Synthesis, molecular structures and solution NMR studies of N-heterocyclic carbene–amine silver complexes

The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH2CH2ImMe]X [2]X (X = I, PF6) and their straightforward transformation into [NH2CH2CH2ImMe]X [3]X is reported. The reaction between [2]X and Ag2O leads to the formation in the solid state of three different bonding motifs: a biscarbene salt [(NHC–NHBoc)2Ag]PF6 ([4]PF6, NHC–NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC–NHBoc)2]2[Ag2I4], (5), and a polymeric silver “staircase” [(NHC–NHBoc)2–Ag4–I4]n, (6) composed of Ag4I4 clusters. The same reaction carried out with [3]I showed that a primary silver mono-NHC–NH2 carbene complex of the type [(NHC–NH2)AgI] (7) is likely to form but it is unstable in solution. The solid state molecular structures of [4]PF6, 5 and 6 were determined by X-ray diffraction analysis, whereas PGSE NMR experiments were employed to investigate the hydrodynamic dimension of the imidazolium salts and silver complexes and, consequently, to gain information on the level of aggregation in solution. PGSE NMR studies were complemented by NOE NMR investigations in order to obtain information on anion–cation relative orientation within aggregates.

Novel N-heterocyclic carbene–amine silver complexes have been prepared and the solid state structures determined by X-ray diffraction analysis have revealed three different bonding motifs: a biscarbene salt [(NHC–NHBoc)2Ag]PF6 (NHC–NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC–NHBoc)2]2[Ag2I4], and a polymeric silver “staircase” [(NHC–NHBoc)2–Ag4–I4]n, composed of Ag4I4 clusters. PGSE and NOE NMR experiments were employed to investigate the level of aggregation and anion–cation relative orientation in solution.Figure optionsDownload as PowerPoint slide