Spectroscopic aspects of the coordination modes of 2,4-dithiohydantoins: Experimental and...

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Spectroscopic aspects of the coordination modes of 2,4-dithiohydantoins:Experimental and theoretical study on copper and nickel complexes ofcyclohexanespiro-5-(2,4-dithiohydantoin)

Anife Ahmedova a,*, Petja Marinova b, Katarzyna Paradowska c, Neyko Stoyanov d, Iwona Wawer c,Mariana Mitewa a

a Faculty of Chemistry, University of Sofia, 1, J. Bourchier av., 1164 Sofia, Bulgariab Faculty of Chemistry, University of Plovdiv, 24, Tzar Assen str., 4000 Plovdiv, Bulgariac Faculty of Pharmacy, Medical University of Warsaw, Banacha 1, 02097 Warsaw, Polandd Rousse University, Technol. Branch – Razgrad, 3, Apr. vast. str., Razgrad-7200, Bulgaria

a r t i c l e i n f o

Article history:Received 5 November 2009Received in revised form 7 June 2010Accepted 16 July 2010Available online 22 July 2010

Keywords:DithiohydantoinMetal complexSolid-state NMR13C CPMAS NMRDFT GIAO calculations

a b s t r a c t

The complexation of cyclohexanespiro-5-(2,4-dithiohydantoin), L, with copper and nickel was studied bymeans of experimental and theoretical methods. The Cu(I) and Ni(II) complexes were synthesized andcharacterized using 13C CPMAS NMR, IR and FAB-MS. Reduction of Cu(II) ions and the formation ofCu(I) complexes with dithiohydantoin was proved. Various coordination modes were investigated onthe basis of calculated (DFT-GIAO) shielding constants of the free ligand and model structures of the com-plexes. General trends in the changes of spectroscopic parameters (NMR chemical shifts, vibrationalmodes) upon different types of coordination were outlined. Dimeric structures for the Cu(I) and Ni(II)complexes were proposed in which the ligands were coordinated in N3^S4- and N3^S2-bridging ways,respectively, acting as monoanions. The results demonstrate that the combined experimental (13C CPMASNMR, IR) and theoretical (DFT) approach can be used to characterize the molecular structure of solid com-plexes for which crystallographic data are not available.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Hydantoin derivatives are well known for their medical applica-tions, e.g. as antiepileptic drugs [1,2]. Antiproliferative activity[3,4], inhibition of aldosoreductase [5] and potential applicationfor treatment of HIV-1 infections [6,7] were also reported. New 2-thioanalogues of hydantoins were synthesized with the aim to re-duce toxicity and their structure and properties were investigated[8,9]. Recently, we described the synthesis of various thioanaloguesof cycloalkanespiro-5-hydantoins [10]. The crystal structures offour cycloalkanespiro-5-(2,4-dithiohydantoins) [11] and two cyclo-alkanespiro-5-(2-thiohydantoins) [12] were determined.

Taking into account medical applications of hydantoin deriva-tives, it is of crucial importance to acquire the knowledge abouttheir interactions with bioavailable metal ions [13,14]. Corre-spondingly, several crystal structures of 5,50-diphenyl-hydantoin,the antiepileptic drug phenytoin, with Cu(II) and Zn(II) are known[15,16]. Novel Pt(II) complexes with cycloalkanespiro-5-hydantoinligands were reported to show moderate antitumour activity;however, the suggested structure of these complexes is ratherimprobable [17]. Therefore, due to the better coordination ability

of the thioanalogues, we undertook a systematic study on thecoordination properties of a series of thioanalogues of the cycloal-kanespiro-5-hydantoins. The complexation of some cycloalkane-spiro-5-(2,4-dithiohydantoins) with Cu(II) ions has recently beenstudied revealing that in alcoholic media the complexation pro-ceeds with concomitant reduction of the metal ion leading to theformation of Cu(I) complexes [18]. However, under the same reac-tion conditions, no reduction was observed when the spiro-5-substituent of the 2,4-dithiohydantoin ligand has a large aromaticring, such as 9-fluorene [19]. Besides the redox properties, thecoordination mode of the cycloalkanespiro-5-(2,4-dithiohydanto-ins) is far more complicated and requires a detailed study.

