Cu(II)-Na(I) heterometallic coordination compounds as photocatalyst for degradation of methylene blue

Article abstract of DOI:10.1016/j.ica.2021.120346

Two new Cu(II)/Na(I) coordination polymers, namely, {[(Cu(L1)Na)₂(ox)(ClO₄)₂]?dmf}_n (1) and {[(Cu(L2)Na)₂(nph)]?H₂O}_n (2) were synthesized by using the Schiff base N-(2-hydroxyet

Full text of DOI:10.1016/j.ica.2021.120346

Inorganica Chimica Acta 522 (2021) 120346
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Research paper
Cu(II)-Na(I) heterometallic coordination compounds as photocatalyst for
degradation of methylene blue
a
b
c
a,*
Aparup Paul , Ennio Zangrando , Valerio Bertolasi , Subal Chandra Manna
a
Department of Chemistry, Vidyasagar University, Midnapore 721102, West Bengal, India
b
Department of Chemical and Pharmaceutical Sciences, University of Trieste, 34127 Trieste, Italy
Dipartimento di ScienzeChimiche e Farmaceutiche, Centro di Strutturistica Diffrattometrica, Universit a` di Ferrara, Via L. Borsari, 46, 44100 Ferrara, Italy
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two new Cu(II)/Na(I) coordination polymers, namely, {[(Cu(L1)Na) (ox)(ClO ) ]∙dmf} (1) and {[(Cu(L2)
2
4
2
n
Cu(II)-Na(I)
Na)
2
(nph)]∙H
2
O}
n
(2) were synthesized by using the Schiff base N-(2-hydroxyethoxy)ethyl-iminomethyl-phenol
L2) with naphthalene-2,6-
Coordination polymer
Synthesis
(
H
2
L1) with oxalate (ox) and (3-methoxy-2-hydroxybenzylidene)propanoic acid (H
2
dicarboxylate anions (nph), respectively. Both the complexes were characterized using elemental analysis, FT-
IR, UV–vis spectroscopic analyses. The solid state structures of complexes 1–2 were determined using single
crystal X-ray crystallography that revealed for 1 the presence of dinuclear copper(II) species, [(L1)Cu(ox)Cu
Crystal structure, photocatalyst
(
2 n
L1)], while 2 consists of stair-like 1D -[CuL2] -(nph)] - polymers. Both these fragments are further connected by
Na(I) cations to form 2D supramolecular assemblies. In both the compounds the copper and sodium atoms
exhibit a square pyramidal and a distorted octahedral coordination geometry, respectively. The catalytic activity
of complexes 1–2 for the degradation of methylene blue (MB) was investigated under UV irradiation and in the
2 2
presence of very small amount of H O . Both the compounds were found to be photo-catalytically active, and the
mechanism of photocatalysis has been discussed in detail.
1
. Introduction
important role in catalytic applications [10], and among these, the
photo-degradation of organic dyes has received significant attention for
reasons related to environmental management [11].
The design and synthesis of hetero-metallic coordination polymers
(
CPs) have attracted extensive research interest not only because of the
Most of the organic dyes are released into water from textile in-
dustries [12]. Due to their high solubility, these dyes restrict waste water
treatment by usual procedures that include synthetic photo-catalysts for
decomposing organic pollutants under UV–vis light [13]. Currently,
commonly used photocatalytic materials with excellent performance are
intriguing structural and topological [1] novelty, but also for their
remarkable potential applications in gas storage, ion exchange,
nonlinear optics, chemical separation and catalysis [2–5]. In the
scheming of coordination polymers, choice of ligands that affords proper
donor atoms to complete the coordination sphere of the transition metal
ions is of most importance [6]. Polycarboxylate ligands as multi dentate
O-donors have been widely used in the construction of CPs [7]. Beside
polycarboxylate linkers, multidentate flexible Schiff base ligands are
frequently used to construct polynuclear complexes, as they can act as
chelating, bridging and charge balance species [8]. Therefore, the use of
multidentate Schiff base ligands in combination with dicarboxylate
anions represents an important strategy for the synthesis of polynuclear
complexes.
