This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7717--7719 7717

Cite this: Chem. Commun.,2013,49, 7717

Oximate metal complexes breaking the limitingesterolytic reactivity of oximate anions†

Paola Gomez-Tagle, Jose Carlos Lugo-Gonzalez and Anatoly K. Yatsimirsky*

Zinc(II) and cadmium(II) complexes of a tridentate oximate ligand

cleave 4-nitrophenyl acetate with rate constants surpassing by two

orders of magnitude those reported as the maximum possible level

for highly basic free oximate anions as a result of removal of the

‘‘solvational imbalance’’ of the nucleophile by metal coordination.

Oximate anions belong to a type of so-called a-nucleophilepossessing two adjacent donor atoms and displaying enhancedreactivity as compared to simple nucleophiles of similar basicity.The kinetics of carboxylic acid and phosphate ester cleavage byoximates has been studied extensively both in pursuing a deeperunderstanding of the nature of the a-effect1 and in connectionwith using oximes as cholinesterase re-activators or detoxicants2

and analytical reagents.3 A characteristic feature of the esterasereactivity of oximate anions is a non-linear Bronsted plot of thelogarithms of second-order rate constants of oximolysis vs. pKa

of oxime, which levels off when the pKa reaches approximately 8for 4-nitrophenyl acetate (NPA) as a substrate.4 The rate con-stants for oximes with pKa 4 8 fluctuate around ca. 50 M�1 s�1

and never surpass 100 M�1 s�1 even with the most basic oximateanions. This effect is attributed to a ‘‘solvational imbalance’’created by the necessity of nucleophile desolvation prior to thenucleophilic attack and is progressively more important formore basic oximate anions.5

The reactivity of coordinated oximates has been studied quiteextensively with 2-pyridine oximes as ligands.6–9 The coordina-tion reduces the pKa of oxime group below 8 and as in a case oforganic oximes one observes a Bronsted type correlation ofreactivity within groups of complexes with ligands of similarstructure.9 Two aspects of reactivity of coordinated oximes are note-worthy. First, metal ions catalyze the hydrolysis of O-acyl oximes10

converting oximes from stoichiometric reactants to catalysts.

Second, on some occasions the reactivity of metal-boundoximates has been found to be several times higher than thatof free anions for as yet unclear reasons.7–9 Here we report theesterolytic reactivity of the coordinated oxime 1, which sur-passes by more than two orders of magnitude the limitingreactivity of free oximate anions and provide a possible expla-nation of this effect.

The ligand 1 (HL) was prepared according to a publishedprocedure (see ESI†).11 The composition and stability of itscomplexes with Zn(II) and Cd(II) were determined via potentio-metric and spectrophotometric titrations (see ESI,† Fig. S1–S4).A complete list of overall stability and protonation constants isgiven in Table S1 (ESI†) and, derived from them, pKa values forfree and coordinated ligand and logarithms of individualformation constants are summarized in Table 1.

The complex formation with Cd(II) follows a simple scheme,which involves the coordination of neutral ligand and itsdeprotonation with the pKa reduced by 2.37 units as comparedwith the free ligand. More electrophilic Zn(II) also forms acomplex with two ligands in addition to a 1 : 1 complex. TheZn(LH)2+ complex undergoes two consecutive deprotonations,which implies deprotonation of coordinated water in additionto the oxime ligand group. The first deprotonation withpKa 7.38 was attributed to the former and the second onewith pKa 8.18 to the latter process (reactions 7 and 8

Universidad Nacional Autonoma de Mexico, Facultad de Quımica, 04510,

Mexico City, Mexico. E-mail: anatoli@unam.mx; Fax: +52 55 56162010;

Tel: +52 55 56223813

† Electronic supplementary information (ESI) available: Synthesis and character-ization of 1; potentiometric and spectrophotometric titration data, overall stabi-lity constants for complexes with Zn(II) and Cd(II); kinetic curves for NPA cleavagewith a 5-fold excess of the ester over 1. See DOI: 10.1039/c3cc43944e

Received 25th May 2013,Accepted 11th July 2013

DOI: 10.1039/c3cc43944e

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7718 Chem. Commun., 2013, 49, 7717--7719 This journal is c The Royal Society of Chemistry 2013

respectively in Table 1) on the basis of UV-Vis spectral data(see ESI,† Fig. S3 and S4).

