Phosphoester hydrolysis promoted by quinoline functionalized Ni(II) and Zn(II) complexes

Article abstract of DOI:10.1080/00958972.2021.1910944

A quinoline functionalized nitrogen-based bidentate ligand (L) was synthesized and used for the preparation of mononuclear Ni(II) and Zn(II) complexes which differ only by the halide ion attached to the respective metal center. All complexes of this serie

Full text of DOI:10.1080/00958972.2021.1910944

Journal of Coordination Chemistry
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Phosphoester hydrolysis promoted by quinoline
functionalized Ni(II) and Zn(II) complexes
Qazi Mohammad Junaid, Popuri Sureshbabu & Shahulhameed Sabiah
To cite this article: Qazi Mohammad Junaid, Popuri Sureshbabu & Shahulhameed Sabiah (2021):
Phosphoester hydrolysis promoted by quinoline functionalized Ni(II) and Zn(II) complexes, Journal
of Coordination Chemistry, DOI: 10.1080/00958972.2021.1910944
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2021.1910944
Phosphoester hydrolysis promoted by quinoline
functionalized Ni(II) and Zn(II) complexes
Qazi Mohammad Junaid, Popuri Sureshbabu and Shahulhameed Sabiah
Department of Chemistry, Pondicherry University, Pondicherry, India
ABSTRACT
ARTICLE HISTORY
Received 26 July 2020
Accepted 15 March 2021
A quinoline functionalized nitrogen-based bidentate ligand (L)
was synthesized and used for the preparation of mononuclear
Ni(II) and Zn(II) complexes which differ only by the halide ion
attached to the respective metal center. All complexes of this ser-
ies show closely related crystal packing regardless of the halide
ion and metal center (Ni/Zn). All the complexes were character-
ized by different spectroscopic techniques such as UV-vis, emis-
sion, IR and NMR spectroscopies, ESI-MS, single-crystal XRD and
thermogravimetry. All the complexes were examined for hydroly-
sis of a phosphoester bond in the DNA-model substrate 4-nitro-
phenyl phosphate (NPP) and bis(4-nitrophenyl)phosphate (BNPP).
All complexes were able to hydrolyze BNPP and NPP and showed
better rate of hydrolysis with diesters as compared
to monoesters.
KEYWORDS
Biphenyl backbone;
quinoline moiety; BNPP and
NPP hydrolysis
1. Introduction
Phosphodiester bonds are central to all life on earth as they make up the backbone of
the strands of nucleic acids. Nucleic acids are the storehouse of biological information
[1]. They are polymeric diesters of phosphoric acid held by phosphodiester linkages
which are remarkably stable under physiological conditions. The half-life for the
hydrolysis of an individual phosphodiester bond in DNA has been estimated to be 30
million years at 25 ꢀC [2]. The remarkable catalytic efficiency of nucleases in cleavage
of the phosphodiester linkages has made it a subject of intense mechanistic studies.
CONTACT Shahulhameed Sabiah
Sabiahs@Gmail.com
Department of Chemistry, Pondicherry University,
Pondicherry 605014, India
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

