Crystal structure of trans-dichlorido(4-nitroaniline-κN 1)(piperidine-κN)platinum(II)

The packing of the title compound features N—H⋯Cl hydrogen bonds and π–π stacking interactions, which form one-dimensional chains of molecules parallel to [001] further linked via N—H⋯O interactions.


Chemical context
The title compound is one of many complexes which have been synthesized for the purpose of potential medical applications (Klein & Hambley, 2009;Wilson & Lippard, 2014;Peng et al., 2014). It is notable that according to the procedure used for the synthesis of complexes of the type cis-[PtCl 2 (piperidine)(another amine)] (piperidine hereafter denoted Pip) (Dinh & Da, 2003;Nguyen Thi Thanh et al., 2014), the reaction between K[PtCl 3 (Pip)] and p-nitroaniline under appropriate conditions gave no cis complex, as expected, but instead gave the trans-[PtCl 2 (p-nitroaniline)(Pip)] derivative, (I).
To explain this we suppose that p-nitroaniline first coordinates with Pt II via the N atom of the amino group to form cis-[PtCl 2 (p-nitroaniline)(Pip)] based on the trans effect. Then, in the reaction solution, the cis complex converts into the trans complex and the thermodynamics of this conversion are currently under investigation by us.
The anticancer activity of the title compound was tested according to the method described by Skehan et al. (1990) against four human cancer cell lines (HepG2, RD,MCF7 and Fl). The IC 50 values calculated based on OD values taken on an Elisa instrument at 515-540 nm are >10, 4.86, >10 and 8.25 mg ml À1 , respectively.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1 and surprisingly shows a trans arrangement of the two Cl ISSN 2056-9890 atoms [Cl8-Pt1-Cl9 = 177.84 (4) ]. The piperidine ring adopts the usual chair conformation, with the N2-Pt1 bond in the equatorial position. The piperidine ring is oriented nearly perpendicular to the coordination plane of the Pt II atom, thereby reducing the van der Waals repulsion; the dihedral angle between the least-squares mean planes through the piperdine ring and the four atoms coordinated to the Pt atom is 89.6 (2) . One short intramolecular contact is observed, i.e. H7BÁ Á ÁCl8 = 2.83 Å . The mean planes through the piperidine ring and the benzene ring make a dihedral angle of 89.0 (3) .
The dihedral angle between the mean planes of the nitro substituent and the benzene ring is 16.6 (3) .

Supramolecular features
In the crystal, inversion dimers are formed via N-HÁ Á ÁCl interactions between the aniline N atom and both Cl atoms, resulting in chains of molecules along the [001] direction ( Fig. 2 and Table 1). Within these chains,interactions occur between the aromatic rings [CgÁ Á ÁCg iv = 3.801 (3) Å ; Cg is the centroid of the C11-C16 ring; symmetry code: (iv) Àx, y, Àz + 3 2 ; Fig. 2]. Neighbouring chains are linked via N-HÁ Á ÁO hydrogen bonds between the piperidine N atom and a nitro O atom ( Fig. 2 and Table 1). The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level.  Symmetry codes: (i) x þ 1 2 ; Ày þ 3 2 ; z À 1 two N atoms gave 713 hits. The majority of these Pt complexes display a cis coordination of the Cl atoms (474 structures), with the remaining 239 structures showing a trans coordination. There is no difference in the Pt-Cl distances between both configurations. The average Pt-Cl distances are 2.300 (15) and 2.299 (12) Å for the cis and trans arrangements, respectively, and correspond to the observed distances of 2.3039 (11) and 2.2917 (12) Å for Pt1-Cl8 and Pt1-Cl9, respectively.

Synthesis and crystallization
The starting complex K[PtCl 3 (piperidine)] (0.425 g, 1 mmol), prepared according to the synthetic procedure of Da et al. (2001) with slight modifications, was dissolved in water (10 ml) and filtered to afford a clear solution. To this solution, pnitroaniline (1 mmol) in ethanol (10 ml) was added gradually while stirring at 413-318 K. After 1 h, a brown powder appeared and the reaction mixture was then stirred further for 24 h until all the precipitate was completely dissolved. The solvent was removed in vacuo to give a brown-yellow product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were placed at idealized positions and refined in riding mode, with U iso (H) values assigned as 1.2U eq of the parent atoms, with C-H distances of 0.95 (aromatic) and 0.99 Å (methylene), and N-H distances of 0.93 (NH) and 0.92 Å (NH 2 ).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.