Synthesis, crystal structures and phosphodiesterase activities of alkoxide-bridged asymmetric dinuclear nickel(II) complexes

Article abstract of DOI:10.1007/s11243-013-9791-8

An asymmetric dinuclear ligand, N-4-methyl-homopiperazine-N'-[N-(2- pyridylmethyl)-N-2-(2-pyridylethyl)amine]-1,3-diaminopr-opan-2-ol (HL) and two dinuclear Ni(II) complexes [Ni₂L(DNBA)₂]ClO₄ (1) and [Ni₂L(BPP)₂]ClO₄·2H₂O (2) (3,5-dinitrobenzoic acid, bisphenyl phosphate) have been synthesized and characterized. Single crystal X-ray crystallographic analysis reveals that the coordination environments of the two Ni(II) atoms in complexes 1 and 2 are five and six coordinate, respectively. The phosphodiesterase activity of a di-Ni(II) complex Ni₂L formed in situ from a 2:1 mixture of Ni²⁺ and HL was investigated using bis(4-nitrophenyl) phosphate (BNPP) as the substrate. The pH dependence of the rate of BNPP cleavage in aqueous buffer indicates a bell-shaped profile with an optimum at about pH 8.4, which is parallel to the formation of the dinuclear species [Ni₂LOH]²⁺ according to UV-vis spectroscopy. At pH 8.4 and 25°C, the k_cat (7.40 × 10^-5 s^-1) is ca.10⁶-fold higher than that of the uncatalyzed reaction. A possible mechanism for BNPP cleavage promoted by Ni₂L is proposed.

Full text of DOI:10.1007/s11243-013-9791-8

Transition Met Chem (2014) 39:205–211
DOI 10.1007/s11243-013-9791-8
Synthesis, crystal structures and phosphodiesterase activities
of alkoxide-bridged asymmetric dinuclear nickel(II) complexes
•
Ai-zhi Wu Tao Wang
Received: 17 October 2013 / Accepted: 2 December 2013 / Published online: 13 December 2013
ꢀ Springer Science+Business Media Dordrecht 2013
Abstract An asymmetric dinuclear ligand, N-4-methyl-
homopiperazine-N0-[N-(2-pyridylmethyl)-N-2-(2-pyridyl-
ethyl)amine]-1,3-diaminopr-opan-2-ol (HL) and two dinu-
clear Ni(II) complexes [Ni2L(DNBA)2]ClO4 (1) and
[Ni2L(BPP)2]ClO4ꢀ2H2O (2) (3,5-dinitrobenzoic acid, bi-
sphenyl phosphate) have been synthesized and character-
ized. Single crystal X-ray crystallographic analysis reveals
that the coordination environments of the two Ni(II) atoms in
complexes 1 and 2 are five and six coordinate, respectively.
The phosphodiesterase activity of a di-Ni(II) complex Ni2L
formed in situ from a 2:1 mixture of Ni2? and HL was
investigated using bis(4-nitrophenyl) phosphate (BNPP) as
the substrate. The pH dependence of the rate of BNPP
cleavage in aqueous buffer indicates a bell-shaped profile
with an optimum at about pH 8.4, which is parallel to the
formation of the dinuclear species [Ni2LOH]2? according to
UV–vis spectroscopy. At pH 8.4 and 25 ꢁC, the kcat
(7.40 9 10-5 s-1) is ca.106-fold higher than that of the
uncatalyzed reaction. A possible mechanism for BNPP
cleavage promoted by Ni2L is proposed.
the two metal ions [1–3]. Although different coordination
environments are found for the metal centers in this dinu-
clear metalloenzyme, a large number of the enzyme
mimics reported so far are based on symmetric dinucleat-
ing ligands with identical coordination geometries for the
two metal centers [4–13]. Less attention has been paid to
the preparation of asymmetric dinuclear complexes, espe-
cially dinickel(II) complexes [14–18]. The synthesis and
comparison of asymmetric and symmetric dinuclear
enzyme models should provide a deeper insight into the
structure and mechanism of the natural enzymes, and so
help to design more effective metalloenzyme models.
