The first crystal structure of a rhodium complex with the antileukaemic drug purine-6-thione; synthesis and molecular orbital investigation of new organorhodium(III) compounds‡

Abstract

Reactions of [Rh ^III Cl ₂ Ph(SbPh ₃ ) ₃ ] 1 with an excess of purine-6-thione (C ₅ H ₄ N ₄ S) or 1,3-thiazole (C ₃ H ₃ NS) in absolute ethanol gave crystalline [Rh ^III Cl ₂ Ph(C ₅ H ₄ N ₄ S)(SbPh ₃ )] 2 (S trans to Sb), [Rh ^III Cl ₂ Ph(SbPh ₃ )(C ₃ H ₃ NS) ₂ ] 3 and [Rh ^III Cl ₂ Ph(SbPh ₃ ) ₂ (C ₃ H ₃ NS)] 4. The crystal structure of complex 2 has been determined. Two different rotamers, which differ in the orientation of the phenyl ligand around the Rh–C bond axis, are present. The co-ordination geometry of both molecules is pseudo-octahedral and the neutral, N ¹ and N ⁹ protonated, purine ligand behaves as bidentate through S and N ⁷ . The Rh–N ⁷ bonding interaction is much weakened [average 2.262(7) Å] by the high trans influence of the phenyl ligand. The H ⁸ atom of both purine systems points towards the centre of a phenyl ring of SbPh ₃ . The geometrical parameters of the SbPh ₃ molecules show that an attractive interaction between H ⁸ and the phenyl ring is operative for each rotamer. The ¹ H NMR spectrum of 2, in DCON(CD ₃ ) ₂ , shows an upfield shift of 1.37 ppm for H ⁸ , consistent with a shielding effect from a phenyl ring of SbPh ₃ . Therefore, the H ⁸ · · · Ph(Sb) attractive interaction exists also in solution. The crystal structure of 3 has also been determined. The co-ordination geometry is pseudo-octahedral, the metal being linked to two trans chloride ions, one antimony donor from SbPh ₃ , one carbon atom from the phenyl ligand and two nitrogen atoms from thiazole ligands, one of which is trans to Ph [Rh–N 2.245(5) Å]. The ¹ H NMR spectrum shows that the solid-state structure is maintained in CDCl ₃ solution. The signals of the H ² and H ⁵ protons of the thiazole ligands are shifted downfield by 0.65 and 0.63 and 0.45 and 0.45 ppm for the molecules trans and cis to the C donor, respectively, upon complexation. The ¹ H HMR spectrum of 4 is in agreement with the presence of a thiazole ligand trans to Ph. An interaction between the chloride ligands and some protons of the phenyl rings of SbPh ₃ is resposible for a downfield chemical shift of about 0.2 ppm for the relevant ¹ H NMR signals in compounds 1–4. Molecular mechanics analysis based on the crystal structures of 2 and 3 made it possible to set up force-field parameters suitable for this class of molecules. In the case of 3 the rotation of the SbPh ₃ molecule around the Rh–Sb bond is highly hindered; the lowest barrier between minima is higher than 125 kJ mol ^-1 . The rotations of the thiazole ligands have minima consistent with the crystal structure.