An efficient method for selective oxidation of (oxime)Pt(II) to (oxime)Pt(IV) species using N,N-dichlorotosylamide

The oxidation of (oxime)PtII species using the electrophilic chlorine-based oxidant N,N-dichlorotosylamide (4-CH3C6H4SO2NCl2) was studied. The reactions of trans-[PtCl2(oxime)2] (where oxime = acetoxime, cyclopentanone oxime, or acetaldoxime) with this oxidant led to trans-[PtCl4(oxime)2] products. The oxidation of trans-[Pt(o-OC6H4CH = NOH)2] at room temperature gave trans-[PtCl2(o-OC6H4CH = NOH)2], whereas the same reaction upon heating was accompanied by electrophilic substitution of the benzene rings.


Introduction
The oxidations of Pt II species with a variety of electrophilic chlorinating reagents have been reported.The oxidative chlorination of platinum(II) species by molecular chlorine is a common method of synthesis of platinum(IV) complexes [1].The reactions with Cl2 proceed via oxidative addition to the metal center with formation of the appropriate Pt IV complexes.In particular, the platinum(II) complexes [PtCl2(RR'CNOH)2] are converted into [PtCl4(RR'CNOH)2] by treatment with Cl2.The oxime ligands usually remain intact; however, in some cases, the ligands also react with molecular chlorine, and, e.g., passage of Cl2 through a chloroform solution of [Pt(o-OC6H4CH=NOH)2] resulted in both the oxidative addition of chlorine to the platinum(II) center and chlorination of the benzene ring (Scheme 1) [2].However, like chlorine, nitrosyl chloride is a highly toxic gas and this property restricts the usage of these reagents.While the reactions of (oxime)Pt II species with other electrophilic chlorinating reagents have not been previously reported, related transformations of Pt II to Pt IV (or Pt III ) with various chlorinating oxidants have been documented.The oxidations of Pt II complexes with iodobenzene dichloride (PhICl2) [3][4][5], N-chlorosuccinimide (NCS) [5,6], N,N-dichlorobenzenesulphonamide (PhSO2NCl2) [7], PCl5 [8], and SbCl5 [9] have been described.
In this study, we used N,N-dichlorotosylamide as a chlorinating agent for the selective chlorination of [PtCl2(oxime)2] complexes.N,N-Dichlorotosylamide is a relatively active solid reagent, which can be used as a stoichiometric solid chlorine equivalent providing selective oxidation of Pt(II) centers.This reagent is more convenient to use than chlorine and also more stable at room temperature than hypervalent iodine reagents.

Materials and Instrumentation
All reagents and solvents were commercially available and used as received without further purification.N,N-Dichlorotosylamide (dichloramine T) was purchased from Sigma-Aldrich.FTIR spectra were recorded on Shimadzu FTIR-8400S (4000 -400 cm -1 ) and IRAffinity-1S (4000 -300 cm -1 ) spectrometers using KBr pellets. 1 H NMR measurements were performed on a Bruker-DPX 400 instrument at ambient temperature.Electrospray ionization mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with electrospray ionization (ESI) source using MeOH as the solvent.The instrument was operated in both positive and negative ion modes using a m/z range of 50 -3000.The capillary voltage of the ion source was set at -4500 V (ESI + -MS) or 3500 V (ESI --MS) and the capillary exit at ±(70 -150) V.The nebulizer gas flow was 0.4 bar and drying gas flow 4.0 L/min.In the isotopic patterns, the most intense peak is reported.

Synthesis of [PtCl2(o-OC6H4CH=NOH
)2] (8).A solution of complex 4 (0.021 g, 0.045 mmol) in chloroform (5 mL) was treated with N,N-dichlorotosylamide (0.010 g, 0.045 mmol) and the mixture left overnight at ambient temperature.The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel (60 Å; Merck) with chloroform as the eluent to give pure complex 8 as a reddish-brown solid (Rf 0.62).Yield: 60 %.

