Directed synthesis of isomerically pure platinum pyrazole complexes

The complexes K₂[PtCl_n] (n = 4 or 6) react with pyrazoles 3,5-MeRpzH (R = H or Me) in 0.1 M HCl at 20–25 °C to form the isomerically pure cis-[PtCln(3,5-MeRpzH)₂] complexes (n = 2 or 4) (1a,b and 3a,b), whereas a decrease in the acidity of the medium leads to a substantial decrease in selectivity of the reaction. Thermal isomerization of complexes 1a,b and 3a,b both in solution (MeNO₂) and in the solid state affords the trans-[PtCl_n(3,5-MeRpzH)₂] complexes (n = 2 or 4) (2a,b and 4a, b). Platinum(II) complexes 1a,b and 2a,b were also prepared by selective reduction of genetically related Pt^IV compounds (3a,b and 4a,b) with the phosphorus ylide Ph₃P=CHCO₂Me in chloroform. Platinum(IV) complexes (3a,b and 4a,b) were synthesized by oxidation of the corresponding PtII complexes (1a,b and 2a,b) with molecular chlorine. X-ray diffraction study demonstrated that coordination of 3(5)-MepzH to Pt^IV in complex 4a stabilizes the sterically least hindered tautomer in the solid state. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 242—249, February, 2006.