trans-Dichloro-bis-(arylazoimidazole)palladium(II): Synthesis, Structure, Photoisomerization, and DFT Calculation

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• 作者：P. Pratiharp>† ; p> ; T. K. Mondalp>† ; p> ; A. K. Patrap>‡ ; p> ; C. Sinha*p>† ; p>
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：April 6, 2009
• 年：2009
• 卷：48
• 期：7
• 页码：2760-2769
• 全文大小：259K
• 年卷期：v.48,no.7(April 6, 2009)
• ISSN：1520-510X

文摘

Reaction between PdCl₂ and 1-alkyl-2-(arylazo)imidazole (RaaiR′) or 1-alkyl-2-(naphthyl-α/β-azo)imidazole (α/β-NaiR′) under reflux in ethanol has isolated complexes of compositions Pd(RaaiR′)₂Cl₂ (5, 6) and Pd(α/β-NaiR′)₂Cl₂ (7, 8). The X-ray structure determination of one of the molecules, Pd(α-NaiBz)₂Cl₂ (7c), has reported a trans-PdCl₂ configuration, and α-NaiBz acts as monodentate N(imidazole) donor ligand. The spectral (IR, UV−vis, p>1p>H NMR) data support the structure. UV light irradiation (light source: Perkin-Elmer LS 55 spectrofluorimeter, Xenon discharge lamp, λ = 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 (_E→Z) of E-to-Z isomerization are calculated, and 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)Cl₂ 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 (−NN−) function is important to reveal photochromic activity. DFT calculation of Pd(α-NaiBz)₂Cl₂ (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 −NN−Ar motif; thus, the rate of photoisomerization and quantum yield decrease versus the free ligand values.