A family of Fe3+ based double-stranded helicates showing a magnetocaloric effect, and Rhodamine B dye and DNA binding activities

Article abstract of DOI:10.1039/c5dt01569c

Herein, the synthesis, structural characterization, magnetic properties and guest binding activities of four Fe³⁺ based double-stranded helicates namely; [Fe₂(L)₂](ClO₄)(Cl)·4(CH₃OH)·2(H₂O) (1), [Fe₂(L)₂](BF₄)₂·2(H₂O) (2), [Fe₂(L)₂](NO₃)₂·3(CH₃OH)·2(H₂O) (3), and [Fe₂(L)₂](Cl)₂·2(CH₃OH)·4(H₂O) (4) are reported. Complexes 1-4 have been synthesized using the hydrazide-based ligand H₂L (H₂L = N′1,N′4-bis(2-hydroxybenzylidene)succinohydrazide) and the corresponding Fe²⁺ salts. Each of the independent cationic complexes [Fe₂(L)₂]²⁺ shows double-stranded helicates from the self-assembly of the ligand and metal ions in a 2 : 2 ratio, where the individual Fe³⁺ centre is lying on a C₂-axis and the ligand strands wrap around it. In 1-4, ligand L adopts pseudo-C conformations and forms a double-stranded dinuclear helicate with a small cage in between them. Moreover, in 1-4, each of the independent cationic complexes [Fe₂L₂]²⁺ is inherently chiral and possesses P for right-hand and M for left-hand helicity and as a consequence is a racemic solid. Detailed magnetic studies of all the complexes reveal that the Fe³⁺ centres are magnetically isolated and isotropic in nature. Estimation of the Magnetocaloric Effect (MCE) from magnetization data unveils a moderate MCE at a temperature of 3 K with magnetic entropy changes (-ΔS_m) of 22.9, 27.7, 24.1, 26.5 J kg^-1 K^-1 at a magnetic field of 7 T for complexes 1-4, respectively. Also, the variation of the -ΔS_m values was justified by considering the parameter of magnetization per unit mass. Stability of all the complexes in solution phase was confirmed by ESI-mass spectrometric analysis and liquid phase FT-IR spectroscopy. Further, the interaction of the complexes 1-4 with Rhodamine B dye was examined by UV-vis and fluorescence spectroscopic study. The observed blue-shift in the fluorescence study and hyperchromicity and hypochromicity with the appearance of two isosbestic points in the UV-vis study ascertain the interactions of the dye with the complexes. A DNA binding study by absorption spectral titration suggests the weak external intercalation of complex 1 within the nucleotide of calf thymus DNA. Computational study supports the isotropic nature of the metal centres and the consequent high spin multiplicity, which assists the complexes to show significant magnetic entropy changes.

Full text of DOI:10.1039/c5dt01569c

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A family of Fe3+ based double-stranded helicates
showing a magnetocaloric effect, and Rhodamine
B dye and DNA binding activities†
Cite this: DOI: 10.1039/c5dt01569c
Amit Adhikary,* Himanshu Sekhar Jena and Sanjit Konar*
Herein, the synthesis, structural characterization, magnetic properties and guest binding activities of four
Fe3+ based double-stranded helicates namely; [Fe2(L)2](ClO4)(Cl)·4(CH3OH)·2(H2O) (1), [Fe2(L)2](BF4)2·2
(H2O) (2), [Fe2(L)2](NO3)2·3(CH3OH)·2(H2O) (3), and [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) (4) are reported. Com-
plexes 1–4 have been synthesized using the hydrazide-based ligand H2L (H2L = N’1,N’4-bis(2-hydroxy-
benzylidene)succinohydrazide) and the corresponding Fe2+ salts. Each of the independent cationic
complexes [Fe2(L)2]2+ shows double-stranded helicates from the self-assembly of the ligand and metal
ions in a 2 : 2 ratio, where the individual Fe3+ centre is lying on a C2-axis and the ligand strands wrap
around it. In 1–4, ligand L adopts “pseudo-C” conformations and forms a double-stranded dinuclear heli-
cate with a small cage in between them. Moreover, in 1–4, each of the independent cationic complexes
[Fe2L2]2+ is inherently chiral and possesses P for right-hand and M for left-hand helicity and as a conse-
quence is a racemic solid. Detailed magnetic studies of all the complexes reveal that the Fe3+ centres are
magnetically isolated and isotropic in nature. Estimation of the Magnetocaloric Effect (MCE) from magne-
tization data unveils a moderate MCE at a temperature of 3 K with magnetic entropy changes (−ΔSm) of
22.9, 27.7, 24.1, 26.5 J kg−1 K−1 at a magnetic field of 7 T for complexes 1–4, respectively. Also, the vari-
ation of the −ΔSm values was justified by considering the parameter of magnetization per unit mass.
