Synthesis and structure of [Et3NH][Fe(HL)2] [H3L = L -2-(3,5-Di-tert-butyl-2-hydroxybenzylamino)succinic acid] and its catalytic activity towards efficient photodegradation of dyes in the presence of H2O2

Article abstract of DOI:10.1002/ejic.201402314

A new biogenic potentially tetradentate ligand, L-2-(3,5-di-tert-butyl-2-hydroxybenzylamino)succinic acid, has been synthesized. Upon reaction with FeCl₃ in the presence of triethylamine, it afforded the complex [Et₃NH][Fe(HL)₂] (1). The complex was structurally characterized and was used for homogeneous photocatalytic degradation of methylene blue (MB), malachite green (MG), crystal violet (CV) and rhodamine B (RhB) under visible-light irradiation in aqueous solution in the presence of H₂O₂.

Full text of DOI:10.1002/ejic.201402314

FULL PAPER
DOI:10.1002/ejic.201402314
Synthesis and Structure of [Et3NH][Fe(HL)2] [H3L = -2-
L
(3,5-Di-tert-butyl-2-hydroxybenzylamino)succinic Acid]
and Its Catalytic Activity towards Efficient
Photodegradation of Dyes in the Presence of H2O2
Sohaham Dasgupta,[a] Sanghamitra Atta,[a] N. D. Pradeep Singh,*[a]
Dibakar Deb,[a] W. Scott Kassel,[b] and Manish Bhattacharjee*[a]
Keywords: Photodegradation / Green chemistry / Dyes/pigments / Iron / Amino acids / Biomimetic synthesis
A new biogenic potentially tetradentate ligand,
L
-2-(3,5-di-
homogeneous photocatalytic degradation of methylene blue
(MB), malachite green (MG), crystal violet (CV) and rhod-
amine B (RhB) under visible-light irradiation in aqueous
solution in the presence of H2O2.
tert-butyl-2-hydroxybenzylamino)succinic acid, has been
synthesized. Upon reaction with FeCl3 in the presence of tri-
ethylamine, it afforded the complex [Et3NH][Fe(HL)2] (1).
The complex was structurally characterized and was used for
noted that, although [Fe(Pcs)] is available commercially, it
is not very cheap. Thus, reports on efficient photocatalysts
for degradation that contain iron complexes of environmen-
tally friendly biomimetic ligands are not numerous and the
development of such complexes still remains a challenge.
Among the various organic pollutants, different synthetic
dyes are widely used to colour products in many industries
such as textile, paper, rubber, plastics and cosmetics. Such
colouring agents, when mixed with water and used for in-
dustrial processes, produce highly coloured effluent. These
common industrial pollutants are subsequently discharged
mostly into surface water resources. Even at low concentra-
tions, dyes can be detected visually and their presence in
surface water affects aquatic life.[16] These highly persistent
coloured compounds are not only the first visually detect-
able contaminant present in the water; they also inhibit the
admission of sunlight into water and thus affect photosyn-
thesis by aquatic plants.[17] Hence, it has become essential
to develop a suitable process that can mineralize these kinds
of contaminants present in wastewater.
Previous studies suggest that a complex with broad ab-
sorption in the visible region can be utilized in the photo-
Fenton reaction to harvest visible light. It should be noted
that some of the reported complexes employ ligands that
are not environmentally benign. Thus there is a need to
develop iron complexes that contain environmentally
friendly ligands and that show ligand-to-metal charge-
transfer (LMCT) transitions in the visible region. Therefore
we thought of synthesizing a ligand based on a naturally
occurring amino acid that contained a phenolate group.
Herein we report the synthesis of a new amino acid based
ligand, l-2-(3,5-di tert-butyl-2-hydroxybenzylamino)suc-
cinic acid (LH3), and its iron complex, [Et3NH][Fe(LH)2]
Introduction
Iron-promoted photo-Fenton systems have been used for
the photodegradation of pollutants such as phenols,[1–5]
chlorophenoxy herbicides[6] and different dyes[7,8] because
they exhibit an accelerated rate of oxidation.[9] The two im-
portant steps that occur in photo-Fenton processes are (i)
photoreduction of iron(III) to iron(II) by photoinduced
metal–heteroatom bond cleavage[10] or by ligand-to-metal
charge transfer[11] and (ii) reaction of H2O2 with iron(II) to
produce a reactive hydroxyl radical. Photo-Fenton pro-
cesses that involve various iron complexes such as ferrioxal-
ate/H2O2[12] by using UV radiation have been used for effec-
tive photodegradation of wastewater and organic pol-
lutants. Since the major portion of sunlight falls in the vis-
ible region, utilization of visible light to degrade pollutants
is highly desirable. Recently, complexes such as iron tetra-
sulfophthalocyanine, [Fe(PcS)], iron(II) bipyridine complex,
[Fe(bipy)3]2+, and iron salen complex, [FeIII–salen]Cl [salen
= 2,2Ј-ethylenebis(nitrilomethylidene)diphenol], in the pres-
ence of hydrogen peroxide have been shown to be effective
catalysts for the photodegradation of organic pollutants un-
der visible light.[13–15] It has been mentioned in the litera-
ture that [Fe(PcS)] is the best choice, since it is a biomimetic
catalyst and can be anchored to Amberlite. It might be
[a] Department of Chemistry, Indian Institute of Technology,
Kharagpur 721302, India
E-mail: mxb@iitkgp.ac.in;
ndpradeep@chem.iitkgp.ernet.in
http://www.chemistry.iitkgp.ac.in/~mxb/
http://www.chemistry.iitkgp.ac.in/faculty/NDPS/
[b] Department of Chemistry, Villanova University,
Villanova, PA 19085, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402314.
