Reactivity of a half-lantern Pt₂(II,II) complex with triphenylphosphine: selectivity in a protonation reaction

Abstract

The half-lantern Pt₂(II,II) complex [{Pt(ppy)(μ₂-Spy)}₂], 1, in which ppyH = 2-phenylpyridine and pySH = pyridine-2-thiol, is treated with two equivalents of triphenylphosphine ligand (dissociated Pt–N_Spy bonds) to form complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 2. This complex can alternatively be obtained directly from the reaction of complex [Pt(ppy)(PPh₃)Cl], 3, with an ethanolic solution of sodium pyridine-2-thiolate. The transphobia effect (T) controls the geometry of complex 2 and spectroscopic data confirms that the phosphine ligand is positioned trans to the nitrogen atom of the cyclometalating fragment (ppy). The solution dynamics of complex 2 was investigated by NMR spectroscopy and it revealed a fluxional behavior in solution. Besides, the protonation reaction conditions for complex 2 revealed the sensitivity of this complex to the anionic group of the acidic species. For instance, reaction of complex 2 with hydrogen chloride that contains a coordinating anion, resulted in a platinum–sulfur bond cleavage and subsequent formation of the corresponding chloride complex 3. In contrast, upon treatment of complex 2 with HPF₆, involving a non-coordinating counter-anion, the pendant nitrogen atom was protonated and complex [Pt(ppy)(η¹-S-Spy^*) (PPh₃)][PF₆], 5, was obtained.

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Introduction

The divalent platinum(II) complexes of the general formula, [{Pt(C^N) (μ₂-SN)}₂], [C^N = different cyclometalating units and SN = various 2-thiolate ligands], are known as neutral half-lantern complexes.¹ Generally, these type of complexes display an intense ³MMLCT (Metal–Metal to Ligand Charge Transfer) emission^2–5 and this property could play an important role in their applications. For instance, they are used as phosphorescent dopants for the preparation of OLEDs,^6–9 and they are good candidates for detection of poisonous ions¹⁰ such as Hg²⁺ or they are used in oxidative addition reactions.¹¹ In the later example, half-lantern complexes undergo two-electron oxidation upon reaction with high or medium polar species to give the corresponding Pt(III)–Pt(III) complexes.^{1–3,7,8,11–15} The results of this type of oxidation reaction is known to be non-emissive.¹⁶

In spite of different reactivity of such half-lantern platinum(II) complexes in oxidative–addition reaction,¹¹ these complexes are potential candidates in other important reactions, for example in ligand substitution reactions. These complexes contain weak platinum–nitrogen bonds³ and upon treatment with strong donor ligands these weak bonds break up^17,18 and may lead to the formation of interesting compounds that encompass a heterocyclic thiolate group with free nitrogen atom. However, both nitrogen and sulfur donor atoms in these complexes can act as coordinating sites.^19–21 The coordination chemistry of such complexes can be complicated^19,20 and the main attractive point for preparation of these compounds is potential application in diverse area such as construction of luminescent materials,^22,23 catalysis,^24,25 and biology.^26,27

We explored the reactivity of complex [{Pt(ppy)(μ₂-Spy)}₂], 1, ppyH = 2-phenylpyridine and pySH = pyridine-2-thiolate, with PPh₃ and complex [Pt(ppy)(η¹-S-Spy) (PPh₃)], 2, was obtained in acetone solution. The geometry of complex 2 was estimated according to spectroscopic methods and the transphobia (T) effect,^28–31 which is associated with the trans influence of the σ platinum–carbon bond existent in this complex. We found out that complex 2 displayed anionic selectivity upon its protonation reaction with acidic species. When this complex was treated with HCl, the Pt–S bond of thiolate ligand was cleaved. In contrast, when complex 2 was reacted with HPF₆, the nitrogen atom of pyridyl group was protonated.

Result and discussion

Reactivity of complex 1 with triphenylphosphine: synthesis and characterization of complex 2

The molecular structure^3,32 of binuclear half-lantern Pt₂(II,II) complex [{Pt(ppy)(μ₂-Spy)}₂], 1, reveals the geometry of each Pt(ppy) moiety that are linked by two pyridine-2-thiolate groups in a head to tail fashion. The high trans influence of Pt–C bonds resulted in an anti conformation and the nitrogen atom of Spy ligand occupies trans position of the carbon atom.^2,3 Therefore, the Pt–N_Spy distances in this configuration are considerably long (∼2.14 Å)^3,32 and these bonds easily dissociate with strong nucleophiles such as phosphorus donor ligands.

As shown in Scheme 1, the reaction of complex 1 with 2 equivalents of PPh₃, proceeded by dissociation of the nitrogen atoms^33–35 of Spy bridging ligands, resulting in complex [Pt(ppy) (η¹-S-Spy)(PPh₃)], 2, in high yield.

Scheme 1 Reaction of complex 1 with phosphine ligand in different dry solvents and under Ar atmosphere.

