Crystal structures of three platinacyclic complexes bearing isopropyl eugenoxyacetate and pyridine derivatives

Three new platinum(II) complexes bearing an isopropyl eugenoxyacetate and pyridine derivatives have been synthesized and further characterized by single-crystal X-ray diffraction.


Figure 2
The molecular structure of complexes (I), (II) and (III) showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

Figure 1
Reaction scheme for the synthesis of mixed i PrEug-pyridine derivative platinum(II) complexes (I), (II) and (III).

Figure 3
Overlay of the three complexes, showing the different conformation of the pyridine ring for (III). Complex (I) is in black, complex (II) in red and complex (III) in green.
the mutual orientation of the eugenol and pyridine parts is different. The plane through the allyl group makes an angle of 40.9 (3) with the pyridine plane. An overlay of the identical parts in (I) and (III) gives an r.m.s. deviation of 0.5782 Å , while 0.5507 Å for (II) and (III) (Fig. 3).
Comparing the bond distances in the coordination sphere of the central Pt II atom of the three complexes shows that the largest differences occur for the Pt-N distance: 2.139 (2) Å for (I) within experimental error the same as 2.1418 (18) Å for (II), and 2.164 (3) Å for (III).
Complex (II) displays a very similar crystal packing (Fig. 5, Table 2). But, due to the presence of a 4-methylpyridine ring in (II), thestacking is absent [Cg4Á Á ÁCg4 v = 4.312 (1) Å , slippage 2.703 Å ; Cg4 is the centroid of ring N22/C23-C27; symmetry code: (v) 2 À x, 2 À y, 1 À z] and is in fact replaced by two C-HÁ Á Á interactions between the methyl group and the pyridine ring. This slippage of the pyridine ring also results in an additional C8-H8BÁ Á ÁCl21 interaction between the allyl CH 2 group and a neighboring Cl atom.
The carboxylic acid function present in complex (III) is involved in head-to-tail fashion O-HÁ Á ÁO interactions resulting in the formation of chains running in the [101] 1014 Chi  Partial crystal packing of complex (II), showing C-HÁ Á ÁO hydrogen bonding (red dashed lines), C-HÁ Á ÁCl (green dashed lines), C-HÁ Á Á andinteractions (grey dashed lines). Hydrogen atoms not involved in interactions have been omitted for clarity (see Table 2 for symmetry codes). Table 1 Hydrogen-bond geometry (Å , ) for (I).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.41, update of November 2019; Groom et al., 2016) for Pt complexes with the Pt atom coordinated to a Cl atom, N atom and allylaryl ligand (similar to the title complexes) gave eight hits. The C C group and N atom are always in a cis position with respect to each other. All complexes also possess a distorted square-planar coordination for the Pt atom with a deviation of the Pt atom from the best plane through the coordinating Cl, N, C aryl and centroid (Cg) of the C C group between 0.018 Å [chloro-(4,5-dimethoxy-2-prop-2-en-1yl)phenyl-(2-methylaniline)platinum(II), refcode GOYJEL; Da et al., 2015] and 0.048 Å [( 2 -5-hydroxy-4-methoxy-2-(prop-2-en-1-yl)phenyl)-chloro-(4-methylpyridine)platinum(II), CSD refcode VEZJIW; Chi et al., 2018]. Table 4 gives an overview of the four Pt bond distances for each compound. The average Pt-Cl, Pt-N, Pt-C aryl and Pt-Cg distances are 2.324 (8), 2.158 (29), 1.996 (64) and 2.014 (16) Å , respectively. The largest spread is observed for the Pt-C aryl bond (1.843 to 2.109 Å in the two molecules present in the asymmetric unit of chloro-( 2 -6-ethenyl-1,3-benzodioxole-5-yl)piperidineplatinum(II) (CSD refcode OFUREN; Da et al., 2008). The averages correspond to the observed distances for complexes (I)-(III). It is worthwhile to note that upon binding to Pt, the C C bond distance [1.29 (4) Å for allylaryl fragments in the CSD] increased significantly. The average C C bond distance for the complexes in

In vitro cytotoxicity of complexes (I) and (II)
The in vitro cytotoxicity of complexes (I) and (II) was tested according to the method described in Skehan et al. (1990) and Likhitwitayawuid et al. (1993)

Synthesis and crystallization
The synthetic protocol for the three complexes is shown in Fig. 1. The starting complex [Pt(-Cl)( i PrEug)] 2 (1) was synthesized according to the synthetic protocol of Thong & Chi (2014).
[PtCl( i PrEug)(pyridine)] (I). A solution of pyridine (80 mL, 1.0 mmol) in 10 mL ethanol was slowly added with stirring to a suspension of [Pt(-Cl)( i PrEug)] 2 (494 mg, 0.5 mmol) in   Table 3 for symmetry codes). Table 4 Pt bond distances (Å ) for Pt complexes with the Pt atom coordinated to a Cl atom, N atom and allylaryl ligand found in the Cambridge Structural Database. C aryl is the aryl C atom and Cg the centroid of the C C group of the coordinating allylaryl ligand.

CSD refcode
Pt-Cl Pt-N  Chi et al. (2018) temperature (AT) and filtered off after 30 minutes to remove the insoluble part. Subsequently, slow evaporation of the solvent of the obtained solution at AT gave within 10 h transparent crystals, which were suitable for X-ray diffraction and other analyses. The yield was 515 mg (90%). %Pt (found/ [PtCl( i PrEug)(4-methylpyridine)] (II). This complex was prepared starting from [Pt(-Cl)( i PrEug)] 2 (494 mg, 0.5 mmol) and 4-methylpyridine (100 mL, 1.0 mmol) according to the procedure for the synthesis of I. The yield was 539 mg (92%), transparent crystals were suitable for X-ray diffraction and other analyses. %Pt ( [PtCl( i PrEug)(pyridine-4-carboxylic acid)] (III). A mixture of pyridine-4-carboxylic acid (123 mg, 1.0 mmol) and [Pt(-Cl)( i PrEug)] 2 (494 mg, 0.5 mmol) in 10 mL acetone was stirred at AT for 8 h. The resulting precipitate was filtered off and washed consecutively with ethanol (2 Â 5 mL) and cold chloroform (2 Â 5 mL), then crystallized in chloroform to give a light-yellow powder. The yield was 493 mg (80%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation within 8 h from a concentrated chloroform/ ethanol solution at AT. %Pt (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5.
The H atoms were placed in idealized positions and included as riding contributions with U iso (H) values of 1.2U eq or 1.5U eq of the parent atoms, with C-H distances of 0.95 (aromatic), 1.00 (CH), 0.99 (CH 2 ) and 0.98 Å (CH 3 ). The carboxylic acid H atom in (III) was refined as rotating group with a O-H distance of 0.84 Å . The displacement parameters of the bonded atoms in the carboxylic acid and isopropyl groups in (III) were restrained to be similar along the bond. For all structures, molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). 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.

κN)platinum(II) (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.005 Δρ max = 0.38 e Å −3 Δρ min = −0.92 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Pt1 1.00923 (2) 0.54971 (2)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.87 e Å −3 Δρ min = −1.77 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.