Carbon–sulfur bond reductive coupling from a platinum(II) thiolate complex

Abstract

The room temperature addition of electrophilic alkyl halide reagents (RX = MeI, EtI and PhCH₂Br) to complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 1a, in which ppyH = 2-phenylpyridine and pySH = pyridine-2-thiol, resulted in a rapid carbon–sulfur (C–S) bond reductive coupling to produce alkyl sulfides and corresponding halide complexes [Pt(ppy)(PPh₃)X], X = I (2a) and Br (2b). A mechanism for this C–S bond formation reaction was suggested based on ¹H and ³¹P {¹H} NMR spectroscopic analyses. In the suggested mechanism, the reaction proceeded through a binuclear intermediate complex [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]I, 3-I, which was separately synthesized by another counter anion (PF₆) and it was fully characterized by multinuclear NMR spectroscopy and single X-ray crystallography. Also, density functional theory (DFT) calculations were used to theoretically assess the structures of intermediates and transition states in this bond formation reaction.

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Introduction

Several prominent catalytic or stoichiometric systems were developed for C–H,^1,2 C–C^2–8 and C–X^8–17 (X = different heteroatoms) bond formation reactions. Despite these extensive research efforts, C–S bond formation still has significant limitations identical to poisoning and/or deactivation effect of sulfur on reactive species or harsh reaction conditions.^14–16 Such difficulties were addressed in some cases and numerous outstanding systems were established for C–S bond coupling¹⁶ using transition metals such as Ni,¹⁸ Cu,¹⁷ Pd^8,16 and Pt.¹⁹ In addition, the growing interest in C–S bond construction chemistry is mostly due to its potential applications in organic synthesis,²⁰ materials science,²¹ agricultural,¹⁵ and pharmaceutical industry.²²

There are only a few reports on the formation of C–S bond using platinum transition metal complexes.¹⁹ This fact has led us to apply complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 1a, ppyH = 2-phenylpyridine and pySH = pyridine-2-thiol, in C–S bond reductive coupling with electrophilic reagents such as alkyl halides at room temperature. In addition, mechanistic aspect of this bond formation reaction was studied with multinuclear NMR spectroscopy by observing an interesting binuclear complex. Density functional theory (DFT) was employed to optimize structures of the related complexes, which were observed experimentally or proposed theoretically.

Result and discussion

Complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 1a, was prepared according to our recently published procedure (Scheme 1 (path I)),²³ involving treatment of complex [Pt(ppy)(PPh₃)Cl], A,²⁴ with an ethanolic solution of sodium pyridine-2-thiolate (NaC₅H₄NS) under Ar atmosphere.

Scheme 1 (I) The synthetic route for the formation of complexes 1; (II) identified products in the reaction of different alkyl halides with complexes 1; (III) direct route for the formation of complexes 2 by halide substitution. All reactions were carried out under inert atmosphere at room temperature.

The reaction of complex 1a with stoichiometric ratio or an excess of MeI at room temperature was investigated. This reaction was extremely rapid and yielded unexpected products. As represented in Scheme 1 (path II), the reaction underwent fast C–S bond reductive coupling to cleanly produce an organic compound 2-(methylthio)pyridine (pySMe), O1,²⁵ and complex [Pt(ppy)(PPh₃)I], 2a.

Complex 2a was identified from the analysis of NMR spectroscopy data and X-ray single crystal diffraction data (see structural determination). As expected, the ³¹P {¹H} NMR spectrum of complex 2a (in CDCl₃) displayed only a singlet along with platinum satellites at δ = 22.7 with a significant value of ¹J_PtP = 4290 Hz. This large coupling constant confirmed that the phosphine ligand is positioned trans to nitrogen atom, same as complex A.²⁴ In the ¹H NMR spectrum of complex 2a, the existence of a doublet of doublet of doublets for H² of ppy ligand at δ = 10.55 ppm with platinum satellites (³J_PtH = 30.9 Hz) and also another doublet of doublet of doublets at δ = 6.67 ppm accompanied by platinum satellites (³J_PtH = 52.1 Hz) for H⁹ of ppy ligand, confirmed that cyclometalating ligand was chelated in this complex (refer to the Experimental section for signal assignments). The later resonance exhibited a C–H⋯π interaction, as detected in the single X-ray crystal structure of complex 2a. In addition, complex 2a could be achieved directly by a nucleophilic substitution reaction of chloride ligand in complex A by iodide (I⁻) ion (Scheme 1 (path III)).

Other electrophilic substrates have been used to investigate the effect of these kind of reagents on the above reaction (Scheme 1 (path II)). The obtained products were similar to the resultant products from methyl iodide species. For example, addition of ethyl iodide (EtI) or benzyl bromide (PhCH₂Br) to complex 1a resulted in products 2-(ethylthio)pyridine (pySEt), O2,²⁵ and complex 2a or 2-(benzylthio)pyridine O3,²⁵ and complex [Pt(ppy)(PPh₃)Br], 2b, respectively (Scheme 1 (path II)). Complex 2b was fully characterized by NMR spectroscopy and single X-ray crystallography studies (see structural determination, Experimental section). Furthermore, complex 2b was obtained through salt metathesis reaction of complex A with KBr (Scheme 1 (path III)).

