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

Abstract

The synthesis of the Boc-protected 1-(2-aminoethyl)-3-methylimidazolium salts [BocNHCH 2CH 2ImMe]X [2]X (X = I, PF 6) and their straightforward transformation into [NH 2CH 2CH 2ImMe]X [3]X is reported. The reaction between [2]X and Ag 2O leads to the formation in the solid state of three different bonding motifs: a biscarbene salt [(NHC–NHBoc) 2Ag]PF 6 ( [4]PF 6, NHC–NHBoc = 1-(2-BocNH-ethyl)-3-methyl-imidazolin-2-ylidene), a tetranuclear complex [Ag(NHC–NHBoc) 2] 2[Ag 2I 4], ( 5), and a polymeric silver “staircase” [(NHC–NHBoc) 2–Ag 4–I 4] n , ( 6) composed of Ag 4I 4 clusters. The same reaction carried out with [3]I showed that a primary silver mono-NHC–NH 2 carbene complex of the type [(NHC–NH 2)AgI] ( 7) is likely to form but it is unstable in solution. The solid state molecular structures of [4]PF 6, 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.