Synthesis and structural characterization of a new dinuclear platinum(III) complex, [Pt2Cl4(NH3)2{μ-HN=C(O)But}2]

The synthesis and the structural characterization of a new dinuclear platinum(III) complex are presented. Lantern-shaped platinum(III) complexes have been shown to have antitumor activity, so this new complex should be screened through antitumor screening tests.


Introduction
The interest in platinum complexes arises from their anticancer and catalytic activities for the oxidation of olefins. The discovery of the anticancer properties of cisplatin and its clinical introduction in the 1970s represent an important milestone in the history of effective anticancer drugs. Cisplatin, cis-diamminedichloroplatinum(II), was discovered by Rosenberg et al. (1965) during their experiments on the effect of electric fields on the cell division of bacteria. Cisplatin plays a significant role in the treatment of epithelial malignancies and in the cure of testicular cancer. It is used also as a principal component for the treatment of ovarian, head-andneck, esophagus, stomach, colon, bladder, cervix and uterus cancers and as second-line treatment against most other advanced cancers including breast, pancreas, liver, kidney and prostate cancers. The primary cellular target of cisplatin is DNA, and the antitumor effects of platinum complexes are expected to arise from their ability to form several adducts with DNA, blocking its replication and transcription, and inducing cell death. The antitumor properties of cisplatin are attributed to the kinetics of the aquation reactions of its chloride ligands leading to DNA crosslinking activities. This process is responsible of DNA bending, interferes with DNA replication, transcription and other nuclear functions, and blocks cancer cell proliferation and tumor growth (Boulikas et al., 2007). However, its pharmacological potential is limited by its side effects (nephrotoxicity, neurotoxicity, hair loss), and the resistance of many cancer types (O'Dwyer et al., 1999;Von Hoff et al., 1979). In the past 45 years over 3000 platinum complexes have been synthesized in the effort of overcoming cisplatin limitations, but not more than 30 of them exhibited adequate pharmacological advantages over cisplatin. Only five of these latter complexes have been used in clinical applications, three FDA-approved (cisplatin, carboplatin and oxaliplatin), one used in Japan (nedaplatin) and one in China (lobaplatin). For a long time, the cisplatin geometry has been retained as a necessary requisite for a platinum complex to exhibit antitumoral activity (Kelland, 2007). Indeed, the transgeometry of the platin complex and other transplatin analogs do not exhibit antitumor activity because of their kinetic instability which renders the complex unable to bind the target in the active form (Cornacchia et al., 2009). More recently, however, several researchers have reported that the transisomer can gain cytotoxicity comparable to that of the cisisomer and even of cisplatin (Farrell et al., 1992;Natile & Coluccia, 2001;Montero et al., 1999;Kasparkova et al., 2003a,b).
On the other hand, the number of dinuclear Pt III complexes containing a metal-metal single bond is steadily increasing as well (O'Halloran & Lippard, 1985;Matsumoto & Sakai, 1999;Gonzá lez et al., 2000;Saeki et al., 2003). Dinuclear Pt III complexes are interesting for their potential use in catalysis and biomedicine, and their chemistry and reactivity has not yet been explored to the same extent as for Pt II and Pt IV species. The majority of Pt III complexes include a metal-metal single bond supported by two (Matsumoto & Sakai, 1999;Chen & Matsumoto, 2003;Lippert, 1999) or four (Fedotova et al., 1997;Dolmella et al., 2002;Bandoli et al., 2003) bridging ligands ( Fig. 1), namely 'platinum blues'-derived and 'lantern-type', respectively. Dinuclear platinum complexes with three bridging ligands ( Fig. 1) (Abe et al., 1991) or unsupported by covalent bridges (Matsumoto et al., 1996;Lippert et al., 1983) are rare. The chelating chain usually consists of three atoms of the type OXO (X = C,S,P), NCO, NCS, SCS or PXP (X = C,O) resulting in an overall five-membered ring including the platinum-platinum interaction (Cornacchia et al., 2009;Goodgame et al., 1986;Umakoshi et al., 1987;Cotton et al., 1997;Bellitto et al., 1983;Usó n et al., 1994). In addition to the bridging ligands, dinuclear Pt III complexes also have equatorial and axial ligands (Matsumoto, 2003). A characteristic feature of dinuclear Pt III complexes is an unusually long bond distance between the platinum and the axial ligands, which is 10% longer than the corresponding distances in square-planar Pt II and octahedral Pt IV coordinations (O'Dwyer et al., 1999). Therefore, axial ligands are weaker bound, due to the strong trans labialization influence exerted by the intermetallic bond (Kuo et al., 2007;Hartmann & Lipp, 2003;Dolmella et al., 2002), and this characteristic feature may increase the reactivity toward the guanine bases of the DNA. The two-bridge Pt III complexes are characterized by a tilting of the two platinum coordination planes of 25 and a twist about the platinum-platinum vector averaging a torsion angle of 25 (O'Halloran & Lippard, 1985). These distortions are due to the steric interactions between the non-bridging equatorial ligands.
In recent decades, the chemistry of dinuclear platinum complexes has attracted an increasing interest (O'Halloran & Lippard, 1985;Matsumoto & Sakai, 1999), because it has been demonstrated that some of these dinuclear Pt III complexes exhibit antitumor (Cervantes et al., 1997) and catalytic activities for the oxidation of olefins (Matsumoto & Sakai, 1999;Saeki et al., 2003;Ochiai et al., 2004). From this perspective, our research efforts have focused on the synthesis and the structural determination of a new head-to-tail (HT) dinuclear Pt III complex ( Fig. 1 . The complex has been characterized by means of NMR and IR spectroscopies, while the structure was determined using X-ray single-crystal diffraction.

