Hydrolytically active tetranuclear [NiII2]2 complexes: Synthesis, structure, spectroscopy and phosphoester hydrolysis

Article abstract of DOI:10.1039/c5ra16555e

Three tetranuclear nickel(ii) complexes, [Ni₄(H₂chdp)₂(H₂O)₄]Br₂·4CH₃OH·3H₂O (1), [Ni₄(H₂chdp)₂(H₂O)₄](PF_6<

Full text of DOI:10.1039/c5ra16555e

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PAPER
Hydrolytically active tetranuclear [NiII ]
2 2
complexes: synthesis, structure, spectroscopy and
Cite this: RSC Adv., 2015, 5, 99270
phosphoester hydrolysis†
Gopal C. Giri,a Ayan Patra,a Gonela Vijaykumar,b Luca Carrellac
a
*
and Manindranath Bera
Three tetranuclear nickel(II) complexes, [Ni4(H2chdp)2(H2O)4]Br2$4CH3OH$3H2O (1), [Ni4(H2chdp)2
(H2O)4](PF6)2 (2) and [Ni4(H2chdp)2(H2O)4](ClO4)2$3.2CH3OH$0.8H2O (3) have been synthesized by
exploiting the flexibility, chelating ability and bridging potential of a new symmetrical m-bis(tetradentate)
ligand, H5chdp (H5chdp ¼ N,N0-bis[2-carboxybenzomethyl]-N,N0-bis[2-hydroxyethyl]-1,3-diaminopropan-
2-ol). Complexes 1, 2 and 3 have been synthesized by carrying out reaction of the ligand H5chdp with
stoichiometric amounts of NiCl2$6H2O/NaBr, NiCl2$6H2O/NH4PF6, and Ni(ClO4)2$6H2O, respectively, in
methanol–water in the presence of NaOH at ambient temperature. Characterizations of the complexes
have been done using various analytical techniques including single crystal X-ray structure determination
of complexes 1 and 3. Molecular architecture of each complex is built from the self-assembly of two
monocationic [Ni2(H2chdp)(H2O)2]+ units which are exclusively bridged by two benzoate functionalities of
the ligands. Single crystal X-ray structure analyses reveal that the metallic cores of complexes 1 and 3
ꢀ
consist of four distorted octahedral nickel(II) ions with intra-ligand Ni/Ni separation of 3.527(7) A and
2
1
1
ꢀ
3.507(1) A, respectively. Complexes 1 and 3 display a rare m3:h :h :h bridging mode of two benzoate
groups of H2chdp3ꢀ ligand with each bridging among three nickel(II) ions. Mass spectrometric analyses
suggest that all the tetranuclear complexes are stable in solution. Potentiometric titration results and the
corresponding species distribution curves show that all the complexes exist predominantly in their
tetrameric species in solution, in the pH range of 6–12. The catalytic activity of all the three complexes
toward phosphoester hydrolysis has been investigated in methanol–water (1 : 1; v/v) solution by UV-vis
spectrophotometric technique using bis(p-nitrophenyl)phosphate (BNPP) as a model substrate.
Received 17th August 2015
Accepted 10th November 2015
DOI: 10.1039/c5ra16555e
www.rsc.org/advances
promoting reactions, and new electronic interactions might
lead to distinct physical properties. Cooperative interactions
Introduction
have commonly been observed in biological systems, and
nature has constructed numerous multimetallic protein
complexes that perform an extraordinary array of catalytic
transformations.3–6 In this context, there is a growing interest in
the active site structures and catalytic functions of nickel(II)
containing metalloenzymes.7 Among these, urease is an enzyme
that catalyzes the hydrolysis of urea to ammonia and carbamic
acid by utilizing carbamylated lysine bridged dimetallic centers
of nickel(II) ions in the active site.8–11 This enzymatic conversion
of urea to ammonia and carbamic acid has shown a consider-
able importance in the eld of agricultural and medicinal
chemistry.11,12 Moreover, multinuclear nickel(II) complexes are
relatively well-known for exhibiting magnetic interactions that
are antiferro- or ferromagnetic in nature.13–23 In these
complexes, controlling the interactions among the metal ions
provides a way to organize the overall spin ground state.
Consequently, this can have a signicant implication on their
magnetic behavior.24,25
The group of multimetallic complexes is a consolidated family
within the area of supramolecular coordination chemistry.1,2
The stereochemical preferences of metal ions in combination
with diverse binding possibilities of the ligands have produced
a large variety of such complexes. The neighboring metal
centers in multimetallic systems are expected to cooperate in
aDepartment of Chemistry, University of Kalyani, Kalyani, West Bengal-741235, India.
E-mail: mbera2009@klyuniv.ac.in; Web: http://www.klyuniv.ac.in; Fax: +91 33
25828282; Tel: +91 33 25828282 ext. 306
bDepartment of Chemical Sciences, Indian Institute of Science Education & Research
Kolkata, Mohanpur, West Bengal-741246, India
cInstitut fur Anorganische Chemie und Analytische Chemie, Johannes-Gutenberg
¨
Universitat Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany
† Electronic supplementary information (ESI) available: FTIR, 1H and 13C NMR,
and mass spectra of the ligand H5chdp. Representative FTIR, UV-vis, ESI mass
spectra, potentiometric titration plots, species distribution curves and kinetics
plots of the complexes. CCDC 1417838 and 1417839 for complexes 1 and 3,
respectively. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c5ra16555e
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The focus here is on a new symmetrical m-bis(tetradentate)
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ligand, H5chdp incorporating two benzoate and two hydrox-
yethyl functionalities. Similar carboxylate-rich polydentate
ligands have been used as a way to assemble different metal
ions into aggregates with relevance in the area of bioinorganic
chemistry.26–30 For example, a carboxylate-rich polydentate
ligand containing two benzoate and two acetate groups is
known to bind cobalt(II), copper(II) and zinc(II) ions to produce
mono-, di-, tetra- and hexanuclear complexes showing the
bridging potential of carboxylates.26,27,31–33 Recently, the unusual
amide binding modes of a similar amide and carboxylate-rich
polydentate ligand has been explored in a self-assembled hep-
tanuclear zinc(II) complex.34 Very recently, we have also reported
the synthesis, characterization and in vitro biological investi-
gations on human cervical cancer cells (HeLa) of new water
soluble heteronuclear [NaCuII6] metallomacrocyclic sandwich
complexes using a similar carboxylate and pyridine containing
polydentate ligand.29 In this paper, we report the synthesis,
X-ray crystal structure, spectroscopic characterization and
phosphoester hydrolysis of three new tetranuclear nickel(II)
complexes.
Results and discussion
Synthesis and general characterization
The new symmetrical m-bis(tetradentate) ligand, H5chdp has been
synthesized in two step reactions (Scheme 1). The synthesis of the
precursor ligand, H3cdp has been achieved by the condensation
of stoichiometric amounts of 2-carboxybenzaldehyde and 1,3-
diamino-propan-2-ol in the presence of NaOH in methanol under
reuxing conditions for 4 h, followed by the subsequent reduc-
tion using NaBH4. Acidication of the resulting solution by
addition of conc. HCl to pH ꢁ 5 produced a white solid product.
