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) was achieved by reaction of 1-[2-(N-Boc)aminoethyl]imidazole with MeI, followed by thermal deprotection. 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-2-imidazolinylidene), a tetranuclear complex [Ag(NHC-NHBoc)2]2[Ag2I4], (5), and a polymeric silver "staircase" [(NHC-NHBoc)Ag(.mu.-I)AgI]n, (6) composed of Ag4I4 clusters, bearing two carbene ligands at the opposite Ag centers. 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 soln. The solid state mol. structures of [4]PF6, 5 and 6 were detd. by x-ray diffraction anal., whereas PGSE NMR expts. 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 soln. PGSE NMR studies were complemented by NOE NMR investigations in order to obtain information on anion-cation relative orientation within aggregates.