C–H reductive elimination during the reaction of cycloplatinated(II) complexes with pyridine-2-thione: kinetic follow up

Abstract

Substitution reactions of the labile SMe₂ ligand in the cycloplatinated(II) complexes [PtR(ppy)(SMe₂)], 1, in which ppy = 2-phenylpyridinate and R = Me, 1a, or p-MeC₆H₄, 1b, by pyridine-2-thione, C₅H₅SN, were studied. When each of the complexes 1 was treated with 1 equiv. C₅H₅SN, existing as a mixture of thiol (N⁁SH) and thione (HN⁁S) tautomers, a mixture containing the S-bound thiol complex [PtR(ppy)(η¹-S-S⁁NH)] (R = Me, 2a, or R = p-MeC₆H₄, 2b) and the dimeric complex [Pt(ppy)(N⁁S)]₂, 3 (having two bridging deprotonated pyridine-2-thione (N⁁S) ligands), was observed along with free R–H. This mixture finally led to pure complex 3 after 3 days. Pure samples of the complexes 2a and 3 were obtained from the abovementioned 2a+3 mixture by using flash chromatography on silica gel. Kinetics of the reactions were investigated by UV-vis spectroscopy (complexes 1 have a MLCT band in the visible region which was used to easily follow the reactions) and ¹H NMR spectroscopy. On the basis of the results, a mechanism was proposed for the related reactions.

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1. Introduction

The chemistry of cyclometalated complexes has long been the subject of intense study.^1–4 Among the cyclometalated complexes, cycloplatinated complexes have attracted a great deal of attention as a result of their highly versatile obvious applications in many fields.^5–8 Monoalkyl cycloplatinated complexes with a solvent or labile ligand have been reported only rarely.^9–16 These complexes reportedly exhibit various reactivities toward fundamental reactions such as oxidative addition and substitution reactions.^17–22

The rich reactive features of complexes with heterodonor ligands, such as 2-diphenylphosphinopyridine (pyPPh₂)^21,23–26 or pyridine-2-thione, C₅H₅SN,^27–38 have been widely studied for application in different aspects. The latter is an unsymmetrical bidentate ligand with a nitrogen donor atom and a sulfur donor atom. Its pK_a values³⁰ suggest that the parent molecule exists in solution as a mixture containing the thione–thiol tautomers (Ia and Ib in Fig. 1, respectively). This ligand can be deprotonated^27,31 easily to obtain the thiolate form N⁁S (C₅H₄SN), indicated as Ic in Fig. 1.³⁷ Although the thione–thiol tautomeric equilibrium has been shown generally to favor the thione form, the electronic differentiation associated with the hard nitrogen and soft sulfur donors in this reagent directs its reactivity and coordination behavior when it binds to metal (Fig. 2).^37,39 The reported monomeric platinum(II) complexes containing pyridine-2-thione have been shown to be S-bound with pendant pyridyl group,⁴⁰ such as trans-[Pt(η¹-S-C₅H₄NS)₂(PPh₃)₂] or N⁁S chelating, such as [Pt(η²-N,S-C₅H₄NS)(PPh₃)₂]PF₆,⁴¹ with no related example of the N-bound thiol complex.

Fig. 1 Thione-thiol tautomeric equilibrium and thiolate form.

Fig. 2 Coordination modes for C₅H₅SN and C₅H₄SN.

In the present study, we describe reaction of the cycloplatinated(II) complexes [PtR(ppy)(SMe₂)], 1, (R = Me; 1a, R = p-MeC₆H₄, 1b, and ppy = deprotonated 2-phenylpyridine) with pyridine-2-thione (C₅H₅SN) showing that the monodentate S-bound complexes [PtR(ppy)(η¹-S-S⁁NH)], 2, with NH pendant group, are formed along with a dimmeric complex [Pt(ppy)(N⁁S)]₂, 3, resulting from C–H bond reductive elimination. We have also followed up the kinetics of these reactions by using UV-vis spectroscopy and ¹H NMR spectroscopy in different solvents and suggested a mechanism for the related reactions.

2. Experimental section

The ¹H NMR spectra of the complexes were recorded on a Bruker Avance DPX 250 MHz or 400 MHz spectrometer, and TMS (0.00) was used as an external reference. All the chemical shifts and coupling constants are given in units of ppm and Hz, respectively. Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer with temperature control using an EYELA NCB-3100 constant-temperature bath. Pyridine-2-thione (C₅H₅SN) was purchased from commercial sources and the starting complexes, [PtMe(ppy)(SMe₂)], 1a,¹⁰ [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b,¹⁰ and [Pt(ppy)Cl(DMSO)],⁴² in which DMSO is dimethyl sulfoxide, were prepared as reported. The NMR labeling for ppy and C₅H₅SN ligands are shown in Scheme 1.^43,44

Scheme 1 The NMR labeling.