2
Q. M. JUNAID ET AL.
During the last decade, there has been considerable interest in elucidating the role
of metal ions in the hydrolysis of phosphates [3]. Transition metal complexes, lanthan-
ide complexes and actinides have all been utilized as catalysts for the hydrolysis of
DNA and model phosphodiesters [4–19]. Metal ions coordinated to organic frame-
works like pyridyl- and quinoline derivatives offer the advantage of a more easy exam-
ination of the substrate structure-activity relationship (SSAR) and the role of the metal
ion. The metal ion has been suggested to play roles of: (1) Lewis acid activation of the
substrate, (2) creation of a nucleophile and (3) generation of a good leaving group of
the substrate [20]. In recent years, a broad range of mononuclear metal-based artificial
nucleases were synthesized by optimizing ligand design [21–25]. By careful ligand
designing, reaction rates and turnovers can be improved.
Here, in this article, we have focused mainly on quinoline-functionalized ligand with
biphenyl as backbone. The use of biphenyl would provide an overall constraint in
structures with high degree of rigidness. Moreover, ligands with pyridyl and other
related moieties with biphenyl have shown interesting applications and unique ability
to form complexes [6]. These factors prompted us to explore the related chemistry
behind quinoline ligand.
To obtain a better understanding of phosphoester hydrolysis by these complexes,
we systematically changed the metal coordination around the ligand. The goal was to
determine how the changes in metal center associated with a rigid biphenyl
appended ligand scaffold affects the hydrolysis of phosphoesters, which can then be
used to develop more efficient catalysts and may provide insight into the differences
in rates between synthetic and enzymatic nucleases. In this article, we present a
detailed study of NPP and BNPP hydrolysis by quinoline-functionalized complexes
which were characterized using different techniques.
2. Experimental
2.1. Materials and methods
All the reactions were carried out under aerobic conditions unless stated otherwise.
The chemicals were purchased from Sigma Aldrich and used without purification. The
sodium salt of the phosphate ester NPP was purchased from Alfa Aesar.
2.2. Instrumentation
NMR spectra were recorded on a Bruker Avance 400 NMR spectrometer (1H, 400 MHz).
Electronic absorption spectra were recorded on a Shimadzu UV-2450 Model instru-
ment. Emission spectra were recorded on a Shimadzu RF-5301-PC spectrofluorimeter.
FT-IR spectra were obtained on a Thermo Nicolet 6700 spectrophotometer using KBr
pellets. Mass spectra were measured on Waters Quattro MicroV 4.1. X-ray crystallog-
raphy data were collected on X Calibur Oxford Diffraction Ltd. with MoKa radiation
(¼0.71073 Å). Structure solution, refinement and generation of hydrogen atoms
according to stereochemistry were performed with SHELXL-2014/7, SHELXL-2018/3,
SHELXS-2008 and Olex2.solve [26].

JOURNAL OF COORDINATION CHEMISTRY
3
2.3. Synthesis of ligand and complexes
The ligand and complexes were prepared as shown in Schemes 1 and 2.
2.3.1. Synthesis of 6-(quinolin-8-yl)-6,7-dihydro-5H-dibenzo[c,e]azepine as ligand L
A round bottom flask was charged with 8-aminoquinoline (0.72 g, 5 mmol), 2,2-bis(bro-
momethyl)-1,1-biphenyl (1.70 g, 5 mmol) and K2CO3 (4.14 g, 30 mmol) in acetonitrile
(20 mL) and put on reflux overnight under nitrogen atmosphere. Solvent was evapo-
rated by rotavapor and then extracted with DCM (3 ꢁ 5 mL) and water to give 6-(qui-
nolin-8-yl)-6,7-dihydro-5H-dibenzo[c,e]azepine as L, which was further purified by
column chromatography using hexane and ethyl acetate (9:1) as an eluent. Yield
1
(1.23 g, 76%). H NMR (400 MHz, CDCl3), d (ppm): 8.96 (dd, J ¼ 4.2, 1.8 Hz, 1H), 8.17 (dd,
J ¼ 8.3, 1.7 Hz, 1H), 7.57 (dd, J ¼ 7.6, 0.9 Hz, 2H), 7.47 (d, J ¼ 1.3 Hz, 1H), 7.46 (m, 2H),
7.43 (m, 1H), 7.39 (d, J ¼ 8.1 Hz, 1H), 7.31 (td, J ¼ 7.4, 1.2 Hz, 2H), 7.25 (m, 2H), 7.15 (dd,
J ¼ 7.1, 1.7 Hz, 1H), 4.39 (s, 4H). FT-IR (KBr), v (cmꢂ1): 3435 v(O-H), 3043 v(C-H), 1602
v(C ¼ C), 1458 v(C ¼ N), 1309 v(C-N). ESI-MS m/z calcd. for C23H18N2, [M þ H]þ,
323.1548; found 323.1539.
2.3.2. Synthesis of 1
To a suspension of anhydrous NiCl2 (0.017 g, 0.125 mmol) in toluene (7 mL) was added
solution of L (0.041 g, 0.125 mmol) and heated at 60 ꢀC for 6 h to get a light violet pre-
cipitate. The precipitate was filtered and dried to give 1. Yield (0.035 g, 63%). UV-vis
(CH2Cl2) [kmax, nm (e, Mꢂ1cmꢂ1)]: 259 (sharp, 966), 292 (sharp, 1089), 500 (band, 74).
FT-IR (KBr, cmꢂ1): FT-IR (KBr), v (cmꢂ1): 3402 v(O-H), 1623 v(C ¼ C), 1449 v(C ¼ N), 1318
1
v(C-N). H NMR (400 MHz, CDCl3), d (ppm): 8.94 (s, 1H), 8.16 (s, 1H), 7.57 (s, 3H), 7.42 (s,
5H), 7.28 (s, 2H), 7.14 (s, 2H), 4.41 (s, 4H). ESI-MS m/z calcd. for C23H18Cl2N2Ni,
[M þ Na]þ 474.9942; found 475.0078.
2.3.3. Synthesis of 2
A similar procedure to that described for 1 was employed using NiBr2 (0.022 g,
0.1 mmol) and L (0.033 g, 0.1 mmol) giving 2 as pink precipitate. Yield (0.036 g, 67%).
UV-vis (CH2Cl2) [kmax, nm]: 250 (sharp, 932), 300 (sharp, 1119), 500 (band, 74). FT-IR
(KBr), v (cmꢂ1): 3354 v(O-H), 1622 v(C ¼ C), 1448 v(C ¼ N), 1317 v(C-N). 1H NMR
(400 MHz, DMSO), d (ppm): 8.83 (s, 1H), 8.30 (d, J ¼ 7.1 Hz, 1H), 7.57 (s, 2H), 7.48 (s, 4H),
7.35 (d, J ¼ 22.5 Hz, 3H), 7.23 (s, 2H), 7.10 (s, 1H), 4.27 (s, 4H). ESI-MS m/z calcd. for
C23H18Br2N2Ni, [M-Br]þ 460.9986; found 460.9976.
2.3.4. Synthesis of 3
To a methanolic (10 mL) solution of L (0.016 g, 0.05 mmol) was added ZnCl2 (0.007 g,
0.05 mmol) in a round bottom flask. The reaction was left to stir for 12 h at room tem-
perature resulting in the formation of grey precipitate which was filtered and washed
with methanol to afford 3. Yield (0.016 g, 67%). UV-vis (CH2Cl2) [kmax, nm]: 259 (sharp,
966), 300 (sharp, 1119), 358 (band, 1335). FT-IR (KBr), v (cmꢂ1): 3057 v(C-H), 1591
v(C ¼ C), 1450 v(C ¼ N), 1319 v(C-N). 1H NMR (400 MHz, DMSO), d (ppm): 8.86 (dd,
J ¼ 4.0, 1.6 Hz, 1H), 8.32 (d, J ¼ 7.2 Hz, 1H), 7.60 (d, J ¼ 7.4 Hz, 2H), 7.56 (m, 1H), 7.52 (m,