Herein, we describe the synthesis and characterization of a
new asymmetric dinuclear ligand, N-4-methyl-homopiper-
azine-N0-[N-(2-pyridylmethyl)-N-2-(2-pyridylethyl)amine]-
1,3-diaminopr-opan-2-ol (HL), and its dinuclear Ni(II)
complexes [Ni2L(DNBA)2]ClO4 (1) and [Ni2L(BPP)2]-
ClO4ꢀ2H2O (2) which are exogenously bridged by carbox-
ylate and phosphate ester ligands, respectively. Further, the
phosphodiesterase activity of the dinuclear Ni(II) complex
formed in situ from HL and nickel(II) in aqueous solution
was investigated using bis(4-nitrophenyl) phosphate
(BNPP) as a model substrate.
Introduction
Kidney bean purple acid phosphatases belonging to the
family of binuclear metallohydrolases and are involved in a
multitude of biological functions. These enzymes require
structurally asymmetric Fe(III) and Zn(II) or Fe(II) centers
in the active sites, reflecting the different roles played by
Experimental
Materials and methods
1H NMR spectra were measured on a Bruker AM-400
spectrometer. FTIR spectra were recorded on an Equinox
55 spectrophotometer using KBr pellets (4,000–400 cm-1).
Mass spectra were measured on a Thermo Finnigan TSQ
mass spectrometer. Elemental analyses were obtained on a
Vario EL-III instrument. The pH measurements were
A. Wu (&) ꢀ T. Wang
School of Chinese Materia Medica, Guangzhou University of
Chinese Medicine, Guangzhou 510006,
People’s Republic of China
e-mail: wuaizhi@gzucm.edu.cn
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Transition Met Chem (2014) 39:205–211
carried out with an Orion 420A pH meter with an Aldrich
combination pH electrode, calibrated with standard buffers
(pH = 4.01, 7.00 and 10.01, Sigma). Kinetic measure-
ments were recorded using quartz cuvettes (1 cm) with
Teflon stoppers on a U-3100 UV–vis spectrophotometer.
The buffer solutions were prepared with HEPES (N-[2-
and extracted with dichloromethane. The organic phase
was dried over Na2SO4. Evaporation of the solvent yielded
an orange oil which was further purified by silica gel
chromatography using chloroform/methanol (10/1, v/v) as
eluent. The pure ligand HL was obtained as a light orange
oil with 36 % yield. 1HNMR (CDCl3): d = 8.52, 2H; 7.58,
2H; 7.2–7.08, 4H; 4.0–3.85, 3H, 3.05–2.95, 4H, 2.90–2.38,
16H, 1.82–1.78, 2H. FTIR (KBr, cm-1): 3381(w, s),
2939(s), 2811(s), 1661(s), 1594(s), 1475(s), 1435(s),
1366(s), 1309(s), 1126(s), 1051(s), 1001(s), 763(s), 624(s).
Hydroxyethyl]piperazine-N0-[2-ethanesulfonate])
and
CHES ((2-cyclohexylamino)ethanesulfonate) and adjusted
to the desired pH by adding freshly prepared NaOH solu-
tion. HBNPP purchased from Aldrich was recrystallized
from ethanol/water before use. NaBPP was synthesized by
mixing NaOH and bisphenyl phosphate (HBPP) in 1:1 ratio
in aqueous solution, and the pure product was obtained as a
white powder after the evaporation of water. Elemental
analysis was performed to confirm the composition and
purity of NaBPP. N-(2-Pyridyl)methyl-N-2-(2-pyridyleth-
yl)amine (I) was prepared as described in the literature
[17].
