, torsion and twist, resperoups are indicated by subscripts py and am, respectively.

etween the experimental and scaled wavenumbers was 8.8 cm−1

xcluding the four observed NH2 stretching and torsional bands.ue to the above mentioned limitations of the model structuree (and in agreement with the sensitivity of the NH/OH stretch-

ngs and torsions on hydrogen bonding [48]), the asymmetric NH2tretching was overestimated by 194 cm−1 by the SQM force field,hile the deviations for the other three modes were between 47

nd 134 cm−1.

The band positions in the IR spectrum of L·HNO3 agree well with

haracteristic group frequencies [49] and with previous results onyrazole derivatives [22,29]. The character of the NH and NO3 bands

n the spectrum reflect the strong nature of the (NH2)2+· · ·NO3

−

ydrogen bonding interaction [48,49].

aictn

obtained at the B3LYP/6-311++G** level. For details of scaling see text.The abbreviations py, s, as, �, �, �, sci, r, w, �, tw mean pyrazole ring, symmetric,y. In ambiguous cases the C and N atoms belonging to the pyrazole or carboxamidine

.2. FT-IR spectra of the complexes

The computations on the title complexes were performed withnitial Cartesian coordinates from the recent X-ray diffraction anal-sis [39]. As observed for L·HNO3 (vide supra), the optimizedeometries of the complexes differed somewhat from the onen the crystal: in the single-complex model the computationsverestimated the hydrogen bonding interactions of the nitro

nion with respect to that of the crystal, due to the absence ofnter-complex hydrogen bonding with ligands of neighbouringomplexes. This affects primarily the NO3

− and H2O modes andhe NH vibrations of L. However, on the basis of the well-knownature of hydrogen bonding shifts (red-shift for the stretching,

1470 P. Pogany et al. / Spectrochimica Acta

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piaintcaTcwone magnitude than the L → M donations. The back-donation from

ig. 2. The FT-IR spectra of L·HNO3, CoL2(H2O)2(NO3)2 and NiL2(H2O)2(NO3)2.

lue-shifts for the deformation and out-of-plane modes), the over-stimated effects of hydrogen bonding can be accounted for. Onhe other hand, the other vibrations of L should not be consider-bly affected by the overestimated hydrogen bonding, as we foundgood agreement (deviations within 0.01 A) between our com-

uted and the experimental bond distances of L in the complexes39]. Moreover, our present assignment of the FT-IR spectrumf L·HNO3 (with a somewhat related hydrogen bonding pattern)an be of assistance in the analysis of the spectra of the com-lexes.

Regarding the metal–ligand distances we note that the differ-nces in the experimental and the present computed values arearger (up to 0.07 A) than those found for the L moiety (vide supra).imilar differences have been observed recently for related com-lexes computed at the same theoretical level [22]. However, dueo the small wavenumber values of the metal–ligand vibrations, therror of these computed frequencies will be very small. Hence wean perform a straightforward interpretation of the experimentalT-IR spectra of the two title complexes.

The FT-IR spectra of CoL2(H2O)2(NO3)2 and NiL2(H2O)2(NO3)2re shown in Fig. 2, whereas the spectral data are given in Table 3.s outlined above, the assignment was performed by a jointonsideration of the present SQM results on L·HNO3 and the com-uted normal modes of the two complexes. The assignment ofetal–ligand modes was further supported by literature data [50].As expected, most vibrations of L have similar wavenumber val-

es in the IR spectrum of L·HNO3 and in those of the complexes (cf.ables 2 and 3). The largest deviations appear in the NH modes dueo the protonation of the imino group in L·HNO3 and the differentharacter of hydrogen bonding with the nitro anion. According tohe X-ray diffraction data [38,39], the NH· · ·O hydrogen bonds areomewhat weaker in the complexes than in L·HNO3 (the NH· · ·Oistances are ca. 1.9 and 2.2 A, respectively). This trend is welleflected in the higher wavenumbers of the NH stretching and theower wavenumbers of the NH torsional bands in the IR spectra ofhe complexes.

The effects of complex formation on the ligand modes could note well distinguished for the title complexes. As L coordinates tohe metals partly by the NH group, its protonation in L·HNO3 andhe different hydrogen bonding suppresses the weaker IR features

t

ri

Part A 71 (2008) 1466–1473

f the metal coordination. The other coordinating atom, the pyri-ine type ring nitrogen is involved in strongly mixed vibrations,ere the other contributions in the normal modes smoothen theffect.

