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^IIICl₂Ph(SbPh₃)₃] 1 with an excess of purine-6-thione (C₅H₄N₄S) or 1,3-thiazole (C₃H₃NS) in absolute ethanol gave crystalline [Rh^IIICl₂Ph(C₅H₄N₄ S)(SbPh₃)] 2 (S trans to Sb), [Rh^IIICl₂Ph(SbPh₃)(C₃H ₃NS)₂] 3 and [Rh^IIICl₂Ph(SbPh₃)₂(C ₃H₃NS)] 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, N¹ and N⁹ protonated, purine ligand behaves as bidentate through S and N⁷. The Rh–N⁷ bonding interaction is much weakened [average 2.262(7) Å] by the high trans influence of the phenyl ligand. The H⁸ atom of both purine systems points towards the centre of a phenyl ring of SbPh₃. The geometrical parameters of the SbPh₃ molecules show that an attractive interaction between H⁸ and the phenyl ring is operative for each rotamer. The ¹H NMR spectrum of 2, in DCON(CD₃)₂, shows an upfield shift of 1.37 ppm for H⁸, consistent with a shielding effect from a phenyl ring of SbPh₃. Therefore, the H⁸ · · · 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 SbPh₃, 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 ¹H NMR spectrum shows that the solid-state structure is maintained in CDCl₃ solution. The signals of the H² and H⁵ 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 ¹H 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 SbPh₃ is resposible for a downfield chemical shift of about 0.2 ppm for the relevant ¹H 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 SbPh₃ 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.

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