Novel Expanded Ring N-Heterocyclic Carbenes: Free Carbenes, Silver Complexes, And Structures

The addition of aromatic formamidines to dihaloalkanes proceeds in air under mildly basic conditions and in polar solvents to afford the corresponding amidinium salts in high yields. To demonstrate the wide scope of this methodology, several five-, six-, and seven-membered saturated N-heterocyclic amidinium salts have been prepared. The free carbenes 7-Mes, 7-Xyl, 7-Pri, 6-Mes, and 6-Xyl have also been isolated as stable white solids, and for the first time the solid state structures of free seven-membered N-heterocyclic carbenes have been determined (7-Mes and 7-Xyl). The syntheses and characterization of their Ag(I) halide complexes are also described and the X-ray structures for Ag(6-Pri)Br, Ag(7-Mes)Br, Ag(7-Xyl)I, and Ag(Xyl7-Mes)Br determined. On the basis of NMR data the silver halide species form either neutral, Ag(NHC)X, or ionic, [Ag(NHC)2][AgX2] complexes in solution. A feature of the expanded ring carbenes is the extremely large NCN angle (≥120°) and the consequential steric impact of the N-substituents on the metal center. For the free seven-membered ring carbenes the 13C NMR shifts are 258−260 ppm, well down field from those observed for the six- and five-membered ring carbenes.