Stability of all the complexes in solution phase was confirmed by ESI-mass spectrometric analysis and
liquid phase FT-IR spectroscopy. Further, the interaction of the complexes 1–4 with Rhodamine B dye
was examined by UV-vis and fluorescence spectroscopic study. The observed blue-shift in the fluor-
escence study and hyperchromicity and hypochromicity with the appearance of two isosbestic points
in the UV-vis study ascertain the interactions of the dye with the complexes. A DNA binding study
by absorption spectral titration suggests the weak external intercalation of complex 1 within the nucleo-
tide of calf thymus DNA. Computational study supports the isotropic nature of the metal centres and the
consequent high spin multiplicity, which assists the complexes to show significant magnetic entropy
changes.
Received 26th April 2015,
Accepted 21st July 2015
DOI: 10.1039/c5dt01569c
www.rsc.org/dalton
molecules in materials science.1–6 However, the chemical
structures of helicates can be modulated by the use of a variety
Introduction
Over the past few decades, helicates have attracted the atten- of multi-dentate ligands and metal centres. There has been
tion of chemists and biologists not only because of their long standing significant interest not only due to their fasci-
aesthetically pleasing structural beauty but also because nating structural self-assembly7 but also their potential appli-
they are considered to be artificial mimics of protein helices, cations in chiral catalysis8 and supramolecular devices.9
promising biologically active substances, and attractive Although a large number of molecular helicates have been
reported using ditopic or tritopic ligands, designing molecular
assemblies with interesting diverse magnetic, as well as bio-
logical, properties and the existence of both the properties in
Department of Chemistry, IISER Bhopal, Bhopal 462066, MP, India.
a single system, is still a challenging task. Therefore, the
choice of suitable metal ions and ligand system is crucial to
induce different intriguing properties in a single molecular
system.
Most reported helicates contain imines, hydrazones/
hydrazides, bis(bipyridine), dicatechol and benzimidazole
E-mail: skonar@iiserb.ac.in, aadhikary87@gmail.com; Fax: +91-755-6692392;
Tel: +91-755-6692339
†Electronic supplementary information (ESI) available: X-ray crystallographic
data for complexes 1–4 in CIF format, synthesis of the ligand, structural figures,
tables of bond distances and bond angles, hydrogen bonding table and table of
significant magnetic entropy changes. CCDC 995297–995300. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c5dt01569c
This journal is © The Royal Society of Chemistry 2015
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ligand derivatives with a variety of metal centres.2a,b An ade-
quate number of iron based helicates and mesocates have
been reported by Oshio, Willams, Kruger and several others
for their potential use as Spin Cross Over (SCO) materials.10–14
Particularly, helicate complexes of polydentate dihydrazone
ligands have drawn much attention because of their vital role
in DNA binding, cytotoxic activity on human cervix carcinoma
cells, tumor cell lines, etc.15 Similarly, the choice of metal ion
also receives equal importance for designing metal related
functional properties.
Recently, the development of magnetic materials for
magnetic refrigeration is an emerging area of interest because
of their potential application to economically replace high
cost conventional compressor based refrigerants for ultra-low
temperature applications.16,17 The majority of magnetic
refrigerants constitute gadolinium as a metal ion. However,
very few transition metal based magnetic refrigerants have
been reported18–20 and among them very few are Fe3+ based.20
Magnetic refrigeration happens because of a phenomenon
known as the magnetocaloric effect (MCE) and the magnitude
of the MCE of a magnetic material is characterized by ΔSm,
isothermal magnetic entropy change and ΔTad, adiabatic
temperature change following a change in the applied mag-
netic field. A large MCE of a molecule can be achieved
because of (a) the large spin ground state of metal ions,
which provides the largest entropy per single ion which
amounts to R ln(2S + 1), (b) zero orbital momentum, which
implies that the crystal field effects are extremely small, and
Scheme 1 General schematic representation of the formation of the
dinuclear double helicate in complexes 1–4.