Synthesis of [Ni2L(DNBA)2]ClO4 (1)
A solution of Ni(ClO4)2ꢀ6H2O (53 mg, 0.146 mmol) in
methanol (1 mL) was added to a stirred solution of HL
(28 mg, 0.073 mmol) in methanol (1 mL). Next, a solu-
tion of Et3N (10 lL, 0.073 mmol) in methanol (1 mL)
was added where upon a red solution was formed. This
was stirred at room temperature for 1 h, and then, a
solution of DNBA (31 mg, 0.146 mmol) and Et3N
(20 lL, 0.146 mmol) in methanol (1 mL) was added. The
color of the solution rapidly changed from red to green,
and then, a green precipitate was formed slowly. The
mixture was stirred at room temperature for another 1 h,
and then, the crude product was collected by filtration and
washed with methanol. Green block crystals were
obtained by recrystallization from 2:1 MeCN-EtOH
solution (3 mL) with a yield of 65 %. Anal. Calcd for
C36H38ClN9O17Ni2. Found (calcd): C, 38.6 (38.4); H, 3.5
(3.7); N, 12.1 (12.3). FTIR (KBr, cm-1): 3434(w),
3106(s), 2911(s), 2862(s), 1633(s), 1574(s), 1539(s),
1480(s), 1457(s), 1400, (s) 1349(s), 1268(s), 1202(s),
1088(w) (s), 920(s), 869(s), 788(s), 763(s), 724(s), 675(s),
645(s), 622(s), 527(s), 486(s).
Synthesis of N-(2-pyridylmethyl)-N-2-(2-pyridylethyl)
[(3-chloro)(2-hydroxy)] propylamine (II)
A solution of I (2.09 g, 9.8 mmol) in methanol (15 mL)
was added to a stirred solution of epichlorohydrin (0.93 g,
10 mmol) in methanol (15 mL) at 0 ꢁC. After addition, the
reaction mixture was stirred for 72 h at room temperature,
resulting in a reddish solution. The solvent was removed
under reduced pressure at 35 ꢁC, and the residue was dis-
solved in dichloromethane (50 mL). The solution was
extracted with successive portions of brine (25 mL each)
until the aqueous phase became colorless. The organic
layer was dried over anhydrous NaSO4 and filtered; the
solvent was removed under reduced pressure at 30 ꢁC,
1
resulting in a light orange oil with 78 % yield. HNMR
(CDCl3): d = 8.52, 2H; 7.4–7.6, 2H; 7.2–6.8, 4H; 7.08,
2H; 3.95–3.9, 3H; 3.53, 2H; 2.80–3.00, 7H. FTIR (KBr,
cm-1): 3351(w, s), 2954 (s), 2830(s), 1594(s), 1570(s),
1475(s), 1437(s), 1366(s), 1305(s), 1256(s), 1122(s),
1094(s), 1052(s), 1001(s), 894(s), 761(s), 622(s).
Synthesis of [Ni2L(BPP)2]ClO4ꢀ2H2O (2)
A solution of Ni(ClO4)2ꢀ6H2O (73 mg, 0.2 mmol) in water
(1 mL) was added to a stirred solution of HL (28 mg,
0.073 mmol) and Et3N (10 lL, 0.073 mmol) in water
(2 mL). Next, a solution of NaBPP (55 mg, 0.2 mmol) in
water (1 mL) was added. The color of the solution rapidly
changed from red to green, and then, a green precipitate
was formed slowly. The mixture was stirred at room tem-
perature for 1 h, after which the crude product was col-
lected by filtration and washed with water and ethanol.
Green block crystals were obtained by recrystallization
from 2:1 CH3CN-EtOH solution (3 mL) with a yield of
70 %. Anal. Calcd for C46H56ClN5O14P2Ni2. Found
(calcd): C, 48.7 (49.4); H, 3.8 (5.0); N, 6.5 (6.3). IR (KBr,
cm-1): 3445(w), 3066(s), 2859(s), 1593(s), 1489(s),
1443(s), 1320(s), 1259(s), 1205(s), 1162(s), 1092(s),
1023(s), 918(s), 777(s), 764(s), 692(s), 526(s).
Synthesis of N-4-methyl-homopiperazine-N0-[N-(2-
pyridylmethyl)-N-2-(2-pyridylethyl) amine]-1,
3-diaminopropan-2-ol (HL)
A mixture of 1-methyl-homopiperazine (1.14 g, 10 mmol)
and triethylamine (4.16 mL, 30 mmol) in acetonitrile
(20 mL) was added to a stirred solution of II (3.05 g,
10 mmol) in dry acetonitrile (40 mL) at 0 ꢁC. After the
addition, the reaction mixture was stirred for 48 h at room
temperature and then for 72 h at 40 ꢁC. After cooling to
0 ꢁC, the resulting white precipitate was filtered off.
