Synthesis and properties of a dizinc complex of a novel macrocyclic ligand with two alcohol-pendants: A model for alkaline phosphatase

Article abstract of DOI:10.1039/b206033g

A novel 24-membered macrocyclic compound having two hydroxyethyl pendants L, 3,6,9,17,20,23-hexazatricyclo[23.3.1.1.^11,15]triconta-1(29),11 (30),12,14,25,27-hexaene-6,20-bis(2-hydroxyethyl), has been synthesized as a dinucleating ligand. A new

Full text of DOI:10.1039/b206033g

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Synthesis and properties of a dizinc complex of a novel
macrocyclic ligand with two alcohol-pendants:
a model for alkaline phosphatase†
De-xi Yang,a Shu-an Li,a Dong-feng Li,a Jiang Xia,a Kai-bei Yub and Wen-xia Tang*a
a State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093,
P.R. China. E-mail: wxtang@netra.nju.edu.cn
b Analysis Center, Chengdu Branch of Chinese Academy of Sciences, Chengdu 610041,
P.R. China
Received 21st June 2002, Accepted 5th August 2002
First published as an Advance Article on the web 24th September 2002
A novel 24-membered macrocyclic compound having two hydroxyethyl pendants L, 3,6,9,17,20,23-hexaza-
tricyclo[23.3.1.1.11,15]triconta-1(29),11(30),12,14,25,27-hexaene-6,20-bis(2-hydroxyethyl), has been synthesized as
a dinucleating ligand. A new complex [Zn2HϪ2L]Br2(H2O)2 has been synthesized as a model to mimic the active
center of alkaline phosphatase. The bridging coordination of the two alkoxides to two ZnII ions was confirmed by
the crystal structure of [Zn2HϪ2L]Br2(H2O)2. The protonation constants of L as well as complexation constants of L
with ZnII have been determined at 298.1 K by means of potentiometric titration. [Zn2HϪ2L]2ϩ and [Zn2HϪ2L(OH)]ϩ
are the dominant species in aqueous solution at 7 < pH < 9. The kinetics of promoted hydrolysis of p-nitrophenyl
acetate has also been studied; the second-order rate constant is 0.018 MϪ1 sϪ1 at pH = 9.0.
was recently reported.13 Zn-assisted deprotonation of the alco-
Introduction
holic group takes place at neutral pH and at slightly alkaline
Some multinuclear metalloenzymes such as alkaline phos-
pH the species containing both a Zn-bound alkoxide and a
phatase contain two ZnII in each active site region and are
terminally bound hydroxide group as nucleophilic functions
responsible for the cleavage of phosphate monoester.1–3 The
can promote the hydrolysis of phosphate or carboxy ester
two ZnII (ca 3.9 Å separation) show different coordination
cooperatively.13
environments and seem to work cooperatively. The activation
To further explore the influence of the cavity size and rigidity
of the proximal alcohol of Ser-102 under the effect of ZnII at
of the macrocycle ligand on the behavior of corresponding ZnII
the active center is involved in the substrate hydrolysis process
complexes, in addition to investigating the nucleophility of
and a proposed mechanism is shown in Scheme 1.4 It is now
considered that the phosphate substrate is initially attacked by
alkoxide under the effect of two adjacent ZnII, we have syn-
thesized a novel 24-membered hexaza ligand containing two
the deprotonated Ser-102 in 1 to yield a transient phosphoseryl
m-xylyl spacers with 2-hydroxyethyl pendants and its binuclear
intermediate 2 that is attacked intramolecularly by the adjacent
ZnII complex (6 in Scheme 2). Its crystal structure revealed a
ZnII-bound hydroxide in 3 to complete the hydrolysis and
reproduce 1 and thus restart the catalytic cycle.
