A copper diimine-based honeycomb-like porous network as an efficient reduction catalyst

Article abstract of DOI:10.1002/aoc.6065

Nitrophenols are among the widely used industrial chemicals worldwide; however, their hazardous effects on environment are a major concern nowadays. Therefore, the conversion of environmentally detrimental p-nitrophenol (PNP) to industrially valuable p-aminophenol (PAP), a prototype reaction, is an important organic transformation reaction. However, the traditional conversion of PNP to PAP is an expensive and environmentally unfriendly process. Here, we report a honeycomb-like porous network with zeolite-like channels formed by the self-organization of copper, 1,10-phenanthroline, 4,4′-bipyridine, and water. This porous network effectively catalyzed the transformation of hazardous PNP to pharmaceutically valued PAP. In the presence of complex, PNP to PAP conversion occurred in a few minutes, which is otherwise a very sluggish process. To assess the kinetics, the catalytic conversion of PNP to PAP was studied at five different temperatures. The linearity of lnC_t/C_o versus temperature plot indicated pseudo-first-order kinetics. The copper complex with zeolite like channels may find applications as a reduction catalyst both on laboratory and industrial scales and in green chemistry for the remediation of pollutants.

Full text of DOI:10.1002/aoc.6065

Received: 21 July 2020
DOI: 10.1002/aoc.6065
Revised: 20 September 2020
Accepted: 22 September 2020
F U L L P A P E R
A copper diimine-based honeycomb-like porous network as
an efficient reduction catalyst
Abrar Ahmad1 | Syed Niaz Ali Shah1
| Mehwish Arshad1
|
Francine Bélanger-Gariepy2 | Edward R.T. Tiekink3
| Zia ur Rehman1
1Department of Chemistry, Quaid-i-Azam
Nitrophenols are among the widely used industrial chemicals worldwide; how-
ever, their hazardous effects on environment are a major concern nowadays.
Therefore, the conversion of environmentally detrimental p-nitrophenol (PNP)
to industrially valuable p-aminophenol (PAP), a prototype reaction, is an impor-
tant organic transformation reaction. However, the traditional conversion of
PNP to PAP is an expensive and environmentally unfriendly process. Here, we
report a honeycomb-like porous network with zeolite-like channels formed by
the self-organization of copper, 1,10-phenanthroline, 4,40-bipyridine, and water.
This porous network effectively catalyzed the transformation of hazardous PNP
to pharmaceutically valued PAP. In the presence of complex, PNP to PAP con-
version occurred in a few minutes, which is otherwise a very sluggish process. To
assess the kinetics, the catalytic conversion of PNP to PAP was studied at five dif-
ferent temperatures. The linearity of lnCt/Co versus temperature plot indicated
pseudo-first-order kinetics. The copper complex with zeolite like channels may
find applications as a reduction catalyst both on laboratory and industrial scales
and in green chemistry for the remediation of pollutants.
University, Islamabad, 45320, Pakistan
2Department of Chemistry, University of
Montreal, Montreal, Quebec, H3T 1J4,
Canada
3Research Centre for Crystalline
Materials, School of Science and
Technology, Sunway University, Subang
Jay, 47500, Malaysia
Correspondence
Zia ur Rehman, Department of
Chemistry, Quaid-i-Azam University,
Islamabad 45320, Pakistan.
Email: zrehman@qau.edu.pk;
hafizqau@yahoo.com
Funding information
Higher Education Commission of Pakistan
K E Y W O R D S
catalyst, copper complex, porous network, zeolite-like channels
1 | INTRODUCTION
been employed, including adsorption,[5] microbial
degradation,[6]
photocatalytic
degradation,[7]
the
Nitrophenols are among the most widely used industrial
chemicals worldwide because of their applications in
explosives, pesticides, dyes, and rubber materials.[1] How-
ever, owing to their carcinogenic and hazardous nature,
the US Environmental Protection Agency (EPA) has
enlisted them among the top organic pollutants.[2] Once
internalized in the bloodstream, nitrophenols can cause a
variety of problems including methemoglobinemia,
decrease ATP production, damage lungs, and affect the
nervous system, dermatitis, hormonal disorders, renal
failure, skin damages, and eyes irritations.[3]
electro-Fenton method,[8] electrocoagulation,[9] and
electrochemical treatment.[10] The conversion of envi-
ronmentally detrimental p-nitrophenol (PNP) to indus-
trially valuable p-aminophenol (PAP) is
a
very
important organic transformation reaction. The latter is
used in various antipyretic and analgesic drugs such as
paracetamol, corrosion inhibitor, lubricant, and dyeing
agent.[11] However, the traditional conversion of PNP
to PAP is carried out in organic solvent under high
pressure making this an expensive and environmen-
tally unfriendly process.[12]
To remove this stable, water-soluble, and environ-
mentally hazardous[4] material, several strategies have
To ensure the foregoing conversion in an inexpensive
and environmentally friendly manner, a smart strategy is
Appl Organomet Chem. 2020;e6065.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6065