The product has been characterized to be a reduced Schiff base
ligand, H3cdp$H2O by elemental analysis, FTIR and NMR spec-
troscopy. Alkylation of the secondary amines of H3cdp with
2-iodoethanol in 1 : 2.5 molar ratio yielded the symmetrical
m-bis(tetradentate) ligand, H5chdp, in good yield. The ligand,
H5chdp has been fully characterized using the analytical tech-
Scheme 1 Synthesis of the ligand, H5chdp.
The reaction of H5chdp with NiCl2$6H2O and NH4PF6 in 1 : 2 : 1
molar ratio in the presence of NaOH in methanol–water at
room temperature yielded a green tetranuclear complex, [Ni4
(H2chdp)2(H2O)4](PF6)2 (2) (Scheme 2). Similarly, the reaction of
H5chdp with Ni(ClO4)2$6H2O in 1 : 2 molar ratio in the presence
of NaOH, in methanol–water at room temperature produced
a green complex, [Ni4(H2chdp)2(H2O)4](ClO4)2$3.2CH3OH$0.8H2O
(3) (Scheme 2). They represent a new family of m-alkoxo-m3-car-
boxylato bridged tetranuclear nickel(II) clusters. Characterizations
of the complexes were done by elemental analysis, FTIR, UV-vis,
mass spectrometric and potentiometric titration techniques.
Furthermore, the molecular structures of complexes 1 and 3 were
conrmed by single crystal X-ray analyses.
1
niques such as elemental analysis, FTIR (Fig. S1, ESI†), H and
13C NMR (Fig. S2 and S3, ESI†), and ESI mass spectrometric
(Fig. S4, ESI†) techniques. Very recently, we have shown that the
alkylation of the secondary amine at one half of H3cdp with
2-iodoethanol in 1 : 1 molar ratio and intramolecular cyclization
between the secondary amine and the benzoate functionality at
the other half produces the unsymmetrical dinucleating ligand
consisting of an isoindol functionality.35 The ligand, H5chdp has
been chosen for the present investigation because the central
Crystal and molecular structures of complexes 1 and 3
pendant alcoholic arm of this ligand acts as a spacer-cum- Complex 1 crystallizes in a triclinic system and the structure was
ꢁ
bridging unit between the two metal ions in the dinuclear solved in P1 space group. Complex 3 crystallizes in an ortho-
entity. Therefore, we decided to prepare such a ligand that may rhombic system and the structure was solved in Pbca space
adequately mimic the ligand environment of di- and tetranuclear group. The crystal structure of complex 1 consists of a cationic
nickel(II) active sites in metallohydrolases.
[Ni4(H2chdp)2(H2O)4]2+ species along with two Brꢀ ions as the
The reaction of H5chdp with NiCl2$6H2O and NaBr in 1 : 2 : 1 counter anions. Four methanol and three water molecules
molar ratio in the presence of a strong base, NaOH in methanol– cocrystallized with the complex. The crystal structure of
water at room temperature afforded a light green tetranuclear complex 3 consists of a cationic [Ni4(H2chdp)2(H2O)4]2+ species
complex, [Ni4(H2chdp)2(H2O)4]Br2$4CH3OH$3H2O (1) (Scheme 2). along with two ClO4ꢀ ions as the counter anions. Three and one-
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Scheme 2 Synthesis of the complexes.
h methanol, and four-h water molecules cocrystallized bridging among three Ni(II) ions. The syn–syn and syn–anti
with the complex. The structural views of the cations of bidentate bridges are commonly observed in dinuclear or poly-
complexes 1 and 3 are depicted in Fig. 1 and 2. Each cationic nuclear metal carboxylate chemistry.36 However, the tridentate
species contains four Ni(II) ions, two H2chdp3ꢀ ligands and four bridges in metal–carboxylate complexes are not common.37
H2O molecules. The tetranuclear complex cation is formed There are three coordination modes in tridentate bridging
through the self-assembly of two dimeric [Ni2(H2chdp)]+ units (Scheme 3). The mode A is postulated to be an important
exclusively bridged by two m3:h2:h1:h1 benzoate groups of the intermediate in “carboxylate shi” chemistry,37 mode B (a
two H2chdp3ꢀ ligands. Each dimer consists of two Ni(II) ions monodentate and a syn–syn bidentate bridges) has been found
which are bridged by the central alkoxo oxygen atom of in metal–carboxylate complexes,38 and mode C (a monodentate
H2chdp3ꢀ ligand and one benzoate group of the other H2chdp3ꢀ and a syn–anti bidentate bridges) has also been found in metal–
ligand of opposite Ni2 moiety. Each dimer also takes the similar carboxylate chemistry.39 The current study offers a rare example
structural arrangements with respect to the coordination mode with mode B carboxylate binding, in which the carboxylate group
of H2chdp3ꢀ ligand. Two terminal aliphatic alcoholic arms of shows a tridentate bridging mode with a monodentate and
H2chdp3ꢀ ligand within each Ni2 dimer of complexes 1 and 3 a syn–syn bidentate bridges among the Ni(II) ions.
are oriented in a trans fashion (Fig. 3 and 4).
The coordination environment around the nickel(II) centers
Interestingly, complexes 1 and 3 display a rare m3:h2:h1:h1 within each dimer exhibits distorted octahedral geometry. In
tridentate bridging mode of two carboxylate groups with each complex 1, the square base of the octahedral geometry around
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Fig. 1 Thermal ellipsoid (35%) plot of the molecular structure of complex [Ni4(H2chdp)2(H2O)4]Br2$4CH3OH$3H2O (1) with atom numbering
scheme. Hydrogen atoms are omitted for clarity.
the Ni1 center is dened by the O1, O5, O5i and N2 atoms, to the bond distances observed in similar octahedral nickel(II)
whereas that around the Ni2 center is dened by the O1, O4, O9 complexes.7,17,40,41 In complexes 1 and 3, the intra-ligand Ni/Ni
and N4 atoms. Whereas the axial positions of the octahedral distances are 3.527(7) and 3.507(1) A,7,17,40,41 and the inter-ligand
ꢀ
ꢀ
geometry around the Ni1 center are occupied by the O11 and O12 Ni/Ni distances are 3.106(8) and 3.117(1) A, respectively. The
atoms, the axial positions around Ni2 center are taken by the O2 comparable nickel–alkoxo and nickel–carboxylate arms provide
and O10 atoms. Similarly, in complex 3, the square base of the the framework to support the wide Ni–Oalkoxo–Ni bridging angles
octahedral geometry around the Ni1 center is dened by the O20, of 129.1(1) and 127.3(1)ꢂ in complexes 1 and 3, respectively,
O34, O34i and N15 atoms, whereas that around the Ni2 center is which are open enough to afford further coordination of
formed by the O20, O35i, O37 and N11 atoms. The axial positions carboxylates of the H2chdp3ꢀ ligand among the Ni(II) ions in
of the octahedral geometry around the Ni1 center are occupied by a m3:h2:h1:h1 fashion.
the O18 and O36 atoms, and the axial positions around the Ni2
center are occupied by the O23 and O26 atoms. The average Ni–
Spectroscopic studies
O
terminal alcohol, Ni–Oaquo and Ni–Oterminal carboxylate bond distances
The different coordination modes of benzoates, namely, mon-
odentate terminal and tridentate m3:h2:h1:h1 bridging
ꢀ
are 2.124, 2.104 and 2.039 A, respectively, which are comparable
Fig. 2 Thermal ellipsoid (35%) plot of the molecular structure of complex [Ni4(H2chdp)2(H2O)4](ClO4)2$3.2CH3OH$0.8H2O (3) with atom
numbering scheme. Hydrogen atoms are omitted for clarity.