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Scheme 1. Structure of cationic dyes used in the present study.
(1), as well as studies on how compound 1 catalyzed the
The 1H NMR spectrum (CD3OD) (Figure S2 in the Sup-
visible-light-induced photodegradation of four cationic porting Information) of the LH3 shows two signals at δ =
dyes, methylene blue (MB), malachite green (MG), crystal 1.30 (s, 9 H) and 1.42 ppm (s, 9 H) owing to the protons
violet (CV) and rhodamine B (RhB), which are widely used of two –C(CH3)3 groups. The signals for the diastereotopic
as industrial dyes (Scheme 1).
methylene protons of the –CH2COOH appear at δ = 2.85
(dd, J = 18, 8.4 Hz, 1 H) and 3.03 ppm (dd, J = 16, 4 Hz,
1 H). The signal for the –CH proton of the –NH(CH) group
appears at δ = 3.95 ppm (dd, 1 H, J = 16, J = 4 Hz). The
signal for the diastereotopic methylene proton of the –
CH2Ph group appears at δ = 4.23 (d, 1 H, J = 12.8 Hz)
and 4.34 ppm (d, 1 H, J = 13.2 Hz). The signals due to the
aromatic protons appear at δ = 7.21 (d, J = 1.6 Hz) and
7.38 ppm (d, J = 1.6 Hz).
The UV/Vis spectrum of LH2Na in methanol shows two
bands at 230 (ε = 2738) and 273 nm (ε = 1880) owing to
the π–π* transition (Figure S3 in the Supporting Infor-
mation). The FTIR spectrum of LH2Na shows, along with
other peaks, aliphatic C–H stretching at 2960 cm–1 and νas-
(COO) stretching at 1583 and νs(COO) at 1480 cm–1 (Fig-
ure S4 in the Supporting Information). The circular di-
chroism spectrum of LH2Na shows a signal with a positive
Cotton effect at 220 nm and one signal with a negative Cot-
ton effect at 254 nm owing to π–π* transitions (Figure S5
in the Supporting Information).
Results and Discussion
Synthesis and Characterization of Ligand LH3
The sodium salt of the ligand l-2-(3,5-di-tert-butyl-2-
hydroxybenzylamino)succinic acid was synthesized by a
one-pot Mannich reaction of l-aspartic acid, formaldehyde
and 2,4-di-tert-butyl phenol in the presence of sodium
hydroxide (Scheme 2).
The ligand is soluble in methanol and ethanol and spar-
ingly soluble in water. The diacid form was obtained by
acidification of the aqueous solution of the sodium salt by
dilute acetic acid; the resulting form was insoluble in water
and soluble in methanol, ethanol and dimethyl sulfoxide
and sparingly soluble in chloroform. The ligand was char-
1
acterized by H NMR spectroscopy, UV/Vis spectroscopy,
IR and CD spectroscopy, elemental analysis and high-reso-
lution mass spectrometry (HRMS).
The elemental analyses agree well with the proposed
composition. The HRMS of NaH2L shows a molecular-ion
peak at m/z 350.1850, which corresponds to H2L–
(C19H28NO5). The isotope pattern also matches the compo-
sition. Along with this peak another peak appears at m/z
Synthesis and Characterization of Complex
[Et3NH][Fe(HL)2] (1)
Ligand H3L, upon treatment with an aqueous solution
382.1749 (Figure S1 in the Supporting Information). This of FeCl3 in methanol and triethylamine in a 2:1:2 ratio,
corresponds to the methanol adduct of the ligand yielded the mononuclear complex 1.
(C20H32NO6). The isotope pattern matches the methanol
The complex is soluble in common organic solvents and
adduct of the ligand. Thus the elemental analysis and the water. The iron complex was characterized by IR and UV/
mass spectrometry support the formulation of the com- Vis spectroscopy and elemental analyses and its electro-
pound.
chemical properties were studied by using cyclic voltamme-
Scheme 2. Synthesis of l-2-(3,5-di-tert-butyl-2-hydroxybenzylamino)succinic acid.