2
mainly based on TiO , ZnO, metal niobate and titanate and other
semiconductors with wide band gap [14]. However, these semi-
conductors are limited in their reducing power, resulting in the ab-
sorption of a small amount of UV light from sunlight [15]. Thus, in order
to develop photo-catalysts with high efficacy and within a wide range of
wavelengths in the visible region, polynuclear transition metal polymers
have been synthesized. Theoretically, CPs can be very auspicious as
photocatalytic degradation materials since both the ligands and metal
ions can deliver the platform to produce variable band gaps, while
coordinating interactions between metal and ligand can control the
band gap widths [16]. Additionally, the high diversity of CPs structures
permits different behaviors in photocatalytic degradation, and at the
CPs of first series transition metals using suitable organic ligands
have been reported by lots of researchers [9]. Literature survey reveals
that many copper(II) containing CPs of diverse geometries display
*
Corresponding author.
E-mail address: scmanna@mail.vidyasagar.ac.in (S.C. Manna).
https://doi.org/10.1016/j.ica.2021.120346
Received 22 April 2020; Received in revised form 5 March 2021; Accepted 5 March 2021
Available online 23 March 2021
0
020-1693/© 2021 Elsevier B.V. All rights reserved.

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
Table 1
dicarboxylate co-ligands (ox = oxalate and nph = naphthalene-2,6-
dicarboxylate). Their UV–vis absorption, emission spectra and photo-
catalytic properties have been explored.
Crystallographic Data and Details of Refinements for Complexes 1–2.
1
2
Empirical formula
M
C
27
H
35Cl
2
Cu
2
3
N Na
O
2 19
C
34 2 2 2 13
H30Cu N Na O
949.54
Orthorhombic
P ca2
847.66
Triclinic
P1
2. Experimental
Crystal system
Space group
1
2
.1. Materials and measurements
a /Å
b /Å
10.1953(2)
16.4562(2)
22.2449(5)
90.0
6.0634(5)
11.8767(12)
12.4577(13)
108.657(5)
92.701(6)
103.665(6)
818.55(14)
1
High purity 2-(2-aminoethoxy)ethanol (Aldrich), β-alanine (SRL), 2-
c /Å
◦
◦
◦
hydroxybenzaldehyde (Spectrochem-India) and 2-hydroxy-3-methoxy-
benzaldehyde were purchased and used as received. All other chem-
icals used were of analytical grade. Solvents used for spectroscopic
studies were purified and dried by standard procedures before use [17].
Elemental analyses (carbon, hydrogen and nitrogen) were performed
using a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded
as KBr pellets on a Bruker Vector 22FT IR spectrophotometer operating
α
/
β /
γ /
90.0
90.0
3
V /Å
3732.16(12)
4
Z
ꢀ
3
Dcalc /g cm
1.690
1.720
ꢀ
1
μ
/mm
1.389
1.400
F(000)
1936
432
ꢀ 1
Reflections collected
Unique reflections
27,420
9710
7913
from 400 to 4000 cm . Electronic absorption spectra were obtained
3035
with Shimadzu UV-1601 UV–vis spectrophotometer at room tempera-
R
int
0.0551
7949
0.0685
2696
3
ture. Quartz cuvettes with a 1 cm path length and a 3 cm volume were
ReflectionsI > 2
σ
(I)
Parameters
499
245
used for all measurements. Emission spectra were recorded on a Hitachi
F-7000 spectrofluorimeter, and room temperature (300 K) spectra were
obtained using a quartz cell of 1 cm path length. The slit width was 2.5
nm for both excitation and emission. ESI-MS spectra of the compounds
in water were recorded on an Agilent Q-TOF 6500 mass spectrometer
and the Mass hunter software was used for mass analysis.
Goodness-of-fit
1.013
1.101
[
1
a]
R
0.0446
0.1077
0.514, ꢀ 0.797
0. 0449
0.1166
[
a]
wR
Residuals /e Å
a]
2
(I > 2 (I))
σ
ꢀ
3
0.312, ꢀ 0.697
2 2 ½
[
2
2 2
R1 = Σ |Fo|–|Fc|| / Σ|Fo|, wR2 = [Σw (Fo – Fc ) / Σw (Fo ) ] .
same time can impact the degradation efficacy [16].
Considering the above facts, in this article we report the syntheses
and characterizations of two novel CPs, namely {[(Cu(L1)Na) (ox)
ClO ]∙dmf} (1) and {[(Cu(L2)Na) (nph)]∙2H O} (2), based on
Schiff base H
L1 = N-(2-hydroxyethoxy)ethyl-iminomethyl-phenol and
L2 = (3-methoxy-2-hydroxybenzylidene)propanoic acid and two
2.2. X-Ray crystallography
2
(
4
)
2
n
2
2
n
Data collections of the structures reported were carried out on a
Nonius Kappa CCD diffractometer [18] equipped with graphite-
2
H
2
monochromated Mo-K
α radiation (λ = 0.71073 Å) at room tempera-
ture. Cell refinement, indexing and scaling of the data sets were carried
Scheme 1. Synthesis of ligand and complexes.
