Electrophilic substitution of (diamine)tetrahydroxoplatinum(IV) with carboxylic anhydrides. Synthesis and characterization of (diamine)platinum(IV) complexes of mixed carboxylates

A novel series of (diamine)platinum(IV) complexes of mixed carboxylates have been synthesized by electrophilic substitution of the tetrahydroxoplatinum(IV) complex (dach)Pt(OH)₄ (dach = trans-(±)-1,2-diaminocyclohexane) with three different carboxylic anhydrides, pivalic, acetic, and trifluoroacetic anhydrides. Consecutive two-step acylations with two different carboxylic anhydrides in acetone or dichloromethane gave the mixed carboxylate complexes (dach)Pt(O₂CR)(x)(O₂CR')(4-x) (R = C(CH₃)₃ or CF₃, R' = CH₃, x = 1-4) including all the possible stereoisomers, which could be separated and identified by means of HPLC, column chromatography, ¹H NMR, and X-ray crystallography. From analysis of the reaction products we have found that the positions of electrophilic substitution of (dach)Pt(OH)₄ were influenced by the kinds of carboxylic anhydrides exhibiting different electrophilicity or steric effects. The initial substitution by the first reactant occurs more favorably on axial OH, but in the case of pivalic anhydride, equatorial substitution is favored probably because of the bulkiness of the pivalate group. Such a result seems to be related to their stereochemical factors rather than to differences in electrophilicity. The lipophilicity of the title complexes was affected not only by the carbon numbers of substituents but also by the conformation of the resulting compound.