Homometallic silver(I) complexes of a heterotopic NHC-bridged bis-bipyridine ligand

By varying the metal to ligand ratio, stepwise formation of a series of homonuclear silver(I) complexes of a carbene-bridged bis-bipyridine ligand (L) was achieved. In the mononuclear 1:2 complex [AgL ₂]Br (1) only the carbene carbon is involved in the metal coordination, while both of the 2,2′-bipyridine (bpy) arms are free. When the amount of silver(I) ion was increased, isomorphous 2:2 dinuclear complexes with different counteranions, [Ag ₂L ₂]X ₂ (X = Br ^- (2a), PF ₆ ^- (2b), BPh ₄ ^- (2c)), were synthesized from the ligand LX, in which the carbene carbon and one of the bpy units participate in the coordination with silver(I) ions. Further addition of Ag ^I salt afforded the one-dimensional coordination polymer {[Ag ₃L ₂](PF ₆) ₃•4CH ₃CN} _n (3), wherein the hanging bipyridine units also coordinate with Ag ^I and thus all the coordination sites of the ligand are employed. The results reveal the preference of Ag ^I ion for the carbene carbon donor rather than the bpy units. The synthesis, structures, and interconversion of the complexes and the counteranion effects on the structures are reported, and the luminescent properties of the ligand LX and the silver complexes have also been studied.