Synthetic metal complexes acting as model compounds for
novel Zn2(µ-alkoxide)2 center located inside a macrocycle. The
protonation constants of L as well as the complexation con-
stants of L with ZnIIat 25 ЊC have also been studied. The
the hydrolytic metalloenzymes have attracted much atten-
behavior toward the hydrolysis of p-nitrophenyl acetate has
been studied too.
tion.5–17 Much effort to design models for alkaline phosphatase
or nucleases to elucidate the mechanisms by which metal ions
promote hydrolysis has focused primarily on mononuclear zinc
or copper complexes.5–11 For instance, by studying several macro-
cyclic polyamine ZnII complexes bearing an alcohol pendant,
Kimura’s group discovered that the pendant alcohol can be
deprotonated to form a ZnII-bound alkoxide anion, which acts
as a strong nucleophile at physiological pH.5,6 The synthesis
of dinuclear complexes to probe the cooperative mechanism
between two metal ions in native enzymes has also ignited con-
siderable interest.12–17 However, only a few model complexes
combining two ZnII ions with a pendant alcohol inside a macro-
cycle have been synthesized successfully. The first reported
alkoxide OϪ-bridged dinuclear ZnII cryptate is shown as 4 in
Scheme 2.12 In this case, instead of acting as a nucleophile, the
alcohol deprotonates to act as a bridging group to connect two
ZnII. Another dinuclear ZnII complex shown as 5 in Scheme 2
given by a macrocyclic ligand with an ethanolic sidearm
Experimental
Materials
2-[Bis(2-aminoethyl)amino]ethanol was prepared as a colorless
sticky liquid according to the literature method.18 Isophthal-
aldehyde was synthesized and purified by a Schmelet reaction
with slight modifications.19 All of the other chemicals used in
the experiments were of analytical grade from commercial
sources and were used without further purification.
Physicochemical measurements
13C NMR spectra were measured with a Bruker AM 500
spectrometer. Element analysis was performed on a Perkin-
Elmer 240C. ES-MS spectra were obtained from a Finnigan
MAT LCQ ES mass spectrometer with a mass to charge (m/z)
range of 2000 and MeOH/H2O was used as the mobile phase.
† Electronic supplementary information (ESI) available: typical
titration curves for L with and without ZnCl2; possible species present
in solution; species distribution curve as a function of pH; ES-MS
spectrum of ZnII with L in a 2 : 1 ratio. See http://www.rsc.org/suppdata/
dt/b2/b206033g/
Synthesis of the ligand
A solution of isophthalaldehyde (0.804 g, 0.006 mol) in 100 mL
CH3CN was added dropwise to a solution of 2-[bis(2-amino-
4042
J. Chem. Soc., Dalton Trans., 2002, 4042–4047
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b206033g

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Scheme 1
(89 mg, 0.1 mmol). After adjustment to pH = 9.0 by adding 1 M
NaOH, the colorless solution was stirred for 2 hours at 50 ЊC.
The resultant solution was diffused with acetone, and colorless
prisms suitable for X-ray analysis were obtained a week later.
Elemental analysis, Calc. for Zn2C28H48Br2N6O2: C, 40.8%; H,
5.83%; N, 10.2%; Zn, 15.9%, Found: C, 40.6%; H, 5.81%; N,
10.1%; Zn, 16.1%.
X-Ray crystallographic study of Zn2C28H48Br2N6O2
Intensity data for the crystal of Zn2C28H48Br2N6O2 were col-
lected on a Siemens P4 four-circle diffractometer with mono-
chromated Mo-Kα (λ = 0.71073 Å) radiation using the ω/2θ
scan mode at 25 ЊC in the range 2.24 ≤ θ ≤ 25.00Њ. An empirical
absorption correction based on ψ scans was also applied. Data
were corrected for Lorenz-polarization effects during data
reduction using XSCANS.21 The structure was solved by direct
methods and refined using SHELXL97 software.22 All non-
hydrogen atoms were refined anisotropically by full-matrix least
squares. Crystallographic data and details about the data
collection are presented in Table 1.
