Oxidative Additon of Tin Thiolates Yielding New Pt-Sn Heterobimetallic Complexes

Abstracts

The reactions between CODPtCl2, COD = 1,5 cyclooctadiene, and the tin thiolate compounds (Ph)3Sn(SPh) and (Ph)2Sn(SPh)2 have been studied. Reaction between CODPtCl2 and (Ph)3Sn(SPh) yielded two new Pt-Sn heterobimetallic complexes depending on the CODPtCl2/(Ph)3Sn(SPh) molar ratio. For a 1:2 molar ratio, CODPt(SPh)2(Cl)Sn(Ph)3.2CH2Cl2, complex 1, was obtained, together with the homometallic complex CODPt(SPh)Cl.CH2Cl2. For a 1:1 molar ratio the complex formed was CODPt(SPh)(Cl)2Sn(Ph)3.3CH2Cl2, complex 2. Reaction with (Ph)2Sn(SPh)2 did not yielded any heterobimetallic complex, but rather CODPt(SPh)2.4CH2Cl2. The complexes were characterized by IR, ¹H-NMR, 13C-NMR, 195Pt-NMR, 119Sn-NMR and Mössabauer spectroscopies, elemental analysis and atomic absorption.

tin thiolates; oxidative addition; Pt-Sn heterobimetallics

Foram estudadas as reações entre CODPtCl2, COD = 1,5 ciclooctadieno, e os compostos tiolato de estanho (Ph)3Sn(SPh) e (Ph)2Sn(SPh)2. Reação entre CODPtCl2 e (Ph)3Sn(SPh) originou dois novos complexos heterobimetálicos contendo Pt e Sn, dependendo da razão molar CODPtCl2/(Ph)3Sn(SPh). Para uma razão molar de 1: 2 foi obtido CODPt(SPh)2(Cl)Sn(Ph)3.2CH2 Cl2, complexo 1, juntamente com o complexo homometálico CODPt(SPh)Cl.CH2Cl2. Para a razão molar 1:1 o complexo formado foi CODPt(SPh)(Cl)2Sn(Ph)3.3CH2Cl2, complexo 2. Reação com (Ph)2Sn(SPh)2 não originou nenhum complexo heterobimetálico, apenas CODPt(SPh)2. 4CH2Cl2. Os complexos foram caracterizados por espectroscopias Mössbauer, de IV, RMN de ¹H, 13C, 195Pt, 119Sn, análise elementar e absorção atômica.

Article

Oxidative Additon of Tin Thiolates Yielding New Pt-Sn Heterobimetallic Complexes

Rodrigo Herbert Vaz^a, Anuar Abras^b, and Rosalice Mendonça Silva^*a

^aDepto. de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte - MG, Brazil

^bDepto. de Física, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte - MG, Brazil

Received: April 18, 1997

Foram estudadas as reações entre CODPtCl₂, COD = 1,5 ciclooctadieno, e os compostos tiolato de estanho (Ph)₃Sn(SPh) e (Ph)₂Sn(SPh)₂. Reação entre CODPtCl₂ e (Ph)₃Sn(SPh) originou dois novos complexos heterobimetálicos contendo Pt e Sn, dependendo da razão molar CODPtCl₂/(Ph)₃Sn(SPh). Para uma razão molar de 1: 2 foi obtido CODPt(SPh)₂(Cl)Sn(Ph)₃.2CH₂ Cl₂, complexo 1, juntamente com o complexo homometálico CODPt(SPh)Cl.CH₂Cl₂. Para a razão molar 1:1 o complexo formado foi CODPt(SPh)(Cl)₂Sn(Ph)₃.3CH₂Cl₂, complexo 2. Reação com (Ph)₂Sn(SPh)₂ não originou nenhum complexo heterobimetálico, apenas CODPt(SPh)₂. 4CH₂Cl₂. Os complexos foram caracterizados por espectroscopias Mössbauer, de IV, RMN de ¹H, ¹³C, ¹⁹⁵Pt, ¹¹⁹Sn, análise elementar e absorção atômica.