2.1. Reaction of [PtMe(ppy)(SMe₂)], 1a, with C₅H₅SN

To a solution of [PtMe(ppy)(SMe₂)], 1a, (100 mg, 0.23 mmol) in dichloromethane (or benzene) was added 1 equiv. pyridine-2-thione (26.1 mg, 0.23 mmol) at room temperature. The mixture was stirred for 2 h (or 24 h in benzene) and the solvent was then removed under reduced pressure. The residue was triturated with n-hexane to give a pale red solid, which was separated and dried under vacuum. The product was identified as a mixture of [PtMe(ppy)(η¹-S-S⁁NH)], 2a, and [Pt(ppy)(N⁁S)]₂, 3, with the ratio 2a/3 being 1/2. Total yield: 76 mg, 69%. Mixture of the compounds 2a and 3 were separated by flash chromatography with good yields on silica gel eluted with 100 : 2 CH₂Cl₂/MeOH, to afford pure [PtMe(ppy)(η¹-S-S⁁NH)], 2a, and [Pt(ppy)(N⁁S)]₂, 3, identified as follows:

2.1.1. Identification of the complex 2a. M.P. 254 °C (decomp.), Anal. calcd for C₁₇H₁₆N₂SPt, C, 41.45; H, 3.38; N, 5.89. Found: C, 41.04; H, 3.06; N, 5.27; ¹H NMR in CDCl₃; δ 1.86 (s, ²J_PtH = 69.9 Hz, Me–Pt, 3H), aromatic protons: ppy ligand, 6.80 (m, ³J_PtH^3′ = 24.5 Hz, ³J_H^3′H^4′ = 7.0 Hz, CH group adjacent to coordinated C atom, 1H^3′_ppy), 7.82 (m, ³J_PtH⁶ = 15.8 Hz, ³J_H⁶H⁵ = 5.7 Hz, CH group adjacent to coordinated N atom, 1H⁶_ppy), 7.99 (d, ³J_H⁶H⁵ = 5.8 Hz, CH group adjacent to coordinated N atom, H⁶_spy), 8.72 (broad singlet, H atom directly bond to N atom), 6.90–7.81 (other aromatic protons of ppy and S⁁N groups). ¹H NMR data for the complex 2a in C₆D₆, to be used when monitoring the related reaction in benzene (vide infra): δ 2.36 (s, ²J_PtH = 69.9 Hz, Me–Pt, 3H), aromatic protons: ppy ligand, 7.79 (dd, ³J_PtH^3′ = 24.5 Hz, ³J_H^3′H^4′ = 7.4 Hz, ⁴J_H^3′H^5′ = 1.1 Hz, CH group adjacent to coordinated C atom, 1H^3′_ppy), 8.76 (dd, ³J_PtH⁶ = 16.2 Hz, ³J_H⁶H⁵ = 5.7 Hz, ⁴J_H⁶H⁴ = 1.8 Hz, CH group adjacent to coordinated N atom, 1H⁶_ppy).

2.1.2. Identification of the complex 3. This was identified by comparison of its ¹H NMR spectrum with that of the complex 3 prepared by direct method as follows:

The complex 3 has been reported to be synthesized by the reaction of (Bu₄N)[PtCl₂(ppy)] with C₅H₅SN and was characterized in solid state by single crystal X-ray crystallography.⁴⁵ In the present work, [Pt(ppy)Cl(DMSO)] (100 mg, 0.22 mmol) was added to an ethanolic solution of sodium pyridine-2-thiolate, NaC₅H₄NS (see below), under inert atmosphere condition. ¹H NMR data in CDCl₃, δ 8.09 (d, ³J_PtH⁶ = 19.1 Hz, ³J_H⁶H⁵ = 5.6 Hz, CH group adjacent to coordinated N atom, 2H⁶_ppy), 7.47 (d, ³J_PtH^3′ = 32.8 Hz, ³J_H^3′H^4′ = 5.7 Hz, CH group adjacent to coordinated C atom, 2H^3′_ppy), 7.53 (t, 2H), 7.49 (d, 2H), 7.86 (d, ³J_PtH⁶ = 16.2 Hz, ³J_H³H⁴ = 7.5 Hz, 2H⁶_N⁁S), 7.13 (t, 4H), 6.91 (t, 4H), 6.74 (t, 2H), 6.71 (t, 2H), 6.43 (t, 2H). Other notes related to complex 3:

(1) Single crystal of dimer 3 was grown in a concentrated benzene solution of the 2a + 3 mixture product obtained from mixing of complex 1a and C₅H₅SN by slow diffusion of diethylether and its structure was confirmed by single crystal X-ray analysis (see Fig. 3).

Fig. 3 Crystal structure of the complex [Pt(ppy)(N⁁S)]₂, 3, from crystallization of the product obtained from mixing of complex 1a and C₅H₅SN. Selected bond distances (Å) and angles (°): Pt2–C22 1.977(11), Pt2–N2 2.064(9), Pt2–N3 2.144(10), Pt2–S2 2.286(3), Pt2–Pt1 2.8513(11), Pt1–C11 1.982(14), Pt1–N1 2.006(11), Pt1–N4 2.170(9), Pt1–S1 2.287(4), S2–C27 1.758(12), S1–C32 1.715(12), C22–Pt2–N2 80.7(4), C22–Pt2–N3 174.2(4), N2–Pt2–N3 93.4(4), C22–Pt2–S2 95.3(3), N2–Pt2–S2 172.8(3), N3–Pt2–S2 90.5(2), C22–Pt2–Pt1 95.6(3), N2–Pt2–Pt1 99.7(3), N3–Pt2–Pt1 85.4(3), S2–Pt2–Pt1 86.57(9), C11–Pt1–N1 81.1(6), C11–Pt1–N4 173.9(5), N1–Pt1–N4 93.6(5), C11–Pt1–S1 95.4(5), N1–Pt1–S1 172.7(4), N4–Pt1–S1 89.6(3), C11–Pt1–Pt2 98.9(4), N1–Pt1–Pt2 100.9(3), N4–Pt1–Pt2 84.9(3), S1–Pt1–Pt2 85.89(9).