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Fig. 3 Thermal ellipsoid (35%) plot of the molecular structure of the dinuclear unit of complex 1 viewing the two aliphatic alcoholic arms of the
ligand in trans-orientations.
cmꢀ1 for complexes 1, 2 and 3, respectively. The signicantly
higher differences of D (D ¼ nas(COOꢀ) ꢀ ns(COOꢀ)) of ꢁ245 and
194, 225 and 208, and 229 and 187 cmꢀ1 between the asym-
metric and symmetric stretching vibrations are attributed to the
monodentate terminal and monodentate bridging coordina-
tions of benzoate groups in complexes 1, 2 and 3, respectively.43
The relatively lower values of D at ꢁ132, 128 and 138 cmꢀ1
between the asymmetric and symmetric stretching vibrations
are characterized by the syn–syn bidentate bridging (m2:h1:h1) of
benzoate groups in complexes 1, 2 and 3, respectively.43 In the
FTIR spectrum of complex 2, a prominent broad band centered
at ꢁ845 cmꢀ1 and a strong sharp band at ꢁ565 cmꢀ1 can be
assigned to the stretching and bending modes of the PF6ꢀ ion,
respectively.44 A characteristic strong band (nClO ꢀ) has been
Fig. 4 Thermal ellipsoid (35%) plot of the molecular structure of the
4
observed at ꢁ1089 cmꢀ1 in the spectrum of complex 3, sug-
dinuclear unit of complex 3 viewing the two aliphatic alcoholic arms of
the ligand in trans-orientations.
gesting the presence of perchlorate ion outside the coordination
sphere.45 In addition, the FTIR spectra of all the complexes
exhibit a broad absorption band at ꢁ3413 cmꢀ1 which is
assignable to the n(O–H) vibration of the coordinated alcohol
groups and/or water molecules.
The electronic spectra of complexes 1, 2 and 3 in methanol–
water (1 : 1; v/v) at pH ꢁ 7.5 display a broad absorption band in
the visible region at 683 nm (3, 138 Mꢀ1 cmꢀ1), 685 nm (3, 146
Mꢀ1 cmꢀ1) and 658 nm (3, 154 Mꢀ1 cmꢀ1), respectively, due to
Scheme 3 Binding modes of carboxylates in tridentate fashion.
the weak d–d transitions (Fig. S6(a), ESI†). This is typical for d–
d transitions of Ni(II) ions with an octahedral coordination
coordination in the complexes have been established by their
environment. In addition, the spectra also show the distinct
FTIR spectra (Fig. S5, ESI†). Deacon and Phillips have carefully
scrutinized the FTIR spectra of many carboxylate containing
metal complexes with known X-ray crystal structures and drawn
nickel(II) ion bound ligand-based charge transfer transitions at
276 nm (3, 1391 Mꢀ1 cmꢀ1) and 220 nm (3, 6971 Mꢀ1 cmꢀ1), 277
nm (3, 1430 Mꢀ1 cmꢀ1) and 223 nm (3, 7530 Mꢀ1 cmꢀ1), and 279
nm (3, 1285 Mꢀ1 cmꢀ1) and 225 nm (3, 6529 Mꢀ1 cmꢀ1),
useful conclusion for the correlation between carboxylate
stretching frequencies and their geometries.42 In the FTIR
respectively (Fig. S6(b), ESI†). Thus, spectral comparison of
three complexes indicates that they are structurally alike.
spectra, three strong asymmetric nas(COOꢀ) vibrations were
observed at 1631, 1586 and 1559 cmꢀ1; 1629, 1593 and 1548
To be better acquainted with the structural features of the
cmꢀ1; 1630, 1589 and 1555 cmꢀ1; whereas three strong
complexes in solution, the ESI mass spectra of all the complexes
symmetric ns(COOꢀ) vibrations were observed at 1454, 1386 and
were recorded in methanol–water at pH ꢁ 7.5. The solutions of
1365 cmꢀ1; 1465, 1404 and 1340 cmꢀ1; 1451, 1401 and 1368
complexes 1, 2 and 3 were positive-ion electrosprayed into
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a quadrupole ion-trap mass spectrometer and subjected to MS/MS spectrum correspond to the species obtained upon
collision-induced dissociation. The mass spectra of all the successive decarboxylation (loss of four CO2 molecules) of
complexes exhibit the signal at m/z ¼ 1119 corresponding to the {[Ni4(H2chdp)(Hchdp)]}+ species having m/z ¼ 1119. Therefore,
tetranuclear {[Ni4(H2chdp)(Hchdp)]}+ species. The spectra also the mass spectrometric results suggest that the tetrameric forms
display an intense signal at m/z ¼ 560 (1119/2 ¼ 559.5) matching of all the three complexes are stable in solution.
to the tetranuclear {[Ni4(H2chdp)(Hchdp)]}2+ species. The
representative mass spectra of complexes 1, 2 and 3 are shown in
Fig. S7–S9 of the ESI.† The experimental and simulated spectra
Potentiometric titrations
Potentiometric titrations of all the complexes were performed to
determine the pKa values of the coordinated water molecules
in MeOH–H2O (1 : 1; v/v); the ionic strength was maintained at
I ¼ 0.1 M NaClO4. The curves obtained from the titration of
complexes 1, 2 and 3 with NaOH are shown in Fig. 5, S13 and
S14.† The titration results indicate that the neutralization
occurs with 2 mol of NaOH per mol of each complex in the pH
range of 6–12. Two deprotonation constants have been evalu-
ated for each complex by treating the potentiometric titration
data. The pKa values of the coordinated water molecules were
calculated to be 8.15 and 9.94 for 1, 7.98 and 8.42 for 2, and 8.34
and 10.32 for 3. These pKa values are more or less comparable to
the value obtained for nickel(II)-bound water (cf. the pKa of
[Ni(H2O)6]2+ is approximately 9.9).46 The representative species
distribution curves for complexes 1 and 3 are shown in Fig. 6
and S15.† The species distribution curves show that all the three
complexes exist predominantly in their corresponding tetra-
meric species in solution, in the pH range of 6–12. From the
potentiometric titration results, we are prone to believe that the
two pKa values observed for each complex are due to the
of the main peaks of complexes 1 and 3 are provided in Fig. S10
and S11 of the ESI.† The distribution patterns between the
experimental and the simulated data are in good agreement to
one another. To make sure whether any dimeric species is
present in solution or not, MS/MS study of the peak at m/z ¼
1119 has been preformed for complex 1. The MS/MS spectrum of
complex 1 is shown in Fig. S12 of the ESI.† The MS/MS spectrum
indicates that there is no any dimeric form of complex 1 present
in solution. The signals at m/z ¼ 1075, 1031, 987 and 943 in the
conversion of “NiII2(H2O)2–NiII2(H2O)2” to “NiII2(H2O)2–NiII
-
2
(H2O)(OH) species (pKa1) and “NiII2(H2O)2–NiII2(H2O)(OH) to
“NiII2(H2O)(OH)–NiII2(H2O)(OH) species (pKa2) (Scheme 4).