The reactions between CODPtCl₂, COD = 1,5 cyclooctadiene, and the tin thiolate compounds (Ph)₃Sn(SPh) and (Ph)₂Sn(SPh)₂ have been studied. Reaction between CODPtCl₂ and (Ph)₃Sn(SPh) yielded two new Pt-Sn heterobimetallic complexes depending on the CODPtCl₂/(Ph)₃Sn(SPh) molar ratio. For a 1:2 molar ratio, CODPt(SPh)₂(Cl)Sn(Ph)₃.2CH₂Cl₂, complex 1, was obtained,together with the homometallic complex CODPt(SPh)Cl.CH₂Cl₂. For a 1:1 molar ratio the complex formed was CODPt(SPh)(Cl)₂Sn(Ph)₃.3CH₂Cl₂, complex 2. Reaction with (Ph)₂Sn(SPh)₂ did not yielded any heterobimetallic complex, but rather CODPt(SPh)₂.4CH₂Cl₂. The complexes were characterized by IR, ¹H-NMR, ¹³C-NMR, ¹⁹⁵Pt-NMR, ¹¹⁹Sn-NMR and Mössabauer spectroscopies, elemental analysis and atomic absorption.

Keywords: tin thiolates, oxidative addition, Pt-Sn heterobimetallics

Introduction

We have been using thiolate compounds of tin, a main group metal, to react with niobium compounds to obtain heterobimetallic complexes. Reaction between (h⁵-C₅H₅)₂NbCl₂ and (Ph)₃Sn(SPh) yielded a heterobimetallic complex bridged by a sulfur atom, instead of the thiolate ligand, due to a C-S bond cleavage¹.Bis(thiolato) complexes of titanium, an early transition metal like niobium, are known to react with halide complexes of platinum, a late transition metal, yielding thiolate complexes, both terminal or bridging, but none with a sulfur atom, no C-S bond cleavage was reported². One aspect that seemed important to verify was whether reaction between platinum and thiolate compounds of tin would activate C-S bonds. There also existed the possibility that a heterobimetallic complex could be formed as in the case with niobium¹. In this paper we report the results obtained from reactions between CODPtCl₂ and the tin compounds (Ph)₃Sn(SPh) and (Ph)₂Sn(SPh)₂, yielding the complexes CODPt(SPh)₂ (Cl)Sn(Ph)_3.2CH₂Cl₂, complex 1, CODPt(Cl)₂(SPh)Sn (Ph)3.3CH₂Cl₂, complex 2, products of oxidative addition reactions, as well as CODPt(SPh) (Cl).CH₂Cl₂, complex 3, and CODPt(SPh)₂.4CH₂Cl₂, complex 4.

Experimental

General comments

All operations were carried out under pure dinitrogen, using Schlenk and vacuum techniques. Nitrogen was predried over an in line columm consisting of molecular sieves, calcium chloride and calcium sulfate. Dichloromethane was distilled from calcium hydride, pentane and hexane from sodium/benzophenone and petroleum ether was left over molecular sieves before being used. All solvents were used immediately following distillation or stored under nitrogen over the appropriate molecular sieves. CODPtCl₂, (Ph)₃Sn(SPh) and (Ph)₂Sn(SPh)₂ were prepared according to literature procedures^3,4. ¹¹⁹Sn Mössbauer spectroscopy was performed in a constant acceleration equipment moving a CaSnO₃ source at room temperature. The samples were measured at liquid nitrogen temperature. All spectra were computer-fitted assuming Lorentzian lineshapes. Infrared spectra were recorded on a Perkin-Elmer 283B spectrophotometer in the range 4000- 200 cm^-1, in CsI pellets. ¹H-, ¹³C-, ¹⁹⁵Pt- and ¹¹⁹Sn-NMR spectra were recorded on a Bruker AC-400 and referenced to internal SiMe₄. C and H analyses were performed using a Perkin-Elmer PE-2400 CHN microanalyser. Atomic absorption for platinum and tin was performed on a Hitachi Z-8200 Polarized Zeeman Atomic Absorption Spectrophotometer.

