Selective hydrolysis of 4-nitrophenyl phosphate by a dinuclear Cu(II) complex

Article abstract of DOI:10.1246/cl.2000.156

A flexible dinuclear Cu(II) complex with 1,3-bis(1,4,7-triaza-1-cyclononyl)propane, hydrolyzes 4-nitrophenyl phosphate under mild conditions, with 300 times rate enhancement over the corresponding mononuclear Cu(II) complex. In contrast, there is no difference in the reactivity in hydrolyzing bis(4-nitrophenyl)phosphate.

Full text of DOI:10.1246/cl.2000.156

156
Chemistry Letters 2000
Selective Hydrolysis of 4-Nitrophenyl Phosphate by a Dinuclear Cu(II) Complex
Jung Hee Kim
Department of Chemistry, Sunmoon University, #100 Kalsan, Tangjeong, Asan-si, Chungnam, 336-840, Korea
(Received November 9, 1999; CL-990963)
A flexible dinuclear Cu(II) complex with 1,3-bis(1,4,7-tri-
aza-1-cyclononyl)propane, hydrolyzes 4-nitrophenyl phos-
phate under mild conditions, with 300 times rate enhancement
over the corresponding mononuclear Cu(II) complex. In con-
trast, there is no difference in the reactivity in hydrolyzing
bis(4-nitrophenyl)phosphate.
There have been considerable research efforts in develop-
ing efficient catalysts carrying two or three metal ions for
phosphate ester hydrolysis.1 In order to achieve a possible
cooperativity among metal ions, it is crucial to design effective
di- and trinucleating ligands. The common feature of the lig-
ands in reported binuclear Zn(II), Cu(II), Co(III) complexes
and trinuclear Zn(II) complexes is that the linkers connecting
two or three monomeric units have functional groups such as
phenyl, hydroxyphenyl and hydroxyalkyl groups to place
metal ions in a close proximity.2
Here we report that a dinuclear Cu(II) complex with 1,3-
bis(1,4,7-triaza-1-cyclononyl)propane(L1), hydrolyzes 4-nitro-
phenyl phosphate(NPP) efficiently under mild conditions.
L1 contains two triazacyclononane moieties that bind Cu(II)
BDNPP hydrolysis by the copper complexes are also listed in
Table 1. The Cu-L2 complex is slightly more efficient in
hydrolyzing BNPP and BDNPP than the Cu-L1 complex. The
results indicate that the two copper ions in the Cu-L1 complex
act independently as like two equivalents of the Cu-L2 com-
plex in hydrolyzing phosphate diesters, such as BNPP and
BDNPP.
with a high stability and its equilibrium and structural data
have been reported.3 The Cu-L1 and the Cu-L2 complexes
were prepared according to the known method.4 Hydrolysis of
NPP by the Cu-L1 and Cu-L2 complexes was performed at 30
o
0.5 C under 1:1 and 2:1 molar ratio of the metal complex
and NPP.5 The Cu complexes catalyzed hydrolysis of NPP to
an inorganic phosphate, yielding 1 equivalent of 4-nitrophenol.
Hydrolysis of NPP by the Cu-L1 complex was first-order in
the complex concentration.6 The second order rate constants
for hydrolysis of NPP are listed in Table 1. The Cu-L1 com-
plex is over two orders of magnitude more reactive than the
Cu-L2 complex in hydrolyzing NPP.
The rate constants were measured at the pH near the maxi-
mum rate obtained from pH-rate profile shown in Figure 1.7 In
contrast to the NPP hydrolysis, the Cu-L1 complex hydrolyzed
bis(4-nitrophenyl)phosphate(BNPP) and bis(2,4-nitrophenyl)
phosphate(BDNPP) in almost the same rates as the Cu-L2
complex did. Hydrolysis of BNPP by the two complexes was
carried out under the same conditions used above. Hydrolysis
of BDNPP was performed under the pseudo-first order condi-
tions with the metal complexes in excess. As a result of the
hydrolysis of BDNP, two equivalents of 2,4-dinitrophenol
were released. The second order rate constants for BNPP and
Participation of two metal ions in hydrolyzing phosphate
monoesters has been reported by several research groups.
Czarnik and Vance reported that a rigid Co(III) dimer showed
about 10 fold rate enhancement in hydrolyzing NPP, but not in
hydrolyzing BNPP.8 Recently, Kimura et al. showed that a
