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

Reactions of [(RhCl2Ph)-Cl-III(SbPh3)(3)] 1 with an excess of purine-6-thione (C5H4N4S) or 1,3-thiazole (C3H3NS) in absolute ethanol gave crystalline [(RhCl2Ph)-Cl-III(C5H4N4S)(SbPh3)] 2 (S trans to Sb), [(RhCl2Ph)-Cl-III(SbPh3)(C3H3NS)(2)] 3 and [(RhCl2Ph)-Cl-III(SbPh3)(2)(C3H3NS)] 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) Angstrom] 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 SbPh3. The geometrical parameters of the SbPh3 molecules show that an attractive interaction between H-8 and the phenyl ring is operative for each rotamer. The H-1 NMR spectrum of 2, in DCON(CD3)(2) shows an upfield shift of 1.37 ppm for H-8, consistent with a shielding effect from a phenyl ring of SbPh3. 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 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) Angstrom]. The H-1 NMR spectrum shows that the solid-state structure is maintained in CDCl3 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 H-1 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 H-1 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.