Mechanistic Implications in the Phosphatase Activity of Mannich-Based Dinuclear Zinc Complexes with Theoretical Modeling

An “end-off” compartmental
ligand has been synthesized
by an abnormal Mannich reaction, namely, 2-[bis­(2-methoxyethyl)­aminomethyl]-4-isopropylphenol
yielding three centrosymmetric binuclear μ-phenoxozinc­(II) complexes
having the molecular formula [Zn2(L)2X2] (Zn-1, Zn-2, and Zn-3),
where X = Cl–, Br –, and I –, respectively. X-ray crystallographic analysis shows
that the ZnO3NX chromophores in each molecule form a slightly
distorted trigonal-bipyramidal geometry (τ = 0.55–0.68)
with an intermetallic distance of 3.068, 3.101, and 3.083 Å (1–3, respectively). The spectrophotometrical
investigation on their phosphatase activity established that all three
of them possess significant hydrolytic efficiency. Michaelis–Menten-derived
kinetic parameters indicate that the competitiveness of the rate of
P–O bond fission employing the phosphomonoester (4-nitrophenyl)­phosphate
in 97.5% N,N-dimethylformamide is 3 > 1 > 2 and the kcat value lies in the range 9.47–11.62
s–1 at 298 K. Theoretical calculations involving
three major active
catalyst forms, such as the dimer-cis form (D-Cis), the dimer-trans
form (D-Trans), and the monoform (M-1 and M-2), systematically interpret
the reaction mechanism wherein the dimer-cis form with the binuclear-bridged
hydroxide ion acting as the nucleophile and one water molecule playing
a role in stabilizing the leaving group competes as the most favored
pathway.