2

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
out using Denzo and Scale pack package [19]. The structures were
solved by direct methods (SIR97) [20] and subsequent Fourier analyses
2
and refined by the full-matrix least-squares method based on F with all
observed reflections [21].One molecule of dimethylformamide and one
of water (at half occupancy) were found in the Fourier maps of 1 and 2,
respectively. In the latter no H atoms were assigned to the slightly
disordered water species. All the calculations were performed using the
WinGX System, Ver 1.80.05. [22] Pertinent crystallographic data and
refinement details are summarized in Table 1.
2
.3. Synthesis of ligand
2
The ligand N-(2-hydroxyethoxy)ethyl-iminomethyl-phenol (H L1)
was prepared by the condensation reaction of 2-(2-aminoethoxy)ethanol
2 mmol, 0.21 g) and 2-hydroxybenzaldehyde (2 mmol, 0.244 g) in
methanol (20 mL), refluxing the solution for 3 h, whereas (3-methoxy-2-
hydroxybenzylidene)propanoic acid (H L2) was prepared by the
(
2
condensation reaction of β-alanine (2 mmol, 0.178 g) and 2-hydroxy-3-
methoxybenzaldehyde (2 mmol, 0.304 g) in water : methanol (1:9, 20
mL), following the same procedure as for HL1 (Scheme 1). The yellow
colored methanolic solutions were used directly for the synthesis of
complexes.
Fig. 1. Dinuclear copper complex 1 (ellipsoid probability at 35%; C atoms not
labelled for sake of clarity; dmf molecule not shown).
2
.4. Synthesis of complexes
6
33 (vw), 605 (vw).
Caution! Perchlorate salts of metal with organic ligands are poten-
2
.5. Photocatalytic experiment
tially explosive. Only a small amount of material should be prepared,
and it should be handled with care.
The photocatalytic performance of 1 and 2 was investigated for the
The complexes have been synthesized by adopting the procedures
schematically given in Scheme 1.
degradation of methylene blue (MB) in a typical process. A crystalline
sample (5 mg) of each complex was mixed with 100 mL of MB aqueous
-
1
solution (1 mg litre ). The mixture was stirred for 30 min in a dark
environment. After that 50 µL of H 30% were added to the solution
2
2 4 2 n
.4.1. Synthesis of {[(Cu(L1)Na) (ox)(ClO ) ]∙dmf} (1)
2 2
O
Methanolic solution (5 mL) of triethylamine(1 mmol, 0.101 g) was
and stirred continuously under UV irradiation of a 125 W mercury lamp
at a distance of 4–5 cm from the liquid surface. At intervals of 30 min, 3
mL of the sample were taken from the container and analyzed by UV–vis
spectroscopy. The MB concentration changes were monitored by
measuring the absorbance intensity at its characteristic absorbance
wavelength of λ = 664 nm. The experimental data were analyzed by a
first order kinetic model. The degradation rate and the kinetic rate
constant (k) were calculated [23] using the equations:
added drop wise to a methanolic solution (10 mL) of HL1 (1 mmol, 10
mL) under stirring condition for 10 min. To the resulting mixture, drop
wise addition of methanolic solution (15 mL) of copper perchlorate
hexahydrate (1 mmol; 0.370 g) yielded a deep green solution. To this
solution an aqueous solution of disodium oxalate (0.5 mmol, 0.067 g)
was added after 1 h and additionally stirred for 1 h and filtered. The
filtrate was kept in open atmosphere for slow evaporation obtaining
after a few days a green crystalline material, not suitable for X-ray
structural analysis. The green compound was dissolved in dmf and
recrystallized to provide suitable crystals for X-ray diffraction. Yield: 72
Degradation (%) = (1 ꢀ A/A
0
) × 100 and ꢀ ln(C/C
0
) = k × t
%
2 2 3 2
. C27H35Cl Cu N Na O19 (949.54): C, 34.15; H, 3.72; N, 4.43 (%).
where A and A
0
are the absorbance at time t and t = 0, respectively,
ꢀ
1
Found: C, 34.37; H, 3.69; N, 4.38 (%). IR (cm ): 3573 (s, br), 3420 (s,
br), 2922 (vw), 1662 (vs), 1621 (vs), 1595(s), 1543 (s), 1473 (w), 1446
0
corresponding to the relative concentrations C and C .