Therefore, in essence, we have proposed the formation of the
species “NiII2(H2O)(OH)–NiII2(H2O)(OH)” as the active species
during phosphoester hydrolysis.
Phosphoester hydrolysis
Fig. 5 Potentiometric titration curve obtained by titrating complex 1
with 0.01 M NaOH. The experimental points (black squares) are in
good agreement with the theoretical curve (red line).
In order to evaluate the phosphatase-like activity47–50 of
complexes 1, 2 and 3, the hydrolysis of bis(p-nitrophenyl)
Fig. 6 Species distribution curves of complex 1 as a function of pH.
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Scheme 4 Conversion of the tetranuclear species of complexes 1, 2 and 3 in MeOH–H2O (1 : 1; v/v) solution.
phosphate (BNPP) was examined in methanol–water (1 : 1; v/v) keeping the pseudo-rst-order reaction conditions (excess
solution at pH ꢁ 10.5 and 11.8. Each of these complexes are complex concentration), the following second-order rate
suitable candidates for the catalytic hydrolysis, as it is observed constants (k2) were calculated from the slope of the straight
to possess loosely bound water molecules where suitable lines of kobs versus complex concentration plots (Fig. 9): k2 ¼
substrates may bind. The hydrolysis experiments have been 0.11 and 0.14 Mꢀ1 sꢀ1 for 1, k2 ¼ 0.19 and 0.26 Mꢀ1 sꢀ1 for 2,
carried out spectrophotometrically by monitoring the increase and k2 ¼ 0.05 and 0.09 Mꢀ1 sꢀ1 for 3, at pH ꢁ 10.5 and 11.8,
in absorption band of the liberated p-nitrophenolate anion at respectively.
400 nm under the pseudo-rst-order reaction conditions.
The dependence of rate on the substrate concentration and
In the rst set of experiments, the pH dependence of cata- various kinetic parameters were evaluated by treating the
lytic activities of the complexes was investigated in the pH range solution of each complex with different concentrations of the
of 8.0–12.5 at 30 ꢂC. As shown in Fig. 7, the rate versus pH plots substrate under aerobic conditions at pH ꢁ 10.5 and 11.8. For
for all the three complexes have sigmoidal shapes, character- all the complexes, a rst-order dependence on the substrate
istic of a kinetic process controlled by an acid–base equilibrium. concentration was observed at lower concentration of substrate
The experimental data reveal that the rate is highly inuenced and the saturation kinetics were observed at higher concentra-
by the pH of solution, the reaction rate increases with increase tion of substrate. Applying the Michaelis–Menten approach of
in pH, and nally gets saturated above pH ꢁ 11.8.
The kinetics data of hydrolysis of BNPP were obtained using 1/Vmax, we have obtained the Lineweaver–Burk (double recip-
initial rate method by monitoring the increase of p-nitro- rocal) plots and the values of various kinetic parameters kcat
enzyme kinetics and using the equation 1/V ¼ (Km/Vmax)(1/[S]) +
,
phenolate band at 400 nm as a function of time, keeping the Km, Vmax, kcat/Km and kcat/kuncat (Table 1) at pH ꢁ 10.5 and 11.8.
substrate concentration at least 10 times larger than that of the The rate versus substrate concentration plots and the Line-
catalyst to maintain the pseudo-rst-order reaction conditions. weaver–Burk plots for the complexes are shown in Fig. 10–12
The dependence of the initial rate on complex and substrate and S16–S18.† The turnover rates (kcat) for complexes 1, 2 and 3
concentrations was examined in order to elucidate the reac- are 1.68 ꢃ 10ꢀ5, 2.26 ꢃ 10ꢀ5 and 1.18 ꢃ 10ꢀ5 sꢀ1 at pH ꢁ 10.5,
tivity. The kinetics data reveal that for all the three complexes and 7.51 ꢃ 10ꢀ5, 14.42 ꢃ 10ꢀ5 and 2.68 ꢃ 10ꢀ5 sꢀ1 at pH ꢁ 11.8,
the rate of hydrolysis is linearly dependent on the concentration
of complexes at both the pH ꢁ 10.5 and 11.8 (Fig. 8). Again,
Fig. 7 Dependence of the rate on pH for the hydrolysis of BNPP in the Fig. 8 Dependence of the rate on the concentration of complexes 1
absence of complex ( ) and in the presence of complexes 1 ( ), 2 ( ) ( ), 2 ( ) and 3 ( ) for the hydrolysis of BNPP. [BNPP] ¼ 25 ꢃ 10ꢀ3 M;
and 3 ( ). [Complex] ¼ 25 ꢃ 10ꢀ5 M; [BNPP] ¼ 25 ꢃ 10ꢀ3 M; [buffer] ¼ [buffer] ¼ 20 ꢃ 10ꢀ3 M; pH ꢁ 10.5; I ¼ 0.1 M (NaClO4) in MeOH–H2O
20 ꢃ 10ꢀ3 M; I ¼ 0.1 M (NaClO4) in MeOH–H2O (1 : 1; v/v) at 30 ꢂC.
(1 : 1; v/v) at 30 ꢂC.
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Fig. 10 Dependence of the rate on the concentration of substrate
(BNPP) for complex 1. Inset shows Lineweaver–Burk plot. [Complex] ¼
25 ꢃ 10ꢀ5 M; [buffer] ¼ 20 ꢃ 10ꢀ3 M; pH ꢁ 10.5; I ¼ 0.1 M (NaClO4) in
MeOH–H2O (1 : 1; v/v) at 30 ꢂC.
Fig. 9 Dependence of the observed rate constant (kobs) on the
concentration of complexes 1 ( ), 2 ( ) and 3 ( ) for the hydrolysis of
BNPP. [BNPP] ¼ 5 ꢃ 10ꢀ5 M; [complex] ¼ (25–125) ꢃ 10ꢀ5 M; [buffer]
¼ 20 ꢃ 10ꢀ3 M; pH ꢁ 10.5; I ¼ 0.1 M (NaClO4) in MeOH–H2O (1 : 1; v/v)
at 30 ꢂC.
reactive than complex 1, followed by the complex 3 (Table 1). As
expected, for any complex to act as an efficient catalyst for
hydrolysis of the substrate, the incoming substrate should rst
get bound successfully to the catalyst, which is generally
directed by a combination of geometrical and electronic factors
of the metal–ligand complex. This occurrence should be fol-
lowed by the attack of the metal-coordinated hydroxide to the
substrate. In our present study, the different ability of
complexes 1, 2 and 3 to hydrolyze BNPP could be most possibly
due to the difference in effective concentrations of the active
tetranickel(II) species [Ni4(H2chdp)2(H2O)2(OH)2] in solution in
the pH range of 10.5–11.8, as revealed by the species distribu-
tion curves. Therefore, the higher catalytic efficiency of 2
compared to 1 and 3 toward BNPP hydrolysis could be due to
the higher effective concentration of the active species, in
solution.
respectively. From these kcat values, it can be believed that the
catalytic activities of the complexes are signicantly higher at
pH ꢁ 11.8 than that at pH ꢁ 10.5. The potentiometric titration
results and the corresponding species distribution curves reveal
that the species which is mostly present and stable at pH ꢁ 10.5
and 11.8, in solution, is [Ni4(H2chdp)2(H2O)2(OH)2]. The higher
catalytic activities at pH ꢁ 11.8 compared to that at pH ꢁ 10.5,
could be due to the two reasons: (i) the effective concentration
of the active tetranickel(II) species [Ni4(H2chdp)2(H2O)2(OH)2] at
pH ꢁ 11.8, in solution, is higher than that at pH ꢁ 10.5, as
revealed by the species distribution curves, and (ii) the forma-
tion of strong hydrogen bond with the complexes exhibiting
higher affinities toward the BNPP substrate is more facilitated
at pH ꢁ 11.8 compared to that at pH ꢁ 10.5.
