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

Lena J. Daumann, Kristian E. Dalle, Gerhard Schenk, Ross P. McGeary, Paul V. Bernhardt, David L. Ollis, Lawrence R. Gahan
2012 Dalton Transactions  
Analogues of the ligand 2,2¢-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis((pyridin-2-ylmethyl)azanediyl)diethanol (CH 3 H 3 L1) are described. Complexation of these analogues, 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (CH 3 HL2), 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 (NO 2 HL2) and
more » ... n-2-ylmethyl)amino)methyl)phenol (CH 3 HL3) with zinc(II) acetate afforded [Zn 2 (CH 3 L2)(CH 3 COO) 2 ](PF 6 ), [Zn 2 (NO 2 L2)(CH 3 COO) 2 ](PF 6 ), [Zn 2 (BrL2)(CH 3 COO) 2 ](PF 6 ) and [Zn 2 (CH 3 L3)(CH 3 COO) 2 ](PF 6 ), in addition to [Zn 4 (CH 3 L2) 2 (NO 2 C 6 H 5 OPO 3 ) 2 (H 2 O) 2 ](PF 6 ) 2 and [Zn 4 (BrL2) 2 (PO 3 F) 2 (H 2 O) 2 ](PF 6 ) 2 . The complexes were characterized using 1 H and 13 C 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 [Zn 2 (CH 3 HL1)(CH 3 COO)(H 2 O)](PF 6 ). Functional studies of the zinc complexes with the substrate bis(2,4-dinitrophenyl) phosphate (BDNPP) showed them to be competent catalysts with, for example, [Zn 2 (CH 3 L2)] + , k cat = 5.70 ± 0.04 ¥ 10 -3 s -1 (K m = 20.8 ± 5.0 mM) and [Zn 2 (CH 3 L3)] + , k cat = 3.60 ± 0.04 ¥ 10 -3 s -1 (K m = 18.9 ± 3.5 mM). Catalytically relevant pK a s of 6.7 and 7.7 were observed for the zinc(II) complexes of CH 3 L2and CH 3 L3 -, respectively. Electron donating para-substituents enhance the rate of hydrolysis of BDNPP such that k cat p-CH 3 > p-Br > p-NO 2 . Use of a solvent mixture containing H 2 O 18 /H 2 O 16 in the reaction with BDNPP showed that for [Zn 2 (CH 3 L2)(CH 3 COO) 2 ](PF 6 ) and [Zn 2 (NO 2 L2)(CH 3 COO) 2 ](PF 6 ), as well as [Zn 2 (CH 3 HL1)(CH 3 COO)(H 2 O)](PF 6 ), the 18 O 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). combinations such as Zn(II)/Fe(II) likely. 7 The enzymes are also catalytically active as Co(II), Cd(II), Mn(II) or Ni(II) substituted forms. 8,9 The active site is typically composed of a nitrogen/oxygen coordination environment for two divalent metal ions coordinated by nitrogen donors located on histidine ligands, oxygen donors typically from an aspartate, and often including a metal ionbridging hydroxide ion. 10-14 Model complexes capable of reproducing the electronic, structural and reactivity characteristics of OP-hydrolyzing metalloenzyme systems generally exhibit less substrate specificity and are often more robust than the metalloenzymes themselves. 15-18 The question of the nucleophilic agent in these models, and often with the enzymes themselves, requires examination. In most hydrolytic enzymes a metal-hydroxide is suggested as the nucleophile in the key hydrolysis step, although exogenous alcohol moieties are also implicated. 19,20 Examples of model systems with either or both a metal-OH and a metal-OR present, which are This journal is View Article Online 3 days at room temperature, filtered and concentrated in vacuo. After purification by column chromatography (EtOAc/MeOH, 8 : 1, FeCl 3 stain, R f = 0.76) a yellow oil (0.5 g) was obtained in 68% yield. 1 H NMR (CDCl 3 , 500.13 MHz); d 2.80 (t, 3H, NCH 2 , J = 5.6 Hz); 3.29 (s, 6H, OCH 3 ); 3.52 (t, 4H, CH 2 OCH 3 , J = 5.6 Hz); 3.85 (s, 4H, arCH 2 N); 3.91 (s, 4H, NCH 2 py); 7.16 (dd, 2H, pyCH, J = 6.9 Hz); 7.42 (d, 2H, pyCH, J = 7.8 Hz); 7.65 (td, 2H, pyCH, J = 7.8, 1.7 Hz); 8.14 (s, 2H, arCH); 8.52 (d, 2H, pyCH, J = 4.4 Hz). 13 C NMR (CDCl 3 , 100.62 MHz); d 53.2 (NCH 2 ); 54.6 (arCH 2 N); 58.8 (OCH 3 ); 60.3 (NCH 2 py); 70.6 (CH 2 OCH 3 ); 122.2 (pyCH), 123.0 (pyCH); 124.5 (pyCH); 125.0 (arCH); 136.7 (pyCH); 139.7 (CNO 2 ); 148.9 (pyCH); 158.5 (pyC); 162.5 (COH). FT-IR spectroscopy (v, cm -1 ) 3372.3 (m, O-H str); 2927.1, 2877.8, 2820.8 (m, C-H str); 1591.6 (m, C C str); 1513.7 (m, N O asym. str); 1471.4 (m, C-H def); 1329.8 (s, N O sym. str); 1094.5 (m, C-O str); 845.3 (w, Ar-H); 752.3 (s, py-H). ESI mass spectrometry (methanol) m/z 496.25 [C 26 H 33 N 5 O 5 +H] + . 4-Bromo-2,6-bis(hydroxymethyl)phenol. 4-Bromo-2,6-bis(hydroxymethyl)phenol was prepared by a modification of a previously published procedure. 71 4-Bromophenol (17.3 g, 0.1 mol) in methanol (25 mL), aqueous sodium hydroxide solution (25%, 50 mL) and formaldehyde were combined and left at room This journal is
doi:10.1039/c1dt11187f pmid:22169932 fatcat:2smxaniutnbyrp2kyixufkudru