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try and differential pulse voltammetry (DPV) and mag- nitrogen atoms, two phenoxide oxygen atoms and two carb-
netic-susceptibility measurements. The elemental analyses oxylate oxygen atoms from two ligands. Therefore, although
agree well with the proposed composition. The IR spectrum the ligand is potentially tetradentate, it acts like a tridentate
of the compound along with other bands shows a broad ligand with a pendant protonated carboxylic acid group.
band at 3445 cm–1 that is due to hydrogen-bonded O–H Two secondary nitrogen atoms (N1, N2) and two phenolate
stretching of the free carboxylate of the monomer that is oxygen atoms (O1, O6) occupy the corners of the equatorial
involved in dimer formation (see below). The other sharp plane of the coordination polyhedron and the axial posi-
band at 1628 cm–1 is due to the stretching of the coordi- tions are occupied by the two carboxylate oxygen atoms,
nated carboxylate to metal centre. The marked shift of the O2 and O7. Unlike tridentate Schiff base ligands, HL2– in
carboxylate stretching from those of the ligand clearly 1 adopts the facial coordination mode, presumably on ac-
shows that the ligand is bonded to the metal centre through count of its stereochemical flexibility. The iron centre is in
the carboxylate oxygen (Figure S6 in the Supporting Infor- a highly distorted octahedral environment as evidenced by
mation). The UV/Vis spectrum of a solution of the complex the observed bond angles [ЄO6–Fe1–O1 = 97.90(12)°,
in methanol shows an absorption at 520 nm (ε = 20400) ЄO1–Fe1–N1 = 86.62(12)°, ЄN1–Fe1–N2 = 90.11(12)°,
along with a shoulder at 335 nm (ε = 32370). These two ЄO2–Fe1–O7 = 157.17(13)°]. The bond lengths Fe1–O2
transitions can be attributed to the phenolate-to-metal [2.037(3) Å] and Fe1–O7 [2.037(3) Å] are comparable to
charge-transfer transitions. Along with these two bands, those observed in the iron complexes of related ligands
two strong absorptions are observed at 290 (ε = 36310) and (Fe–Ocarboxylate ≈ 1.95–2.09 Å)[19–21] The iron phenolate
260 nm (ε = 36220). These bands are due to intraligand π– oxygen bond lengths, Fe1–O1 [1.900(3) Å] and Fe1–O6
π* transitions (Figure S7 in the Supporting Information). [1.883(3) Å], are similar to those reported for the six-coordi-
The CD spectrum of the compound in methanol shows a nate high-spin iron(III) phenolate complexes. Typically,
band with a negative Cotton effect at 210 nm and two high-spin iron(III)–phenolate distances range from 1.88 to
bands with a positive Cotton effect at 245 and 275 nm. A 1.93 Å.[21–24] Likewise, the iron–nitrogen bond lengths, Fe1–
comparison with the ligand CD spectrum indicates that N1 [2.187(4) Å] and Fe1–N2 [2.217(3) Å], are comparable
these signals are due to an intraligand π–π* transition. In to those observed in similar complexes (Fe–Namine ≈ 2.16–
addition to these intraligand transitions, two bands with a 2.38 Å) in octahedral geometry.[19,20]
negative Cotton effect appear at 290 and 330 nm and a
band with a positive Cotton effect appears at 530 nm (Fig-
ure S8 in the Supporting Information). These bands might
be assigned to ligand-to-metal charge-transfer transitions.
The electron-transfer behaviour of the complex has been
studied in acetonitrile by using cyclic voltammetry. The cy-
clic voltammogram of the complex is not very well resolved;
however, differential pulse voltammetry shows three oxidat-
ive responses at 0.80, 0.97 and 1.31 V (Figure S9 in the Sup-
porting Information). These oxidative responses are attrib-
uted to the following oxidation process. The first and sec-
ond responses are due to the ligand-based oxidation (as
both have the same ionization potential (ip) value and have
a similar potential to known one-electron oxidation pro-
cesses under identical conditions), which is phenoxide-to-
Figure 1. ORTEP view of [FeL2]– cation. Hydrogen atoms and
phenoxyl-radical oxidation and can be utilized to generate
mono- and bis-iron(III) phenoxyl radical. The third re- [Et3NH]+ have been omitted for clarity.
sponse might be due to FeIII Ǟ FeIV oxidation. The room-
temperature magnetic moment of the compound, measured
The proton on the nitrogen of [Et3NH]+ (N3H) shows a
by the Gouy method, was found to be 5.9 BM, which is hydrogen-bonding interaction with one of the iron-bound
typical of a high-spin iron(III) centre.
carboxylate oxygen atoms, (O8), [N3···O8 2.814(5) Å and
H3···O8 1.926 Å]. Apart from this, intramolecular hydrogen
bonding exists between the hydrogen atoms on the ligand
nitrogen atoms, N1 and N2, and oxygen atoms, O4 and
O10, of nonbonded carboxylic acid groups, respectively. In
Single-Crystal X-ray Structure of 1
The compound crystallizes in chiral orthorhombic space addition, intermolecular hydrogen bonding exists in the so-
group P21212. The asymmetric unit consists of an iron cen- lid state between two molecules. Two molecules are
tre coordinated to two protonated ligands and a triethyl- hydrogen-bonded with each other through hydrogen-bond-
ammonium ion (Figure 1). The observed absolute structure ing interactions between the free –COOH groups. The
(Flack) parameter [0.03(2)][18] clearly shows that the coordi- hydrogen bonded to the free carboxylic acid group, H4, of
nates correspond to the absolute structure of the molecules one molecule is hydrogen-bonded with the –OH oxygen of
in the crystal. The iron(III) centre is coordinated to two –COOH, O10, of the second molecule. Similarly, the
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hydrogen atom of the free –COOH group of the second at regular time intervals, which is shown in Figure 3 (A).
molecule, H10, is hydrogen-bonded with O4 of the first The kinetics of the degradation of MB under various degra-
molecule [H–O10 1.8200 Å; O4–O10 2.536(6) Å; ЄO4–H4– dation conditions is depicted in curves 1–6 of Figure 3 (B).