Furthermore, these kcat values also indicate that the catalytic
activities of complexes 1, 2 and 3 are comparable to some re-
ported dimetallic nickel(II),17,40,51 zinc(II)52,53 and iron(III)–zin-
c(II)54 model complexes, but are quite lower than those reported
by others.55,56 It is also important to note that complex 2 is more
Table 1 Kinetic parameters of complexes 1, 2 and 3 for the hydrolysis
of BNPP
Kinetic parameter pH
1
2
3
kcat (sꢀ1
Km (M)
)
10.5 1.68 ꢃ 10ꢀ5
10.5 2.66 ꢃ 10ꢀ3
10.5 4.20 ꢃ 10ꢀ9
10.5 6.32 ꢃ 10ꢀ3
10.5 1.79 ꢃ 102
11.8 7.51 ꢃ 10ꢀ5
11.8 2.40 ꢃ 10ꢀ3
2.26 ꢃ 10ꢀ5
2.99 ꢃ 10ꢀ3
5.60 ꢃ 10ꢀ9
7.55 ꢃ 10ꢀ3
2.41 ꢃ 102
1.18 ꢃ 10ꢀ5
1.98 ꢃ 10ꢀ3
2.94 ꢃ 10ꢀ9
5.96 ꢃ 10ꢀ3
1.26 ꢃ 102
Vmax (M sꢀ1
)
kcat/Km (Mꢀ1 sꢀ1
kcat/kuncat
)
)
kcat (sꢀ1
Km (M)
)
14.42 ꢃ 10ꢀ5 2.68 ꢃ 10ꢀ5
18.09 ꢃ 10ꢀ3 27.93 ꢃ 10ꢀ3
Vmax (M sꢀ1
)
11.8 18.78 ꢃ 10ꢀ9 36.05 ꢃ 10ꢀ9 6.70 ꢃ 10ꢀ9
Fig. 11 Dependence of the rate on the concentration of substrate
(BNPP) for complex 2. Inset shows Lineweaver–Burk plot. [Complex] ¼
25 ꢃ 10ꢀ5 M; [buffer] ¼ 20 ꢃ 10ꢀ3 M; pH ꢁ 10.5; I ¼ 0.1 M (NaClO4) in
MeOH–H2O (1 : 1; v/v) at 30 ꢂC.
kcat/Km (Mꢀ1 sꢀ1
kcat/kuncat
11.8 31.2 ꢃ 10ꢀ3
11.8 3.38 ꢃ 102
7.97 ꢃ 10ꢀ3
6.49 ꢃ 102
0.95 ꢃ 10ꢀ3
1.20 ꢃ 102
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The hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) in
methanol–water (1 : 1; v/v) solution under the similar experi-
mental conditions but in the absence of any catalyst has been
executed. Therefore, an acceleration (kcat/kuncat) of BNPP
hydrolysis has been calculated. All the three complexes are
capable of cleaving the phosphoester bond in BNPP with an
average acceleration of hydrolysis up to 1.82 ꢃ 102 fold at pH ꢁ
10.5 and 3.69 ꢃ 102 fold at pH ꢁ 11.8 over the background
reaction. Again, the control experiments have been performed
under similar experimental conditions to examine the hydro-
lysis of BNPP using free metal salts NiCl2$6H2O and
Ni(ClO4)2$6H2O. As expected, the hydrolysis of BNPP is not
promoted by these free metal salts in solution, because most
possibly a precipitate of Ni(OH)2 is formed at such higher pH.
In recent years, a number of investigations have been per-
formed to test the potential of dinickel(II) complexes with vacant
labile coordination site(s) for the hydrolysis of phosphate
esters.41,49,50 The proposed mechanism for the hydrolysis of
bis(p-nitrophenyl)phosphate (BNPP) by complexes 1, 2 and 3 is
given in Scheme 5, based on the results obtained from mass
spectrometric, potentiometric titration and kinetic analyses.
The potentiometric titration results indicate that the possible
Fig. 12 Dependence of the rate on the concentration of substrate
(BNPP) for complex 3. Inset shows Lineweaver–Burk plot. [Complex] ¼
25 ꢃ 10ꢀ5 M; [buffer] ¼ 20 ꢃ 10ꢀ3 M; pH ꢁ 10.5; I ¼ 0.1 M (NaClO4) in
MeOH–H2O (1 : 1; v/v) at 30 ꢂC.
Scheme 5 Proposed mechanism for the hydrolysis of BNPP by the complexes. Half part of each of the complexes is shown for clarity.
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1
active species responsible for the catalytic hydrolysis is [Ni4- N, 7.44%. Found: C, 60.25%; H, 6.28%; N, 7.33%. H NMR for
(H2chdp)2(H2O)2(OH)2]. These results also show that the nick- the sodium salt of the compound (400 MHz, D2O, 25 ꢂC, d): 7.46–
el(II)-bound hydroxide plays a crucial role in this catalysis 7.32 (m, 8H), 3.92–3.80 (m, 1H), 2.63 (q, 4H), 2.55 (q, 4H).