The substitution reaction described in Scheme 1 only proceeded in dry solvent and under inert argon atmosphere. This is because of the phosphine ligand being sensitive to water content in the solvent and molecular oxygen in the environment.^36–38 The type of solvent also plays a major role in the overall outcome of the reaction. When non-halogenated solvents like acetone or dimethylsulfoxide (DMSO) were used, PPh₃ ligand clearly ruptured Pt–N_Spy bonds in complex 1 and produced complex 2. Phosphine displacement did not occur if halogenated solvents such as chloroform (CHCl₃) or dichloromethane (CH₂Cl₂) was used as reaction media. Therefore, reaction of complex 1 with halogenated solvents yielded corresponding Pt₂(III,III)X₂ complex [{Pt(ppy)(μ₂-Spy)Cl}₂], 1a (Scheme 1)^2,3,11 and phosphine ligand did not play any role in this reaction. It is notable that oxidation reaction leading to complex 1a could happen in the absence of PPh₃.^8,11 Therefore, the parallel oxidation reaction competes with desired bond breaking reaction. This observation suggested that complex 1 require a vacant site for initial coordination of PPh₃ ligand to the platinum center.^17,18,39 To confirm this hypothesis, the substitution reaction between complex 1a, which lacks an open coordination site, and the phosphine ligand was investigated. This reaction did not yield any product and only PPh₃ ligand was converted to phosphine oxide,³⁶ (Fig. S1 and S2†).

Further investigation was carried out using UV-vis spectroscopy. In this study, complex 1 displayed a distinctive band centered at 500 nm (acetone), which was assigned to ¹MMLCT transition and believed to be the main reason for the red color of complex 1 (Fig. 1).³ This band was used to monitor the substitution reaction. After addition of an excess amount of phosphine ligand to a solution of complex 1, the intensity of the peak decreased progressively, finally vanished and at the same time a new band was observed at 420 nm (Fig. 1 and S3†). According to time-dependent DFT (TD-DFT) calculations (see below), and previous assignments this new profile can be attributed to an admixture of ¹L'LCT/¹MLCT (Ligand (Spy) to Ligand (ppy) Charge Transfer/Metal to Ligand (ppy) Charge Transfer) transitions.^23,40 Besides, the red color of mixture gradually disappeared and eventually a yellow solution was obtained. Therefore, the new band at 420 nm and the appearance of yellow color confirmed the formation of complex 2 and complete consumption of complex 1 (Fig. 1 and S3†).

Fig. 1 UV–vis spectra of (a) complex [{Pt(ppy)(μ₂-Spy)}₂], 1, (3 × 10⁻⁴ M); (b) the end point of reaction of excess PPh₃ with complex [{Pt(ppy)(μ₂-Spy)}₂], 1, (c) the synthesized complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 2, (2.5 × 10⁻⁴ M) in acetone at 25 °C.

To deduce the nature of the lower energy electronic transition of complex 2, TD-DFT calculations were conducted in acetone solution. The frontier molecular orbitals involved in this transition were shown in Fig. S4.† The relative compositions of these energy levels in terms of composing fragments are collected in Table S1.† In complex 2, the HOMO is mainly localized on the pyridine-2-thiolate (80%) ligand with partial contribution of the platinum (12%) metal center. The LUMO is predominantly located on the ppy (86%) cyclometalating ligand (Fig. S4 and Table S1†). The calculated value for the low energy absorption 433 nm is close to the experimental value of 420 nm, with a small red shift. The main contribution to this low energy profile (∼70%) including the HOMO → LUMO transition (Table S2†), can be allocated to a mixture of ¹MLCT (Pt → ppy) with a notable contribution of ¹L'LCT (Spy → ppy).

Complex 2 was also obtained using a different method via treatment of complex [Pt(ppy)(PPh₃)Cl], 3,^41,42 with an ethanolic solution of sodium pyridine-2-thiolate (NaC₅H₄NS) under Ar atmosphere (Scheme 2, step b').

Scheme 2 Potential routes for the preparation of complex 2 using complex A as a precursor.

The above two reaction routes demonstrate an interesting point. As presented in Scheme 2, complexes 1³² (step a) and 3^41,42 (step a') were prepared by similar platinum(II) starting complex [Pt(ppy)(DMSO)(Cl)], A.⁴³ This suggests that in the first step, addition of either phosphine or sodium thiolate ligand to complex A was not crucial. Consequently, path (I) and (II) yielded the same complex 2 at the end of reactions (Scheme 2).

Complex 2 was inferred from common analytical methods (Elemental analyses and Mass) and its integrity in solution was studied by multinuclear (¹H, ³¹P {¹H}, in different solvents) NMR spectroscopy. The details are given in the Experimental section.

The ESI-MS spectrum of complex 2 (positive ions), illustrated three main fragment peaks assigned to the Pt species [Pt(ppy)(η¹-S-SpyH)(PPh₃)]⁺ (m/z 722, 100%), [Pt(ppy)(PPh₃)]⁺ (m/z 611, 94%) and [{Pt(ppy)(PPh₃)}₂(μ₂-Spy)]⁺ (m/z 1332, 53%) (Fig. S5 and S6†). The observed value (m/z 722) and isotropic pattern of this peak (Fig. S6†) is originated from the protonated form of complex 2 [Pt(ppy)(η¹-S-SpyH)(PPh₃)]⁺, which obviously establishes this complex in the gas phase. Moreover, the occurrence of the fragment ion at m/z 611 demonstrates that Spy ligand in complex 2 is labile and it easily undergoes Pt–S bond cleavage in the gas phase (refer to reactivity of complex 2 with HCl). This lability can be attributed to high trans influence of the σ Pt–C bond of ppy ligand which was observed recently in the cycloplatinated complexes having Pt(C^C*) moiety.²⁸ The presence of a fragment peak at m/z 1332 suggests the uncoordinated nitrogen atom in thiolate ligand probably reacts with other reagents (see reactivity of complex 2 with HPF₆). The low intensity peak (below 5%) at m/z 919 which is related to complex 1,² may suggest that complex 2, through phosphine dissociation converts to complex 1 (see stability of complex 2 in solution). Other low intensity fragments at m/z 831, 1028, and 1070 could be assigned to [Pt(ppy)(η¹-S-Spy)(PPh₃)(pySH)]⁺, [{Pt(ppy)(μ₂-Spy)}₂(pySH)]⁺ and [{Pt(ppy)}₂(PPh₃) (μ₂-Spy)]⁺, respectively (Fig. S5†).