The aim of this work was to investigate the spectroscopicfingerprints of each possible coordination mode of the cyclohex-anespiro-5-(2,4-dithiohydantoin), presented in Scheme 1, and tocharacterize the structure of newly synthesized complexes. Themain challenge was a variety of possible ways of coordination forcyclic thioamide ligands, which was reviewed by Raper [20,21]and more recently by Akrivos [22], basing on crystallographic data.It should be noted that the dithiohydantoins, i.e. imidazolidine-2,4-dithiones, present even more different ways of coordination due tothe presence of two thioamide groups in a ligand (see Scheme 1).Moreover, the available crystallographic data of their metal com-plexes are rather scarce. To the best of our knowledge, the only

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* Corresponding author. Fax: +359 2 962 5438.E-mail address: [email protected] (A. Ahmedova).

Inorganica Chimica Acta 363 (2010) 3919–3925

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X-ray data for a metal complex of 2,4-dithiohydantoin derivativeare those of Cu(I) complex of 5,50-dimethyl-2,4-dithiohydantoin[23]; its structure shows the formation of a polymeric complexinvolving the S-atoms of the ligand. Spectroscopic structural stud-ies of these complexes are additionally complicated by the fact thatall possible donors: two S-atoms and two N-atoms, are conjugatedin a five-membered heterocyclic ring. Therefore, the problem ofdithiohydantoin coordination with metal ions will be subjectedto a combined approach of theoretical (quantum chemical) andexperimental (spectroscopic) methods. Theoretical data will be re-lated to the observed 13C CPMAS NMR and IR spectral parametersof copper and nickel complexes of cyclohexanespiro-5-(2,4-dith-iohydantoin), L. Such approach was recently applied for Cu(I) andNi(II) complexes of similar hydantoin ligands [24]. Presently, wewill also describe the spectroscopic features of each possible modeof coordination of the studied dithiohydantoins, so that they can beuseful in future applications of spectroscopic methods for struc-tural determinations of metal complexes of this class of heterocy-clic polydentate ligands.

2. Experimental

2.1. Methods and calculations

IR spectra were recorded on a Perkin–Elmer FTIR-1600 spectro-photometer (KBr pellets). Elemental analyses were performed on aVario EU III instrument. FAB-MS spectra were obtained using aFisons VG Autospec. 1H and 13C NMR spectra of the ligand were re-corded on a Bruker DRX-250 spectrometer for DMSO-d6 solutions.Solid-state cross polarization (CP) magic angle spinning (MAS) 13CNMR spectra were recorded on a Bruker Avance DSX-400 instru-ment at 100.61 MHz; the samples were spun at 10 kHz in a4 mm ZrO2 rotor (contact time 2–9 ms, repetition time 30 s, 1024scans). 13C chemical shifts were calibrated indirectly through theglycine CO signal recorded at 176.0 ppm relative to TMS.

Quantum chemical calculations were performed using GAUSSIAN

03 package of programs [25]. Geometries of the studied com-pounds were optimized using the DFT method with the hybridB3LYP functional [26,27]. In all calculations the 6-31G** basis setwas used. Vibrational frequencies and intensities were computedat the same level of theory for the ligands and their complexes.The isotropic NMR shielding constants, ri, were calculated usingthe gauge-including atomic orbitals (GIAO) DFT method [28] atthe B3LYP/6-31G** level and converted to 13C chemical shifts usingthe equation: di = 191.818 � ri.

2.2. Synthesis

All chemicals were analytical grade reagents and were usedwithout additional purification. The ligand, L, was synthesized

according to a previously described procedure [10] and was recrys-tallized from the methanol/water solution.

The copper complex was obtained by mixing methanol solu-tions of the Cu(CH3COO)2�H2O (1.2 mmol, 0.2396 g in 10 mL) withsixfold excess of the ligand (7.2 mmol, 1.442 g in 50 mL) in order toensure complete reduction of the Cu(II) ion. The reaction mixturewas stirred at 40–50 �C for 1 h. Red precipitate was formed, filteredoff, washed repeatedly with ethanol and dried over P4O10 for twoweeks; yield 0.258 g; 82% with respect to the metal. Elementalanalyses: Cu–L. Anal. Calc. for Cu2(L�)2 [C16H22Cu2N4S4, 525.71]:C 36.56, H 4.22, N 10.66, S 24.39; Found C 37.62, H 4.57, N 10.81,S 25.01. FAB+ MS: [M]+ 525.0 (20) m/z, [M+Cu]+ 588.8 (100) m/z.

The Ni(II) complex was obtained from Ni(CH3COO)2�4H2O andthe ligand in the 1:2 metal-to-ligand ratio; i.e 0.6 mmol(0.1493 g) of the metal salt were dissolved in 10 mL methanoland mixed with solution of the ligand (1.2 mmol, 0.2404 g) in10 mL methanol. The reaction mixture was stirred at 40–50 �Cfor a couple of hours, which resulted in the formation of brownprecipitate; yield 0.263 g; 92% with respect to the metal. Elementalanalyses: Ni–L. Anal. Calc. for Ni2(L�)4�2H2O [C32H48N8Ni2O2S8,950.65]: C 40.43, H 5.09, N 11.79, S 26.98; Found C 40.15, H 5.06,N 11.63, S 26.16. FAB+ MS: [M]+ 914.0 (20) m/z, [M�L]+ 715.3(100) m/z.

