Syntheses, X-ray structures, electrochemical properties and biological evaluation of mono- and dinuclear N2O2-donor ligand-Fe systems

Article abstract of DOI:10.1007/s11243-019-00322-6

The present work reports the synthesis, spectroscopic and structural characterizations of Fe(III) complexes of types [Fe(L)(H₂O)(NCS)] (1), [Fe(L)(1-methylimidazole)₂]ClO₄ (2) and [(L)Fe(μ-O)Fe(L)]·3H₂O·MeOH (3)

Full text of DOI:10.1007/s11243-019-00322-6

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00322-6
Syntheses, X‑ray structures, electrochemical properties and biological
evaluation of mono‑ and dinuclear N O ‑donor ligand‑Fe systems
2
2
1
1
1
1
1
1
M. Venkata Nikhil Raj · Kishalay Bhar · Surbhi Jain · Monika Rana · Tanveer A. Khan · Anuj K. Sharma
Received: 18 January 2019 / Accepted: 4 April 2019
©
Springer Nature Switzerland AG 2019
Abstract
The present work reports the synthesis, spectroscopic and structural characterizations of Fe(III) complexes of types [Fe(L)
H O)(NCS)] (1), [Fe(L)(1-methylimidazole) ]ClO (2) and [(L)Fe(μ-O)Fe(L)]·3H O·MeOH (3) containing a known planar
(
2
2
4
2
N O -donor salphen Schiꢀ base, H L (N,N′-bis(3-methoxysalicylidene)phenylene-1,2-diamine). In mononuclear complexes 1
2
2
2
2−
and 2, iron(III) centre adopts a distorted octahedral geometry where planar N O -donor L ligand forms equatorial plane and
2
2
varied co-ligands (aqua and thiocyanate in 1 and 1-methylimidazole in 2) occupy the axial sites. The μ-oxo-bridged dinuclear
complex 3 is a new solvatomorph of [(μ-O)(Fe(vanophen)) ]·2H O [vanophen=N,N′-bis(3-methoxysalicylidene)phenylene-
2
2
1
,2-diamine] reported by Jana et al. where a marked diꢀerence in Fe–O–Fe bond angle is noticed. The electrochemical
behaviours of H L and complexes 1–3 have been examined to ascertain the nature of electron transfer processes. The binding
2
interactions of 1–3 with ct-DNA as well as with Bovine serum albumin (BSA) have been investigated using ꢁuorescence
spectroscopy in T E buꢀer (pH=7.8). All the complexes show good binding propensity with ct-DNA probably via partial
1
0 1
intercalation mode. Furthermore, the complexes quench the intrinsic ꢁuorescence of BSA by a static quenching mechanism.
Introduction
potential anticancer activities of such iron(III)-salen/sal-
phen complexes towards various human cancer cell lines
(HOS, MCF-7, A549, HeLa, A2780 and G-361) have been
well documented [7]. Importantly, the majority of com-
plexes are highly cytotoxic and several fold more active
than cisplatin [7], the well-known anticancer drug that has
clinical use. In general, anticancer activity in such com-
plexes depends upon the structure of the salen, nature and
position of substituent on salen and co-ligands attached
to the metal centre [15]. The presence of aromatic bridge
between two imine N atoms in salen increases the antican-
cer activity [15], i.e. salphen is more eꢀective than salen.
Moreover, the redox properties associated with both metal
and ligands in such complexes [9] may provide unusual
mechanistic pathway for selective apoptosis and cyto-
toxicity towards cisplatin-resistant cancer cells. Of late,
Lange et al. have reported an iron(III)–salphen complex,
Design and synthesis of iron(III) complexes with var-
ied nuclearities is of continuous attention owing to their
potential applications in catalysis [1, 2], enzyme mimick-
ing [3, 4], molecular magnetism [5, 6] and drug designing
[
7–9]. Planar salen/salphen-type Schiꢀ bases [10] are well
suited for preparation of iron(III) complexes with diverse
co-ligands like halide/pseudohalide and azole-based het-
erocycles. These metal–salen complexes containing planar
aromatic rings have excellent binding ability with DNA
typically through non-covalent (intercalation, electro-
static or groove binding) interactions [11–14]. Further,
such redox-active transition metal complexes facilitate the
generation of reactive oxygen species (ROS) under reduc-
ing environment causing the damage of DNA [12]. The
[
Fe(L)(H O)(Cl)] (H L=N,N′-bis(3-methoxysalicylidene)
2 2
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-019-00322-6) contains
supplementary material, which is available to authorized users.
phenylene-1,2-diamine) which shows selective cytotoxic
eꢀects [16] on human platinum-resistant ovarian cancer
cells (SKOV-3 and OVCAR-3). Keeping in mind, all the
above mentioned factors, particularly the drug design-
ing and structural aspect of iron(III) salen/salphen com-
plexes, here we have studied the coordination behaviour
*
Anuj K. Sharma
anuj.sharma@curaj.ac.in
1
Department of Chemistry, School of Chemical Sciences
and Pharmacy, Central University of Rajasthan, NH-8,
Bandarsindri, Ajmer District, Rajasthan 305817, India
of H L ligand [16] towards iron(III) in combination with
2
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
thiocyanate and 1-methylimidazole as co-ligands. Suc-
cessfully, we have isolated two mononuclear compounds
Synthesis
[
Fe(L)(H O)(NCS)] (1) and [Fe(L)(1-methylimidazole) ]
Synthesis of N,N′‑bis(3‑methoxysalicylidene)
phenylene‑1,2‑diamine) (H L)
2
2
ClO (2), and a μ-oxo-bridged diiron(III) complex of the
4
2
type [(L)Fe(μ-O)Fe(L)]·3H O·MeOH (3). The complexes
2
are characterized through various physico-chemical and
spectroscopic methods. The redox properties, ct-DNA
and BSA binding studies of complexes 1–3 are delineated
herein.