X-ray crystal structure determinations
Crystals of complexes 1, 6, and 7 were immersed in cryo-oil, mounted in a nylon loop, and analysed at a temperature of 100 K.The X-ray diffraction data were collected on a Bruker Axs Smart Apex II or Nonius KappaCCD diffractometer using Mo Ka radiation (l = 0.710 73 Å).The Apex2 [14] or Denzo-Scalepack [15] program packages were used for cell refinements and data reductions.The structures were solved by direct methods using SHELXS-97 [16] with the WinGX [17] graphical user interface.A semi-empirical absorption correction (SADABS) [18] was applied to all data.Structural refinements were carried out using SHELXL-97 or Bruker SHELXTL [16].In all structures, the OH hydrogen atoms were located from the difference Fourier maps, but constrained to ride on their parent oxygen with Uiso = 1.5×Ueq (parent atom).Other hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H = 0.95-0.99Å and Uiso = 1.2-1.5 Ueq(parent atom).The crystallographic details are summarized in Table 1.

Results and discussion
The reactions of (oxime)Pt II complexes with N,N-dichlorotosylamide were studied.N,N-Dichlorotosylamide was used as a solid source of "positive chlorine".To the best of our knowledge, this is the first example of use of N,Ndichlorotosylamide for selective oxidation of platinum(II) species; the only precedent being the application of N,Ndichlorobenzenesulfonamide for the oxidative chlorination of trans-[PtCl2(HN=C(OMe)Et)2] [7].It is known that N,Ndichlorotosylamide is relatively reactive, but can nevertheless be easily handled [19].In order to study the reactivity of N,N-dichlorotosylamide in the oxidative chlorination of (oxime)Pt II complexes, we chose a series of coordination compounds with different oxime ligands, i.e. aldoximes (acetaldoxime and salicylaldoxime) and ketoximes (acetoxime and cyclopentanone oxime).The (oxime)Pt II complexes were synthesized by the previously published reaction between K2[PtCl4] and oximes in a 1:2 molar ratio in water.The prepared compounds were characterized by elemental analyses, HRMS (ESI), IR and 1 H NMR spectroscopies; complex 1 was also characterized by X-ray single-crystal diffraction (Fig. 1).
In complex 1, each Pt atom has a centrosymmetric square-planar environment provided by two acetoxime N and two Cl atoms (Pt-N 2.008(2) Å, Pt-Cl 2.3019(6) Å, N-Pt-Cl 89.91(7)°) (Table 2).The N(1)-C(2) bond length in 1 .Thus, only the oxidative chlorination of the platinum center is observed and the oxime ligands remain unchanged.All our attempts to grow crystals of complex 5 suitable for single-crystal X-ray diffraction failed.However, crystals of good quality were obtained for complexes 6 and 7 by slow crystallization from chloroform.
In complex 6, the platinum has a slightly distorted octahedral geometry with two axial acetaldoxime ligands and four chloride ligands (Fig. 3).The values of the Pt-Cl bond lengths agree well with those of previously   of Pt(II) to Pt(IV) was observed and the salicylaldoximato ligands remained intact (Scheme 2).Several attempts were made to obtain suitable crystals of complex 8 for X-ray diffraction studies.Unfortunately, crystals of sufficient quality could not be obtained.The reaction product was characterized by HRMS (ESI) and elemental analyses.Also, the infrared spectrum of this compound included a Pt-Cl band at 346 cm -1 .The 1 H NMR spectrum confirmed that electrophilic substitution in the benzene ring does not proceed.

Conclusions
In conclusion, the chlorination of (oxime)Pt II (where oxime = aldoxime, cyclic or acyclic ketoxime) with N,Ndichlorotosylamide leads to selective oxidation of the Pt II center, whereas the oxime ligands remain intact.The oxime ligands, which contain benzene rings activated to electrophilic substitution, do not react with TsNCl2 under mild conditions.Furthermore, N,N-dichlorotosylamide can find a wide synthetic application in selective chlorination of the metal centers in the complexes, where the ligands may be easily subjected to electrophilic substitution processes.

Table 1
Crystal Data.