reaction within the specied experimental conditions. In the
A solution of 2-iodoethanol (2.145 g, 12.50 mmol) in 10 ml
proposed mechanism, it has been suggested that during the water was added drop wise to a solution of N,N0-bis(2-
catalytic hydrolysis the substrate preferably binds as nucleo- carboxybenzomethyl)-1,3-diaminopropan-2-ol, H3cdp (1.790 g,
phile with one nickel(II) center of an intra-dinuclear [NiII2] unit 5.00 mmol) and NaOH (0.400 g, 10.00 mmol) in 25 ml water. The
in the active species of the complexes. Then, the nickel(II)- reaction mixture was reuxed for ꢁ4 h. While reuxing, more
bound hydroxide of the same intra-dinuclear [NiII2] unit NaOH (0.400 g, 10.00 mmol) was added in portions to maintain
undergoes a nucleophilic attack to the phosphorous atom of the the pH ꢁ 11. The resulting solution was cooled and acidied
substrate resulting in the formation of free phosphate and p- with conc. HCl to pH ꢁ 5. The solution was then evaporated to
nitrophenolate ions in solution. Similar binding-cum-hydrolytic dryness under vacuum to isolate an off-white solid product
events of bis(p-nitrophenyl)phosphate with dinuclear nickel(II) which was washed with acetone. The product was then extracted
and zinc(II) complexes have been reported in the literature.51,57 with dry methanol to discard the undesired salts and the
methanol extract was evaporated to obtain an oily product. The
oily product was triturated with 60–120 mesh silica gel and was
Conclusion
subjected to column chromatography using CHCl3–MeOH
In conclusion, the present study unveils the results on the (10 : 1; v/v) aer washing the column by glacial acetic acid for 3–
synthesis, structure, spectroscopic characterization and evalu- 4 times. The product was re-crystallized as an off-white solid
ation of catalytic phosphoester hydrolysis of three new tetra- from MeOH–H2O (1 : 1; v/v) solution to make it free from acetic
nuclear [NiII
] complexes. The metallic cores of complexes 1 acid. The product was conrmed by the elemental analysis,
2 2
and 3 consist of four distorted octahedral nickel(II) ions with FTIR, 1H and 13C NMR, and mass spectrometry. Anal. calcd for
ꢀ
ꢀ
intra-ligand Ni/Ni separation of 3.527(7) A and 3.507(1) A, C23H30N2O7: C, 61.87%; H, 6.77%; N, 6.27%. Found: C, 61.75%;
respectively. The complexes show a rare m3:h2:h1:h1 bridging H, 6.86%; N, 6.14%. FTIR (cmꢀ1): n ¼ 3452(b), 1651(s), 1590(s),
mode of two benzoate groups of H2chdp3ꢀ ligands with each 1562(s), 1475(s), 1457(s), 1399(s), 1307(s), 1214(s), 1154(s),
1
bridging among three nickel(II) ions. The complexes maintain 1091(s), 868(s), 740(s). H NMR (400 MHz, D2O, room tempera-
ꢀ
the average intra-ligand nickel–nickel separation of 3.517 A, ture, d): 7.47–7.50 (m, 2H), 7.34–7.41 (m, 6H), 3.98 (d, 2H), 3.79–
which is the optimum cooperativity between two nickel centers 3.86 (m, 3H), 3.59–3.65 (m, 4H), 2.72–2.80 (m, 4H), 2.64 (d, 2H),
for mimicking the structural and functional models to the 2.47–2.52 (m, 2H). 13C NMR (100 MHz, D2O, room temperature,
active site of phosphoester hydrolases. In a simulated metal– d): 177.89, 139.61, 133.00, 131.18, 129.06, 128.12, 127.57, 64.86,
metal distance and NO5 coordination environment, all the three 57.93, 57.64, 57.57, 55.68. Mass spectrum (ESI): m/z 507 (M+ ¼
complexes exhibit moderate catalytic activity toward the phos- {H5chdp$3H2O + Li}+).
phoester hydrolysis. The catalytic efficiency of all the complexes
is comparable toward the hydrolysis of bis(p-nitrophenyl)
phosphate. The present investigation will positively provide the
valuable insights and directions for the future design of
hydrolytically active polynuclear transition metal complexes.
Synthesis of [Ni4(H2chdp)2(H2O)4]Br2$4CH3OH$3H2O (1)
A methanol (10 ml) solution of NiCl2$6H2O (0.266 g, 1.12 mmol)
was added to a solution of H5chdp (0.250 g, 0.56 mmol) and NaOH
(0.067 g, 1.68 mmol) in methanol (10 ml) with magnetic stirring
during a period of 10 min. The reaction mixture was then stirred
for 2 h resulting in a light green solution. The solution was ltered
to discard any insoluble precipitate. Then, 2 ml aqueous solution
of NaBr (0.058 g, 0.56 mmol) was added drop wise and the reac-
tion mixture was stirred for another 1 h. The solution was ltered
again to discard any insoluble precipitate. The X-ray quality green
plate shaped single crystals were obtained by slow ether diffusion
into the clear ltrate aer ꢁ15 days. Yield: 0.298 g (75%). Anal.
calcd for C47H69N4O20.5Br2Ni4 (1–3CH3OH$1.5H2O): C, 39.96; H,
4.92; N, 3.97; Ni, 16.62; found: C, 40.74; H, 4.98; N, 4.02; Ni, 16.49.
FTIR (KBr, cmꢀ1): n ¼ 3412(b), 1631(s), 1586(s), 1559(s), 1454(s),
1386(s), 1365(s), 1155(s), 1121(s), 1045(s), 995(s), 963(s), 762(s),
672(s). UV-vis (MeOH–H2O): lmax (3, Mꢀ1 cmꢀ1) ¼ 683 (138), 276
(1391), 220 (6971). Mass spectrum (ESI): m/z 1119 (M+ ¼ {[Ni4
(H2chdp)(Hchdp)}+), 560 (M+ ¼ {[Ni4(H2chdp)(Hchdp)}2+).
Experimental section
Synthesis of N,N0-bis[2-carboxybenzomethyl]-N,N0-bis
[hydroxyethyl]-1,3-diaminopropan-2-ol, H5chdp
A solution of 2-carboxybenzaldehyde (4.643 g, 30.00 mmol) and
NaOH (1.200 g, 30.00 mmol) in 100 ml methanol was added to
1,3-diamino-propan-2-ol (1.424 g, 15.00 mmol) in 20 ml meth-
anol. The yellowish solution obtained was heated to 60 ꢂC while
stirring for ꢁ4 h. Then, the solution was cooled in an ice-bath.
Excess NaBH4 (1.500 g, 39.50 mmol) was added in portions to
the cold solution while stirring. The yellow color was slightly
discharged. Aer 30 min, 2 ml conc. HCl was added drop wise to
destroy the excess NaBH4. Acidication of the solution to pH ꢁ 5
by the addition of more conc. HCl resulted in the precipitation of
a white crystalline solid. The white solid was ltered, washed
with H2O and methanol, and dried at ꢁ80 ꢂC. Yield: 4.95 g (87%).
The compound was recrystallized from warm MeOH–H2O (1 : 1;
Synthesis of [Ni4(H2chdp)2(H2O)4](PF6)2 (2)
v/v) solution and conrmed by the elemental analysis as H3- This compound was prepared as microcrystalline powder
cdp$H2O. Anal. calcd for C19H22N2O5$H2O: C, 60.63%; H, 6.43%; following the above procedure using NiCl2$6H2O, NaOH and
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NH4PF6. Yield: 0.307 g (71%). Anal. calcd for C48H70N4O20P2- strength to 0.1 M of NaClO4. To balance the expected methanol–
F12Ni4 (2$2CH3OH): C, 37.25; H, 4.56; N, 3.62; Ni, 15.17; found: water liquid junction potential, a correction of pH value of
C, 37.04; H, 4.39; N, 3.56; Ni, 15.55. FTIR (KBr, cmꢀ1): n ¼ ꢁ0.051 units were subtracted from the measured pH readings.58
3392(b), 1629(s), 1593(s), 1548(s), 1465(s), 1404(s), 1340(s), Computations were performed with the HYPERQUAD 2000
1212(s), 1159(s), 1105(s), 854(s), 845(b), 762(s), 590(s), 565(s). program and species distribution curves were obtained using
UV-vis (MeOH–H2O): lmax (3, Mꢀ1 cmꢀ1) ¼ 685 (146), 277 (1430), the program HySS.59 To determine the pKa values of the coor-
223 (7530). Mass spectrum (ESI): m/z 1119 (M+ ¼ {[Ni4(H2- dinated water molecules in complexes 1, 2 and 3, a typical pH-
chdp)(Hchdp)}+), 560 (M+ ¼ {[Ni4(H2chdp)(Hchdp)}2+).
metric titration was executed as follows: 1 mM MeOH–H2O
(1 : 1; v/v) solutions of the complexes were titrated separately
with 0.01 M NaOH solution. The ionic strength of the solution
was maintained at I ¼ 0.1 M of NaClO4.