Experimental
Materials and methods
The reagents were used as received from Sigma Aldrich and
Co. without any further purification. Magnetic susceptibility
and magnetization measurements were carried out on a
Quantum Design SQUID-VSM magnetometer. Direct current
magnetic measurements were performed using an applied
field of 0.1 T in the 1.8 K–300 K temperature range. The
measured values were corrected for the experimentally
measured contribution of the sample holder, while the derived
susceptibilities were corrected for the diamagnetism of the
samples, estimated from Pascal’s tables.22 BVS calculations
were done following the procedure given by Liu and Thorpe.23
The elemental analyses were carried out on an Elementar
Micro vario Cube Elemental Analyzer. FT-IR spectra
(4000–400 cm−1) were recorded as KBr pellets using a Perkin
Elmer Spectrum BX spectrometer.
6
(c) weak superexchange interactions. Fe3+ has a S5/2 ground
state term (S = 5/2, L = 0) with high spin and zero orbital
momentum. If the magnetic centres are separated by a large
distance there will be negligible magnetic exchange. More
precisely, none of the iron based helicates/mesocates have
been explored towards their application in magnetic refriger-
ation and hence, their exploration will be an exciting and
timely attempt.
In our earlier report, we synthesized various hydrazide
based molecular systems using copper and lanthanide metal
ions and explored their magnetic properties.21 Herein, we
present the synthesis, structural analysis, magnetic, Rhoda-
mine B, and CT-DNA binding studies of four Fe3+ based
double-stranded helicates (Scheme 1), namely [Fe2(L)2](ClO4)
(Cl)·4(CH3OH)·2(H2O) (1), [Fe2(L)2](BF4)2·2(H2O) (2), [Fe2(L)2]
(NO3)2·3(CH3OH)·2(H2O) (3), and [Fe2(L)2](Cl)2·2(CH3OH)·4
X-ray crystallography
(H2O) (4) using the hydrazide based ligand H2L (H2L = N′1, Data collection of the complexes 1, 3 and 4 was performed on
N′4-bis(2-hydroxybenzylidene)succinohydrazide) and the a Bruker Smart Apex II CCD diffractometer using graphite
corresponding Fe2+ or Fe3+ salts. Magnetic study of 1–4 reveals monochromated MoKα (λ) (0.71073 Å) radiation and complex 2
a moderate magnetocaloric effect. DNA binding studies were on a Bruker D8 venture CCD diffractometer using graphite
investigated by UV-vis study, which suggests the weak external monochromated Cu-Kα (λ = 1.54718 Å) radiation at 120 K
intercalation of complex 1 within the nucleotide of calf using a cold nitrogen stream. Data reduction and cell refine-
thymus DNA. In addition, the interaction of complexes 1–4 ments were performed with the SAINT program24 and the
with the dye Rhodamine B was examined by UV-vis and fluo- absorption correction program SADABS25 was employed to
rescence spectroscopic study. DFT analysis shows that two Fe3+ correct the data for absorption effects. Crystal structures were
centres are well separated from each other and experience no solved by direct methods and refined with full-matrix least-
interaction between them, which suggests the isotropic nature squares (SHELXL-14)25b and the OLEX software26 with atomic
of each Fe3+ centre.
coordinates and anisotropic thermal parameters for all non-
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hydrogen atoms. X-ray crystallographic data in CIF format are for C40H52Cl2Fe2N8O18: C 43.07, N 10.04, H 4.69; found C
available in CCDC 995297–995300.
43.18, N 10.12, H 4.82; Selected IR data (KBr pellet;
4000–400 cm−1): 3411(b), 2930(w), 1601(s), 1440(m), 1390(s),
1313(s), 1202(s), 1102(m), 901(m).