The NMR spectroscopic data (typically in CD₂Cl₂) of complex 2 endorsed the formation of an isomer with the phosphine ligand and the nitrogen of ppy group in trans position. This feature is consistent with the transphobia effect (T),^28–31 an expression that was previously agreed by several scientists to elucidate the stable geometry of square-planar d⁸ transition metal complexes. On the basis of T effect, the softer ancillary ligand, i.e. PPh₃, must be located in cis position of Pt–C bond of cyclometalating ligand as the phosphorus atom has greater trans influence than the sulfur atom in thiolate group. The ³¹P {¹H} NMR spectrum of complex 2 (Fig. S7†) includes a singlet resonance with platinum satellites (¹J_PtP = 4335 Hz). This high coupling constant value confirmed the trans disposition of the triphenylphosphine ligand with the N_ppy atom.^42,44 It also supports the higher trans influence of cyclometalated carbon respect to nitrogen. It is worthwhile mentioning that the proton NMR signals corresponding to both thiolate and ppy ligands in complex 2 are distinctly modified compared to complex 1 due to dissociation of Pt–N_spy bonds with the phosphine ligand. Three characteristic resonances (H², H⁹ and H^6′, the assignments were made on the basis of 2D experiment and schematic labeling is given in Fig. 2) are so sensitive to the coordination of PPh₃ ligand. The H² resonance in complex 2 appeared more deshielded⁴² (∼2.3 ppm) than complex 1, which is in agreement with the change of the ligand (from Spy to PPh₃) in trans position of the nitrogen atom of ppy group in both complexes (Fig. 2). In complex 2, the H^6′ peak did not display any coupling with the platinum center and it also indicated upfield shift (∼0.7 ppm). These results confirmed (in the substitution reaction) that the coordinated nitrogen atom of thiolate ligand is separated from metal center and this atom is not involved in the coordination sphere.⁴⁵ The H⁹ signal in complex 2 in relation to complex 1 is shielded (∼0.6 ppm). This upfield shift, perhaps, can be attributed to an anisotropic shielding effect.^28,46,47 It might have occurred by spatial proximity of the aromatic ring current of a phenyl group in the phosphine ligand with respect to the H⁹. This is consistent with C–H⁹⋯π interaction according to the cis embellishment of the Pt–P and Pt–C_ppy bonds in complex 2. Moreover, H² and H⁹ in complex 2 are coupled with phosphorous ligand and appeared as multiplet or doublet of doublets, respectively, as compared with complex 1.

Fig. 2 ¹H NMR spectra of complexes (a) 1 and (b) 2 in CD₂Cl₂ at room temperature. The signal assignments are depicted.

Interestingly, complex 2 in dmso-d₆ has experienced an equilibrium transformation over time (Fig. S8, S9† and Scheme 3 (step a)) and it experienced sluggish phosphine dissociation (detected by ³¹P {¹H} NMR, Fig. S8b†). This observation suggests that perhaps an unstable intermediate [Pt(ppy)(η²-N,S-Spy)], I,^33–35 (Scheme 3 (step a)) was formed during bond breaking. This intermediate probably has a weak intramolecular Pt–N_Spy contact.²⁰ This interaction is often labile in solution²⁰ and easily broken by phosphine ligand and is converted back to complex 2. Furthermore, this suggests that the free nitrogen donor atom was necessary to cause this equilibrium because complex [Pt(ppy)(Sph)(PPh₃)], 4, did not exhibit similar equilibrium.³³ That is why complex 4 does not have pendant nitrogen in molecular structure (see structural determination). While this fluxional behavior occurred in hydrated (hydrous and aerated) solvent, these conditions caused the free phosphine easily oxidize to corresponding phosphine oxide (Fig. S8c–f† and Scheme 3).³⁴ The side oxidation reaction stopped the equilibrium and it gradually transformed complex 2 to complex 1, perhaps by dimerization of two unstable intermediate I (Fig. S9, S10† and Scheme 3 (dimerization step)). This may support the idea that step (b) in Scheme 2 can be reversed under these circumstances. It is also worth mentioning that addition of excessive phosphorous ligand to the solution of complex 2 suppressed all above processes, which indicates that the weak Pt–N_Spy contact in intermediate I is readily dissociated by PPh₃.

Scheme 3 Equilibrium transformation and possible oxidation reaction for complex 2 in solution.