4
Q. M. JUNAID ET AL.
Figure 1. Molecular structures of 1 and 2. Hydrogens were omitted for clarity. Selected bond dis-
tances (Å) and angles (ꢀ) are as follows. (1): Ni-N1 ¼ 1.984(19), Ni-N2 ¼ 2.070(19), Ni-Cl1 ¼ 2.249(9),
Ni-Cl2 ¼ 2.207(8); Cl1-Ni-Cl2 ¼ 129.92(3), N1-Ni-N2 ¼ 84(7). (2): Ni-N1¼ 1.981(4), Ni-N2 ¼ 2.066(4),
Ni-Br1 ¼ 2.376(9), Ni-Br2 ¼ 2.332(9); Br1-Ni-Br2 ¼ 128.35(4), N1-Ni-N2 ¼ 83.76(14).
3H), 7.41 (t, J ¼ 7.8 Hz, 1H), 7.35 (t, J ¼ 7.3 Hz, 2H), 7.23 (d, J ¼ 7.4 Hz, 2H), 7.11 (d,
J ¼ 7.5 Hz, 1H), 4.29 (s, 4H).
2.3.5. Synthesis of 4
A similar procedure to that described for 3 was employed using ZnI2 (0.032 g,
0.1 mmol) and L (0.033 g, 0.1 mmol) giving 4 as an off white precipitate. Yield (0.046 g,
70.6%). UV-vis (CH2Cl2) [kmax, nm]: 250 (sharp, 932), 309 (sharp, 1152). FT-IR (KBr), v
1
(cmꢂ1): 3467 v(O-H), 3059 v(C-H), 1618 v(C ¼ C), 1450 v(C ¼ N), 1316 v(C-N). H NMR
(400 MHz, DMSO), d (ppm): 8.85 (s, 1H), 8.32 (d, J ¼ 7.8 Hz, 1H), 7.59 (d, J ¼ 7.4 Hz, 2H),
7.50 (dd, J ¼ 15.3, 7.7 Hz, 4H), 7.39 (t, J ¼ 7.7 Hz, 1H), 7.33 (t, J ¼ 7.3 Hz, 2H), 7.22 (d,
J ¼ 7.2 Hz, 2H), 7.10 (d, J ¼ 7.4 Hz, 1H), 4.28 (s, 4H). ESI-MS m/z calcd. for C23H18I2N2Zn,
[M-I]þ 514.9775; found 515.2138.
3. Results and discussion
3.1. X-ray crystallography
Crystals suitable for single-crystal X-ray analysis were obtained by slow evaporation of
dichloromethane and methanol solvents. Plate-shaped brown crystals of L and block-
shaped dark purple crystals of 1 and 2 were obtained from dichloromethane whereas
3 and 4 were crystallized from CH2Cl2/MeOH mixture as colorless plates. The molecular
structures of L and 1-4 were determined by single-crystal X-ray diffraction (Figures 1
and 2).
Complexes 1-3 were crystallized in orthorhombic system with Pbca space group,
whereas L and 4 were crystallized in monoclinic system with P21/c space group.
Complexes 1-4 are neutral four-coordinate mononuclear Ni(II) and Zn(II) complexes
with pseudo-tetrahedral geometry and the metal is chelated by the N-bidentate ligand
L and two terminal halide ions. The bond angles between N1–M–N2 and X1–M–X2 are
different from the bond angles in ideal tetrahedral geometry. The bond distance