Evaporation of the solvent under reduced pressure pro-
duced a brown oil, which was dissolved in water (30 mL)
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Transition Met Chem (2014) 39:205–211
207
Table 1 Crystal data and
structure refinement for
complexes 1 and 2
Complexes
1
2
Empirical formula
Formula weight
Crystal size (mm)
T (K)
C36H38ClN9Ni2O17
1021.62
C46H56ClN5Ni2O14P2
1117.77
0.18 9 0.08 9 0.03
298(2) K
0.30 9 0.20 9 0.02
173(2) K
˚
0.71073 A
˚
0.71073 A
Wavelength
Crystal system
Space group
Monoclinic
Monoclinic
P2(1)/c
P2(1)/c
˚
a (A)
15.8271(10)
14.5728(9)
19.1305(9)
20.0043(12)
90ꢁ
90.710(5)ꢁ
90ꢁ
5576.5(5)
˚
b (A)
18.7181(8)
˚
c (A)
15.4805(10)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90ꢁ
113.527(7)ꢁ
90ꢁ
4204.9(4)
3
˚
V (A )
Z
4
4
Density (g cm-1
)
1.614
1.331
F (000)
2104
2328
Absorption coefficient (mm-1
Data/restraints/parameters
h range for data collection (ꢁ)
Reflections collected
)
2.449
2.354
7757/0/582
8016/0/622
3.03–67.00
14676
3.85–72.34
19243
Independent reflections
GOF on F2
7757(Rint = 0.0887)
0.843
8016(Rint = 0.0495)
1.148
Final R indices [I [ 2r(I)]a
R indices (all data)a
R1 = 0.0584, wR2 = 0.0956
R1 = 0.1475, wR2 = 0.1205
0.366 and -0.310
R1 = 0.1151, wR2 = 0.3181
R1 = 0.1654, wR2 = 0.3510
1.349 and -0.558
-3
)
˚
Largest different peak and hole (e.A
Single crystal X-ray crystallography
detecting the increase in the maximum absorbance at
400 nm of the 4-nitrophenolate anion (NPat), using an
extinction coefficient of 18,700 L mol-1 [20]. In a typical
experiment, a solution of Ni2L was prepared by thoroughly
mixing 40 lL of stock solution of HL (50 mM in water)
and Ni(ClO4)2 (100 mM in water) in containing 1.92 mL
of 0.05 M buffer solution for 30 min at 25 ꢁC. An aliquot
of 50 lL of BNPP solution (40 mM in water) was then
added to the solution of the complex in a cuvette and mixed
well. The final concentrations of Ni2L and BNPP were both
1 mM. After addition, the mixture was allowed to equili-
brate for 5 min and the reactions were followed up to 5 %
BNPP cleavage. The absorbance values were converted
into the concentration of the NPat ion, and the total analytic
4-nitrophenol product was calculated using the buffer pH
and the pka of 4-nitrophenol (7.15) [21].
The green crystals of 1 and 2 were mounted on glass fibers
and used for data collection. Single X-ray diffraction data
were obtained on a Bruker Smart Apex CCD area detector
using graphite monochromated Mo Ka radiation
˚
(k = 0.71073 A). The structure was solved by direct
methods and refined on F2 by full-matrix least squares
techniques with SHELXTL-97 [19]. All non-hydrogen
atoms were refined anisotropically, and all hydrogen atoms
of the ligands were located and included at their calculated
positions. Crystallographic data for complexes 1 and 2
have been deposited at the Cambridge Crystallographic
Data Center, CCDC No. 966148 for 1, and 966149 for 2,
respectively. The crystal data and structure refinement are
summarized in Table 1, and selected bond lengths and
bond angles are listed in Table 2.