Zn(II)-cryptate complex was able to selectively recognize and
hydrolyze NPP over other phosphate di- and triesters.9 In the
above cases, the suggested mechanisms involved a bridging of
phosphate monoesters to the two metal centers. The efficiency
of this double Lewis acid activation has been estimated in
hydrolyzing phosphate diesters.1a In the present study, it is
quite surprising that a selective recognition of phosphate
monoesters by the Cu-L1 complex is observed.
The remarkable selectivity of NPP over BNPP by Cu-L1
complex could be due to the higher basicity of NPP than BNPP
as Breslow pointed out in his dinuclear Zn complex system.10
However the differences in the basicity can not explain the
observed 300 fold rate enhancement in this study.11 We recent-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
157
ly have shown that a dinuclear Zn(II)L complex(L=1,3-
bis(1,5,9-triaza-1-cyclododecyl)propane) hydrolyzed NPP only
4 times faster than the corresponding mononuclear complex
did.12 The hydrolysis of NPP by the Cu-L1 complex most like-
ly proceeds via bridging NPP to the two Cu ions followed by
the attack of available nucleophiles, yielding an inorganic
phosphate. The similar mechanism has been suggested in the
case of ApA and 2,3-cyclic AMP hydrolysis by a rigid Cu(II)
dimer.11 The bell-shaped pH-rate profile indicates that the
monohydroxy form of the dinuclear complex or its kinetic
equivalent is the active species in hydrolyzing NPP.13
In conclusion, the Cu-L1 complex efficiently and selec-
tively hydrolyzes phosphate monoester NPP. Two metal ions in
the Cu-L1 complex interact cooperatively, where about 300
fold rate enhancement is observed over its corresponding
mononuclear Cu-L2 complex. The present results may well be
relevant to dinuclear metal centers in phosphomonoesterases
such as an alkaline phosphatase.
4
5
W. F. Schwindinger, T. G. Fawcett, R. A. Lalancette, J. A.
Potenza, and H. J. Schugar, Inorg. Chem., 19, 1379
(1980).
In a typical kinetic run, 2 mL of the Cu complex (0.2-1
mM) in a buffer solution [MES (pH 6-6.6), MOPS (pH
6.7-7.4)] of an appropriate pH (20 mM, I=0.1 M KNO3)
was incubated at 30
0.5 °C for 5 min and to which was
added 5-10 µL of NPP or BNPP (0.1 M stock solution).
The reaction was followed by monitoring the increase in
the absorbance at 400 nm. The concentration of 4-nitro-
phenol released was calculated from the known coefficient
(ε = 18,800 M-1cm-1), with the absorbance corrected for
pH. The rates were obtained by the initial slopes method
(0.5-3% completion, correlation coefficient R>0.99), and
each kinetic run was reproducible within 5% error. The
second rate constants were obtained by dividing the rate
with substrate and catalyst concentrations.
6
7
A plot of log k (s-1) versus log of the catalyst concentra-
tion gives a slope of 0.91.
According to the pH-rate profile for the Cu-L2 complex
catalyzed hydrolysis of NPP, the rate reaches its maximum
near pH 7.3.
D. H. Vance and A. W. Czarnik, J. Am. Chem. Soc., 115,
12165 (1993).
T. Koike, M. Inoue, E. Kimura, and M. Shiro, J. Am.
Chem. Soc., 118, 3091 (1996).
I thank Kiseob Yun (Chungnam National University) for
helping potentiometric titration experiments. This work was
supported by the Korea Science and Engineering Foundation
(97-03-03-01-035).
8
9
References and Notes
1
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2
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13 In titration experiments, the apparent pKa values of the
Cu-L1 and Cu-L2 complexes were 6.4 and 7.3, respective-
ly. These are not the pKa of the coordinated water, but
rather a composite of multiple pH-dependent equilibria
(ref. 3a and 3f). The kinetic pKas of the Cu-L1 and Cu-L2
complexes are consistent with those from the titration
experiment. Titration was performed with 0.5 mM solu-
tion of the Cu complex and 0.5 M KOH in water. The
ionic strength was maintained at 0.15 M KCl.
3