New Platinum ( II ) Complexes of Cycloalkanespiro-5-( 2-thiohydantoins ) . Synthesis and Quantum Chemical Investigation

Synthesis and characterization of new Pt(II) complexes of cyclohexanespiro-5-(2-thiohydantoin) (L1) and cycloheptanespiro-5-(2-thiohydantoin) (L2) are discussed. The new complexes are studied by elemental analysis, IR and H NMR spectroscopy. The free ligands are investigated by UV-Vis, IR, H NMR, C NMR and Raman spectroscopy. The ground-state equilibrium geometries of the ligands L1 and L2 and their complexes with Pt(II) are optimized at the BLYP/CEP-31G theoretical level.


Introduction
Platinum-based drugs, and in particular cis-diamminedichloroplatinum(II) (best known as cisplatin), are employed for the treatment of a wide array of solid malignancies, including testicular, ovarian, cervical, head and neck, colorectal, bladder and lung cancers.3][4] In addition, certain cancer types are resistant to cisplatin therapy.This resistance is either intrinsic or developed during prolonged treatment. 5,6New platinum complexes have been pursued and investigated for their antitumor properties in order to circumvent these problems.Although well over a thousand complexes have been prepared and tested so far, 7 only two other platinum drugs are approved for clinical use worldwide, and three additional compounds are approved for regional use in individual nations in Asia. 8These complexes (nedaplatin, lobaplatin and heptaplatin) operate with a mechanism of action similar to that of cisplatin, which involves DNA binding and transcription inhibition.Several platinum(IV) complexes have undergone clinical trials, but to date none have been approved for use in the United States.Examples include iproplatin, tetraplatin and satraplatin.Recently, J. Wilson et al. presented the synthetic methods for the preparation of platinum anticancer complexes. 9Quiroga discussed the potential and limitations of the non-classical metallodrugs with platinum as metal. 10Despite the fact that cisplatin is one of the most effective and commonly used agents, nephro-, neuro-and ototoxicities are the main side effects of this drug. 11Recently, N. Stojanovi} et al. investigated the cytotoxic activity of Pt(II) complexes with diazenecarboxamide against human cervical carcinoma HeLa cells. 12M. Saeidifar et al. synthesized a new watersoluble Pd(II) anionic complex and studied its cytotoxicity against human leukemia cells. 13arinova et al.: New Platinum(II) Complexes ...
In the field of non-platinum compounds exhibiting anticancer properties, ruthenium complexes are very promising, showing activity on tumors which developed resistance to cisplatin or in which cisplatin is inactive.The first ruthenium compound NAMI-A (imidazolium transimidazoledimethylsulfoxidetetrachloro-ruthenate) entered phase I clinical trials in 1999 as an antimetastatic drug, 14,15 whereas the ruthenium complex KP1019 (transtetrachlorobis(indazole)ruthenate(III)) entered phase I clinical trials in 2003 as an anticancer drug which is among others very active against colon carcinomas and their metastases. 16Complexes such as RM175 (ONCO4417 ), prepared by P. J. Sadler, break the rule of the "activation by reduction mechanism", since they are based on the ruthenium at +2 oxidation state then having no need to be reduced to be active. 17 Hydantoin derivatives are well known for their medical applications, e.g. as antiepileptic drugs. 18,19Antiproliferative effects, 20,21 inhibition of aldosoreductase 22 and potential application for treatment of HIV-1 infections 23,24 were also described.Recently, we reported the synthesis of various thioanalogues of cycloalkanespiro-5-hydantoins. 25 The crystal structures of four cycloalkanespiro-5-(2,4-dithiohydantoins), with different size of the saturated ring 26 and two cycloalkanespiro-5-(2-thiohydantoins) 27 were determined by means of single-crystal X-ray crystallography.Taking into account medical applications of hydantoin derivatives, it is of crucial importance to acquire the knowledge about their interactions with bioavailable metal ions. 28lthough hydantoin compounds are studied extensively, there is not much research on their anticancer activities.In a previous work of ours, we have reported a method for obtaining 4'-bromo-(9'-fluorene)-spiro-5-(2,4-dithiohydantoin). 29In the study cited above, we have investigated cytotoxic activities of the compound on the retinoblastoma cell line WERI-Rb-1 and antibacterial effects towards both, Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria, as well as yeasts Candida albicans.The preliminary results of the cytotoxicity assay have shown that the compound could serve as a potential anticancer agent.Further investigations are needed to elucidate exact mechanisms of this action and to exclude any cytotoxic effect on normal cells.The results for the compound have shown no antimicrobial activity towards the bacteria Escherichia coli, Staphylococcus aureus and no activity towards Candida albicans.In another previous paper of ours, we have described a method for synthesis, examination of cytotoxicity and antibacterial effects of 3-amino-9'-fluorenespiro-5-hydantoin 30 and new Pt(II) complexes of (9'-fluorene)-spiro-5-hydantoin and its 2-thio derivative. 31The two platinum complexes show significant effects on cancer cell growth compared to their ligands.Recently, we studied the complexation properties of cyclohexanespiro-5-(2,4-dithiohydantoin) with copper and nickel. 32In a previous work of ours, we have presented the synthesis of Nsubstituted tetralinspiro-5-hydantoins. 33Quantum-chemical calculations at DFT level are also performed to elucidate their structure.
That is why, the research described here is focused on the synthesis of Pt(II) complexes of cyclohexanespiro-5-(2-thiohydantoin) (L1) and cycloheptanespiro-5-(2thiohydantoin) (L2) and their characterization by elemental analysis, IR, ATR FTIR spectroscopy.The free ligands are described by UV-Vis, IR, and Raman spectroscopy.The QM calculations are performed with full geometrical optimization without any symmetry restrictions.The structures of the organic compounds used in this study are described in Scheme 1.
It should be noted that the spirothiohydantoins, i.e. imidazolidine-2-thiones, present even more different ways of coordination (monodentate-A, bridging-B and chelating-C) due to the presence of two thioamide groups in a ligand (see Scheme 2).