Reaction between CODPtCl₂ and (Ph)₃Sn(SPh): a- 200 mg (0.534 mmol) of CODPtCl₂ were put to react with 490 mg (1.07 mmol) of (Ph)₃Sn(SPh), in CH₂Cl₂, under reflux. Immediatelly after the addition of the solvent the reaction color turned to yellow. After two hours of reaction the solvent was removed under vacum leaving a yellow residue that was washed three times with petroleum ether. The solvent separated and a white solid precipitated, (Ph)₃SnCl. The yellow residue was redissolved in dichloromethane and upon addtion of hexane (COD)(SPh)₂ ClPtSn(Ph)₃.2CH₂Cl_2, complex 1_, precipitated as a yellow solid. Yield: 49.4%. The mother liquor was separated and submitted to the same procedure from which (COD)Pt (SPh)Cl.CH₂Cl₂, complex 3, was obtained. Yield: 8.36%. (Ph)₃SnCl was characterized by comparision of the m.p, IR, Mössbauer data and ¹H-NMR of an authentic sample. (COD)(SPh)₂ClPtSn(Ph)₃.2CH₂Cl₂:Anal. Calc. for C₄₅H₄₁ S₂Cl₅PtSn (found): C = 44.58 (44.55), H = 3.80(3.81), Pt = 18.11(20.69). IR (cm^-1, CsI): 3050, 3040, 3000, 2940, 1570, 1470, 1430, 1410, 1060, 1050, 1005, 980, 715, 670, 660, 455, 420, 305, 300, 235; ¹H-NMR (d, CDCl₃): 7.68-7.44(m, 25H, 5C₆H₅), 5.60(br, 4H, CH=CH, ²J_Pt-H = 67.89 Hz), 2.68-2.37 (m, 8H, CH₂); ¹³C-NMR (d, CDCl₃): 137.31, 136.12, 130.46, 129.14 (aromatics), 99.98 (CH, J_Pt-C not observed), 30.88 (CH₂); ¹¹⁹Sn-NMR (d, CDCl₃): -50.85; ¹⁹⁵Pt-NMR (d, CDCl₃): -3834.8; Mössbauer: (mm/s): d = 1.34 ± 0.02 and D = 2.54 ± 0.01. (COD)Pt(SPh)(Cl).CH₂Cl₂: Anal. Calc. for C₁₅H₁₉SCl₃Pt (found): C = 35.80 (36.86), H = 3.48 (2.79); IR (cm^-1, CsI): 3080, 3070, 3015, 3005, 2980, 1590, 1480, 1020, 1000, 725, 670, 470, 430, 315, 220; ¹H-NMR (d, CDCl₃): 7.70-7.10 (m, 5H, C₆H₅), 5.63 (br, 4H, ²J_Pt-H = 66.41 Hz), 2.71-2.25 (m, 8H, CH₂); ¹³C-NMR (d, CDCl₃): 136.41, 130.88, 129.55, 126.28 (aromatics), 100.45 (CH, J_Pt-C = 600.8 Hz), 31.31 (CH₂).

b- The reaction described in a was repeated using a 1: 1 molar ratio of the reactants, 0.544 mmol of each, under room temperature. After a few minutes of reaction a yellow solid started to precipitate. The reaction was left stirring for one hour. After this time the solvent was removed under reduced pressure leaving a yellow residue that was first washed three times with hexane and then dissolved in acetone. Upon addition of hexane a yellow solid precipitated, COD(SPh)(Cl)₂PtSn(Ph)₃.3CH₂Cl₂, complex 2. Yield: 25%. Anal. Calc. for C₃₅H₃₈SCl₈PtSn (found): C = 38.88(38.37), H = 3.49 (3.07); IR (cm^-1, CsI): 3150, 2995, 2925, 2900, 2860, 2815, 1570, 1460, 1430, 1335, 1325, 1300, 1230, 1220, 1210, 1155, 1070, 1050, 1005, 980, 895, 850, 800, 780, 715, 680, 670, 475, 440, 415, 305, 285, 240; ¹H-NMR (d, (CD₃)₂CO): 7.47-7.22 (m, 4C₆H₅), 5.50 and 5.37 (m, 4H, 2CH=CH, ²J_Pt-H = 58.49 and 54.14 Hz), 2.81- 2.16 (m, 8H, CH₂); ¹³C-NMR (d, (CD₃)₂CO): 136.82, 129.15, 128.80, 127.87 (aromatics), 105.07, 101.65, 100.21, 95.81 (CH, J_Pt-C not observed), 31.57, 31.42 (CH₂). ¹⁹⁵Pt (d, (CD₃)₂CO)= -3653.05; Mössbauer (mm/s): d = 1.44 ± 0.02 and D = 2.44 ± 0.05.

Recation between CODPtCl₂ and (Ph)₂Sn(SPh)₂: a- 160 mg (0.427 mmol) of CODPtCl₂ and 430 mg (0.880 mmol) of (Ph)₂Sn(SPh)₂ were put to react under room temperature, in CH₂Cl₂, for 20 min. After this time the solvent was removed under reduced pressure leaving a yellow residue. This residue was washed three times with petroleum ether, yielding small amounts of a white solid. The yellow residue was redissolved in CH₂Cl₂ and upon addition of hexane CODPt(SPh)₂.4CH₂Cl₂, complex 4, precipitated as a yellow solid. Yield: 75%. Anal. Calc. for C₂₈H₃₀S₂Cl₈Pt (found): C = 36.96 (34.07), H = 3.30 (2.14); IR (cm^-1, CsI): 3080, 3060, 2960, 2890, 2840, 1590, 1470, 1440, 735, 690, 680, 490, 470, 455, 305, 290; ¹H-NMR (d, CDCl₃): 7.55-7.13 (m, 10H, 2C₆H₅), 4.66( br, 4H, CH, ²J_Pt-H = 55.64Hz), 2.52-2.09 (m, H, CH₂); ¹³C-NMR (d, CDCl₃): 136.16, 135.81; 128.09; 126.83; (aromatics), 98.86 (CH, J_Pt-C = 481.60 Hz), 30.27 (CH₂). Yield: 75%.