We could identify several metal–ligand vibrations below00 cm−1. In agreement with literature data on relatedonor–acceptor interactions, the M· · ·O stretching vibrationsppear as major contribution in bands between 400 and 350 cm−1

50,51]. The M· · ·N stretching modes appear in bands between 320nd 250 cm−1 [24,50,52]. Similarly to the stretching vibrations,rom the two type of coordinating atoms (O, N) the formers giveigher-wavenumber deformation modes (around 270 cm−1), whilehe N· · ·M· · ·N bendings appear in bands around 100 cm−1 as majorontributions [24]. Note that all the metal–ligand vibrations aretrongly mixed with each other or with low-wavenumber ligandibrations.

.3. Bonding properties of the complexes

Our analysis of the charge distribution and charge transferffects in the complexes aimed to draw qualitative conclusionsn the nature of the bonding in these molecules. The atomicharges demonstrate the magnitude of ionic interactions betweenhe metal ions and the coordinating atoms of the ligands, while theharge transfer effects give information on the covalent compo-ent, i.e., on the metal–ligand donor–acceptor interactions. Notehat the presented values refer to the isolated complexes, devi-ting somewhat from the situations in the crystal. The strongerydrogen bonding interactions of the NO3

− ions with both the2O and L ligands in the computed structures with respect to

hat in the crystals enhances the charge separation in the com-lex and the charge transfer interactions between these moieties.he enhanced hydrogen bonding has much less impact on theetal–L interactions, hence the data obtained for the latter inter-

ction may provide a good description of the metal–L bonding.oreover, we can perform a straightforward comparison of the

wo metals, as the error of the computations is systematic in thisespect.

The natural atomic charges [33] of CoL2(H2O)2(NO3)2 andiL2(H2O)2(NO3)2 are compiled in Table 4. The atomic charges

eveal very strong ionic interactions in the complexes. The chargef the metals, around +1.4 e, is closer to the M2+ ion than to the neu-ral atom (this latter would be achieved by a hypothetical net chargeransfer of two electrons). Among the facing atoms of the donorshe oxygens of H2O have an atomic charge of −1.0 e, the NH nitro-en of L ca. −0.85 e, while the pyrazole nitrogen N9 −0.4 e. All thesemply that in the present metal–ligand interactions the electrostaticttraction is the major factor.

A quantitative assessment of the donor–acceptor interactions isossible by the second-order perturbation energies [33] depicted

n Table 5. Both the two coordinated nitrogens of L (N5, N6, N8, N9)nd the oxygens of H2O donate a considerable charge to the M2+

ons. From the imino and pyridine type nitrogens of L the iminoitrogens (N6, N8) make a somewhat larger donation. Note thathese nitrogens have a considerable charge excess (cf. the atomicharges). The total donations of the four nitrogens of the two L lig-nds are ca. twice larger than the donation of the two H2O ligands.he back-donation from the metals to anti-bonding orbitals of Lonsists of several minor (generally below 1 kJ/mol) contributions,hich add up to a total back-donation charge transfer weaker by

he metals to H2O is negligible.A comparison of the data of the two metals in Tables 4 and 5

eveals the somewhat stronger character of the M–L donor acceptornteractions in the Ni complex. The total N → M charge trans-

P. Pogany et al. / Spectrochimica Acta Part A 71 (2008) 1466–1473 1471

Table 3Assignment of FT-IR spectra of the complexes

Co Ni Assignmentc

Experimentala Calculatedb Experimentala Calculatedb

3476 s 3879(45) 3427 sh 3867(81) �OH3330 s 3621(36) 3364 s, br 3618(72) �NH2

3276 sh 3573(28) 3312 s 3585(44) �NH3229 sh 3159(576) 3241 s, br 3164(464) �NH2

3193 s, br 3144(692) 3198 s, br 3158(740) �NH2

3007 w 3278(1) �CH2993 w 3155(79) 2992 w 3155(229) �asCH3

2973 w 3138(41) 2973 w 3149(31) �asCH3

2924 w 3065(16) �asCH3

1678 m 1720(722) 1685 s 1722(755) �Cam N, �sciNH2

1656 sh 1697(91) 1675 sh 1698(95) �H2O, �sciNH2

1606 m 1668(418) 1611 m 1668(381) �sciNH2, �Cam N1571 m 1623(159) 1572 m 1625(147) �py, �CH1493 m 1542(657) 1489 m 1542(631) �asNO3, �py, �H2O1469 w 1534(96) 1480 sh 1535(117) �asCH3, �py1450 w 1511(80) 1459 m 1535(117) �asCH3, �py1418 sh 1460(157) 1416 sh 1460(153) �py, �asCH3

