Rhenium complexes of bidentate, bis-bidentate and tridentate N-heterocyclic carbene ligands

Abstract

A series of eight Rhenium(I)-N-heterocyclic carbene (NHC) complexes of the general form [ReCl(CO)₃(C^C)] (where C^C is a bis(NHC) bidentate ligand), [ReCl(CO)₃(C^C)]₂ (where C^C is a bis-bidentate tetra-NHC ligand) and [Re(CO)₃(C^N^C)]⁺[X]⁻ (where C^N^C is a bis(NHC)-amine ligand and the counter ion X is either the ReO₄⁻ or PF₆⁻) have been synthesised using a Ag₂O transmetallation protocol. The novel precursor imidazolium salts and Re(I) complexes were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy and the molecular structures for two imidazolium salt and six Re(I) complexes were determined by single crystal X-ray diffraction. These NHC ligand systems are of interest for possible applications in the development of Tc-99m or Re-186/188 radiopharmaceuticals and as such the stability of two complexes of the form [ReCl(CO)₃(C^C)] and [Re(CO)₃(C^N^C)][ReO₄] were evaluated in ligand challenge experiments using the metal binding amino acids L-histidine or L-cysteine. These studies showed that the former was unstable, with the chloride ligand being replaced by either cysteine or histidine, while no evidence for transchelation was observed for the latter suggesting that bis(NHC)-amine ligands of this type may be suitable for biological applications.