Exploring (bio)catalytic activities of structurally characterised Cu(ii) and Mn(iii) complexes: Histidine recognition and photocatalytic application of Cu(ii) complex and derived CuO nano-cubes

Article abstract of DOI:10.1039/c8dt03007c

Structurally characterised two polymeric complexes of Cu(ii) (3) and Mn(iii) (4) of the ligand L (2-(((2-(phenylamino)ethyl)imino)methyl)phenol) have been explored for catecholase-like activity, fluorescence recognition of histidine and catalytic activity towards coupling of aryl iodides with benzamide, leading to N-arylbenzamides. CuO nano-cubes (NCs) prepared by thermal decomposition of the Cu(ii) complex function as photocatalysts for the degradation of methylene blue. Cube-like morphology of CuO nanocrystal and flake-shaped micrometer order Cu(ii) complex have been established from the field emissive scanning electron microscopy (FESEM) images.

Full text of DOI:10.1039/c8dt03007c

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Exploring (bio)catalytic activities of structurally
characterised Cu(II) and Mn(III) complexes: histidine
recognition and photocatalytic application of Cu(II)
complex and derived CuO nano-cubes†
Cite this: DOI: 10.1039/c8dt03007c
Babli Kumari,a Samapti Kundu,b Kajari Ghosh,a Mahuya Banerjee,a
d
Swapan Kumar Pradhan,b Sk. Manirul Islam, *c Paula Brandão, d Vítor Félix
*
a
and Debasis Das
*
Structurally characterised two polymeric complexes of Cu(II) (3) and Mn(III) (4) of the ligand L (2-(((2-(phe-
nylamino)ethyl)imino)methyl)phenol) have been explored for catecholase-like activity, fluorescence reco-
gnition of histidine and catalytic activity towards coupling of aryl iodides with benzamide, leading to
N-arylbenzamides. CuO nano-cubes (NCs) prepared by thermal decomposition of the Cu(II) complex
function as photocatalysts for the degradation of methylene blue. Cube-like morphology of CuO nano-
crystal and flake-shaped micrometer order Cu(II) complex have been established from the field emissive
scanning electron microscopy (FESEM) images.
Received 23rd July 2018,
Accepted 2nd September 2018
DOI: 10.1039/c8dt03007c
rsc.li/dalton
These facts inspired us to report polymeric complexes of
copper and manganese of a Schiff base derivative and study
Introduction
Schiff’s base-derived copper and manganese complexes are their activities such as molecular sensing, catecholase activity
used as multi-functional materials.1–7 Recently, copper com- and catalytic activity.
plexes of Schiff’s bases have found potential applications as
sensors and in catalytic, anti-viral and anti-bacterial activities. for human growth, particularly for children’s growth and repair
They may deactivate HIV or H1N1 virus also.8 of human tissues.13 His plays a vital role in several proteins
It is well-known that histidine (His) is an essential amino acid
On the other hand, manganese is an essential trace metal through metal–ligand coordination or acid–base catalysis with
for the development and functioning of the brain.9 It acts as a the imidazole moiety collectively with carbonic anhydrase.14
cofactor for several enzymes such as decarboxylase, hydrolase
The unusual level of His-rich protein causes several diseases
and kinase that are involved in the metabolism of protein, in humans.15 The impaired nutritional state of patients with
lipid and carbohydrate.10 Unusual concentration of manga- chronic kidney disease may be ascribed to the deficiency of His.16
nese in brain, especially in basal ganglia, results in neurologi- It is reported as a neurotransmitter or neuromodulator in the
cal disorders identical to Parkinson’s disease.11 In human and mammalian central nervous system.17 It is pertinent to mention
animal tissues, the concentration of manganese is less that the biological importance of His has opened up an active
than 1 µg g−1 wet weight,12 whereas its function is poorly research area for the development of optical probes in recent
understood.