(2) NaC₅H₄NS (as used in direct synthesis of complex 3, see above) was prepared by dissolving of sodium (6 mg 0.25 mmol) in 10 mL of absolute ethanol following by treatment with pyridine-2-thione (25 mg, 0.22 mmol) to immediately give a red solution that after stirring for 6 h at room temperature a red solid was precipitated which was separated and dried under vacuum. Yield: 73 mg, 74%. This compound gave ¹H NMR data similar to the reported data in DMSO-d₆.⁴⁵

(3) The complex 3 is not stable in CDCl₃ (or CH₂Cl₂) and slowly forms [Pt(ppy)(N⁁S)(Cl)]₂ complex,⁴⁵ giving ¹H NMR data in CDCl₃: δ 9.54 (d, ³J_PtH⁶ = 20.8 Hz, ³J_H⁶H⁵ = 5.6 Hz, CH group adjacent to coordinated N atom, 2H⁶_ppy), 8.15 (d, ³J_PtH^3′ = 28.2 Hz, ³J_H^3′H^4′ = 6.2 Hz, CH group adjacent to coordinated C atom, 2H^3′_ppy), 7.56 (t, 2H), 7.37 (t, 2H), 7.29 (d, 2H), 7.17 (t, 4H), 6.97 (t, 4H), 6.90 (t, 2H), 6.71 (t, 2H), 6.77 (t, 2H). These ¹H NMR data are similar to those reported for the complex [Pt(ppy)(N⁁S)(Cl)]₂ in DMSO-d₆.⁴⁵

(4) When the reaction of [PtMe(ppy)(SMe₂)], 1a, with pyridine-2-thione in benzene was stirred for 3 days, only dimer 3 as a pure product was formed.

(5) ¹H NMR data for the complex 3 in C₆D₆: ppy ligand, δ 7.49 (m, ³J_H^3′H^4′ = 5.3 Hz, ⁴J_H^3′H^5′ = 1.0 Hz, ³J_PtH^3′ = 34.2 Hz, CH group adjacent to coordinated C atom, 1H^3′_ppy), 8.09 (m, ³J_H⁶H⁵ = 6.0 Hz, ⁴J_H⁶H⁴ = 1.0 Hz, ³J_PtH⁶ = 20.4 Hz, CH group adjacent to coordinated N atom, 1H⁶_ppy); 7.84 (d, ³J_H⁵H⁶ = 7.8 Hz, br Pt sat, CH group adjacent to coordinated N atom, 1H⁶_N⁁S).

2.2. Monitoring reaction of 1a with C₅H₅SN by ¹H NMR spectroscopy

To a small sample (10 mg, 0.023 mmol) of 1a dissolved in C₆D₆ (0.75 mL) in a sealed NMR tube, 1 equiv. pyridine-2-thione (2.6 mg, 0.023 mmol) was added. NMR spectra of the solution at 27 °C were recorded several times showing gradual conversion to a mixture containing the complexes 2a and 3 and free CH₄.

2.3. Reaction of [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, with C₅H₅SN

To a solution of [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, (100 mg, 0.19 mmol) in dichloromethane (15 mL) was added 1 equiv. pyridine-2-thione (22.1 mg, 0.19 mmol) at room temperature. The mixture was stirred for 5 h, the solvent was then removed under reduced pressure and the residue was triturated with n-hexane to give a pale red solid, which was separated and dried under vacuum. The product was identified as a mixture of [Pt(p-MeC₆H₄)(ppy)(η¹-S-S⁁NH)], 2b, and [Pt(ppy)(N⁁S)]₂, 3, with the ratio 2b/3 being 1/1. Total yield: 68 mg, 65%. Due to low solubility, the compounds formed in this reaction could not be separated and therefore elemental analysis was not useful. ¹H NMR data in CDCl₃, 2b: δ 2.29 (s, Me of p-tolyl, 3H), aromatic protons: p-tolyl ligand, 6.88 (d, ³J_H^mH^o = 7.6 Hz, 2H^m), 7.17 (d, ³J_H^oH^m = 7.6 Hz, 2H^o); ppy ligand, 6.41 (d, ³J_H^3′H^4′ = 7.5 Hz, ³J_PtH^3′ = 24.6 Hz, CH group adjacent to coordinated C atom, 1H^3′_ppy), 8.60 (d, ³J_H⁶H⁵ = 6.8 Hz, ³J_PtH⁶ = 16.1, CH group adjacent to coordinated N atom, 1H⁶_ppy); η¹-S-S⁁NH ligand, 9.07 (d, ³J_H³H⁴ = 5.6 Hz, 1H³). 3 : 8.07 (d, ³J_PtH⁶ = 19.2 Hz, ³J_H⁶H⁵ = 5.6 Hz, CH group adjacent to coordinated N atom, 1H⁶_ppy), 7.49 (d, ³J_PtH^3′ = 32.0 Hz, ³J_H^3′H^4′ = 5.8 Hz, CH group adjacent to coordinated C atom 1H^3′_ppy), 7.89 (d, ³J_PtH⁶ = 16.0 Hz, ³J_H³H⁴ = 7.5 Hz, H⁶_N⁁S), 6.50–7.60 (other aromatic protons of ppy and S⁁N groups).

