First observation of metal-mediated nitrile–imine coupling giving ligated 1,3-diaza-1,3-dienes

Abstract

Coupling between benzophenone imine and coordinated organonitriles in the platinum(IV) complexes [PtCl4(RCN)2] (R = Me or Et) proceeded rapidly under mild conditions to afford the 1,3-diaza-1,3-diene compounds [PtCl4{NHC(R)NCPh2}2] in good yield and this reaction is the first observation of metal-assisted nucleophilic addition of an imine to a ligated nitrile. [PtCl4{NHC(R)NCPh2}2] were reduced to the corresponding platinum(II) complexes [PtCl2{NHC(R)NCPh2}2] by the carbonyl-stabilized phosphorus ylide Ph3PCHCO2Me. The [PtCl2{NHC(Me)NCPh2}2] complex was also detected in a mixture formed in the reaction between the platinum(II) nitrile complex [PtCl2(MeCN)2] and Ph2CNH, but the selectivity of the platinum(II)-mediated nitrile–imine coupling was enhanced by employing for the reaction with benzophenone imine the phenyl cyanide complex [PtCl2(PhCN)2] to give [PtCl2(NHCPh2){NHC(Ph)NCPh2}]. Liberation of 1,3-diaza-1,3-dienes was exemplified by the reaction of [PtCl2{NHC(Et)NCPh2}2] with two equivalents of 1,2-bis(diphenylphosphino)ethane in CHCl3 to give, along with free NHC(Et)NCPh2 retained in solution, the solid complex [Pt(dppe)2]Cl2. All isolated metal complexes were characterized by elemental analyses, FAB mass spectrometry, IR, 1H, 13C-{1H} and 195Pt NMR spectroscopies and cis-[PtCl4{Z-NHC(Et)NCPh2}2] also by X-ray single-crystal diffraction.