Platinum(II) complexes of nitroimidazoles: synthesis, characterisation, and X-ray crystal structures of cis-dichlorobis[1-(2′-hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole]platinum(II) and trans-dichlorobis[1-(2′-hydroxy-3′-methoxypropyl)-2-nitroimidazole]platinum(II)

Abstract

A range of complexes of the type [PtL₂X₂](where L is a substituted 5-nitroimidazole, and X₂ is a dihalide or a dicarboxylate) has been prepared and characterised by a variety of methods, including ¹⁹⁵Pt n.m.r. spectroscopy. These 5-nitroimidazole complexes had a cis stereochemistry as exemplified by the X-ray crystal-structure determination of cis-dichlorobis[1-(2′-hydroxyethyl)-2-hydroxymethyl-5-nitroimidazole]platinum(II)[orthorhombic crystals: a= 8.643(1), b= 24.052(3), c= 9.119(1)Å, Z= 4, space group Pcan]. In addition, a number of analogous complexes of 2- and 4-nitroimidazoles were prepared. The 2-nitroimidazoles appeared to form the thermodynamically favoured trans complexes, rather than the kinetically favoured cis products. This was verified for trans-dichlorobis[1-(2′-hydroxy-3′-methoxypropyl)-2-nitroimidazole]platinum(II)[monoclinic crystals: a= 8.134(1), b= 13.014(1), c= 11.323(2)Å, β= 91.469(9)°, Z= 2, space group P2₁/a]. This complex showed an unusual loss of planarity between the nitro-group and the imidazole ring, giving a dihedral angle of 45.6°. The geometry of the 4-nitroimidazole complexes was not determined. Co-ordination of the nitroimidazole ligand to Pt^II lowered the wavelength of the π–π* electronic absorption band, and reduced the polarographic reduction potential by ca. 0.15–0.2 V.