1,10-Phenanthroline Analogue Pyridazine-Based N-Heterocyclic Carbene Ligands

Synthesis of a planar, pi-conjugated pyridazine-based biscarbene is reported. Starting from 3,6-dimethylpyridazine, the bisimidazolium salt 1*2HPF(6) was prepared in a four-step synthesis by chlorination, amination, formylation, and cyclization. The free carbene 1 can be generated in situ by addition of base. Despite the rigid annelated tricycle, the carbene ligand turns out to be highly flexible upon coordination of transition-metal complexes. With silver(I) oxide or copper(I) oxide binuclear carbene complexes with a bridging coordination mode of the ligand are obtained. Transmetalation of both complexes to the respective gold complex is described. The bridging coordination mode of the carbene ligand is similar to that of 2,2'-bipyridine. Reaction of the bisimidazolium salt 1*2HPF(6) with potassium acetate and [RhCl(COD)](2) leads to a mononuclear rhodium complex with the chelating binding mode of 1, resembling strongly the coordination properties of 1,10-phenanthroline.