2.4. Kinetic study

In a typical experiment, a solution of complex 1a in dichloromethane (3 mL, 3 × 10⁻⁴ M) in a cuvette was thermostated at 25 °C and a 0.01 molar solution (90 µL) of pyridine-2-thione was added using a microsyringe. After rapid stirring, the absorbance at λ = 500 nm was monitored.

2.5. Theoretical methods

Geometry optimizations were performed with the program suite Gaussian03 at the DFT/B3LYP level.⁴⁶ The effective core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ)^47,48 was chosen to describe Pt and the 6-31G* basis set was used for other atoms. To evaluate and ensure the optimized structures of the molecules, frequency calculations were carried out using analytical second derivatives.

3. Results and discussion

3.1. Synthesis and characterization of the complexes

As shown in Scheme 2, reaction of the Pt(II) starting complexes [PtR(ppy)(SMe₂)], 1, in which R = Me (1a) or p-MeC₆H₄ (1b) and ppy = deprotonated 2-phenylpyridine, with pyridine-2-thione (C₅H₅SN) in a 1 : 1 molar ratio, followed by the replacement of the labile ligand SMe₂ to give a mixture containing the S-bound thione Pt(II) complex [PtR(ppy)(η¹-S-S⁁NH)], 2, in which R = Me (2a) or p-MeC₆H₄ (2b), the dimeric complex [Pt(ppy)(N⁁S)]₂, 3, with two bridging deprotonated pyridine-2-thione (N⁁S) ligands, and R–H, i.e. CH₄ (in case where R = Me) or MeC₆H₅ (in case where R = p-MeC₆H₄). When reaction of 1 with C₅H₅SN is allowed to stir for 3 days in benzene, only 3 along with free R–H are formed.

Scheme 2 Reactions studied in this work.

The products were characterized by ¹H NMR and X-ray crystallography (for the complex 3) and full data are collected in the Experimental Section. In the ¹H NMR spectrum of complex [PtMe(ppy)(η¹-S-S⁁NH)], 2a, in C₆D₆ at room temperature, the methylplatinum resonance occurred at δ = 2.36 as a singlet which coupled to ¹⁹⁵Pt with ²J_PtH = 69.9 Hz (the value in the range expected for a methylplatinum(II) complex with methyl being trans to nitrogen^49–52). The CH group adjacent to ligating C atom of the ppy ligand, i.e. H^3′, appeared as a doublet of doublets at δ = 7.79 with ³J_H^3′H^4′ = 7.4 Hz and ⁴J_H^3′H^5′ = 1.1 Hz, and with ³J_PtH^3′ = 24.5 Hz, while the hydrogen related to the CH group adjacent to ligating N atom of the ppy ligand, i.e. H⁶, as expected was appeared further down field as a doublet of doublets at δ = 8.76 with ³J_H⁶H⁵ = 5.7 Hz and ⁴J_H⁶H⁴ = 1.8 Hz, and with ³J_PtH⁶ = 16.2 Hz. The H³ proton of the η¹-S-S⁁NH ligand appeared as a doublet at δ = 7.92 with ³J_H³H⁴ = 5.5 Hz, with no observable coupling to Pt center probably due to the related pyridine plane being perpendicular to square geometry of the complex.¹⁸ Notice also that formation of CH₄ is confirmed by the observation of a singlet at 0.16. The complex [Pt(ppy)(N⁁S)]₂, 3, is also characterized with signals due to H^3′_ppy, H⁶_N⁁S and H⁶_ppy at 7.49 (³J_PtH^3′ = 34.2 Hz), 7.84 (br Pt sat) and 8.09 (³J_PtH⁶ = 20.4 Hz) ppm, respectively.

In the ¹H NMR spectrum of complex [Pt(p-MeC₆H₄)(ppy)(η¹-S-S⁁NH)], 2b, in CDCl₃ at room temperature, a singlet signal was observed at δ = 2.29 for the methyl group on the para-tolyl ligand. The meta and ortho protons of the para-tolyl ligand appeared as two doublets at δ = 6.88 and 7.17, respectively, each with ³J_H^mH^o = 7.6 Hz. A doublet signal at δ = 6.41 with ³J_H^3′H^4′ = 7.5 Hz, accompanied by platinum satellites (³J_PtH^3′ = 24.6 Hz), is attributed to hydrogen atom of C–H group adjacent to the coordinated C atom of ppy ligand, i.e. H^3′. A doublet signal at δ = 8.60 (with ³J_H⁶H⁵ = 6.8 Hz and ³J_PtH⁶ = 16.1 Hz) is assigned to CH group adjacent to coordinated N atom of ppy ligand, i.e. H⁶. The H³ proton of the η¹-S-S⁁NH group appeared as a doublet at δ = 9.07 with ³J_H³H⁴ = 5.6 Hz. The dimer complex, i.e. [Pt(ppy)(N⁁S)]₂, 3, with similar ¹H NMR data as mentioned above (see also Experimental section), was also detected. Besides, formation of R–H (i.e. MeC₆H₅) is confirmed by the observation of a singlet at 2.31.

