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Journal of the Brazilian Chemical Society

Print version ISSN 0103-5053

J. Braz. Chem. Soc. vol.16 no.6a São Paulo Nov./Dec. 2005

http://dx.doi.org/10.1590/S0103-50532005000700014 

ARTICLE

 

Platinum complexes with non-symmetric fluorodithioethers. Molecular structures of cis-[PtCl2(CH3SCH2CH 2SRf)] (Rf = C6F5, C6H4F-3) and trans-[PtCl2(CH3SCH2CH 2SC6F5)2]

 

 

Amaranta RamírezI; José G. AlvaradoII; Sylvain BernsIII; Luís Ortiz-FradeI; Ezequiel HuipeIV; Hugo Torrens*, I

IDEPg, Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México D.F., Mexico
IICIQ, Universidad Autónoma del Estado de Hidalgo, Unidad Universitaria, Pachuca, Hidalgo, Mexico
IIICQIC, Universidad Autónoma de Puebla, A.P. 1613 72000 Puebla, Puebla, México
IVInstituto Tecnológico de Morelia, Morelia, Michoacan, Mexico

 

 


ABSTRACT

This paper describes the synthesis and characterization of the non-symmetric ligands CH3SCH2CH2SRf (Rf = C6F5, C6HF4-4, C6H4F-2, C6H4F-3, C6H4F-4) as well as their platinum(II) derivatives cis-[PtCl2(CH3SCH2CH2SR f)] (Rf = C6F5, C6HF4-4, C6H4F-2, C6H4F-3, C6H4F-4) and trans-[PtCl2(CH3SCH2CH 2SRf)2] (Rf = C6F5, C6HF4-4). 19F NMR of these complexes show the presence of syn and anti isomers, consistent with a fast flipping of the metallacycle ring and a slow inversion of configuration at the dithioether sulfur atoms. The molecular and crystalline structures of the compounds cis-[PtCl2(CH3SCH2CH2SC 6F5)], cis-[PtCl2(CH3SCH2CH2SC 6H4F-3)] and trans-[PtCl2(CH3SCH2CH 2SC6F5)2], solved by X-ray diffraction are also described.

Keywords: fluorodithioeteres, platinum, 19F NMR, X-ray diffraction


RESUMO

Este trabalho descreve a síntese e caracterização dos ligantes não simétricos CH3SCH2CH2SRf (Rf = C6F5, C6HF4-4, C6H4F-2, C6H4F-3, C6H4F-4) bem como seus derivados de platina(II) cis-[PtCl2(CH3SCH2CH2SR f)] (Rf = C6F5, C6HF4-4, C6H4F-2, C6H4F-3, C6H4F-4) e trans-[PtCl2(CH3SCH2CH 2SRf)2] (Rf = C6F5, C6HF4-4). O espectro de RMN de 19F desses complexos mostra os isômeros syn e anti, consistente com um rápido dobramento do anel metalacíclico e uma inversão lenta de configuração nos átomos de enxofre do tioéter. As estruturas molecular e cristalina dos compostos cis-[PtCl2(CH3SCH2CH2SC 6F5)], cis-[PtCl2(CH3SCH2CH2SC 6H4F-3)] e trans-[PtCl2(CH3SCH2CH 2SC6F5)2], resolvida por difração de raios-X, são também descritas.


 

 

Introduction

The co-ordination chemistry of sulfur containing ligands-thiolates, thioetheres etc, is a traditional area still growing at a fast speed. Its interest steams from the importance of these compounds in catalytic processes, oil dehydrodesulfurization, biological systems and chemical synthesis amongst others. The equivalent chemistry involving fluoro-containing ligands,1,2 is younger, proportionally less known, but not less interesting. Fluorinated compounds are becoming of increasing importance in modern pharmacology,3 design of new materials4 and catalysis of fluorinated organic compounds.5

Substitution of hydrogen by fluorine has two general consequences. The first one is topological, as a result of the different dimensions between hydrogen and fluorine atoms. The second is an electronic one, caused by the difference on the electroatractor character of these two elements. It is possible to generalize in a broad sense, by stating that a fluorinated molecule is stereochemically similar to its normal hydrogenated analogue but considerably different as far as its electronic properties is concerned. The chemical reactivity derived from morphological changes is hardly altered whereas chemical behavior depending on electronic parameters is profoundly affected.