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 2. Molecular structures of 3 and 4. Hydrogens were omitted for clarity. Selected bond dis-
tances (Å) and angles (ꢀ) are as follows. (1): Zn-N1 ¼ 2.049(14), Zn-N2 ¼ 2.134(13), Zn-
Cl1 ¼ 2.227(6), Zn-Cl2 ¼ 2.191(5); Cl1-Zn-Cl2 ¼ 122.36(2), N1-Zn-N2 ¼ 82.16(5). (2): Zn-N1 ¼ 2.074(3),
Zn-N2 ¼ 2.152(3), Zn-I1 ¼ 2.502(12), Zn-I2 ¼ 2.544(14); I1-Zn-I2 ¼ 120.48(4), N1-Zn-N2 ¼ 81.63(13).
between Ni-N1 is shorter than Ni-N2 in all four complexes as expected due to the
bulky biphenyl backbone. There is appreciable difference in the bond distances of two
halide ions from the metal center. The bond distance between Ni-N is shorter than
Zn-N as usual. Also the bite angles are close to each other with 84.00ꢀ and 83.71ꢀ in 1
and 2, respectively. A closer look at the crystal structures of these four complexes
revealed only small differences in the atomic distances and angles.
Non-covalent intermolecular interactions like hydrogen bonding and p-p stacking
were not observed between molecules in the crystal lattice. The coordination geom-
etry and bond parameters of Ni(II) and Zn(II) complexes are analogous with reported
structures [27, 28]. A summary of the crystallographic data and selected bond lengths
and angles for 1-4 can be found in Table 1.
3.2. Spectroscopic characterization
1
L and 1-4 were characterized by NMR, ESI-MS and UV-Vis. H NMR spectrum of L was
recorded in CDCl3 as well as DMSO-d6, whereas spectra of the complexes were
1
recorded in DMSO-d6 due to their poor solubility in CDCl3, except 1. In H NMR spec-
trum of L, a singlet peak at 4.39 ppm (in CDCl3) corresponds to four methylene pro-
tons attached to biphenyl backbone, which suggests that molecule has no C2-
symmetry. The highly upfield signals above 8 ppm are typical of quinoline moiety
while rest of the peaks in normal aromatic region correspond to both biphenyl and
quinoline rings. The chemical shift values of 3 slightly shifted upfield and 4 shifted
downfield as compared to ligand, which may be due to the complexation with the
1
respective metal cation. The H NMR spectrum of Zn(II) complexes can be interpreted
similar to that of ligand with small chemical shift differences. For high-spin Ni(II) com-
plexes, electronic relaxation times are typically in the range 10ꢂ10–10ꢂ12 s, leading to
broadening of signals with no proper splitting pattern [29].
The ESI-MS of L displays a base peak at m/z 323.1573 corresponding to
[C23H19N2]þ, 1 shows molecular-ion peak at m/z 475.0078 corresponding to [M þ Na]þ,

