Synthesis, characterization and crystal structures of platinum(II) saccharinate complexes with 1,5-cyclooctadiene

Two new platinum(II) complexes, namely [PtCl(sac)(COD)] (1) and [Pt(sac)2(COD)] (2) (sac = saccharinate and COD = 1,5-cyclooctadiene), were synthesized and characterized by elemental analysis, IR, NMR, ESI-MS spectroscopic and thermal analysis (TG/DTA) methods. The platinum(II) complexes were prepared from the reaction of [PtCl2(COD)] with Na(sac)•2H2O. The addition of the sac ligand resulted in the replacement of 1 and 2 chlorido ligands in [PtCl2(COD)] to yield 1 and 2, respectively. The structures of the complexes were determined by single crystal X-ray diffraction and showed a distorted square planar coordination geometry around platinum(II). COD acted as a π-donor ligand, while sac was N -coordinated in both complexes. The TG/DTA data indicated that both complexes were thermally stable up to 220 °C in air and their thermal decompositions yielded Pt as a final product. Complexes 1 and 2 were also designed as possible precursors to synthesize new mixed-ligand platinum(II) sac complexes in a one-pot reaction.


Introduction
After the discovery of the cell growth inhibition effect of cisplatin by Rosenberg [1][2][3], the design and synthesis of new platinum(II) complexes received increasing attention. For this purpose, several synthetic routes were used. One way is to synthesize precursors to prepare new platinum(II) complexes. The reaction of a simple platinum(II) complex (K 2 [PtCl 4 ]) with the mono-, di-or tridentate ligands usually leads to the formation of chloridoplatinum(II) complexes bearing these ligands. In a subsequent reaction, the chlorido ligands in the precursor can be exchanged by another ligand to yield the desired platinum(II) complex. Another alternative way is to use [PtCl 2 (COD)] (COD =1,5-cyclooctadiene), which serves as an excellent starting complex for a wide range of organoplatinum(II) compounds [4]. The chlorido ligands in [PtCl 2 (COD)] can be replaced by other higher halides (Br − or I − ) or anionic organic ligands due to the high π trans effect of COD [5].
In this study, the synthesis, characterization and crystal structures of 1 and 2 were described. Both complexes can also be used as precursors for the preparation of new platinum(II) sac complexes. The COD ligand in 1 and 2 can further be exchanged by σ -donor neutral ligands to give novel mixed-ligand chlorido platinum(II) sac or bis(sac)platinum(II) complexes. The synthetic method seems to be an attractive alternative way to synthesize new platinum(II) sac complexes, because it may be regarded as a one-pot reaction with high yields. However, the other procedures include multistep synthesis and usually result in low yields. The 1 H NMR spectra in Figure 1 display the phenyl protons of sac in the range of 7.94-7.73 ppm. The 2 =CH protons of COD in 1 appear as 2 signals at 5.84 and 5.08 ppm, while those in 2 occur at 5.52 ppm. The notable difference between the 2 olefinic chemical shifts in 1 is due to the trans influence of the chlorido and sac ligands. On the other hand, a COD ligand together with 2 sac ligands in 2 results in only 1 signal for the 2 equivalent =CH protons. The splitting of these signals is evident because of coupling with the Pt nucleus. The 2 J P t,H (=CH, COD) coupling constants are found as 36 and 64 Hz for 1 and consistent with the somewhat higher trans influence of sac [15,29]. The corresponding coupling constant is approximately 40 Hz for 2. In addition, the signals between 2.78 and 2.08 ppm are assigned to the methylene protons of COD. The

Structures of the platinum(II) complexes
Slow evaporation of the saturated solutions of 1 and 2 at rt afforded white prisms. Complexes 1 and 2 crystallize in the monoclinic space group P 2 1 / c and orthorhombic space group Pna2 1 , respectively. The molecular structures of both complexes were given in Figure 2, while selected bond lengths and angles are given in Table 1. In both complexes, the platinum(II) ion shows a distorted square planar coordination geometry defined by the mid-points of the 2 π -coordinated double bonds of COD, 1 chloride and 1 N -bonded sac anion in 1, and 2 N -coordinated sac ligands in 2 ( Figure 2). In 1 and 2, the COD ligand coordinates to the platinum(II) ion in η 4 -type coordination and in the boat conformation with the double-bond lengths of ca. 1.38 Å. The Pt-C bond distances lie within the typical range for the platinum(II) complexes of COD [17,30,31]. However, 2 sets of the Pt-C bonds were observed in 1, most likely due to the different trans influences of the chlorido and sac ligands. In addition, 1 of the 4 Pt-C bonds in 2 is notably shorter than the rest and consequently, the molecule retains a pseudo C 2 symmetry in the solid state. This is consistent with the similar observations reported for

