Thermodynamic and kinetic studies of zinc(II)-triamine complexes as models of CA and AP

The formation constant (K-ZnL), enthalpy and entropy changes (Delta H and Delta S), and deprotonation constant (K,) of coordinated water for hydrated zinc(II)-triamine (1 : 1) complexes (N-3-Zn-(OH2)(n) (N-3 = diethylenetriamine (dien), N-(2-aminoethyl)- 1,3-propanediamine (epd), dipropylenetriamine (dpt), cis,cis- 1,3,5-triaminocyclohexane (tach), and 1,5,9-triazacyclododecane([12]aneN(3))) were determined by potentiometry. The pK(a) linearly increased along with an increase in -Delta H, and was explained in terms of a ligand-ligand interaction through the N-3-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 pK(a) and -Delta H. 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)](C1O(4)), [{Zn(dpt)}(3)(CO3)](ClO4)(4) . NaClO4, and [Zn(OAc)(tach)](C1O(4)), are also reported.