The dimeric complex [Pt(ppy)(N⁁S)]₂, 3, was also prepared by direct route by reaction of the complex [Pt(ppy)Cl(DMSO)]⁴² with an ethanolic solution of sodium pyridine-2-thiolate (NaC₅H₄NS) under inert atmosphere condition and a purified sample of it (as checked by microanalysis) was characterized by ¹H NMR spectroscopy. In the ¹H NMR spectrum of complex 3 in CDCl₃, the CH group adjacent to coordinated N atom of ppy appeared at δ = 8.09 as a doublet (³J_H⁶H⁵ = 5.6 Hz) which coupled to ¹⁹⁵Pt with ³J_PtH⁶ = 19.1 Hz. The resonance of CH group adjacent to coordinated C atom of ppy is occurred at δ = 7.47 as a doublet with Pt satellite (³J_PtH^3′ = 32.8 Hz, ³J_H^3′H^4′ = 5.7 Hz) and the CH group adjacent to coordinated N atom of N⁁S chelate was observed at δ = 7.86 as a doublet signal (³J_H³H⁴ = 7.5 Hz, ³J_PtH⁶ = 16.2 Hz), which confirmed the coordination of N⁁S as chelate. The complex 3 was also characterized by single-crystal X-ray diffraction analysis as illustrated in Fig. 3.⁴⁵

3.2. Kinetics and mechanism of the reactions

On the basis of the data obtained from UV-vis and ¹H NMR spectroscopic studies (see below), a mechanism (being depicted in Scheme 3) for reaction of the complex [PtR(ppy)(SMe₂)], 1, with 1 equiv. pyridine-2-thione (C₅H₅SN) is proposed. The reaction is suggested to involve initial displacement of the SMe₂ ligand by C₅H₅SN, during which S donor site of the thione tautomer (HN⁁S) attacks Pt center of the complex 1 to displace the SMe₂ ligand, giving either the product 2a (or 2b) or its isomeric form, 2a′ (or 2b′), in which the py ring has twisted by 90° as compared to that in 2a (or 2b); in the latter case, N–H group has the opportunity to interact with the Pt center to form Pt⋯H–N hydrogen bonding as confirmed by ¹H NMR studies (vide infra). The complex 2a′ (or 2b′) then performs an intramolecular N–H oxidative addition to form the intermediate I (or I′) following by CH₃–H (or MeC₆H₅) reductive elimination to give the complex 3.

Scheme 3 Suggested mechanism for reaction of the complex [PtR(ppy)(SMe₂)], 1, with pyridine-2-thione.

3.2.1. Kinetic studies by UV-vis spectroscopy. Reaction kinetics, using UV-vis spectroscopy, of equimolar amounts of the complex [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, and pyridine-2-thione in CH₂Cl₂ solution to form a mixture containing the complexes [Pt(p-MeC₆H₄)(ppy)(η¹-S-S⁁NH)], 2b, and [Pt(ppy)(N⁁S)]₂, 3, along with the free MeC₆H₅, is typically presented here. A series of spectra recorded during the reaction is shown in Fig. 4. On mixing, the characteristic peaks of complex 1b (MLCT band at 365 nm) and pyridine-2-thione (n–π* at 375 nm)^53,54 disappeared and a new broad absorption band due the intermediate 2b′ appeared (see Scheme 3). The decay of 2b′ to finally form 3 could be studied by monitoring formation of absorption band of the final product 3 at 500 nm. Typical plots of absorbance at λ = 500 nm versus time are shown in Fig. 5.

Fig. 4 The changes in the UV-vis spectrum during the reaction of [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, with pyridine-2-thione (each 3 × 10⁻⁴ M) in CH₂Cl₂ at 25 °C: (a) pure 1b; (b) pure pyridine-2-thione; (c) spectrum at t = 0; successive spectra recorded at intervals of 1 min.

Fig. 5 Absorbance (at 500 nm)–time curves for the reaction of [Pt(p-MeC₆H₄)(ppy)(SMe₂)], 1b, with pyridine-2-thione, using 1 : 1 stoichiometry, in CH₂Cl₂ at temperatures of 15, 20, 25, 30 and 35 °C (temperature increases reading upward).

The abs-time data at λ = 500 (i.e. formation of the product 3) is fitted to the equation A_t = A + (A₀ − A)exp(−kt), and the calculated rate constants at different temperatures are given in Table 1. The activation parameters were also obtained from measurement at different temperatures using Eyring equation (Fig. 6). Rates of the reactions are sensitive to the nature of R ligand and it is lower when the tolyl complex 1b is involved as compared to the case where the methyl complex 1a is used. The intermediate 2b′ then performs an intramolecular N–H oxidative addition reaction to form intermediate I′ (see Scheme 3) following by C–H reductive elimination to form toluene and the dimer complex 3.

