Bridged dinucleating N-heterocyclic carbene ligands and their double helical mercury(II) complexes

A series of six 3,6-bis(imidazolium-3-yl)pyridazine derivatives with different imidazole-N substituents have been synthesized and isolated as the salts [H2L]Cl2 (1a)–(6a) and [H2L](PF6)2 (1b)–(6b). Solid state structures have been determined crystallographically for eleven out of the twelve compounds, revealing diverse hydrogen bonding patterns that involve the imidazolium-C2H units and the anions. N-heterocyclic carbene (NHC) mercury(II) complexes [Hg2L2](PF6)4 (7)–(9) are readily formed in good yields from ligand precursors [H2L](PF6)2 and Hg(OAc)2, as long as imidazole-N substituents are not too bulky. X-ray crystallography reveals double helical bimetallic arrangements for the stable [Hg2L2]4+ cations. Ligand scrambling in [Hg2L2]4+ occurs only in the presence of free carbene precursor, presumably via an associative mechanism.