title: trans-Dichloro-bis-(arylazoimidazole)palladium(II): synthesis, structure, photoisomerization, and DFT calculation.
abstract: Reaction between PdCl(2) and 1-alkyl-2-(arylazo)imidazole (RaaiR') or 1-alkyl-2-(naphthyl-alpha/beta-azo)imidazole (alpha/beta-NaiR') under reflux in ethanol has isolated complexes of compositions Pd(RaaiR')(2)Cl(2) (5, 6) and Pd(alpha/beta-NaiR')(2)Cl(2) (7, 8). The X-ray structure determination of one of the molecules, Pd(alpha-NaiBz)(2)Cl(2) (7c), has reported a trans-PdCl(2) configuration, and alpha-NaiBz acts as monodentate N(imidazole) donor ligand. The spectral (IR, UV-vis, (1)H NMR) data support the structure. UV light irradiation (light source: Perkin-Elmer LS 55 spectrofluorimeter, Xenon discharge lamp, lambda = 360-396 nm) in a MeCN solution of the complexes shows E-to-Z isomerization of the coordinated azoimidazole unit. The reverse transformation, Z-to-E, is very slow with visible light irradiation. Quantum yields (phi(E-->Z)) of E-to-Z isomerization are calculated, and phi is lower than that of the free ligand but comparable with those of Cd(II) and Hg(II) complexes of the same ligand. The Z-to-E isomerization is a thermally induced process. The activation energy (E(a)) of Z-to-E isomerization is calculated by controlled-temperature experimentation. cis-Pd(azoimidazole)Cl(2) complexes (azomidazole acts as N(imidazole) and N(azo) chelating ligand) do not respond upon light irradiation, which supports the idea that the presence of noncoordinated azo-N to make free azo (-N=N-) function is important to reveal photochromic activity. DFT calculation of Pd(alpha-NaiBz)(2)Cl(2) (7c) has suggested that the HOMO of the molecule is constituted of Pd (32%) and Cl (66%), and hence photo excitation may use the energy of Pd and Cl instead of that of the photofunctional -N=N-Ar motif; thus, the rate of photoisomerization and quantum yield decrease versus the free ligand values.
authors.affiliation: Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India.
authors.forename: P
authors.initials: P
authors.lastname: Pratihar
authors.forename: T K
authors.initials: TK
authors.lastname: Mondal
authors.forename: A K
authors.initials: AK
authors.lastname: Patra
authors.forename: C
authors.initials: C
authors.lastname: Sinha
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D007093:Imidazoles / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D010165:Palladium / Q000737:chemistry*; D010777:Photochemistry; D013237:Stereoisomerism
chemical_list: D001391:Azo Compounds; D007093:Imidazoles; D009942:Organometallic Compounds; D010165:Palladium