1. Instrumentation and Methods
A metal salt ((NH 4 ) 2 [PtCl 4 ] -Sigma-Aldrich) and solvents used for synthesis of the complexes were of p.a. quality.Electronic spectra were registered on a Lambda 9 Perkin-Elmer UV/Vis/NIR Spectrophotometer from 200 nm to 1000 nm.The IR spectra of all compounds were re- gistered in KBr pellets on a Bruker FT-IR VERTEX 70 spectrometer from 4000 cm -1 to 400 cm -1 at resolution 2 cm -1 with 25 scans.The Raman spectra of the free ligands were measured on a spectrometer RAM II (Bruker Optics) with a focused laser beam of 20 mW and 200 mW power of Nd:YAG laser (1064 nm) from 4000 cm -1 to 400 cm -1 at resolution 2 cm -1 with 25 scans).The NMR spectra were taken on a Bruker Avance II+ 600 MHz NMR spectrometer operating at 600.130 and 150.903MHz for 1 H and 13 C, respectively, using the standard Bruker software.Chemical shifts were referenced to tetramethylsilane (TMS).Measurements were carried out at ambient temperature.

3. Theoretical Methods
The ground-state equilibrium geometries of the ligands L1 and L2 and their complexes PtL1 (C1) and Pt-L2 (C2) were optimized at the BLYP/CEP-31G theoretical level.No symmetry and coordinate restrictions were applied (fully relaxed geometries during the optimization).The ligands L1 and L2 were optimized with spin multiplicity singlet while for the complexes C1 and C2 the multiplicity was set to quintet.The calculations were performed with the GAUSSIAN 03 program package. 53