Table 1 Rate constants^a and activation parameters for R–H reductive elimination from the complexes 2a′ or 2b′ to give dimer 3 in CH₂Cl₂ solution

R	10^2k s^−1 at different temperatures							ΔH^‡/kJ mol^−1	ΔS^‡/J K^−1 mol^−1
	5 °C	10 °C	15 °C	20 °C	25 °C	30 °C	35 °C
a Estimated errors in k values are ±5%. b The value in benzene 10^2k = 0.32 s^−1.
Me	0.16	0.28		0.53	1.05^b	1.14	1.16	53.4 ± 3.9	−105 ± 13
p-MeC6H4			0.012	0.019	0.027	0.039	0.052	50.8 ± 1.9	−142 ± 6

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Fig. 6 Eyring plots for reductive elimination of R–H and formation of dimer 3 from (a) complex 2a′ (R = Me) and (b) 2b′ (R = p-MeC₆H₄) in CH₂Cl₂.

As is shown in Table 1, ΔH^‡ value for methane reductive elimination from the intermediate I is 53.4 kJ mol⁻¹ being considerably lower than the values of 107.4 and 70 kJ mol⁻¹ reported for CH₄ reductive elimination from Pt(IV) complexes [PtMe₃(H)(dppe)] (dppe = 1,2-bis(diphenylphosphino)ethane)⁵⁵ and [PtMe(H)(Cl)₂(PMe₃)₂],⁵⁶ respectively. This lower energy barrier found for [PtMe(H)(ppy)(S⁁N)] may be related to higher ring strain, presented in four-membered S⁁N ring in Pt(IV) intermediate I.

3.2.2. Monitoring the reactions by ¹H NMR spectroscopy. As can be seen in Fig. 7, immediately after addition of pyridine-2-thione to the complex [PtMe(ppy)(SMe₂)], 1a, at 27 °C in C₆D₆, the complexes [PtMe(ppy)(η¹-S-S⁁NH)], 2a and 2a′, were started to appear. The rate of disappearance of complex 1a using the ¹H NMR signal at δ = 1.55 (due to Me group connected directly to Pt in complex 1a) was found to be 1.90 (0.10) × 10³ s⁻¹. According to Fig. 7, for the complex 2a′, a singlet signals for Pt–Me protons was observed at δ 2.02 (with ²J_PtH = 68.2 Hz) with the liberated SMe₂ appearing at δ 1.72, in comparison with the coordinated SMe₂ ligand of 1a (at δ 1.85 with ³J_PtH = 25.1 Hz). Besides, the presence of a doublet signal, appearing at δ = 10.52 with accompanying ¹⁹⁵Pt satellite signals of ³J_PtH = 23.3 Hz (assigned for the H–N of the pyridine-2-thione ring) confirms that the pyridine-2-thione ligand is connected to the Pt center via S atom (which is normally preferred to N atom⁴¹). The observation of this broad signal at a significantly lower field, in comparison with that for corresponding hydrogen of the free ligand (in thione form, Ia in Fig. 1, appearing at δ = 7.64 (ref. 39)), and with a significant ³J_PtH value of 23.3 Hz complies with the formation of Pt⋯H–N hydrogen bonding; this coupling value is smaller than those usually observed in similar hydrogen bondings which could be attributed to the η¹-S-S⁁NH ligand being rather rigid and thus there is some force on the corresponding hydrogen to locate itself above the platinum coordination plane in order to interact with its 5d_z² orbital.⁵⁷

Fig. 7 ¹H NMR spectra (Me region) of reaction of the complex 1a with pyridine-2-thione at 27 °C in C₆D₆; (a) pure 1a, (b) immediately after addition of pyridine-2-thione to 1a, (c) 5 min after addition, (d) 15 min after addition, (e) 1 h after addition, (f) 3 h after addition, (g) 5 h after addition. Signals with satellites are assigned to the complexes 1a, 2a′ and 2a. The peak labeled * is due to water of C₆D₆ solvent.

The ratio of 2a′ to 1a, immediately after addition of pyridine-2-thione to 1a, is close to 1 : 2. As the time was passing on, the signals due to the starting material 1a was disappearing while those due to the complex 2a′ were growing and later started to fade away and meanwhile comparatively weak signals due to the complexes 2a and 3 (being out of the range of Fig. 7) were also observed in the first stages. As the reaction was progressed, the complex 2a′ was disappearing while the final complexes 2a and 3 were forming. As mentioned before, after 3 days the dimer 3 was solely obtained in pure form.

Appearance of the corresponding signal for 2a at δ = 2.36 was used to measure rate of formation of the complex 2a, which was found to be 1.51(0.05) × 10⁻⁴ s⁻¹. The rate of formation of CH₄ (at δ = 0.16) which is equal to disappearance of the intermediate I and formation of the dimer 3 is measured to be 3.0 × 10⁻³ s⁻¹, very close to value of 3.2 × 10⁻³ s⁻¹ obtained from UV-vis study (vide supra).

