1-Alkyl-2-{(o-thioalkyl)phenylazo}imidazole complexes of mercury(II) and their photochromic properties

The reaction of HgX2 (X = Cl, Br, I) with 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole (SRaaiNR′) in MeOH-ethyleneglycol mixture has synthesised [Hg(SRaaiNR′)X2]. The structure of the products has been established by microanalytical and spectral (IR, UV–Vis, 1H NMR) data, and a distorted Td-geometry has been confirmed by a single crystal X-ray diffraction study of [Hg(SMeaaiNEt)I2] (SMeaaiNEt = 1-ethyl-2-{(o-thiomethyl)phenylazo}imidazole). The UV light irradiation in CH3CN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configurations) isomerisation of the coordinated SRaaiNR′ ligand about the –NN– bond. The rate of isomerisation follows: [Hg(SRaaiNR′)Cl2] < [Hg(SRaaiNR′)Br2] < [Hg(SRaaiNR′)I2]. The quantum yields (ϕE→Z) of the isomerisation of the complexes are lower than those of the free ligands. These may be due to increased mass and rotor volume of the complexes. The electronegativity sequence of X (I < Br < Cl) may regulate the molecular association and hence the effective mass of the rotor and the photoisomerisation rates. Thermal isomerisation determines the rate, activation energy (Ea), activation enthalpy (ΔH∗) and activation entropy (ΔS∗) of the Z → E transformation of the coordinated SRaaiNR′. The Eas of the thermal isomerisation of the complexes are lower than those of the free ligands.

Hg(SRaaiNR′)X2 (SRaaiNR′ = 1-alkyl-2-{(o-thioalkyl)phenylazo}imidazole) complexes are established by different spectral data and by a single crystal structure in one case. UV light irradiation in CH3CN solution of the complexes shows E-to-Z (E and Z refer to trans and cis-configuration) isomerisation of the coordinated SRaaiNR′ ligand about the –NN– bond. The rate of isomerisation follows: [Hg(SRaaiNR′)Cl2] < [Hg(SRaaiNR′)Br2] < [Hg(SRaaiNR′)I2]. The quantum yields (ϕE→Z) of the isomerisation of the complexes are lower than those of free ligands. The decrease in electronegativity of X increases the rate of E-to-Z photoisomerisation. The Z-to-E isomerisation is a thermally driven process. The Eas of the thermal isomerisation of the complexes are lower than those of the free ligands.Figure optionsDownload as PowerPoint slideHighlights
► Hg(II)-(thioalkylimidazolylazoimidazole)X2 complexes are spectroscopically characterized.
► UV light irradiation shows E-to-Z isomerisation of the coordinated SRaaiNR′ ligand.
► The Z-to-E isomerisation is a favored thermal process.
► The Ea of the Z-to-E isomerisation is lower in the complexes than in the free ligands.
► The decrease in electronegativity of X increases the rate of E-to-Z photoisomerisation.