Scheme 2
ethyl)amino]ethanol (0.882 g, 0.006 mol) in 150 mL CH3CN
under magnetic stirring over 8 hours at 0 ЊC and a white sus-
pension appeared. After stirring for another 12 hours, a white
microcrystalline precipitate appeared and was filtered off. The
microcrystals were dissolved in 100 mL CH3CH2OH at 45 ЊC
and 2.5 g NaBH4 was added little by little over 3 hours under
stirring. After removing the solvent under reduced pressure,
5 ml H2O and 100 ml CH2Cl2 were added sequentially to extract
the product. A colorless viscous oil was obtained after remov-
ing the CH2Cl2 from the organic phase. Several hours after the
addition of 5 ml 48% HBr to the product at 0 ЊC, white micro-
crystals appeared with a yield of 1.61 g (60%). Colorless crys-
tals of C28H46N6O2ؒ4HBrؒ4H2O were obtained by evaporating
an aqueous solution of the white microcrystals described
above.20 The whole procedure is shown in Scheme 3. The 500
MHz 13C NMR spectrum in D2O gave peaks at δ = 42.6 (CH2
attached to phenyl), 51.0 and 57.1 ( NCH2CH2OH ), 55.4 and
49.8 (NCH2CH2N ), 131.0, 131.4, 131.5 and 132.0 (phenyl).
Elemental analysis, Calcd for C28H46N6O2ؒ4HBrؒ4H2O: C,
37.6%; H, 6.48%; N, 9.39%, Found: C, 37.5%; H, 6.36%; N,
9.28%.
CCDC reference number 176430.
See http://www.rsc.org/suppdata/dt/b2/b206033g/ for crystal-
lographic data in CIF or other electronic format.
Potentiometric measurements
Equilibrium data for the potentiometric experiments were
determined by using the procedure described in our previous
works with an Orion 91-04 glass-combined pH electrode on an
microprocessor ionalyser/901.11,17 The pH meter was calibrated
using commercial pH standards prior to each titration. A
humidified nitrogen gas stream was introduced to protect the
solution from the effect of air and the solution temperature was
maintained at 25.0 0.1 ЊC. Double-distilled water with pH =
6.4 was used in all solutions. NaNO3 ( 1.0 M) was used to
maintain the ionic strength at I = 0.1 M, the Zn(NO3)2 stock
solution was calibrated with EDTA. 2.00 mM pure ligand with
6 equiv of HCl (12.0 mM) in the presence or absence of 2 equiv
of Zn(NO3)2 was employed in the titration measurement in the
pH range 2.5–11 by addition of NaOH (0.1 M, CO2 free) with a
spiral micro-injector. Program BEST23 was used to determine
each equilibrium constant and about 90 data points were calcu-
lated every time, using Kw ( = aHϩaOHϪ) = 10Ϫ13.69 for 25 ЊC.11 All
data represent the average of at least two independent titration
experiments.