O10 142.00°]. Thus two molecules form a double-helical
structure through hydrogen bonding (Figure 2). It should
be noted that hydrogen-bonded helical structures are not
numerous in coordination compounds.
Figure 3. (A) Absorption spectra of MB dye during the degrada-
tion under visible light in the presence of 1 (8.5ϫ10–5 m) + H2O2
(3 mm) at different times. (B) Degradation kinetics of MB dye
(60 mgL–1) under different conditions: (1) Only dye + light; (2) dye
+ 1 + light; (3) dye + 1 + H2O2 in the dark; (4) dye + H2O2
+
light; (5) dye + 1 + H2O2 + iPrOH + light; (6) dye +FePcS + H2O2
+ light; (7) dye + 1 + H2O2 + light; and (8) dye + Fe–salen + H2O2
+ light.
When the aqueous solution of only MB and that of MB
and 1 were exposed to visible light, no degradation could
be observed (Figure 3, B, curves 1 and 2). Similarly, we
could detect only 5% degradation of MB in the presence of
1 and H2O2 in the dark (Figure 3, B, curve 3). When an
aqueous solution of MB and H2O2 was irradiated with vis-
ible light, a very small amount (10%) of degradation was
observed (Figure 3, B, curve 4). However, when an aqueous
solution of MB, 1 and H2O2 was irradiated with visible
light, the degradation was found to be efficient (Figure 3,
B, curve 7, 90% after 4 h) and displayed first-order reaction
kinetics with a rate constant of k = 5.36ϫ10–4 s–1 (Fig-
ure 4).
Figure 2. View of the inter- and intramolecular hydrogen bonding
in 1. Colour code: Fe, yellow; O, red; N, light blue; C, grey; H, red;
hydrogen bonding, blue dotted line.
Degradation of Methylene Blue (MB) Dye Using the
Iron(III) Complex 1
The UV/Vis spectrum of 1 shows a strong charge-trans-
Hence the experimental results suggest that the iron cata-
fer band in the visible region, which led us to examine the lyst and H2O2 are necessary ingredients for the degradation
efficacy of 1 as a catalyst for the photodegradation of dyes. reaction under visible-light irradiation. Again, to verify the
As a test case we first examined 1-catalyzed degradation of formation of reactive oxygen species, hydroxyl radicals, in
methylene blue (MB). The UV/Vis spectrum of MB in water this photo-Fenton process, the degradation reaction was
shows three absorption peaks at 246 and 291 nm and a studied in the presence of a well-known hydroxyl radical
high-intensity peak at 663 nm. The peaks at 291 and scavenger, 2-propanol, under visible-light conditions. It was
663 nm were used for the analysis during decolourization found that in the presence of 2-propanol, degradation of
and degradation of MB. The change in the absorption spec- MB was lower (35%, 4 h, Figure 3, B, curve 5). This experi-
tra of the solution was monitored by UV/Vis spectroscopy mental result indicates that hydroxyl radical generated in
Figure 4. Degradation kinetics [ln(C0/Ct) versus time] of MB, CV, RhB and MG dye.
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the reaction medium is the key species responsible for the absence of 1. But, as expected, with an increase in the con-
degradation. centration of 1, the percentage of degradation was found to
We have compared the degradation efficiency of 1 with increase and reached a maximum value at a concentration
that of two reported iron complexes, [Fe(PcS)] and [FeIII of 8.5ϫ10–5 m (complete degradation: 4 h). With a further
–
salen] (Figure 3, B, curve 6 and 8, respectively) and the effi- increase in the concentration of the catalyst, the percentage
ciency of 1 was found to be comparable to that of [Fe(PcS)] of degradation of MB remained almost unchanged. Thus
and [FeIII–salen].
for subsequent photodegradation experiments 8.5ϫ10–5
m
catalyst was used as the optimum catalyst concentration.
