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

Synthesis of a planar, π-conjugated pyridazine-based
biscarbene
is reported. Starting from 3,6-dimethylpyridazine, the bisimidazolium
salt 1*2HPF6 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*2HPF6 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.