Reaction of complex 2 with HX (X = Cl and PF₆)

Complex 2 reacted very rapidly with HCl in stoichiometric ratio at room temperature. Remarkably, Pt–S bond was cleaved through the reaction (see Mass assignment of complex 2) and corresponding chloride complex 3 was formed with the loss of pySH ligand (Scheme 4).

Scheme 4 (a) Reaction of complex 2 with HCl and HPF₆ at room temperature; (b) other potential products for reaction of complex 2 with HCl.

The formation of these products in this condition suggested that step (b') in Scheme 2 can be inverted. Comparable product observed with complex 3, has already been published by Zucca and co-workers^48,49 in the reaction of rollover complex [Pt(bipy-H)Me(DMSO)] with HCl, which gave the analogous chloride complex [Pt(bipy-H)Cl(DMSO)] and free methane.⁴⁸ Under this condition, the protonation of the free nitrogen atom was not detected. Also, in the absence of nitrogen atom in complex 4, it reacted smoothly with HCl and rendered complex 3 and free thiophenol (PhSH).⁵⁰ This might have occurred with the protonation process via an S_E(ox) or an S_E2 mechanism^51–54 and probably uncoordinated nitrogen atom did not play an important role in the reaction.^48,49 It should be noted that these two mechanisms are not relevant on present discussion.

Other possible routes for reaction of complex 2 with HCl are represented in Scheme 4. These routes are the protonation of the uncoordinated nitrogen atom of Spy ligand for preparation of the cationic adduct complex [Pt(ppy)(η¹-S-Spy^*)(PPh₃)][Cl], 2*, or oxidation of platinum metal center in order to form Pt(IV) complex [Pt(ppy)(η²-N,S-Spy)(PPh₃) (H)][Cl], 2**, but these proposed products were not formed (see below).

In the reaction of rollover complex [Pt(bipy-H)Me(DMSO)]^48,49 with acidic species when coordinating anion (Cl⁻) was substituted with non-coordinating anion (BF₄⁻), selective protonation occurred on the free nitrogen atom and platinum center remained intact.^48,49,55 Thus, in the above protonation reaction (Scheme 4), reaction with HPF₆ containing uncoordinating PF₆⁻ anion, caused the protonation of pendant nitrogen atom. When complex 2 treated with HPF₆, complex [Pt(ppy)(η¹-S-Spy^*)(PPh₃)][PF₆], 5, was yielded in which Spy^* ligand can be envisaged as a tautomeric form of the neutral thiol ligand. Therefore, it demonstrates that the formation of complex 2* (Scheme 4) requires a non-coordinating anion.⁴⁸

Complex 5 was characterized by multinuclear (¹H and ³¹P {¹H}) NMR spectroscopy in solution. The ³¹P {¹H} NMR spectrum of complex 5 (Fig. S11†) indicated two different phosphorus resonances. The first signal appeared at δ = 22.3 ppm as a singlet resonance flanked by platinum satellites (¹J_PtP = 4156 Hz) which was assigned to PPh₃ ligand. It is well known that coupling constant is a good indicator of the kind of atom in trans position of phosphorus atom. Therefore, this large coupling constant supports a P–Pt–N_ppy trans arrangement in complex 5, indicating a good agreement with coupling constant that was found for complex 2, only less by ca. 190 Hz. This reduction can be attributed to the cis influence of the N-protonated 2-thiolate fragment which is considerably increased compared to that of the 2-thiolate ligand.^56–61 Similar behavior was previously reported for complex trans-[PtBr(C₅H₅N-C₂)(PPh₃)₂] in reaction with HClO₄.⁵⁸ The second signal at δ = −144.3 was assigned to PF₆ counter-anion which is coupled to fluorine nucleus to give a septet and was separated by 708 Hz. In the ¹H NMR spectrum, a broad signal was detected at down field (δ = 13.11 ppm) which is related to N–H proton (Fig. S12†),^48,57,58 and it disappeared immediately upon treatment with D₂O (Fig. S13†).⁵⁷ The protonation of the nitrogen atom led all protons of Spy ligand shift to down field of about 0.2–0.9 ppm, as anticipated from the existence of a positive charge.^49,57 With the reduction of trans influence of protonated thiolate ligand, coupling constant value of H⁹ was increased by ca. 8 Hz.

Templeton, et al. reported an interesting protonation reaction for the preparation of platinum–hydride complexes.⁶² In this reaction, κ² complexes [(κ²–Tt^R)PtMe₂], which include a free nitrogen, were treated with a strong acid (HBF₄) and yielded κ³ complexes [(κ³–Tt^R)PtMe₂(H)]BF₄. In these Pt–H complexes all three nitrogens were coordinated to the platinum center and produced two stable chelates. In the case of reaction of complex 2 with HCl, if complex 2** was formed as a presumptive product, the thiolate ligand in complex 2** acts as a four membered chelate (η²-N,S-Spy). The chelates formed by 2-N,S-thiolate ligands are intrinsically strained²⁰ and perhaps this structural strain prevented formation of complex 2** (Scheme 4).

Complex 5 can be obtained in another synthetic route involving treatment of complex 3 with AgPF₆ (in acetonitrile solvent) to abstract the chloride ligand from the platinum center as AgCl precipitate which is separable from solution by filtration. Subsequently, pyridine-2-thiol was added to the resulting solution to form complex 5. Moreover, addition of equimolecular amount of HCl to a solution of complex 5 rendered complex 3 (Scheme 4).

