ABSTRACT
Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time-dependent density functional calculations as largely due to different excited-state dynamics of [6](2+) and [9](2+) . The geometries of both radicals [6](.+) and [9](.+) and excited states {[6](2+) }* and {[9](2+) }* are substantially different from those of the parent ground-state molecules.
ABSTRACT
A new and highly flexible biscarbene ligand with two imidazolin-2-ylidene moieties in 6,6'-position of a 2,2'-bipyridine was prepared and structurally characterised by X-ray analysis. Various silver complexes were prepared from the imidazolium salts. Depending on the counterion, the coordination mode of the ligand can be mono- or bidentate. Pd complexes are feasible directly from the imidazolium salt using palladium(II)-acetate or via transmetallation from silver carbene complexes with [PdCl(2)(CH(3)CN)(2)] or [PdCl(2)(COD)]. Controlled by the structure of the silver complexes the biscarbenebipyridine ligand is a di- or tetracoordinating ligand forming mono- or dinuclear complexes. The high flexibility of this ligand is confirmed by transmetallation from silver to copper, which results in formation of a dinuclear copper complex with two bridging biscarbene ligands.
