Hydrolysis of phosphate esters catalyzed by copper(II)-triamine complexes. The effect of triamine ligands on the reactivity of the copper(II) catalysts

Add time:09/03/2019    Source:sciencedirect.com

Hydrolyses of lithium 2,4-dinitrophenyl ethyl phosphate (7) and 2,4-dinitrophenyl diethyl phosphate (8) catalyzed by copper(II)—triamine complexes (triamine = cis,cis-triaminocyclohexane (1), 1,5,9-triazacyclododecane (2), 1,4,7-triazacyclononane (3), dipropylenetriamine (4), N-(2-aminoethyl)-1,3-propanediamine (5), diethylenetriamine (6)) were investigated at pH 7.0–9.5, and the reactivity of the catalysts was found to increase in the order 5 ∼ 6 ∼ 4 < 3 < 1 < 2. From the pH dependence of the apparent rate constants, and the deprotonation constant (pKa) of the coordinated water, the active species was confirmed to be CuL(OH)+ for all of the catalysts. Furthermore, the reactions were confirmed to proceed via a phosphate ester-coordinated intermediate on the basis of a good fit between the reciprocal of the stability constant (Kester−1) of a mixed complex of CuL(OH)+ with diethyl phosphate (9) and the Michaelis constant KM for 1–4, and of crystal structure analysis of μ-(diethyl phosphato)-(dipropylenetriamine)copper(II) hexafluorophosphate (10). In addition, it was found that the increasing order (5 ∼ 6 ∼ 4 ≪ 3 < 1 ∼ 2) in the estimated rate constant (kcat) corresponded closely to the decreasing order in the coordination strength of triamine ligands. The effect of triamine ligands was discussed in relation to the Lewis acidity of central copper(II) ion and the reaction mechanism.

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