3. Results and discussion

3.1. Evaluation of the applied theoretical method for structural andspectroscopic description – experimental (X-ray) and calculatedstructure and spectra of the cyclohexanespiro-5-(2,4-dithiohydantoin), L, and a related Cu(I) complex

Structural parameters for the 2,4-dithiohydantoin ring of L ob-tained from X-ray data [11] and from our calculations (B3LYP/6-31G**) are compared in Fig. 1. The 13C NMR and IR spectra forthe ligand L were recorded and the corresponding spectroscopicparameters were calculated using the quantum chemical DFTmethod. Nuclear magnetic shielding constants were computedusing the geometry taken from the available crystal structure ofthe ligand (without optimization) and were compared with thechemical shifts obtained for the B3LYP/6-31G** optimized geome-try and with the experimental data. In this way we could estimatethe performance of the quantum chemical method used for pre-dicting the 13C NMR chemical shifts. Additionally, the resulting dif-ferences due to geometry optimization could be evaluated. As canbe seen from Fig. 1, the known trend of the DFT method to predictlonger bonds is clearly observed in the presented structure. Theonly exception is the C2@S bond length, which is by 0.008 Å short-er in the optimized structure (the calculations were performed foran isolated molecule). The reason is that the C2@S group isinvolved in the formation of two intermolecular S� � �H–N bondsin the crystal while the C4@S group does not participate in such

N

N

S

S

M

H(CH2)n

M

1 2

345

N

N

S

S

H

M(CH2)n

M1

23

45

N

N

S

S

H

M(CH2)n

M

123

45

N1^S2 N3^S2 N3^S4

Scheme 1. Possible bridging ways of coordination of cycloalkanespiro-5-(2,4-dithiohydantoins) with metal ions (M).

3920 A. Ahmedova et al. / Inorganica Chimica Acta 363 (2010) 3919–3925


interactions. This causes elongation of the C2@S bond, which is by0.037 Å longer than the C4@S bond, according to X-ray data. Thepresence of intermolecular hydrogen bonding in the crystal alsoexplains the largest deviation observed for the calculated C4@S(0.018 Å) and N1–C2 bond (0.028 Å). Correspondingly, the experi-mental NMR chemical shifts measured for solution and solid state(see Table 1), confirmed that the heteroatoms of hydantoin ringswere involved in intermolecular interactions. In comparison withthe data for solution, the 13C chemical shifts for carbons from thecycloalkane rings (C5–C10) are larger in the solid-state spectra,whereas the C2 and C4 show an upfield shift. Apparently, the ob-served differences in the solution and solid-state NMR spectracould be explained by elongation of the bonds in the hydantoinring in solution, presumably due to strong intermolecular interac-tions involving the solvent (DMSO) molecules. The chemical shiftscalculated (DFT-GIAO) using the crystal structure of L are predictedfairly well for C2, C4 and C5, but fail in predicting the shifts of thecarbon atoms from the cyclohexane ring. The reason is the rela-tively short C–C bonds obtained from the X-ray data (1.513–1.530 Å). This problem is overcome after geometry optimization,

leading to C–C bonds in the range 1.535–1.549 Å and acceptableresults for the calculated 13C NMR shifts (see Table 1). These datademonstrate how sensitive are the calculated NMR shifts on theused geometry. In general, the calculated NMR shielding constantsfor the relaxed gas-phase structure show a downfield shift for all C-atoms when compared with that for the X-ray structure.

In comparison with the experimental data, the calculated 13CNMR chemical shifts for L predict a weaker shielding which is inaccordance with the structural features discussed above. Excellentcorrelation was obtained between the calculated (B3LYP/6-31G**)and the experimental (CPMAS) 13C NMR shifts (correlation factorof 0.999). The calculated differences D = dcalc � dexp_solid amountto 3.8, and 2.2, for the C2- and C4-atoms, respectively. Smallerdifference, of 1.5 ppm, is observed for the C5-atom.