Mechanism of C–S coupling from complexes 1

In an effort to achieve more insight into the mechanism of reaction (carbon–sulfur bond formation) that was described in Scheme 1 (path II), it was monitored at different temperatures with NMR spectroscopy. The reactants were mixed and firstly NMR spectra were recorded at 25 °C. In ³¹P {¹H} NMR spectra (Fig. 1), immediately after addition of MeI, the signals of the starting complex 1a (δ = 22.7 ppm) quickly disappeared (Fig. 1c) and a signal corresponding to complex 2a (δ = 22.4 ppm) grew rapidly as time elapsed. Apart from resonances of complex 1a (precursor) and complex 2a (product), an interesting Pt(II)–Pt(II) binuclear intermediate species was detected, suggesting the presence of two different phosphorus ligands with high platinum coupling constants (Fig. 1). Its structure was determined as a binuclear complex [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]I, 3-I. Therefore, two singlet signals, one at δ = 21.5 ppm (¹J_PtP = 4407 Hz, P^a) and another one at δ = 21.0 ppm (¹J_PtP = 4245 Hz, P^b) were assigned to complex 3-I and phosphine ligands in this complex perhaps were located in cis position to the nitrogen and sulfur atoms of bridging thiolate ligand, respectively. This assignment is related to high cis influence of S atom over N atom.^26,27 As time passed, the signal intensities of complex 3-I decreased while the signal intensities of complex 2a increased until full conversion (ca. 6 h) was obtained as compared to the pure form of complex 2a.

Fig. 1 Reaction of complex 1a with 3 eq. of MeI as monitored by ³¹P {¹H} NMR spectroscopy in CD₂Cl₂ at room temperature. (a) Pure complex 1a, (b) immediately after addition of MeI, (c) 5 min (d) 15 min, (e) 30 min, (f) 60 min, (g) 120 min, (h) 180 min, (i) 240 min and (j) 480 min. The signal assignments are shown.

The related ¹H NMR spectra (Fig. S1†) for this reaction were also obtained simultaneously at room temperature. This observation confirmed the formation of complex 2a, and especially compound O1, as products and complex 3-I as an intermediate by observing their expected signals. As shown in Fig. S1,† new resonances with increased signals at δ = 2.58 ppm and 8.46 ppm were assigned to methyl and H^6′ (CH group adjacent to nitrogen atom) of compound O1,²⁵ respectively. When complex 1 was treated with CD₃I under the same conditions (CD₂Cl₂, room temperature), the methyl signal disappeared in the ¹H NMR spectrum (Fig. S2†).

To detect other plausible intermediates, the reaction was monitored at low temperature (223 K). In this temperature the rate of reaction became very slow and an excess of MeI was used. However, no further intermediates were observed (Fig. S3†) and similar room temperature NMR spectra were obtained. This spectroscopic observation suggested that other possible intermediates were very short lived and they were quickly transformed to the products.

Attempts to prepare resemble of complex 3-I were successful. The synthetic strategy involved treatment of complex A with AgPF₆ to abstract the chloride (Cl⁻) ligand from the platinum center as AgCl (separated from solution by filtration). Subsequently, complex 1a was added to the resulting solution and the binuclear complex [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]PF₆, 3-PF₆, was formed as an orange solid. The multinuclear NMR spectroscopy and crystal structure determination confirmed the formation of this complex. The ³¹P {¹H} NMR spectrum (Fig. 2) of complex 3-PF₆ exhibited two singlet resonance flanked by platinum satellites at similar chemical shift (with equal coupling ¹J_PtP constant) of complex 3-I. It is notable that the counteranion (I⁻ or PF₆⁻) did not have impact effect on the cationic part of complex 3 in solution and only PF₆⁻ facilitated crystallization process. As expected, the ¹⁹⁵Pt {¹H} spectrum (Fig. 2) of complex displayed two doublets at −4081 (Pt^a) and −4296 (Pt^b) ppm with ¹J_PtP values of 4413 and 4256 Hz, respectively, close to the values obtained from the ³¹P {¹H} NMR spectrum.

Fig. 2 (a) ³¹P and (b) ³¹Pt NMR spectra of complex 3-PF₆ in CD₂Cl₂ at room temperature.

The synthesized complex 3-I was stable in solution for several days and it did not display any decomposition products or rearrangements to complexes 1a and 2a. Also, complex 3-I was treated with MeI and it easily was reacted and lead to complex 2a and compound O1. When complex 3-PF6 was treated with MeI it did not show any reaction after three hours. According to this data, the binuclear complex 3, perhaps, after the reaction with MeI and subsequent C–S bond coupling needs two coordinating anion like complex 3-I (one from counterion and another from MeI). But complex 3-PF₆ has a non-coordination counterion (PF₆⁻), probably, this point stops desire bond formation.

Initial coordination site of alkyl halide species in complex 1a is important, because complex 1a contains three potential sites (nitrogen, sulfur of Spy and platinum center) for initial coordination of alkyl halide reagents. Therefore, some tests were conducted to clarify the nucleophilicity of each coordination site in complex 1a against these substrates (see below). The results from these experiments were helpful in proposing a mechanistic pathway for the observed C–S bond formation reaction in this study.

