Effect of structural variation on enzymatic activity in tetranuclear (Cu4) clusters with defective cubane core

Article abstract of DOI:10.1080/07391102.2021.1924263

The stimulus to the modeling of enzyme functioning sites comes from their potential to give insight into the natural enzyme’s mechanistic pathways, ascertain the role of that different metal ion in the active site and construct better catalysts motivated by nature. The presence of metal ion leads to the activation of molecular oxygen in the metalloenzymes. The metalloenzymes such as the catechol oxidase (CO) enzyme that oxidizes the catechol to corresponding quinones which eventually protect damage tissues from plant and pathogen. Thus, the design and characterization of catalysts used as selectively and efficiently oxidation reactions have grown to be unique challenges for modern inorganic chemists. In this work, two novel tetranuclear complexes (1 and 2) have been synthesized in excellent yield. The complexes were characterized using various spectroscopic techniques such as FTIR, UV–Visible and PXRD pattern. The structure of 1 and 2 was elucidated by SC-XRD (single crystal X-ray diffraction) analysis. The magnetic study reveals the presence of the antiferromagnetic nature of 1 and 2. Both 1 and 2 shows a very good catecholase-like activity by oxidizing the catechol to analogous quinone in methanolic solution. Thus, a structure-activity relationship can further help us design other substituted tetranuclear complexes with enhanced catecholase like activity. Communicated by Ramaswamy H. Sarma.

Full text of DOI:10.1080/07391102.2021.1924263

Journal of Biomolecular Structure and Dynamics
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tbsd20
Effect of structural variation on enzymatic activity
in tetranuclear (Cu ) clusters with defective
4
cubane core
M. Shahnawaz Khan, Mohd Khalid, M. Shahwaz Ahmad, Samrah Kamal, M.
Shahid & Musheer Ahmad
To cite this article: M. Shahnawaz Khan, Mohd Khalid, M. Shahwaz Ahmad, Samrah Kamal, M.
Shahid & Musheer Ahmad (2021): Effect of structural variation on enzymatic activity in tetranuclear
(
Cu ) clusters with defective cubane core, Journal of Biomolecular Structure and Dynamics, DOI:
4
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JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
https://doi.org/10.1080/07391102.2021.1924263
Effect of structural variation on enzymatic activity in tetranuclear (Cu ) clusters
4
with defective cubane core
a
a
a
a
a
M. Shahnawaz Khan
Ahmad
, Mohd Khalid , M. Shahwaz Ahmad , Samrah Kamal , M. Shahid
and Musheer
b
a
b
Functional Inorganic Materials Lab (FIML), Department of Chemistry, Aligarh Muslim University, Aligarh, India; Department of Applied
Chemistry (ZHCET), Aligarh Muslim University, Aligarh, India
Communicated by Ramaswamy H. Sarma
ABSTRACT
ARTICLE HISTORY
The stimulus to the modeling of enzyme functioning sites comes from their potential to give insight
into the natural enzyme’s mechanistic pathways, ascertain the role of that different metal ion in the
active site and construct better catalysts motivated by nature. The presence of metal ion leads to the
activation of molecular oxygen in the metalloenzymes. The metalloenzymes such as the catechol oxi-
dase (CO) enzyme that oxidizes the catechol to corresponding quinones which eventually protect
damage tissues from plant and pathogen. Thus, the design and characterization of catalysts used as
selectively and efficiently oxidation reactions have grown to be unique challenges for modern inor-
ganic chemists. In this work, two novel tetranuclear complexes (1 and 2) have been synthesized in
excellent yield. The complexes were characterized using various spectroscopic techniques such as FTIR,
UV–Visible and PXRD pattern. The structure of 1 and 2 was elucidated by SC-XRD (single crystal X-ray
diffraction) analysis. The magnetic study reveals the presence of the antiferromagnetic nature of 1 and
Received 14 October 2020
Accepted 24 April 2021
KEYWORDS
Tetranuclear complexes;
copper cluster; defective
cubane; magnetism;
catecholase-like activity
2. Both 1 and 2 shows a very good catecholase-like activity by oxidizing the catechol to analogous
quinone in methanolic solution. Thus, a structure-activity relationship can further help us design other
substituted tetranuclear complexes with enhanced catecholase like activity.
1
. Introduction
Sessoli, Tsai, et al. 1993), this proposal has become very
admirable as it declares very skilled in constructing the large
metal ion assemblies.
The coordination chemistry of the multinuclear complexes is
nowadays most touted attractions owing to their potential
topological specialties biological importance and material
applications. Metal cluster plays a prominent role in catalysis
Furthermore, the oxidation process plays an influential
role in the relevant synthesis of organic compounds viz.
pharmaceuticals, agrochemicals, and some extra superior
chemicals (Fontecave & Pierre, 1998; Que & Tolman, 2008). In
the present scenario, consistent demands from society neces-
sitate the progression of sustainable and atmosphere benign
processes in manufacturing synthesis through the conven-
tional oxidants method replacement, which is lethal and pro-
ducing environmental pollution. Nowadays, there is an
accelerating need for the discriminatory oxidation of natural
compounds utilizing the dioxygen as a primary oxidant for
manufacturing appliances because of its profitable and envir-
onmental reward. Though, the organic oxidation reaction by
molecular oxygen is callous because of the inertness of
molecular oxygen, which is associated with the triplet
(
Khan et al., 2019; 2020; Wegner et al., 2001), bioinorganic
chemistry (Ashafaq et al., 2020; Holm et al., 1996; Iman et al.,
019; Mariyam et al., 2020), and molecular magnetism
Ahamad et al., 2020; Khan et al., 2019; 2020). Amongst poly-
2
(
nuclear cages and clusters, tetranuclear complexes of copper
with cubane cores drag special attention due to their specific
magnetism and catalytic properties (Papadakis et al., 2013).
The tetranuclear complex also embodies a prototype system
for metalloenzymes (Koval et al., 2006). In the last several
years, the magnetic property of Cu O -type of complex with
phenoxo, alkoxo, and hydroxo bridges has been investigated
by various research groups by theoretical and experimental
4
4
approaches (Scheme 1) (Fujita et al., 2001; Gungor et al., ground state of molecular oxygen; and this high-energy
014; Mergehenn & Haase, 1977). After the development of block is the way of nature to shield the organic compounds
first-ever single molecular magnets [Mn O (O CR) (H O) ] which are characteristically in the singlet ground state from
2
1
2
12
2
16
2
4
(
R ¼ Ph, Me), which were synthesized unexpectedly by the unfavorable oxidation (Khan et al., 2019). Life has produced
reaction between Mn(III) and permanganate (Efthymiou et an artistic way to examine oxidations beneath normal cir-
al., 2011; Kitos et al., 2011; Sessoli, Gatteschi, et al., 1993; cumstances in which metalloenzymes do activation of
CONTACT Mohd Khalid
02002, India
Supplemental data for this article can be accessed online at https://doi.org/10.1080/07391102.2021.1924263.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group
khalid215@gmail.com
Functional Inorganic Materials Lab (FIML), Department of Chemistry, Aligarh Muslim University, Aligarh,
2