The kinetics of NPA cleavage were monitored spectrophoto-metrically by the appearance of 4-nitrophenol (NP) in 0.01–0.05 M MOPS, CHES or CAPS buffer solution containing 2%MeCN at 25 1C. Reactions were followed until complete con-sumption of NPA and observed first-order rate constants (kobs)were calculated by fitting the kinetic curves to the respectivefirst-order integral equation. The kinetics of NPA cleavageby free ligand were studied under similar conditions byinitial rates.

The second-order rate constant for NPA cleavage by freeligand anion kox = 60 � 2 M�1 s�1 is in the range of kox valuesreported for other oximate anions of oximes with pKa 4 8 (seeabove). Additions of Zn(II) or Cd(II) accelerate cleavage of NPAby 1 enormously. For instance, in the presence of 1 mM 1 at pH8 the half-life of NPA is about 10 h, but it drops to just a fewseconds on addition of 1 mM Zn(II) or Cd(II). Among othertested metal ions, Cu(II) produced an inhibitory effect whileMn(II) and Ni(II) produced small acceleration effects.

In order to identify the reactive species, reaction rates in thepresence of Zn(II) or Cd(II) at constant metal to ligand ratio 1 : 1were measured as a function of pH and the kobs vs. pH profileswere superimposed with the species distribution diagrams,Fig. 1. In separate experiments at several fixed pH values itwas established that kobs is a linear function of total complexconcentration (ESI,† Fig. S5).

Inspection of Fig. 1 clearly shows that observed reactivity canbe assigned to Zn(L)(OH) and Cd(L) complexes containingthe deprotonated oximate form of 1. The second-order rateconstants for NPA cleavage by oximate complexes (kMox) werecalculated as the slopes of kobs vs. Zn(L)(OH) or Cd(L) concen-trations and were found to be equal (8.7 � 0.4) � 103 and(1.09 � 0.06) � 104 M�1 s�1 for Zn(II) and Cd(II) respectively.Thus the esterolytic reactivity of coordinated oximate anionssurpasses that of free oximate by more than two orders ofmagnitude.

An additional set of experiments was performed underconditions of excess NPA over oximate complexes. Mixing of10 mM 1 with 10 mM Zn(II) and 1 mM NPA at pH 9 resulted inliberation of 0.4 mM NP (40 equivalents with respect to 1) after10 min. In a similar experiment with Cd(II) instead of Zn(II) theliberation of 0.35 mM NP was observed. Thus, reactions in

the presence of metal ions proceed with high turnover throughthe catalytic cycle illustrated in Scheme 1.

A more detailed kinetic study was performed at lower 50 mMNPA concentration in the presence of a 10 mM 1 : 1 mixture of 1and metal ions. Fig. S6 (ESI†) shows the representative resultsfor the reaction in the presence of Cd(II) at different pH values.Kinetic curves show an initial ‘‘burst’’ in the production of 1equivalent of NP followed by a slower reaction. Such behavior ischaracteristic of the enzyme hydrolysis of esters with goodleaving groups.12 Reaction rates after the ‘‘burst’’ limited bythe deacylation step strongly increase with an increase in pH inagreement with the reported first-order in free hydroxidekinetics of the deacylation of O-acetyl-2-pyridine oximes inthe presence of metal ions10 reflected in Scheme 1.

The unusually high reactivity of Zn(II) and Cd(II) complexesof 1 can be rationalized by analyzing the Bronsted plot includ-ing rate constants and pKa values for all reported to dateoximate metal complexes with ligands 2–4 together with com-plexes of 1, Fig. 2.