Zn2C28H48Br2N6O2
An aqueous solution (1 ml) of ZnSO4ؒ7H2O (58 mg, 0.2 mmol)
was added to an aqueous solution (2 ml) of Lؒ4HBrؒ4H2O
J. Chem. Soc., Dalton Trans., 2002, 4042–4047
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Scheme 3
Table 1 Crystallographic data for Zn2C28H48Br2N6O2
Table 2 Selected bond lengths [Å] and angles [Њ] for Zn2C28H48-
Br2N6O2
Formula weight
Crystal system
Space group
T /K
823.28
Orthorhombic
Pbca
Zn–O1A
Zn–O1
Zn–N3
1.976(3)
2.031(3)
2.109(3)
Zn–N2
Zn–N1
Zn ؒ ؒ ؒ ZnA
2.119(3)
2.229(3)
3.0669(10)
298(2)
λ/Å
0.71073
Colorless, prism
18.149(2)
8.3650(10)
21.169(3)
90
3213.8(7)
4
4.032
Color of crystal, habit
a/Å
b/Å
c/Å
O1A–Zn–O1
O1A–Zn–N3
O1–Zn–N3
O1A–Zn–N2
O1–Zn–N2
O1A–Zn–ZnA
N3–Zn–ZnA
N1–Zn–ZnA
80.12(12)
112.60(13)
120.18(13)
106.46(12)
128.58(12)
40.73(8)
O1–Zn–N1
N3–Zn–N2
O1A–Zn–N1
N3–Zn–N1
N2–Zn–N1
O1–Zn–ZnA
N2–Zn–ZnA
79.51(11)
104.39(13)
159.16(12)
81.74(13)
83.14(13)
39.39(7)
α = β = γ/Њ
V/Å3
Z
125.49(10)
118.78(9)
126.55(9)
µ/mmϪ1
Reflections collected
Independent reflections
Data/restraints/parameters
R1a [I > 2σ(I)]
wR2b [I > 2σ(I)]
3520
2814 (Rint = 0.0357)
2814/0/191
0.0349
0.0685
a R1
= Σ||Fo| Ϫ |Fc||/Σ|Fo|, wR2 = Ϫ .
|Σw(|Fo|2 |Fc|2)|/Σ|w(Fo)2|1/2
2
b w = 1/[(Fo )2 ϩ (0.0354P)2 ϩ 0.0000P], where P = (Fo2 ϩ 2Fc2)/3.
Hydrolysis of p-nitrophenyl acetate
The hydrolysis rate of p-nitrophenyl acetate (NA) was meas-
ured by the initial slope method following the increase in the
400 nm absorption of the released p-nitrophenolate in 10%
(v/v) aqueous CH3CN at 25.0 0.5 ЊC on a UV-3100 spectro-
photometer by using the procedure previously reported.11,17
Buffered solutions containing 20 mM Tris buffer ( pH 7.5, 8.5 )
or 20mM CHES buffer ( pH 9.0 ) were used, and the ionic
strength was adjusted to I = 0.10 M with 1 M NaNO3. For
the initial rate determination, the following procedure was
employed. Each time, after the NA (0.4–1.0 mM) and the Zn
complex (0.5–2.0 mM) were mixed in the buffered solution, the
UV absorption at 400 nm was recorded immediately and fol-
lowed generally until 2% formation of p-nitrophenolate which
gave the total velocity νtotal. After subtracting the velocity of
spontaneous hydrolysis of p-nitrophenyl acetate (promoted by
buffered solutions which were identical in composition to the
Fig. 1 An ORTEP24 drawing of Zn2C28H48Br2N6O2. All hydrogen
atoms are omitted for clarity.
located in the middle of the Zn and ZnA ions. The two zinc
ions show analogous coordination environments and each
Zn is penta-coordinated. Analysis of the Zn coordination
polyhedron according to the method of Muetterties25 and
Galy26 provide the degree of distortion ∆ from an ideal trigonal
bipyramid (∆ = 0) to a square pyramid (∆ = 1), and each Zn ion
has a coordination geometry close to trigonal bipyramid in the
N3O2 coordination environment, indicated by ∆ = 0.202. The
Zn ion deviates 0.313 Å from the equatorial plane defined by
N2, N3 and O1 with O1–Zn–N3 = 120.18(13)Њ, N2–Zn–N3 =
104.39(13)Њ, O1–Zn–N2 = 128.58(12)Њ, and the axial sites are
coordinated by O1A from the pendant alcohol and N1 from
the diethylenetriamine moiety with the angle O1A–Zn–N1 =
159.16(12)Њ.
above except lacking the Zn complex) denoted as νbuffer, νZn
=
νtotal Ϫ νbuffer was obtained. The slope of νZn vs. [NA] was
denoted as the first-order rate constant kobs (sϪ1) and the slope
of kobs vs. [Zn() complex]total was denoted as the second-order
rate constant kNA (MϪ1 Ϫ1). Experiments were conducted at
s
least in duplicate.