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AHMAD ET AL.
usually adopted, that is, via a catalytic reduction in aque-
ous media. In this context, various metal-based
nanocatalysts including those of Au,[12,13] Ag,[14] Pd,[15]
Cu,[16] Zn, and Ni[17] have been used. To further enhance
the stability and catalytic efficacy, these materials can be
supported on polymers, oxides, resins, TiO2, SiO2, and
carbon. Furthermore, some of these metals (e.g., Au, Pd,
and Pt) are too expensive for practical applications on a
large scale. Literature studies reveal that photocatalytic
reduction of PNP is mostly carried out by heterogeneous
catalysis using various metal-based nanostructures.[18–20]
However, no homogenous catalyst has been reported so
far to reduce PNP to PAP.
catalytic potential,[21,22] which indeed has inspired a large
number of recent reviews on the utility of both transition
metal- and lanthanide-based coordination polymers for
catalytic applications.[33] Literature survey shows that the
incorporation of PNP and BH4− into the pores of a porous
structure has also been observed in case of certain porous
zeolites[34] and MOF-supported photocatalyst.[35]
During our on-going studies in copper chemistry,[36]
a
zero-dimensional copper diimine complex, whereby the
dimer is connected into a three-dimensional architecture
via conventional hydrogen bonding interactions to gener-
ate a honeycomb-like porous network, has been synthe-
sized, characterized, and investigated as a catalyst for
PNP reduction to PAN.
Sparked by the challenge that coordination chemistry
could be exploited to mimic naturally occurring three-
dimensional materials,[21] crystal engineers have
responded with a literal plethora of metal-organic frame-
work (MOF) sustained by a combination of covalent/
dative bonding interactions between practically all avail-
able metals and a huge diversity in ligands.[22] The oppor-
tunities for tuning/fine-tuning synthetic outcomes by
combining metal centers, for example, choice of metal,
oxidation state, and coordination geometry preference,
with ligands, for example, charged or neutral, variable
donor atoms, and denticity, are enormous. Conversely,
this variety of options coupled with the observation that
reaction outcomes are sometimes dependent on the sol-
vent, temperature, concentration of reagents, etc.[23]
means that there is practically an unlimited number of
MOFs that can be generated. All this, of course, ignores
two-dimensional coordination polymers, the focus of the
present report, let alone one-dimensional chains. Links
between lower dimensional architectures can be anyone
or a combination of noncovalent interactions,[24] with
hydrogen bonding obviously being prominent[25] but hal-
ogen bonding[26] and interactions involving “meta-
lloligands”[27] also being important.
Recently, copper-based catalysts, owing to their
impressive advantages over other transition metal cata-
lysts, have received a significant attention. Their lower
cost, readily availability, insensitivity to air, easy han-
dling, and generation of less waste make them versatile
catalysts in various organic reaction like synthesis of
enzymes,[28] oxidative polymerization of aniline,[29]
rearrangement reactions of aldoxime,[30] and reductions
of nitroarenes.[20] Furthermore, copper is the second
abundant transition metal inside human body and can be
easily metabolized by the body metabolic system.[31] Its
antimicrobial nature and biocompatible features make it
a suitable catalyst for biological applications.[32]
2 | MATERIALS AND METHODS
Analytical grade chemicals were purchased from Merck
(copper(II)
chloride
dihydrate),
Sigma-Aldrich
(1,10-phenanthroline [phen], 4,40-bipyridine [bipy],
sodium borohydride, silver nitrate, and ethanol), and
Fluka (PNP) companies.
2.1 | Synthesis of the copper diimine
complex
An ethanolic solution (25 mL) of phen (0.58 g, 2.9 mmol)
was added dropwise to a CuCl2.2H2O (0.50 g, 2.9 mmol)
solution (25 mL) in the same solvent, and the mixture
was stirred for 30 min at room temperature. The light
blue precipitates thus obtained were filtered off and dried
in the open air. The precipitates (0.714 g, 2.27 mmol)
were re-dissolved in distilled water in a two-necked flask
and treated with AgNO3 (0.77 g, 4.51 mmol) in the dark
and stirred for 24 h. To remove AgCl, the solution was fil-
tered, and to this, an ethanolic solution of bipy (0.35 g,
2.27 mmol) was added dropwise followed by reflux at
110ꢀC till the turbidity cleared (Scheme 1). Blue crystals
were obtained from the slow evaporation of the ꢀfinal fil-
tered solution. Yield 78% (0.54 g); m.p. 270–272 C; Fou-
rier transform infrared (FT-IR) (cm−1): 3,441 ν (O H),
3,040 ν (C H)aromatic; 1,659 ν (C N); 1,612 ν (C C)
aromatic; 1,522 ν (ring vibration; 1,494) ν (N O)asym
1,305 ν (N O)sym
;
.
2.2 | Characterization
Among the myriad of practical applications of MOF
materials, owing to their zeolite-like pores, subsequent
high surface area, and stability, relates to their high
Gallenkamp (UK) electrothermal melting point appara-
tus, ultraviolet–visible (UV–vis) spectrophotometer
(Model TCC-240A, CAT. No. 204-05557) Shimadzu 1800