2
M. S. KHAN ET AL.
molecular oxygen (Banu et al., 2009; Punniyamurthy et al., 2.2. Single-crystal X-ray analysis
005). In recent times, many efforts have been employed
2
Single-crystal XRD analysis of 1 and 2 was done at 100 K
employing a Bruker SMART APEX CCD diffractometer. The
data were monitored using graphite monochromated Mo-Ka
radiation with the k ¼ 0.71073 Å. The data reduction with
integration has been made using the software named SAINT.
The Empirical absorption adjustment was made with
SADABS, and the determination of space group was per-
formed with XPREP. DIFX commands fixed bond length
parameters. The structures solution was done employing
from a bioinorganic chemist to treat an artificial analog
approach on enzyme active-site (Castro et al., 2015;
Dasgupta et al., 2017). From direct enzymatic reactions, this
kind of information and insight is not available. Furthermore,
extremely skilled biomimetic models may afford efficient bio-
mimetic catalysts for the dilemma of conventional lethal
inorganic catalysts for the industrialized process. Here it
should be noted that the great catalytic action of metalloen-
zymes is chiefly correlating with the essential responsibility
of protein chains that support substrate recognition and sta-
bilization of the intermediate through various non-covalent
forces (Dasgupta et al., 2017). Hence, the evolution of artifi-
cial analogs exceeding the first coordination sphere could be
the right way to promote efficient biomimetic catalysts.
Considering all the above things in our mind in the present
work, we have synthesized two novel tetranuclear complexes
using copper(II) metal salt employing amino alcohol and
carboxylate ligands. The tetranuclear complexes are synthe-
sized with the same experimental condition but a different
synthetic procedure. In both the complexes, we have pro-
vided the basic medium using the NaOH, a strong base. The
role of NaOH is crucial in forming both the complexes as it
provides the template for the formation of defective cubane
2
SHELXL-97, and refinement was performed on F with full
matrix least squares with SHELXL-97 ( Ibers and Hamilton,
1
974; Sheldrick, 2002; Bourhis et al., 2015 ). Anisotropic dis-
placement parameters were used for refining H atoms. Table
illustrates the crystal data and refinement parameters for 1
1
and 2. The CCDC number for 1 and 2 are 2036610
and 2036611.
2
.3. Catecholase-like activity
Tetranuclear copper complexes were further checked to their
biomimetic activity, such as the catecholase-like activity. In a
methanolic solution standard substrate such as 3,5-di-tert-
butyl catechol (3,5-DTBC) has been used to monitored the
process of 3,5-DTBC oxidation to 3,5-di-tert-butylquinone (3,5-
like structure. 1 was synthesized at pH ¼ 8 while the 2 was DTBQ). For monitoring the oxidation of 3,5-DTBC to 3,5-
isolated at little higher pH of 10. The synthesized complexes DTBQ, a Perkin-Elmer k-45 absorption spectrophotometer
were characterized using various spectroscopic techniques was used. At 401 nm, the appearance of 3,5-DTBQ was
and with the help of single crystal X-ray technique the exact observed after particular time intervals. Furthermore, the kin-
structure of 1 and 2 was proposed. The design defective etics of catecholase like activity was also performed. In an
ꢀ
5
cubane established better catecholase-like activity and can experimental setup at a constant concentration (ꢁ1.0 ꢂ 10
be employed for magnetic materials in future endeavors.
M) of complexes, and varying the concentration of 3,5-DTBC
was taken in cuvettes and monitored it on UV-Visible spec-
trophotometer with different time intervals (Scheme 2). The
curve between slopes of the absorbance versus time plot
determined the underlying rate for catalyst-substrate com-
bination. Michaelis–Menten method was used to understand
the kinetic analyses, and Lineweaver–Burk plots were used to
obtain all the critical kinetic parameters (Khan et al., 2020).
2
. Experimental protocols
2.1. General materials and measurements
All the chemicals used in this work are commercially avail-
able and used without any further purification. The copper
nitrate trihydrate, 2-pyridine methanol, sodium hydroxide,
benzoic acid, and 2-methoxy benzoic acid are reagent grade
of Sigma-Aldrich and used as received. The whole character-
ization experiments were conducted at room temperature
and pressure. The GX automatic recording spectrometer
Perkin-Elmer spectrum was employed to understand the FTIR
2
.4. Synthetic protocols
The tetranuclear complexes 1 and 2 were synthesized using
similar reaction conditions except for the pH of reactions. In
case of 1, the pH of the reaction was kept at 8 and in case
of 2 the pH of the reaction was kept at 10 using aqueous
NaOH (Scheme 3).
ꢀ
1
spectrophotometer as KBr disks from 4000 to 400 cm
.
"Miniflexll X-ray diffractometer" having Cu-Ka radiation was
used to determine the PXRD patterns of complexes. The
Perkin-Elmer Lambda-45 UV–visible spectrophotometer was
used to check the electronic transition in 1 and 2 between
1
mmol of CuSO .5H O was poured in a round bottom
4 2
flask with 15 mL of distilled water and 2-pyridine methanol
2 mmol). The whole reaction was stirred for almost an hour
before adding the aqueous solution of 2 mmol benzoic acids
(
8
00–200 nm ranges at room temperature. The cuvettes use
in the experiment was 1 cm path length. For the magnetic (1) or 2-methoxy benzoic acid (2). The pH of the reaction
study, the temperature variable has been performed on an was kept little basic to provide the template for the forma-
MPMS-XL quantum design SQUID magnetometer. The mag- tion of tetranuclear complexes. Using the aqueous NaOH in
netic study was taken under an applied field of 0.1 T in the 1 the reaction pH was kept at 8 and in 2 the pH of the reac-
temperature range of 2–300 K. Further, the pascal constants tion was kept at 10 and the whole mixture was stirred again
were used for diamagnetic corrections.
for 3 h. After the reaction was completed, the solution was

JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
3
Table 1. Crystallographic parameters for 1 and 2.
Identification code
1
2
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C
52
H
46Cu
4
N
4
O
13
48 4 4 13
C H43Cu N O
1189.15
296(2)
1138.08
100(2)
monoclinic
P2 /c
monoclinic
-P 2ybc
1
20.819(5)
13.021(4)
19.561(5)
90
12.0799(8)
b/Å
24.6243(16)
19.7876(14)
90
c/Å
ꢃ
a/
ꢃ
b/
112.380(7)
90
103.959(2)
90
ꢃ
c/
3
Volume/Å
4904(2)
4
5712.2(7)
4
Z
3
q
calcg/cm
1.6106
1.782
1.3233
1.527
-
1
l/mm
F(000)
2430.5
2322.4252
3
Crystal size/mm
Radiation
0.35 ꢂ 0.27 ꢂ 0.18
0.4 ꢂ 0.29 ꢂ 0.18
Mo Ka (k ¼ 0.71073)
ꢀ16 ꢄ h ꢄ 16, ꢀ32ꢄ k ꢄ 32, ꢀ26 ꢄ l ꢄ 26
1.0436
Mo Ka (k ¼ 0.71073)
Index ranges
ꢀ20 ꢄ h ꢄ 24, ꢀ15 ꢄ k ꢄ 12, ꢀ23 ꢄ l ꢄ 22
2
Goodness-of-fit on F
1.050
Final R indexes [I> ¼2r (I)]
Final R indexes [all data]
R
R
1
¼ 0.0703, wR
¼ 0.1923, wR
2
¼ 0.1010
¼ 0.1352
R
R
1
¼ 0.1589 wR
¼ 0.1474, wR
1
¼ 0.3178
¼ 0.3069
1
2
1
2
Scheme 1. Class of cubane as described in the literature.
filtered off to remove the turbidity and kept in a beaker for alcohol ligand with varying coordination chemistry. The lig-
slow evaporations. After several weeks blue color well- and was previously used to synthesize numerous mono-, di-
shaped crystals were obtained from the beakers.
and polynuclear coordination complexes due to its unusual
binding sites (Scheme 4). The synthetic procedure involves a
simple method at ambient conditions to isolate two novel
tetranuclear compounds containing Cu(II) metal ion. 1 was
synthesized using the same environment at pH ¼ 8 while 2
was isolated by changing the pH from 8 to 10 using aqueous
NaOH. In the basic medium, the protonation of amino alco-
hol and benzoate ligand takes place very quickly. Moreover,
3
. Results and discussions
3.1. Synthetic approach
To understand the catalytic behavior of tetranuclear Cu(II)
complexes we have design two defective cubane like struc-
ture in the presence of rigid amine 2-pyridine methanol the sturdy base, such as NaOH, provides the templates in the
(
Hhmp) as primary ligand and benzoate ligand as a second- formation of tetranuclear copper complexes, which were fur-
ary ligand. In aqueous medium, sodium hydroxide (NaOH) ther, explored for its unusual magnetic and catalytic proper-
has been used as pH regulator. Hhmp is a rigid amino ties. The distinctive structural features of 1 and 2 force us to

4
M. S. KHAN ET AL.
Scheme 2. Conversion of 3,5 DTBC into 3,5 DTBQ in presence of molecular oxygen.
Scheme 3. Synthetic route for 1 and 2.
Scheme 4. Different coordination modes of Hhmp ligand.
explore the chemistry associated with the defective cubane robust, which is due to the presence of various non-covalent
structure. 1 and 2 were thoroughly characterized, employing interactions in the compounds, especially C … H–p and p–p
various spectroscopic and SC-XRD technique. With the assist- interactions. 1 and 2 were then employed to check if they
ance of single-crystal data, the molecular structure of 1 and can catalyze the organic reaction in the formation of 3,5-
2
was established. The tetranuclear complexes are quite DTBQ from 3,5-DTBC in the presence of molecular oxygen.

JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
5
Figure 1. Molecular structure diagram of 1(a) and incomplete cubane core of 1 (b).
3
.2.1. Crystal description of [Cu (hmp) (ba) (Hba) (H O)] (1)
4 4 2 2 2
The molecular structure of 1 is shown as Figure 1 (a), and
the selected bond angles and bond distances are repre-
sented in Table S1 and S2 (supplementary material) ESI. The
tetranuclear copper complexes were observed to be mono-
clinic having space group P21/c. The single-crystal X-ray data
revealed that 1 is a tetranuclear copper (II) complex with the
formula [Cu (hmp) (ba) (Hba) H O] units and some solvent
4
4
2
2 2
molecule also present in the lattice. The adjoining Cu blocks
4
disclose a single-open [Cu (l -O)(l -O) ] cubane core, as
4
2
3
3
shown in Figure 1(b). The tetranuclear complex can be
formed in various ways (Scheme 5). The tetranuclear
copper(II) unit consists of four symmetrical Cu atoms with
different coordination environments, two completely proto-
2-
nated chelating benzoic acid ligands (ba ), two partially pro-
-
tonated Hba moiety and four hmp ligand. The five-
coordinated Cu2 center is very similar to the five coordinated
Cu4 metal ion. Cu4 atom confers a distorted square-pyram-
idal with fCu O Ng environment with s ¼ 0.14 and 0.214 for
4
4
Cu2 and Cu4 metal ions. For an ideal square-pyramidal
geometry, the Addison Tau parameter (s) value is 0, and it is
close to 1 for trigonal bipyramidal geometry. In both Cu2
and Cu4 center, the value of s is close to 0, purporting the
distorted square pyramidal geometry around the Cu2 and
Cu4 metal ions (Bondi, 1964; Yang et al., 2007). Additionally,
the four bond distances between the Cu2 or Cu4 and adjoin-
ing atom are nearly the same, but one bond is comparatively
large, authenticating the square pyramidal geometry around
Cu2 and Cu4 metal ions. The coordination environment
around Cu2 ion filled by N2, O4, and O9 by amino alcohol
ligand, O6 and O7 atom of benzoate ligand and N1, O4, and
O10 by amino alcohol ligand, O1 and O2 by benzoate ligand.
This equatorial N2, O4, O6, and O9 atom fills the position
around Cu2 ion, [Cu2-N2 1.996(6) Å, Cu2-O4 1.939(5) Å, Cu2-
O6 1.916(6) Å, Cu -O9 1.974(5) Å] while the axial position is
2
occupied by O7 atom [Cu2-O7 2.300(5) Å]. Similarly, the
equatorial position around Cu4 atom is occupied by N4, O9,
O10 and O11 atom [Cu4-N4 1.980(6) Å, Cu4-O9 2.312(5) Å,
Cu4-O10 1.924(4) Å, Cu4-O11 1.941(5) Å]. On the other hand,
the Cu1 and Cu3 metal centers are present in distorted octa-
hedral fCuN O g atmosphere, filled by the N1 and N3 of
2
4
Figure 2. Formation of H-bonding (a), C-H-p bonding (b) and 3-D supra-
molecular network (c) in 1.
amino alcohol ligand, O1, O2, O4, O15 and O4, O8, O9, O10

6
M. S. KHAN ET AL.
Scheme 5. Different type of cubane finds in the tetranuclear complexes.
of benzoate ligand. The axial position in Cu1 is invaded by differences result in the formation of a slightly different open
N1, O1, O10 and O15 atom [Cu1-N1 1.995(6) Å, Cu1-O1 cubane core [Cu (l -O)(l -O) ] in 2, as shown in Figure 3b.
4
2
3
3
2
.397(5) Å, Cu1-O10 2.449(5) Å, Cu1-O15 1.960(5) Å] and the In 2, resembling distorted square-pyramidal geometry was
equatorial places are occupied by O2 and O4 atom [Cu-O2 shown by Cu1 and Cu3 centers with fCuO Ng environments
4
1
.939(5) Å, Cu1-O4 1.939(5) Å]. The angle between having s ¼ 0.12 and 0.22, respectively (Bondi, 1964; Yang et
O4–Cu1–O2, O10–Cu1–O1 and N3-Cu3-O9, O10-Cu3-O8 are al., 2007). In the case of Cu1 center, the coordination sphere
71.7(2), 165.97(17) and 166.8(2), 171.7(2) which deviates is filled by three O atoms of the three protonated amino
1
from a perfect linearity thus Cu1 and Cu3 metal centers alcohol ligands [Cu1-O4 1.925 Å; Cu1-O6 2.347 Å; Cu1-O5
ascertaining the coordination geometry as distorted octahe- 1.943 Å], one O atom from the 3Meba moieties [Cu1-O1
dral. Some of the bond distances [i.e. Cu1-O1 2.397(5) Å, 1.944 Å] and one N atom of amino alcohol ligand [Cu1-N2
Cu3-O9 2.312(5) Å] are relatively long, but these are compar- 1.963 Å] bond distances. On the other hand, the five-coordin-
able to the sum of O and Cu atoms van der Waals radii ation sphere around the Cu3 center is filled by two O atom
(
ꢁ2.92 Å) (Bondi, 1964; Yang et al., 2007). As displayed in
of amino alcohol ligand, one N of amino alcohol ligand, and
one atom of monoprotonated deprotonated 3Meba ligands
[Cu3-O6; Cu3-O9; Cu3-O11; Cu3-O7; Cu3-N3]. The coordin-
ation sphere around the Cu2 center is six coordinated dis-
Scheme 5, all the metal ions are interconnected jointly to
each other via [Cu (l -O)(l -O) ] cubane core having CuꢅꢅꢅCu
4
1
3
3
separation in the range of 2.9587(15) to 3.1007(15) Å (avg
3
.329 Å). Several Ni , Co , Zn , and Cu complexes based on
4 4 4 4
torted octahedral geometry filled with fCuO Ng. The angle
5
the 2-pyridine methanol have been documented in the lit-
erature (Lawrence et al., 2007; Yang et al., 2006; Zhang et al.,
2
molecules displayed non-covalent interactions to form 1-D
chains, but the discrete 0-D construction of 1 dispense a
neutral tetra copper(II) [Cu (hmp) (ba) (Hba) H O] unit.
Furthermore, the various non-covalent interactions (C-H p,
C -H O, and p-p) can also be seen in the complex, contribu-
ting to forming a supramolecular network (Figure 2). One
dimeric unit is connected to others through O-H O hydro-
between N1-Cu2-O6, O4-Cu2-O8, and O2-Cu2-O7 deviates
ꢃ
ꢃ
ꢃ
ꢃ
from linearity 180 to 169.67 , 173 , and 152 further con-
firming the distorted octahedral geometry around the Cu2
metal ion. The coordination sphere is filled by one N of
amino alcohol ligand, three O atom of amino alcohol ligand,
and two fully protonated O atom of 3Meba ligands [Cu2-N1
1.992(10) Å; Cu2-O7 1.920(8) Å; Cu2-O8 ¼ 1.950(8) Å; Cu2-
O4 ¼ 1.960(10) Å; Cu2-O6 ¼ 1.929(7) Å; Cu2-O2 ¼ 2.445(10) Å].
The Cu4 center is present in square planer geometry around
the central metal ion in fCu NO g with one N atoms of
013). These tetranuclear complexes having discrete cubane
4
4
2
2᭿
2
᭿᭿᭿
᭿᭿᭿
᭿
᭿᭿
4
3
gen bonds; as a result, the 1-D polymeric chain is formed in
compound 1 (Gronert & Keeffe, 2005; Mueller-Westerhoff et
al., 1981).
amino alcohol ligand and three O atom of amino alcohol lig-
and [Cu-N4 1.981(14) Å; Cu-O5 1.929(11) Å; Cu-O7 1.954(8) Å].
Several weak O-H, C–H p and p-p interactions are also
᭿᭿᭿
found in 2 (Figure 4) which contribute to crystal robustness;
the C–H p interactions are also created in the range of
᭿᭿᭿
3
.2.2. Crystal description of [Cu (hmp) (2Meba) (2MeHba)].
4 4 2
2
.801–2.810 Å between the one benzene ring of amino alco-
solvent (2)
hol ligand and other rings of benzoate ligand. The p-p inter-
action specially provides the robustness of the structure
formed between the two-benzene ring of amino alcohol lig-
and and a benzoate ligand results in a 1 D polymer unit. H-
bonding beside various non-covalent interactions is also
being seen in the molecule, which contributes to the forma-
tion of a 3-D supramolecular network.
The discrete structure of 2 is present in a neutral tetranuclear
form bearing the formula [Cu (hmp) (2Meba) (2MeHba)].
4
4
2
solvent, as depicted in Figure 3a. Although this tetranuclear
complex somehow resembles 1, there are some differences
between the two structures. Complex 2 formed due to the
contribution of four 2-pyridine methanol ligand, which binds
to each copper ion into the protonated form, two 2-methoxy
benzoic acid ligand in deprotonated form and one 2-
methoxy benzoic acid ligand into its monoprotonated form.
There are also changes in the coordination environment
3
.3. FTIR spectra of 1 and 2
around each copper metal atom. The Cu1 and Cu3 center is The Fourier Transform Infrared spectrum of designed tetranu-
five coordinated systems with square pyramidal geometry, clear complexes and have been recorded from
Figure
1
2
ꢀ
1
Cu2 is six coordinated distorted octahedron system, and Cu4 4000–400 cm
is four coordinated with square planer geometry. These few 3410 cm observed in 1 and 3390 cm in 2 were due to
.
5
shows the new band near
ꢀ
1
ꢀ1

JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
7
ꢀ1
Table 2. Rate constants (s ) for the catechol oxidation reaction of previously reported tetranuclear Cu(II) complexes against substrate
,5-DTBC.
3
ꢀ
1
S. No.
Complex
O) ]5H
Solvent
k
cat (s
)
Ref. No.
1
.
[Cu
4
(L)(mOH)
2
(H
2
4
2
O
MeCN–water
0.08
Mijangos et al. (2008)
(
pH ¼ 7.5)
2
3
.
.
[Cu
[Cu
4
(L
(L
2
)
4
(MeOH)
2
(ClO
4
)
2
]
MeCN
MeCN;
0.045
0.0218
Dasgupta et al. (2017)
Dasgupta et al (2017)
4
1
)(m-O)(OAc)
4
]
MeCN–water
Cl
MeOH
DMSO
MeOH
MeOH
4.
5.
6.
7.
8.
[Cu
[Cu
[Cu
[Cu
[Cu
4
2
4
4
4
(L
2
)(OH)
4
(Cl)
)(H
(Ac)
2
](PF
O)] (CF
2
6
)
2
CH
2
0.0006
0.0002
S ꢀe n ꢁe que et al. (2002)
Koval et al. (2006)
Das et al. (2012)
This work
(HL)(CO
(O)(L)
(hmp)
(hmp)
3
2
3
SO
3
)
4
.2CH
3 2
CN. 4H O
2
4
]
4
(ba)
2
(Hba)
2
]
0.36
0.20
4
(2Meba)
2
(2MeHba)]. solvent
This work
Figure 3. Molecular labeling diagram of 2(a), and incomplete cubane core of 2(b).
Figure 4. H-bonding (a), C-H-p bonding (b), robustness provided due to p ꢀ p bonding (c) and formation 3-D network due to various non covalent interactions (d)
in 2.
m(OH) in the complexes. Due to the amino alcohol ligand, m(C-O) (Gatehouse et al., 1958; Lever, 1984; Nakamoto, 1986).
ꢀ
1
the band is often seen in the three regions for 1 and 2, i.e. The vibration band in the region of 1610-1595 cm
is
ꢀ
1
1
) medium band in the 3400 cm region is due to the alco- assigned to m(C-C) due to the aromatic ring in 1 and 2. The
ꢀ
1
holic group. 2) The mass(C-H) and msym(C-H) vibration were two consecutive bands at 1570/1384 cm
in 1 and 1506/
ꢀ
1
ꢀ1
ꢀ1
-
also observed in the range of 2944 cm and 2904 cm ; 3) 1325 cm in 2 are the characteristics band for mass(COO )/
the three progressive bands around 1050 cm assigned to msymm(COO ) of the carboxylate group attached to benzoate
ꢀ
1
-