We have been interested in the chemistry of transition metal compounds with fluorinated ligands for a long time.6 One of the areas that has particularly attracted our attention is that related with sulfur containing polydentate ligands, especially those systems in which each sulfur atom has a different coordinating ability. In this paper we describe the synthesis and characterization of the non-symmetric ligands CH3SCH2CH2SRf (Rf = C6F5 L1, C6HF4-4 L2, C6H4F-2 L3, C6H4F-3 L4 and C6H4F-4 L5) as well as their platinum(II) derivatives cis-[PtCl2(CH3SCH2CH2SR f)] (Rf = C6F5 1, C6HF4-4 2, C6H4F-2 3, C6H4F-3 4 and C6H4F-4 5) as chelating ligands and trans-[PtCl2(CH3SCH2CH 2SRf)2] (Rf = C6F5 6 and C6HF4-4 7) as monodentate ligands. The 19F NMR spectra of complexes 1-5, show the presence of syn and anti isomers, consistent with a fast flipping of the metallacycle ring and a slow inversion of configuration at the dithioether sulfur atoms. The molecular and crystalline structures of the compounds cis-[PtCl2(CH3SCH2CH2SC 6F5)] 1, cis-[PtCl2(CH3SCH2CH2SC 6H4F-3)] 4 and trans [PtCl2 (CH3SCH2CH2SC6F5) 2] 6 , solved by X-ray diffraction are also described.

 

Results and Discussion

CH3SCH2CH2SRf

Reactions of CH3SCH2CH2Cl with lead fluorothiolates Pb(SRf)2 (Rf = C6F5,7 C6HF4-4, C6H4F-2, C6H4F-3 and C6H4F-4) yield the corresponding dithioethers CH3SCH2CH2SRf (Rf = C6F5 L1, C6HF4-4 L2, C6H4F-2 L3, C6H4F-3 L4 and C6H4F-4 L5) as clear, dense oils. IR, 1H and 19F NMR as well as EI-MS data, shown in Table 1, confirm the identity of each ligand.

[PtCl2(CH3SCH2 CH2SRf)]

The reactions of bisulphides CH3SCH2CH2SRf , L1-L5, with alcohol-water solutions of K2[PtCl4], reaction 1, yield the compounds cis-[PtCl2(CH3SCH2CH2SR f)], (Rf = C6F5 18 C6HF4-4 2, C6H4F-2 3, C6H4F-3 4 and C6H4F-4 5), as air stable, yellow microcrystalline solids, soluble in acetone and dichloromethane. As expected for planar platinum(II) complexes, compounds 1-5 are diamagnetic. Physical and spectroscopic data for compounds 1-7 are collected in Table 2.

Infrared spectra

As expected for a cis configuration, IR spectra of compounds 1-5, show two absorptions in the 330-310 cm-1 range, assigned to the symmetric and asymmetric Pt-Cl vibrations. Dn = nsymnasym, span from 11 to 15 cm-1 but without apparent correlation with the fluorinated substituent at the sulfur atom. The IR spectra are useful in terms of characterization since they exhibit the absorptions associated to both the ligands and the cis-PtCl2 fragment. Unfortunately since absorptions are relatively wide, this spectroscopy is insensitive to structural changes and therefore, for identification purposes, only characteristic bands are reported for compounds 1 to 7.

NMR spectra

Compounds with general formulae [PtX2 (RSCH2 CH2SR)] can give rise to different isomers depending, as shown in Figure 1, on the relative position of the sulfur substituents -syn and anti- and also, depending of the relative configuration of the PtS2C2 cycle, A, B, C and D isomers.

 

 

In solution at room temperature, the A, B or C, D isomers are, in general, NMR indistinguishable because flipping of the PtS2C2 ring is too fast on the time scale of this spectroscopy. There are some examples, with large R substituents, that allow distinct configurations of the SCR2CR2S- skeleton to be measured at low temperatures.8 Normally however, all one can detect is a dynamic average between both stereoisomers shown in Figure 1.

Isomers syn and anti -Figure 1- can also interconvert to each other through a process known as inversion of configuration9 at the sulfur atoms. This inversion requires energies between 40 and 80 kJ mol-1 and therefore at room temperature both isomers can, normally, be experimentally detected by NMR.10

Each of the sulfur substituents used in this work show second order magnetic systems. Experimentally the expected multiplicity for each absorption has been obtained. 19F NMR parameters for compounds 1 to 5 are shown in Table 2.

Compounds 1 to 5 show NMR spectra consistent with the presence of syn and anti isomers. As an example, Figure 2 shows the spectrum of cis-[PtCl2(CH3SCH2CH2SC 6F5)] 1. The presence of only two absorptions (isomers) requires fast (in the NMR time scale) conformational changes at the metallacycle otherwise up to four isomers would have been detected, see Figure 1.

 

 

At room temperature both isomers are in equilibrium with nearly equal abundances for compounds 1 to 4 and with a ca. 2:1 relative abundance for compound 5. It is worth mentioning that the presence of both isomers implies that inversion of configuration could not be only and simultaneously at both sulfur atoms.