Synthesis of [Ni4(H2chdp)2(H2O)4](ClO4)2$3.2CH3OH$0.8H2O
(3)
This compound was prepared following the above procedure
using Ni(ClO4)2$6H2O and NaOH. The X-ray quality green block
shaped single crystals were obtained by slow evaporation of the
clear ltrate aer ꢁ10 days. Yield: 0.281 g (71%). Anal. calcd for
Kinetic measurements
Phosphatase-like activities of complexes 1, 2 and 3 were deter-
mined through the hydrolysis reaction of the model substrate,
bis(p-nitrophenyl)phosphate (BNPP) under pseudo rst-order
reaction conditions. The kinetic experiments were performed
in 3 ml UV cells in MeOH–H2O (1 : 1; v/v) solution. The solution
was buffered using 20 mM CAPS buffer (pH ꢁ 10.5 and 11.8).
The ionic strength was maintained at I ¼ 0.1 M NaClO4. 1 mM
stock solution of each complex was added separately in requi-
site amount to maintain the concentration of each complex at
0.25 mM in cuvette and 50 mM BNPP stock solution was added
in requisite amounts to vary the concentrations from 5 mM to
C
47H66N4O27Cl2Ni4 (3–2.2CH3OH$0.8H2O): C, 39.62; H, 4.67; N,
3.93; Ni, 16.48; found: C, 39.78; H, 4.67; N, 3.91; Ni, 16.49. FTIR
(KBr, cmꢀ1): n ¼ 3435(b), 1630(s), 1589(s), 1555(s), 1451(s),
1401(s), 1368(s), 1089(s), 941(s), 897(s), 759(s), 636(s), 627(s).
UV-vis (MeOH–H2O): lmax (3, Mꢀ1 cmꢀ1) ¼ 658 (154), 279 nm (3,
1285 Mꢀ1 cmꢀ1) and 225 nm (3, 6529 Mꢀ1 cmꢀ1). Mass spectrum
(ESI): m/z 1119 (M+ ¼ {[Ni4(H2chdp)(Hchdp)}+), 560 (M+
¼
{[Ni4(H2chdp)(Hchdp)}2+).
Caution
Perchlorate salts of metal complexes are potentially explosive
and should be handled in small quantities with great care.
Table 2 Crystal data and structure refinement for complexes 1 and 3a
Materials
1
3
2-Carboxybenzaldehyde, 1,3-diamino-2-propanol, iodoethanol,
Empirical formula
Formula weight
Crystal system
Space group
C50H84N4O25Br2Ni4
1535.87
Triclinic
ꢁ
P1
C49.2H76.4N4O30Cl2Ni4
1509.62
Orthorhombic
Pbca
17.6138(3)
17.4519(3)
19.6556(5)
90.00
bis(p-nitrophenyl)phosphate
and
ammonium
hexa-
uorophosphate were purchased from Sigma-Aldrich Chemie
GmbH, Germany. Nickel(II)chloride hexahydrate, sodium
hydroxide and sodium bromide were purchased from Merck,
India. CAPS buffer was purchased from SRL, India. Nickel(II)
perchlorate hexahydrate was prepared by treating nickel(II)
carbonate with 1 : 1 perchloric acid and crystallized aer
concentrating on water bath. All other chemicals and solvents
were reagent grade materials and were used as received from
commercial sources without further purication.
ꢀ
a, A
10.7231(14)
11.7791(16)
13.9580(19)
109.907(2)
93.565(3)
107.606(3)
1553.0(4)
1
ꢀ
b, A
ꢀ
c, A
a, deg
b, deg
g, deg
Volume
90.00
90.00
6042.0(2)
4
Z
Density (calculated)
Wavelength
Temperature
F(000)
Absorption
coefficient
q range for
data collection
Reections collected
Independent
reections
R(F obsd data)
[I > 2s(I)]
1.642 Mg mꢀ3
1.670 Mg mꢀ3
ꢀ
ꢀ
0.71073 A
1.54184 A
273(2) K
794
293(2) K
3160
Physical measurements
Microanalyses (C, H, N) were performed using a Perkin-Elmer
2400 CHNS/O Series II elemental analyzer. FTIR spectra were
obtained on a Perkin-Elmer L120-000A spectrometer (400–4000
cmꢀ1). 1H and 13C NMR spectra were obtained in D2O solution
on a Bruker AC 400 NMR spectrometer. ESI mass spectra were
recorded using a Micromass Q-Tof Micro™ (Waters) mass
spectrometer. UV-vis spectra were recorded on a Shimadzu UV
1800 (190–1100 nm) (1 cm quartz cell) spectrophotometer.
2.563 mmꢀ1
3.037 mmꢀ1
1.96 to 24.78ꢂ
5.02 to 66.19
5331
4388
5032
4536
0.0356
0.0587
wR(F2 all data)
Goodness-of-t on F2
Largest diff.
peak and hole
P
0.0965
1.051
0.1609
1.046
+1.900 to ꢀ0.642 e A
Potentiometric measurements
ꢀꢀ3
+1.015 to
ꢀꢀ3
ꢀ0.478 e A
Potentiometric titrations of the complexes were carried out at 30
ꢂC using a Mettler Toledo Seven Compact S220 digital Ion/pH
meter in MeOH–H2O (1 : 1; v/v) solution by adjusting the ionic
P
P
P
a
2
wR2 ¼ { [w(Fo ꢀ Fc2)2]/ [w(Fo2)2]}1/2, R1 ¼ ||Fo| ꢀ |Fc||/ |Fo|.