Ortho-vanillin (1.52 g, 10 mmol) dissolved in MeOH (10 ml)
was added slowly to the solution of ortho-phenylenediamine
(0.54 g, 5 mmol) in MeOH (10 ml) and stirred for 3 h at
room temperature. Then, the orange crystalline solid was
ꢂltered oꢀ, washed with cold methanol and dried in vacuum.
1
Yield: 1.31 g (70%). H-NMR (CDCl ): 13.22 (s, 2H, –OH),
3
8
.63 (s, 2H, –CH=N–), 7.34 (m, 2H, –Ph), 7.20 (m, 2H,
Experimental
–Ph), 7.02 (dd, 2H, –Ph), 6.98 (dd, 2H, –Ph), 6.87 (t, 2H,
−
1
–
Ph), 3.90 (s, 6H, –OCH ). FTIR (KBr disc, cm ): ν(HO)
3
Materials and methods
3440, ν(C=N+C=C) 1626, 1584. UV–Vis [DMSO, λ
/
max
3
−1
−1
4
4
nm (ε/dm mol cm )]: 282 (3.0×10 ), 334 (2.3×10 ).
High-purity (NH ) SO .FeSO .6H O (Merck, India),
4
2
4
4
2
Fe(ClO ) .6H O (Alfa Aesar, India), ortho-phenylenedi-
4
2
2
amine (SRL, India), ortho-vanillin (Merck, India), 1-meth-
ylimidazole (Spectrochem, India) and potassium thiocy-
anate (Merck, India) were used as received. The Schiꢀ base,
N,N′-bis(3-methoxysalicylidene)phenylene-1,2-diamine
Synthesis of [Fe(L)(H O)(NCS)] (1)
2
To a light yellow aqueous-methanolic solution (10 ml) of
NH ) SO .FeSO .6H O (0.39 g, 1 mmol), an orange solu-
(
4
2
4
4
2
(
H L) was prepared using a reported method as described
2
tion (15 ml) of H2L (0.37 g, 1 mmol) in dichloromethane
was added slowly followed by a colourless methanolic solu-
tion (10 ml) of KSCN (0.09 g, 1 mmol). The resulting dark
green solution was ꢂltered and kept for slow evaporation.
After two days, dark plate-like crystals of 1 were isolated.
Yield: 0.30 g (60%). Analytical calcd. for C H N O SFe
elsewhere [16, 17]. All other chemicals and solvents used
were AR grade and used as received. The synthetic reac-
tion and work-up were done in open air. Elemental analy-
ses (CHNS) were performed on a Thermo-scientiꢂc ꢁash
2
000 elemental analyser. Infrared spectra were recorded at
2
3
20
3
5
room temperature using PerkinElmer FTIR spectrometer (in
(506.33): C, 54.5; H, 3.9; N, 8.3; S, 6.3%. Found: C, 53.9;
−
1
−1
KBr disc) in 4000–400 cm range. The NMR spectra were
H, 3.8; N, 8.4; S, 6.5%. FTIR (KBr disc, cm ): ν(HO)
recorded in CDCl solvent on Bruker Avance III 500 MHz
3400, ν(NCS) 2060, ν(C=N+ C=C) 1605, 1577. UV–Vis
3
3
−1
−1
4
spectrometer at 25 °C. UV–Vis spectra and kinetic studies
were performed at room temperature on Agilent Technolo-
gies Cary 100 UV–Vis spectrophotometer equipped with
multiple cell holders. Fluorescence measurements were done
on Agilent Technologies Cary Eclipse ꢁuorescence spectro-
photometer at 25 °C. Thermogravimetric analysis (TGA)
was performed on a Discovery Thermogravimetric analyser
by TA instruments-waters lab. All electrochemical experi-
ments were performed with Autolab PGSTAT 302N work-
station (Eco-Chemie BV, Netherlands). The electrochemical
studies of ligand and complexes were carried out in dry ace-
tonitrile and/or dry methanol, respectively, under nitrogen
atmosphere at room temperature, at scan rate of 0.1 V/s in
[DMSO, λ /nm (ε/dm mol cm )]: 312 (3.7×10 ), 343
max
4
4
3
(2.4×10 ), 397 (1.3×10 ), 604 (2.3×10 ).
Synthesis of [Fe(L)(1‑methylimidazole) ]ClO (2)
2
4
To an orange solution (20 ml) of H L (0.20 g, 0.53 mmol)
2
in dichloromethane, a solution of Fe(ClO ) (0.14 g,
4
2
0.53 mmol) in methanol (10 ml) was added slowly. The col-
our of the mixture turned dark green. Then, a methanolic
solution (10 ml) of 1-methylimidazole (0.09 ml, 1.06 mmol)
was added to that mixture with continuous stirring. A dark
greenish-brown solid was precipitated out immediately. The
solid was isolated by ꢂltration. The ꢂltrate was kept for slow
evaporation. After two days, dark green crystals of com-
plex 2 were obtained. Yield: 0.20 g (55%). Analytical calcd.
for C H N O ClFe (693.90): C, 51.9; H, 4.3; N, 12.1.