(
(
s), 1348 (w), 1317 (s), 1242 (vw), 1200 (vw) 1081 (vs), 898 (w), 760
w), 662 (vw).
2
.6. Band-gap calculations
The band gap of complexes was examined by the UV–vis diffuse
reflection measurement process at room temperature, and the band-gap
2
.4.2. Synthesis of {[(Cu(L2)Na)
Methanolic solution (5 mL) of triethylamine(1 mmol, 0.101 g) was
added dropwise to a methanol : water (9:1) solution of H L2 (1 mmol,
0 mL) in stirring condition for 10 min. To this resulting mixture,
methanolic solution (10 mL) of copper nitrate trihydrate (1 mmol;
.241 g) was added and left for stirring for 30 min. A deep green solution
was obtained. After that an aqueous solution of disodium naphthalene-
,6-dicarboxylate (0.5 mmol, 0.13 g) was added and transferred the
2 2 n
(nph)]∙H O} (2)
2
energy evaluated from the Tauc plot (
α
h
ν
) versus photon energy (h ) for
ν
2
a particular sample according to equation.
1
n
(α
hν) = K(h
ν ꢀ Eg)
0
where
α
is the absorption co-efficient, h
ν
is the discrete photon energy, K
is the band
is a constant which is considered as 1 for ideal case, and E
g
2
gap energy [13]. To determine the direct band gap, the value of the
resulting solution to a Teflon lined stainless steel reactor and heated to
◦
exponent ‘n’ in the above equation has been considered equal to 1/2
8
0
C for six hours, followed by cooling slowly to room temperature.
[
13b]. The extrapolated value along the X axis of h
ν
at
α = 0 gives the
After 18 h the solution was filtered and the filtrate was kept in open
atmosphere for slow evaporation and green color single crystal suitable
for X-ray diffraction was obtained after a few days. Yield: 77 %.
absorption edge energies corresponding to E
g
[23c–23d]. The point of
inflection in the Tauc plot gives the particular value of the band-gap
energy.
C
34
H
30Cu
2 2 2
N Na O13 (847.66): C, 48.17; H, 3.57; N, 3.30 (%). Found: C,
ꢀ 1
4
8.11; H, 3.61; N, 3.32 (%). IR (cm ): 3596 (w, br), 3417 (w, br), 2912
(
(
w), 1605 (vs), 1473 (vs), 1448(s), 1367 (vs), 1325 (vs), 1238 (s), 1216
vs), 1190 (w), 1074 (s), 1039 (w) 961 (vs), 862 (s), 780 (vs), 743 (vs),
3

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
Table 2
3. Results and discussion
.1. Synthetic aspect
◦
Selected bond distances (Å) and angles ( ) for complex 1.
3
Cu(1)-O(1)
1.874(4)
Cu(2)-O(4)
1.872(4)
Cu(1)-N(1)
Cu(1)-O(7)
Cu(1)-O(8)
Cu(1)-O(11)
Na(1)-O(2)
Na(1)-O(3)
1.943(4)
Cu(2)-N(2)
Cu(2)-O(9)
Cu(2)-O(10)
Cu(2)-O(17)
1.941(4)
2.019(3)
1.991(4)
2.534(6)
2.414(4)
2.529(4)
2.323(6)
2.384(4)
2.520(6)
2.394(7)
85.03(15)
167.73(13)
96.10(17)
87.93(14)
82.82(14)
177.35(18)
92.17(14)
95.97(16)
94.27(13)
90.27(15)
156.46(18)
169.6(2)
171.7(2)
The Schiff base ligands, N-(2-hydroxyethoxy)ethyl-iminomethyl-
phenol (H L1) and (3-methoxy-2-hydroxybenzylidene) propanoic acid
(H L2) were synthesized by adopting the condensation reaction between
corresponding amine and aldehyde in methanol under reflux condition.
Then H L1 and H L2 were used for the synthesis of complexes 1 and 2.