Furthermore, to understand the effect of H2O2 and light,
the iron complex (without dye) was irradiated in the pres-
ence of H2O2 under visible light. The irradiated complex
solution was analyzed by UV/Vis spectroscopy (Figure 6,
B), which shows a gradual decrease in the absorbance at
500 nm and gradual increase in the absorbance in the
400 nm region, which indicates the formation of iron-coor-
dinated phenoxyl radical species.[26]
Effect of Hydrogen Peroxide on Degradation
In the Fenton system, concentration of H2O2 has an im-
portant role in the generation of hydroxyl radicals. To find
out the optimum concentration of H2O2 required, the newly
prepared iron complex (8.5ϫ10–5 m) was used in the photo-
degradation of MB (1.87ϫ10–4 m) under visible light in the
presence of different amounts of H2O2 (1–8 mm). It has
been observed that with an increase in the concentration of
H2O2, the rate of degradation of MB increases. The maxi-
mum degradation (90%, 4 h) has been observed at a 3 mm
concentration of H2O2, and with a further increase in the
concentration, the rate of the degradation decreases (Fig-
ure 5). This is because of the fact that in the presence of a
·
low concentration of H2O2, the formation of the OH radi-
cal is lower, thus leading to the inefficient degradation.
Similarly, when a high concentration of H2O2 is used, the
·OH radicals produced react with the excess amounts of
H2O2 to produce ·OOH radical and H2O, and consequently,
the rate of degradation decreases.[25] Thus the optimum
concentration of H2O2 was found to be 3 mm for the degra-
dation of MB (1.87ϫ10–4 m).
Figure 6. (A) Effect of different concentration of iron catalyst on
the degradation of MB. (B) Change in iron complex alone by H2O2
in the presence of light.
After visible-light irradiation of the complex for 30 min
in the presence of H2O2, the solution was treated with con-
centrated hydrochloric acid, the ligand was extracted with
chloroform and upon evaporation the chloroform solution
afforded a white solid. The 1H NMR spectrum of the solid
was recorded and the spectrum was found to be exactly the
same as that of the ligand. Thus it is clear that there is no
degradation of the ligand. Also, it has been shown that this
kind of ligand produces stable metal-coordinated phenoxyl
radical.[26]
Figure 5. Effect of different concentrations of hydrogen peroxide
on the degradation of MB after 2 h.
Effect of pH on Degradation
It has been shown that the pH of the solution also has
an important role in degradation.[27,28] To determine the ef-
fect of pH on the degradation reaction, first the stability of
the iron complex was determined under different pH condi-
Effect of the Concentration of the Iron Complex on
Degradation
Since both iron(II/III) and H2O2 are responsible for the tions. It was found that the newly prepared iron complex is
production of the highly oxidizing species, hydroxyl radical, stable in the pH range of 3–10. Therefore photodegradation
in any Fenton or photo-Fenton system, in addition to the of MB was examined in the pH range of 3.12 to 9.88 (Fig-
concentration of H2O2, the concentration of iron species is ure 7).
also important. Therefore, to determine the optimum con-
In all experiments, the pH of the solution was adjusted
centration of the catalyst, the photodegradation of MB by adding an appropriate amount of NaOH or HCl solu-
(1.87ϫ10–4 m) was carried out, by varying the concentra- tion. It was observed that with an increase in pH, from 7.0
tion of 1 (0–9ϫ10–5 m) in the presence of H2O2 (3 mm) to 9.82, the percentage of degradation also increases. This
(Figure 6, A). From the experimental results it was ob- is probably because of the fact that in an alkaline medium
served that after a long period of irradiation, the percentage the iron complex becomes a negatively charged species and
degradation of MB was very low (only 10%, 4 h) in the as the dye MB is cationic in nature, there is an electrostatic
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less availability of the hydroxyl radicals in the reaction me-
dium. The degradation rate of MB has also been found to
decrease in the presence of sodium carbonate because it acts
like a free-radical scavenger.[30]
Detection of Active Oxidant Species
In any photo-oxidative degradation reaction, the detec-
tion of reactive oxygen species involved is very important
as it helps to understand the degradation mechanism and
the pathways. Hence, to search for the reactive oxygen spe-
cies produced in the present degradation study, a series of
reactions was conducted by using different selective scaven-
gers. Some specific agents such as 2-propanol (2-propanol/
H2O = 1:20), methanol (CH3OH/H2O = 1:4) and benzo-
quinone (BQ; c = 1 mmolL–1) were added to the reaction
solution separately to detect the active species in the degra-
dation process. No significant change in the degradation
rate was observed when BQ (O2·– quencher) was added into
the reaction medium. Thus involvement of superoxide radi-
cal anions (O2·–) can be ruled out. However, in the presence
of 2-propanol, a hydroxyl radical scavenger, the rate of de-
gradation was found to decrease drastically (only 35% de-
gradation of MB occurred over 4 h). This indicates that hy-
droxyl radicals are the key active oxidant that is responsible
for the degradation. The same thing was observed when
another hydroxyl radical scavenger, methanol, was used.