The free nitrogen of thiolate ligand in complex 1a with alkyl halide reagents could potentially undergo Menschutkin reaction²⁸ to produce N-alkylated complexes (Scheme S1,† nitrogen path), although those were not observed in NMR spectroscopy studies (Fig. S1†). The possible N-methylated complex 1-N from the reaction of complex 1a with MeI could display a low field signal owing to the coordinated methyl on nitrogen.²⁶ Supposedly, cationic complex 1-N was not stable and could not be detected with spectroscopic methods and it was converted to complex 2a and compound O1-t (N-methyl-2-thiopyridone).²⁹ But, ¹H NMR spectra²⁹ did not show any signal for the formation of compound O1-t and only compound O1 was observed. It is notable that the conversion of compound O1-t to compound O1 (referenced to the thione-thiol tautomer)^29,30 needs harsher reaction conditions.²⁹ To solve this challenge, complex [Pt(ppy)(SPh)(PPh₃)], 1b,²³ in which PhSH = thiophenol, with no pendant nitrogen atom was selected for reaction with MeI.³¹ This reaction produced complex 2a and its corresponding C–S coupling product i.e. thioanisole O4 ³² (Scheme 1 (path II)). Therefore, this test suggested that the uncoordinated nitrogen atom does not have an important role in the formation of yielded compounds and perhaps it has some effects on the formation of complex 3-I from complex 1a. Consequently, when the test reaction was followed with NMR spectroscopy (Fig. S4 and S5†) it merely displayed transformation of complex 1b to complex 2a without any detectable intermediates in comparison with complex 1a (Fig. 1). All above observations suggested that the nitrogen site has weak nucleophilicity.³¹ Comparable type of systems, each having at least one free nitrogen atom on the coordinated ligand to platinum center remain intact in reaction with alkyl halide because other sites of these complexes are stronger nucleophiles than N atom.^31,33–35 Consequently, a different site in complexes 1a and 1b is involved in the initial coordination of alkyl halide reagents.

An alternative possibility for preliminary coordination of alkyl halides to complex 1a is sulfur atom which is known as S-alkylation.^36–40 It is related to the metal complexes containing thiolate ligands especially dithiolate group,^38–41 while these are treated with alkyl halides species (in thermal condition for most cases)^38,39,42 S-alkylation is a plausible route.^36,37,43 As shown in Scheme S1† (sulfur path), in the case of MeI, the presumed S-methylated complex 1-S ought to exhibit a signal with platinum satellite in relation to coordinated methyl to sulfur atom but this was not observed (Fig. S1†). Thus, another experiment was designed for unravelling this inconsistency. While complex 1a reacted with iodine or bromine reagents, it produced complexes 2a or 2b and its corresponding S–I or S–Br coupling products⁴⁴ (Scheme 1 (path II)). This reaction was followed by NMR spectroscopy and an intermediate (complex 3-I) similar to the reaction of methyl iodide was detected (Fig. 1). Therefore, sulfur atom is not incipient coordination site and S-alkylation can be ruled out.⁴³ Also, there are numerous reports on the oxidation of metal centers by alkyl substrates^45–48 than sulfur oxidation (thiolate alkylation). For example, sicilia research group reported that the reaction of half-lantern compound (containing thiolate ligand) with MeI leaded to the oxidized complex and S-alkylation did not occur.⁴⁸ Therefore, the proposed routes in Scheme S1† are not appropriate and platinum center is initial coordination site for alkyl halide substrates and oxidative addition path is adequate for suggested C–S coupling mechanism (Scheme 2).

Scheme 2 Proposed mechanism for the reaction of complex 1a with methyl iodide.

We also investigated the effect of phosphine ligand on the C–S bond formation by changing PPh₃ with PPhMe₂, and complex [Pt(ppy)(η¹-S-Spy)(PPhMe₂)], 1c, was prepared (Scheme 1 (path I)). When alkyl halide regent (MeI) was added to complex 1c in acetone-d₆ at room temperature and the reaction was followed by NMR spectroscopy, rapid and complete conversion to complex [Pt(ppy)(PPhMe₂)I], 2c, and O1 was observed (Scheme 1 (path II)). This reaction pathway is analogous to the reaction with complex 1a which contains PPh₃, but C–S bond reductive elimination occurs at a faster rate. The increased rate of reaction is consistent with the greater nucleophilicity of complex 1c than complex 1a.^33,49

Structural determination

Single-crystal X-ray diffraction investigation was carried out on complexes A, 2a, 2b and 3-PF₆ to approve their molecular structures. Crystallographic data are collected in Table S1,† and selected bond distances and angles are quoted in Table S2.† The perspective drawing of these complexes are illustrated in Fig. 3 and S6.†

Fig. 3 Representations of the X-ray crystal structure of complex [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]PF₆, 3-PF₆, showing all non-hydrogen atoms as 40% thermal ellipsoids. Hydrogen atoms, PF₆ and a CH₂Cl₂ solvent molecule have been omitted for clarity. Pt1–C11 1.982(14), Pt2–C45 2.000(17), Pt1–N1 2.070(12), Pt2–N2 2.066(12), Pt1–N3 2.117(11), Pt1–P1 2.235(3), Pt2–P2 2.226(3), Pt2–S1 2.418(4), C11–Pt1–N1 81.0(5), C45–Pt2–N2 80.4(6), C11–Pt1–P1 95.0(4), C45–Pt2–P2 97.9(5), C11–Pt1–N3 172.2(4), C45–Pt2–S1 160.4(5), N1–Pt1–P1 173.5(3), N2–Pt2–P2 165.7(4), N1–Pt1–N3 91.2(4), N2–Pt2–S1 94.6(4), N3–Pt1–P1 92.8(3), P2–Pt2–S1 91.41(13).

All complexes A, 2a, 2b and 3-PF₆ reveal a distorted square planar coordination environment around Pt(II) center. This distortion is due to small bite angle [81.0(5)–78.7(13)°] of the ppy cyclometalated ligand. This narrow bite angle with bond distances for cyclometalated chelate (i.e. Pt–C_ppy and Pt–N_ppy) are comparable to those observed in other five membered rings of 2-phenylpyridinate platinum(II) complexes.^50–53 In each structure, the metalated carbon atom of the ppy ligand accommodates the coordinated phosphine ligand in cis arrangement of Pt–C_ppy bond, as a result of high trans influence of carbon atom.⁵⁴ Therefore, PPh₃ ligands are located trans to nitrogen atom of ppy ligand and bond distances of the Pt–P are notably shorter than platinum–phosphorous bond lengths, positioned trans to carbon atom of cyclometalated ligand.⁵⁰ Furthermore, halogen ligand completes the coordination sphere of platinum atom in complexes A, 2a and 2b or nitrogen and sulfur atoms of bridging Spy ligand in complex 3-PF₆. Also, in the binuclear complex 3-PF₆ the pyridine-2-thiolate group acts as a bridging ligand and binds two Pt centers through S^N coordination. The nitrogen and sulfur atoms of Spy ligand in this complex occupies a position trans to the Pt–C_σ bond of cyclometalated ligands.

