Cadmium(II) Complexes of (Arylazo)imidazoles: Synthesis, Structure, Photochromism, and Density Functional Theory Calculation

Reaction between CdX2 and 1-alkyl-2-(phenylazo)imidazole (RaaiR‘) has isolated complexes of composition Cd(RaaiR‘)2X2 in MeOH or MeCN. Crystallization of Cd(RaaiR‘)2I2 from N,N-dimethylformamide (DMF) has separated
[Cd(RaaiR‘)I2·DMF], while Cd(RaaiR‘)2X2 (X = Cl and Br) remains unchanged in its composition upon crystallization
under identical conditions. The structure has been established by spectral (UV−vis and 1H NMR) data and confirmation
in the latter case by a single-crystal X-ray diffraction study of [Cd(TaiMe)I2·DMF] [where TaiMe = 1-methyl-2-(p-tolylazo)imidazole]. UV-light irradiation in a MeCN solution of Cd(RaaiR‘)2I2 and [Cd(RaaiR‘)I2·DMF] shows trans-to-cis isomerization of coordinated azoimidazole. The reverse transformation, cis-to-trans, 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 values than their cadmium(II) iodo complexes. The cis-to-trans isomerization is a thermally induced process. The
activation energy (Ea) of cis-to-trans isomerization is calculated by a controlled-temperature experiment. The effects
of the anions (Cl-, Br-, I-, and ClO4-) and the number of coordinated azoimidazoles (RaaiR‘) [Cd(RaaiR‘) or
Cd(RaaiR‘)2] on the rate and quantum yields of photochromism are established in this work. A slow rate of
photoisomerization of [Cd(RaaiR‘)4](ClO4)2 compared to Cd(RaaiR‘)I2 or Cd(RaaiR‘)2X2 may be associated with the
increased mass and rotor volume of the complexes. The rate of isomerization is also dependent on the nature of
X and follows the sequence Cd(RaaiR‘)2Cl2 < Cd(RaaiR‘)2Br2 < Cd(RaaiR‘)2I2. It may be related to the size and
electronegativity of halide, which reduces the effective molar association in the order of I < Br < Cl and hence the
rate. Gaussian 03 calculations of representative complexes and free ligands are used to explain the difference in
the rates and quantum yields of photoisomerization.