For a better understanding, we carried out spin-trapping
EPR experiments using 5,5-dimethyl-1-pyrroline N-oxide
(DMPO) as a spin trap. Unfortunately, no characteristic
peaks of the DMPO–OH adduct were detected. Similar re-
sults are also reported in the literature.[35]
Figure 7. Effect of pH of medium on the degradation of MB.
interaction between the catalyst and MB that facilitates
them coming in close proximity.[29] The degradation rate of
MB was also investigated at pH 6.12, and at this pH it was
observed that the degradation rate was higher than that un-
der the neutral (pH ≈ 7) conditions. The percentages of de-
gradations were found to be 85.2, 90.4, 84.9 and 76.6% at
pH 3.45, 9.88, 6.12 and 7.12, respectively, after 2 h of irradi-
ation. It can be seen that the rate of degradation increases
with a decrease in pH from 6.95 to 3.45. This is due to the
fact that generation of FeII increases with the decrease in
pH. Thus it can be concluded that at constant time, the
percentage of degradation varies with the pH of the solu-
tion.[30]
Effect of Salts on Degradation
In the textile industry, various types of auxiliaries are
employed with dye materials in colourization processes.
Therefore textile waste streams contain elevated levels of
other salts and ions in addition to dyes.[31] To evaluate the
individual effect of added salts in the decolourization and
degradation of dyes, photodegradation of MB by 1 was car-
ried out in the presence (0.01m) of sodium salts of chloride,
nitrite, nitrate and carbonate ions while maintaining all
other experimental conditions fixed as before.
The effect of different added ions in the degradation of
MB is shown in Figure 8. It has been observed that, in the
absence of any salt, MB degrades 90.4% after 2 h of irradia-
tion but in the presence of sodium salts of chloride and
nitrite the percentage of degradation decreases. Similar ob-
servations were also made in some recent studies.[32–34] Gen-
erally, anions such as chloride and nitrite react with the
reactive oxygen species, hydroxyl radicals. This results in
Furthermore, to confirm the formation of hydroxyl radi-
·
cals in the present system, the detection of OH radicals
was carried out by means of the benzoic acid hydroxylation
method.[36] Benzoic acid is known to react with hydroxyl
radical and form salicylic acid, which reacts with iron(III)
to produce a violet-coloured salicylatoiron(III) complex
that shows an intense band at 520 nm. Thus, benzoic acid
was added to a Pyrex tube. Hydrogen peroxide and 1 were
added to this and the reaction solution was irradiated with
visible light. At intervals of 20 minutes the solution was
analyzed by means of UV/Vis spectroscopy (Figure 9). The
Figure 9. Absorption spectra of tetraaquosalicylatoiron(III) com-
Figure 8. Effect of different salts on the degradation of MB after plex generated by complexation of iron(III) by salicylic acid at dif-
3 h of irradiation.
ferent irradiation times.
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spectrum shows a characteristic peak of the salicylate com- acts with hydrogen peroxide to produce the hydroxyl radical
plex of iron at λmax = 520 nm.[35] It was found that the in- that is responsible for MB degradation. A similar kind of
tensity of the peak increases with an increase in irradiation mechanism has been predicted in the 5-sulfosalicylic acid
time of photolysis (Figure 9). Thus it is clear that the hy- complex of iron(III).[38]
droxyl radical produced reacts with benzoic acid to afford
salicylic acid, which forms a complex with iron(III).
The FTIR Analysis of the Intermediates During the
Photodegradation of MB
Probable Degradation Mechanism
To study the degradation mechanism of the dyes, the in-
The control experiments clearly indicate that the hy-
termediates produced during the photodegradation reaction
droxyl radical is the oxidizing agent in the 1-catalyzed pho-
of MB at different reaction times were examined by means
todegradation of MB by hydrogen peroxide. To gain insight
of infrared spectroscopy (Figure S10 in the Supporting In-
into the reaction, we carried out experiments to establish
formation). In this experiment, a high-concentration aque-
the generation of iron(II) in the reaction. Thus, the aqueous
ous solution of MB was chosen for degradation to observe
solution of 1 (8.50ϫ10–5 m) in the presence of 3 mm of
the clear change during degradation. The FTIR spectrum
H2O2 was irradiated under visible light. After a certain in-
of pure MB shows characteristic bands of the C=N ring
terval of time, a measured (2 mL) amount of aliquot was
stretch at 1600 cm–1, C=C at 1491 cm–1, the symmetric
taken and mixed with an aqueous solution of 1 (2 mL) and
C–N stretch at 1395 cm–1 and the N–CH3 accompanied
10-phenanthroline and the mixture was preserved in the
with CH3 bending vibrations at 2815, 2861 and 2949 cm–1.
dark for 1 h. Then UV/Vis spectra of the solutions were
The band for the C–S stretch appears in the 700–570 cm–1
recorded. The concentration of FeII ions was determined
region.[39,40] The band at 2861 and 2815 cm–1 gradually de-
from the absorbance at 510 nm.[37] The experimental results
creases with an increase in irradiation time. This phenome-
reveal that under visible-light irradiation the iron(III) com-
non has also been found in earlier reports to indicate de-
plex can produce an iron(II) complex in the presence of
methylation of the –N(CH3)2 group that is present in MB
H2O2.
dye. During degradation, a broad band in the range of
In the literature it has been shown that dyes can absorb
3500–3400 cm–1 was observed, which might be due to the
visible light and become excited into a higher-energy state
formation of various functional groups such as –NH2,
from which they can transfer an electron to iron(III) to pro-
–COOH and –OH. The FTIR spectrum after 4 h of irradia-
duce iron(II) species. However, it is clear from the experi-
tion (required for complete decolourization) shows the dis-
mental results that in this case iron(II) is produced from the
appearance of the characteristic peaks at 1600 cm–1 (for
iron(III) complex in the reaction medium in the presence of
C=N), at 1491 cm–1 (for C=C) and peaks in the range of
light. The iron(II) complex produced reacts with H2O2 and
700–570 cm–1 (responsible for the C–S bond). A new peak
generates hydroxyl radicals by means of a classical Fenton
appears at 1648 cm–1 that might be due to the formation of
reaction. These hydroxyl radicals are responsible for the de-
gradation of organic dyes.