Cleavage of BNPP
Results and discussion
The cleavage of BNPP was monitored in a buffered
The synthetic route to the asymmetric ligand HL is illus-
trated in Scheme 1. The introduction of the asymmetry, the
aqueous solution (I = 0.1 M NaClO4, T = 25 ꢁC) by
123

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Transition Met Chem (2014) 39:205–211
˚
Table 2 Selected bond lengths (A) and angles (ꢁ) for complexes 1
and 2
1
2
Bond lengths
Ni(1)–O(1)
Ni(1)–O(41)
Ni(1)–O(51)
Ni(1)–N(1)
Ni(1)–N(11)
Ni(1)–N(21)
Ni(2)–O(1)
Ni(2)–O(42)
Ni(2)–O(52)
Ni(2)–N(2)
Ni(2)–N(3)
Ni(1)…Ni(2)
2.023(3) Ni(1)–O(1)
2.127(3) Ni(1)–O(14)
2.070(3) Ni(1)–O(22)
2.109(5) Ni(1)–N(30)
2.082(4) Ni(1)–N(34)
2.099(5) Ni(2)–O(1)
1.957(3) Ni(2)–O(13)
1.986(3) Ni(2)–O(23)
1.991(3) Ni(2)–N(1)
2.051(4) Ni(2)–N(10)
2.056(4) Ni(2)–N(20)
1.953(8)
1.975(7)
1.992(7)
2.037(7)
2.064(10)
2.057(6)
2.086(7)
2.188(8)
2.101(8)
2.098(8)
2.102(10)
3.558
3.305
Ni(1)…Ni(2)
Fig. 1 ORTEP drawing of the cationic structure in 1. Thermal
ellipsoids for the non-hydrogen atoms are drawn at the 30 %
probability level. All hydrogen atoms are omitted for clarity
Bond angles
O(1)–Ni(1)–N(21) 166.98(16) O(1)–Ni(1)–N(30)
86.1(3)
98.3(3)
77.6(4)
100.0(3)
80.0(3)
165.8(3)
84.0(3)
91.7(3)
103.1(3)
89.7(3)
125.1(3)
O(1)–Ni(1)–O(41)
N(1)–Ni(1)–N(11)
O(1)–Ni(1)–N(1)
N(1)–Ni(1)–O(41)
O(1)–Ni(2)–O(52)
87.78(13) O(1)–Ni(1)–O(14)
82.9(2) N(30)–Ni(1)–N(34)
78.96(18) O(1)–Ni(1)–O(22)
98.80(19) N(1)–Ni(2)–N(20)
97.47(13) O(1)–Ni(2)–N(10)
O(1)–Ni(2)–O(42) 103.13(14) O(1)–Ni(2)–O(23)
N(2)–Ni(2)–O(52) 163.05(15) O(23)–Ni(2)–O(13)
O(1)–Ni(2)–N(3)
N(2)–Ni(2)–N(3)
156.35(16) O(1)–Ni(2)–N(20)
78.29(17) N(10)–Ni(2)–N(20)
Ni(1)–O(1)–Ni(2) 112.27(15) Ni(1)–O(1)–Ni(2)
NH2
i
NH
CHO
+
N
N
N
N
(I)
ii
Fig. 2 ORTEP drawing of the cationic structure in 2. Thermal
ellipsoids for the non-hydrogen atoms are drawn at the 30 %
probability level. All hydrogen atoms are omitted for clarity
N
N
iii
Cl
N
N
OH
N
OH
N
N
N
CH3
Crystal structure analysis of complexes 1 and 2 shows
that they both crystallize in a monoclinic P2(1)/c space
group with a mono-cation of [Ni2L(DNBA)2]? and
[Ni2L(BPP)2]?, respectively, and one ClO-4 acting as the
counter anion. Ellipsoid drawings of the cationic cores in 1
and 2 are depicted in Figs. 1 and 2, respectively. In com-
plex 1, the two nickel atoms are bridged by the ionized
alkoxide oxygen of L and two carboxyl groups from the
DNBA ligand in l-bridging mode. In the bipyridyl pen-
dant, the six-coordinated environment around Ni(1) is
comprised of the N3O donor set from L and two oxygen
(II)
HL
Scheme 1 Synthetic route to the ligand HLa
key step in the synthesis, is realized through opening of the
epoxyethane ring in the epichlorohydrin, which also offers
a chloromethyl group for the alkylation of a second amine
different from the first. The overall synthesis involves three
reactions and affords the desired ligand in gram quantities
with an overall yield of ca. 32 %.