Additional examination on the molecular packing of the crystal structures displayed the presence of inter- and intramolecular interactions (see below) such as C–H⋯π interaction, π⋯π stacking, without any notable Pt⋯Pt contacts. All molecular structures reveal a moderate short C–H⋯π intramolecular interaction (Fig. S7†) between hydrogen atom adjacent to coordinated carbon atom of cyclometalated ligand and the phenyl ring of the triphenylphosphine ligand (C–H⋯π; C2⋯C_ph (PPh₃) = 3.320–3.416 Å (A), 3.237–3.401 Å (2a), 3.291–3.365 Å (2b), for 3-PF₆, C–H⋯π; C11 or C45⋯C_ph (PPh₃) = 3.244–3.493 Å, C–H⋯π; C1⋯C_Spy = 3.462 Å).

The crystal structure of complexes A, 2a and 2b show that the halogen ligand is involved in a substantial intramolecular interaction with C–H group adjacent to coordinated nitrogen atom of ppy ligand (Fig. S8†). This C–H⋯X interaction causes deshielding of the proton involved in hydrogen bonding^55,56 (∼1.3 to 1.9 ppm) in ¹H NMR (Fig. S9†) relative to the hydrogen in free ppy ligand.⁵⁷ The shift values has also increased according to atomic radii of halogen ligands in the order of Cl < Br < I.⁵⁸

In complex 3-PF₆ the ring of the pyridine-2-thiolate ligand exhibits weak intramolecular π⋯π interaction with one of the phenyl groups of the PPh₃ ligand bound to the Pt1 with distance between the centroids of 3.888 Å (Fig. S10†). Complexes A and 2b are stacked through short intermolecular π⋯π interactions with an inter-planar distance of 3.335 Å and 3.329 Å, respectively. These interactions lead to formation of a dimer in a head to tail style, which are analogous to other platinum complexes having ppy fragments⁵¹ (Fig. S11†). Furthermore, in crystal network of complex 2b, each Br atom is bound to one hydrogen atom of the phosphine ligand from neighboring unit. In this molecular structure, each Pt(ppy) moiety is arranged through π⋯π interactions with another Pt(ppy) fragment in nearby chain and thus giving rise to a 2D network (Fig. S12†). Therefore, this complex indicates that the C–H groups (see above discussion) are involved in the formation of intra- and intermolecular hydrogen bonds⁵⁹ through the coordinated Br ligand with bond distance of 2.583 Å and 3.045 Å, respectively (Fig. S8c and S12†). Complex 2a displays the comparatively long interlayer distances between nearby ppy ligands that are longer than 3.8 Å,⁶⁰ demonstrating the lack of π⋯π interactions in this complex.

DFT investigation

In order to verify this approach (according to the experimental data), we calculated the energy and reaction profiles for the oxidative addition of Me–I to complex 1a and carbon–sulfur bond formation from the suggested intermediate structures (Scheme 2 and Fig. 4). It is also worth mentioning that the pyridyl group and its peripheral nitrogen donor atom could be orientated in four different fashions (Fig. S13†). In these modes, the py ring and N atom are located at a plane above or below of the molecule and near or far from the platinum center, respectively. However, theoretical calculations suggest all possible orientations have small variation in energy (∼2 kJ mol⁻¹), perhaps due to free rotation around S–C_py bond.⁶¹ But, the form a (Fig. S13†) is preferred over other structures since it is in good agreement with further theoretical calculations (see below).

Fig. 4 Calculated structures and relative energies for probable intermediates and transition states arising from the reaction of complex 1 and MeI in acetone solution.