AHMAD ET AL.
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SCHEME 1 Synthesis of homobimetallic copper
complex, 1
FIGURE 1 Molecular
structure of the binuclear copper
complex, 1
double beam (Japan) and Perkin Elmer Spectrum 1000
(USA) instruments were used for recording melting
point, time-dependent UV–vis and FT-IR spectra,
respectively.
TABLE 1 Selected bond lengths (Å) for 1
2.3 | Single crystal study
Parameters
Cu1
Cu3
Cu N (bridging bipy)
Cu N (terminal bipy)
Cu N (phen, trans to terminal bipy)
Cu N (phen, trans to bridging bipy)
Cu O (aqua)
2.009(3)
2.010(3)
2.018(3)
2.024(3)
2.265(3)
2.745(3)
1.995(3)
2.019(3)
2.015(3)
2.037(3)
2.280(3)
2.718(3)
Intensity data for a blue block (0.08 × 0.20 × 30 mm)
were measured at 100 K on a Bruker Venture Metal jet
diffractometer using Ga Kα radiation (λ = 1.34139 Å) so
that 2θmax = 54.9ꢀ. A total of 69,840 absorption-
corrected[37] data were collected of which 11,307 were
unique (Rint = 0.051), and of these, 9,908 satisfied the
I ≥ 2σ (I) criterion. The structure was solved by
SHELXT[38] and refined on F2 with SHELXL-2014/7[39]
Cu O (nitrate)
Abbreviation: bipy, 4,40-bipyridine.

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AHMAD ET AL.
FIGURE 2 Molecular packing for
1: (a) a view of the supramolecular
chain along the a-axis mediated by
aqua-O Hꢁ ꢁ ꢁO (nitrate) hydrogen
bonding, shown as orange dashed lines,
(b) a view of the supramolecular layer in
the ac-plane with aqua-O Hꢁ ꢁ ꢁN
(pyridyl) hydrogen bonds shown as blue
dashed lines (nonparticipating H atoms
have been omitted), and (c) a view in
projection down the a-axis of the unit
cell contents highlighting the channels
parallel to the a-axis