+
a three-electrode assembly using non-aqueous Ag/Ag as
reference electrode, glassy carbon as a working electrode
and platinum wire as a counter electrode, with tetrabutylam-
monium perchlorate (TBAP) as the supporting electrolyte in
the potential range from −1 to 2 V, and were uncorrected for
3
0
30
6
8
−
1
Found: C, 51.6; H, 4.1; N, 11.8. FTIR (KBr disc, cm ):
+
junction contributions. The value for the F –F couple under
ν(ClO ) 1090, 623, ν(C=N + C=C) 1605, 1577. UV–Vis
c
c
4
3
−1
−1
4
our conditions is 0.47 V in MeOH and 0.44 V in MeCN [18].
CV data analyses were done using Nova 1.10.1.9 module
provided with Autolab.
[DMSO, λ /nm (ε/dm mol cm )]: 311 (3.5×10 ), 343
max
4
4
3
(2.3×10 ), 397 (1.2×10 ), 604 (1.9×10 ).
1
3

Transition Metal Chemistry
Synthesis of [(L)Fe(μ‑O)Fe(L)]·3H O·CH OH (3)
unit cell determination, the single crystal was exposed to
X-rays for 10 s in three sets of frames. The detector frames
were integrated using SAINT program, and the multi-scan
absorption corrections were performed using SADABS pro-
gram [19]. The structures were solved by SHELXT [20] and
2
3
Complex 3 was isolated as dark brown crystalline solid
by dropwise addition of water in excess to a dark green
methanolic solution (0.20 g, 0.29 mmol) of complex 2
with continuous stirring until precipitation was completed.
Yield: 0.10 g (71%) Analytical calcd. for C H N O Fe
2
reꢂned by full-matrix least-squares methods based on F
using SHELXL [21]. All non-hydrogen atoms were reꢂned
with anisotropic displacement parameters, whereas hydro-
gen atoms were placed in calculated positions when pos-
sible and given isotropic U values 1.2 times that of the atom
to which they are bonded. Materials for publication were
prepared using PLATON [22] and OLEX2 [23] programs.
A summary of the crystallographic data and structure deter-
mination parameters is given in Table 1.
4
5
46
4
13
2
(
962.56): C, 56.1; H, 4.8; N, 5.8. Found: C, 55.9; H, 4.4; N,
−
1
5
1
.5. FTIR (KBr disc, cm ): ν(HO) 3419, ν(C=N + C=C)
603, 1580, ν(Fe–O–Fe) 854. UV–Vis [DMSO, λ /nm
max
3
−1
−1
4
4
(
(
ε/dm mol cm )]: 309 (6.5×10 ), 346 (4.6×10 ), 408
4
3
2.2×10 ), 632 (1.9×10 ).
X‑ray structure reꢀnement
Crystallographic data of compounds 1–3 were collected on
DNA binding ꢁuorescence quenching assay
a Bruker Kappa APEX-II CCD diꢀractometer at 293(2) K
(
for 1), 296(2) K (for 2) and 100(2) K (for 3) using graph-
The steady-state ꢁuorescence quenching experiments were
performed with ct-DNA on Agilent Technologies Cary
ite monochromated Mo Kα radiation (λ=0.71073 Å). For
Table 1 Crystallographic data and reꢂnement parameters for 1–3
Crystal parameters
1
2
3
Formula
Formula weight
Crystal system
Space group
C H FeN O S
C H ClFeN O
693.90
Triclinic
P1̄
C H Fe N O
13
962.56
Monoclinic
2
3
20
3
5
30 30
6
8
45 46
2
4
506.33
Triclinic
P1̄
P2 /c
1
a/Å
b/Å
c/Å
α°
8.8035(3)
11.4953(3)
11.8428(3)
106.500(2)
99.689(2)
105.302(2)
1069.36(6)
0.71073
1.572
10.3441(5)
10.4721(5)
15.1434(7)
99.255(2)
107.122(2)
93.316(3)
1537.64(13)
0.71073
1.499
13.5531(15)
22.894(3)
14.7625(15)
90
112.774(4)
90
4223.5(8)
0.71073
1.504
β°
γ°
V/Å3
λ/Å
ρcalcd/gm cm−3
Z
2
2
4
T/K
293(2)
0.845
296(2)
0.639
100(2)
0.760
μ (mm−1)
F(000)
θ ranges (°)
522
2.684–25.993
718
2.073–25.000
1976
2.413–24.999
h/k/l
−10,10/−14,14/−14,14
17,375
4191
4191/0/314
1.013
R1 =0.0369
−12,12/−12,12/−17,17
34,277
5389
5389/6/427
1.056
R1 =0.0422
−16,16/−27,27/−17,17
89,784
7405
7405/4/615
1.095
R1 =0.0449
Reꢁections collected
Independent reꢁections
Data/restraints/parameters
Goodness-of-ꢂt on F2
Final R indices [I>2σ(I)]
wR =0.0823
R1 =0.0531
wR =0.1177
R1 =0.0455
wR =0.1171
R1 =0.0537
2
2
2
R indices (all data)
wR =0.0904
0.344 and −0.335
1550383
wR =0.1219
0.736 and −0.460
1862333
wR =0.1252
1.004 and −0.609
1560868
2
2
2
Largest peak and hole (eÅ−3
CCDC No.