+ [38,40]
an amide group or R–NH3
.
The results obtained
from the FTIR study indicate that the cleavage of the core
ring structure of the MB dye occurs along with demethyl-
ation.
On the basis of the degraded products of MB and the
information from previous reports, a probable mechanistic
pathway of degradation has been proposed that is shown in
Scheme 3. We suggest that the iron(III) complex 1 under
visible-light irradiation undergoes internal electron transfer
to produce an iron(II)-coordinated phenoxyl radical and
the iron(II) complex produced in the reaction medium re-
Probable Degradation Products of Methylene Blue
To identify the degradation end products, liquid
chromatography–mass spectrometry (LC–MS) analysis was
carried out after the complete photodegradation reaction
of MB solution. Identification of all compounds present in
the solution could not be carried out. The LC–MS spec-
trum (Figure S11 in the Supporting Information) shows an
intense peak at m/z 123 owing to the formation of N-meth-
ylaniline as a major product. The peak at m/z 108 revealed
the presence of another degraded product, 1,4-diamino-
benzene. Other small peaks at m/z 73, 85 and 100 showed
that newly prepared iron(III) complex 1 can efficiently de-
grade the organic dye MB.[30,41] On the basis of the degra-
dation products (identified by LC–MS) it has been found
that the MB dye undergoes successive demethylation fol-
lowed by aromatic ring cleavage during the degradation
process.
Scheme 3. Proposed mechanism for the degradation of dye by com-
plex 1 under visible light.
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Degradation of Other Dyes Such as Crystal Violet (CV),
Malachite Green (MG) and Rhodamine B (RhB) by Using 1
To further prove the versatility of the complex in photo-
degradation, the degradation of CV, MG and RhB dyes was
carried out in the presence of 1. A 1.42ϫ10–4 m solution
of CV was used for photodegradation under same reaction
conditions as described above for the degradation of MB.
The UV/Vis spectrum of CV shows a peak at 585 nm. The
change in the absorption spectra of CV during photocata-
lytic degradation is shown in Figure 10 (A). The kinetic
study under different conditions is represented in curves 1–
6 of Figure 10 (B). On the basis of the results it was found
that the complete decolourization of CV took place in
3.5 hours under the optimum conditions.
Figure 12. (A) Absorption spectra of RhB during the degradation
under visible light in the presence of 1 with H2O2 at different times.
(B) Degradation RhB under different conditions: (1) Dye + light;
(2) dye + 1 + light; (3) dye + 1 + H2O2; (4) dye + H2O2 + light;
(5) dye + 1 + H2O2 + iPrOH + light; and (6) dye + 1 + H2O2 +
light.
the TOC of the initial solution (dye/catalyst/H2O2) and de-
graded solution were measured before and after photode-
gradation. Then the percentage of mineralization was ob-
tained for different dye solutions from the difference of ini-
tial and final TOC values. From this result it was observed
that TOC values dropped by 31.5, 43.5, 46.7 and 87% for
MB, MG, CV and RhB, respectively, after complete decol-
ourization. The result shows that this iron complex medi-
ated visible-light photo–Fenton degradation provided a sig-
nificant demineralization of synthetic organic dyes.
Figure 10. (A) Absorption spectra of CV during the degradation
under visible light in the presence of 1 with H2O2 at different times.
(B) Degradation CV under different conditions: (1) Dye + light;
(2) dye + 1 + light; (3) dye + 1 + H2O2; (4) dye + H2O2 + light;
(5) dye + 1 + H2O2 + iPrOH + light; and (6) dye + 1 + H2O2 +
light.
Conclusion
In summary, we have synthesized a new amino acid
based ligand and its iron(III) complex, which has been
structurally characterized. This complex has been shown to
be an efficient photocatalyst for the degradation of dyes
such as MB, CV, MG and RhB under visible-light irradia-
tion. Complete degradation and decolourization took place
within 3–6 hours for different dyes. The optimal catalyst
Similarly, MG (6.50ϫ10–4 m, λmax = 618 nm) (Figure 11)
and RhB (2.83ϫ10–4 m and λmax = 554 nm) (Figure 12)
dyes were degraded under the same conditions as those de-
scribed above for the degradation of MB. It has been ob-
served that 4 and 6 hours are required for the complete de-
gradation of MG and RhB, respectively. The pseudo-first-
order rate constants for the degradation of MG, CV and
RhB were found to be 2.79, 2.87 and 3.04ϫ10–4 s–1, respec-
tively (Figure 4).
concentration for degradation was found to be 8.5ϫ10–5
m
and the optimal pH was 6.5.
