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.