8
M. S. KHAN ET AL.
ligand which has functional binding modes. It can bind to authenticated by the SC-XRD of the complexes. There is no
ꢀ
1
the central metal ion in bidentate or monodentate mode band in the range of 1385 cm , justifying the absence of
Scheme S1 ESI), which depends upon the difference any anions present in 1 and 2. The weak vibrational band of
(
–
–
between Zm ¼ masym(COO )–m (COO ). So, it can be said C ¼ C shifted at lower energy regions in case 1 and 2
sym
ꢀ1
that 1 and 2 both are unsaturated carboxylates and can (1620 cm ), symbolizing the significant influence of conjuga-
adopt bridging mono- or bidentate coordination modes with tion on the Hhmp aromatic ring. The aromatic ring hydro-
ꢀ
1
Zm values observed at around 170 cm . The magnitude of Zv gens give rise to wagging vibration bands in
ꢀ
1
ꢀ1
specifies that both 1 and 2 have an asymmetric bidentate 1459–1276 cm and at 761 cm (Khan et al., 2021; Krause
mode and the anionic monodentate mode in 2, which is also et al., 1961). The C–C bending vibrations of in-plane and out-
ꢀ
1
of-plane are remarked at 650 and 416 cm . The band at
ꢀ
1
9
50 cm in 1 and 2 are characteristic of M–O–M bridging.
ꢀ
1
The bands that appeared at 590–800 cm
are related to
deformation modes of the benzoate (ba) ligand. The weak
ꢀ
1
intensity bands at 875–955 cm are endorsed to the C–C
vibrations. The strong to medium bands in the region
ꢀ
1
9
53–1255 cm may be due to the C–O stretching vibrations
ꢀ
1
ꢀ1
of O–CH . The bands at 470–500 cm (1) and 574–571 cm
2
(
(
2) represents m(M–N) and m(M–O) vibrations, respectively
Frank & Rogers, 1966; Khan et al., 2021).
3
.4. Electronic spectroscopic study
To understand the various transitions, involve in the tetranu-
clear complex, we have performed the electronic spectra of
9
ꢀ3
1
and 2 (d ) in 10 M concentration of methanol at ambi-
ent condition. The absorption spectra of the complexes
exhibited well-resolved absorption bands at 212, 261, and
2
90 nm in 1 and 225 and 261 nm in 2 (Figure 6a). The two
beautiful bands in 1 and 2 are assignable to intraligand
ꢆ ꢆ
n!p and p!p ) charge transfer transitions. The broad
(
absorption bands near 300 nm are characteristic of LMCT (lig-
and-to-metal charge transfer) in 1 and 2. These peaks are
consistent with the distorted octahedral geometry around
Figure 5. FTIR spectra of 1 and 2.
Figure 6. Electronic spectra (a) and PXRD pattern (b), (c) in 1 and 2.

JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
9
3
ꢀ1
Figure 7. Plots of molar magnetic susceptibility (v
M B
/cm mol ) and effective magnetic moment (meff/m ) versus temperature for 1(a) and 2(b). The experimental
values are represented by hollow triangles and squares. The solid lines represent the best fit of the data.
ꢀ
4
ꢀ1
ꢀ1
the central metal ion. The slightly blue solution of 1 and 2 (R¼ 4.12 ꢂ 10 ) for 1 and J ¼ ꢀ44.61 cm , Jˊ ¼ ꢀ3.11 cm
,
ꢀ4
displayed low-intensity bands near 665 nm, which can be g ¼ 2.06, and x¼ 0.013 (R ¼ 4.12 ꢂ 10 ) for 2, where x¼ a
assigned to the d–d transition band (Amim et al., 2008; small paramagnetic impurity.
Ansari et al., 2016; Lever, 1984; Sama et al., 2017).
H ¼ ꢀ2JðS1S2 þ S3S4Þ–2JðS1S3 þ S1S4 þ S2S3 þ S2S4Þ
(1)
2
2
2u
ꢀ2u
ꢀ2v
ctet ¼ ½Nb g =4kTꢇð10e þ 2e
þ 4e Þ=
3.5. PXRD pattern of 1and 2
2u
ꢀ2u
þ eꢀ4u þ 6eꢀ2v þ eꢀ4v
ð5e þ 3e
Þ
(2)
The bulk purity of complexes was checked by performing
powder XRD on as-synthesized crystals of both the com-
plexes. These patterns were further compared with the simu-
lated pattern, which was taken from the crystal data file.
From Figure 6b and c, it is clear that the PXRD pattern of
synthesized and simulated crystals is almost identical,
authenticating the bulk purity of 1 and 2 (Kamal et
al., 2020).
where u ¼ J/KT and v ¼ Jˊ/KT
2
2
vM ¼ ð1 ꢀ xÞv þ ½x:Nb g =4kTꢇ þ Na
(3)
tet
P
calc
exp
M
2
2
ðv T ꢀ v TÞ
M
R ¼
P
exp
ðvM TÞ
The data reveal the presence of both strongly antiferro-
magnetic (between bridged Cu(II) centers) and weakly anti-
ferromagnetic interactions (between weakly bridged Cu(II)
centers). The present results are quite different from the
reported similar cubane complexes (Tan et al., 1999; Walz et
al., 1983) which exhibit both ferromagnetic and antiferro-
magnetic interactions. The difference in the J values in the
two cubane (1 and 2) could be due to the presence of the
inductive effect group and the slight variations in the bond
angles which bridge the two neighboring Cu(II) centers. The
geometry, i.e., distorted octahedral may also be the reason
of the presence of only antiferromagnetic interactions rather
than both antiferro and ferro- magnetic (as reported in litera-
ture in such cubane cores) (Tan et al., 1999; Walz et
al., 1983).
3.6. Magnetic study
The magnetic susceptibility measurements were performed for
1
and 2 in the temperature range of 2–300 K under a 0.1 T
magnetic field. As shown in Figure 7, with when temperature is
decreased, the susceptibility increases steadily to a maximum
up to 144 K (1) or, 156 K (2) and then decreases rapidly. On the
other hands, the meff decreases continuously upon cooling upto
43 K in 1 and 47 K in 2 and indicating an antiferromagnetic
behavior. The susceptibility increases slightly at less than 30 K,
probably due to the presence of a small amount of paramag-
netic impurity (Merz & Haase, 1978; 1980). At room tempera-
3
ꢀ1
3
ture, the v T values for 1 and 2 are 1.27 and 1.21 cm kmol
M
which are slightly lesser than the expected value (1.50 cm
3
.7. Catecholase like activity
ꢀ
1
2þ
1
kmol ) for uncoupled four spin-only Cu ions (S ¼ =2). For
the incomplete cubane structures of 1 and 2, the isotropic Catechol oxidase is a dinuclear copper complex, and in the
exchange integral J is used to describe the coupling interac- appearance of molecular oxygen, it catalyzes the oxidation
ij
tions, by which the Heisenberg spin–spin exchange of catecholase to the analogous quinone. In the latter several
Hamiltonian equation (1) has been produced. Considering the years, a considerable number of mononuclear, dinuclear, and
paramagnetic impurities and temperature independent mag- polynuclear copper and other transition metal complexes are
netic susceptibility Na, the van Vleck equations (2) and (3) of designed for the model of catecholase oxidase (Liu et al.,
the appropriate molar magnetic susceptibility can be applied 2015). The preferential oxidation of catechol to its corre-
to fit the experiment plot. The fitting parameters provide, J ¼ sponding quinone is of exceptional significance for the hor-
ꢀ
1
ꢀ1
ꢀ47.33 cm
,
Jˊ
¼
ꢀ2.97 cm
,
g¼ 2.06, x¼ 0.015 monally vigorous catecholamines (Ahamad et al., 2020;