Results and Discussion
Complexation with Pt(II) using a metal salt namely (NH 4 ) 2 [PtCl 4 ] at molar ratio M:L = 1:1 for PtL were conducted under alkaline conditions.Neutral complexes were synthesized and isolated as precipitates.The PtL1 (C1) and PtL2 (C2) complexes obtained have yellow colour.All complexes were investigated by IR spectroscopy and elemental analysis.Elemental analyses data was found to be in good agreement (+0.5%) with the calculated values.It was found that the molar ratio metal:ligand is 1:2.Selected vibrational frequencies observed in the IR spectra of the complexes were compared with those of the free ligands in Table 1.
In the IR spectrum of the free ligand L1 bands at 3346 cm -1 and 3234 cm -1 were observed and referred to the stretching vibrations of the two N-H groups of the hydantoin ring (see Table 1).In the spectrum of the PtL1 complex the band resulting from the oscillation of the one of the two N-H groups was observed at 3446 cm -1 , which is about 100 cm -1 shifted to the larger frequencies as compared to the free ligand spectrum.The second band was missed.In the spectrum of free ligand L1, the bands at 1738 cm -1 and 1062 cm -1 could be assigned to vibrational oscillations of C 4 =O and C 2 =S groups of the hydantoin ring.The band resulting from the oscillation of the C 4 =O group in the IR spectrum of PtL1 complex is shifted to the lower frequencies by 30 cm -1 as compared to that of the free ligand.The vibrational oscillation of C 2 =S group of the hydantoin ring was not change in the IR spectrum of PtL1 complex.
In the IR spectrum of the free ligand L2 bands at 3447 cm -1 and 3163 cm -1 were observed which we referred to the stretching vibrations of N-H groups of the hydantoin ring.In the spectrum of the PtL2 complex the same band was observed at 3104 which is shifted to the lower frequencies by 59 cm -1 as compared to the free ligand spectrum.One of the two bands was missing in the spectrum of the PtL2 complex.In the spectrum of free ligand L2 the bands at 1738 cm -1 and 1035 cm -1 could be assigned to vibrational oscillations of C 4 =O and C 2 =S groups of the hydantoin ring.The band resulting from the oscillation of the C 4 =O group in the IR spectrum of PtL2 complex is shifted to the higher frequencies by 28 cm -1 as compared to that of the free ligand.In the spectrum of Pt-L2 complex the band at 1028 cm -1 , which could be attributed to vibrational oscillation of C 2 =S group of the hydantoin ring was not change.
It was not possible to measure Raman spectra of the complexes -the sample bumed even at 1 mW laser power.Only the Raman spectra of the free ligands L1 and L2 were measured and discussed in the current paper (Table 2).The C 4 =O stretching vibration of L1 appears at 1736 cm -1 .The C 2 =S stretching vibration is appears at 1065 cm -1 in the Raman spectrum.Several bands in the Raman spectrum (2943, 2929, 2872, 2861 and 2843 cm -1 ) and in the IR spectrum (2948-2859 cm -1 ) were assigned to the stretching vibrations of CH 2 in the cyclohexane ring.The two vibrational bands of ν(N 1 -H) and ν(N 3 -H) in the Raman spectrum were not registered.
In the Raman spectrum of L2 the C 4 =O stretching vibration appears at 1726 cm -1 .The C 2 =S stretching vibration appears as a weak band at 1048 cm -1 in the same spectrum.Several bands in the Raman spectrum (2937, 2898 and 2853 cm -1 ) and in the IR spectrum (2932-2854 cm -1 ) are for stretching vibrations of CH 2 in the cycloheptane ring.One of the two vibrational bands in the Raman spectrum for ν(N 1 -H) and ν(N 3 -H) appears only lower frequency band (at 3159 cm -1 with a very low intensity).
IR technique is excellent for carbonyl species while the Raman analysis is quite variable. 54The carbonyl C=O stretching vibration results in strong characteristic IR bands.Raman bands for thess vibrations are typically moderate to weak with some structures resulting in a strong C=O stretch.The carbonyl C=O stretching band was easily identified in the IR spectrum because of its intensity and its lack of interference with most of the other group frequencies.
The chemical structure of L1 and L2, as well as their Pt(II) complexes was established through 1 H and 13 C NMR spectroscopy.The 1 H NMR spectra of L1, L2 show   Optimized geometries in Fig. 1 show that the ligand L1 has a chair conformation of the cyclohexane ring and a planar structure of the aromatic residue.We have identified a chair conformation of the cycloheptane ring either.
Regarding the complexes C1 and C2, one can see that the chair conformations of the cyclohexane / cycloheptane rings are kept.Two valent bonds are formed between Pt(II) and the nitrogen, and oxygen atoms from the planar rings.Weak bonds (over 3 Å) are formed between the remaining nitrogen and the oxygen atoms of the rings.These bonds are a bit longer in the complex C2 than in the complex C1, probably due to the influence of the large hydrocarbon ring (steric hindrance).
In the two complexes one of the sulfur atoms (the ring which forms coordination valent bond between the carbonyl oxygen atom and Pt(II)) is considerably deviated from the aromatic ring: <SCNN=129° (in C1) and <SC-NN=130° (in C2).We explain this fact with the strong repulsion between the sulfur atom and the weakly bonded carbonyl oxygen atom of the other ring to Pt(II).The complexes C1 and C2 are planar with respect to the bonded atoms to Pt(II).

resonance signals at 1
.3-2.0 ppm (CH 2 protons of cycloalkane residue, multiplet) and two broad signals at 10.0-12.0ppm characteristic of NH protons.In the 1 H NMR spectra of PtL1 and PtL2 show resonance signals at 1.5-2.0ppm (CH 2 protons of cycloalkane residue, multiplet) and one broad signal at 10.49 and 10.39 ppm characteristic of NH proton, respectively.This fact shows that the one of the two NH groups participates in the coordination with the metal ion.The 13 C NMR spectra of L1 and L2 show resonance peaks of C-2, C-4 and C-5 of the thioanalogues of spirohydantoins at 179 and 180 ppm, 181 ppm and 65.7 and 68.2 ppm, respectively.