Additionally, we performed geometry optimization with thesame method (B3LYP/6-31G**) of the only available crystal struc-ture of a metal complex of dithiohydantoin; Cu(I) complex of5,50-dimethyl-2,4-dithiohydantoin [23]. The optimized structuralparameters are compared with the experimental ones in order toevaluate the performance of the selected method to describe

Fig. 1. Molecular structure of cyclohexanespiro-5-(2,4-dithiohydantoin), L, together with the experimental (X-ray) (normal) and the calculated bond lengths (B3LYP/6-31G**)in bold.

Table 113C NMR data for L and the Cu(I) and Ni(II) complexes. The 13C CPMAS data are given in bold and the differences D0exp ¼ dexp

compl � dexpfreeL – in parentheses; the calculated data (GIAO-

B3LYP/6-31G**) for the complexes are presented as the differences D0calc ¼ dcalccompl � dcalc

freeL.

L Cu–L Ni–L

Exp. B3LYP CPMAS B3LYP CPMAS B3LYP

DMSO CPMAS X-ray str. Opt. str. Model 1 Model 2

C2 180.0 178.8 173.00 174.99 188.6 (9.8) 11.25 13.11 187.2 (8.4) 12.10C4 211.8 209.2 205.00 207.04 209.4 (0.2) 9.46 1.97 216.8 (7.6) 11.02C5 77.1 79.5 74.38 78.06 79.0 (�0.5) �1.00 �3.21 80.1 (0.6) 0.10C6 36.7 39.9 17.42 38.86 37.3 (�2.6) �1.68 �0.69 �36 (�3.9) �0.03C7 20.8 22.9 1.99 24.60 22.8 (0.1) 0.38 0.44 23.1 (0.3) 0.58C8 24.4 22.8 1.88 26.03 22.8 (0.0) 0.12 0.24 23.1 (0.3) 0.43C9 20.8 22.8 1.72 24.60 22.8 (0.0) 0.32 0.49 23.1 (0.3) 0.58C10 36.7 35.1 18.96 38.87 37.3 (2.2) �0.66 �0.68 �36 (0.9) �0.04

A. Ahmedova et al. / Inorganica Chimica Acta 363 (2010) 3919–3925 3921


correctly the structures of metal complexes, too. The calculatedspectroscopic (IR and NMR) characteristics of the complex will bediscussed in the following section in relation to the spectroscopicevidences for the different coordination modes. According to thecrystal structure described in [23], all Cu(I) centers have tetrahe-dral geometry and are coordinated with three sulfur atoms and achloride ion. Two different modes of coordination of the 5,50-di-methyl-2,4-dithiohydantoin are realized, monodentate – throughS2-atom and bridging – through both S-atoms: S2 and S4. Thecoordinated chloride ions participate in three intermolecularhydrogen bondings with the N–H groups from the coordinatedligands. In this way polymeric catena is formed. In our optimizedmodel a small fragment of the crystal structure is used havingthe composition Cu2L4Cl2H2O (replacing one of the ligands withan water molecule). The B3LYP/6-31G** optimized structure of thismodel is presented in Supplementary material – SP-Fig. S1 and se-lected bond lengths are compared with the crystallographic data(SP-Table S1). Moreover, the structural parameters of the complexare compared with that of the free ligand in order to follow thechanges resulting from coordination, as obtained from the X-raydata and from the B3LYP/6-31G** optimizations. According to theX-ray data, the only bonds from the hydantoin ring that get elon-gated upon S2-monodentate and S2,S4-bridging coordination areC4–N3 and C2@S2 bonds. The same trend is observed in the opti-mized structures, except the C4–N3 bond elongation in theS2,S4-bridging coordination. In the latter case elongation is ob-served for the C4@S4 bond. In contrast to the general trend knownfor the DFT methods to predict longer bonds, the optimized Cu–Sbonds are shorter than that in the crystal (see SP-Table S1). How-ever, the Cu–Cl bond is much longer after optimization, and thiscan be explained with the strong intermolecular H-bonding involv-ing the N–H groups from the three ligands coordinated to the cop-per center. In this respect it is worth mentioning that due to thestructural feature of the studied hydantoins, intermolecular inter-actions are always present in the crystals of the free ligand as wellas in the complexes. Therefore the obtained results should be takenwith caution.

3.2. Models and calculations

The main complication in describing spectroscopic evidence forcomplexation arises from the presence of two thioamide groups inthe ligand’s molecule that are conjugated in a ring. The variety ofways of coordination of L is a possible reason why no general trendin the experimental NMR spectra of its complexes was observed. Inorder to find the most plausible structure of the complexes, char-acterized by 13C CPMAS NMR and IR spectra, we applied the follow-ing methodology: (i) various possible structures of Cu(I) and Ni(II)complexes of L were modelled and optimized with B3LYP/6-31G**and their 13C shielding constants and IR vibrational frequencieswere computed; (ii) general trends in the spectroscopic changesupon particular way of coordination were summarized; (iii) thelatter were used to suggest the most probable structures of the ob-tained complexes of L showing the best agreement between theexperimental and theoretical data.