Structural determination

Single-crystal X-ray diffraction investigation was carried out on complex 4 to approve its molecular structure. Crystallographic data is collected in Table S3,† and the perspective drawing of this complex is illustrated in Fig. 3.

Fig. 3 Representations of the X-ray crystal of complex [Pt(ppy)(Sph)(PPh₃)], 4, showing all non-hydrogen atoms as 40% thermal ellipsoids. Pt1–C1 2.065(12), Pt1–N1 2.073(9), Pt1–P1 2.244(3), Pt1–S1 2.361(3), C1–Pt1–N1 81.9(4), C1–Pt1–P1 97.2(3), C1–Pt1–S1 169.5(3), N1–Pt1–P1 178.2(3), N1–Pt1–S1 90.2(3), P1–Pt1–S1 90.95(11).

Complex 4 reveals a distorted square planar coordination environment around Pt(II) center. This distortion is due to small bite angle [81.9(4)°] 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.^23,43,63,64 In this structure, the metalated carbon atom of the C^N ligand accommodates the coordinated phosphine ligand in cis arrangement of Pt–C_ppy bond, as a result of high trans influence of carbon atom.⁴⁴ Therefore, PPh₃ ligand is located trans to nitrogen atom of ppy ligand and bond distance of the Pt–P is similar to those observed in complex [Pt(ppy)(PTA)Cl]⁴² and notably shorter than platinum–phosphorous bond lengths, positioned trans to carbon atom of cyclometalated ligand.⁶³ Also, sulfur ligand completes the coordination sphere of platinum atom. The Pt–SPh distance is normal and fall in the typical range for cycloplatinated(II) complexes containing ppy moieties with those positioned trans to the σ-bound carbon atoms. The Pt–C metalated bond lengths is noticeably changed according to the trans influence of the SPh ligand occupying its trans site.²⁸

The molecular structure of complex 4 reveals that the thiolate ligand is bent toward phosphine ligand (Fig. 3). The similar orientation has already been observed in (arylthiolato) palladium complexes with thiolate ligands.⁶⁵

Additional examination on the molecular packing of the crystal structure of complex 4 displayed the presence of intramolecular interactions (see below) such as C–H⋯π interaction, π⋯π stacking, without any notable Pt⋯Pt contact. This molecular structure reveals a moderate short C–H⋯π intramolecular interaction (Fig. S14†) between the 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) = 3.312 Å). This interaction has caused the coordinated hydrogen atom of ppy ligand to be more shielded in NMR spectroscopy (Fig. 2).²⁸

In complex 4, the aromatic ring of the thiolate ligand exhibits weak intramolecular π⋯π interaction with one of the phenyl groups of the PPh₃ ligand and the distance between two centroids is 3.837 Å (Fig. S15†), which are located nearly parallel to each other (15.6(7)° angle between two aromatic rings).

Conclusions

The half-lantern platinum(II) complex 1 is a useful precursor for substitution reaction with triphenylphosphine ligand. This process proceeded via bond dissociation between platinum and nitrogen atom of thiolate bridging ligand and yielded platinum(II) complex 2, which is comprised of the free pyridyl group. Also, the transphobia effect directs the entering phosphine donor ligand to cis position of carbon atom of cyclometalating fragment, as concluded from spectroscopic data.

Complex 2 underwent protonation reaction and the type of anion played an important role in the outcome of the protonation reaction. Thus, its reaction with an acid involving a coordinating anion such as chloride (Cl⁻), proceeded through the cleavage of the Pt–Spy bond and formation of complex 3. In contrast, reaction of complex 2 with an acid containing an uncoordinating anion such as PF₆⁻, proceeded by the protonation of the pendant nitrogen atom and formation of complex 5 as a cationic species.

Experimental section

General procedures and materials

All NMR spectra (¹H and ³¹P {¹H}) were recorded on a Brucker Avance DPX 400 MHz instrument and are referenced to the residual peak of the solvent, i.e. CD₂Cl₂, CDCl₃, dmso-d₆ and acetone-d₆, 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.⁶⁶ 2-Phenylpyridine (ppyH), hexafluorophosphoric acid solution (HPF₆), silver hexafluorophosphate (AgPF₆), pyridine-2-thiol (pySH) and thiophenol (PhSH) were purchased from Aldrich or Acros. The complexes [Pt(ppy)(DMSO)(Cl)], A,⁴³ [{Pt(ppy)(μ₂-Spy)}₂], 1,^3,32 [{Pt(ppy)(μ₂-Spy)Cl}₂], 1a,³ [Pt(ppy)(PPh₃)Cl], 3,^41,42 were prepared as reported in literature. The NMR labeling are shown in Fig. 2 for clarifying the chemical shift assignments. Additional data for complex 3: ¹H NMR (400 MHz, dmso-d₆, 20 °C, δ): 9.67 (m, ³J_PtH = not resolved, 1H, H²), 8.20–8.13 (m, 2H, H⁴, H⁵), 7.75 (dd, ³J_HH = 7.9, ⁴J_HH = 0.9, 1H, H⁶), 7.71–7.66 (m, 6H, H^o of PPh₃), 7.59–7.55 (m, 1H, H³), 7.52–7.43 (m, 9H, H^p and H^m of PPh₃), 6.95 (td, ³J_HH = 8.0, ⁴J_HH = 1.2, 1H, H⁷), 6.59–6.46 (m, 2H, H⁸, H⁹). ³¹P {¹H} NMR (162 MHz, dmso-d₆, 20 °C, δ): 22.8 (s, ¹J_PtP = 4341, 1P). ¹H NMR (400 MHz, CD₂Cl₂, 20 °C, δ): 9.82 (m, ³J_PtH = not resolved, 1H, H²), 7.93 (td, ³J_HH = 8.1, ⁴J_HH = 1.6, 1H, H⁴) 7.85 (d, ³J_HH = 8.1, 1H, H⁵), 7.82–7.76 (m, 6H, H^o of PPh₃), 7.56 (dd, ³J_HH = 7.8, ⁴J_HH = 1.3, 1H, H⁶), 7.49–7.32 (m, 10H, H³, H^p and H^m of PPh₃), 6.96 (td, ³J_HH = 7.8, ⁴J_HH = 1.1, 1H, H⁷), 6.66 (dd, ³J_PtH = 53.6, ³J_HH = 7.8, ⁴J_PH = 2.2, 1H, H⁹), 6.51 (td, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, CD₂Cl₂, 20 °C, δ): 23.358 (s, ¹J_PtP = 4338, 1P).

