Influence of the Coordination Environment of Zinc(II) Complexes of Designed Mannich Ligands on Phosphatase Activity: A Combined Experimental and Theoretical Study

A mononucleating (HL1)
and a dinucleating (HL2) “end-off” compartmental
ligand have been designed and synthesized by controlled Mannich reaction
using p-cresol and bis­(2-methoxyethyl)­amine, and
their formation has been rationalized. Six complexes have been prepared
on treating HL1 and HL2 with ZnIIX2 (X = Cl–, Br–,
I–) with the aim to investigate their hydrolytic
activity on phosphoester bond cleavage. Interestingly, the mononucleating
ligand was observed to yield dinuclear complexes, [Zn2(L1)2X2] (1–3), while the potential dinucleating ligand generated mononuclear
complexes, [Zn­(HL2)­X2] (4–6). Four (1–4), out of six
complexes studied, were characterized by single-crystal X-ray diffraction
(XRD): the Zn ion exhibits trigonal bipyramidal and tetrahedral coordination
spheres in the di- and mononuclear complex, respectively. The hydrolytic
kinetics, followed spectrophotometrically with 4-nitrophenylphosphate
(4-NPP) in buffered dimethylformamide (DMF) (97.5% DMF, v/v) because
of solubility reasons, under excess substrate conditions (substrate:complex
= 20:1), indicated that the complexes enormously accelerate the rate
of phosphomonoester hydrolysis with first order rate constants (kcat) in the range 2–10 s–1 at 25 °C. In each case kinetic data analyses have been run
by Michaelis–Menten treatment. The efficacy in the order of
conversion of substrate to product (p-nitrophenolate
ion) follows the trend 1 > 2 > 3 > 4 > 5 > 6, and the ratio of kcat of an analogous
dinuclear to mononuclear complex is ≃2. An electrospray ionization-mass
spectrometry (ESI-MS) study has revealed the dissociation of the centrosymmetric
dinuclear complex to two mononuclear species instead of a syn-cooperative
catalysis. Density functional theory (DFT) calculations have been
performed to rationalize our proposed mechanistic pathway for phosphatase
activity. The comparative analysis concludes the following facts under
experimental conditions: (1) the halide bound to the active site affects
the overall rate in the order: Cl– > Br– > I– regardless of nuclearity; (2) dinuclear
complexes prevail over the mononuclear ones.