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Transition Met Chem (2014) 39:205–211
209
35
30
25
20
15
10
5
6
5
4
3
2
1
10
8
6
4
2
0
400
800
1200
1600
2000
1/[BNPP] (M-1
)
0
0
1
2
3
4
5
0
1
2
3
4
5
[BNPP] (mM)
[Ni2L] (mM)
Fig. 4 Saturation kinetic experiments for the cleavage of BNPP by
[Ni2L] at pH 8.4 at 25 ꢁC. I = 0.10 M (NaClO4), [Ni2L] = 0.02 mM
and buffer solution (50 mM HEPPS). Inset Lineweaver–Burk double-
reciprocal plot
Fig. 3 Dependence of kobs on the concentration of [Ni2L] in buffer
solution (50 mM HEPPS and 100 mM NaClO4 in water, pH = 8.4) at
25 ꢁC, [BNPP] = 1 mM
atoms from the bridging carboxyl groups, while in the
homopiperazine pendant, the N2O donor set from L, plus
two bridging oxygen atoms, provides the distorted square-
pyramidal geometry around Ni(2), with the atom O(42)
occupying the apical position in the coordination sphere.
Complex 2 is almost isostructural with 1, except that the
two exogenous carboxyl bridges are replaced by two
phosphate groups of BPP. The bond lengths of Ni(1)–
O(1)and Ni(2)–O(1) in complex 2 are similar to those
observed in 1. However, the Ni–O–Ni angle and Ni_Ni
distance are slightly larger than those observed in 1, which
is partly caused by the bulky size of the BPP bridges in 2.
To bind the nickel atom in a chelating mode, the seven-
membered homopiperazine ring has to adopt a less stable
boat-shape conformation, and the corresponding bond
angles N(2)–Ni(2)–N(3) in 1 and N(30)–Ni(1)–N(34) in 2
are 78.3(2)ꢁ and 77.6(4)ꢁ, respectively. These values
deviate significantly from the normal bond angle observed
in complexes with square-pyramidal geometry, suggesting
that the homopiperazine pendant is in a strained geometry.
8.4, the UV–vis spectrum of Ni2L in buffer solution
exhibited a strong absorption band at 505 nm, with the
extinction coefficient of 90 M-1cm-1, assigned to the
existence of square-planar four-coordinated Ni2? in Ni2L,
as shown in an analogous asymmetric dinickel(II) system
[18]. These results strongly suggest that [Ni2L(OH)]2? is
the active species responsible for cleaving BNPP. Figure 3
shows the effect of Ni2L concentrations on the value of kobs
for the cleavage of BNPP at pH 8.4 and 25 ꢁC. The rate of
BNPP cleavage initially increases linearly with increasing
Ni2L concentration, indicating the reaction is pseudo-first
order for [Ni2L]. The calculated pseudo-second-order rate
constant for Ni2L determined from the plot is
8.94 9 10-3 M-1 s-1
.