years.18 Therefore, several methods for the determination of His
have been suggested, viz., liquid chromatography,19,20 capillary
electrophoresis,21,22 voltammetry,23 spectrofluorimetry24–30 and
spectrophotometry.31,32 However, recognition of His by fluo-
rescence spectroscopy is demanding due to its operational simpli-
city, rapidity, non-invasiveness and less expensive methodology,
direct and instant visual perception, selectivity and sensitivity.33,34
Several metal complexes have been used as fluorescence and
colorimetric probes for His.35,36 In the light of previous studies,
we have prepared a Cu(II) complex that acts as a turn on, highly
selective probe for His over other amino acids.
aDepartment of Chemistry, The University of Burdwan, Burdwan, 713104 W.B.,
India. E-mail: ddas100in@yahoo.com
bDepartment of Physics, The University of Burdwan, Burdwan, 713104 W.B., India
cDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, India
dDepartment of Chemistry, CICECO – Aveiro Institute of Materials, University of
Aveiro, 3810-193 Aveiro, Portugal
†Electronic supplementary information (ESI) available: Cotains NMR, FTIR,
UV-Vis, fluorescence spectra. CCDC 1052606 (3) and 1827168 (4). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt03007c
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Catechol oxidase catalyses oxidation of a wide range of
o-diphenols (catechols) to corresponding o-quinones.37–44 This
catecholase activity depends on several factors such as metal–
metal distance, structure of ligands, its electrochemical pro-
perties, and pH of the medium. Literature suggests that some
Mn(III),45 Mn(IV)46 and a few Mn(II) complexes47 exhibit catecho-
lase-like activity, whereas phenol-based ligands have rarely been
used. Mohanta et al.48 have reported a series of Mn(III) complexes
that show catecholase activity with turnover (Kcat) numbers in
between 17.0–41.7 h−1 in DMF and 16.9–137.3 h−1 in MeCN.
Several mononuclear49–51 and dinuclear52–55 Cu(II) complexes
show significant catecholase-like activities. However, catecholase
activity of polymeric metal complexes is relatively rare.
Fig. 1
A
single polymeric chain [CuLCl]n of
3
in crystalline state.
On the other hand, the most proficient and potent method
for the synthesis of N-aryl amide is transition metal-catalyzed
cross coupling reaction between aryl halide and amide, and it
has immense importance in biological, pharmaceutical and
material research.56
Thermal ellipsoid drawn at the 50% probability level and the N–H⋯O
hydrogen bonds are drawn as violet dashed lines. *Symmetry operator:
x, 1/2 − y, 1/2 + z.
It is well-known that amidation of aryl halides has efficien-
tly been achieved by using palladium catalyst.57 However,
copper-catalysed amidation is rare.58 The low cost, environ-
mentally friendly nature and easy availability allow copper to
be an attractive catalyst for C–N cross-coupling reactions.
Surface and ground water contamination by organic pollu-
tants released from textile industries is a serious concern and
highly alarming for human and wild life. Photo-catalysis has
emerged as a promising green technique for the degradation of
organic pollutants in wastewater.59 Copper oxide, a p-type semi-
conductor having a narrow band gap (1.2–2.5 eV), acts as an
excellent photo-catalyst for the degradation of organic pollu-
tants.60 Thermal decomposition of copper complex is an in-
expensive, well-known and relatively new method for the prepa-
ration of highly pure CuO nanostructures, which prompted us to
prepare CuO nanostructures from our new Cu(II) complex.61,62
Overall, we report the synthesis of two polymeric complexes
of Cu(II) and Mn(III) using a reported Schiff’s base.63 The struc-
tures of the complexes are confirmed by single crystal X-ray
diffraction analysis. The catecholase-like activities of the com-
plexes have been demonstrated. The Cu(II) complex selectively
recognizes His by fluorescence spectroscopy, supported by
other techniques. Moreover, the Cu(II) complex acts as an
efficient catalyst for coupling several aryl iodides with benz-
amide to yield the respective N-arylbenzamides. Additionally,
CuO nano-cubes (NCs) prepared by thermal decomposition of
the Cu(II) complex function as an efficient photo-catalyst for
visible light-driven decomposition of a representative organic
contaminant: methylene blue (MB) dye.
Fig. 1 and 2 including thermal ellipsoids and [ML2Cl]n poly-
meric chains formed in solid state. Selected bond lengths and
angles around metal centres are summarized in Table 1.