1
0
M. S. KHAN ET AL.
Figure 8. Formation of 3,5-DTBQ with increasing time in 1(a) and 2(b).
Figure 9. Kinetics of 1(a) and 2(b).
Ahmad et al., 2020; Sama et al., 2017). In this work, we have presence of copper nitrate, no conversion was observed after
studied the oxidation of 3,5-DTBC to 3,5-DTBQ in the metha- 120 min. This indicates that 1 and 2 can act as a catalyst in
nolic solution. The selective oxidation of 3,5-DTBC is funda- the translation of 3,5-DTBC to 3,5-DTBQ, proving the effi-
mental to monitor and is frequently used as a diagnostic ciency of 1 and 2. The way copper metal linking to the
reaction for new biomimetic copper complexes. The UV-Vis benzoate and amino alcohol ligand helped to sustain the
scan used for observing the conversion of 3,5-DTBC to 3,5- solid structure favoring the catalytic routine, although the
DTBQ by seeing the absorbance at 401 nm for the construc- degree to which this configuration is imperative for catalysis
tion of 3,5-DTBQ as a function of time. In a laboratory setup, is unidentified. One thing should be noted that the way
ꢀ
4
ꢀ2
1
0
M solutions of 1 or 2 were added in 10 M solution of Cu(II) differently bind to the ligands can also have some role
ꢃ
the 3,5-DTBC into a 1:1 ratio. At 25 C and the experiment in the catecholase like activity of 1 and 2. The different cata-
was performed in a methanol solvent. The reaction was fol- lytic activity for earlier documented model complexes is fre-
lowed for 2 h after the addition of 3,5-DTBC into the solution quently ascribed to the redox potentials, intermetallic Cu–Cu
of 1 and 2. Both 1 and 2 were active to serve as a mimic for distance, coordination spheres, and neighboring groups. A
catechol oxidase. Initially, 1, and 2 does not dispense band large number of mono-, di- and polynuclear complexes of
in the range of 401 nm, but after 3,5-DTBC, it shows a devel- Cu(II) ion as functional mimics for catechol oxidase have
oping peak as manifested in Figure 8. The formation of quin- been well documented in the literature (Adak et al., 2016;
one at 401 nm provides the distinct sign for 3,5-DTBQ and Ahmad et al., 2020; Ording-Wenker et al., 2015), but with this
signifies that 1 and 2 are catalytically active. The other cata- combination of ligands, this is a very first example of cate-
lytic performance of the complexes may be due to the cholase like activity was explored.
unusual coordination environments around 1 and 2 in an
open cubane structure. The results indicate that in both
cases, the oxidation of 3,5-DTBC to the analogous 3,5-DTBQ
3
.8. Kinetics study of complexes
was highly dependent on the time of reaction. In the pres- Understanding the mechanism inside the catalytic reaction,
ence of molecular oxygen, 1 and 2 shows excellent catalytic kinetic experiments were carried out on the tetranuclear
ꢀ
4
activity, but when the same reaction was performed in the complexes (taking a constant concentration of 10 mol