)
1
3

Transition Metal Chemistry
Eclipse ꢁuorescence spectrophotometer at room tempera-
ture. The samples were excited at 480 nm, and the emission
(NH ) SO ·FeSO .6H O, H L and KSCN. Complex 2 was
4
2
4
4
2
2
isolated as dark green crystalline solid by reaction of a 1:1:2
was recorded from 490 to 800 nm (λ
=600 nm). The
molar ratio of Fe(ClO ) ·6H O, H L and 1-methylimidazole
emission
4 2
2
2
excitation and emission slit width were ꢂxed at 15 nm and
in DCM-MeOH. The dark brown crystalline compound 3
was synthesised by dropwise addition of measured excess
water to a methanolic solution of complex 2 with continu-
ous stirring (Scheme 1). All the compounds are air stable
and completely/partially soluble in common organic sol-
vents like methanol, acetonitrile, dimethylsulphoxide and
dimethylformamide, but insoluble in water. Elemental analy-
ses of compounds 1–3 show good correspondence with the
formulations.
1
5 nm, respectively. Concentrated stock solutions (5 mM)
of complexes 1–3 were prepared in DMSO. The titrations
were carried out by adding increasing amount of complex
into the solution of ct-DNA saturated with EtBr. The con-
−
5
centration of ct-DNA was ꢂxed at 3.7×10 M. The DNA-
EtBr solution was prepared in 10 mM Tris-1 mM EDTA
(T E ) buꢀer. A large range of complex-to-DNA molar ratio
10 1
range was covered by varying the concentration of the com-
plexes. DNA concentration per nucleotide was determined
by recording the absorption spectra using 1 cm path length
cuvettes. DNA solutions in 10 mM Tris/1 mM EDTA buꢀer
gave a single peak for UV absorbance at 260 nm. The DNA
concentration was determined by taking the molar absorp-
Spectroscopic and thermal studies
In FTIR spectrum, free Schiff base (H L) displays
2
−
1
ν(OH) stretching at 3440 cm along with characteristic
−
1
−1
−1
tion coeꢃcient (ɛ ) of ct-DNA as 6600 M cm . The
ν(C=N+C=C) bands at 1626 and 1584 cm [17]. Complex
2
60
concentration was calculated using the Beer–Lambert’s Law:
1 exhibits characteristic ν(C=N+C=C) peaks at 1605 and
−
1
1
577 cm of the deprotonated Schiꢀ base [17]. In addition,
A = ꢀcl
−1
1
shows peak at 3400 cm corresponding to ν(HO) stretch-
where A is the absorbance of the solution, ɛ is the molar
absorption coeꢃcient, c is the concentration of the solution
and l is the path length of the cuvette. The concentration of
DNA thus determined was 3.75×10− M.
ing of water ligand [16]. In 1, the metal bound thiocyanate
−1
bands for ν(NCS) and ν(C–S) appearing at 2060 cm and
−1
7
30 cm , respectively, are indicative of N-bonded coordina-
5
tion [17] of thiocyanate to metal. In complex 2, the stretch-
ing bands related to perchlorate counter anion appear at
−1
BSA binding ꢁuorescence quenching assay
1
090 and 623 cm along with characteristic ν(C=N+C=C)
2
−
−1
peaks of L at 1605 and 1577 cm [17]. FTIR spectrum
−
1
The steady-state ꢁuorescence quenching experiments were
carried out on Agilent Technologies Cary Eclipse ꢁuores-
cence spectrophotometer at room temperature. The excita-
tion and emission slit widths were ꢂxed at 15 and 3 nm,
respectively. The concentrated stock solutions of all the
three complexes were prepared in DMSO. Concentration
of BSA was ꢂxed at 3 μM, while the complex concentration
was gradually increased from zero to nearly 50 μM which
covered a large protein-to-complex ratio. The tryptophan
residue of BSA was excited at 280 nm, and emission spec-
tra were recorded from 285 to 510 nm. The ꢂnal amount of
DMSO in the solution was so small to aꢀect any changes in
the protein structure or conformation. The binding constant
of interaction of iron complexes with BSA was determined
by monitoring the intrinsic ꢁuorescence of BSA solution
with increasing complex concentration.
of 3 shows moderately strong band at 854 cm correspond-
ing to ν(Fe–O–Fe) stretching [17]. In addition, broad band
−
1
at around 3419 cm is found in 3 which may correspond
to ν(OH) stretching of crystalline water and methanol mol-
ecules [17]. In all cases, the characteristic ν(C=N+C=C)
stretching frequency of Schiꢀ base is shifted towards a lower
energy region indicating involvement of the imine N atom
in metal coordination [17]. In order to conꢂrm the presence
of crystalline solvent molecules in 3, the thermogravimetric
analyses (TGA) were carried out under N atmosphere in
2
the temperature range 34–700 °C. The TGA plot (Fig. 1)
clearly shows the mass loss of 9.10% within the temperature
range 37–100 °C corresponding to the loss of one methanol
and three water molecules (calcd. 8.94%). Moreover, com-
plex 3 shows thermal stability up to 310 °C and then started
to decompose gradually. The electronic spectra of ligand
(
H L) and complexes (1–3) were recorded in DMSO at
2
room temperature (Figure S1, supporting information). The
Results and discussion
ligand (H L) shows two bands around 282 nm and 334 nm
2
which are attributed to π → π* transitions [1, 24]. Hexa-
Synthesis and formulation
coordinated mononuclear complexes [Fe(L)(NCS)(H O)]
2
(
1) and [Fe(L)(1-methylimidazole) ](ClO ) (2) exhibit one
2
4
The hexa-coordinated mononuclear complex 1 was obtained
broad band and three shoulder bands. The ꢂrst two bands
observed at ~310 nm and ~345 nm are assigned to ligand-
centred π→π* transitions [1, 24]. The two other low-energy
as a dark green crystalline solid in DCM-MeOH-H O
2
(
2:2:1) solvent mixture containing a 1:1:1 molar ratio of
1
3

Transition Metal Chemistry
Scheme 1 Synthetic routes for
preparation of complexes 1–3
ꢂrst two bands observed at 309 nm and 346 nm can be
assigned to ligand-based π → π* transitions [1, 24]. The
shoulder band appeared at 408 nm is probably attributed to
pπ
→dσ* phenolate ligand-to-metal charge trans-
Fe
phenolate
fer (LMCT) transition [24–28]. The second shoulder band
appeared at 632 nm can be attributed to pπ
→dπ*
Fe
phenolate
LMCT and weaker oxo-to-iron CT transition [24–28].
