Kinetics and mechanism of promoted hydrolysis of 4-nitrophenyl acetate by zinc(II)-tripod: Different roles of mononuclear and trinuclear species
✍ Scribed by Yan-He Guo; Qing-Chun Ge; Hai Lin; Hua-Kuan Lin; Shou-Rong Zhu
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- English
- Weight
- 122 KB
- Volume
- 36
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
Abstract
Coordination studies on Zn(II) complexes of 1,3,5‐tri(2,5‐diazahexyl)benzene (L) show that by comparison with the non‐deprotonation of complex ZnL in a 1:1 system, the three‐dimensional complexiaton decreases the p__K__~a~ of the Zn‐bound water molecule, that is, p__K__~a~ = 7.47 for trinulclear complex Zn~3~L in a 3:1 metal–ligand ratio. These two types of zinc(II) complexes have been examined as catalysts for the hydrolysis of 4‐nitrophenyl acetate (NA) in 10% (v/v) CH~3~CN at 298 K, I = 0.10 mol dm^−3^ KNO~3~ at pH range 6.5–8.2 and 8.5–10, respectively. Kinetic studies show that the second‐order rate constants of NA‐hydrolysis catalyzed by complexes ZnL, Zn~3~L, and Zn~3~LH~−1~ are 0.021, 0.0082, and 0.342 mol^−1^ dm^3^ s^−1^, respectively. In all the cases, the pH‐dependent observed first‐order rate constant, k~obs~, shows sigmoidal pH–rate profile. The 1:1 complex ZnL–H~2~O undergoes NA hydrolysis by direct rate‐determining hydrolysis to produce 4‐nitrophenol(ate) (NP^−^) and ZnL(OOCCH~3~); while in the 3:1 system the oxygen atom of acetic group forms a H‐bond with the Zn(II)‐bound water of the second branch of tripod indicating that the polynuclear centers are associated and bi‐functional. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 41–48 2004