Results and discussion
Crystal structure of Zn2C28H48Br2N6O2
A perspective view of Zn2C28H48Br2N6O2 is shown in Fig. 1.
Selected bond lengths and bond angles relevant to the Zn
coordination sphere are given in Table 2.
A chair conformation is adopted by the macrocyclic com-
plex. The whole structure is centrosymmetric with the center
A Zn2(µ-alkoxide)2 structure is located inside the macrocycle
with the angle Zn–O–ZnA = 99.88Њ. The two zinc ions are
bridged by the two deprotonated alcohol pendants located on
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J. Chem. Soc., Dalton Trans., 2002, 4042–4047

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Scheme 4
two different sides of the plane defined by the nitrogen atoms of
in aqueous solution at 298.1 K have been determined by
the macrocycle; the Zn, ZnA, O1, O1A atoms are coplanar. The
distance between Zn and ZnA is 3.0669(10) Å and this short
distance is primarily governed by the nature and mode of
coordination of the bridging groups. Each alkoxide almost
symmetrically bridges the two ZnII ions with Zn–O1 = 2.031(3)
Å and Zn–O1A = 1.976(3) Å. Compared with the structure of
the ligand,20 the whole macrocycle is distorted. The distance
between the two tertiary amino N atoms N1 and N1A is only
6.541 Å and the two alcohol O atoms are 2.579 Å away from
each other. The two tertiary amino N atoms deviate from the
plane defined by the four secondary amino N atoms with a
value of 2.277 Å. Two factors contribute to the distortion of the
conformation: i) the bridging coordination has brought the two
alcohol O atoms together and accordingly the size of the cavity
shrinks; ii) by adopting a bipyramidal coordination geometry,
Zn has folded the N1, N2, N3 plane, and distance between N1
and N1A is thereby shortened because of the coordination of
the ZnII ions to O1, O1A, N1 and N1A.
potentiometric titration; 6 equiv HCl was initially added to
acidify the pure ligand. A typical pH titration curve is shown in
the ESI (Supplement 1(b)†).
The titration curve reveals the formation of stable complexes
corresponding to the buffer region at 6 < pH < 10. In this
region, two continuous inflection points at a = 8 and a = 9 are
observed. The inflection at a = 8 corresponds to the completion
of the neutralization of the six protonated ammonium groups
and deprotonation of the two pendant hydroxyethyl groups
during the formation of dinuclear Zn complexes. Further
deprotonation was evidenced by the buffer region between a = 8
and a = 9, and the inflection point at a = 9 is ascribed to the
completion of deprotonation of a ZnII-bound water molecule.
The buffer region above a = 9 suggests further deprotonation of
water molecules and the appearance of a white precipitate,
Zn(OH)2, at pH > 12 suggests a partial decomposition of the
dinuclear ZnII complex at higher pH.
The program BEST was used to analyze the titration data.
All possible species used for model construction shown in the
ESI (Supplement 2†) have been used for calculation. Reasonable
results were given by BEST, with a σ fit value optimized to be
0.003. Four species: [ZnH2L]4ϩ, [Zn2HϪ2L]2ϩ, [Zn2HϪ2L(OH)]ϩ,
[Zn2HϪ2L(OH)2] were confirmed to be dominant species under
the employed experimental conditions, and other species were
less than 1 ppm in concentration.
Te two aromatic rings are parallel to each other and tilt with
an angle of 48.5Њ to the plane defined by the two Zn and two O
atoms. The distance between the two aromatic rings is 8.103 Å,
therefore π–π stacking interactions do not exist.