Some of the possible bridging ways of coordination of the stud-ied ligands are depicted in Scheme 1. We performed geometryoptimization on model monomeric structures of Cu(I) and Ni(II)complexes with L coordinated in all possible monodentate ways(not shown in Scheme 1) and computed their IR vibrational fre-quencies and 13C NMR shielding constants. In all model structuresthe complexes are non-charged, linear Cu(I) and square planarNi(II). The L coordinates as neutral molecule in S2- and S4-coordi-nation or as monoanion in N1- and N3-coordination. The totalcharge of the complexes and the coordination number of the met-als were kept the same in all models by adding chloride ions and/or

water molecules. The optimized structures, together with the cal-culated NMR parameters and the vibrational analyses, can be ob-tained from the authors.

Presently, the general trends in spectroscopic changes that arecaused by different ways of coordination will be described. Themonodentate S2-coordination leads to a downfield 13C NMR shiftfor C2 (by ca. 8 ppm) and an upfield shift for C4 (ca. 3 ppm)whereas the S4-coordination leads to opposite changes. More pro-nounced changes are observed when the ligand is coordinated bythe N-atoms after deprotonation; N1-coordination leads to down-field (almost equal) shifts for C2 and C4, although the latter is moredistant from the site of coordination (D0calc ¼ dcalc

compl � dcalcfreeL in the

Cu(I) complex is 5.5 and 5.6 ppm for C2 and C4, respectively, andin the Ni(II) complex it is 8.0 and 7.1 ppm for C2 and C4, respec-tively). On the other hand, the N3-coordination leads to moreremarkable shifts (D0calc) for C2 than for C4, which in the Cu(I) com-plex are 14.0 and 9.1 ppm, respectively.

It should be noted however that for cyclic thioamide ligands themonodentate coordination by the thione group leads to an upfieldshift of the corresponding C-atom signal, according to NMR datasupported by crystal structures [29–31] and to solely spectroscopicstudies [32,33]. This is in contradiction with the results from ourcalculations and experimental spectra, and illustrates the complex-ity of structural determination of the complexes when the ligandscontain two conjugated thioamide groups.

Before we suggest the most plausible structures of the com-plexes, we would like to emphasize the spectroscopic features thatare characteristic of each possible bridging way of coordination. InN3^S4 bridging coordination the largest shift occurs for C2 (ca.11 ppm), whereas the downfield shift for C4 is ca. 8 ppm and C5signal is shifted to higher field by ca. 1 ppm. The specific featurefor N3^S2-coordination is that both C2- and C4-atoms show almostequal downfield shifts in the complexes, amounting to 13.6 and12.1 ppm, respectively. The shift for the spiro-type C5-atom is only1.6 ppm.

The corresponding changes in the IR vibrational frequenciesresulting from different ways of complexation could be summa-rized as follows: (i) monodentate coordination with a thione groupalways leads to high-frequency shifting of the intensive C2–N1stretching vibration (see m(N–C) in Table 2 for L) while the sameis true for the stretching vibrations of N3–C2 in S2-coordinationand N3–C4 in S4-coordination; (ii) the coordination with an N-atom results in the absence of strong stretching and deformationalvibrations for the corresponding deprotonated H–N group (seem(N–H) and d(N–H) in Table 2). Another specific feature is the shiftof all C–N stretching vibrations to lower frequencies, which clearlydistinguishes this type of coordination from the S-monodentatecoordination; (iii) chelate formation is also characterized by theshifting of C–N stretching vibrations to lower frequencies; (iv) ina bridging way of coordination, the C–N stretching vibrations ofthe involved atoms are shifted to higher frequencies, while thoseunaffected by the coordination exhibit a low-frequency shift. Sincethere are almost no pure vibrations in the dithiohydantoin ring, itis risky to make further generalizations and each case must be re-garded separately (see for example Table 2, in Cu–L the ligands areN3^S4 bridging).