1384 s 1373(475) 1384 s 1373(662) �asNO3, �sCH3

1351 w 1390(110) 1353 m 1391(110) �py, �sCH3

1343 sh 1385(1) �py, �sCH3

1245 m 1271(209) 1234 m 1272(201) �py, �NH, �rNH2

1206 w 1214(61) 1217 sh 1213(62) �CH, �NH, �py1134 m 1153(77) 1129 m 1154(69) �py, �py, �Cam N1128 sh 1150(9) �py, �py, �Cam N1074 w, br 1077(22) 1082 sh 1077(43) �sNO3, �H2O1053 m 1074(53) 1051 m 1075(41) �py, �C–CH3

1004 m 1018(41) 1000 m 1018(15) �C–CH3, �CH, �H2O987 sh 1017(39) 992 sh 1018(15) �C–CH3

824 m 824(31) 823 m 823(30) �CH808 m, br 1021(178) 807 m, br 1017(308) �H2O786 m 812(7) 800 sh 812(7) �py, �–CH3

721 sh 702(56) 725 sh 700(73) �Npy–Cam

714 m 831(67) 716 m 831(68) �NO3, �NH698 m 694(86) 695 m 695(81) �sciCN2am, �py550–750 s, br 728(77) 550–750 s, br 728(84) �rH2O, �sNO3, �NH550–750 s, br 935(347) 550–750 s, br 931(296) �NH2, �NH550–750 s, br 928(366) 550–750 s, br 929(410) �NH2, �NH580 w 644(51) 580 m 658(73) �rH2O, �py502 m 493(13) 504 m 498(15) �py, �C–CH3

416 s 404(63) �C–CH3, �C–NH2, �H2O396 m 392(37) 400 m 424(156) �H2O, �M· · ·O356 m 345(135) 356 w 366(57) �M· · ·O, �H2O314 w 305(34) 320 sh 303(8) �M· · ·N, �C–CH3

300 m 304(34) 304 s 314(24) �C–CH3, �Npy–Cam

279 s 249(144) 282 m 262(131) �O· · ·M· · ·O, �M· · ·O263 sh 280(9) 275 sh 286(2) �M· · ·N, �O· · ·M· · ·O, �C–CH3

255 sh 233(6) 258 sh 241(22) �CH3, �CNam

217 w 220(19) 235 w 227(11) �CH3

205 sh 187(4) 206 sh 204(11) �Npy–Cam, �CH3

190 w 195(4) 188 w 196(7) �CH3, �Npy–Cam, �N· · ·M· · ·N115 m 129(4) 120 m 126(10) �N· · ·M· · ·N, �CH3, �Npy–Cam

97 m 98(13) 100 w 115(7) �N· · ·M· · ·N, �H· · ·ON75 w 64(3) 76 w 66(2) �Co· · ·py (butterfly), �H· · ·ON

a In cm−1. The abbreviations vs, s, m, w, vw, br mean very strong, strong, medium, weak, very weak and broad, respectively.b Computed at the B3LYP/6-31G** level using relativistic effective core potentials for the metals. Computed IR intensities (km/mol) are given in parentheses.

. Thes guousi

fC3cpeinrr

rLoe

c Major contributions deduced from visual observation of the computed vibrationstretch, deformation, bend, scissoring, rock, wag and torsion, respectively. In ambindicated by subscripts py and am, respectively.

er is larger by ca. 100 kJ/mol than in the Co complex. Althougho receives somewhat more charge from the H2O ligands (by ca.5 kJ/mol) than Ni, it does not compensate for the larger L → Mharge transfer of the latter. As a result, Co has a somewhat largerositive atomic charge than Ni (cf. Table 4). These shown differ-

nces in the donor–acceptor interactions lead to an opposite ordern the ionic interactions: the larger positive charge of Co – accompa-ied with the slightly more negatively charged adjacent nitrogens –esults in larger electrostatic attraction in CoL2(H2O)2(NO3)2 withespect to NiL2(H2O)2(NO3)2.