AHMAD ET AL.
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FIGURE 3 Time-dependent ultraviolet–visible (UV–vis) spectra for the reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) at
five different temperatures: (a) 25ꢀC, (b) 30ꢀC, (c) 35ꢀC, (d) 40ꢀC, and (e) 45ꢀC
integrated within Olex.[40] Nonhydrogen atoms were
refined anisotropically, and nonacidic H atoms were
included in the model in their calculated positions.
The water-H atoms were located and refined without
restraint. At this stage of the refinement, the presence
of heavily disordered solvent molecules, that is, etha-
nol and water, were indicated. These were modeled
employing the SQUEEZE routine[41] with full details
given in the deposited CIF. The final refinement
Dx = 1.244 g cm−3, μ = 4.166 mm−1. CCDC deposition
number: 1580602.
3 | RESULTS AND DISCUSSIONS
3.1 | Crystal and molecular structures
The molecular structure along with crystallographic
numbering scheme for the binuclear copper complex is
shown in Figure 1. Although the structure is devoid of
crystallographic symmetry, it has pseudo two fold sym-
metry with the axis bisecting the central C C bond of
the bridging bipy ligand and lying in the plane of defined
by Cu1, Cu3, N1, and N31 atoms. The coordination
geometry for the Cu1 atom is defined by N atoms of the
chelating phen ligand, an N atom of one end of the bridg-
ing bipy ligand, an N atom of a terminally bound bipy
ligand, an aqua-O atom and one O atom of the nitrate
anion. The immediate environment of the Cu1 atom is
(701 parameters) had a weighting scheme of the form
2
w
=
1/[s2(Fo )
+
0.052P2
+
13.335P] where
P = (Fo + 2Fc )/3) on F2 and converged with
R = 0.065 and wR2 = 0.171. The molecular structure
diagram was generated with ORTEP for Windows[42] at
the 35% probability level, and the packing diagrams
were drawn with DIAMOND.[43]
2
2
Crystal data for C54H44Cu2N12O8 (1); exclusive of
unresolved solvent) M = 1,116.09, monoclinic space
group P21/c, a = 7.3006(2), b = 43.8747(10), c = 18.6593
(4) Å, β = 94.4600(10)ꢀ, V = 5958.7(2) Å3, Z = 4,

6 of 9
AHMAD ET AL.
FIGURE 4 lnCt/Co versus time graph for p-nitrophenol (PNP) conversion to p-aminophenol (PAP) at different temperatures
defined by the N4O2 donor set and is best described as
5 + 1 as the Cu O63 bond length is significantly longer
than the other bonds as seen from the data collated in
Table 1; an analogous coordination geometry is seen for
the Cu3 atom. Nevertheless, the coordination geometry is
based on an octahedron. There are a small number of lit-
erature precedents for the aforementioned structure.
Thus, the Cu2(phen)2(bipy)3 fragment has been observed
in the structure of the copper(II) complex [(L-valinato-
N,O)(bipy)(H2O)Cu(bipy)Cu(L-valinato-N,O)(bipy)(OH2)]
(NO3)2.2H2O[44] but where the terminal bipy ligands have
an antidisposition as opposed to syn in 1; the Cu atoms
have square pyramidal geometries with aqua ligands in
the apical positions. An antidisposition is also found in
the structure of [(phen)(bipyH)(H2O)Cu(bipy)Cu(phen)
(bipyH)(OH2)][Mo12O40P]2.2H2O,[45] that is, with proton-
ated terminally bound bipy ligands, where the bulky
counter ions cannot approach the copper centers
resulting in square pyramidal geometries again with the
aqua ligands in the apical positions. Of particular interest
in the structure of 1 is the mode of supramolecular aggre-
gation occurring in the crystal.
both pairs of independent aqua molecules and nitrate
anions, leads to the formation of supramolecular chains
along the a-axis. As viewed from Figure 2a, the chain is
linear, and as both terminal bipy molecules are oriented
to the same side, the topology is of a U-tube. The
remaining aqua-H atoms form hydrogen bonds to the
noncoordinating pyridyl-N atoms derived from two
symmetry-related U-tubes, of opposite orientation, to
generate layers in the ac-plane and, crucially, approxi-
mately square channels parallel to a-axis direction
(Figure 2b). The channels have disparate edge lengths
and the maximum dimensions of the face, based on the
Cuꢁ ꢁ ꢁCu separations, are 8.8 × 11.0 × 12.0 × 13.1 Å. The
layers stack along the b-axis with the primary connec-
tions between them being of the type phen-C Hꢁ ꢁ ꢁO
(nitrate) (Table S1). In this way, additional channels are
formed, which is more symmetrical with each edge, again
based on Cuꢁ ꢁ ꢁCu separations, being 10.7 Å (Figure 2c).
3.2 | Catalytic activity
As anticipated, conventional hydrogen bonding plays
a significant role in the molecular packing of 1 (Table S1).
Thus, aqua-O Hꢁ ꢁ ꢁO (nitrate) hydrogen bonds, involving
The catalytic ability of 1 was evaluated for the PNP's
reduction into PAP using NaBH4, a prototype reaction.
The reduction was followed readily by UV–vis