Experimental Section
General: Chemicals used were reagent-grade products. Elemental
analyses were performed with a Perkin–Elmer model 2400 C, H, N
1
analyzer. H and 13C NMR spectra were recorded in D2O with a
Bruker Avance II instrument. The IR spectra were recorded with a
Perkin–Elmer model 883 spectrometer. UV/Vis spectra were re-
corded in solution with a Shimadzu model UV-1601 spectropho-
tometer. EPR spectra were recorded with a Bruker model ESP
300E spectrometer. LC–MS measurements were carried out with
an Agilent 1100 series spectrometer equipped with VWD and DAD
detectors and with a C8 column in a variable-temperature range of
30–280 °C. TOC values for the degradation of dyes (aqueous solu-
tion) were measured by WinTOC, using an OI Analytical Solids
TOC Analyzer (model 1030) with attached autosampler (model
1088).
Figure 11. (A) Absorption spectra of MG during the degradation
under visible light in the presence of 1 with H2O2 at different times.
(B) Degradation MG under different conditions: (1) Dye + light;
(2) dye + 1 + light; (3) dye + 1 + H2O2; (4) dye + H2O2 + light;
(5) dye + 1 + H2O2 + iPrOH + light; and (6) dye + 1 + H2O2 +
light.
Synthesis of H3L: l-Aspartic acid (1.33 g, 10 mmol) was dissolved
in water (30 mL) that contained NaOH (0.8 g, 20 mmol). This was
added to solution of 2,4-di-tert-butylphenol in ethanol (2.06 g,
The extent of mineralization in terms of total organic
carbon (TOC) for the degradation of dyes (aqueous solu-
tion) was measured by WinTOC. During this experiment, 10 mmol in 30 mL of ethanol). Aqueous formaldehyde (8 mL;
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40%) was added to it. The resulting mixture was heated to reflux to identify the degraded products. Generation of hydroxyl radical
for 6 h at 90 °C and was kept for slow evaporation, which gave a
milky white slurry. Treatment of the slurry with a large excess
amount of acetone resulted in a white precipitate, which was fil-
tered and washed by aqueous ethanol and finally hexane (to re-
move unreacted phenol) to obtain the sodium salt of the ligand.
The acid form of the ligand, H3L, was obtained by acidification of
the aqueous solution of NaHL by dilute CH3COOH. Finally the
product was recrystallized from a mixture of ethanol and water
(4:1); yield 82% (0.288 g). C19H28NNaO5: calcd. C 61.11, H 7.56,
N 3.75; found C 61.06, H 7.49, N 3.67.
was determined by means of the benzoic acid hydroxylation
method.[44] In this method, FeIII–salicylic acid complex [tetraaquo-
salicylatoiron(III) complex, λmax = 522 nm] was formed and iden-
tified by means of UV/Vis spectroscopy.
LC–MS Analysis of MB Degradation: The sample was prepared as
follows. The mixture of MB dye solution was irradiated with visible
light. When the solution was completely decolourized, the degrada-
tion products were extracted several times with ethyl acetate. The
ethyl acetate extract was evaporated to dryness with a rotary evapo-
rator. The dry residue obtained was dissolved in methanol and ana-
lyzed by LC–MS analysis.
Synthesis of [Et3NH][Fe(LH)2] (1): Ligand LH3 (0.351 g, 1 mmol)
was dissolved in a mixture of methanol (20 mL) and water (5 mL)
in a beaker. A solution of anhydrous FeCl3 (0.08 g, 0.5 mmol) in
methanol and triethylamine (0.101 g; 1 mmol, 0.14 mL) was added.
The resulting blue-violet solution was stirred for an hour and then
kept for slow evaporation at room temperature. Upon standing for
about a week the solution gave the product, which was washed with
water and then redissolved in acetonitrile, from which red-violet
single crystals suitable for X-ray diffraction were obtained; yield
0.25 g; 58.2%. C44H68FeN3O10 (854.86): calcd. C 61.82, H 8.02, N
4.92; found C 61.76, H 7.92, N 4.86.
Supporting Information (see footnote on the first page of this arti-
cle): Spectra, electrochemical data and tables with important bond
lengths and bond angles.
Acknowledgments
The authors thank the Department of Science and Technology
(DST), Government of India, New Delhi for use of its NMR spec-
troscopic and single-crystal X-ray facility.
Crystal Structure Determination: Crystal data for 1 was collected
at 100 K with a Bruker Smart CCD APEX diffractometer with
Mo-Kα (λ = 0.71073 Å) radiation using a cold nitrogen stream. The
crystal structure was solved by direct methods and refined with
full-matrix least-squares cycles using SHELX[42] with atomic coor-
dinates and anisotropic thermal parameters for all non-hydrogen
atoms.
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Published Online: September 5, 2014
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