Crystal structure of chlorido(piperidine-κN)(quinoline-2-carboxylato-κ2 N,O)platinum(II)

The platinum(II) complex with notable antitumor activity shows a slightly distorted square-planar coordination and intramolecular C—H⋯Cl and intermolecular N—H⋯Cl and C—H⋯O hydrogen bonds.


Chemical context
The title compound belongs to a series of platinum(II) complexes bearing piperidine (pip) as a ligand, which exhibit notable antitumour activity (Da et al., 2001;Rounaq Ali Khan et al., 2000;Solin et al., 1982). In comparison with the earlier reported complex [PtCl 2 (pip)(quinoline)] (Nguyen Thi Thanh et al., 2014), the quinoline ligand is replaced by an N,Obidentate quinaldate ligand. It is interesting to note that in the [PtCl 2 (pip)(quinoline)] complex, the quinoline and piperidine ligands are arranged in cis positions (Nguyen Thi Thanh et al., 2014). In the title compound, the quinoline ring of the quinaldate ligand occupies a trans position with respect to the piperidine ring. We suggest that in the reaction solution there exists a chemical equilibrium between the neutral and bipolar forms of quinaldic acid. Thus, the quinaldic acid in its ionic form coordinates with Pt II via the O atom of the carboxylate group first and in a cis position with respect to piperidine based on the trans effect. In a second step, the quinaldic acid coordinates with Pt II also via its N atom, resulting in the cyclic complex.
The anticancer activity of the title compound was tested according to the method described in Skehan et al. (1990)

Structural commentary
The title complex crystallizes with one molecule per asymmetric unit (Fig. 1). The Pt II cation is surrounded by two N atoms, one O atom and one Cl atom, resulting in a slightly distorted square-planar coordination environment [angles around platinum: O1-Pt1-N1 81.38 (9), O1-Pt1-N2 88.26 (9), Cl1-Pt1-N2 84.26 (7) and Cl1-Pt1-N1 106.11 (7) ]. The Cl À and the Pt II atoms are displaced from the least-squares plane of the quinoline ring and all other coordinating atoms by 0.2936 (7) and 0.0052 (1) Å , respectively. The piperidine ring adopts a chair conformation and is almost perpendicular to the coordination plane of the Pt II cation [dihedral angle between the best plane through the piperidine ring and the four atoms coordinating to the Pt II cation = 79.66 (13) ]. Bond lengths are normal and agree well with related platinum compounds (Cambridge Structural Database, version 5.34; Allen, 2002). There is an intramolecular hydrogen bond between atom Cl1 and atom H8 ( Fig. 1 and Table 1).

Supramolecular features
The crystal packing is characterized by N-HÁ Á ÁCl and C-HÁ Á ÁO hydrogen bonds (Table 1). Molecules are arranged into columns along the c axis ( Fig. 2) with the piperidine rings all directed towards the center of the column, favouring hydrophobic interactions.

Synthesis and crystallization
The starting complex K[PtCl 3 (piperidine)] (0.425 g, 1 mmol), prepared according to the synthetic protocol of Da et al. (2001), was dissolved in water (10 ml) and filtered to afford a clear solution. To this solution, quinaldic acid (1.2 mmol) in an aqueous ethanol solution (5 ml, 1:1 v/v) was added gradually View of the molecular structure of the title compound, showing the atomlabelling scheme, with ellipsoids drawn at the 50% probability level. The intramolecular C-HÁ Á ÁCl hydrogen bond is shown as a green dashed line.

Refinement
All H atoms were refined using a riding model, with C-H = 0.95 Å for aromatic, C-H = 0.99 Å for CH 2 and N-H = 0.93 Å for amino H atoms, with U iso = 1.2U eq (C,N). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.80 e Å −3 Δρ min = −0.53 e Å −3 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.