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 massspectrometry, and IR and 1H and 13C{1H} NMR spectroscopies.