Although at first glance the correlation seems unacceptablypoor, it is obvious that complexes of ligands 4a,b constitute aseparate correlation line (red squares) shifted by ca. 3 logarithmic

Table 1 Logarithms of formation constants (log K) of metal complexes and pKa

values for free and coordinated 1 at 25 1C and ionic strength 0.1 M. The numberin parentheses is the standard error in the last significant digit

Reaction log K or pKa

1 HL $ H + L 11.74(9)2 H2L $ H + HL 6.96(5)3 H3L $ H + H2L 2.6(2)

Zn(II) Cd(II)4 M + HL $ M(HL) 5.04(5) 4.46(7)5 M(HL) + HL $ M(HL)2 3.94(4)6 M(HL) $ M(L) + H 9.37(5)7 M(HL) $ M(HL)(OH) + H 7.38(5)8 M(HL)(OH) $ M(L)(OH) + H 8.17(8)9 M(HL)2 $ M(L)(HL) + H 6.35(7)

Fig. 1 First-order rate constants of NPA cleavage by 0.1 mM 1 in the presence of0.1 mM Zn(II) (black squares) or Cd(II) (blue circles) superimposed with speciesdistribution plots (dashed lines) for the respective oximate complexes.

Scheme 1 Mechanism of catalytic NPA cleavage by coordinated 1.

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units downward from other complexes and must be discussedseparately. The reason for this decreased reactivity is not clear atthe moment. The results for other complexes (black squares)clearly show a linear trend with a slope of about unity. Thevariations in reactivity for coordinated oximate ions of similarbasicity are significant but not larger than those for free organicoximates.4 The most important point is that there is no tendencyto level off in the range of pKa 7–8, as is observed for free oximates(dashed line). One can conclude therefore that the metal coordi-nation of the oximate anions removes the ‘‘solvational imbalance’’around the nucleophile apparently by changing its solvation andthis allows further increase in reactivity for more basic oximates. Itshould be mentioned that Zn(II) and Cd(II) complexes of 1 are thefirst reported oxime complexes with pKa values above 8 andfurther studies with more basic oximate complexes are neededto see whether the levelling off may still be observed at higher pKa

values. Remarkably, the reactivity reached by complexes of 1 is ofthe order of magnitude typical for NPA cleavage by the mostpowerful biological catalysts. Representative data for NPA cleavageby a-chymotrypsin,14 human carbonic anhydrase II15 and humanserum albumin16 are shown for comparison in Fig. 2 (bluecircles).

An interesting question is: what is the magnitude of thea-effect for coordinated oximes? As an estimate one can com-pare the reactivity of coordinated oximes with the reactivity ofcoordinated hydroxide. The respective data for metal hydroxo-complexes (ML(OH)) are shown in Fig. 2 as open squares. Thecorrelation line for ML(OH) has a similar slope close to 1, but

passes about 4 logarithmic units below that for M(Ox) com-plexes. For comparison, the a-effect for free oximes estimatedin a similar way, but using the reactivity of phenolate anions asa reference, is between two and three orders of magnitude bothin water and aqueous DMSO.5

In conclusion, the results of this study demonstrate thatcoordination of oximate anions to metal ions can remove the‘‘solvational imbalance’’ around the nucleophile, which is themajor obstacle to the development of highly efficient oximate-based nucleophilic reactants. The reactivity of oxime complexesis very sensitive to the structure of the ligand and the natureof metal ion due to factors that are poorly understood atthe moment, but most probably are related to solvation of thecoordinated oxime group. With a properly chosen ligand theesterolytic reactivity of a moderately basic complex with a pKa ofaround 9 can reach that of the most efficient biocatalysts.Finally, coordinated oximate can operate in a catalytic regimewith a high turnover.

Financial support by CONACyT (project 82927) is gratefullyacknowledged.

