Hydrolysis of Lipophilic Esters Catalyzed by a Zinc(II) Complex of a Long Alkyl-Pendant Macrocyclic Tetraamine in Micellar Solution

A novel lipophilic zinc(II) complex with 1-hexadecyl-1,4,7,10-tetrazacyclododecane (hexadecylcyclen, L) has been synthesized, which is almost insoluble in water, but becomes soluble in the presence of Triton X-100 surfactant. Analysis of the potentiometric pH titration of Zn II −hexadecylcyclen (1 mM) in the presence of Triton X-100 (10 mM) disclosed monodeprotonation of the Zn II -bound H 2 O, yielding an OH - -bridged complex (ZnL) 2 OH - (p K d = −log([(ZnL) 2 OH - ] a H + /[ZnL] 2 ) = 3.92 ± 0.05) and a monomeric Zn II −OH - complex ZnL−OH - (p K a = −log([ZnL−OH - ] a H + /[ZnL]) = 7.56 ± 0.05) at 25 °C with I = 0.10 (NaNO 3 ). The zinc(II) complex (ZnL) possesses higher catalytic activity than the parent zinc(II) complex of 1,4,7,10-tetrazacyclododecane (cyclen, L‘) in hydrolysis of 4-nitrophenyl acetate (NA), bis(4-nitrophenyl) phosphate (BNP - ), and tris(4-nitrophenyl) phosphate (TNP) in aqueous comicellar solution with 10 mM Triton X-100. The NA hydrolysis activity of Zn II −hexadecylcyclen increased with the pH and leveled off at pH > 10 and 25 °C, from which the active species was estimated to be a ZnL−OH - complex. The second-order rate (first-order in [NA] and [ZnL−OH - ]) of NA hydrolysis ( k NA = 5.0 ± 0.2 M -1 s -1 ) in the presence of 10 mM Triton X-100 is 50 times greater than that with a reference ZnL‘−OH - in 10% (v/v) CH 3 CN aqueous solution. Hydrolysis of TNP in 10% (v/v) MeOH aqueous solution with ZnL−OH - and 10 mM Triton X-100 ( k TNP = (1.1 ± 0.1) × 10 3 M -1 s -1 ) is 290 times more efficient than with ZnL‘−OH - (3.8 ± 0.2 M -1 s -1 ) at 25 °C. The higher effective molarity of the lipophilic substrate coexisting with ZnL−OH - in the micelles accounts for the extraordinary catalytic activity. From comparison with previously reported metal catalysts, the present lipophilic zinc(II) complex with hexadecylcyclen is probably one of the best candidate catalyst for detoxification of poisonous phosphotriesters.