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25 mM in cuvette. Required amount of buffer was added to crystal of complex 1 with approximate dimensions of 0.39 ꢃ
make up the volume. All the ingredients were mixed in a ther- 0.24 ꢃ 0.16 mm and a green block shaped single crystal of
ꢂ
mostatic cell (30 C), and the visual spectrum was recorded at complex 3 with approximate dimensions of 0.26 ꢃ 0.18 ꢃ 0.13
400 nm, where the molar extinction coefficient for the hydro- mm were selected for structural analysis. Intensity data for
lysis product p-nitrophenolate is 18 500 Mꢀ1 cmꢀ1
. The total complex 1 were collected using a diffractometer with Bruker
60
amount of p-nitrophenol/phenolate was determined by using SMART CCD area detector61 and graphite-monochromated Mo
the pKa (7.15).60
Ka radiation (l ¼ 0.71073 A). Similarly, intensity data for
ꢀ
complex 3 were collected using a diffractometer with Super-
Nova, Dual, Cu at zero, Eos area detector and graphite-
X-ray crystallography and data analysis
ꢀ
monochromated Cu Ka radiation (l
¼
1.54184 A). For
Crystal data and renement parameters for both the complexes
1 and 3 are summarized in Table 2. Selected bond distances and
bond angles are given in Table 3. A green plate shaped single
complex 1, a total of 5331 data were measured with Miller
indices hmin ¼ ꢀ12, hmax ¼ 12, kmin ¼ ꢀ13, kmax ¼ 13, lmin
¼
ꢀ16, lmax ¼ 16, in the range of 1.96 < q < 24.78ꢂ using u oscil-
lation frames. The data were corrected for absorption by the
multi-scan method62 giving minimum and maximum trans-
mission factors. The data were merged to form a set of 4388
independent reections with R ¼ 0.0356. The residual electron
ꢀ
Table 3 Selected bond lengths [A] and angles [deg] in complexes 1
and 3
ꢀꢀ3
(1)
(3)
density is in the range of +1.015 to ꢀ0.478 e A . For complex 3,
a total of 5032 data were recorded with Miller indices hmin
¼
ꢀ
Bond lengths [A]
Ni(1)–O(1)
Ni(1)–O(5)
Ni(1)–N(2)
Ni(1)–O(12)
Ni(1)–O(11)
Ni(1)–O(5i)
Ni(2)–O(1)
Ni(2)–O(2)
Ni(2)–O(4)
Ni(2)–N(4)
Ni(2)–O(9)
Ni(2)–O(10)
ꢀ19, hmax ¼ 20, kmin ¼ ꢀ20, kmax ¼ 14, lmin ¼ ꢀ19, lmax ¼ 23, in
the range of 5.02 < q < 66.19ꢂ using u oscillation frames. The
data were corrected for absorption by the multi-scan method62
giving minimum and maximum transmission factors. The data
were merged to form a set of 4536 independent reections with
R ¼ 0.0587. The residual electron density is in the range of
1.943(2)
2.058(2)
2.075(3)
2.105(3)
2.127(3)
2.131(2)
1.964(3)
2.032(3)
2.068(2)
2.087(3)
2.090(3)
2.147(3)
Ni(1)–O(20)
Ni(1)–O(34)
Ni(1)–N(15)
Ni(1)–O(36)
Ni(1)–O(34i)
Ni(1)–O(18)
Ni(2)–O(20)
Ni(2)–O(26)
Ni(2)–O(35i)
Ni(2)–O(23)
Ni(2)–N(11)
Ni(2)–O(37)
1.955(3)
2.069(3)
2.079(4)
2.111(3)
2.120(3)
2.133(3)
1.957(3)
2.048(3)
2.070(3)
2.089(3)
2.100(4)
2.110(3)
ꢀꢀ3
+1.900 to ꢀ0.642 e A . The structures were solved by direct
methods using the soware SIR-97,63 and rened by full-matrix
least-squares methods on F2 using the program SHELXL64
embedded in the routine Olex2.65 Hydrogen atom positions
were initially determined by geometry and rened by riding
model. Non-hydrogen atoms were rened with anisotropic
displacement parameters. All hydrogen atoms were generated
Bond angles [deg]
O(1)–Ni(1)–O(5)
O(1)–Ni(1)–N(2)
O(5)–Ni(1)–N(2)
O(1)–Ni(1)–O(12)
O(5)–Ni(1)–O(12)
N(2)–Ni(1)–O(12)
O(1)–Ni(1)–O(11)
O(5)–Ni(1)–O(11)
N(2)–Ni(1)–O(11)
O(12)–Ni(1)–O(11)
O(1)–Ni(1)–O(5i)
O(5)–Ni(1)–O(5i)
N(2)–Ni(1)–O(5i)
O(12)–Ni(1)–O(5i)
O(11)–Ni(1)–O(5i)
O(1)–Ni(2)–O(2)
O(1)–Ni(2)–O(4)
O(2)–Ni(2)–O(4)
O(1)–Ni(2)–N(4)
O(2)–Ni(2)–N(4)
O(4)–Ni(2)–N(4)
O(1)–Ni(2)–O(9)
O(2)–Ni(2)–O(9)
O(4)–Ni(2)–O(9)
N(4)–Ni(2)–O(9)
O(1)–Ni(2)–O(10)
O(2)–Ni(2)–O(10)
O(4)–Ni(2)–O(10)
N(4)–Ni(2)–O(10)
O(9)–Ni(2)–O(10)
177.55(11)
82.12(11)
97.82(11)
87.82(11)
89.83(10)
105.40(12)
93.83(11)
88.61(10)
84.55(12)
170.05(10)
96.22(10)
84.29(10)
169.38(12)
84.96(10)
85.11(10)
89.28(11)
90.16(10)
93.63(10)
84.61(12)
93.53(12)
171.08(12)
172.28(11)
90.89(11)
82.12(10)
103.08(12)
92.23(11)
172.55(11)
93.66(11)
79.35(12)
88.60(11)
O(20)–Ni(1)–O(34)
O(20)–Ni(1)–N(15)
O(34)–Ni(1)–N(15)
O(20)–Ni(1)–O(36)
O(34)–Ni(1)–O(36)
N(15)–Ni(1)–O(36)
O(20)–Ni(1)–O(34i)
O(34)–Ni(1)–O(34i)
N(15)–Ni(1)–O(34i)
O(36)–Ni(1)–O(34i)
O(20)–Ni(1)–O(18)
O(18)–Ni(1)–O(34)
N(15)–Ni(1)–O(18)
O(18)–Ni(1)–O(36)
O(18)–Ni(1)–O(34i)
O(20)–Ni(2)–O(26)
O(20)–Ni(2)–O(35i)
O(26)–Ni(2)–O(35i)
O(20)–Ni(2)–O(23)
O(26)–Ni(2)–O(23)
O(35i)–Ni(2)–O(23)
O(20)–Ni(2)–N(11)
O(26)–Ni(2)–N(11)
O(35i)–Ni(2)–N(11)
O(23)–Ni(2)–N(11)
O(20)–Ni(2)–O(37)
O(26)–Ni(2)–O(37)
O(35i)–Ni(2)–O(37)
O(23)–Ni(2)–O(37)
N(11)–Ni(2)–O(37)
178.74(12)
81.88(13)
96.90(13)
90.74(12)
89.95(11)
107.12(13)
97.26(12)
83.85(12)
168.25(13)
84.59(12)
92.51(12)
87.01(11)
83.99(13)
168.77(12)
84.34(11)
90.24(13)
89.57(12)
88.93(12)
91.93(13)
175.87(13)
94.59(13)
84.73(13)
94.18(13)
173.51(13)
82.53(14)
173.07(13)
95.00(13)
86.01(13)
83.13(13)
99.37(14)
ꢀ
at ideal positions (C–H, 0.96 A) and xed with isotropic thermal
parameters.
Acknowledgements
The authors would like to acknowledge the Council of Scientic
& Industrial Research (CSIR) (Grant No. 01(2732)/13/EMR-II),
New Delhi for funding and nancial support. The authors
would also like to acknowledge the DST-FIST and UGC-SAP
program for providing the instrumental facilities in the
Department of Chemistry, University of Kalyani. A. P. thanks the
University of Kalyani for providing URS fellowship. Finally, the
authors would like to thank the Reviewers of this article for their
fruitful comments and valuable suggestion to improve its
quality.
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