Crystal structures of [Fe(L)(H O)(NCS)] (1) [Fe(L)
2
(
1‑methylimidazole) ]ClO (2), [(L)Fe(μ‑O)
2 4
Fe(L)]3H O·CH OH (3)
2
3
Single crystal X-ray structures have been determined to
unravel the coordination geometry and nuclearity of com-
plexes 1–3.
Complex 1 is the thiocyanate analogue of a chloro com-
plex [Fe(L)(H O)(Cl)] reported [16] by Lange et al. Struc-
2
Fig. 1 TGA graph of compound
3 in the temperature range
tural analysis of 1 reveals an asymmetric unit consisting of
3
4–700 °C
a [Fe(L)(H O)(NCS)] molecule. The coordination polyhe-
2
dron around iron(III) is best described as a distorted octahe-
bands observed at ~ 400 nm and ~ 600 nm can probably
be attributed to ligand-to-metal charge transfer (LMCT)
dron with a FeN O chromophore. The coordination sphere
3
3
includes two imine N atoms (N2 and N3) and two O atoms
(O1 and O2) of Schiꢀ base ligand, one N atom (N3) of ter-
minal NCS unit and one O atom (O5) of water (Fig. 2). Two
imine N atoms (N2 and N3) along with two O atoms (O1 and
O2) of the Schiꢀ base deꢂne the equatorial plane, while one
transitions [1, 24–28] of the types pπ
→ dσ* and
Fe
phenolate
pπ
→ dπ* , respectively. The penta-coordinated
phenolate
Fe
dinuclear complex [(L)Fe(μ-O)Fe(L)]·3H O·MeOH (3)
2
also shows one broad band and three shoulder bands. The
1
3

Transition Metal Chemistry
Fig. 2 Molecular structure of 1
aqua molecule (O5) and one thiocyanate (N1) anion are in
the axial positions. Fe(III) centre deviates (0.148 Å) slightly
from the mean basal plane towards axial thiocyanate. The
Fe–O and Fe–N bond distances (Table S1) are in line with
the high-spin Fe(III) [17]. The structural features of complex
Structural analysis shows that complex [Fe(L)(1-methylimi-
̄
dazole) ]ClO (2) crystallizes in a triclinic system with P1
2
4
space group. The Fe(III) centre adopts a distorted octahedral
geometry coordinated by N O -donor set (N3, N4, O1, O2)
2
2
of the deprotonated Schiꢀ base along the basal plane and
two N atoms (N1, N2) of two 1-methylimidazole moieties in
axial positions (Fig. 3). The Fe–O and Fe–N bond distances
(Table S1) are in line with the high-spin Fe(III) [17]. The
1
are comparable with the chloro analogue [16].
Complex 2 is the 1-methylimidazole analogue of a
reported imidazole iron(III) complex [17] by Jana et al.
Fig. 3 Molecular structure of
+
[
Fe(L)(1-methylimidazole) ]
2
in 2
1
3

Transition Metal Chemistry
Fe1 centre deviates (0.009 Å) slightly from the mean basal
plane. Two 1-methylimidazole rings are almost orthogonal
136.86(13)° in 3. On the contrary, the orientation of FeN O
2
2
cores is in between eclipsed and staggered orientations in
the reported solvatomorph [17] which prevents such intra-
molecular π–π stacking interaction. This results in a higher
Fe–O–Fe bond angle [154.3(2)°] in the reported solvato-
morph. In 3, the structural distortion indexes are found as
τFe1=0.19 and τFe2=0.02, respectively, which indicates
that Fe1 and Fe2 polyhedra are all close to a distorted square
pyramid. To the best of our knowledge, the Fe–O–Fe bond
angle [136.86(13)°] and Fe···Fe distance (3.326 Å) in 3 are
shortest compared to those of structurally related dinuclear
complexes (Table S2) with the Fe–O–Fe bridge.
(dihedral angle: 65.16°) to each other. The structural features
of complex 2 are comparable with the imidazole analogue
17].