6
Q. M. JUNAID ET AL.
Table 1. Crystallographic data for L and 1-4.
Parameters
L
1
2
3
4
CCDC No.
Formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
1963212
C23H18N2
323.40
293(2)
Monoclinic
P21/c
10.0676(7)
12.3011(7)
14.0132(9)
90.00
93.003(6)
90.00
1733.05(19)
4
1.239
1963213
C23H18Cl2N2Ni
452.00
293(2)
Orthorhombic
Pbca
16.6214(16)
13.8035(7)
17.4942(9)
90.00
90.00
90.00
4013.8(5)
8
1.496
1.244
1856.0
1963214
C23H18Br2N2Ni
540.92
293(2)
Orthorhombic
Pbca
16.4738(17)
14.2112(10)
17.8300(5)
90.00
90.00
90.00
4174.2(6)
9
1.937
5.365
2412.0
1963215
C23H18Cl2N2Zn
458.70
273
Orthorhombic
Pbca
16.6086(6)
13.6797(4)
17.7086(7)
90.00
90.00
90.00
4023.4(3)
37
1.5144
1.498
1877.3
1963216
C23H18I2N2Zn
641.56
293(2)
Monoclinic
P21/c
9.705(6)
16.828(9)
13.507(7)
90.00
94.74(5)
90.00
2198(2)
4
1.939
3.937
1224.0
23697
b/Å
c/Å
a/ꢀ
b/ꢀ
c/ꢀ
Volume/Å3
Z
qcalc (mg/mm3)
l/mm–1
F(000)
0.073
684.0
9465
Reflections
collected
Independent
reflections
13468
12976
44237
4052 [R(int)
¼ 0.0272]
4726 [R(int)
¼ 0.0375]
4902 [Rint
¼ 0.0703]
5161 [Rint ¼
0.0264, Rsigma
¼ 0.0171]
5366 [Rint
¼
0.0348, Rsigma
¼ 0.0319]
Data/
4052/0/226
4726/0/253
4902/0/253
5161/0/253
5366/0/253
restraints/
parameters
Goodness-of-fit
on F2
1.164
1.046
1.040
1.060
1.025
Final R indexes
[I> ¼2r (I)]
Final R indexes
[all data]
R1 ¼ 0.0652,
wR2 ¼ 0.1634
R1 ¼ 0.1088,
wR2 ¼ 0.1757
0.17/–0.45
R1 ¼ 0.0395,
wR2 ¼ 0.0872
R1 ¼ 0.0700,
wR2 ¼ 0.1020
0.39/–0.43
R1 ¼ 0.0504,
wR2 ¼ 0.1172
R1 ¼ 0.1062,
wR2 ¼ 0.1617
0.82/–1.11
R1 ¼ 0.0312,
wR2 ¼ 0.0778
R1 ¼ 0.0452,
wR2 ¼ 0.0864
0.39/–0.52
R1 ¼ 0.0385,
wR2 ¼ 0.0786
R1 ¼.0610,
wR2 ¼ 0.0868
1.38/–1.06
Largest diff.
peak/
hole /e Å–3
2 at m/z 460.9976 corresponding to [M-Br]þ and the isotopic patterns are in agree-
ment with the theoretical ones. However 4, [M-I]þ having molecular weight of
514.6955 shows base peak at 519.2419 and unfortunately we were unable to get ESI-
MS of 3.
3.3. Photophysical properties
The UV spectra of L and the complexes (Figure 3) were recorded in dichloromethane.
High-energy band below 300 nm (e ¼ 14,520 Mꢂ1cmꢂ1) in L have been assigned to lig-
ꢃ
and centered p-p transitions within the aromatic rings and a band at 370 nm
(e ¼ 1380 Mꢂ1cmꢂ1) to intraligand charge-transfer transitions. The band due to p–p
ꢃ
transitions remains almost unchanged in the spectra of metal complexes, while the
ꢃ
other band due to n–p transition shifted to shorter wavelength as a result of coordin-
ation with the metal atom. The UV–cis spectra of 1 and 2 display a broad band
around 500 nm (e ¼ 74 Mꢂ1cmꢂ1) only at high concentration, which may be due to
3
3T1(F)! T1(P) transition in tetrahedral geometry [30]. The electronic spectral data con-
form to the structures determined by X-ray crystallography and reported structures
[31]. The UV–vis spectra of all the complexes remained unchanged for almost one