b- The reaction described in a was repeated using a 1: 1 molar ratio of the reactants, 0.427 mmol of each, at room temperature. After a few minutes of reaction a yellow solid started to precipitate. The reaction was left stirring for 30 min. After this time the solvent was removed under reduced pressure leaving a yellow residue that was first washed three times with hexane and then dissolved in acetone. Upon addition of hexane complex 4 precipitated.

Results and Discussion

Reaction between CODPtCl_2, COD = 1,5 cyclooctadiene_, and (Ph)₃Sn(SPh), yielded two different heterobimetallic complexes depending on the CODPtCl₂ / (Ph)₃Sn(SPh) molar ratio. For a 1:2 molar ratio complex 1 was obtained and for a 1:1 molar ratio complex 2 was obtained. Together with complex 1, a homometallic complex, complex 3, was obtained in minor amounts. Reaction of CODPtCl₂ with (Ph)₂Sn(SPh)₂ yielded a homometallic complex, complex 4, Fig. 1; irrespective of the reagent molar ratio, no heterometallic complex was obtained. All the complexes were yellow and air stable.

Important IR data for the complexes are shown in Table 1. For complexes 1 and 3 the n_Pt-S were observed as a doublet⁵. For complex 1 this is indicative of the cis position of the two thiolate ligands. The same is true for complex 2 regarding the two observed n_Pt-Cl streching frequencies⁶.

Figure 2 showns the ¹H-NMR spectra of complex 1. The signal due to the olefinic portion of the COD ligand was observed as a broad singlet together with the Pt satellites. The values of the ²J_Pt-H coupling constants are given in Table 2.

For complex 2, two of these coupling constants could be observed which is in accordance with the fact that the ^-SPh ligand and one of the Cl^- ligands are cis to each other, exerting different trans influences on the CH=CH portion of the COD ligand. The resonance of the hydrogens of the olefin is observed as a multiplet, indicating that they are not equivalent. It is worth mentioning that although complex 4 is used as a starting material in the reactions of bis(thiolato)titanocene with (COD)PtCl₂² no spectroscopic data or elemental analysis was reported for 4 and we did not find such data in the literature. Also, complex 3 was reported as an intermediate but the literature states it could not be isolated. The ²J_Pt-H coupling constants for complex 1 and 2 are within the range observed for other Pt(IV)-Sn complexes⁷. In the ¹³C-NMR spectrum of complex 2, four resonances are observed for the carbons of the olefin and two resonances for the CH₂ group. However, we were not able to determine the J_Pt-C coupling constants for complexes 1 and 2, as for complexes 3 and 4. Although the signals in the ¹³C-NMR were well defined, they were of little intensity which precluded the determination of this coupling constant.

We encountered solubility problems especially with complexes 1 and 2. In all cases an emulsion was obtained. Low solubility has been observed for other Pt(IV) complexes^2,7a,8. However, the low solubility was the factor that permitted us to isolate complexes 1 and 2. As pointed out by others, many Pt(IV) complexes are formed but they are rarely isolated because they have a tendency, in solution, to undergo rapid reductive elimination, yielding Pt(II) complexes, both soluble and insoluble⁹. This behavior could be observed in the present work. If complexes 1 to 4 were left for long periods of time (> 0.5 h) in solution, a yellow insoluble solid precipitated on the bottom of the flask. In both the ¹H- and ¹³C-NMR spectra of the four solutions, small peaks at d = 4.60 (²J_Pt-H = 71.8 Hz₎, 99.0, 30.79 could be observed. Complex 1 was the most soluble of the two heterobimetallic complexes, and for this reason we could obtain a ¹¹⁹Sn-NMR spectrum of this complex^#. ¹⁹⁵Pt-NMR spectra could be obtained for both complexes, the values were d = -3834.8 and d = -3653.05 for complexes 1 and 2 respectively, and are in accordance with the fact that since complex 1 is surrounded by less electro-ndonating ligands, d ¹⁹⁵Pt is observed at higher field. These values are found at lower field as compared to the resonance found for heterobimetallic Pt(II)-Sn complexes, e.g., trans-PtCl (SnCl₃)(PEt₃)_2, d = -4790.0 andtrans-Pt(SnCl₃)₂(PEt₃)₂, d = -5093.0¹⁰. The data obtained from the Mössbauer spectra were indicative of a tetrahedral environment around the tin atom as well as being bonded to another metal.Table 4 gives Mössbauer parameters for other Pt(IV)-Sn compounds for comparison. For all the complexes a singlet could be observed around d = 5.2, in the ¹H-NMR spectrum, that was attributed to the hydrogens of the solvent CH₂Cl₂. The structures proposed for the complexes, shown in Fig. 1, were based also on the elemental analysis and atomic absorption.

