Synthesis and crystallographic characterization of multi-donor N-heterocyclic carbene chelating ligands and their silver complexes: Potential use in pharmaceuticals

The potential for N-heterocyclic carbenes (NHCs) to be used as novel chelating ligands for bio-inorganic pharmaceuticals is discussed. In this paper, we design, synthesize and characterize two NHC precursors, 6 and 7, that we believe have potential for use as metal chelators for pharmaceuticals. The NHC precursors are composed of imidazolium and pyridine rings that would form mixed donor NHCs upon metallation with medicinally relevant metals. The exploration of the silver chemistry of 6 yielded the dimeric silver NHC complex 8[BPh4]2. The study of the silver chemistry of 7 gave 9[1/3(Ag4Br7)] and 10[NO3]3. Complex 9[1/3(Ag4Br7)] appears to be a silver biscarbene charge balanced by a silver bromide anionic cluster. Complex 10[NO3]3 is a trinuclear silver cluster that is stabilized by NHCs and pyridine rings. Silver NHCs have shown themselves to be excellent transmetallation agents for access to other metal NHC systems. It is envisioned that the silver NHCs 8[BPh4]2, 9[1/3(Ag4Br7)] and 10[NO3]3 will readily transfer to medicinally relevant metals, such 105Rh.