Computational study
All the theoretical calculations were performed using density
Synthesis of [Fe2(L)2](BF4)2·2(H2O) (2). 35.4 mg (0.1 mmol)
functional theory (DFT) with Becke’s three-parameter hybrid of ligand H2L and 67.4 mg (0.2 mmol) of Fe(BF4)2·6H2O were
exchange functional and the Lee–Yang–Parr correlation func- taken in 10 mL of methanol in a round bottomed flask and
tional (B3LYP). A dicationic structure of the complexes 1–4 was the reaction mixture was stirred for 4 h. The solution was fil-
considered for geometry optimization and calculations were tered and the filtrate was kept undisturbed at room tempera-
carried out in the gaseous phase. The double-ζ basis set of Hay ture for slow evaporation of the solvent. After a few days, dark
and Wadt (LanL2DZ) with a small core (1s2s2p3s3p3d4s4p4d) brown single crystals suitable for X-ray diffraction were
effective core potential (ECP) was used for all the atoms. obtained from the solution. The crystals were washed with
A quadratic convergence method was employed in the self- cold methanol and dried in air. Elemental analysis calcd (%)
consistent field (SCF) process. A high performance cluster was for C36H32B2F8Fe2N8O10: C 42.31, N 10.96, H 3.16; found C
used for running the jobs. The orbital and spin density plots 42.22,
N 10.81, H 3.03; Selected IR data (KBr pellet;
were generated by means of the Gaussian 09 package.27
4000–400 cm−1): 3425(b), 2967(w), 1601(s), 1390(s), 1301(s),
1202(s), 1085(m), 903(m), 754(m).
Rhodamine B binding studies
Synthesis of [Fe2(L)2](NO3)2·3(CH3OH)·2(H2O) (3). Complex
Four 50 mL stock solutions of the complexes 1–4 of concen- 3 was synthesized following the same procedure as for 2
tration 1 mg mL−1 were prepared in UV grade DMF solvent. by using Fe(NO3)3·9H2O (80.8 mg, 0.2 mmol) instead of
Stock solutions of Rhodamine B of concentration 8 μg mL−1 Fe(ClO4)2·xH2O.
Elemental
analysis
calcd
(%)
for
were prepared in the same solvent. Different sets of solutions C39H48Fe2N10O19: C 43.67, N 13.06, H 4.51; found C 43.55,
were prepared by mixing Rhodamine B and sample solutions N 13.21, H 4.63; Selected IR data (KBr pellet; 4000–400 cm−1):
varying the volume of Rhodamine B from 50 μL to 1 mL. After 3437(b), 2916(w), 1601(s), 1390(s), 1302(s), 1202(s), 1099(m),
keeping the solutions for 24 h without disturbing, UV-vis and 903(m).
fluorescence studies were performed.
Synthesis of [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) (4). Complex 4
was synthesized following the same procedure as for 1
by using FeCl2·4H2O (39.6 mg, 0.2 mmol) instead of
DNA binding studies
A 2.7 × 10−6 (M) concentrated stock solution of complex 1 was Fe(ClO4)2·xH2O.
Elemental
analysis
calcd
(%)
for
prepared by dissolving it in 90% DMF/5 mM Tris-HCl/50 mM C38H50Cl2Fe2N8O14: C 44.51, N 10.92, H 4.92; found C 43.92, N
NaCl buffer at pH 7.1. Solutions of CT-DNA in a buffer of 10.81, H 4.73; Selected IR data (KBr pellet; 4000–400 cm−1):
5 mM Tris HCl/50 mM NaCl in water provided a ratio of UV 3412(b), 2922(w), 1601(s), 1393(s), 1302(s), 1207(s), 1099(m),
absorbance at 260 and 280 nm, A260/A280, of 1.9, indicating 902(m).
that the DNA was sufficiently free from protein. Stock solutions
were stored at 4 °C and used on the same day. Maintaining a
constant concentration of the complex and varying the nucleic
acid concentration absorption, spectral titration experiments
were carried out.
