The role of Zn–OR and Zn–OH nucleophiles and the influence of para-substituents in the reactions of binuclear phosphatase mimetics

Abstract

Analogues of the ligand 2,2′-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis((pyridin-2-ylmethyl)azanediyl)diethanol (CH₃H₃L1) are described. Complexation of these analogues, 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (CH₃HL2), 4-bromo-2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (BrHL2), 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (NO₂HL2) and 4-methyl-2,6-bis(((2-phenoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (CH₃HL3) with zinc(II) acetate afforded [Zn₂(CH₃L2)(CH₃COO)₂](PF₆), [Zn₂(NO₂L2)(CH₃COO)₂](PF₆), [Zn₂(BrL2)(CH₃COO)₂](PF₆) and [Zn₂(CH₃L3)(CH₃COO)₂](PF₆), in addition to [Zn₄(CH₃L2)₂(NO₂C₆H₅OPO₃)₂(H₂O)₂](PF₆)₂ and [Zn₄(BrL2)₂(PO₃F)₂(H₂O)₂](PF₆)₂. The complexes were characterized using ¹H and ¹³C NMR spectroscopy, mass spectrometry, microanalysis, and X-ray crystallography. The complexes contain either a coordinated methyl- (L2 ligands) or phenyl- (L3 ligand) ether, replacing the potentially nucleophilic coordinated alcohol in the previously reported complex [Zn₂(CH₃HL1)(CH₃COO)(H₂O)](PF₆). Functional studies of the zinc complexes with the substrate bis(2,4-dinitrophenyl) phosphate (BDNPP) showed them to be competent catalysts with, for example, [Zn₂(CH₃L2)]⁺, k_cat = 5.70 ± 0.04 × 10⁻³ s⁻¹ (K_m = 20.8 ± 5.0 mM) and [Zn₂(CH₃L3)]⁺, k_cat = 3.60 ± 0.04 × 10⁻³ s⁻¹ (K_m = 18.9 ± 3.5 mM). Catalytically relevant pK_as of 6.7 and 7.7 were observed for the zinc(II) complexes of CH₃L2⁻ and CH₃L3⁻, respectively. Electron donating para-substituents enhance the rate of hydrolysis of BDNPP such that k_catp-CH₃ > p-Br > p-NO₂. Use of a solvent mixture containing H₂O¹⁸/H₂O¹⁶ in the reaction with BDNPP showed that for [Zn₂(CH₃L2)(CH₃COO)₂](PF₆) and [Zn₂(NO₂L2)(CH₃COO)₂](PF₆), as well as [Zn₂(CH₃HL1)(CH₃COO)(H₂O)](PF₆), the ¹⁸O label was incorporated in the product of the hydrolysis suggesting that the nucleophile involved in the hydrolysis reaction was a Zn–OH moiety. The results are discussed with respect to the potential nucleophilic species (coordinated deprotonated alcohol versus coordinated hydroxide).