JOURNAL OF BIOMOLECULAR STRUCTURE AND DYNAMICS
11
ꢀ
ꢀ
3
ꢀ3
ꢀ2
dm ) and varying the concentration from 10 to 10 mol electron-donating capability (Sreenivasulu et al., 2006). We
3
dm of 3,5-DTBC. The whole experiment was monitored by can take another example in which the research group of
UV–vis spectrophotometry. First, the stock solutions of 1, 2, Torelli elaborates on the catalytic activity of some dinuclear
and substrate were prepared in methanol. The UV–vis quartz copper complexes using H-BPMP-type ligands. In that
cell was filled with 2 ml of a substrate having an exact con- research article, they saw the drastic modulation of activities
centration formed by diluting the stock solution. The quartz by the substituent of para-positions. Using the methyl-substi-
cell was left for a while inside the cell holder attached with a
thermostat maintaining the 25 C temperature. A known
tuted ligand as a reference, the activity was found to be
inhibited to a moderate extent when substituted by a strong
electron-withdrawing F atom, and the opposite was reported
ꢃ
amount of stock solution of 1 or 2 added to it to attain a
ꢀ
4
ꢀ3
preferred concentration of 1 and 2 as 10 mol dm with
increasing time formation of 3,5-DTBQ was monitored. The
classical rate method was employed to verify the rate con-
stant for each substrate concentrations, and each experiment
was repeated three times. For both the complexes, the value
of the average rate constant point toward the rate is first
order at low concentrations of the substrate. It is clear from
Figure 9 dependencies on the initial rate on the concentra-
tion of the substrate. This type of saturation reaction
depends on catechol concentration and can be effortlessly
inherent by applying the Michaelis–Menten equation for kin-
etics. The above mechanism leads to the well-known
Michaelis–Menten equations: (Banu et al., 2009)
to be true for the electron-donating -OCH group (Belle et
3
al., 2002). Based on previous literature and our understand-
ing of chemistry, we can draw a definite conclusion about
the effect of substituent and their electronegativity on the
catecholase like activity of 1 and 2. To deeply understand
the steric encumbrance on the a-carbon of the carboxylate
group also affect the oxidation of 3,5-DTBC to 3,5-DTBQ.
Likewise, Karlin and Casella suggested that the mechanism
associated with the oxidation of catechol to the correspond-
ing quinone by some dinuclear complexes, and they pro-
posed that in the second step, two electrons of the
catecholase reduce the copper(II) to copper(I) after transfer-
ence of two protons to the bridging alkoxide groups (Karlin
et al., 1994; Karlin et al., 1993; Battaini et al., 2000 ). The kin-
etic parameters of these two newly designed tetranuclear
Vmax½Sꢇ
V ¼
(4)
Km þ ½Sꢇ
KM ¼ ðk2 þ k3Þ=k1
(5) complexes have been compared with those of some of the
tetranuclear copper(II) complexes already reported in the lit-
erature (Table 2). The catecholase activity of the present
complexes is moderate for oxidizing 3,5-DTBC into corre-
sponding quinone (Das et al., 2012).
where V denotes initial rate, [S] denotes the substrate con-
centration, Km is Michaelis–Menten constant for the tetranu-
clear complex, and Vmax ¼ maximum initial rate for a specific
concentration the complexes in the incidence of 3,5-DTBC.
To plot the data, the Michaelis–Menten equation can be
modified into other easy forms. The most widely used trans-
form equation known as Lineweaver–Burk equation as fol-
lows:
4. Conclusion
In summary, two discrete assemblies of copper atoms have
been lucratively designed employing a polyalcohol and carb-
oxylate ligand via the slow evaporation method. 1 and 2 are
constructed in the basic medium using strong base NaOH as
a pH regulator in the reaction. 1 and 2 were characterized
by different spectral techniques. Tetranuclear copper com-
1
V
Km
1
1
¼
:
þ
(6)
Vmax ½Sꢇ
Vmax
From the above equation, the value of KM and Vmax can
be calculated, and the value of kcat was calculated by divid-
ing the Vmax values with complex concentration. After the
plexes have scaffold Cu O4 cubane [4 þ 2]-type structure,
4
ꢀ
1
calculation for complex 1 the values of k ¼ 1297.6 h
,
cat
verified by single-crystal XRD analysis. The molecular formula
ꢀ
4
ꢀ1
ꢀ4
Vmax ¼ 0.3604 (10 M s ), and Km ¼ 0.8702 (10 M) and
of 1 and 2 were formulated as [Cu (hmp) (ba) (Hba)2H O]
4
4
2
2
ꢀ
1
for complex 2 the values are K
¼ 748.58 h , Vmax
¼
cat
and [Cu (hmp) (2Meba) (2MeHba)] solvent. The open cubane
4
4
2
ꢀ
4
ꢀ1
ꢀ4
0
.2078 (10 M s ), and Km ¼ 0.6292 (10 M), respectively.
like structures of 1 and 2 having unusual catalytic and mag-
netic properties. Magnetic data elaborate on the moderate
antiferromagnetic nature of the tetranuclear complexes. The
tetranuclear complexes have the same structural features
and can act like those of naturally occurring catecholase
enzymes. The catecholase like activity of both 1 and 2 was
elaborated, and their kinetic parameters were also calculated.
When the kinetic parameters of 1 and 2 were compared, it
was found that 1 shows better Kcat value than that of 2. This
difference between the kinetic parameters is due to the dif-
ferent groups situated on the benzoate ligand. The methoxy
group on the ortho position of benzoic acid has a great
effect on the catalytic activity of 2. Due to H atom’s presence
on the ortho position of 1, it is obvious that one enhances
the catalytic activity. The phenomenon can be easily justified
by considering the different electron-donating substituents
capabilities on the ortho positions of the auxiliary ligand.
This hypothesis can be further verified by the work published
by S. Gao et al., where the catalytic activity follows the CH3
Acknowledgments
The author thanks chairperson, Department of Chemistry, AMU, Aligarh,
for providing the necessary research facilities. M. S. Khan thanks UGC,
New Delhi, for providing a Non-NET Fellowship. M. Khalid gratefully
acknowledges UGC, New Delhi, for providing start up grant. Author also
>
H > OH > Cl order. In this case, the complex containing
the CH group shows better catalytic activity due to higher thank to UGC DRS-SAP (II) and DST-FIST program for financial assistance.
3

1
2
M. S. KHAN ET AL.
complexes), structural properties, and catecholase activities. Inorganic
Chemistry, 41, 479–491.
Bondi, A. (1964). van der Waals Volumes and Radii. The Journal of
Disclosure statement
No potential conflict of interest was reported by the author(s).
Physical
Chemistry,
68(3),
441–451.
https://doi.org/10.1021/
j100785a001
ORCID
Bourhis, L. J., Dolomanov, O. V., Gildea, R. J., Howard, J. A. K., &
Puschmann, H. (2015). Crystal structure of 5,11-diEntities not
definedhydroEntities not definedpyrido[2,3-b][1,4]benzodiazepin-6-
one. Acta Crystallographica, Section A, 71, 59–60.
Castro, K. A. D. F., Sim o~ es, M. M. Q., Neves, M. G. P. M. S., Cavaleiro,
J. A. S., Ribeiro, R. R., Wypych, F., & Nakagaki, S. (2015). Synthesis of
new metalloporphyrin derivatives from [5,10,15,20-tetrakis (penta-
fluorophenyl)porphyrin] and 4-mercaptobenzoic acid for homoge-
neous and heterogeneous catalysis. Applied Catalysis A: General, 503,
M. Shahnawaz Khan
M. Shahid
Musheer Ahmad
http://orcid.org/0000-0002-3631-1914
http://orcid.org/0000-0002-7446-0232
http://orcid.org/0000-0003-3817-0286
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