Protonation constants of L
A typical titration curve of 2.0 mM pure ligand with 6 equiv
HCl with the addition of 0.1 M NaOH solution at 298.1 K is
shown in the ESI (Supplement 1(a)†). Only one inflection point
at a = 2 (where a = moles of added base per mole of ligand) is
shown in the potentiometric equilibrium curve, which reveals
that the two tertiary nitrogen atoms are of very low basicity and
release their protons easily into aqueous solution at low pH.23
The buffer region occurring at higher pH corresponds to the
neutralization of the remaining four ammonium groups of the
ligand. The titration data were analyzed for equilibria (1)–(4).
The protonation constants K1–K4 (aHϩ is the activity of Hϩ) are
defined as follows:
The species distribution as a function of pH based on the
calculation results is shown in the ESI (Supplement 3†). The
mononuclear complex [ZnH2L]4ϩ appears in the pH range
5.5–7.2 (maximum at pH 6.4, 38.6%). The binuclear Zn com-
plex with two alkoxide bridges [Zn2HϪ2L]2ϩ exists in the pH
range 6–8 (maximum at pH 7.0, 51.3%). As the pH is raised, a
binucleating complex with a Zn-bound hydroxide [Zn2HϪ2L-
(OH)]ϩ is formed with a preferred pH of 9.0, above which it is
gradually converted to [Zn2HϪ2L(OH)2]. To illustrate the pro-
cess clearly, the stepwise complexation of L with ZnII via these
four species is shown in Scheme 4.
The complexation constants K and deprotonation constants
Ka for the four species are defined as follows:
L ϩ Hϩ
HLϩ ϩ Hϩ
HLϩ K1 = [HLϩ]/[L]aHϩ
(1)
(2)
2Hϩ ϩ L ϩ ZnII = ZnH2L4ϩ K(ZnH2L4ϩ) =
[ZnH2L4ϩ]/aHϩ2[L][ZnII] (5)
H2L2ϩ K2 = [H2L2ϩ]/[HLϩ]aHϩ
2ϩ
2ϩ
ZnH2L4ϩ ϩ ZnII = Zn2H
L
Ϫ2
ϩ 4Hϩ K(Zn2H
L ) =
H2L2ϩ ϩ Hϩ
H3L3ϩ ϩ Hϩ
H3L3ϩ K3 = [H3L3ϩ]/[H2L2ϩ]aHϩ (3)
H4L4ϩ K4 = [H4L4ϩ]/[H3L3ϩ]aHϩ (4)
Ϫ2
2ϩ
[Zn2H
L
]aHϩ4/[ZnH2L4ϩ][ZnII] (6)
Ϫ2
4ϩ
Zn2H
L
ϩ H2O = Zn2HϪ2L(OH)3ϩ ϩ Hϩ Ka3
[Zn2HϪ2L(OH)ϩ]aHϩ/[Zn2H
=
Ϫ2
Ϫ2
2ϩ
L
]
(7)
Logarithms of the protonation constants logK1–logK4 are
9.39 0.02, 9.18 0.02, 7.86 0.03, 7.59 0.02, respectively.
Zn2HϪ2L(OH)ϩ ϩ H2O = Zn2HϪ2L(OH)2 ϩ Hϩ Ka4
=
[Zn2HϪ2L(OH)2]aHϩ/[Zn2HϪ2L(OH)ϩ] (8)
Complexation constants of L with ZnII
The complexation constants of the species formed by L and
Zn() in a 1 : 2 ( L : Zn() ) system with I = 0.10 M (NaNO3)
[ZnH2L]4ϩ is a mono-Zn complex with log K(ZnH2L) = 23.9.
As another ZnII ion binds to [ZnH2L]4ϩ, the dinuclear ZnII
J. Chem. Soc., Dalton Trans., 2002, 4042–4047
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complex [Zn2HϪ2L]2ϩ is formed directly and several processes,
involving deprotonation of two substituted ammonium groups,
deprotonation of two alcohols and dimerization of two tri-
podal moieties in the macrocycle, take place simultaneously.
ES-MS spectra (shown as ESI Supplement 4†) for the complex
at pH = 7.0 in aqueous solution evidences the stable existence of
[Zn2HϪ2L]2ϩ (m/z = 313.3) with a relative abundance of 100%.