3.3. Experimental data and suggested structures

The coordination properties of the ligand L towards Cu(II) andNi(II) ions were studied employing various spectroscopic methods,such as IR, FAB-MS, 13C CPMAS NMR and EPR. The results con-firmed our previous observation, based on EPR and XPS data, thatin alcohol medium the cycloalkanespiro-5-dithiohydantoins re-duce Cu(II) ions to the diamagnetic Cu(I) [18], and therefore noEPR signal was observed for the obtained copper complex of L. This



result is also consistent with the available crystallographic data ofthe Cu(I) complex of 5,50-dimethyl-2,4-dithiohydantoin [23],which was obtained from CuCl2. In this case the complex has apolymeric structure with tetrahedral geometry of the Cu(I) ionscoordinated to a chloride ion and three sulfur atoms. However, inour syntheses we used Cu(II) and Ni(II) acetates as starting salts.Therefore, in order to explain the formation of neutral complexes,we assumed that some of the ligands coordinate as monoanionsafter deprotonation. This supposition was confirmed by the IRspectra of the complexes where one of the characteristic strongstretching vibrations for the N–H groups, at 3159 cm�1, is missing.Moreover, no experimental evidence for the presence of acetates ascounter ions was seen in the IR and NMR spectra. Due to poor sol-ubility of the isolated complexes, no single crystals of sufficientquality could be obtained. Therefore, we focused the structuralcharacterization of the complexes on solid-state NMR and IR spec-troscopy supported by quantum chemical DFT calculations.

In order to explain the observed downfield shifts in the 13CCPMAS NMR spectra of the complex, we will focus on the resultsobtained for dimeric structures in which the ligands act as bridgingmonoanions. In the case of Cu(I) complex, model structures withdifferent composition were optimized, namely Cu2(L�)2,Cu2(L�)2(L) and Cu4(L2�)(L�)2. For Ni(II) complexes the composi-tion of the model structures was Ni2(L�)4 in different combinationof N1,3^S2,4 bridging coordination of L�, and in cis- and trans-iso-meric forms of the square planar Ni(II) centers. For all optimizedstructures the 13C NMR shifts and IR vibrational frequencies werecalculated. As an illustration, the results of vibrational energy dis-tribution analyses are listed in Table 2; the data for the L and Cu–Lare included.

The 13C CPMAS NMR spectra of the free ligand L and those of itsCu(I) and Ni(II) complexes are presented in Fig. 2. The correspond-ing chemical shifts and the calculated differences D0exp, defined asD0exp ¼ dexp

compl � dexpfreeL, are listed in Table 1. Unfortunately, no clear

trend can be seen in the NMR spectra of the complexes. Therefore,various model structures that are consistent with the experimentaldata from elemental analyses and IR spectroscopy were optimized

and the corresponding 13C NMR shifts were computed. In order toestimate the correctness of the suggested models the experimentaland the calculated values for the shifts due to complexation(D0exp and D0calc, defined as D0exp ¼ dexp

compl � dexpfreeL and D0calc ¼ dcalc

compl�dcalc

freeL, respectively) were compared.Although the 13C CPMAS NMR spectrum of Cu–L shows changes

due to complexation only for the C2-atom, monodentate S2-coor-dination is ruled out by the formation of a non-charged complexand the absence of acetate ions as counter ions (as discussedabove). Therefore, we should assume that ligand molecules coordi-nate with the N-atoms as well, after deprotonation. That is why wesuggest a dimeric structure with a composition Cu2(L�)2 in whichthe ligand molecules act as bridging monoanions. The model de-picted in Fig. 3 gives the best initial agreement betweenD0exp and D0calc. Both ligand molecules coordinate in the N3^S4bridging mode, and the calculated D0calc are given in Table 1. In allother model structures the calculated shift for C4 and C5 are muchlarger, which makes them less probable. As far as the polymericstructure of the complexes was highly probable, the effect ofcoordination of neighbouring molecules was also examined. Theobtained D0calc show better agreement with the experimental ones(see Model 2 for Cu–L in Table 1) if a thione group from anotherligand coordinates to each metal center from the dimer. In thisway, the C4–S4 bonds get shortened by 0.014 Å and the value forD0calc becomes 1.97 ppm for C4. Due to the better agreement withthe experimental data in this case, we suppose that the metal cen-ters from the suggested dimeric structure (shown in Fig. 3) interactwith the additional thione group from neighbouring ligand mole-cules. The structure of the complex is symmetric with identicalstructural parameters of both coordinated ligands. Selected struc-tural characteristics of all suggested structures of the complexesare given in Table 3.

The Ni(II) complex was modelled as a dimer coordinated withfour singly deprotonated ligand molecules. In the solid-stateNMR spectrum a downfield shift was observed for all C-atoms fromthe hydantoin ring (see Table 1 and Fig. 2). Based on our theoreticalconsiderations it can be suggested that the most probable ways of

Table 2Calculated (B3LYP/6-31G**) vibrational frequencies (in cm�1), intensity (in km/mol) and analysis of potential energy distribution for L and Cu–L and experimental values (in bold)for L, Cu–L and Ni–L obtained in KBr.