[Pt(ppy)(η¹-S-Spy)(PPh₃)], 2

Method A. A red solution of complex 1 (100 mg, 0.11 mmol) in acetone (15 mL) was treated with PPh₃ (57.1 mg, 0.22 mmol, 2 equiv.) at room temperature. A lemon suspension was formed immediately and then the mixture was stirred for 6 h. The resulting yellow solid was filtered, washed with cold acetone (2 × 2 mL) and dried under vacuum. Yield: 108 mg, 69%; mp 214 °C. MS ESI(+): m/z (%) 611.33 [M − Spy]⁺ (94); 722.15 [M + H]⁺ (100); 1332.27 [M₂ − Spy]⁺ (53). Elem. anal. calcd for C₃₄H₂₇N₂PPtS (721.13): C, 56.58; H, 3.77; N, 3.88; found: C, 56.34; H, 3.71; N, 3.86. ¹H NMR (400 MHz, dmso-d₆, 20 °C, δ): 9.72 (m, ³J_PtH = not resolved, 1H, H²), 8.17 (d, ³J_HH = 8.0, 1H, H⁵), 8.08 (td, ³J_HH = 8.0, ⁴J_HH = 1.3.1H, H⁴), 7.88 (d, ³J_HH = 4.6, 1H, H^6′), 7.81 (d, ³J_HH = 7.7, 1H, H⁶), 7.70–7.65 (m, 6H, H^o of PPh₃), 7.44–7.32 (m, 10H, H³, H^p and H^m of PPh₃), 6.99–6.91 (m, 3H, H⁷, H^3′, H^4′), 6.65 (dd, ³J_PtH = not resolved, ³J_HH = 7.8, ⁴J_PH = 2.4, 1H, H⁹), 6.59–6.52 (m, 2H, H⁸, H^5′). ³¹P {¹H} NMR (162 MHz, dmso-d₆, 20 °C, δ): 22.8 (s, ¹J_PtP = 4337, 1P). ¹H NMR (400 MHz, CD₂Cl₂, 20 °C, δ): 9.91 (m, ³J_PtH = 30.5, 1H, H²), 7.96 (ddd, ³J_HH = 4.9, ⁴J_HH = 1.9, ⁵J_HH = 0.8, 1H, H^6′), 7.90–7.83 (m, 2H, H⁴, H⁵) 7.77–7.72 (m, 6H, H^o of PPh₃), 7.63 (dd, ³J_HH = 7.9, ⁴J_HH = 1.2, 1H, H⁶), 7.40–7.36 (m, 3H, H^p of PPh₃), 7.31–7.27 (m, 6H, H^m of PPh₃), 7.19 (m, 1H, H³), 7.10 (dt, ³J_HH = 8.1, ⁴J_HH + ⁵J_PH = 1.0, 1H, H^3′), 7.00 (td, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁷), 6.88 (ddd, ³J_HH = 6.1, ⁴J_HH = 2.0, ⁵J_HH = 0.8, 1H, H^4′), 6.76 (ddd, ³J_PtH = 41.9, ³J_HH = 7.7, ⁴J_HH = 0.9, ⁴J_PH = 2.3, 1H, H⁹), 6.59 (td, ³J_HH = 7.7, ⁴J_HH = 1.3, 1H, H⁸), 6.55 (ddd, ³J_HH = 6.1, ⁴J_HH = 2.1, ⁵J_HH = 0.8, 1H, H^5′). ³¹P {¹H} NMR (162 MHz, CD₂Cl₂, 20 °C, δ): 22.7 (s, ¹J_PtP = 4335, 1P). ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 10.04 (m, ³J_PtH = 30.7, 1H, H²), 8.12–8.05 (m, 2H, H⁴, H⁵), 7.90 (ddd, ³J_HH = 4.8, ⁴J_HH = 1.9, ⁵J_HH = 0.8, 1H, H^6′), 7.81–7.76 (m, 7H, H⁶, H^o of PPh₃), 7.45–7.30 (m, 10H, H³, H^p and H^m of PPh₃), 7.00–6.85 (m, 3H, H⁷, H^3′, H^4′), 6.79 (dd, ³J_PtH = 42.1, ³J_HH = 7.8, ⁴J_PH = 2.3, 1H, H⁹), 6.56–6.51 (m, 2H, H⁸, H^5′). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): 23.1 (s, ¹J_PtP = 4340, 1P).

