Directed synthesis of isomerically pure platinum pyrazole complexes

Abstract

The complexes K2[PtCln] (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-[PtCln(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 PtIV 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 PtIV in complex 4a stabilizes the sterically least hindered tautomer in the solid state.