L Cu–L Ni–L

(km/mol) (cm�1) Assignment PED (%) EXP. (km/mol) (cm�1) Assignment PED (%) EXP. EXP.

55.6 3682.3 100 m(N1–H) 3200 s 7.3 3681.4 100 m(N1–H) 3451 br 3348 br78.9 3641.7 100 m(N3–H) 3159 s 133.0 3681.3 100 m(N1–H) 3230 br 3128 m780.9 1551.6 28 m(N1–C2); 18 d(H1–N1–C2); 1538 vs 589.3 1511.0 21 m(N1–C2); 14 d(H1–N1–C2) 1499 vs

21 8(H3–N3–C2); 10 m(N3–C2) + m(S2@C2)34.7 1518.7 89 d(HCH) 1458 sh 204.6 1507.4 24 m(N1–C2); 56 d(HCH) 1480 vs 1458 vs262.8 1446.7 32 m(N3–C4); 10 m(S4@C4);

12 d(H1–N1–C2); 21 d(H3–N3–C2)1427 vs,1378

55.6 1505.7 10 m(N1–C2); 60 d(HCH)

76.6 1363.8 26 m(N1–C2); 13 m(N3–C2);17 d(H1–N1–C2)

1340 w 248.4 1473.6 75 m(N3–C4); 1456 vs 1346 m

22.3 1316.2 11 m(N3–C4); 20 d(HCC) 1270 w 28.6 1463.8 81 m(N3–C4) 1385 m 1314 m65.5 1295.8 34 d(HCC) 1216 m 31.5 1385.8 34 d(HCC)74.8 1246.7 10 m(N3–C4); 11 d(C5–N1–C2) 1157 117.4 1340.2 14 m(N1–C2); 18 d(H1–N1–C2);

11 d(HCC)1312 vs 1270 s

26.7 1177.9 11 m(N3–C2) + m(S2@C2); 12 d(HCC); 1142 m 242.4 1290.1 11 m(N1–C2); 12 d(C4–N3–C2); 1261 vs 1221 m14 s(C–CCC) 18 d(H1–N1–C2)

215.9 1121.9 21 m(N3–C2) + m(S2@C2); 16 m(S4@C4);22 d(H3–N3–C2)

1117 vs 21.6 1246.2 23 d(HCC); 12 d(C5–N1–C2) 1227 s 1176 vs

8.0 1064.7 20 d(C2–N1–C5) 1053 w 340.9 1201.8 26 m(N3–C2); 18 m(S2@C2) 1174 vs 1134 s16.3 937.6 16 d(N1–C2–N3); 15 d(CCC) 929 m 157.9 1153.7 11 d(H1–N1–C2) 1128 vs 1048w,13.8 714.7 13 d(N1–C2–N3) 735 11.9 1001.9 32 m(N1–C2); 15 s(H–CCC) 1019 w 1031w94.3 668.0 91 s(H3–NCN) 710 m 24.7 991.8 23 m(N3–C2); 10 m(S4@C4); 945 w, 905 w

25 d(N1–C2–N3)12.0 617.2 71 s(S2–CNN) + s(S4–CNC) + s(C–N1CC) 626 m 37.3 956.4 10 m(CC); 16 d(C5–N1–C2); 901 w 852 w, 722 w12.7 547.8 27 d(C5–N1–C2) + d(CCC) 542, 19.7 721.9 11 m(S2@C2); 10 d(N1–C2–N3) 711 w 675 w, 622 w48.8 422.5 64 s(H1–NCN) + s(C5–NCN); 18 s(H1–NCN) 453 m 116.4 427.4 85 s(H–NCN) 511 w 541 m, 470 w

vs – Very strong, s – strong, m – medium, w – weak, br – broad, sh – shoulder; (m(CH) vibrations are omitted for clarity).



coordination are the N3^S2 and N3^S4 bridging modes. The sug-gested structure for Ni–L consists of N3^S2 bridging ligands andshows the best agreement between the calculated and experimen-tal NMR shifts (see Fig. 3). Although the suggested isolated-mole-cule models give reasonable results, it seems probable thatintermolecular interactions leading to the formation of polymericspecies are present in the solids. The optimized structure of thiscomplex is also symmetric, and selected bond lengths are listedin Table 3.