The stable existence of [Zn2HϪ2L]2ϩ at pH = 7.0 indicated that
deprotonation of hydroxyethyl bound to ZnII happened below
pH = 7.0, that is, the pKa value for ZnII bridged hydroxyethyl
denoted as pKa1,2 is less than 7.0.
It is worthy of mention that the simultaneous formation of
binuclear Zn complexes together with the formation of bridg-
ing alkoxide or hydroxide has also been reported previously in
other 24-membered macrocycles.12,27 The simultaneous depro-
tonation of a water molecule with a pKa value less than 7 to
bridge two ZnII was reported in the work of Bencini et al. for
the ligand [24]aneN8.27 Kimura et al., also observed the simul-
taneous coordination of two Zn ions to the propanol-bridged
[24]aneN8.12
pKa3 = 7.3 for equilibrium (7) is almost identical with the
pKa value for H2O bound to ZnII[12]andN3 (pKa = 7.3)9 and
may correspond to the formation of a terminal ZnII-bound
hydroxide ion to give [Zn2HϪ2L (OH)]ϩ, a situation encountered
by many dinuclear ZnII complexes with or without pend-
ant alcohol groups.13,14 During this process, the binding of
hydroxide to a ZnII ion will lead to detachment of a chelating
alkoxide from that ZnII ion to a terminal position by binding to
the other ZnII, or cause a weakening of the binding of Zn–OR
as sketched in Scheme 4. pKa4 = 10.6 for equilibrium (8) is
assigned to the deprotonation of another water molecule
coordinated to the second ZnII ion.
Fig. 2 Change in kobs (sϪ1) as a function of total concentration of Zn
complex [Zn() complexes]total at pH = 7.5, 8.5 and 9.0 with I = 0.1 M
(NaNO3) at 25 ЊC in the presence of 10% (v/v) aqueous CH3CN.
Spontaneous hydrolysis by the buffered solution alone has been
subtracted at each pH and the slope of the straight line illustrated is
denoted as kNA
.
The low activity of [Zn2HϪ2L]2ϩ containing Zn2(µ-alkoxide)2 is
to some degree similar to the inactivity of the dimers of some
mono-copper model complexes Cu[9–11]aneN3X2.7,8 In these
complexes, the activity is decided by the concentration of
monomers controlled by the monomer–dimer equilibrium, with
the monomer being the active species and the dimer being an
inactive form of corresponding complexes.
Hydrolysis of p-nitrophenyl acetate
The reactivity of the dinuclear complex toward the hydrolysis
of phosphate ester or carboxy ester has been checked. Since
phosphomonoester underwent low hydrolysis with the com-
plex, here we only describe the hydrolytic cleavage of p-nitro-
phenyl acetate (NA). The activity of the binuclear species has
been investigated by following the hydrolysis of p-nitrophenyl
acetate (0.4 mM–1.0 mM) promoted by the Zn complex
(0.5 mM–2.0 mM) at different pH (7.5, 8.5, 9.0) as described in
the Experimental section, and the spontaneous hydrolysis of
p-nitrophenyl acetate by the buffered solution alone has been
subtracted from the Zn-complex promoted hydrolysis. An
initial slope method has been applied to the data processing
procedure and the initial velocity νtotal, spontaneous hydrolysis
velocity νbuffer, Zn complexes induced hydrolysis velocity νZn,
first-order rate constant kobs (sϪ1), and second-order rate
constant kNA (MϪ1 sϪ1) are defined as follows:
A higher activity is achieved when the pH is raised above 8.0
with kNA = 0.013 MϪ1 sϪ1 at pH = 8.5, and kNA = 0.018 MϪ1 sϪ1 at
pH = 9.0, which parallels the increase in the concentration of
[Zn2HϪ2L(OH)]ϩ. The obvious correlation between concen-
tration of [Zn2HϪ2L(OH)]ϩ and hydrolysis rate at 7.0 < pH <
9.0 suggests that the latter complex which is characterized as