2.002(3)
1.984(4)
2
d
c
2.540(4)
2
c
2.502(4)
Na(2)-O(1)
2.342(5)
Na(2)-O(5)
Na(2)-O(6)
d
2
2
Na(1)-O(4)
2.378(4)
d
c
Na(1)-O(9)
2.364(4)
Na(2)-O(7)
a
Na(1)-O(12)
Na(1)-O(18)
2.486(6)
Na(2)-O(13)
b
2.580(11)
85.91(14)
168.52(14)
96.56(16)
93.46(12)
82.72(14)
176.59(16)
92.29(11)
94.74(16)
88.57(12)
89.91(11)
153.70(17)
157.5(3)
Na(2)-O(16)
3
.2. Crystal structure description of {[(Cu(L1)Na)
2 4 2 n
(ClO ) (ox)]∙dmf}
O(1)-Cu(1)-O(7)
O(1)-Cu(1)-O(8)
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-O(11)
O(7)-Cu(1)-O(8)
O(7)-Cu(1)-N(1)
O(7)-Cu(1)-O(11)
O(8)-Cu(1)-N(1)
O(8)-Cu(1)-O(11)
N(1)-Cu(1)-O(11)
O(4)-Cu(2)-O(9)
O(4)-Cu(2)-O(10)
O(4)-Cu(2)-N(2)
O(4)-Cu(2)-O(17)
O(9)-Cu(2)-O(10)
O(9)-Cu(2)-N(2)
O(9)-Cu(2)-O(17)
O(10)-Cu(2)-N(2)
(
1)
d
d
c
c
The X-ray structural analysis reveals that the asymmetric unit of
compound 1 comprises a dinuclear Cu(II) complex, two perchlorate
anions, two sodium ions (Fig. 1) and a lattice dmf molecule. In the
complex each copper atom is chelated by a Schiff base through the imino
N and the phenolate O atoms and these fragments are further connected
by an oxalate anion in a bis-bidentate fashion with metals separated by
5.235(1) Å.
d
d
c
O(10)-Cu(2)-O(17)
c
N(2)-Cu(2)-O(17)
d
c
O(2)-Na(1)-O(4)
O(5)-Na(2)-O(1)
b
a
O(3)-Na(1)-O(18)
O(12)-Na(1)-O(9)
O(6)-Na(2)-O(13)
The copper atoms exhibit a square pyramidal geometry completing
their coordination sphere by a perchlorate anion at apical position. The
Cu–O(phenol) bond distances are of 1.874(4), 1.872(4) Å, while the
Cu–O(oxalate) ones are slightly different, being of 1.984(4) and 1.991
d
c
175.65(19)
O(16)-Na(2)-O(7)
Symmetry codes: a = x, -1+y, z; b = x, 1+y, z; c = 1/2+x, -y, z; d = - 1/2+x, 1-y,
z.
(
4) Å those in trans to the phenol oxygen, and of 2.002(3) and 2.019(3)
those trans located to the imino nitrogen (Table 2). Finally the Cu–N
bond distances are of 1.943(4) and 1.941(4) Å, while the Cu–O ones of
Fig. 2. The 2D polymer of compound 1 (Na-O bonds are indicated as dotted lines; lattice dmf molecules not shown).
4

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
Fig. 3. 2Dpolymer of compound 1 viewed down axis b.
apical perchlorate ligand present the longest values, of 2.540(4) and
.534(6) Å. The dinuclear entity shows an almost centro-symmetrical
2
fashion, the main difference being exhibited by a different conforma-
tion of the (2-hydroxyethoxy)ethyl chains that pend hedgewise. The
copper complexes are connected by sodium cations to form a 2D su-
pramolecular assembly parallel to the crystallographic ab plane (Fig. 2).
In fact both the sodium ions, embedded among the complexes, result
coordinated by the oxygen atoms of one (2-hydroxyethoxy)ethyl arm
and in addition chelated by the oxalate and phenol oxygen atoms of a
symmetry related complex and finally bound by two oxygen atoms of
perchlorate anions, giving rise to a highly distorted octahedral geometry
(
Figs. 2 and 3).
The Na-O bond lengths fall in a range from 2.323(6) to 2.580(11) Å
Table 2), in agreement with values measured in other copper(II)/so-
(
dium(I) compounds [24]. The O-Na-O coordination bond angle values of
trans located O oxygen atoms (Table 2) indicate the deviations from the
ideal octahedral values. Thus the perchlorate anions act as bridging
units between the sodium and copper atoms which are separated by
3.279(2) Å (Na(1)… Cu(2) at ꢀ 1/2 + x, 1-y, z) and 3.340(2) Å (Na(2)…
Cu(1) at 1/2 + x, -y, z). Both alcoholic groups O3 and O6 (from different
Fig. 4. The asymmetric unit (Ortep drawing, 30% ellipsoid probability) of
compound 2 with the naphthalene-dicarboxylate species located on a center of
′
′
′
symmetry (O4 at 1-x,-y, 1-z; O5 ’/O6 ’ at 1-x,-1-y,-z).
Fig. 5. The 1D chain built by the (CuL2)
2
centrosymmetric dimers connected by the nph anions.