As far as the calculated IR spectra of the complexes are con-cerned, an illustrative example for the vibrational frequenciesand VEDA analyses is presented in Table 2 for L and its coppercomplex. The experimental data for all studied compounds are alsoincluded. It can be seen from the calculations that the most intensebands for L due to stretching and deformational vibrations involv-ing the N1-atom (1551 and 1363 cm�1) are shifted to lower fre-quencies upon coordination (1511, 1507 and 1340 cm�1) for theN3^S4 bridging mode, as suggested for Cu–L. Correspondingly,

200 150 100 50 ppm

4 2

5 610

7 89

a

b

c

Fig. 2. 13C CPMAS NMR spectra of L – (a.), Cu–L – (b.) and Ni–L – (c.); (C-atoms numbering, see Fig. 1).

Fig. 3. Optimized structures for the Cu(I) and Ni(II) complexes of cyclohexane-spiro-5-(2,4-dithiohydantoin), Cu–L and Ni–L, respectively.



vibrational modes involving the N3-atom (1446) shift to higherfrequency due to coordination (1473 and 1463 cm�1). These spec-tral features can also be seen inspecting the data from the experi-mental spectra (Table 2). Another characteristic feature in thecalculated spectra of L and Cu–L that also finds its confirmationin the experimental data is the shift and splitting of the strongband at 1117 cm�1, due to vibrations involving the N3-atom, to1128 and 1174 in Cu–L (Table 2). The general trends outlined forL and Cu–L can also be found in the experimental data for the Ni(II)complex.

4. Conclusion

The study demonstrated the applicability of 13C CPMAS NMRand IR spectroscopy to determine solid-state structures of metalcomplexes of the cyclohexanespiro-5-(2,4-dithiohydantoin). Thedetailed description is based on the calculated (DFT) and experi-mental 13C NMR shifts as a more reliable source of informationthan the IR spectroscopy, due to the complexity of vibrationalmodes of the studied ligands. Different possible ways of coordina-tion were studied with respect to the resulting spectroscopicchanges and general trends are outlined. According to our calcula-tions, the characteristic feature of the 13C NMR spectra of coordi-nated dithiohydantoin ligand is that monodentate coordinationwith a thione group leads to a downfield shift of the signal forthe corresponding C-atom, which is in contrast to the reported up-field shifts in the 13C NMR spectra of metal complexes of cyclic thi-oamide ligands. This result shows the main difference arising fromthe fact that in the dithiohydantoins there are two thioamidegroups conjugated in one cycle. Moreover, the presented experi-mental data together with our recent reports [19,24] are the only13C NMR data for metal complexes of heterocyclic ligands contain-ing two thioamide groups conjugated in a ring. The experimentallyobserved 13C CPMAS NMR spectra for L, and its Cu(I) and Ni(II)complexes were used to suggest the best possible structures ofthe complexes supported by the quantum chemical (DFT) calcula-tions. For the Cu(I) complex, Cu–L, the dimeric structure is sug-gested with the N3^S4-bridging coordination. Additionalinteraction with neighbouring ligand molecules is also proposedleading to better agreement between the calculated and experi-mental 13C NMR shifts. For the Ni(II) complex the dimeric structureis suggested with four ligands coordinated in the N3^S2-bridgingway forming square-planar geometry of both Ni(II) centers. The ap-plied combined approach (13C CPMAS NMR and DFT) demonstratesits potential as an alternative way to determine the solid-statestructure of metal complexes even for non-trivial ligand molecules.

Acknowledgements

Financial support by the National Science Fund of Bulgaria, Con-tract BYX-01.05, is gratefully acknowledged. A.A. acknowledges a

postdoctoral fellowship (DO-02-255, NSFB). The calculations wereperformed in the Interdisciplinary Centre for Mathematical andComputational Modelling of Warsaw University under the compu-tational Grant G14-6.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2010.07.050.

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Table 3Selected optimized (B3LYP/6-31G**) bond lengths for the Cu(I) and Ni(II) complexesof cyclohexanespiro-5-(2,4-dithiohydantoin), Cu–L and Ni–L, respectively.

L Cu–L (Model 1) Cu–L (Model 2) Ni–L

C5–N1 1.472 1.466 1.464 1.466C2–N1 1.348 1.350 1.349 1.343C2–N3 1.398 1.409 1.396 1.364C4–N3 1.360 1.327 1.334 1.367C5–C4 1.536 1.521 1.522 1.534C2@S2 1.657 1.660 1.677 1.705C4@S4 1.646 1.703 1.689 1.650M–N – 1.852 1.894 1.898M–S – 2.167 2.217 2.265


Spectroscopic aspects of the coordination modes of 2,4-dithiohydantoins: Experimental and...

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Transcript of Spectroscopic aspects of the coordination modes of 2,4-dithiohydantoins: Experimental and...