Synthesis
(i) K 2 PtCl 4 (2.1641 g, 5.21 mmol, M r = 415.06 g mol À1 ) and KI (4.3874 g, 26.4 mmol, M r = 165.99 g mol À1 ) were dissolved in water (15 ml). The solution was stirred at 55 C until a brown suspension consistent with the formation of the tetraiodo salt, having formula K 2 PtI 4 , was observed. To this aqueous dispersion of potassium salt, an aqueous solution of NH 4 OH was added dropwise and the pH was controlled to not exceed 7.5 during the addition. The obtained solution was warmed at 55 C under stirring for 20 min to allow the formation of a yellow precipitate of cis-[PtI 2 (NH 3 ) 2 ].
(ii) cis-[PtI 2 (NH 3 ) 2 ] (1.5229 g, 3.15 mmol, M r = 482.92 g mol À1 ) was dissolved in water (20 ml), stirred with AgNO 3 (1.0713 g, 6.31 mmol, M r = 169.88 g mol À1 ) at 70 C for 20 min in the dark, and AgI was then removed by filtration from the yellowish suspension. The mother liquor was treated with NCBu t (2 ml) and stirred at 70 C for 1 h, affording a blue solution of cis-[Pt(NH 3 ) 2 (NCBu t ) 2 ]. The resulting nitrile complex was stirred at room temperature for 1 h with KI (2.6192 g, 17.78 mmol) to give a green precipitate of trans-  (a) 'Lantern-type' diplatinum(III) complex, (b) dinuclear Pt III with three bridging ligands and (c) a 'platinum blues' derivate (this work). In this study, YZX = OCN, axial L = Cl, HHT is head-head-to-tail and HT is head-to-tail.
(iii) trans-[PtI 2 (NH 3 )(NCBu t )] (0.9457 g, 1.72 mmol, M r = 549 g mol À1 ), dissolved in acetone (20 ml), was first stirred at 60 C to give a yellow solution, and then stirred with AgNO 3 (0.5859 g, 3.45 mmol) at 70 C for 20 min in the dark, affording a green solution that was filtered. The mother solution was treated with KCl (1.2892 g, 17.3 mmol, M r = 74.56 g mol À1 ) and then taken to dryness by evaporation of the solvent under reduced pressure. The solid residue was dissolved in water (20 ml), stirred at 50 C for 1 h, and dried in vacuum to obtain the formation of a green precipitate of trans- and trans-[PtI 2 (NH 3 )(NCBu t )] (0.0415 g, 75.6 mmol) were first stirred at 60 C for 6 h in a water solution (10 ml), giving a brown suspension, and then heated for 20 h at 60 C. The solid residue was separated from the solution and crystallized in water by slow evaporation at room temperature. After three days, red crystals were formed (Fig. 2). It is worth noting that in the final complex only chlorine ligands are in the equatorial and axial positions. This can be explained by assuming that the iodide anion (from the Pt II complex) is a stronger base with respect to chlorine, and as a consequence exhibits a larger reactivity with H 3 O + cations (water after reacting with the platinum precursors releases hydrogen ions).

