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

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)]+, kcat = 5.70 ± 0.04 × 10−3 s−1 (Km = 20.8 ± 5.0 mM) and [Zn2(CH3L3)]+, kcat = 3.60 ± 0.04 × 10−3 s−1 (Km = 18.9 ± 3.5 mM). Catalytically relevant pKas 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 kcatp-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)