It is worth to mention that complex 3 isolated here
[
is a solvatomorph [29] of the oxo-bridged compound
reported [17] by Jana et al. Complex 3 is a μ-oxo-bridged
dinuclear Fe(III) compound. The asymmetric unit of 3
consists of a [(L)Fe(μ-O)Fe(L)] dimeric unit with a crys-
talline methanol and three water molecules (Fig. 4). The
iron(III) centres (Fe1 and Fe2) surrounded by the four
coordinating atoms N O of the ligand, extend towards the
2
2
bridging oxygen atom as much as 0.565 Å and 0.560 Å,
respectively. Two FeN O cores are in staggered orienta-
Redox behaviour of H L and complexes 1–3
2
2
2
tion relative to the oxo-bridge to minimize interligand
steric repulsions. The overlay plot of two solvatomorphs
is given in Fig. 5a demonstrating different orientations
Cyclic voltammograms were recorded for H L (in ace-
2
tonitrile) and complexes [Fe(L)(NCS)(H O)] (1), [Fe(L)
2
(1-methylimidazole) ](ClO ) (2) and [(L)Fe(μ-O)
2
4
(
Green: complex 3 and Pink: reported solvatomorph). It is
Fe(L)]·3H O·MeOH (3) (in methanol) to investigate ligand
2
found that two FeN O cores in 3 are in staggered orien-
and/or metal-centred redox behaviours. Ligand, H L hav-
2
2
2
tation which enables two vanillin units from each FeN O
ing two phenolate groups shows two irreversible oxidative
2
2
core to involve in both intra- [Cg(9)–Cg(10): 3.600(2) Å,
dihedral angle (α): 8.86(18)°, slippage: 1.056 Å, symme-
try code: x, y, z; Cg(9): C15–C16–C17–C18–C19–C20;
Cg(10): C21–C22–C23–C24–C25–C26] and intermo-
lecular [Cg(9)–Cg(9): 3.676(2) Å, dihedral angle (α): 0°,
slippage: 1.639 Å, symmetry code: − x, 1 − y, 1 − z] π–π
stacking interaction (Fig. 5b). Such staggered orientation
and intramolecular π–π stacking are primarily responsible
in stabilizing the bent μ-oxo-bridge with a Fe–O–Fe angle
responses (Figure S2a) at 0.99 V and 1.54 V versus Ag/
+
Ag in acetonitrile probably due to formation of phenoxy
radicals. Similarly, complexes 1–3 show two irreversible
oxidative responses (Figures S2b–S2d) in the potential
+
range 1.10–1.40 V versus Ag/Ag in methanol. Such val-
ues are in line with some reported complexes containing
salen/salphen-type ligand systems [30]. Two mononuclear
complexes 1 and 2 in methanol show one irreversible reduc-
+
tive response at − 0.49 V and − 0.55 V versus Ag/Ag ,
Fig. 4 A view of μ-oxo-bridged
dinuclear structure in 3
1
3

Transition Metal Chemistry
Fig. 5 a An overlay view of
complex 3 (green) and reported
complex of Jana et al. [17]
(
pink) (CIF BEHJIJ 870310
of the reported complex by
Jana et al. [17] was obtained
from CCDC and the structures
were drawn using OLEX2 [23]
program); b A view of intramo-
lecular π–π stacking in 3 which
is primarily responsible in stabi-
lizing the bent μ-oxo-bridge
respectively (Figure S3), which may be originated from the
metal-centred reduction of Fe(III)→Fe(II). A similar type
of cathodic response was observed in reported [Fe(dmsalen)
DNA‑binding interactions
Transition metal complexes can bind to DNA via both cova-
lent (replacement of a labile coordinating ligand by nitrogen
base of DNA) and/or non-covalent (intercalation, electro-
static or groove binding) interactions [32]. Many important
applications emerge if the complexes can bind to DNA via
(
H O)(Cl)] complex [26]. The dinuclear complex [(L)
2
Fe(μ-O)Fe(L)]·3H O·MeOH (3) in methanol also shows one
2
irreversible reductive response at −0.47 V (Fig. 6) which is
attributed to the metal-centred reduction of Fe(III)→Fe(II)
[
31].
1
3

Transition Metal Chemistry
Fig. 6 Cyclic voltammogram of [(L)Fe(μ-O)Fe(L)]·3H O·MeOH (3)
2
−
3
(
0.5×10 M) at scan rate of 0.1 V/s recorded in methanol using Ag/
+
Ag as reference electrode, Glassy carbon as working electrode, Pt
wire as a counter electrode and TBAP as supporting electrolyte
an intercalative mode [32]. Therefore, the interaction of
metal complexes, especially when containing planar aro-
matic heterocyclic ligands which can insert and stack them-
selves into the base pairs of the DNA duplex has attracted
considerable attention [32]. A competitive ethidium bromide
(
EtBr or 3,8-Diamino-5-ethyl-6-phenylphenanthridinium
bromide) binding study has been carried out with ꢁuores-
cence experiment in order to investigate if the complex could
displace EtBr from its EtBr-DNA complex. EtBr is a typi-
cal indicator of intercalation [11–14]. The molecular ꢁuo-
rophore EtBr forms soluble complexes with nucleic acids
and emits intense ꢁuorescence in the presence of ct-DNA
due to the intercalation of the planar phenanthridinium ring
between adjacent base pairs on the double helix. In order to
investigate the binding ability of iron complexes 1–3, they
were titrated with the solution of ct-DNA-EtBr (see experi-
mental). The ꢁuorescence of EtBr enhances upon inter-
calation with ct-DNA which can be hampered by another
DNA-binding molecule which in our case are the synthe-
sized iron(III) complexes. The competitive binding of the
complexes 1–3 to ct-DNA quenched the emission intensity
of EtBr. The ꢁuorescence intensity of EtBr at 600 (480 nm
excitation) with an increasing amount of the complex con-
centration was recorded. The ꢁuorescence quenching curve
of EtBr-bound ct-DNA in the presence of complexes 1–3
is in good agreement with the classical linear Stern–Vol-
mer equation [11, 14]: F /F=1+K [complex], where F
Fig. 7 Fluorescence intensity versus wavelength plot for DNA-EtBr
solution at diꢀerent concentrations of mononuclear complexes 1
(
Top) and 2 (Bottom). The arrow shows change in intensity of DNA-
EtBr emission upon increasing amount of compound. Inset: the plot
of F /F versus the complex concentration
0
changes (Figs. 7 and 8). The K values of the complexes
SV
3
were calculated as 1.88×10 (R=0.987 for initial six points)
3
for 1, 5.78×10 (R=0.984 for initial six points) for 2 and
3
9.92 × 10 (R = 0.990 for initial six points) for 3. By con-
7
−1
sidering a DNA-binding constant of 1.0×10 M for EtBr
and the complex concentration of the value at a 50% reduc-
tion in the ꢁuorescence intensity of EtBr, an apparent DNA-
5
−1
binding constant K of the complexes (1.77×10 M for
app
6
−1
6
−1
1, 4.41×10 M for 2 and 5.96×10 M for 3) was derived
from the equation: K [EtBr] = K [complex], which is
EtBr
app
less than the binding constant of classical intercalations.
