Crystal structures of two platinum(II) complexes containing ethyl eugenoxyacetate and 2-aminopyridine

The synthesis and crystal structures of two platinum(II) complexes containing one or two Cl atoms, an eugenol derivative and 2-aminopyridine as ligand are described. The central PtII atom displays a distorted square-planar coordination.


Chemical context
Since the discovery of the anticancer activity and subsequent clinical success of cisplatin {cis-[PtCl 2 (NH 3 ) 2 ]}, platinumbased compounds have been widely synthesized and studied as potential chemotherapeutic agents (Wong & Giandomenico, 1999). Despite the great success in treating certain kinds of cancer, there are several side effects, and both intrinsic and acquired resistance limit the organotropic profile of the drug (Chabner & Roberts, 2005;Kelland, 2007;Wilson & Lippard, 2014). Hence, there is continuing interest in the development of new platinum complexes that have high activities but low toxicity (Johnstone et al., 2014).
Several natural arylolefins, such as safrole (in sassafras oil), eugenol (in clove oil) and anethole (in anise and fennel oil), and their derivatives have been used as important intermediate materials to synthesize many compounds that have various applications in the flavouring, food and pharmaceutical industries (Jadhav et al., 2004). Recently, a number of Pt II complexes containing natural arylolefins as ligands, i.e. safrole or derivatives of eugenol such as methyleugenol and alkyleugenoxyacetate, have been prepared (Da et al., 2010(Da et al., , 2012Da, Chi et al., 2015;Da, Hai et al., 2015;Nguyen Thi Thanh et al., 2016;Le Thi Hong et al., 2016). The insertion of these natural arylolefins into the coordination with Pt II and their transformations formed complexes with novel structures and ISSN 2056-9890 high applicability. In particular, many of these organoplatinum(II) complexes exhibit significant inhibitory activities against human cancer cells (Da et al., 2012;Da, Chi et al., 2015;Da, Hai et al., 2015).

Structural commentary
In both title complexes, the central Pt II metal atom displays a distorted square-planar coordination (Fig. 1). In addition to the two Cl atoms in dichloride complex (I), the pyridine N atom and the C C double bond of the eugenol ligand coordinate to the central Pt II atom. In monochloride complex (II), one Cl atom is replaced by a C atom of the eugenol phenyl group. An overlay of the Pt-2-amino fragment present in both structures clearly shows the differences in coordination (Fig. 2). Where in (I) the Cl atoms are trans with respect to each other, this is the case for the two aromatic rings in (II). One Cl and the C C coordinations in (I) are replaced by, respectively, C C and a phenyl C atom in (II). In both cases, the 2-aminopyridine ligand only interacts via the ring N atom. In (I), the CH 2 -CH CH 2 fragment is disordered, with population parameters of 0.614 (14) and 0.386 (14) for the two positions of the central C atom. The dihedral angles between the planes through the two aromatic rings are 78.5 (2) and   51.10 (13) for (I) and (II), respectively. In (I), the H atoms of the amino group are involved in a weak intramolecular N-HÁ Á ÁO interaction (N8-H8BÁ Á ÁO25, Table 1) and an N-HÁ Á Á interaction (N8-H8AÁ Á ÁCg1, Table 1; Cg1 is the centroid of the C14-C19 ring). Similar interactions are not possible in (II) due to the different orientation of the ligands.

Supramolecular features
The complexes crystallize in different space groups, viz. P1 for dichloride complex (I) and P2 1 /c for monochloride complex (II).
The crystal packing of (II) is built up by C-HÁ Á ÁO, N-HÁ Á ÁCl and C-HÁ Á Á interactions (Table 2 and Fig. 5). Two types of inversion dimers are created by C-HÁ Á ÁO interactions enclosing R 2 2 (10) and R 2 2 (16) ring motifs, and resulting in the formation of chains parallel to the b axis. Nointeractions are observed in the packing of (II).

Database survey
The Pt-N distances of 2.066 (3) Å in (I) and 2.143 (2) Å in (II) agree well with the average Pt-N distance of 2.06 (7) Å for Pt-pyridine fragments present in the Cambridge Structural Database (CSD, Version 5.38, last update February 2017; Groom et al., 2016).
The CSD contains 34 Pt complexes with Pt coordinated by Cl, pyridine and C C, with 28 complexes having an additional Cl atom as the fourth ligand (27 trans and one cis coordination), three a C atom and another three an N atom.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were placed in idealized positions and refined in the riding mode, with U iso (H) values assigned as 1.2U eq of the parent atoms (1.5 times for methyl groups), and with C-H distances of 0.95 (aromatic and CH 2 ), 0.98 (CH 3 ), 0.99 (CH 2 ) and 1.00 Å (CH), and N-H distances of 0.88 Å (NH 2 ).

yl]benzene}platinum(II)
Crystal data [PtCl 2 (C 5  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.41 e Å −3 Δρ min = −2.25 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.