Method B. Complex 3 (100 mg, 0.15 mmol) was added to an ethanolic solution of sodium pyridine-2-thiolate ligand (NaC₅H₄NS) under inert atmospheric condition [NaC₅H₄NS prepared by dissolving of sodium (4.6 mg, 0.20 mmol) in 10 mL of absolute ethanol and was treated with pyridine-2-thiol (16.8 mg, 0.15 mmol)]. Yellow solution was formed and after stirring for 4 h at room temperature a yellow solid precipitated which was separated, washed with ethanol (3 × 2 mL) and cold acetone (2 × 2 mL) and dried to yield complex 2 (61 mg, 56%).

[Pt(ppy)(SPh)(PPh₃)], 4

Complex 3 (100 mg, 0.15 mmol) was added to an ethanolic solution of sodium phenylthiolate ligand (NaC₆H₅S) under inert atmospheric condition [NaC₆H₅S prepared by dissolving of sodium (4.6 mg, 0.20 mmol) in 10 mL of absolute ethanol and is treated with thiophenol (15.3 μL, 0.15 mmol)]. Bright orange solution was formed and after stirring for 5 hours at room temperature an orange solid precipitated which was separated, washed with ethanol (3 × 2 mL) and cold acetone (2 × 2 mL) and dried to obtain complex 4. Yield: 57 mg, 53%; mp 210 °C. Elem. anal. calcd for C₃₅H₂₈NPPtS (720.14): C, 58.32; H, 3.92; N, 1.94; found: C, 58.16; H, 3.87; N, 1.98. ¹H NMR (400 MHz, dmso-d₆, 20 °C, δ): 9.81 (m, ³J_PtH = not resolved, 1H, H²), 8.14 (d, ³J_HH = 8.2, 1H, H⁵), 8.05 (td, ³J_HH = 8.2, ⁴J_HH = 1.4, 1H, H⁴), 7.80 (d, ³J_HH = 7.8, 1H, H⁶), 7.67–7.62 (m, 6H, H^o of PPh₃), 7.46–7.34 (m, 10H, H³, H^p and H^m of PPh₃), 7.15 (dd, ³J_HH = 8.3, ⁴J_HH = 1.2, 2H, H^2′), 6.98 (td, ³J_HH = 7.8, ⁴J_HH = 1.1, 1H, H⁷), 6.75 (t, ³J_HH = 7.9, 2H, H^3′), 6.68–6.65 (m, 2H, H⁹, H^4′), 6.57 (td, ³J_HH = 7.8, ⁴J_HH = 1.1, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, dmso-d₆, 20 °C, δ): 23.5 (s, ¹J_PtP = 4295, 1P). ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): 10.05 (dddd, ³J_PtH = 31.2, ³J_HH = 5.7, ⁴J_HH = 1.6, ⁵J_HH = 0.7, ⁴J_PH = 3.9, 1H, H²), 8.11–8.03 (m, 2H, H⁴, H⁵), 7.81–7.73 (m, 7H, H⁶, H^o of PPh₃), 7.44 (tdd, ³J_HH = 7.4, ⁴J_HH = 1.4, ⁵J_PH = 2.0, 3H, H^p of PPh₃), 7.37–7.31 (m, 7H, H³, H^m of PPh₃), 7.23 (ddd, ³J_HH = 8.3, ⁴J_HH = 1.1, ⁵J_PH = 3.4, 2H, H^2′), 6.99 (td, ³J_HH = 7.8, ⁴J_HH = 1.2, 1H, H⁷), 6.81 (ddd, ³J_PtH = 42.7,³J_HH = 7.9, ⁴J_HH = 0.9, ⁴J_PH = 3.2, 1H, H⁹), 6.72 (t, ³J_HH = 7.8, 2H, H^3′), 6.66 (td, ³J_HH = 7.7, ⁴J_HH = 1.2, 1H, H^4′), 6.57 (td, ³J_HH = 7.7, ⁴J_HH = 1.2, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): 23.9 (s, ¹J_PtP = 4301, 1P). ¹H NMR (400 MHz, CD₂Cl₂, 20 °C, δ): 9.96 (m, ³J_PtH = 31.1, 1H, H²), 7.89–7.81 (m, 2H, H⁴, H⁵), 7.75–7.71 (m, 6H, H^o of PPh₃), 7.63 (dd, ³J_HH = 7.8, ⁴J_HH = 1.1, 1H, H⁶), 7.42–7.38 (m, 3H, H^p of PPh₃), 7.34–7.25 (m, 8H, H^2′ and H^m of PPh₃), 7.16–7.13 (m, 1H, H^4′), 7.01(td, ³J_HH = 7.4, ⁴J_HH = 1.1, 1H, H³), 6.83–6.69 (m, 4H, H⁷, H⁹, H^3′), 6.60 (td, ³J_HH = 7.7, ⁴J_HH = 1.4, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, CD₂Cl₂, 20 °C, δ): 23.4 (s, ¹J_PtP = 4304, 1P).