Results and discussion
Synthetic aspects
Albrecht et al. proposed an empirical odd–even rule, which
Synthesis
demonstrates that the spacer of the ligand plays an important
The ligand H2L was synthesized following previously reported role in the formation of helicates or mesocates.29 It was
procedures.28
suggested that an even number of C atoms or an odd number
Caution! Although we encountered no problem, appropriate of C atoms of the alkyl linker can facilitate the formation of a
care should be taken in the use of the potentially explosive per- helicate or a mesocate, respectively.29 Also, the anion-induced
chlorate salts.
resolution of helicates and mesocates was reported which sig-
Synthesis of [Fe2(L)2](ClO4)(Cl)·4(CH3OH)·2(H2O) (1). nifies that small spherical anions such as chloride, bromide
35.4 mg (0.1 mmol) of ligand H2L and 50.8 mg (0.2 mmol) of and iodide and trigonal planer nitrate ions result in triple heli-
Fe(ClO4)2·xH2O were taken in 10 mL of methanol in a round cate formation whereas tetrahedral shaped perchlorate ions
bottomed flask and the reaction mixture was allowed to stir for result in triple mesocate formation.30
4 h. During the reaction a small amount of precipitate was
Taking the above design criteria into consideration we have
formed which was dissolved by the addition of a few drops of synthesized a hydrazine-based ligand that has two methylene
1 M HCl. The filtrate was kept undisturbed at room tempera- spacers between two amide functions (H2L) by the conden-
ture for slow evaporation of the solvent. After a few days, dark sation of succinic dihydrazide and salicylaldehyde in a 1 : 2
brown colored single crystals suitable for X-ray diffraction were ratio. It exhibits two tridentate (N2O) coordination sites away
obtained from the solution. The crystals were washed with from each other and two amide (C(O)–NH) functionalities. The
cold methanol and dried in air. Elemental analysis calcd (%) tridentate bridging unit restricts the two metal centres away
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of the ligands. So, it can be inferred that the ligands are tetra-
basic hexadentate and coordinate through the deprotonation
of all the labile protons in the enolic forms and bonding
taking place through the phenolic and enolic oxygen and azo-
methine nitrogen. The bands occurring in the 603–589,
530–519, 490–483, 456–430 and 371–343 cm−1 regions have
been assigned to ∼(Fe–O) (phenolic), ∼(Fe–O) (enolic) and
∼(Fe–N) vibrations, respectively.
Structural descriptions for 1–4
Scheme 2 Schematic representations for the formation of dinuclear
The crystal structures of the four complexes [Fe2L2](ClO4)(Cl)·4
(CH3OH)·2(H2O) (1), [Fe2L2](BF4)2·2(H2O) (2), [Fe2L2](NO3)2·3
(CH3OH)·3(H2O) (3) and [Fe2L2](Cl)2·2(CH3OH)·5(H2O) (4),
were investigated using single-crystal X-ray diffraction. The
crystallographic data and refinement parameters are summar-
double-stranded helicates for complexes 1, 2, and 4 (starting salt Fe2+
and for complex 3 (starting salt Fe3+) with both P and M helices.
)
ˉ
ized in Table 1. Complexes 1, 3 and 4 crystalize in the P1 space
group and each asymmetric unit contains one [Fe2L2]2+ cation,
one anion and some lattice solvent molecules. Whereas,
complex 2 crystalizes in the P2/c space group and its asym-
metric unit contains one [FeL]+ cation, one tetrafluoroborate
anion and one water molecule. BVS calculation further sup-
ports the oxidation state of the Fe3+ ion in each of the com-
plexes (Table S1†).
The structural analysis reveals that complexes 1–4 contain
similar dinuclear cationic units [Fe2L2]2+ with different anions
and number of solvent molecules. In all of the complexes, the
individual Fe3+ centre lies on the C2-axis and the ligand
strands wrap around it. A ball & stick representation of the cat-
ionic double-stranded helicate [Fe2L2]2+ found in 1 is illus-
trated in Fig. 1(a). The cationic structures of complexes 1, 3,
and 4 are distorted double helicates due to the absence of a
centre of symmetry. However, complex 2 exhibits a perfect
double-stranded helicate structure (Fig. 1(b)).
from each other, whereas the amide functionalities act as
H-donor sites for stabilizing anions or solvent in the lattice.