In 3, Cu(II) centres are covalently linked by chloride with
non-equivalent Cu–Cl distances of 2.286(2) and 2.849(3) Å and
an angle Cu–(μ-Cl)–Cu of 105.22(8)°, leading to the formation
of one dimensional polymeric chain with a ladder shape
(Fig. 1). Furthermore, the phenolate and amine groups of the
adjacent ligands (L) interact through almost linear hydrogen
bonds (N⋯O = 2.967(9) Å and ∠N–H⋯O = 170.0°). This
structure has a distorted square-pyramidal geometry with a
structural index of trigonality τ = (β − α)/60° = 0.32, where
α and β are the trans-angles N(11)–Cu–Cl and N(1)–Cu–Cl of
172.2(2) and 152.8(2)°, respectively.64 The basal plane is
composed of nitrogen and phenol oxygen donors from the tri-
dentate L and bridging chloride with shorter Cu–Cl distance.
The apical position is occupied by the remaining bridging
chloride at much longer Cu–Cl distance (ca. 0.6 Å difference)
than that expected for a d9 Cu(II) complex due to Jahn–Teller
distortion.
Results and discussion
Single crystal X-ray structures of Cu(II) complex (3) and Mn(III)
complex (4)
Single crystals of 3 and 4 suitable for X-ray diffraction were
grown from methanol–dichloromethane mixture (1 : 1, v/v).
Fig. 2 A single [MnL2Cl]n polymeric chain of 4 in crystalline state.
*Symmetry operator: −1/2 + x, y, 3/2 − z. Remaining details are given in
The most relevant structural features of 3 and 4 are depicted in Fig. 1.
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Table 1 Selected bond lengths (Å) and angles (°) in 3 and 4
Complex 3
Cu–Cl
Cu–O(3)
Cu–N(11)
2.286(2)
1.912(6)
1.967(6)
107.64(8)
91.8(3)
172.2(2)
97.44(18)
81.7(3)
Cu–Cl(b)
Cu–N(14)
2.849(3)
2.060(7)
Cl–Cu–Cl(b)
O(3)–Cu–N(11)
O(3)–Cu–N(14)
Cl–Cu–N(14)
N(11)–Cu–N(14)
Cl(b)–Cu–N(14)
Complex 4
Cu–Cl–Cu(a)
O(3)–Cu–Cl
N(11)–Cu–Cl
Cl(b)–Cu–O(3)
Cl(b)–Cu–N(11)
105.22(8)
90.32(16)
152.75(19)
94.86(17)
99.26(18)
82.01(19)
Mn–Cl
Mn–O(31)
2.6127(10)
1.864(2)
Mn–Cl(a)
Mn–N(39)
2.6080(10)
2.032(3)
Mn–N(19)
2.034(3)
Mn–O(11)
1.869(2)
Fig. 3 Changes in fluorescence spectra of 3 (5 µM) in HEPES buffer
(0.1 M, ethanol/water, 1/1, v/v, pH 7.4) upon gradual addition of His
(λex = 380 nm; λem = 440 nm).
Mn–Cl–Mn(b)
Cl–Mn–Cl(a)
O(31)–Mn–N(39)
130.39(3)
179.25(4)
90.33(10)
N(39)–Mn–N(19)
O(31)–M–O(11)
O(11)–Mn–N(19)
179.08(11)
178.93(11)
90.71(10)
For complex 3, (a) and (b) stand for symmetry operators x, 1/2 − y, −1/2
+ z and x, 1/2 − y, 1/2 + z, respectively. For complex 4, (a) and (b)
denote the symmetry operators −1/2 + x, y, 3/2 − z and 1/2 + x, y, 3/2 −
z, respectively.