We suggest that the formation of the complex 3 proceeds through an intramolecular N–H oxidative addition, involving the η¹-S-S⁁NH ligand in 2a′, to the platinum center^58,59 to form the platinum(IV) hydride intermediate [PtMe(H)(ppy)(S⁁N)], I, followed by reductive elimination of methane to form the complex 3. Our attempts to observe the intermediate I (by following the reaction at low temperatures) were not successful. However, evidence in favor of formation of this hypothesized Pt(IV) complex is the methane production, appearing as a singlet signal at δ = 0.16.^60–62 Formation of the dimeric complex [Pt(ppy)(N⁁S)]₂, 3, was confirmed by the observation of a doublet signal at δ = 7.84 with ³J_H⁶H⁵ = 7.6 Hz, due to H⁶ of the N⁁S bridge, that experiences coupling with Pt with the satellites being broad and so the related coupling value was not measurable. Formation of the dimer 3 was also confirmed by X-ray crystal determination of suitable crystal of 3 obtained from crystallization of the mixture. Formation of the product complex [PtMe(ppy)(η¹-S-S⁁NH)], 2a, in which pyridine-2-thione adopts the thione form,^63–66 suggests a pathway including direct substitution of labile SMe₂ by sulfur of thiolate ligand. Same experiment for the reaction of 1a with pyridine-2-thione was also carried out in CD₂Cl₂ as NMR solvent and found that the reaction in dichloromethane is faster than in benzene (in agreement with data obtained from the UV-vis spectroscopy, see Table 1).

3.3. Geometry optimizations

To get some insights in structures of the synthesized complexes 2 and 3, calculations in the DFT method were carried out and their geometries were optimized using the B3LYP function. Optimized geometric parameters of the complexes 2 and 3 are given in Table 2 and the DFT-optimized structures for these complexes are shown in Fig. 8. The computed structural details are in good agreement with corresponding experimental parameters obtained for the related complexes. Mean error for the bond lengths is 0.026 Å for the complex 3. One reason for the small bond length discrepancy may be due to differences in the geometrical parameters calculated in the gas phase with those obtained experimentally in the solid state.

Table 2 Selected calculated bond distances (Å) and angles (°) for complexes 2 and 3

2a		2b		3		3 (exp)
Pt–S(S⁁NH)	2.514	Pt–S(S⁁NH)	2.521	Pt2–N3(N⁁S)	2.233	2.144(10)
Pt–C(Me)	2.064	Pt–C(Ar)	2.024	Pt2–S2(N⁁S)	2.370	2.286(3)
Pt–N(ppy)	2.193	Pt–N(ppy)	2.190	Pt2–C22(ppy)	2.009	1.977(11)
Pt–C(ppy)	2.012	Pt–C(ppy)	2.165	Pt2–N2(ppy)	2.093	2.064(9)
C(Me)–Pt–S(S⁁NH)	91.4	C(Ar)–Pt–S(S⁁NH)	173.4	Pt1–Pt2	3.009	2.8513(11)
S(S⁁NH)–Pt–N(ppy)	95.0	S(S⁁NH)–Pt–N(ppy)	94.9	C22(ppy)–Pt2–S2(N⁁S)	95.0	95.3(3)
N(ppy)–Pt–C(ppy)	79.7	N(ppy)–Pt–C(ppy)	79.6	S2(N⁁S)–Pt2–N3(N⁁S)	89.1	90.5(2)
C(ppy)–Pt–C(Me)	93.9	C(ppy)–Pt–C(Ar)	95.4	N2(ppy)–Pt2–N3(N⁁S)	94.9	93.4(4)

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Fig. 8 DFT optimized structures of complexes 2 and 3. The H atoms are omitted for clarity.

The complex 2a contains a square planar platinum(II) center being connected to the Me group, ortho C and N atoms of the ppy ligand and S atom of the S⁁NH. The two carbon ligating atoms are mutually cis in the square-planar structure, as expected, and largest deviation from ideal geometry is the angle N(ppy)-Pt-C(ppy) = 79.7, which is associated with the Pt(ppy) chelate ring. N atom of the S⁁NH ligand is not coordinated and is positioned opposite to the platinum center. Similar to 2a, the complex 2b has also a square-planar stereochemistry at platinum and the p-tolyl ligand lies roughly orthogonal to the square plane and to the ppy ligand. The Pt–S (S⁁NH) distances are 2.514 and 2.521 Å in 2a and 2b, respectively. The dimeric complex 3 has a square planar stereochemistry at each Pt center and has a head-to-tail configuration with PtSNPt coordination, in which S atom of one of the SN ligands is in a cis arrangement with N atom of the other SN ligand (Fig. 8); the coordination geometry around each platinum center is completed by nitrogen and ortho C atoms of the ppy ligand. This complex with a head-to-tail arrangement of the two SN ligands has been formulated as containing a Pt–Pt distance of 3.009 Å which is close to the experimental value of 2.8513(11) Å. This separation is amongst the shortest Pt–Pt interactions observed in binuclear platinum complexes bridged with two ligands.⁴⁵ The calculated bond distance Pt–N(N⁁S) = 2.233 Å is slightly longer than Pt–N(ppy) = 2.093 Å due to a higher trans influence of the coordinating C atom of ppy as compared to coordinating S donor atom of N⁁S. These values are comparable with experimental values of 2.144(10) and 2.064(9) Å for Pt2–N3(N⁁S) and Pt2–N2(ppy), respectively. The two nitrogen atoms are cis disposed to one another (N(ppy)PtN(N⁁S) = 94.9°), indicating that angles around the Pt center are rather close to the ideal angle of 90°. The experimental value of this bond angle is equal to 93.4(4)°.

3.4. Energy profile for the product formation

To gain more insights into the species involved in the reaction of complexes 1 with C₅H₅SN (see Scheme 3), DFT calculations were carried out to show structures and energies for the related complexes in CH₂Cl₂ solution (Fig. 9 and 10).