The metallation of H2L with three different Fe2+ salts results in
complexes 1, 2, and 4 and with Fe(NO3)3·9H2O results in
complex 3 (Scheme 2) and during the formation of the com-
plexes, Fe2+ was oxidized to Fe3+ in air.
All the complexes have similar cationic units ([Fe2L2]2+).
Although for the synthesis of complexes 1 and 4 tetrahedral
shaped perchlorate and tetrafluoroborate anions were used, we
did not observe any mesocate formation. Hence in all of the
complexes, the double-stranded helicate formation is indepen-
dent of the anions used. FT-IR spectral analysis of the com-
plexes (Fig. S1–S2†) reveals that bands in the range of the
3500–3250 cm−1 region are assigned to ν(OH) vibrations while
the bands in the range of 3250–3100 cm−1 are assigned to
ν(NH) vibrations. The bands in the 1531–1510 cm−1 region are
due to the –NvC(OH)– group which suggests the enolization
Table 1 Summary of the crystallographic data and refinement parameters for complexes 1–4
Molecule
1
2
3
4
Formula
Mr
Temperature/K
C
40H52Cl2Fe2N8O18
C36H36B2F8Fe2N8O10
1026.05
120
C39H48Fe2N10O18.75
1068.57
120
C38H50Cl2Fe2N8O14
1025.44
120
1115.50
120
Crystal system
Space group
Triclinic
P1
Monoclinic
P2/c
Triclinic
P1
Triclinic
P1
ˉ
ˉ
ˉ
a/Å
b/Å
c/Å
α/°
β/°
γ/°
V/Å
Z
10.7106(11)
12.6687(14)
19.429(2)
81.790(3)
83.552(3)
67.758(3)
2410.3(4)
2
12.280(4)
11.817(4)
15.788(5)
90
108.274(8)
90
12.1418(13)
12.4085(13)
15.8390(18)
74.689(4)
89.892(4)
85.089(4)
2292.6(4)
2
12.8832(6)
13.5040(6)
15.2650(7)
106.9900(17)
92.5610(17)
113.645(2)
2286.53(19)
2
2175.5(12)
2
λ (Å)
0.71073
1.537
0.794
1.54178
1.566
6.229
0.71073
1.548
0.721
0.71073
1.489
0.826
Dc/g cm−3
µ/mm−1
Reflection measured
Unique reflections
GOF
11 005
11 000
1.187
0.0693, 0.2032
4989
4217
1.162
0.0794, 0.2136
8326
5952
1.062
0.0664, 0.1847
9336
6862
1.031
0.0419, 0.1251
b
R1a, wR2
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo2 − Fc2)/∑(Fo2)2]1/2
.
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self-assembly might be due to the flexibility of the ligand and
the presence of the even number of flexible (–CH2–CH2–)
spacers in it.
In all of the cases, the Fe⋯Fe distances are in the range of
6.2–6.6 Å (Table 2) and the C2 axis passes through the Fe⋯Fe
vector (Fig. 2). In all of the complexes, both the meridional
ligand strands are bent at the –C(O)–CH2–CH2–C(O)– moieties
to have a “pseudo-C” conformation and the two –C(O)–NH–
functions in each ligand strand are orientated at an angle in
the range of ∼69°–90° (Table 2).
Moreover, in 1–4, each of the independent cationic com-
plexes [Fe2L2]2+ are inherently chiral and possess P for right-
hand and M for left-hand helicity (Fig. 3 and S3–S5†). Based
on the fact that all of the complexes crystalized in achiral
space groups, the unit cell accommodates an equal amount of
P and M enantiomers of the cationic complexes [Fe2L2]2+, thus
forming a racemic solid overall. In all of the complexes, four
hydrazide-H atoms (vN–NH–C(O)–) are involved in strong
Fig. 1 Ball-stick model of cationic units [Fe2(L)2]2+ found (a) in complex
1 and (b) in complex 2 illustrating double-stranded helicate formation.
Anions and solvents are omitted for clarity. Colour codes: C (grey), N
(blue), O (red), Fe (orange).