5

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
Table 3
whereas complex 2 exhibits bands at 361 and 404 nm upon excitation at
273 nm (Fig. S3). The positions of emission bands remain unaffected
when λex is varied within ± 10 nm.
◦
Selected bond distances (Å) and angles ( ) for complex 2.
Cu–O(1)
1.897(2)
Na-O(2)a
2.699(3)
Cu–O(3)
1.951(2)
Na-O(3)
2.430(3)
3
.5. ESI mass spectrometry
Cu–N(1)
1.952(3)
Na-O(4)c
2.735(3)
Cu–O(5)
1.967(2)
Na-O(5)a
2.457(3)
Cu–O(4)e
2.441(3)
Na-O(6)
2.271(3)
ESI mass spectra of complexes were recorded in water. The ESI mass
Na-O(1)a
2.274(3)
Na-Cu_a
3.1462(14)
92.60(11)
90.53(10)
173.18(11)
84.43(11)
100.98(12)
124.49(11)
145.39(12)
61.34(9)
spectrum of 1 (Fig. S4) shows a peak at m/z = 876.0177, corresponding
O(1)-Cu–O(3)
O(1)-Cu–O(5)
O(1)-Cu–N(1)
O(1)-Cu–O(4)e
O(3)-Cu–O(5)
O(3)-Na-O(6)
O(3)-Na-O(1)a
O(3)-Na-O(2)a
O(3)-Na-O(5)a
O(3)-Na-O(4)c
O(6)-Na-O(1)a
O(6)-Na-O(2)a
174.66(10)
83.64(10)
92.45(11)
86.83(10)
91.50(10)
79.73(10)
117.92(11)
96.10(11)
140.04(11)
73.68(9)
O(3)-Cu–N(1)
O(3)-Cu–O(4)e
O(5)-Cu–N(1)
O(5)-Cu–O(4)e
N(1)-Cu–O(4)e
O(6)-Na-O(5)a
O(6)-Na-O(4)c
O(1)a-Na-O(2)a
O(1)a-Na-O(5)a
O(1)a-Na-O(4)c
O(2)a-Na-O(5)a
O(2)a-Na-O(4)c
O(5)a-Na-O(4)c
+
to [C24
H
28Cl
2
2
Cu N
2
Na
2
O
18
]
(calc. m/z = 876.46). Alternatively the
spectrum of 2 (Fig. S5) shows a peak at m/z = 847.0412, corresponding
+
to [C34
H
30Cu
2
N
2
Na
2
O
13
]
(calc. m/z = 847.68). These results evidences
that the species of 1 and 2 in water are composed of dinuclear copper
fragments (bound to two Na cations), a structure formally similar to the
building units forming the polymers detected in solid state.
65.86(9)
73.12(9)
3.6. Band-gap studies
140.43(13)
82.81(11)
116.47(11)
121.29(10)
69.87(9)
The difference in the band-gap sizes influences the photo-catalytic
activity of coordination polymers. For this reason, we were motivated
to estimate the band-gap value of complexes at room temperature using
the UV–vis diffuse reflectance measurement method. The calculated
values of band gap (Fig. S6) of 1 and 2 are 2.87 ± 0.03 and 3.01 ± 0.06
eV, respectively, indicating that these materials are semiconductors in
nature. These results give emphasis to the relative strength of complexes
for photocatalytic applications. In the complexes with lower degree of
electronic band-gap, charge transfer from 2p bonding orbitals (valence
band) of oxygen and nitrogen to empty orbital (conduction band) of
copper takes place easily, thus favouring photo-catalytic activity [23b].
Symmetry codes: a = -1 + x,y,z; b = 1 + x,y,z; c = -x,-y,1-z; d = 1-x,-1-y,-z; e = 1-
x,-y,1-z.
complexes) interacts with the dmf oxygen through H-bond interactions
(
O… O distances of 2.788(8) Å).
.3. Crystal structure description of {[(Cu(L2)Na)
The asymmetric unit of compound 2, shown in Fig. 4, consists of a
3
2 2 n
(nph)]∙H O} (2)
CuL2 unit, half naphthalene-2,6-dicarboxylato anion, a sodium ion, and
a residual interpreted as half lattice water molecule. For the latter,
hydrogen atoms were not located. The crystal packing comprises of
3
.7. Photocatalytic activity
Methylene blue (MB), a chemically stable and poor biodegradable
[
2
CuL2] dimers further connected by the naphthalene-2,6-dicarboxylato
species, typically difficult to decompose in waste water [25], was
anions to form a stair-like 1D polymeric arrangement (Fig. 5).