The DFT calculations suggest that the electron rich platinum center of complex 1a (as a nucleophile) attacks on carbon atom of MeI (as an electrophile) to form transition state TS1 and energy barrier for this state is +52.9 KJ mol⁻¹. This transition state illustrates a linear arrangement for I⋯C_MeI⋯Pt with the bond angles close to 90° and 180° between H–C_MeI⋯Pt (90–93°) and I⋯C_MeI⋯Pt (178.5°), respectively. The most important modifications in bond distances are detected for I⋯C_MeI and Pt⋯C_MeI in transition state TS1. These calculations suggested that the bond distance increases for I⋯C_MeI (2.73 Å) in this transition state as compared to MeI (2.20 Å), while Pt⋯C_MeI (2.49 Å) bond length is reduced.^62,63 This data supports the hypothesis that the formation of transition state TS1 is followed by concurrent bond breaking of methyl-iodide and a new bond formation between platinum and methyl, then the cationic 5-coordinate intermediate IM1 is obtained. This intermediate is equilibrated with the comparable intermediate IM2 with phosphine ligand and Me group being in trans disposition at platinum center (Fig. 4). Both intermediates IM1 and IM2 could potentially convert to the 6-coordinate compounds P1 and P2, respectively (Scheme 2). The calculation indicates that the energy barriers for these compounds are high (+42.9 and +50.3 kJ mol⁻¹ for compounds P1 and P2, respectively) and subsequent C–S bond coupling is not favorable. It is recommended that this concerted elimination from the five-coordinate d⁶ metal complexes is more straightforward than direct elimination from the saturated six-coordinate metal complexes.^11,64,65 Therefore, the carbon–sulfur bond formation from intermediate IM1 (selected based on its good agreement with theoretical calculations) proceeds via transition state TS2 as a concerted reductive elimination⁶⁶ (Scheme 2). In TS2, the bond distances between platinum with methyl (2.42 Å) or thiolate (2.47 Å) groups are lengthened and the bond angle of C_Me–Pt–S_Spy (61.3°) is decreased and it suggested that this elimination is appropriate. Another possibility for the reductive elimination of C–S bond, probably, is the microscopic reverse of the popular S_N2 mechanism.^66,67 In this mechanism, pervious mechanistic evidences provided, the nucleophilic groups like NR⁻, OR⁻, I⁻ dissociated from central metal, and then perform S_N2 attack upon an alkyl ligand (Me group) of the cationic intermediates.^{12,65,68–70} Therefore, by addition of excess of nucleophilic groups to the reaction mixture the rate of reaction should be increased. By this strategy, when Spy⁻ was added to the reaction mixture, the desire C–S bond coupling reaction was stopped. Due to Spy⁻ was reacted with MeI and compound O1 and KI salt were formed without accelerating rate of reductive elimination of C–S from intermediate IM1. Consequently, the possibility of S_N2 mechanism is not proper for this C–S bond formation. As compound O1 dissociates from this transition state (TS2), the new cationic T-shaped intermediate IM3 ^11,65,69,71 is produced (Scheme 2). Intermediate IM3 can then react by two routes: (i) migration of iodide to the platinum center and formation of complex 2a as a very stable complex (−71.3 KJ), or (ii) reaction with complex 1a to yield a cationic complex 3-I. The latter pathway is slower (deduced from NMR, Fig. 1 and S1†) and more favorable (−51.8 kJ, Fig. 4). Subsequently, complex 3-I in the presence of MeI acts as a nucleophile (perhaps through a classical S_N2 mechanism)⁴⁸ and leads to the formation of an intermediate IM4 ⁴⁸ (Pt(II)–Pt(IV)–Me). Finally, intermediate IM4 through a reductive elimination process (Me–Spy, compound O1) generates the stable final products (Fig. 4 and Scheme 2).

Conclusion

The platinum(II) complex 1a reacted readily with electrophilic alkyl sources at ambient temperature and through carbon–sulfur reductive coupling reaction produced corresponding halide complexes 2a or 2b and alkyl sulfides. This reaction was monitored by NMR spectroscopy at different temperatures and the results revealed (Scheme 2) an interesting binuclear complex 3-I was formed during this process. Also, we proposed that the platinum center is a stronger nucleophile than peripheral nitrogen or sulfur atom. This claim was approved by selection of complex 1c, which lacks a free nitrogen atom, and it underwent a similar C–S bond formation reaction. Moreover, in this experiment complex 1a did not display any Pt(IV) compound(s) and perhaps such intermediate(s) is(are) energetically unfavorable as supported by DFT calculations. Therefore, platinum–thiolate complexes of these nature served as an appropriate platform for oxidative addition reaction that impressively enhanced the formation of alkyl sulfides.

Experimental section

General procedures and materials

All NMR spectra (¹H, ³¹P {¹H} and ¹⁹⁵P {¹H}) were recorded on a Brucker Avance DPX 400 MHz instrument. References were TMS or the residual peak of the solvent, i.e. CD₂Cl₂, CDCl₃ and acetone-d₆ (¹H), 85% H₃PO₄ (³¹P), and aqueous Na₂PtCl₆ (¹⁹⁵Pt). The chemical shifts (δ) being reported as ppm and coupling constants (J) expressed in Hz. The microanalyses were performed using a vario EL CHNS elemental analyzer. Electrospray ion mass spectrum (ESI-MS) was recorded by a HP-5989B spectrometer using methanol–water as the mobile phase. All solvents were purified and dried according to standard procedures.⁷² 2-phenylpyridine (ppyH), pyridine-2-thiol (pySH), thiophenol (PhSH), silver hexafluorophosphate (AgPF₆), triphenylphosphine (PPh₃) and dimethylphenylphosphine (PPhMe₂) were purchased from Aldrich or Acros. Complexes [Pt(ppy)(DMSO)(Cl)],⁵³ [Pt(ppy)(PPh₃)Cl], A,²⁴ [Pt(ppy)(η¹-S-Spy)(PPh₃)], 1a,²³ [Pt(ppy)(SPh)(PPh₃)], 1b,²³ were prepared as reported in literature. The NMR labeling for all ligands are shown in Scheme 3 for clarifying the chemical shift assignments.

Scheme 3 Representative ligands with position labeling.

[Pt(ppy)(PPhMe₂)Cl], B

To a solution of [Pt(ppy)(DMSO)Cl] (100 mg, 0.22 mmol) in acetone (10 mL) was added PPhMe₂ (30.8 μL, 0.22 mmol). The mixture was stirred at room temperature for 1 h. Then, the solvent was removed under reduced pressure and the residue was triturated with n-hexane (3 × 2 mL) and the resultant yellow solid was dried under vacuum. Yield: 94 mg, 83%. Elem. anal. calcd for C₁₉H₁₉ClNPPt (522.87): C, 43.64; H, 3.66; N, 2.68; found: C, 43.91; H, 3.74; N, 2.57. ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 9.80 (d, ³J_PtH = 24.1, ³J_HH = 5.8, 1H, H²), 8.11–8.04 (m, 4H, H⁴, H⁵ and H⁰ of PPhMe₂), 7.69 (dd, ³J_HH = 7.8, ⁴J_HH = 1.3, 1H, H⁶), 7.53–7.48 (m, 4H, H³, H^p and H^m of PPhMe₂), 7.00 (td, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁷), 6.87 (d, ³J_PtH = 56.1, ³J_HH = 7.8, 1H, H⁹), 6.75 (td, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁸), 2.00 (d, ²J_PH = 10.4, ³J_PtH = 39.7 Hz, 6H, Me group of PPhMe₂). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): −8.1 (s, ¹J_PtP = 4131, 1P).