X-ray structure determination
The selected crystal ( Fig. 1) was mounted on a Bruker AXS X8 APEX CCD diffractometer equipped with a four-circle Kappa goniometer and a 4 K CCD detector (radiation Mo K). Data reduction and unit-cell refinement were carried out with the SAINT package (Bruker, 2003). A total of 29306 reflections ( max = 20.28 ) was collected. The reflections were indexed, integrated and corrected for Lorentz, polarization and absorption effects with the program SADABS (Sheldrick, 2010). The unit-cell parameters were calculated from all reflections. The structure was solved using direct methods in space group C2/c and the model refined using full-matrix leastsquares. The ADPs of non-hydrogen atoms were refined anisotropically, while hydrogen atoms were located by Fourier difference, except for those of the tert-butyl group which have been placed at calculated positions, and ADPs refined isotropically. All calculations and molecular graphics were  Single crystal selected for the X-ray diffraction characterization.
ORTEP-3 for Windows packages (Ferrugia, 2012). Details of the experiment and crystal data are listed in Table 1. Atomic positions are listed in Table 2. Selected bond lengths, bond angles and atomic displacement parameters are given in the CIF (see supporting information). The CIF file has been deposited in the Crystallography Open Database (Gražulis et al., 2009;ref. 3000395).

Results and discussion
NMR and IR characterization. The 1 H-NMR spectrum at 295 K in DMSO-d 6 (Fig. S1) exhibits signals at frequencies $1.20, $5.00 and $6.70 ppm assigned to the tert-butyl  -195 Pt] HSQC-NMR heterocorrelate spectrum (Fig. S2) recorded in DMSO-d 6 , exhibits two NH signals at 5.01 and 6.74 ppm correlated with the platinum signal at À347 ppm, indicative of a Pt III cation in a N 2 Cl 2 OPt coordination environment. The occurrence of the tert-butyl and ammine ligands in the complex is coherent with the IR spectrum with bands at 2964-2918 and 3283-3405 cm À1 , respectively (Fig. S2).

Figure 3
ORTEP drawing of the final structure model of the [Pt 2 Cl 4 (NH 3 ) 2 -{-HN C(O)Bu t }] complex. Symmetry code: (i) 1 À x, y, 1 2 À z. Ellipsoids drawn at the 30% probability level. Atom color coding: white for hydrogen, green for chlorine, blue for nitrogen, gray for carbon and red for oxygen. Pt III molecule is characterized by a twist around the platinumplatinum vector [N2-Pt1-Pt1-O2] with an average torsion angle of 19.58 . The steric interactions between the ammine (bound to the first platinum) and the chloride (bound to the platinum in the second subunit) ligands are responsible for these distortions. In the amidate moiety, the C-X distances are 1.294 (3) Å and 1.293 (3) Å for the C1-N2 and C1-O1 bonds, respectively, in accordance with an extensive -bond delocalization over the O-C-N moiety. The complex exhibits an intramolecular hydrogen bond involving the ammine hydrogen and the chlorine ligand (Fig. 4) The molecular packing (Fig. 5) is governed by intermolecular hydrogen bonds (Table 3) and van der Waals interactions. (i) Intermolecular hydrogen bonds occur between the N(H) hydrogen of the ammine group and the chlorine ligand (see Fig. 6); and between the axial chlorine and the N(H) hydrogen of the amidate chelating group (see Fig. 7). (ii) van der Waals intermolecular forces involve chlorine and ammine ligands in equatorial position (see Fig. 8).
The structural results from the spectroscopic and diffraction analyses confirm that we have successfully designed the synthesis of the expected complex, and the absence of iodine ligands as hypothesized. Crystal packing of [Pt 2 Cl 4 (NH 3 ) 2 {-HN C(O)Bu t } 2 ], viewed along [010]. Ellipsoids drawn at the 30% probability level. Atom color coding: white for hydrogen, green for chlorine, blue for nitrogen, gray for carbon and red for oxygen. . Ellipsoids drawn at the 30% probability level. Atom color coding: white for hydrogen, green for chlorine, blue for nitrogen, gray for carbon and red for oxygen. Table 3 Hydrogen-bond geometry (Å , ).  (2)  138 Symmetry codes: (i) 1 + x, y, Àz + 1 2 ; (ii) 1 À x, Ày, 1 À z; (iii) x, 1 À y, z + 1 2 .

Conclusion
The necessity to overcome the limitations associated with the use of cisplatin, such as the occurrence of toxic side effects and drug-resistance behaviors, has led to the synthesis of new complexes with higher pharmacological properties. In this study, we have identified a strategy for synthesizing a novel dinuclear Pt III complex from the pivalonitrile derivatives of Pt II and Pt IV as precursors. The complex has been characterized by NMR ( 1 H and 195 Pt) and IR spectroscopies. The crystal structure was determined by X-ray crystallography. To our knowledge this complex is the first example of dinuclear Pt III species with two bridging ligands in HT configuration. Since it has been demonstrated that 'lantern-shaped' Pt III complexes exhibit antitumor activity, it will be interesting to investigate the activity of our complex with biological assays.