From NMR data it is not possible to assign unambiguously a set of signals to a particular isomer (syn or anti). It has been suggested before11 that the more abundant isomer corresponds frequently to the anti isomer, since this geometry minimizes the steric interactions. This does not seem to be the case for the complexes studied in this work, since they do not show significant differences between the relative syn:anti proportions observed. The relative sizes of both substituents -CH3 and Rf- are similar for all 1 to 5 complexes and apparently the steric hindrance of the methyl group has no effect on the orientation adopted by the fluorinated benzene ring.

19F NMR spectrum of compound [PtCl2(CH3SCH2 CH2SC6F5)] 1, Figure 2, exhibits two sets of absorptions arising from the syn and anti isomers –magnetic system A2B2C– with nearly equal relative intensities. Each group includes three absorptions corresponding to fluorine nuclei at ortho, meta and para positions with relative intensities of 2:2:1 respectively.

[PtCl2(CH3SCH2CH2SC 6F4H)] 2 exhibits a 19F NMR spectrum with two groups of signals assigned to the syn and anti isomers –magnetic system A2B2X– with 1:1 relative intensities. Each set shows two absorptions arising from the fluorine nuclei in ortho and meta positions with nearly equal relative intensities. 19F NMR spectra of compounds [PtCl2(CH3SCH2CH2SC 6H4F)] 3 to 5, with a single fluorine atom in ortho, meta or para positions display the expected single absorption for each isomer.

Molecular structures

cis-[PtCl2(CH3SCH2CH2SC 6F5)] 1 and cis-[PtCl2(CH3 SCH2CH2SC6H4F-3)] 4 were studied by X-ray diffraction methods. Table 3 shows the principal bond angles and distances from these molecules. ORTEP diagrams are shown in Figures 3 and 4.

 

 

 

 

 

 

 

 

Molecules 1 and 4 have platinum atoms bonded to both sulfur atoms of the chelating ligands with practically equal Pt-S bond lengths. 1: Pt-S1 2.244(2), Pt-S2 2.252(2) and 4: Pt-S1 2.257(3), Pt-S2 2.252(3) Å. Despite the presumed electronic differences between C6F5 and C6H4F-3 there is no evidence of cis-effect since the Pt-S1 and Pt-S2 are practically equal for both compounds.

In contrast, trans bond distances, 1: Pt-Cl1 2.306(2), Pt-Cl2 2.330(2) and 4: Pt-Cl1 2.318(3), Pt-Cl2 2.322(3) Å, are significantly different and probably reflect a larger trans-influence of the SC6F5 as compared with that of SC6H4F-3. Bond angles around the central platinum atoms as well as bond distances with sulfur and chlorine ligands define an almost perfect square.

Both substituents at the sulfur atoms are located above the S2Cl2 plane, adopting a syn configuration, probably as a result of crystal packing effects. Similar anti isomers are found in the solid state structures of [PtCl2(CF3SCH(CH3)CH2SCF 3)],12 [PtCl2(CH3SCH(CF3) CH(CF3)SCH3)],13 [Pt(SC6F5)2(CH3SCH(CF 3)CH(CF3) SCH3)]14 and [PtCl2(CH3SCF2CH2SCH 3)].15

H3C9-S-Pt and RfC1-S-Pt bond angles (108º, average) suggest the geometry around the sulfur atoms is pyramidal, close to a tetrahedral arrangement (109.5º) and therefore each sulfur atom could be considered as having sp3 hybridization.

A noticeable feature on these structures is that the crystal seems to be stabilized by intermolecular interactions in which the ortho-fluorine atoms of C6F5 in a molecule have short contacts with the para-fluorine atoms of C6F5 in a neighbor molecule.

Trans-[PtCl2(CH3SCH2CH2SR f)2]

Reactions of compounds cis-[PtCl2(CH3SCH2CH2 SC6F5)] 1 and cis-[PtCl2(CH3SCH2 CH2SC6HF4-4)] 2 with additional stoichiometric amounts of CH3SCH2CH2SC6 F5 or CH3SCH2CH2SC6 HF4-4, respectively, in acetone and room temperature yield the trans compounds [PtCl2(CH3SCH2CH2SC 6F5)2] 6 and [PtCl2(CH3SCH2 CH2SC6HF4-4)2] 7. Clearly, the presence of free ligand allows the donor sulfur atoms in CH3S- and RfS- to compete. Despite the chelating effect, the more basic CH3S group displaces the labile RfS moiety and trans-[PtCl2(CH3SCH2CH 2SRf)2] compounds are formed predominantly as shown in Figure 5.

 

 

X-ray diffraction data for compound trans-[PtCl2(CH3SCH2CH 2SC6F5)2] 6 are shown in Table 2. The corresponding ORTEP diagram is shown in Figure 6.

 

 

Experimental

All reactions were carried out under an atmosphere of oxygen-free dinitrogen using Schlenk techniques. Solvents were purified and degassed prior to use according to published methods.16 Thin-layer chromatography (TLC) (Merck, silica gel 60 F254 and neutral aluminium oxide 60 F254) was used to monitor the progress of the reactions under study. All reagents were purchased from Aldrich and used as received.

