Di­chloro(D-me­thionine-N,S)plati­num(II) at 130 K

The use of transition metal complexes, in particular those of platinum group metals, in cancer chemotherapy has been widely reported (Rosenberg et al., 1965; Sherman & Lippard, 1987; Lippert, 1989). Methionine inhibits nephrotoxicity of the cis-dichlorodiammineplatinum(II) complex (Burchenal et al., 1978), which is in turn widely used clinically as an antiviral and antitumoral agent against a range of human cancers (Farrell, 1989). Consequently, there is a great interest in the study of the interaction between methionine and platinum(II). To date the X-ray structures of the [PtCl₂(dl-Met)] and [PtCl₂(l-Met)] complexes have been described (Wilson et al., 1992; Freeman & Golomb, 1970). Wilson and co-workers (1992) have redetermined the structure of [PtCl₂(l-Met)] reported by Freeman & Golomb (1970) by using high-resolution data. However, the crystals used by Wilson et al. (1992) belonged to the monoclinic space group P2₁ and those used in the original structure analysis by Freeman & Golomb (1970) belonged to the triclinic space group P1. In this paper, we report and compare with the complexes described above the X-ray structure of [PtCl₂(d-Met)], (I), which has been determined at two different temperatures, 294 and 130 K. \sch

The lattice constants of (I) at 294 K are a = 7.339 (1), b = 8.384 (1), c = 8.903 (2) Å, α = 74.52 (2), β = 86.38 (1) and γ = 78.25 (1)° and the corresponding lattice constants at 130 K are given in the experimental section of this paper. Crystal data at both temperatures are consistent and indicate that the crystals studied in the present work belong to the triclinic space group P1. Coordination of methionine to platinum creates a new chiral centre at the S atom, which gives rise to two diastereoisomers, S and R. The asymmetric unit comprises two independent molecules which correspond to the two diastereoisomers mentioned above, that is, with the same chirality at C2A and C2B carbon atoms, but with S5A and S5B sulfur atoms having opposite chiralities. This is a clear case where checking programs alert for a possibly missed centre of symmetry; however, the difference between both molecules in the asymmetric unit is clear when examining the C2A and C2B atoms. The anomalous scattering contribution also indicates to the d-enantiomer, as is shown by the absolute structure parameter -0.006 (12) (Flack, 1983), confirming the chirality of the crystal. Perspective views of the two independent molecules, including the atom labelling, are presented in Figs. 1a and b. The bond distances and angles with their s.u's are given in Table 1. The cell parameters of the triclinic dichloro(l-methionine-N,S)platinum(II) complex obtained at room temperature (Freeman & Golomb, 1970), a = 7.31 (1), b = 8.91 (1), c = 8.39 (1) Å, α = 74.47 (3), β = 78.13 (3), γ = 86.40 (3)°, correspond well with (I) indicating that the structures are similar except for a chiral inversion at the carbon atom linked to the carboxyl group. Wilson and co-workers (1992) have presented a monoclinic dichloro(l-methionine-N,S)platinum(II) complex similarly exhibiting two diastereoisomeric molecules with their S atoms having opposite chiralities. In the structure of the racemic crystal (monoclinic space group P2₁/c; Freeman & Golomb, 1970), dichloro(dl-methionine-N,S)platinum(II), the S atoms in the centrosymmetrically related independent molecules of the asymmetric unit have obviously opposite chiralities.

In (I), the Pt atom is coordinated by the methionine N (amino) and S (thioether) atoms and by two chlorines. Models based on this structure show that considerable strain would be involved in tridentate coordination of methionine through the amino, thioether, and carboxyl groups. Coordination of methionine to platinum(II) through N and S atoms in dichloro(d-methionine-N,S)platinum(II) was predicted on the basis of IR, ¹H and ¹³C NMR spectra (Caubet et al., 1992). The geometry around platinum is only slightly distorted from a square-planar coordination. The deviations from the plane defined by N, S and Cl atoms are, for molecule (A) N2A 0.006 (4), S5A -0.006 (4), Cl1A 0.006 (4), Cl2A -0.006 (4), Pt1A -0.023 (3) Å; and molecule (B) N2B 0.033 (4), S5B -0.030 (4), Cl1B 0.029 (4), Cl2B -0.032 (4), Pt1B -0.004 (4) Å. The amino acid ligand acts in a bidentate manner and together with the platinum atom forms a six-membered ring. The resulting platinum-methionine chelate ring adopts a half-chair conformation with the carboxyl group equatorial to the ring. The planes N2/C2/C4/S5 and C2/C3/C4 form an angle of 63.5 (6)° in molecule (A) and the equivalent planes in molecule (B) form an angle of 60.6 (8)°. The Pt—S bond lengths in the two independent molecules of the asymmetric unit, 2.234 (4) and 2.274 (4) Å, are not equivalent but are not significantly different from the values of 2.246 (2) and 2.247 (2) Å in dichloro(l-methionine-N,S)platinum(II) (Wilson et al., 1992). In both diastereoisomers the Pt—Cl bond distances trans to the S atoms, 2.329 (3) and 2.310 (3) Å, are similar to the Pt—Cl bond distances trans to the N atoms, 2.312 (3) and 2.317 (3) Å, respectively. In contrast to dichloro(l-methionine-N,S)platinum(II) (Wilson et al., 1992) and dichloro(O-methyl-l-methionine-N,S)platinum(II) (Hambley & Webster, 1994), where the Pt—Cl bond distances trans to S [2.323 (2), 2.320 (2) and 2.314 (2) Å] are marginally longer than those trans to N [2.309 (2), 2.310 (3) and 2.288 (2) Å, respectively], there is no structural trans effect in (I).

The distances N2A···O2A and N2B···O2B at 2.662 (14) and 2.600 (13) Å, respectively, suggest the presence of intramolecular hydrogen bonds. On the other hand, intermolecular distances show all acidic hydrogen atoms to be involved in hydrogen bonding. The crystal packing is depicted in Fig. 2. Different diastereoisomers form dimers by means of strong hydrogen bonds between their carboxyl groups. The lengths of the O···O hydrogen bonding distances are 2.663 (12) and 2.633 (10) Å. The shortest Pt···Pt distance is 3.5642 (6) Å. Similar pairing arrangements of molecules in pairs due to strong hydrogen bonding are common in molecules containing carboxyl groups. In (I), d chirality at carbon atoms bonded to carboxyl groups together with opposite chirality at the sulfur atoms of the two diastereoisomers prevents the formation of a geometry center between the carboxyl groups.