Notes and references1 (a) F. Terrier, P. Rodriguez-Dafonte, E. Le Guevel and G. Moutiers,

Org. Biomol. Chem., 2006, 4, 4352; (b) I.-H. Um, E.-J. Lee andE. Buncel, J. Org. Chem., 2001, 66, 4859; (c) I.-H. Um, S.-J. Hwangand E. Buncel, J. Org. Chem., 2006, 71, 915; (d) F. Terrier, E. LeGuevel, A. P. Chatrousse, G. Moutiers and E. Buncel, Chem. Commun.,2003, 600.

2 (a) D. Kiderlen, F. Worek, R. Klimmek and P. Eyer, Arch. Toxicol.,2000, 74, 27; (b) H. Morales-Rojas and R. A. Moss, Chem. Rev., 2002,102, 2497; (c) G. Saint-Andre, M. Kliachyna, S. Kodepelly, L. Louise-Leriche, E. Gillon, P.-Y. Renard, F. Nachon, R. Baati and A. Wagner,Tetrahedron, 2011, 67, 6352.

3 K. J. Wallace, R. I. Fagbemi, F. J. Folmer-Andersen, J. Morey,V. M. Lyntha and E. V. Anslyn, Chem. Commun., 2006, 3886.

4 F. Terrier, P. Mc Cormack, E. Kizilian, J. C. Halle, P. Demerseman,F. Guir and C. Lion, J. Chem. Soc., Perkin Trans. 2, 1991, 153.

5 E. Buncel, C. Cannes, A. P. Chatrousse and F. Terrier, J. Am. Chem.Soc., 2002, 124, 8766.

6 R. Breslow and D. Chipman, J. Am. Chem. Soc., 1965, 87, 4195.7 (a) J. Suh, M. Cheong and H. Han, Bioorg. Chem., 1984, 12, 188;

(b) J. Suh and W. J. Kwon, Bioorg. Chem., 1998, 26, 103.8 A. K. Yatsimirsky, P. Gomez-Tagle, S. Escalante-Tovar and L. Ruiz-

Ramırez, Inorg. Chim. Acta, 1998, 273, 167.9 F. Mancin, P. Tecilla and U. Tonnellato, Eur. J. Org. Chem., 2000,

1045.10 J. Suh, B. N. Kwon, W. Y. Lee and S. H. Chang, Inorg. Chem., 1987,

26, 805.11 L. Catalano, R. Dreos, G. Nardin, L. Randaccio, G. Tauzher and

S. Vuano, J. Chem. Soc., Dalton Trans., 1996, 4269.12 M. L. Bender, M. Luisa Begue-Canton, R. L. Blakeley, L. J. Brubacher,

J. Feder, C. R. Gunter, F. J. Kezdy, J. V. Killheffer Jr., T. H. Marshall,C. G. Miller, R. W. Roeske and J. K. Stoops, J. Am. Chem. Soc., 1966,88, 5890.

13 A. K. Yatsimirsky, Coord. Chem. Rev., 2005, 249, 1997.14 M. L. Bender, G. E. Clement, F. J. Kezdy and H. d’A. Heck, J. Am.

Chem. Soc., 1964, 86, 3680.15 L. L. Kiefer and C. A. Fierke, Biochemistry, 1994, 33, 15233.16 P. Ascenzi, M. Gioia, G. Fanali, M. Coletta and M. Fasano, Biochem.

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Fig. 2 Bronsted plot of reactivity of coordinated oximate anions M(Ox)(M = Mn(II), Ni(II), Cu(II), Zn(II), Cd(II), Pb(II)) in NPA cleavage. Black squares –complexes with ligands 1–3 (present work and ref. 6–9), red squares – complexeswith ligands 4a,b.9 The dashed line shows the plot for free oximate anions.4

Open squares are for NPA hydrolysis by metal hydroxo complexes.13 k2 is thesecond-order rate constant for NPA cleavage by the respective nucleophile.

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