Hydrolytic Metal-Mediated Coupling of Dialkylcyanamides at a Pt(IV) Center Giving a New Family of Diimino Ligands

Addition of excess R₂NCN to an aqueous solution of K₂[PtCl₄] led to the precipitation of [PtCl₂(NCNR₂)₂] (R₂ = Me₂ 1; Et₂ 2; C₅H₁₀ 3; C₄H₈O, 4) in a cis/trans isomeric ratio which depends on temperature. Pure isomers cis-1−3 and trans-1−3 were separated by column chromatography on SiO₂, while trans-4 was obtained by recrystallization. Complexes cis-1−3 isomerize to trans-1−3 on heating in the solid phase at 110 °C; trans-1 has been characterized by X-ray crystallography. Chlorination of the platinum(II) complexes cis-1−3 and trans-1−4 gives the appropriate platinum(IV) complexes [PtCl₄(NCNR₂)₂] (cis-5−7 and trans-5−8). The compound cis-6 was also obtained by treatment of [PtCl₄(NCMe)₂] with neat Et₂NCN. The platinum(IV) complex trans-[PtCl₄(NCNMe₂)₂] (trans-5) in a mixture of undried Et₂O and CH₂Cl₂ undergoes facile hydrolysis to give trans-[PtCl₄{N(H)C(NMe₂)OH}₂] (9; X-ray structure has been determined). The hydrolysis went to another direction with the cis-[PtCl₄(NCNR₂)₂] (cis-5−7) which were converted to the metallacycles [PtCl₄{NHC(NR₂)OC(NR₂)NH}] (11−13) due to the unprecedented hydrolytic coupling of the two adjacent dialkylcyanamide ligands giving a novel (for both coordination and organic chemistry) diimino linkage. Compounds 11−13 and also 14 (R₂ = C₄H₈O) were alternatively obtained by the reaction between cis-[PtCl₄(MeCN)₂] and neat undried NCNR₂. The structures of complexes 11, 13, and 14 were determined by X-ray single-crystal diffraction. All the platinum compounds were additionally characterized by elemental analyses, FAB mass-spectrometry, and IR and ¹H and ¹³C{¹H} NMR spectroscopies.