acmta

abbreviations py, s, as, �, �, �, sci, r, w, � mean pyrazole ring, symmetric, asymmetric,cases the C and N atoms belonging to the pyrazole or carboxamidine groups are

Table 5 lists also the computed charge transfer interactionselated to the hydrogen bonding between the nitrate anion andand H2O. As the hydrogen bonding between these moieties is

verestimated in the computed model, these data are also over-stimated with respect to the situation in the crystal, hence they

re primarily of theoretical interest. In the isolated complexes theharge transfers to the �*NH and �*OH orbitals are comparable inagnitude (but somewhat smaller) to the ligand-to-metal dona-

ions. The atomic charges reveal that ca. 0.18 e is transferred fromnitrate anion to its hydrogen bonding partners (cf. Table 4). The

1472 P. Pogany et al. / Spectrochimica Acta

Table 4Selected computed natural charges (e) in the complexesa

Fragment Atom CoL2(H2O)2(NO3)2 NiL2(H2O)2(NO3)2

M M 1.427 1.372L N3 −0.209 −0.208

N5 −0.402 −0.390N6 −0.881 −0.873

H2O O23 −1.035 −1.023�H2O −0.003 0.005�NO3

− −0.821 −0.822

a For the numbering of atoms see Scheme 1. Because of the symmetrical arrange-ment only the data of one ligand are depicted.

Table 5Main charge transfer interactionsa in the complexes

Interaction CoL2(H2O)2(NO3)2 NiL2(H2O)2(NO3)2

L → ME(2) LP(N6) → M 179.3 205.3E(2) LP(N5) → M 159.5 187.7� E(2) LP(N) → M 677.6 786.0

H2O → ME(2) LP(O23) → M 163.8 146.0� E(2) LP(O) → M 327.6 292.0

NO3− → L

E(2) LP(O43) → �*NH 93.5 105.8

NO3− → H2O

E(2) LP(O40) → �*OH 291.3 284.0

a Second-order perturbation energies (E(2) donor → acceptor, kJ/mol). The abbre-viations LP, and �* mean lone pair, and anti-boding � orbitals, respectively. Becauseof the symmetrical arrangement only the charge transfer interactions of one ligandas

tn

5

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tctst

bt(cai

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mamhI

A

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[[[35] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[36] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270.[37] A.W. Ehlers, M. Bohme, S. Dapprich, A. Gobbi, A. Hollwarth, V. Jonas, K.F. Kohler,

R. Stegmann, A. Veldkamp, G. Frenking, Chem. Phys. Lett. 208 (1993) 111.

re depicted. Items starting with � represent the total charge transfer from/to bothymmetrically arranged ligands. For the numbering of atoms see Scheme 1.

ransfer to H2O compensates for the H2O → M transfer, resulting inearly neutral H2O ligands in the complexes.

. Conclusions

In the present study we performed a joint experimentalnd theoretical investigation of 3,5-dimethyl-1H-pyrazole-1-arboxamidine (L) and its complexes CoL2(H2O)2(NO3)2 andiL2(H2O)2(NO3)2. We recorded and interpreted the solid-phaseT-IR spectra of the three compounds. Using a DFT-derived SQMorce field we carried out a complete normal coordinate analy-is of L. We achieved the assignment of 50 fundamentals withn average deviation of 8.8 cm−1 between the scaled and exper-mental frequencies (excluding the NH stretching and torsion

odes).The FT-IR spectra of the complexes have been assigned on

he basis of the presently obtained assignment of L and DFTomputations on the complex molecules. We were able to iden-ify numerous metal–ligand modes below 400 cm−1, generallytrongly mixed with each other or low-wavenumber ligand vibra-ions.

The bonding properties of the title complexes were assessedy NBO analysis. They indicated the major importance of attrac-ive electrostatic interactions between the metal cations and thepartially) negatively charged N and O donors of the ligands. Theomputed atomic charges proved the slightly stronger ionic inter-

ctions in CoL2(H2O)2(NO3)2 while the charge transfer is strongern NiL2(H2O)2(NO3)2.

[

Part A 71 (2008) 1466–1473

cknowledgements

Financial support from the Ministry for Science and Environ-ental Protection of the Republic of Serbia (Grant No. 142028)

nd Provincial Secretariat for Science and Technological Develop-ent of Vojvodina is gratefully acknowledged. The computations

ave been performed on the SUN supercomputer of the Nationalnformation Infrastructure Development Program of Hungary.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.saa.2008.05.002.

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