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 3. Electronic absorption spectra of 1-4 in CH2Cl2.
Figure 4. Emission spectra of L and 1-4.
week in both polar as well as nonpolar solvents like dimethylsulfoxide and dicholoro-
methane, respectively, indicating their fair stability in these solvent systems.
The emission spectra of 1-4 (Figure 4) were measured in the solution state with
DMSO as solvent at room temperature. All complexes upon excitation at 305 nm dis-
played dual emission bands around 340–360 nm and small shoulders at 430 nm and
observed hypsochromic shift with decrease in fluorescence intensity when compared
with ligand. Ligand shows a broad emission band at 450 nm at room temperature,
probably centered on the quinoline moiety (ꢀ(C ¼ N) or ꢀ(C ¼ C) stretching modes)
[32]. The quenching in fluorescence intensity occurred due to the presence of

8
Q. M. JUNAID ET AL.
Figure 5. TGA curves of 1-4.
Scheme 1. Synthesis of L.
unpaired electrons in the case of 1 and 2 [33]. Metal ions such as Cu2þ, Pb2þ, Hg2þ
,
Cd2þ and Ni2þ show quenching in the fluorescence spectra due to charge transfer
process [34]. On the other hand Zn2þ having d10 is photophysically inactive. However,
the Zn2þ ion can interact with neighboring fluorophore, which may lead to fluores-
cence regeneration or enhancement [35].
IR spectra of L, 1, 2 and 4 exhibit a broad peak around 3300–3400 cmꢂ1 which is
assigned as a characteristic peak due to moisture content in KBr. Strong absorption
bands in all the cases around 1550–1630 cmꢂ1 were observed due to C ¼ C stretching
vibrations. The presence of quinoline moiety is confirmed by bands of v(C ¼ N) and
v(C-N) around 1450–1300 cmꢂ1. The absorptions at 3200–3000 cmꢂ1 are assigned to
CH stretching modes.
3.4. Thermal analysis
The thermal stabilities of all complexes were investigated by thermogravimetric ana-
lysis (TGA) under nitrogen atmosphere. The TGA data (Figure 5) indicate that all com-
plexes showed different behavior towards increasing temperature. Complex 4 was

JOURNAL OF COORDINATION CHEMISTRY
9
Scheme 2. Synthesis of 1-4.
Scheme 3. Schematic representation of the hydrolysis reactions of NPP and BNPP by 1-4.
stable up to 360 ꢀC, while the rest of the complexes showed some sort of decompos-
ition from 45 ꢀC onwards. Complexes 1-4 started decomposition at 100 ꢀC, 100 ꢀC,
45 ꢀC and 360 ꢀC, respectively, indicating more stability of 4. Complex 4 exhibited no
mass loss up to almost 360 ꢀC. In the TGA curve of 1, an estimated mass loss of 6%
was observed at 330 ꢀC. All complexes showed decomposition mainly from 320
to 600 ꢀC.
3.5. Phosphate ester hydrolysis
The hydrolytic cleavage of BNPP and NPP was monitored spectrophotometrically by
appearance of hydrolysis product like 4-nitrophenolate anion at 400 nm. BNPP is ini-
tially hydrolyzed to nitrophenol (NP) and 4-nitrophenyl phosphate (NPP) which is fur-
ther hydrolyzed to a second NP molecule and phosphate (P) (Scheme 3). The
efficiency of hydrolysis is greatly influenced by the electrophilicity of metal centers
and nucleophilic character of metal bound hydroxide [36].