In 4, Mn(III) centres are also covalently assembled in poly-
meric zigzag chains (Fig. 2) by chloride bridges with slightly
different Mn–Cl distances of 2.608(1) and 2.613(1) Å and an
obtuse Mn–Cl–Mn angle of 130.39(3)°. Moreover, each
Mn(III) centre exhibits an octahedral environment with the
chloride bridges occupying the axial positions (∠Cl–Mn–Cl =
179.25(4)°), whereas the four equatorial sites are occupied by
nitrogen and oxygen donors from 2-(iminomethyl) phenol
moieties of two independent L ligands. In other words, in con-
trast to that observed for the Cu(II) complex, both L ligands
adopt bidentate behaviour and therefore, their amino nitrogen
centres are away from the manganese centre. The Mn–O
distance of 1.864(2) Å and Mn–N distances of 2.035(3) and
2.032(3) Å are similar to those observed in the manganese
polymeric complex of 2-ethoxy-6-[([2-(phenylamino)ethyl]
imino) methyl] phenolato (Mn–N, 2.044(2) and Mn–O, 1.862(1)
Å), in which octahedral Mn(III) centres are also linked by chlor-
ide bridges with Mn–Cl distances of 2.617(1) Å.65
Fig. 4 Relative emission intensities of 3 upon addition of different AAs
(100 µM) in said medium, λex = 380 nm.
pH of 7.4 has been selected for entire studies. No significant
interference from other AAs is observed (Fig. S3, ESI†). The
lowest detection limit and binding constant for His are 5.66 ×
10−8 M and 4.34 × 104 M−1, respectively (Fig. S4 and 5, ESI†).
The plot of emission intensity of 3 vs. [His] is shown in Fig. S6
(ESI†), the linear region of which is useful for the determi-
nation of an unknown concentration of His. The fluorescence
quantum yield of 3 increases from 0.24 to 0.72 in the presence
of His.
His extracts out Cu(II) from 3 and releases free ligand.
Hence, emission intensity that is enhanced at 440 nm is due
to free ligand.67
The ESI-MS spectrum of the mixture of His and 3 provides
a molecular ion peak at 371.17 (Fig. S7, ESI†), indicating the
formation of [2His + Cu2+] adduct. An intense peak at 241.53
indicates the release of free ligand. Job’s plot also indicates
the same stoichiometry (Fig. S8, ESI†). The probable sensing
mechanism is illustrated in Scheme 2.
Interactions with amino acids (AA)
The fluorescence response of L (5 μM) towards Cu2+ was tested
in HEPES (20 mM, 0.1 M, ethanol/water, 1/1, v/v, pH 7.4)
(Fig. S1, ESI†). Upon gradual addition of Cu2+, the emission
intensity of L at 440 nm gradually decreases and gets finally
quenched. The fact that Cu2+ complex selectively distinguishes
His from other AAs66 made us test 3 as a potential candidate
for fluorescence recognition of His. The emission intensity of
3 enhances eleven-fold upon interaction with His (λem
=
440 nm, λex = 380 nm) (Fig. 3), whereas other natural AAs (Trp,
Cys, Phe, Gly, Thr, Ile, Pro, His, Arg, Ger, Met, Lys, Leu, Glu,
Gla, Asp, Asn, Gln, Val, Tyr) do not exhibit any interference
(Fig. 4). However, 4 does not show any fluorescence enhance-
Catecholase-like activity
ment upon interaction with these AAs. Emission intensities of Catecholase-like activities of 3 and 4 have been studied using
3 have been examined in the presence and absence of His at 3,5-di-tertiarybutyl catechol (DTBC) for its low redox potential
different pH values (Fig. S2, ESI†). However, the physiological and easy monitoring of absorbance at ∼400 nm (corres-
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Table 2 Kinetic parameters of catecholase-like activity of 3 and 4
Complex
Vmax (M s−1
)
KM (M)
kcat (h−1
)
3
4
2.72 × 10−4
2.79 × 10−4
6.36 × 10−3
1.13 × 10−3
9.79 × 103
1.00 × 104
ponding o-quinone, DTBQ). The rate constant (turn over
number, Kcat), maximum velocity (Vmax) and binding constant
(KM) are calculated from the plot of 1/V vs. 1/[S] (Fig. S9 to S16,
ESI† and Table 2) using the equation 1/V = {KM/Vmax} × {1/[S]}
Fig. 5 Optimization of reaction temperature (iodobenzene, 1.0 mmol;
benzamide, 1.2 mmol; K2CO3, 1.3 mmol; catalyst, 10 mg; time, 18 h;
solvent free). The yield is monitored with GC.
+ 1/Vmax
.
