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) A] 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) A]. 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.