0
SV
0
is the emission intensity of ct-DNA-EtBr in the absence of
This suggests that the complexes interact moderately with
complex and F is the emission intensity of ct-DNA-EtBr
ct-DNA. The apparent DNA-binding constant K of some
app
in the presence of complex. The linear plot of F /F ver-
reported salen/salphen Schiꢀ base complexes [11–13, 33,
0
4
6
−1
sus [complex] gives a measure of the ꢁuorescence intensity
34] falls in the range 1 ×10 –1.6×10 M . As compared
1
3

Transition Metal Chemistry
than one binding site for interaction of 1–3 with ct-DNA. It
also indicates the existence of moderate interactions between
complexes 1–3 and ct-DNA. Further, binding aꢃnity of 1–3
with DNA follows the order 3>2>1. These results are in
good agreement with the geometry and nuclearity of the
complexes. The penta-coordinated dinuclear complex 3 has
higher binding aꢃnity as compared to mononuclear hexa-
coordinated complexes 1 and 2. Moreover, the binding aꢃn-
ity of 2 is higher than 1 which can probably be due to the
presence of aromatic 1-methylimidazole moieties in axial
positions.
BSA binding studies
Serum albumins are the carriers of essential metal ions in
the body and are the most abundant proteins in blood and
cerebrospinal ꢁuid. Interactions between serum albumin and
chemicals have attracted many researchers as serum albumin
plays a crucial role in drug transport and drug metabolism
[11, 14, 35]. Serum albumin is also well known to bind small
molecules with aromatic moieties. Bovine serum albumins
Fig. 8 Fluorescence intensity versus wavelength plot for DNA-EtBr
solution at diꢀerent concentrations of dinuclear complex 3. The
arrow shows change in intensity of DNA-EtBr emission upon increas-
ing amount of compound. Inset: the plot of F /F versus the complex
0
concentration
(
BSA) is the most extensively studied serum albumin due to
with the reported examples, the apparent binding constant
its structural homology with human serum albumin (HSA)
[11, 14, 35]. Their binding with drugs is important as it can
alter the eꢃciency of the drug. A strong ꢁuorescence emis-
sion at 345 nm is observed in BSA solutions when excited
at 280 nm due to the presence of tryptophan residues. The
ꢁuorescence intensity of BSA solution decreases gradually
by adding an increasing amount of iron(III) complexes (1–3)
(Figs. 9 and 10). The observed quenching may be attributed
to substrate binding, subunit association, denaturation of
protein or changes in the secondary protein conformation
[11, 14, 35]. The ꢁuorescence quenching is quantitatively
calculated by the Stern–Volmer equation, F /F=1+K τ [
(
K
) values of complexes 1–3 indicate that the complexes
app
partially intercalate with the DNA helix [11, 14, 33]. Also,
the relatively higher binding propensity of complexes 1–3
to ct-DNA can be ascribed due to the presence of planar
aromatic moieties. Further, the values of binding constant
(
K ) and number of binding sites (n) for ct-DNA–complex
a
interactions were calculated from the ꢁuorescence data using
the Scatchard Eq. (1) [11].
Fo − F
ꢀ
ꢁ
log
= log K + n log[Q]
(1)
a
F
0
q 0
Q]=1+K [Q], where F and F represent the ꢁuorescence
where F and F are ꢁuorescence intensities of ct-DNA-EtBr
SV
0
o
intensities in the absence and in the presence of quencher, K
solution in the absence and presence of varying concentra-
tions of complexes 1, 2 or 3. [Q] is the concentration of the
quencher. The Scatchard plots for complexes 1–3 are given
in supporting information (Figures S4 and S5). The values
q
is the quenching rate constant, τ the average lifetime of the
0
−
8
biomolecule without quencher (about 10 s) [11], K the
SV
Stern–Volmer quenching constant, and [Q] the concentration
of quencher. The calculated values of K and K are given
of K and n obtained from intercept and slope of Scatchard
SV
q
a
in Table 2. The calculated K value for the BSA complex
plot are summarized in Table 2. The binding constant values
q
1
3
−1 −1
4
5
−1
systems of 1–3 is in the magnitude of 10
M
s , which
are in the range of 10 –10 M which shows the possibil-
ity of intercalative binding [11]. The value of n is in the
range of 1.39–1.56, which suggests the presence of more
1
0
−1 −1
is threefold higher than 2.0× 10
M
s , the maximum
scatter collision quenching constant of quenchers with BSA.
