Thermodynamic and Kinetic Studies of Zinc(II)–Triamine Complexes as Models of CA and AP

• Published in: Bulletin of the Chemical Society of Japan 1996-05, Vol.69 (5), p.1265-1274
• Main Authors: Itoh, Toyofumi, Fujii, Yuki, Tada, Toshiji, Yoshikawa, Yuzo, Hisada, Hiroyuki
• Format: Article
• Language: English
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.69.1265

The formation constant (KZnL), enthalpy and entropy changes (ΔH and ΔS), and deprotonation constant (Ka) of coordinated water for hydrated zinc(II)–triamine (1 : 1) complexes (N3–Zn-(OH2)n (N3 = diethylenetriamine (dien), N-(2-aminoethyl)-1,3-propanediamine (epd), dipropylenetriamine (dpt), cis,cis-1,3,5-triaminocyclohexane (tach), and 1,5,9-triazacyclododecane ([12]aneN3)) were determined by potentiometry. The pKa linearly increased along with an increase in -ΔH, and was explained in terms of a ligand–ligand interaction through the N3–Zn-O bond on the bases of a thermodynamic analysis and strain-energy calculation (MM2). The zinc(II) complex-promoted hydrolysis of 2,4-dinitrophenyl diethyl phosphate was investigated in 1% (v/v) methanol–water; the rate constants linearly increased along with decreases of pKa and −ΔH. This fact indicates that the hydrolysis proceeds via a concerted direct nucleophilic attacking mechanism of the coordinated hydroxide ion, in which the phosphate ester coordinates to the zinc(II) ion. X-ray structure analyses for synthesized model complexes, [Zn(OAc)(dien)](ClO4), [{Zn(dpt)}3(CO3)](ClO4)4·NaClO4, and [Zn(OAc)(tach)](ClO4), are also reported.