The initial rate of BNPP cleavage has also been deter-
mined as a function of the substrate concentration at pH
8.4. The data depicted in Fig. 4 reveal saturation kinetics
with Michaelis–Menten-like behavior, typical for native
metalloenzymes. The data points were fitted to the
Michaelis–Menten model by means of a Lineweaver–Burk
double-reciprocal plot, which resulted in a Michaelis con-
stant (KM) of 6.95 9 10-3 M and a catalytic rate constant
(kcat = Vmax/[Ni2L]) of 7.40 9 10-5 s-1. The second-
order rate constant KBNPP (kcat/KM) for BNPP is
1.06 9 10-2 M-1 s-1. The reciprocal of the Michaelis–
Menten constant could be treated as the substrate binding
constant, that is, Kb = 1/KM = 143.9 M-1. The low value
of Kb is in agreement with the observations that BNPP is a
weak ligand for Ni2L [22, 23]. At pH 8.4 and 25 ꢁC, the
first-order rate constant (k = kOH[OH-]) for automatic
hydrolysis of BNPP is calculated to be 1.46 9 10-11 s-1
Phosphodiesterase activity of Ni2L
Bis(4-nitrophenyl) phosphate is often used as a DNA
model compound in the investigation of phosphodiesterase
activity. The tests for phosphodiesterase activity were
carried out in buffers in order to mimic biological condi-
tions. The reaction was monitored by following the release
of NP from BNPP. The dependence of the reaction rate on
pH for the BNPP cleavage promoted by dinuclear species
Ni2L formed in situ from a 2:1 mixture of Ni2? and ligand
HL in aqueous solution shows a bell-shaped profile, with
an optimum at about pH 8.4. The ES-MS spectrum of Ni2L
in aqueous solution displayed a peak at m/z = 615, corre-
sponding to the [Ni2L(OH)(ClO4)]? ion. Moreover, at pH
with kOH = 5.8 9 10-6 M-1 s-1 for BNPP (Vauto
OH[BNPP][OH-]) [24]. Therefore, it can be concluded
=
k
that at pH 8.4 and 25 ꢁC, Ni2L promotes the hydrolysis of
BNPP by a factor of 5.1 9 106. Under the same conditions,
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Transition Met Chem (2014) 39:205–211
Scheme 2 Proposed mechanism for
hydrolysis of BNPP by Ni2L
N
N
O
ROPO3H-
-
N
Ni
R2O2PO2
Ni
Py
O
H
Py
k
OH-
(A)
N
N
N
N
O
O
(B)
(D)
N
Ni
Py Ni
N
Ni
Ni
Py
O
H
Py
Py
O
O
O
O
P
P
OR
RO
RO
OH
(C)
N
N
RO-
O
N
Ni
O
Py Ni
O
H
Py
O
P
RO
OR
the catalytic activity of complex 1 was also measured, and
it promoted the hydrolysis of BNPP by a factor of
7.8 9 104, indicating that the bridging carboxyl group is an
inhibitor of the catalytic species that reduces the rate of
catalytic hydrolysis. This result further suggests that BNPP
coordinates the two nickel centers by a bidentate bridging
mode in the catalytic reaction.
22]. In addition, the two nickel centers bring together the
two reactants (substrate BNPP and nucleophile OH-),
neutralizing the electrostatic repulsion that would occur for
two negatively charged species.
Conclusions
Mechanism of catalytic phosphodiester hydrolysis
In summary, we have developed a convenient and versatile
procedure for the synthesis of a new asymmetric alkoxide-
based dinucleating ligand. In the dinickel(II) complexes 1
and 2, the ligand forces the two metal ions into proximity
with different chemical environments, as in related natural
dinuclear metalloenzymes. At 25 ꢁC, the Ni2L species
formed in situ from a 2:1 mixture of Ni2? and HL in
aqueous solution at pH 8.4 shows efficient phosphodies-
terase activity.
On the basis of the above studies, a double Lewis acid
activation mechanism is proposed for the catalytic hydro-
lysis of BNPP by Ni2L, which is similar to previously
reported cases [18, 25, 26]. As shown in Scheme 2, the
catalytic hydrolysis cycle may contain the following steps.
First, the active species A is formed at pH 8.4, and then
BNPP coordinates to the two nickel centers as a bidentate
ligand, forming the BNPP adduct B (similar to the structure
of 2). Next, the bridging hydroxide acts as a nucleophile
and attacks BNPP, forming a transition complex C. Third,
the P–O bond in D breaks to release p-nitrophenolate fol-
lowed by recovery of the starting complex A, accomplished
by the replacement of NPP with hydroxide. In the first step,
an equilibrium may exist between A and B, since BNPP is
a weak ligand for Ni2L. The intrinsic activity of a dinuclear
catalyst is highly dependent upon the metal–metal distance
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