This result is indicative of the existence of static quenching.
Table 2 DNA and BSA binding
DNA
BSA
KSV (M−1
parameters of complexes 1–3
Complex KSV (M−1
)
Kapp (M−1
)
K (M−1)
N
)
K (M−1s−1
)
K (M−1)
fa
a
q
a
3
5
4
5
5
5
13
1.27×105 1.09
1
2
3
1.88×10
5.78×10
9.92×10
1.77×10
4.41×10
5.96×10
4.17×10
5.38×10
7.72×10
1.39 1.61×10
1.56 1.68×10
1.50 3.04×10
1.61×10
1.68×10
3.04×10
3
3
6
5
5
13
5
1.41×10
1.05
6
13
1.42×105 1.21
1
3

Transition Metal Chemistry
Fig. 10 Fluorescence intensity versus wavelength plot for BSA
solution at diꢀerent concentrations of complex 3. The arrow shows
change in intensity of BSA emission upon increasing amount of com-
pound. Inset: the plot of F /F versus the complex concentration
0
complex BSA binding. Complexes show good binding aꢃn-
ity towards BSA with binding constant value in the order of
5
−1
1
0 M . The value of f is close to 1 indicating the presence
a
of single binding site for interaction of 1–3 with BSA. In
comparison with some reported iron(III)–salen complexes
4
5
−1
[
11] (3.6×10 –1.36×10 M ), the binding constant values
for complexes 1–3 are similar.
Conclusion
Fig. 9 Fluorescence intensity versus wavelength plot for BSA solu-
tion at diꢀerent concentrations of complexes 1 (Top) and 2 (Bottom).
The arrow shows change in intensity of BSA emission upon increas-
In summary, we have synthesized and characterized two
hexa-coordinated mononuclear complexes [Fe(L)(H O)
2
(
NCS)] (1) and [Fe(L)(1-methylimidazole) ]ClO (2), and
2
4
ing amount of compound. Inset: the plot of F /F versus the complex
0
one penta-coordinated dinuclear μ-oxo-bridged complex
concentration
[
Fe (L) (μ-O)]·3H O·CH OH (3) of iron(III) in combina-
2
2
2
3
tion with a salphen (N O -donor) Schiꢀ base. The dinuclear
2
2
Further, the binding constant (K ) was calculated using
complex 3 was isolated using a new synthetic strategy that
a
modiꢂed Stern–Volmer (MSV) Eq. (2) [35].
resulted in a solvatomorph of the reported complex [(μ-O)
(
Fe(vanophen)) ]·2H O by Jana et al. Structural studies show
2 2
F0
1
1
=
+
K f [Q] fa
a marked diꢀerence in Fe–O–Fe bond angle (136.86(13)°)
in 3 compared to the reported solvatomorph (154.3(2)°).
The intramolecular π–π stacking in staggered orientation
(2)
F0 − F
a a
where F is the initial tryptophan ꢁuorescence intensity of
o
between two vanillin rings of FeN O cores plays a major
BSA, F is the tryptophan ꢁuorescence intensity of BSA after
2
2
role for such lowering in Fe–O–Fe bridging angle. The ct-
DNA and BSA binding interactions of the mono- and dinu-
clear iron(III) (1–3) complexes were studied. The dinuclear
iron(III) complex 3 showed signiꢂcant ct-DNA and BSA
binding propensity than the mononuclear iron(III) com-
plexes 1 and 2. The studies suggested that co-ligands, nucle-
arity and geometry of iron(III) complexes play noticeable
role in the binding interactions with both ct-DNA and BSA.
addition of complexes 1, 2 or 3 acting as a quencher, f is
a
the fraction of the tryptophan that is initially accessible to
the complex and [Q] the concentration of the quencher. The
linearity in the MSV plots of all three complexes is also
indicative of possible static quenching mechanism [11]. The
ratio of intercept to slope of MSV plots (Figures S6 and S7)
gave the eꢀective binding constant (K ) values (Table 2) for
a
1
3

Transition Metal Chemistry
Further, biological studies especially anticancer studies are
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Acknowledgements AKS acknowledges Science and Engineering
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and DST-INSPIRE research grant (IFA-13, CH-97). VNR is grate-
ful to Central University of Rajasthan for University fellowship. SJ
thanks DST-INSPIRE for SRF; MR is thankful to DST-SERB for fel-
lowship; TAK is grateful to the UGC for SRF; KB is grateful to SERB
for NPDF (PDF/2017/000929) fellowships. Authors acknowledge the
instrumental facilities at Department of Chemistry, Central University
of Rajasthan supported by DST-FIST (ref. no. SR/FST/CSI-257/2014
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USIC, The University of Burdwan for X-ray crystallographic facilities.
Paper is dedicated to the memories of Dr. Sunil G. Naik. The authors
also acknowledge the Reviewers for their valuable suggestions in the
revision stage.
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Compliance with ethical standards
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Chem 68:766–773
Conflicts of interest There are no conꢁicts of interest to declare.
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Met Chem 37:431–437
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