The copper atom exhibits a square pyramidal geometry being
chelated by the Schiff base through the phenolato oxygen (Cu–O(1) =
selected for evaluating the photocatalytic properties of complexes 1 and
2
toward the decomposition of organic pollutants. We are aware that
polymeric complexes 1 and 2 are dissociated in solution to form simpler
species, and in fact ESI mass spectra in water indicate complex frag-
ments as the building blocks forming the polymers observed in solid
state. With this premise, Fig. 7 illustrates the time dependent electronic
absorption spectra of the MB solution degraded by the complexes, by
monitoring the process of photo degradation at the characteristic ab-
sorption of MB at 664 nm. The spectra of Fig. 7 clearly demonstrate that
the absorption peak of MB gradually decreased with irradiation time in
presence of the metal complexes. The fading of color could be simply
detected by naked eye in the photocatalytic process (inset of Fig. 7). The
1
.897(2) Å), the imino nitrogen (Cu–N(1) = 1.952(3) Å), and a
carboxylate oxygen of the propanoate chain (Cu–O(3) = 1.951(2) Å),
completing the basal plane with an oxygen of the naphthalene-2,6-
dicarboxylato anion (Cu–O(5) = 1.967(2) Å). Two of these fragments
are arranged about a crystallographic center of symmetry in such a way
that one propanoate oxygen of each complex coordinates the metal of
the other at apical position (Cu–O(4)’= 2.441(3) Å, O(4)’ at 1-x,-y,1-z).
The copper atom is slightly displaced by 0.03 Å from the best-fit NO
plane towards the apical oxygen donor.
3
The sodium cation presents a six-coordination in a distorted trigonal
prismatic molecular geometry with Na-O distances in a wide range from
plot of Fig. 8 shows the variation of C/C
is the initial concentration of MB and C is the apparent concentration
at different intervals of time).
0
against irradiation time (where
C
0
2
.271(3) to 2.735(3) Å, involving oxygen atoms O(1–6) from three
symmetry related copper complexes. The O-Na-O coordination bond
angles reported in Table 3 are indicative of the highly distorted octa-
The degradation rate estimates that about 63% of MB degradation
occurred after 150 min in presence of complex 1 under UV irradiation,
and 61% for complex 2. Some parallel experiments were conducted on
the degradation of dye solution under the following reaction conditions:
6
hedral geometry. Fig. 6 shows the NaO polyhedra connecting the
polymeric chains of Fig. 5 to form a 2D architecture parallel to the bc
plane. No short phenyl ring interaction with centroid-to-centroid dis-
tance shorter than 4.5 Å is detected in the crystal packing.
(
i) in presence of H
and light irradiation (Fig. S7); (iii) in presence of complexes and light
irradiation in the absence of H (Fig. S8); (iv) in presence of isopropyl
alcohol, H and light irradiation (Fig. S9). Experimental results show
that after 150 min tiny degradation of MB can be detected in presence of
in the dark. The degradation rate under Hg lamp is 30% with H
2 2 2 2
O in the dark (Fig. S7); (ii) in presence of H O
2 2
O
3
.4. Electronic absorption and emission spectra of complexes
2 2
O
The electronic spectra of complexes 1 and 2 were recorded in water.
H
2
O
2
2 2
O
The spectrum of 1 (Fig. S1) shows significant transitions at 215 nm (
ε
~
~
only, and 27 and 22% in the presence of only complexes 1 and 2,
5
ꢀ 1
ꢀ 1
5
ꢀ 1
ꢀ 1
1
9
.65 × 10 M cm ), 235 nm (
ε
~ 1.36 × 10 M cm ), 266 nm (
ε
respectively, but these values increase to 63 and 61% upon addition of
4
ꢀ 1
ꢀ 1
4
ꢀ 1
ꢀ 1
.26 × 10
M
cm ) and 354 nm (
ε
~ 2.84 × 10
M
cm ).
H
2
O
2
(Fig. 8) under UV light irradiation. Photocatalytic activities of the
solutions of mixtures of (i) Cu(ClO O and H L1 (1:1), and (ii) Cu
⋅6H
(NO O and H L2 (1:1) have also been investigated under same
⋅3H
Conversely, the absorption spectrum of 2 (Fig. S1) exhibits significant
4
)
2
2
2
5
ꢀ 1
ꢀ 1
transitions at 237 nm (
ε
~ 1.63 × 10 M cm ), 273 nm (
ε
~ 4.98 ×
3
)
2
2
2
4
ꢀ 1
ꢀ 1
4
ꢀ 1
ꢀ 1
1
0
M
cm ) and 364 nm (
ε
~ 1.12 × 10
M
cm ). Both the
experimental conditions to compare with the catalytic activities with
complexes 1 and 2. Experimental results show that (Fig. S10) after 150
min very small amount of MB degradation were detected (11% for the
complexes display red shifted emission. On excitation at 354 nm com-
plex 1 displays luminescence bands at 407 and 498 nm (Fig. S2),
6

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
6
Fig. 6. (A) The 2D architecture built by NaO polyhedra; (B) perspective view of the 2D supramolecular structure.