AHMAD ET AL.
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FIGURE 5 Plots of (a) Kapp versus temperature, (b) Inkapp versus 1/T, and (c) Inkapp/T versus 1/T
spectroscopy as both reactant (PNP) and product (PAP)
absorb in the UV–vis region at different λmax values.[46]
Without a catalyst, the only obvious change noted was
deepening of the yellow appearance of PNP upon the
the peak of PNP (400 nm) and the emergence of a
new peak for PAP at 290 nm[48] (Figure 3a–e). In the
induction period, the time required for the diffusion of
reagents and catalyst lnCt/Co value remains
unchanged. Excess NaBH4 was used, and the linearity
of lnCt/Co versus temperature relationship indicated
pseudo-first-order kinetics.[49] Afterward, the catalytic
conversion of PNP to PAP was studied at five different
temperatures (Figure 3a–e). The induction time
decreases as does the reaction time, that is, from
22 (25ꢀC) to 5 min (45ꢀC). Furthermore, a threefold
addition of NaBH4 accompanied by
a red shift
(317–400 nm), signifying the formation of PNP ions
(Figure S1). The absence of a peak pertinent to PAP, even
after keeping the solution for several days, indicated that
the reaction required a catalyst to proceed.[47] However,
in the presence of 1, PNP to PAP conversion occurred
after a few minutes as signified by the diminishing of
TABLE 2 Kinetic parameters for the conversion of PNP to PAP
Temp (ꢀC)
Kapp (min−1
0.259
)
Ea (kJ mol−1
)
ΔH# (kJ mol−1
)
ΔS# (J mol−1 K−1
)
TOF (min−1
0.0024
)
25
30
35
40
45
40.65
—
38.09
—
−197.51
—
0.428
0.0040
0.532
—
—
—
0.0075
0.652
—
—
—
0.0087
0.759
—
—
—
0.010
Abbreviation: TOF, turn over frequency.

8 of 9
AHMAD ET AL.
increase in the apparent rate constant was witnessed
as the temperature rose from 25ꢀC to 45ꢀC (Figure 4).
This correlated with an increase in the average kinetic
energy of molecules at elevated temperature, which in
turn increases the diffusion rate of the reactants.
Hence, an increase in collision frequency and a fast
diffusion rate triggered the conversion of reactants into
products.[50] The Arrhenius equation, that is, lnk = ln
A − Ea/RT, was used to calculate Ea by plotting lnk
versus 1/T gives a slope Ea/RT that corresponds Ea
CONFLICT OF INTEREST
There is no conflict of interest.
DATA AVAILABILITY STATEMENT
The authors confirm that the data supporting the find-
ings of this study are available within the article and its
supplementary materials.
ORCID
Syed Niaz Ali Shah https://orcid.org/0000-0001-9902-
857X
Edward R.T. Tiekink https://orcid.org/0000-0003-1401-
1520
Zia ur Rehman https://orcid.org/0000-0002-1436-3131
value of 40.65 kJ mol−1 [51]
The kinetic parameters
.
such as activation entropy (ΔS#) and enthalpy (ΔH#)
were calculated using the Erying equation, that is, lnk/
T = ln (kb/h) + ΔS#/R − ΔH#/R (1/T),[52] where T is
the absolute temperature, kb is the Boltzmann con-
stant, h is the Planck constant, and R is the ideal gas
constant. The plot of lnkapp/T versus 1/T gives a
straight line with slope −ΔH#/R (Figure 5) and inter-
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4
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Ahmad A, Shah SNA,
Arshad M, Bélanger-Gariepy F, Tiekink ERT, ur
Rehman Z. A copper diimine-based honeycomb-
like porous network as an efficient reduction
catalyst. Appl Organomet Chem. 2020;e6065.
https://doi.org/10.1002/aoc.6065
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