Isostructural Dinuclear Phenoxo-/Acetato-Bridged Manganese(II), Cobalt(II), and Zinc(II) Complexes with Labile Sites: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate

Abstract

Using the dinucleating phenol-based ligand 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol] (HL(2)), in its deprotonated form, the six new dinuclear complexes [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][PF(6)] (M = Mn (2a), Co (3a), Zn (4a)) and [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][BPh(4)] (M = Mn (2b), Co (3b), Zn (4b)) have been synthesized. Crystallographic analyses on 2b·2MeCN, 3b·2MeCN, and 4b·2MeCN reveal that these complexes have closely similar μ-phenoxo bis(μ-carboxylato) structures. The physicochemical properties (absorption and ESI-MS spectral data, 2a,b, 3a,b, and 4a,b; (1)H NMR, 4a,b) of the cations of 2a-4a are identical with those of 2b-4b. Each metal ion is terminally coordinated by a pyrazole nitrogen and a pyridyl nitrogen from a 3-(pyridin-2-yl)pyrazole unit and a solvent molecule (MeCN). Thus, each metal center assumes distorted-octahedral M(II)N(3)O(3) coordination. Temperature-dependent magnetic studies on Mn(II) and Co(II) dimers reveal the presence of intramolecular antiferromagnetic (J = -8.5 cm(-1)) for 2b and ferromagnetic exchange coupling (J = +2.51 cm(-1)) for 3b, on the basis of the Hamiltonian H = -JS(1)·S(2). The exchange mechanism is discussed on the basis of magneto-structural parameters (M···M distance). Spectroscopic properties of the complexes have also been investigated. The pH titration and kinetics of phosphatase (transesterification) activity on 2-hydroxypropyl-p-nirophenylphosphate (HPNP) were studied in MeOH/H(2)O (33%, v/v) with 2a-4a, due to solubility reasons. This comparative kinetic study revealed the effect of the metal ion on the rate of hydrolysis of HPNP, which has been compared with what we recently reported for [Ni(II)(2)(L(2))(μ-O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (1a). The efficacy in the order of conversion of substrate to product (p-nitrophenolate ion) follows the order 4a > 3a > 2a > 1a, under identical experimental conditions. Notably, this trend follows the decrease of pK(a) values of M(II)-coordinated water (7.95 ± 0.04 and 8.78 ± 0.03 for 1a, 7.67 ± 0.08 and 8.69 ± 0.06 for 2a, 7.09 ± 0.05 and 8.05 ± 0.06 for 3a, and 6.20 ± 0.04 and 6.80 ± 0.03 for 4a). In this work we demonstrate that the stronger the Lewis acidity (Z(eff)/r) of the metal ion, the more acidic is the M(II)-coordinated water and the greater is the propensity of the metal ion to catalyze hydrolysis of the activated phosphate ester HPNP. Notably, the observed k(2) values (M(-1) s(-1)) for Mn(II) (2a, 0.152), Co(II) (3a, 0.208), and Zn(II) (4a, 0.230) complexes (1a, 0.058; already reported) linearly correlate with Z(eff)/r values of the metal ion. In each case a pseudo-first-order kinetic treatment has been done. Kinetic data analysis of complexes 2a-4a were also done following Michaelis-Menten treatment (catalytic efficiency k(cat)/K(M) values 0.170 M(-1) s(-1) for 2a, 0.194 M(-1) s(-1) for 3a and 0.161 M(-1) s(-1) for 4a; for 1a the value is 0.089 M(-1) s(-1)). Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transeste...

Phosphatase models: Synthesis, structure and catalytic activity of zinc complexes derived from a phenolic Mannich-base ligand

“…The coordination patterns and binding models of the catalystsubstrate complex have been largely explored in literature [15,16,20,30,42] and after rigorous considerations two plausible catalytic cycles having reasonably good agreement with our kinetic results have been postulated from the previous representative mechanisms. The substrate molecule binds as a dianionic mononitrophenyl phosphate to the complex having phosphatase activity.…”

“…Over the years, synthetic Zn(II) (and also other transition metal) complexes have been judiciously studied as phosphate ester models taking into account their extraordinary Lewis acidity, redox rigidity, nucleophile generation, leaving group stabilization and physiological relevancy [11,12]. However, among the numerous catalytically promiscuous Zn(II) derivatives [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] some complexes show low activity, others require rigorous synthetic conditions or have high toxicity. Therefore the development of new efficient metallocatalysts is still a tough challenge in the field of biocatalysis.…”

Spectroscopic and mechanistic studies of dinuclear metallohydrolases and their biomimetic complexes

“…Since Sargeson's early work other examples of functional or spectroscopic model complexes for several dicobalt(III)/(II) complexes have been reported. 149,[176][177][178][179][180][181][182][183][184][185] For the Co(II) complexes in the present studies the activity towards organophosphoesters using BDNPP, was investigated (Table 2). 136 All complexes are good functional mimics for phosphodiesterases and show one pK a relevant for hydrolysis.…”

Hydrolysis of the RNA model substrate catalyzed by a binuclear ZrIV-substituted Keggin polyoxometalate

“…12 In addition, it was shown that the monomeric [α-PW 11 O 39 Zr-(µ-OH)(H 2 O)] 4− (ZrK 1 : 1) species is present in aqueous solution at near-neutral pD and that this species is responsible for the hydrolysis of the phosphoester bond in BNPP. 19 As a previous study showed that the solution speciation of this POM is highly dependent on several factors such as pD, temperature, initial concentration, ionic strength and substrate concentration, 12 the speciation behavior of ZrK 2 : 2 at the conditions used to study HPNP hydrolysis was studied in detail. Theoretical studies have shown that when ZrK 1 : 1 interacts with NPP or BNPP, it prefers to form monodentate complexes which are more stable than the corresponding bidentate complexes.…”

“…reaction show that cyclic phosphate (CycP), with a 31 P resonance at 18 ppm, is formed 19,28. At the end of hydrolysis, no HPNP resonances were detected in both 1 H and 31 P NMR spectra, indicating full conversion of HPNP into cyclic phosphate and NP.Several control experiments were performed.…”