having a Zn-bound alkoxide and a terminally bound hydroxide
group has higher hydrolytic activity than [Zn2HϪ2L]2ϩ. Since no
intermediate has been isolated the hydrolysis mechanism is still
uncertain. A possible catalytic process is suggested as being
similar to that described in the work of Bencini et al., where
both Zn–OHϪ and Zn–OR are involved in the p-nitrophenyl
acetate hydrolysis.13
For comparison, we have recently synthesized a di-Zn com-
plex of a novel 26-membered binucleating macrocyclic ligand
[Zn2L1(CH3CO2)2](ClO4)2(H2O)2 (7 in Scheme 2), and it is
considerably more efficient in promoting hydrolysis of NA with
kNA = 0.32 MϪ1 sϪ1 at pH = 9.0,28 a value that is remarkably
higher than that of the title complex. This can be explained by
considering the structural differences between the two com-
pounds. The rigidity of the 26-membered macrocycle has kept
the two Zn() ions 8.74 Å apart and therefore cannot form the
species containing Zn2(µ-alkoxide)2 while the cavity size and
rigidity of the 24-membered macrocycle facilitates the form-
ation of the inactive species containing Zn2(µ-alkoxide)2. The
difference in their hydrolytic activity is to some degree similar to
some copper complexes described previously whose acitvities
are determined by the concentration of the active monomer
species in the monomer–dimer equilibrium.7,8
νtotal = νZn ϩ νbuffer
νbuffer = (kOHϪ[OHϪ]ϩ. . .)[NA]
νZn = kobs[NA] = kNA[Zn() complex]total[NA]
Initial velocities νtotal were collected by following the absorb-
ance at 400 nm increasing up to 2% formation of p-nitro-
phenolate with the slope being νtotal. Zn complex induced
hydrolysis velocity νZn was obtained by substracting the spon-
taneous hydrolysis velocity promoted by the buffered solution
(νbuffer) from νtotal. The first-order rate constant kobs (sϪ1) was
calculated from the slope of the straight line νZn vs. [NA]. The
plot of kobs vs. [Zn() complexes]total gave a straight line with the
slope denoted as kNA (MϪ1 Ϫ1).17 The pH-dependant curve of
s
Conclusion
kobs vs. [Zn() complexes]total is shown in Fig. 2.
The second-order rate constant kNA is 0.0046 MϪ1 sϪ1 at pH =
7.5, where [Zn2HϪ2L]2ϩ is the main species in aqueous solution.
A new macrocyclic ligand containing two pendant alcohol
groups was synthesized and characterized by potentiometric
4046
J. Chem. Soc., Dalton Trans., 2002, 4042–4047

View Article Online
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analysis. The corresponding dizinc complex was crystallized
from aqueous solution. X-Ray crystal analysis reveals that the
two pendant alcohol groups are deprotonated to bridge two
ZnII ions from two different sides of the plane determined by
the nitrogen atoms of the macrocycle. Potentiometric investi-
gation shows that the two alcohol groups deprotonate simul-
taneously with pKa1,2 < 7 at 298K. Further deprotonation of the
terminally coordinated water molecule was observed with pKa3
= 7.3 and pKa4 = 10.6 respectively. The catalyzed hydrolysis rate
of p-nitrophenyl acetate by the complex has been investigated at
7.0 < pH < 9.0 and results show that the catalytic activity of
[Zn2HϪ2L (OH)]ϩ is higher than [Zn2HϪ2L]2ϩ
.
Acknowledgements
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This work is supported by the National Natural Science
Foundation of China. We are also thankful to Professor
Wei-yin Sun and Dr Yan Xu (State Key Laboratory of
Coordination Chemistry, Nanjing University, P.R. China) for
their kind help and useful suggestions.
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