[Pt(ppy)(η¹-S-Spy)(PPhMe₂)], 1c

Complex B (100 mg, 0.19 mmol) was added to an ethanolic solution of sodium pyridine-2-thiolate ligand (NaC₅H₄NS) under inert atmospheric condition [note: NaC₅H₄NS was prepared by dissolving sodium (5.8 mg, 0.25 mmol) in 10 mL of absolute ethanol and subsequent treatment with pyridine-2-thiol (21.3 mg, 0.19 mmol)]. The resulting orange colored mixture was allowed to react while stirring for 12 h. Then, solvent was removed under reduced pressure the residue was extracted with CH₂Cl₂ (20 mL). The obtained orange solution was filtered through celite and the filtrate was concentrated to a small volume (1 mL). Finally, n-hexane (5 mL) was added to precipitate complex 1c as an orange solid. Yield: 71 mg, 62%. Elem. anal. calcd for C₂₄H₂₃N₂PPtS (597.57): C, 48.24; H, 3.88; N, 4.69; found: C, 47.96; H, 3.74; N, 4.76. ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 10.05 (m, ³J_PtH = 26.2, 1H, H²), 8.15–8.03 (m, 5H, H⁴, H⁵, H^6′ and H⁰ of PPhMe₂), 7.90 (dd, ³J_HH = 7.8, ⁴J_HH = 1.0, 1H, H⁶), 7.57 (d, ³J_HH = 8.0, 1H, H^3′), 7.48–7.36 (m, 4H, H³, H^p and H^m of PPhMe₂), 7.19 (td, ³J_HH = 7.8, ⁴J_HH = 1.9, 1H, H⁷), 7.02 (t, ³J_HH = 7.1, 1H, H^4′), 6.98 (d, ³J_PtH = 46.1, ³J_HH = 7.8, 1H, H⁹), 6.80 (td, ³J_HH = 7.8, ⁴J_HH = 1.7, 1H, H⁸), 6.71 (dd, ³J_HH = 5.1, ⁴J_HH = 0.9, 1H, H^5′), 2.01 (d, ²J_PH = 10.9, ³J_PtH = 40.1 Hz, 6H, Me group of PPhMe₂). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): −8.5 (s, ¹J_PtP = 4178, 1P).

[Pt(ppy)(PPh₃)I], 2a

Method A. To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL) was added an excess of MeI (174.3 μL, 2.8 mmol, 20 equiv.) at room temperature. The color of the mixture turned red instantly and then the mixture was stirred for 2 h. Then, the solvent was removed under reduced pressure and the residue was triturated with n-hexane (3 × 2 mL). The resulting green solid was dried under vacuum. Yield: 82 mg, 78%; mp 225 °C. Elem. anal. calcd for C₂₉H₂₃INPPt (738.03): C, 47.15; H, 3.14; N, 1.90; found: C, 47.81; H, 3.37; N, 1.97. ¹H NMR (400 MHz, CDCl₃, 20 °C, δ): 10.55 (ddd, ³J_PtH = 30.9, ³J_HH = 5.8, ⁴J_HH = 0.9, ⁴J_PH = 4.3, 1H, H²), 7.87–7.77 (m, 8H, H⁴, H⁵, H⁰ of PPh₃), 7.50 (dd, ³J_HH = 7.9, ⁴J_HH = 1.1, 1H, H⁶), 7.43–7.34 (m, 9H, H^p and H^m of PPh₃), 7.20 (tdd, ³J_HH = 7.3, ⁴J_HH = 1.3, ⁴J_PH = 1.4, 1H, H³), 6.99 (td, ³J_HH = 7.9, ⁴J_HH = 0.9, 1H, H⁷), 6.67 (ddd, ³J_PtH = 52.1, ³J_HH = 7.3, ⁴J_HH = 0.8, ⁴J_PH = 4.0, 1H, H⁹), 6.52 (td, ³J_HH = 8.0, ⁴J_HH = 1.3, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, CDCl₃, 20 °C, δ): 22.7 (s, ¹J_PtP = 4290, 1P). ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 10.54 (ddd, ³J_PtH = 31.1, ³J_HH = 5.7, ⁴J_HH = 1.0, ⁴J_PH = 4.3, 1H, H²), 8.13–8.11 (m, 2H, H⁴, H⁵), 7.86–7.81 (m, 6H, H⁰ of PPh₃), 7.70 (dd, ³J_HH = 7.9, ⁴J_HH = 1.4, 1H, H⁶), 7.50–7.40 (m, 10H, H³, H^p and H^m of PPh₃), 6.99 (td, ³J_HH = 7.9, ⁴J_HH = 1.1, 1H, H⁷), 6.69 (ddd, ³J_PtH = 52.7, ³J_HH = 7.4, ⁴J_HH = 0.8, ⁴J_PH = 4.0, 1H, H⁹), 6.49 (td, ³J_HH = 79, ⁴J_HH = 1.5, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): 22.8 (s, ¹J_PtP = 4296, 1P). ¹H NMR (400 MHz, CD₂Cl₂, 20 °C, δ): 10.49 (m, ³J_PtH = not resolved, 1H, H²), 7.92–7.78 (m, 8H, H⁴, H⁵, H⁰ of PPh₃), 7.53 (dd, ³J_HH = 7.8, ⁴J_HH = 1.1, 1H, H⁶), 7.44–7.38 (m, 9H, H^p and H^m of PPh₃), 7.25 (t, ³J_HH = 7.4, 1H, H³), 7.00 (t, ³J_HH = 7.9, 1H, H⁷), 6.65 (dd, ³J_PtH = 52.4, ³J_HH = 7.5, ⁴J_PH = 4.1, 1H, H⁹), 6.52 (t, ³J_HH = 7.9, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, CD₂Cl₂, 20 °C, δ): 22.4 (s, ¹J_PtP = 4288, 1P).