Mechanistic Studies

The mechanism that is generally proposed for reactions between Pt(II) and Sn(IV) compounds leading to heterobimetallic Pt(IV)-Sn complexes is the oxidative addition of tin chlorides to the platinum nucleophilic center^7a,7b,11. The formation of complex 1 could be explained perfectly using such a mechanism, as represented in Eqs. 1 and 2.

First, there ought to be a nucleophilic displacement of halide by the thiolate, as shown in Eq. 1, followed by the oxidative addition of (Ph)₃SnCl, yielding complex 1, Eq. 2. However, when the reaction shown in Eq. 2 was performed independently using first, stoichiometric quantities and then excess of tin chloride both at room temperature or under reflux, no heterobimetallic complex was obtained. Reaction between CODPtCl₂ and (Ph)₂Sn(SPh)₂ yielded only CODPt(SPh)₂, as a platinum containg product; no heterobimetallic complex was formed in the reaction, despite the reaction being more favourable as Ph₂SnCl₂ is a better electrophile than Ph₃SnCl₂. The reaction shown in Eq. 3, also did not yield any heterobimetallic complex:

According to the suggested mechanism we could not explain the formation of complex 2. These facts suggested to us that the mechanism for the reactions decribed here must involve oxidative additions of the tin thiolate instead of the tin chloride. The proposed mechanism for the formation of complex 1 is represented in Eqs. 4 and 5 as follows.

First, it must involve the nucleophilic displacement on only one metal-chloride bond by the thiolate, yielding complex 3 which is a likely intermediate in this reaction. The oxidative addition of (Ph)₃Sn(SPh) to the intermediate yields complex 1. The oxidative addition of the tin thiolate explains the formation of complex 2, as represented in Eq. 6. It involves the direct oxidative addition of the tin thiolate to CODPtCl₂.

Thus it is possible to explain why reaction between CODPtCl₂ and (Ph)₂Sn(SPh)₂ did not yield a product due to the oxidative addtion reaction. As (Ph)₂Sn(SPh)₂ is a worse eletrophile than (Ph)₃Sn(SPh), it does not add oxidatively to the platinum nucleophilic center. Also, the reaction represented in Eq. 2 does not occur because, although (Ph)₃Sn(SPh) is the best eletrophile, CODPt(SPh)₂ is a poor nucleophile as compared to CODPtCl₂.

Conclusions

The reactions described here involve a rare type of oxidative addition of tin thiolates. In fact just one case has been described recently in the literature and according to the authors^7c and to the present work these oxidative additions are cis instead of the trans type of oxidative addtions of tin chlorides. The present work allowed us to conclude that reactions between platinum compounds, a late transition metal, with tin thiolates do not cleave C-S bonds but a new route was found for obtaining Pt(IV)-Sn heterobimetallic complexes. Pt(IV)-Sn heterobimetallic complexes are known to participate in many homogeneous catalytic processes, especially in the hydroformylation of olefines¹².

Acknowledgments

To the agencies that support our work: FAPEMIG, CNPq/PADCT, FINEP and PRPq. To Prof. Carlos Alberto Lombardi Filgueiras for valuable suggestions and to Wagner Magno Teles for his help with the synthesis of CODPtCl₂.

References

1.Results to be published

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^# The natural abundance of

The natural abundance of ¹¹⁹Sn and ¹¹⁷Sn isotopes are respectivelly 8.58% and 7.61%. This low abundance requires longer periods of time to obtain a good spectrum.

Publication Dates

• Publication in this collection
  17 Mar 2008
• Date of issue
  Feb 1998

History

• Received
  18 Apr 1997
• Accepted
  18 Apr 1997