Fig. 7. Change in the UV–vis spectra of the degradation of MB in the presence of complex 1 (A) and 2 (B) under UV irradiation. Inset: Color change photograph of MB
solutions in the presence of complexes taken at intervals of 30 min.
7

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
Scheme 2. Formation of 2-hydroxy terephthalic acid in the presence of hy-
droxyl radicals.
Fig. 8. The MB degradation with different photocatalytic conditions.
Fig. 10. The emission spectra of TAOH formed by the reaction of TA with
hydroxyl radicals generated from different photocatalytic system under UV
light for 30 min.
2 2
Where k(H O +1/2) is the rate constant of the complex in presence of H O
2
2
under irradiation, kH O2 is the rate constant in presence of H
2
O
2
under
irradiation without any catalyst, and k1/2 is the rate constant in presence
of complex 1 (or 2) under irradiation without H . The calculated SI
2
2 2
O
values are 1.338 and 1.444 for complex 1 and 2, respectively, which can
Fig. 9. The reaction kinetics of MB over the two complexes at
describe the degree of synergy with the electron acceptor.
different conditions.
solution of Cu(ClO
4
)
2
⋅6H
2
L2. All the above results reveal that the presence of
2
O and H
2
L1; 14% for the solution of Cu
3.9. Reaction mechanism of photocatalytic degradation of methylene blue
(
NO O and H
3
)
2
⋅3H
2
the metal complex is essential for effective degradation of the organic
dye MB, and both the complexes show high photocatalytic activity.
The reaction mechanism for the degradation of MB may proceed via
two possible pathways: (i) a catalytically promoted decomposition by
the Cu(II) complexes to produce hydroxyl radicals (∙OH) through a
Fenton-like reaction, where Cu(II) reacts through a Cu(II)-Cu(I) redox
cycle; [26] (ii) by absorption of energy ≥ the band gap of the Cu(II)
complexes and consequent electron promotion from the valence band
3
.8. Kinetics of the catalytic photodegradation reaction
In order to obtain a better understanding of the reaction kinetics of
+
the MB degradation catalyzed by the complexes, the experimental data
were fitted by Langmuir-Hinshelwood model (a first-order kinetic
model). The value of the rate constant (k) usually provides an indication
of the photocatalytic activity of the complexes. Fig. 9 shows the linear
(VB) to the conduction band (CB), leaving holes (h ) in the valence band
2 2
(eq. (1)). The electrons reduce O to ∙O and finally produce hydroxyl
radicals (∙OH) (eq. (3)). On the other hand photoexcited holes have
ꢀ
strong oxidant ability and can oxidize hydroxyl ions (OH ) or adsorbe
relationship between ln(C/C
0
) against the irradiation time for MB
2 2
organic molecules to ∙OH (eq. (2)). H O is an electron acceptor which
degradation under conditions indicated.
provides more ∙OH by accepting electrons from CB within the photo-
catalytic system (eq. (4)). The formed radical ∙OH possesses the ability
to effectively decompose methylene blue [27].
The rate constants of 1 and 2 for the photodegradation of MB in
ꢀ 1
presence of H O are 0.00629 and 0.00611 min , respectively. The
2 2
result showed that addition of hydrogen peroxide electron acceptor can
easily increase the photo-catalytic activities. The synergy index (SI) was
calculated using the following equation: [23b].
Complex 1/2 + h
ν
→h+ + eꢀ
(1)
(2)
(3)
+
ꢀ
h + OH →⋅OH
SI = k
(H O2+1/2)
2
eꢀ + O
2
→⋅O
2
k
H
O
+ k1/2
2
2
8

A. Paul et al.
Inorganica Chimica Acta 522 (2021) 120346
eꢀ + H
→OHꢀ + ⋅OH
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O
2
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hydroxyterephthalic acid (TAOH), which emits at 426 nm (Scheme 2).
Upon 30 min irradiation of TA (0.5 mM in KOH solution) in the
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