Structural Studies and Photochromism of Mercury(II)−Iodo Complexes of (Arylazo)imidazoles

Neat reaction between HgI2 and 1-methyl-2-(phenylazo)imidazole (Pai-Me) under microwave irradiation has isolated
a novel compound whose structure shows intercalated HgI2 in the layers of Pai-Me. They exist independently in
interpenetrated arrays. In a solution phase study, the same reaction has synthesized an iodo-bridged azoimidazole−Hg(II) complex, [Hg(RaaiR‘)(μ-I)(I)]2 (RaaiR‘ = 1-alkyl-2-(arylazo)imidazole). The structures have been characterized
by X-ray diffraction studies. Chloro-bridged Hg(II) complexes of azoimidazoles, [Hg(RaaiR‘)(μ-Cl)(Cl)]2, are also
known. These complexes upon irradiation with UV light show trans-to-cis isomerization. The reverse transformation,
cis-to-trans isomerization, is very slow with visible light irradiation. Quantum yields (φt→c) of trans-to-cis isomerization
are calculated, and the free ligand shows higher φ than their Hg(II) complexes. The cis-to-trans isomerization is a
thermally induced process. The activation energy (Ea) of cis-to-trans isomerization is calculated by controlled
temperature reaction. The Ea's of free ligands are much higher than that of halo-bridged Hg(II)−azoimidazole
complexes. Chloro-bridged Hg(II) complexes show lower Ea's than those of iodo-bridged complexes. DFT calculation
has been adopted to rationalize the experimental results.