10
Q. M. JUNAID ET AL.
Figure 6. Spectral profile showing comparative hydrolysis of BNPP [10 mM] and NPP [10mM] by
complexes 1-4 [5 mM] at 50 ꢀC.
Table 2. % Hydrolysis of BNPP and NPP by 1-4.
Complex
BNPP
NPP
1
2
3
4
17
14
64
33
94
85
47
80
Complexes 1-4 were tested for complete hydrolysis of BNPP and NPP in wet DMSO
for 24 h at 50 ꢀC (Figure 6, Table 2). The concentration of 4-nitrophenolate was calcu-
lated from the extinction coefficient (18,700 Mꢂ1cmꢂ1) [4]. The activity of 1-4 showed
a different trend in both substrates (BNPP, NPP). Hydrolysis of BNPP followed the order
3 > 4 > 1 > 2 and the order in NPP hydrolysis being 1 > 2 > 4 > 3. Complex 1 was
able to hydrolyze 94% of NPP substrate while 3 hydrolyzed 64% of BNPP substrate.
Also, 3 showed moderate activity with both substrates. All complexes 3 showed better
activity with monoesters over diesters. Moreover, varying temperatures from physio-
logical to 50 ꢀC had no profound effect on BNPP hydrolysis. It is noteworthy to men-
tion that the steric hindrance of the complex plays a crucial role in hydrolysis.
3.5.1. Kinetics of NPP and BNPP hydrolysis
The hydrolysis rate of NPP and BNPP was measured at pH 8.4 by an initial slope
method in aqueous DMSO solutions of complexes at 50 ꢀC. The initial rate of reaction
was monitored spectrophotometrically at 400 nm by the production of 4-nitropheno-
late. The concentration of 4-nitrophenolate was calculated from the extinction coeffi-
cient. The initial first-order rate constants were obtained directly from a plot of the p-
nitrophenolate concentration versus time. Complexes 1 and 4 (Figure 7, Table 3)
showed similar rates in NPP hydrolysis with rate constant equal to 3.7 ꢁ 10ꢂ5 sꢂ1
,
while same rate was observed by 2 in BNPP hydrolysis, which was least among all the
complexes. Complex 3 in BNPP hydrolysis (Figure 8, Table 4) showed better activity
with rate and rate constant as 3.8 ꢁ 10ꢂ4 mM sꢂ1 and 7.5 ꢁ 10ꢂ5
s
ꢂ1. These complexes
showed hydrolytic activity comparable to many reported nickel and zinc complexes [5,
10, 37]. The rate acceleration in the case of BNPP hydrolysis is higher than that of NPP

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 7. (a) Spectral profile showing hydrolysis of NPP by 4 after every 30 min. (b) Plot of concen-
tration vs. time.
Table 3. Observed values for NPP hydrolysis by 1-4.
Complex
Rate (mM s–1
)
Rate constant, K (s–1
)
1
2
3
4
1.9 ꢁ 10–4
1.3 ꢁ 10–4
1.2 ꢁ 10–4
1.9 ꢁ 10–4
3.7 ꢁ 10–5
2.6 ꢁ 10–5
2.3 ꢁ 10–5
3.7 ꢁ 10–5
Figure 8. (a) Spectral profile showing hydrolysis of BNPP by 3 after every 30 min. (b) Plot of con-
centration vs. time.
Table 4. Observed values for BNPP hydrolysis by 1-4.
Complex
Rate (mM s–1
)
Rate constant, K (s–1
)
1
2
3
4
2.8 ꢁ 10–4
1.9 ꢁ 10–4
3.8 ꢁ 10–4
3.0 ꢁ 10–4
5.6 ꢁ 10–5
3.7 ꢁ 10–5
7.5 ꢁ 10–5
5.9 ꢁ 10–5
hydrolysis. Rate constants calculated for the hydrolysis of BNPP were found to be
approximately two times higher than the rate constants for the corresponding reac-
tions with NPP. This increase can be attributed to the fact that NPP contains only one
4-nitrophenyl group while BNPP contains two 4-nitrophenyl groups.

12
Q. M. JUNAID ET AL.
4. Conclusion
We report the hydrolysis of phosphoesters by Ni(II) and Zn(II) complexes. The hydro-
lytic nature of 1-4 was proven by the complete hydrolysis of NPP and BNPP. All com-
plexes showed better rate of hydrolysis with diesters as compared to monoesters. In
terms of complete hydrolysis of substrate, complexes showed better hydrolysis with
monoesters. All complexes were characterized by spectroscopic methods like single-
crystal XRD, NMR, ESI-MS and UV-vis. The spectral data were consistent with the tetra-
hedral structure from X-ray analysis.
Acknowledgement
We gratefully acknowledge UGC for fellowship, Central Instrumentation Facility, Pondicherry
University for NMR spectra, DST-FIST for the single-crystal X-ray diffraction facility and the
Department of Chemistry, Pondicherry University for other facilities.
Disclosure statement
No potential conflict of interest was reported by the authors.
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