Thermal analysis
The thermal stability and decomposition behaviour of the complexes were studied by TG and DTA in air. As shown in Figure 3, complexes 1 and 2 are thermally stable up to ca. 220°C. 1 decomposes in 2 steps, which do not correspond to the removal of any species. Its DTA curve presents a small endothermic and a highly exothermic effects centred at 250 and 340°C, respectively. The latter is likely to due to combustion of the sac moiety. The decomposition of this complex ends at 360°C to give metallic Pt as a residue (found: 36.2% and calcd: 37.5%). On the other hand, complex 2 also exhibits a 2-stage exothermic decomposition centred at 253 and 331°C in the DTA curve. The decomposition of 2 also yields metallic Pt as an end product (found: 30.6% and calcd: 29.2%) at 390°C. a X1 and X2 refer to the centroids in the middle of olefinic bonds C1-C2, and C5-C6, respectively.

Conclusions
Two new platinum(II) complexes containing COD and sac ligands were successfully synthesized via the progres- (2) has approximate C 2 symmetry, which results in a signal for the olefinic proton and carbon atoms in 1 H and stability in air. Finally, both complexes can be considered as precursors for the synthesis of new mixed-ligand platinum(II) sac in a one-pot reaction, in which COD in the complexes is readily replaced by the other ligands.

Materials and measurements
[PtCl 2 (COD)] was prepared according to the reported methods [4]. Elemental analyses for C, H, and N were performed using a Costech elemental analyser. FTIR spectra were recorded on a Perkin Elmer Spectrum Two FT-IR spectrophotometer in the frequency range 4000-400 cm −1 . NMR spectra were obtained using a Bruker Avance III (400.13 MHz 1 H and 100.62 MHz 13 C) NMR spectrometer in DMSOd 6 . ESI-MS spectra were recorded using a Bruker Daltonics Microtof II-ESI-TOF mass spectrometer. Thermal analysis curves (TG and DTA) were obtained from a Seiko Exstar TG/DTA 6200 thermal analyser in a flowing atmosphere of air with a heating rate of 10 K min −1 using a sample size of ca. 10 mg and platinum crucibles. The electrical conductivity measurements of the complexes in MeOH were carried out with HANNA HI 5521 at rt. Melting points are measured using a BUCHI 560 instrument.

Synthesis of [PtCl(sac)(COD)] (1)
A solution of [PtCl 2 (COD)] (0.25 mmol, 94 mg) in CH 2 Cl 2 (10 mL) was added to a solution of Na(sac)•2H 2 O (0.50 mmol, 121 mg) in MeOH (20 mL). The resulting solution was refluxed at 70°C for 24 h and then the solvent was removed using a rotary evaporator to yield a white powder. The powder was washed with water twice and dried in air. Single crystals of the complex were obtained by slow crystallization from a mixture of

Synthesis of [Pt(sac) 2 (COD)] (2)
The complex was prepared using a similar method explained for 1.

X-ray crystallography
The intensity data of the platinum(II) complexes were collected on a Rigaku Xcalibur X-ray diffractometer with EOS CCD detector using Mo-Kα radiation (0.71073 Å) with ω -scan mode. The structures were solved by direct methods using SHELXT [34], and refined by a full-matrix least-squares minimization with SHELXL [35], using Olex2 [36]. All nonhydrogen atoms were refined anisotropically, while all hydrogen atoms were located at calculated positions and refined by using riding model. Crystals of 2 displayed twinning and therefore, its structure was refined as a 2-component twin with a transformation matrix of [-1 0 0 / 0 1 0 / 0 0 -1]. Using an HKLF4 refinement in SHELXL, the fractional contribution of the major twin component was refined to 0.54 (4). Details of the data collection and structure refinement are given in Table 2.

Supplementary data
Crystallographic data and refinement parameters of 1 and 2 have been deposited at the Cambridge Crystallographic Data Centre with CCDC numbers 1981760 and 1981761, respectively. These data can be obtained