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

Daumann, Lena J. and Dalle, Kristian E. and Schenk, Gerhard and McGeary, Ross P. and Bernhardt, Paul V. and Ollis, David L. and Gahan, Lawrence R. (2012) The role of Zn–OR and Zn–OH nucleophiles and the influence of para-substituents in the reactions of binuclear phosphatase mimetics. Dalton Transactions, 41. pp. 1695-1708. ISSN 1477-9226

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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)]+, 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).

Item Type: Article
Keywords: Zn–OR; Zn–OH; nucleophiles; binuclear phosphatase mimetics;
Subjects: Science & Engineering > Chemistry
Item ID: 3689
Depositing User: Gary Schenk
Date Deposited: 28 May 2012 15:30
Journal or Publication Title: Dalton Transactions
Publisher: Royal Society of Chemistry
Refereed: Yes
URI:

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