Catalytic activity
On the other hand, solvent also plays a crucial role towards
product yield. Hence, both polar and non-polar solvents have
been screened. However, no significant improvement has been
observed (Table S3, ESI,† entries 1–3). Hence, the reaction is
screened under solvent-free conditions and interestingly,
better efficiency is observed (Table S3,† entry 4). The catalytic
reaction does not occur at room temperature. While gradual
increase of temperature increases the yield, the optimum
temperature has been observed as 110 °C (Fig. 5) (Table 3).
The catalytic activities of 3 towards N-arylation of amide have
been tested by reacting iodobenzene and benzamide under
solvent-free conditions (Scheme 1).
As bases play a key role in the efficiency of the N-amidation
reaction between aryl iodide and benzamide, the reactions are
monitored by varying moderate to strong bases. It is observed
that the yield is highest with K2CO3, which is a mild base
(Table S2, ESI,† entry 5) (Scheme 3).
Nanostructure characterization of CuO nano-cubes (CuO NCs)
Fig. 6 shows the Rietveld analysis of PXRD of CuONCs derived
by calcination of 3. The peak positions match well with the
Table 3 N arylation reaction with Cu catalyst
Entry
1
R
Product
Yield (%)
96
H
2
3
4-OMe
4-Me
89
92
Scheme 1 Synthesis of 3 and 4.
4
5
6
2-Me
89
88
97
2-NH2
4-NO2
Scheme 2 Probable sensing mechanism.
7
4-Cl
78
Reaction conditions: Iodobenzene (1.0 mmol), benzamide (1.2 mmol),
K2CO3 (1.3 mmol), solvent (2.5 mL), catalyst (10 mg), 18 h, 110 °C. GC
yield.
Scheme 3 A model N-arylation reaction using 3 as catalyst.
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to form a bundle. Cube-like CuO nano-crystals have been pro-
duced uniformly after calcination of 3 (Fig. 7b–d). A single dis-
tinct cube shown in the inset of Fig. 7d has side length of
∼214 nm, which is well corroborated with the Rietveld refine-
ment value (Table S4, ESI†). PXRD analysis of the residue
obtained after calcination of 3 indicates CuO. Thus, a morpho-
logical evolution from continuous 2D micro-flakes to 3D cube-
like CuO NCs with preferential growth along <002> has been
observed.
Photo-catalytic activity of CuONCs under visible light
The photo-catalytic activity of CuO NCs for the degradation of
methylene blue (MB), an organic pollutant dye, has been
explored. The absorbance of MB decreases gradually with
increasing time upon visible light illumination (Fig. 8a). In the
presence of CuO NCs, the absorbance of MB reduces to ∼20%
within 4 h. The reaction follows pseudo first order kinetics
ln(C0/C) = kt, where C0 and C are concentrations of MB before
and after visible light illumination, respectively. The first order
rate constant (k) has been found to be 6.84 × 10−3 min−1
(Fig. 8b). The mechanism of photo-degradation of MB is por-
trayed in Scheme 4. The visible solar radiation has much
higher energy than the band gap of CuO photo-catalyst
(∼2.62 eV).
Absorbtion of photons with energies equal to or exceeding
the band gap of CuO NCs generates electron–hole pairs. For
MB degradation, photo-induced holes in the VB readily react
with the hydroxyl ions (hole acceptors) to form hydroxyl rad-
icals (•OH) with the generation of corresponding photo-excited
electrons at CB, which react with dissolved O2 to generate
Fig. 6 Observed (red) PXRD patterns and corresponding Rietveld
powder structure refinement output patterns (black) of CuONCs. (Io − Ic)
is the difference between observed and calculated patterns.
monoclinic polycrystalline CuO phase (JCPDF No. 41-0254;
space group, C2/c, monoclinic; a, 4.685 Å; b, 3.423 Å; c, 5.132 Å
and β, 99.52°). All peaks are indexed accordingly. The well-
resolved sharp peaks indicate crystallite (coherently diffracting
domain) CuO, which is almost free from lattice strain. The
most interesting part in this pattern is the intensity of the
(002) reflection, which is almost the same as the strongest
reflection of CuO powder. The ideal I002 : I111 for CuO powder
is 0.6 (JCPDF No. 41-0254), which becomes 0.95 for Cu NCs,
indicating orientation of cubes along the (002) plane. The
results confirm preferential growth of NCs along <002>, which
is obtained by incorporating the March–Dollase function in
the Rietveld refinement.68 The refined structural and micro-
structural parameters are presented in Table S4 (ESI†).