Method B. To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL) was added a solution of I₂ (175.8 g, 0.70 mmol, 5 equiv.) in acetone (5 mL) at room temperature. A fast color change to red was observed. The resulting solution was stirred for 2 h. Then, the solvent was reduced under vacuum to a small volume (1 mL) and n-hexane (5 mL) was added to precipitate complex 2a as a green powder (73 mg, 71%).

Method C. To a green suspension of complex A (100 mg, 0.15 mmol) in acetone/CH₂Cl₂ (10 : 5 mL) was added an excess of KI (74.7 g, 0.45 mmol, 3 equiv.) in acetone (2 mL) at room temperature. The resulting mixture was allowed to react while stirring for 24 h. Then, the solvent was removed under reduced pressure and the residue was extracted with CH₂Cl₂ (20 mL). The obtained green solution was filtered through Celite and the filtrate was concentrated to a small volume (1 mL). Finally, n-hexane (5 mL) was added to yield complex 2a as a green solid which was dried under vacuum (48 mg, 43%).

[Pt(ppy)(PPh₃)Br], 2b

Method A. To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL) was added benzyl bromide (166.3 μL, 1.4 mmol, 10 equiv.) at room temperature and an orange solution was obtained quickly. The reaction mixture was stirred for 2 h. Then, the solvent was evaporated under vacuum and the residue was washed with n-hexane (3 × 2 mL), resulting in a green solid which was dried under vacuum. Yield: 86 mg, 89%; mp 221 °C. Elem. anal. calcd for C₂₉H₂₃BrNPPt (690.04): C, 50.43; H, 3.36; N, 2.03; found: C, 51.01; H, 3.59; N, 2.26. ¹H NMR (400 MHz, CDCl₃, 20 °C, δ): 10.18 (ddd, ³J_PtH = 30.8, ³J_HH = 5.7, ⁴J_HH = 1.0, ⁴J_PH = 4.1, 1H, H²), 7.89–7.77 (m, 8H, H⁴, H⁵, H^o of PPh₃), 7.51 (dd, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁶), 7.44–7.35 (m, 9H, H^p and H^m of PPh₃), 7.27 (m, 1H, H³, this signal has overlapping with CHCl₃ NMR solvent), 6.97 (td, ³J_HH = 8.0, ⁴J_HH = 0.9, 1H, H⁷), 6.67 (ddd, ³J_PtH = 52.4, ³J_HH = 7.4, ⁴J_HH = 0.8, ⁴J_PH = 3.9, 1H, H⁹), 6.52 (td, ³J_HH = 8.0, ⁴J_HH = 1.4, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, CDCl₃, 20 °C, δ): 22.7 (s, ¹J_PtP = 4328, 1P).

Method B. To a yellow suspension of complex 1a (100 mg, 0.14 mmol) in acetone (15 mL) was added a solution of Br₂ (36 μlit, 0.70 mmol, 5 equiv.) in acetone (5 mL) at room temperature. A fast color change to red was observed. The resulting solution was stirred for 2 h. Then, the solvent was reduced under vacuum to a small volume (1 mL) and n-hexane (5 mL) was added to precipitate complex 2b as a green powder (61 mg, 64%).

Method C. To a green suspension of complex A (100 mg, 0.15 mmol) in acetone/CH₂Cl₂ (10 : 5 mL) was added an excess of KBr (53.6 g, 0.45 mmol, 3 equiv.) in acetone (2 mL) at room temperature. The resulting mixture was allowed to react while stirring for 30 h. Then, the solvent was removed under reduced pressure and the solid residue was extracted with CH₂Cl₂ (20 mL). The obtained green solution was filtered through celite and the filtrate was concentrated to a small volume (1 mL). Finally, n-hexane (5 mL) was added to yield complex 2b as a green solid (39 mg, 38%).

[Pt(ppy)(PPhMe₂)I], 2c

To an orange solution of complex 1c (100 mg, 0.17 mmol) in acetone (15 mL) was added an excess of MeI (208.3 μL, 3.4 mmol, 20 equiv.) at room temperature. The solution color turned red instantly and the mixture was stirred for 1 h. Then, the solvent was removed under reduced pressure and the resulting residue was triturated with n-hexane (3 × 2 mL). Finally, the resulting green solid was dried under vacuum. Yield: 64 mg, 62%. Elem. anal. calcd for C₁₉H₁₉INPPt (614.32): C, 37.15; H, 3.12; N, 2.28; found: C, 37.31; H, 3.24; N, 2.21. ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 10.41 (m, ³J_PtH = 32.3, 1H, H²), 8.10–8.05 (m, 4H, H⁴, H⁵ and H⁰ of PPhMe₂), 7.70 (dd, ³J_HH = 7.7, ⁴J_HH = 1.4, 1H, H⁶), 7.51–7.43 (m, 4H, H³, H^p and H^m of PPhMe₂), 7.05 (td, ³J_HH = 7.7, ⁴J_HH = 1.1, 1H, H⁷), 6.84 (ddd, ³J_PtH = 55.3, ³J_HH = 7.6, ⁴J_HH = 1.0, ⁴J_PH = 3.8, 1H, H⁹), 6.77 (td, ³J_HH = 7.7, ⁴J_HH = 1.3, 1H, H⁸), 2.21 (d, ²J_PH = 10.8, ³J_PtH = 39.9 Hz, 6H, Me group of PPhMe₂). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): −9.0 (s, ¹J_PtP = 4108, 1P).

