Chiral Silver and Gold Rings: Synthesis and Structural, Spectroscopic, and Photophysical Properties of Ag and Au Metallamacrocycles of Bridging NHC Ligands

The synthesis and silver(I) and gold(I) coordination chemistry of a new chiral, bidentate N-heterocyclic carbene (NHC) dehydrohexitol derivative (3) are reported. The imidazolium salt [H23][PF6]2 reacts with Ag2O and Au(tht)Cl (tht = tetrahydrothiophene) precursors to form the isostructural 18-membered metallamacrocyclic dimers [Ag2(μ-3)2][PF6]2 and [Au2(μ-3)2][PF6]2 and the monocarbene complex [(AuCl)2(μ-3)]. Single-crystal X-ray structures have been determined for the bis-imidazolium precursor [H23][PF6]2 and corresponding Ag(I) and Au(I) complexes of ligand 3. Comparison between the X-ray-derived structures and solution-phase NMR data for [Ag2(μ-3)2][PF6]2 and [Au2(μ-3)2][PF6]2 demonstrate that the complexes adopt a conformation in solution different from that found in the solid state, implying a conformational flexibility of the metallamacrocycle in solution. Both [(AuCl)2(μ-3)] and [Au2(μ-3)2][PF6]2 are emissive in the solid state at ca. 380 nm (λex = 295 nm). Time-resolved luminescence measurements indicate different excited-state lifetimes for the two species, with [(AuCl)2(μ-3)] measured at 35 ns and [Au2(μ-3)2][PF6]2 at 379 ns. The chiroptical properties of the silver and gold NHC complexes have been studied by circular dichroism (CD).