Determination of surface morphology by FESEM
Field emission scanning electron microscopy (FESEM) has
been employed to determine the morphology and size of
micro-flakes of 3 and CuO NCs derived from it. Fig. 7a reveals
that 3 consists of different micro-flakes agglomerated together
Fig. 7 FESEM images of (a) Cu(II) complex, (b–d) CuO NCs at low and
high magnifications.
Fig. 8 (a) Absorption spectra of MB solution in the presence of CuO
NCs upon visible light irradiation, (b) plot of ln(C0/C) vs. irradiation time.
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X18 rotating anode diffractometer equipped with MAR345
image plate for 3 and on a Bruker SMART Apex II diffract-
ometer equipped with a CCD area detector for 4 using Mo-Kα
(0.71073 Å) radiation.
Size and morphology of CuO NCs are determined through
Rietveld refinement of PXRD pattern and FESEM images.
PXRD is recorded by Bruker D8 advanced diffractometer
having Ni-filtered CuKα radiation within the range 2θ = 20–80°.
Synthesis of 2-(((2(phenylamino)ethyl)imino)methyl)phenol
(L)
Scheme 4 CuO NC-catalysed visible light-induced degradation of MB.
L is synthesized by refluxing equimolar mixture of salicylalde-
hyde and N-phenyl ethylenediamine for 6 h in methanol.63
The filtrate is kept at room temperature for slow evaporation.
After a few days, yellow crystals of L are found. The yield is
86%. The purity of L is checked by ESI-MS (Fig. S17, ESI†) and
1HNMR spectra (Fig. S18, ESI†)
superoxide radical •−O2; it further reacts with H2O to increase
hydroxyl radical (•OH) concentration, which is responsible for
MB oxidation. The procedure is illustrated below.
CuO þ hν ! CuO ðeCBꢀ þ hVBþÞ
Synthesis of Cu(II) complex
•
hVBþ þ H2O ! OH þ Hþ
Methanol solution of Cucl2·6H2O (85.24 mg; 0.5 mmol) is
added slowly to the magnetically stirred solution of L (240 mg;
1 mmol) in dichloromethane at room temperature, and stir-
ring is continued for 30 minutes. The solution is then filtered.
Upon slow evaporation of the filtrate, green crystals suitable
for single crystal X-ray diffraction are obtained. The yield is
80%. Significant crystal data and refinement parameters are
given in ESI (Table S5, ESI†). FTIR (cm−1) (Fig. S19; ESI†): ν(N–
H, 2° amine) 3342.64, ν(CHvN) 1600.92, ν(C–O phenyl)
1307.74; UV-Vis. (Fig. S20, ESI†): λ (nm) in ethanol–water (1/1,
v/v), 361 nm, 269 nm and 232 nm.
eCBꢀ þ O2 ! •ꢀO2
•
ꢀO2 þ Hþ ! HO•2
HO•2 þ HO• ! H2O2 þ O2
2
ꢀ
O•2 þ HO• ! O2 þ HO2
2
HO2ꢀ þ Hþ ! H2O2
H2O2 þ hν ! 2•OH
•
The polymeric [L–Cu2+] complex is used as a precursor to
prepare CuO NCs through calcination at 450 °C for 5 h in air.
The absorption and photoluminescence spectra of CuO NCs
are presented in Fig. S21–22 (ESI†).
MB þ OH ! MB mineralization
The detailed mechanistic investigation on photo-catalytic
bleaching of MB is available in literature.69
In addition to direct CuO photo-excitation, dye degradation
can also proceed via its sensitization of the semiconductor.70
MB adsorbed at the surface of CuO NCs may undergo photo-
excitation (absorption maximum, ∼670 nm) and resultant elec-
tron injection into CuO CB, facilitating charge separation.