[{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]PF₆, 3-PF₆

To a solution of complex A (100 mg, 0.15 mmol) in CH₂Cl₂ (15 mL) was added AgPF₆ (51 mg, 0.20 mmol). The reaction mixture was stirred for 5 h in dark at room temperature, and then filtered through celite to remove AgCl. The green filtrate was treated with complex 1a (108.3 mg, 0.15 mmol) in CH₂Cl₂ (5 mL). The resulting orange solution was stirred for 2 h and then the solvent was removed under reduced pressure and concentrated to a small volume (∼1 mL) and n-hexane (3 mL) was added to obtain complex 3-PF₆. Yield: 157 mg, 71%; mp 217 °C. MS ESI(+): m/z 1332.27 [M − PF₆]⁺. Elem. anal. calcd for C₆₃H₅₀F₆N₃P₃Pt₂S (1477.22): C, 51.18; H, 3.41; N, 2.84; found: C, 51.47; H, 3.52; N, 2.91. ¹H NMR (400 MHz, CDCl₃, 20 °C, δ): 8.97 (m, ³J_PtH = 32.7, 2H, H²), 7.98–6.31 (overlapping multiplets, 48H). ³¹P {¹H} NMR (162 MHz, CD₂Cl₂, 20 °C, δ): 21.5 (s, ¹J_PtP = 4407, 1P, P^a cis with nitrogen atom of Spy ligand), 21.0 (s, ¹J_PtP = 4245, 1P, P^b cis with sulfur atom of Spy ligand), −144.4 (septet, ¹J_PF = 712, 1P, P of PF₆). ¹⁹⁵Pt {¹H} NMR (85.7 MHz, CD₂Cl₂, 20 °C, δ): −4081 (d, ¹J_PtP = 4413, 1 Pt, Pt^a bound to the nitrogen atom of Spy ligand), −4296 (d, ¹J_PtP = 4256, 1 Pt, Pt^b bound to the sulfur atom of Spy ligand).

Monitoring the reaction of complexes 1a–c with electrophilic reagents (MeI, CD₃I, Br₂ and I₂) by ¹H and ³¹P {¹H} NMR spectroscopy

To a solution of complexes 1a–c (0.014 mmol) in CD₂Cl₂ (0.75 mL) in an NMR tube was added the appropriate electrophilic reagent at 298 K (0.020 mmol) or at 223 K (0.42 mmol). The tube was then placed in the probe of the NMR spectrometer, and NMR spectra were obtained at appropriate time intervals.

Crystal structure determination and refinement

The X-ray diffraction measurements were carried out on STOE IPDS-2/2T diffractometer with graphite-monochromated Mo Kα radiation. All single crystals were mounted on a glass fiber and used for data collection. Cell constants and an orientation matrix for data collection were obtained by least-square refinement of the diffraction data from 4977, 4763, 4350 and 10 828 for A, 2a, 2b and 3-PF₆, respectively. Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the Stoe X-AREA⁷³ software package. A numerical absorption correction was applied using X-RED⁷⁴ and X-SHAAPE⁷⁵ software. The data were corrected for Lorentz and polarizing effects. The structures were solved by direct methods⁷⁶ and subsequent difference Fourier maps and then refined on F² by a full-matrix least-squares procedure using anisotropic displacement parameters.⁷⁷ Atomic factors are from the International Tables for X-ray Crystallography.⁷⁸ All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. A view of the structure is depicted in Fig. 3 (top) and S6.† All refinements were performed using the X-STEP32, SHELXL-2014 and WinGX-2013.3 programs.^79–86 Also, the suitable crystals were obtained from CHCl₃/n-hexane solution (complexes A, 2a and 2b) or CH₂Cl₂/n-hexane solution (complex 3-PF₆) at room temperature.

Theoretical methods

Gaussian 09 was used⁸⁷ to fully optimize all the structures at the DFT/B3LYP level of density functional theory. The solvation energies were calculated by CPCM model in acetone. The effective core potential of Hay and Wadt with a double-x valence basis set (LANL2DZ) was chosen to describe Pt and I.⁸⁸ The 6-31G(d) basis set was used for other atoms.

Acknowledgements

This work was supported by the Institute for Advanced Studies in Basic Sciences (IASBS) Research Council. Technical support of the Chemistry Computational Center at Shahid Beheshti University is gratefully acknowledged. Thanks are also due to Dr F. Niroomand Hosseini, Islamic Azad University (Shiraz Branch), for helpful assistance and discussions on DFT calculations, Dr A. Neshat, IASBS, for valuable suggestions, Mr A. Biglari, the operator of Bruker NMR instrument at IASBS, for recording the NMR spectra and Prof. Elena Lalinde, Universidad de La Rioja, for ESI-MS measurements. This paper is dedicated to Professor Mehdi Rashidi on the occasion of his 65^th birthday.

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra, crystallographic and computational details (PDF). CCDC 1457809, 1457810, 1457812 and 1496283. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21756g

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