Fig. 9 Calculated structures and relative energies of species involved in reaction of the complex 1a with pyridine-2-thione.

Fig. 10 Calculated structures and relative energies of species involved in reaction of the complex 1b with pyridine-2-thione.

As shown in Fig. 9, replacement of labile ligand SMe₂ in the cycloplatinated(II) complex 1a by pyridine-2-thione (C₅H₅SN) first gives the complex [PtMe(ppy)(η¹-S-S⁁NH)], 2a, having a NH pendant group, with 33.1 kJ mol⁻¹ lower in energy with respect to that for 1a. This step is facilitated by strong trans–influence of the metalated C atom of ppy ligand which helps in weakening the Pt–SMe₂ bond. The complex 2a is obtained in equilibrium with its isomeric form, 2a′, in which the pyridine ring has twisted by 90° as compared to that in 2a to form Pt⋯H–N hydrogen bonding (as confirmed by ¹H NMR studies). The complex 2a′ is more stable than 2a by 14.8 kJ mol⁻¹. In the next step, the complex 2a′ attends in an intramolecular N–H oxidative addition reaction to give the organoplatinum(IV) hydride complex [PtHMe(ppy)(N⁁S)], I, followed by reductive elimination of methane to give the dimer 3. The final product 3 is 139.0 kJ mol⁻¹ more stable than 1a complying with the experimental finding.

In the related reaction profile (see Fig. 10), the complex 1b reacts with pyridine-2-thione to form the S-bonded complexes 2b and 2b′, which are more stable than 1b by 29.9 and 43.3 kJ mol⁻¹, respectively. Coordination of pyridine-2-thione entering ligand through the N atom is not proposed, because the corresponding isomer [Pt(p-MeC₆H₄)(ppy)(η¹-N-N⁁SH)] was calculated to be 42.4 kJ mol⁻¹ less stable than 2b′. Oxidative addition of N–H bond to Pt center of the complex [Pt(p-MeC₆H₄)(ppy)(η¹-S-S⁁NH)], 2b′, gives the Pt(IV) intermediate complex [PtH(p-MeC₆H₄)(ppy)(N⁁S)], I′, with 12.9 kJ mol⁻¹ higher in energy than 1b. At the final step, reductive elimination of toluene gives dimer 3 which is more stable than 1b by 126.2 kJ mol⁻¹.

4. Conclusion

Reaction of the cycloplatinated(II) complexes [PtR(ppy)(SMe₂)], 1, with pyridine-2-thione is suggested to proceed via displacement of the labile SMe₂ ligand. In this reaction pyridine-2-thione, which exists as a mixture of tautomers thiol (N⁁SH) and thione (HN⁁S), attacks Pt center of the complex 1 from S side of the thione form to give the four coordinated square-planar complex 2a (or 2b) and its isomer 2a′ (or 2b′), with a general formula [PtR(ppy)(η¹-S-S⁁NH)]. DFT calculations show that this behavior is much preferred to the case of attacking from N side of the thiol form. The following points are considered for the reaction:

(1) Suggestion in the first step is consistent with the “soft” S atom (from the thione tautomer, HN⁁S) being preferred to be connected to “soft” Pt(II) center as compared to that for the “hard” nature of N atom (from the thiol tautomer, N⁁SH).

(2) Formation of intramolecular Pt⋯H–N hydrogen bonding in one of the product isomers of the first step, i.e. the complex [PtR(ppy)(η¹-S-S⁁NH)], 2a′, (as detected by monitoring the reaction using ¹H NMR spectroscopy) is suggested to provide an extra stabilization as compared with the product isomer 2a.

(3) As was determined by UV-vis spectroscopy (see Table 1), R–H reductive elimination from the complex 2a′ or 2b′ to form the dimer complex 3 is significantly faster when R = Me (with k = 1.05 × 10⁻² s⁻¹ at 25 °C, in CH₂Cl₂) as compared to that when R = p-MeC₆H₄ (with k = 0.027 × 10⁻² s⁻¹ at 25 °C, in CH₂Cl₂). We attributed this to the easier reductive elimination of CH₄ as compare with toluene.

(4) The rate of reaction of [PtMe(ppy)(SMe₂)], 1a, with pyridine-2-thione is significantly faster in CH₂Cl₂ (k = 1.05 × 10⁻² s⁻¹ at 25 °C) than in benzene (k = 0.32 × 10⁻² s⁻¹ at 25 °C). We attribute this to the possibility that CH₂Cl₂ solvent molecules make the species more stable, by solvation, than when solvent molecules are the less polar benzene.

We finally observed the formation of two complexes, the S bounded complex [PtR(ppy)(η¹-S-S⁁NH)], 2, and the dimeric complex [Pt(ppy)(N⁁S)]₂, 3, along with the evolution of free R–H. These observations led us to believe that an intramolecular N–H oxidative addition reaction must have been taken place in the complex [PtR(ppy)(η¹-S-S⁁NH)], 2a′ (or 2b′), to give the Pt(IV) intermediate complex [PtHR(ppy)(N⁁S)], I (or I′), in which the reductive elimination of R–H bond is needed to produce complex 3.

Acknowledgements

We thank the Iran National Science Foundation (Grant no. 92028194 and 93026027), the Shiraz University Research Council and Institute for Advanced Studies in Basic Sciences for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01341k

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