Synthesis of Mn(III) complex
Methanol solution of MnCl2·4H2O (95 mg; 0.5 mmol) is added
slowly to a magnetically stirred solution of L (240 mg, 1 mmol)
in dichloromethane at room temperature, and stirring is con-
tinued for 30 minutes. After filtration, the filtrate is kept for
slow evaporation, and intense brown crystals suitable for X-ray
diffraction are found after several days. Yield is 82%. Selected
crystal parameters and refinement data are given in ESI
Experimental
Materials and methods
High-purity HEPES, salicyaldehyde, N-phenyl ethylenediamine, (Table S5†).
CuCl2 and MnCl2 are purchased from Sigma Aldrich (India).
FTIR (cm−1) (Fig. S23; ESI†): ν(N–H, 2° amine) 3331,
All reagents and solvents are of analytical reagent grade and ν(CHvN) 1598.99, ν(C–O phenyl), 1307.74. UV-Vis. (Fig. S24,
used without further purification. Water with a resistivity of ESI†): λ (nm) in ethanol–water (1/1, v/v) 382 nm, 330 nm,
18.2 MΩ cm obtained from a Milli-Q, Millipore water purifi- 242 nm and 222 nm.
cation system (Bedford, MA) is used in all experiments. A
Catalytic oxidation of 3,5-DTBC
Shimadzu Multi Spec 2450 spectrophotometer is used for
recording UV-vis spectra. FTIR spectra are recorded on a To study the catecholase-like activity, 1 × 10−4 mol dm−3 solu-
Shimadzu FTIR (model IR Prestige 21 CE) spectrophotometer. tions of 3 and 4 were treated with 1 × 10−2 mol dm−3 (100
The steady state emission and excitation spectra are recorded equiv.) of 3,5-DTBC in methanol at room temperature under
with a Hitachi F-4500 spectro-fluorimeter. Systronics digital aerobic condition. For both 3 and 4, absorbance of the
pH meter (model 335) is used for pH measurement. Single quinone band increased significantly with time. The kinetics
crystal X-ray diffraction data are collected on a Rigaku Ultra of the reaction was monitored by the initial rate method. The
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concentration of the complex was fixed at 1 × 10−4 mol dm−3
,
Conflicts of interest
whereas that of 3,5-DTBC was varied from 1 × 10−3 to 1 × 10−2
mol dm−3. Thus, 2 mL of 3,5-DTBC solution was quickly There are no conflicts to declare.
mixed with 0.04 mL of 5 × 10−3 mol dm−3 complex solution so
that the final concentration of the complex in the mixture
remained 1 × 10−4 mol dm−3. The rate constants vs. 3,5-DTBC
concentration plot using Michaelis–Menten approach of enzy-
Acknowledgements
matic kinetics results in the Lineweaver–Burk (double recipro- B. Kumari gratefully acknowledges BU for fellowship. S. Kundu
cal) plot. The rate constant (turn over number, Kcat), maximum is thankful to DST for INSPIRE fellowship. K. Ghosh acknowl-
velocity (Vmax) and binding constant (KM) are calculated from edges SERB-DST NPDF (PDF/2016/000160) for financial assist-
the plot of 1/V vs. 1/[S] using the equation 1/V = {KM/Vmax} × ance. The crystallographic studies were supported by
{1/[S]} + 1/Vmax
.
CICECO-Aveiro Institute of Materials (UID/CTM/50011/2013),
financed by National Funds through the FCT/MEC and co-
financed by QREN-FEDER through COMPETE under the
PT2020 partnership agreement.
Photo-catalytic activity
Visible light-induced CuO NC-assisted photo-degradation of
MB dye has been studied. CuO NCs (30 mg) are added to MB
dye solution (1.2 × 10−4 M, 100 mL) and stirred in the dark for
30 min to establish the adsorption–desorption equilibrium of
MB dye. After recording the absorbance at zero time, the solu-
tion is then irradiated with visible light. The absorbance of the
solution is monitored at 30-minute intervals, and the percen-
tage of dye degradation is calculated using the formula: degra-
dation = [(C0 − Ct)/C0] × 100%, where C0 and Ct are the absor-
bances of MB dye solution at zero and after “t” time,
respectively.
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