[Pt(ppy)(η¹-S-Spy^*)(PPh₃)][PF₆], 5

Method A. To a yellow suspension of complex 2 (100 mg, 0.14 mmol) in acetone (10 mL) was added dropwise an excess of HPF₆ (68.1 μL, 1.4 mmol, 10 equiv., ∼55 wt. % in H₂O) at room temperature. The suspension became quickly red and the reaction mixture was stirred for 1 h under Ar inert atmosphere. The solvent was removed under reduced pressure and concentrated to a small volume (∼1 mL) and Et₂O (3 mL) was added and gave complex 5 as a light brown solid and dried under vacuum. Yield: 86 mg, 71%. Elem. anal. calcd for C₃₄H₂₈F₆N₂P₂PtS (867.10): C, 47.05; H, 3.25; N, 3.23; found: C, 47.61; H, 3.34; N, 3.41. ¹H NMR (400 MHz, acetone-d₆, 20 °C, δ): δ 13.11 (br. s, 1H, NH of Spy^*), 9.61 (ddd, ³J_PtH = 30.4, ³J_HH = 5.9, ⁴J_HH = 1.1, ⁴J_PH = 4.3, 1H, H²), 8.28 (d, ³J_HH = 8.0, 1H, H⁵), 8.23 (td, ³J_HH = 8.0, ⁴J_HH = 1.4, 1H, H⁴), 8.04 (tdd, ³J_HH = 5.7, ⁴J_HH = 1.8, ⁵J_HH = 0.7, 1H, H^6′), 7.89–7.76 (m, 9H, H³, H⁶, H^3′, H^o of PPh₃), 7.54–7.50 (m, 4H, H^4′, H^p of PPh₃), 7.43 (td, ³J_HH = 7.7, ⁴J_PH = 2.3, 6H, H^m of PPh₃), 7.27 (td, ³J_HH = 6.1, ⁴J_HH = 1.2, 1H, H^5′), 7.08 (td, ³J_HH = 7.8, ⁴J_HH = 1.0, 1H, H⁷), 6.76 (ddd, ³J_PtH = 50.1,³J_HH = 7.8, ⁴J_HH = 0.7, ⁴J_PH = 3.6, 1H, H⁹), 6.61 (td, ³J_HH = 7.8, ⁴J_HH = 1.4, 1H, H⁸). ³¹P {¹H} NMR (162 MHz, acetone-d₆, 20 °C, δ): 22.3 (s, ¹J_PtP = 4156, 1P, P of PPh₃), −144.3 (septet, ¹J_PF = 708, 1P, P of PF₆).

Method B. To a solution of complex 3 (100 mg, 0.15 mmol) in acetonitrile (10 mL) was added AgPF₆ (51 mg, 0.20 mmol). The reaction mixture was stirred for 3 h in dark at room temperature, and then filtered through Celite to remove AgCl. The green filtrate was treated with pyridine-2-thiol (17 mg, 0.15 mmol) in acetonitrile (5 mL). The resulting red solution was stirred for 5 h and then the solvent was removed under reduced pressure and concentrated to a small volume (∼1 mL) and Et₂O (3 mL) was added to obtain complex 5.

Reaction of complexes 2 and 5 with HCl

To a solution of complexes 2 or 5 (0.14 mmol) in acetone (20 mL) with vigorous stirring was added HCl (0.14 mmol, 1 mL of aqueous 0.05 M) and after 3 h the solution was concentrated to a small volume (5 mL). The yellow solution was extracted with CH₂Cl₂ (10 mL) and dried with MgSO₄, and concentrated to a small volume (1 mL). Then, n-hexane (10 mL) was added to precipitate complex 3 (confirmed by NMR spectroscopy) as a yellow solid and dried under vacuum.

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 5090 for complex 4. Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the Stoe X-AREA⁶⁷ software package. A numerical absorption correction was applied using X-RED⁶⁸ and X-SHAAPE⁶⁹ software. The data were corrected for Lorentz and polarizing effects. The structures were solved by direct methods⁷⁰ and subsequent difference Fourier maps and then refined on F² by a full-matrix least-squares procedure using anisotropic displacement parameters.⁷¹ Atomic factors are from the International Tables for X-ray Crystallography.⁷² 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). All refinements were performed using the X-STEP32, SHELXL-2014 and WinGX-2013.3 programs.^73–80 Also, the suitable crystals for complex 4 were obtained from acetone/hexane solution.

Theoretical methods. Gaussian 09 was used⁸¹ to fully optimize the structure of complex 2 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.⁸² The 6-31G(d) basis set was used for other atoms. Frequency calculations were carried out at the same level of theory to identify whether the calculated stationary point is a minimum (zero imaginary frequency) or a transition structure (one imaginary frequency).

Acknowledgements

This work was supported by the Institute for Advanced Studies in Basic Sciences (IASBS) Research Council and the Iran National Science Foundation (Grant no. 93026027). Technical support of the Chemistry Computational Center at Shahid Beheshti University is gratefully acknowledged. Thanks are also due to 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. El5ena Lalinde, Universidad de La Rioja, for ESI-MS measurements. This paper is dedicated to Professor S. Masoud Nabavizadeh on the occasion of his 45th birthday.

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Footnote

† Electronic supplementary information (ESI) available: Full NMR spectra, crystallographic and computational details. CCDC 1457811. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra15604e

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