CH3SCH2CH2SC6F5, 7 [PtCl2(CH3SCH2CH2SC 6F5)]8 and the lead thiolates Pb(SRf)2 (Rf = C6F5, SC6HF4-4, SC6H4F-2, SC6H4F-3 and SC6H4F-4,17,18 were synthesized following published reports.

Infrared spectra were recorded on a 750 Nicolet Fourier Transform Magna-IR Spectrometer over the 4000-300 cm-1 range on CsI. Elemental Analyses were determined by Galbraith Laboratories Inc., USA. 1H and 19F-{1H} nuclear magnetic resonance spectra were recorded on a Varian Unity INOVA-300 spectrometer operating at 300 MHz and 282.23 MHz respectively. Chemical shifts are in ppm positive at low field, relative to TMS=0 ppm for 1H and CFCl3=0 ppm for 19F.

FAB+ mass spectra were recorded on a JEOL JMS-SX102A mass spectrometer operating with an acceleration voltage of 10 KV. Samples were desorbed from a 3-nitrobenzyl alcohol matrix using 3KeV xenon atoms. FAB+ mass measurements were carried out with resolution of 3000 using magnetic field scan and the matrix ions as reference or as electric field scan with the sample peak flanked by two poly(ethylene glycol) or CsI ions as reference.

For both, dithioethers -CH3SCH2CH2SRf- and platinum(II) derivatives -cis-[PtCl2(CH3SCH2CH2SRf )] and trans-[PtCl2(CH3SCH2CH2SRf )2]- a detailed experimental procedure is described.

CH3SCH2CH2SRf , L1-L5

CH3SCH2CH2Cl (0.88g, 8 mmol) in acetone (25 mL) were added to a solution of Pb(SRf)2 (4.0 mmol) in acetone (25 mL) and refluxed overnight. The white precipitate (PbCl2) was filtered off and the solvent transferred under vacuum. The oily colorless products were purified through a chromatographic column with silica and (CH3)2CO/CH3COOEt 1:1 as eluent. Yields: 97-98%.

cis-[PtCl2(CH3SCH2 CH2SRf)], 1-5

CH3HSCH2CH2SRf, (0.242 mmol) in acetone (20 mL) was added dropwise to (100.0mg, 0.242mmol) of K2[PtCl4] dissolved in a 1:1 water-acetone mixture (20 mL). The color changes slowly from red to yellow and after 24 h at room temperature the solvent was evaporated under vacuum. The yellow solid of cis-[PtCl2(CH3SCH2CH2SR f)] was washed with cold water and dried under vacuum at room temperature 4 h.

trans-[PtCl2(CH3SCH2CH2SR f)2], 6-7

CH3SCH2CH2SC6F5 L1 (0.28mg, 1 mmol), dissolved in acetone (10 mL) was added to [PtCl2(CH3SCH2CH2 SC6F5)] 1 (0.540g, 1 mmol) dissolved in acetone (10 mL). The color solution changes between yellow tones and after 24 h at room temperature the solvent was evaporated to obtain cis-[PtCl2(CH3SCH2CH2SC 6F5)2] 6 (0.710g, 0.88 mmol).

X-ray diffraction data

Air stable single crystals of complexes 1, 4, and 6 were obtained by slow evaporation of saturated solutions. Crystal data and other crystallographic parameters are listed in Table 5. The diffraction data for 1 was collected at room temperature on a Enraf CAD4 diffractometer using graphite monochromated Mo-Ka radiation (l = 0.71073 Å) and q/2q or w scan mode with variable scan speed. Structures was solved and refined using routine procedures19 on the basis of absorption-corrected data (y-scans). Structure 1 was refined without restraints. H atoms were placed on idealized positions and final least-squares cycles were carried-out using anisotropic displacement parameters for non-H atoms.

 

 

The diffraction data for 4 and 6 were collected at room temperature on a Siemens P4/PC 20,20 diffractometer using graphite monochromated Mo-Ka radiation (l = 0.71073 Å) and q/2q or w scan mode with variable scan speed. Structures were solved and refined using routine procedures19 on the basis of absorption-corrected data (y-scans). Structures 4 and 6 were refined without restraints. H atoms were placed on idealized positions and final least-squares cycles were carried-out using anisotropic displacement parameters for non-H atoms.

 

Supplementary Information

Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 265984 (compound 1), 266404 (compound 4) and 266807 (compound 6). Copies of the data can be obtained, free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK ; fax: +44 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk).

 

Acknowledgments

We are grateful to DGAPA-UNAM (IN119305) and CONACYT (44494) for financial support.

 

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Received: December 5, 2004
Published on the web: September 22, 2005

 

 

* e-mail: torrens@servidor.unam.mx