In all of the cases, ligand L possesses two identical triden- H-bonding interactions with the oxygen atoms of lattice water
tate chelating units (N2O) (homotopic = same denticity, same as well as with the respective anions (Fig. 4). The H-bond para-
connectivity, same donor atoms) dispersed along the helical meters are listed in Table S6.† Besides the above H-bonding
strand in a meridional manner to saturate the stereochemical interactions, various weak interactions such as C–H⋯O,
requirements around the Fe3+ centres. In all of the complexes, C–H⋯N, C–H⋯F, π⋯π and anion⋯π interactions are also
the Fe3+ centres feature the typical octahedral coordination perceptible.31 The packing diagram of 1 illustrates that the
environment where pairs of carbonyl-O and phenoxo-O are cis hydrophilic region consists of anions and solvent molecules
to each other whereas the hydrazide-N are trans to each other. and these are arranged in a 1D zigzag manner down the
In ligand L, the CvO bond distances are in the range of 1.249 b axis (Fig. S6†). The de-solvated de-anionic framework of 1
(5)–1.272(6) Å and the (O)C–NH bond distances are in the consists of small channels with dimensions of 4.5 × 5.0 Å2
range of 1.319(4)–1.337(6) Å which suggest that it is in the di- (Fig. S7†).
anionic keto form during coordination. In addition, the sym-
metrical ligand L, becomes unsymmetrical upon coordination
to the Fe3+ centre and forms a five-membered and a six-
membered chelate ring. Selected bond lengths and angles around
each Fe3+ centre in complexes 1–4 are listed in Tables S2–S5.†
The Fe–N/Fe–O coordinate bond distances and N–Fe–N/O–Fe–
O/N–Fe–O bond angles are in the range of reported dinuclear
Fe3+ helicates.14
In all the cases, ligand L adopts “pseudo-C” conformations
and forms a double-stranded dinuclear helicate structure with
a small cage in between them (Fig. 1). Interestingly the cage is
empty which might be due to the steric congestion inside the
small cage. Hence, it can be stated that in complexes 1–4, the
formation of helicates is independent of the anion present.
The mechanism of the formation of double-stranded helicates
Fig. 2 A view down the Fe⋯Fe vector of 1 emphasizing the double-
is a difficult task to analyse. However, the observed helicate
stranded helical conformation of the molecule.
Table 2 Comparative structural analysis among the complexes 1–4
Fe⋯Fe
Complex separation (Å)
Cage dimensions Dihedral angles between two
Dihedral angle (°) [–C(O)–
CH2–CH2–C(O)–]
Angle between (°) [–C(O)–
NH] functions
(Å2)
meridional plane (°)
1
2
3
4
6.264(1)
6.614(1)
6.252(1)
6. 250(1)
4.5 × 5.0
4.81 × 4.89
4.6 × 4.9
83.09, 83.70
88.49
83.12, 83.71
67.32, 83.98
−70.2(8), −69.3(7)
75.9(5)
−69.2(7), −70.2(8)
60.8(4), 72.8(4)°
79.00, 85.15
88.15
81.60, 89.86
72.91, 73.78
4.32 × 4.19
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The packing diagram of 3 illustrates that the phenoxo
groups of each discrete double-stranded helicate are stacked
through weak π⋯π interactions (4.494 and 5.026 Å) down the
c-axis (Fig. S10†). Similarly, the packing diagram of 4 illustrates
that each discrete double-stranded helicate is involved in the
above noted non-covalent interactions to form a 3D self-assem-
bly. It was found that in the packing, the solvent molecules
and two anions are arranged in a continuous distorted hexa-
gonal shape down the c axis (Fig. S11†).
Magnetic study
Fig. 3 Space-filling representations of the two independent cationic
complexes [Fe2L2]2+ found in complex 1 with M- and P-helicity. The two
strands are differently coloured and hydrogen atoms are omitted for
clarity.
DC susceptibility data of complexes 1–4 were collected using
polycrystalline samples in the temperature range of 1.8–300 K
at 0.1 T and are shown in the form of χMT (χM = molar mag-
netic susceptibility) as a function of temperature in Fig. 5 and
6. The experimental room temperature χMT values of 8.6, 8.3,
8.5, 8.7 cm3 mol−1 K for complexes 1, 2, 3 and 4, respectively
are in good agreement with the theoretical value of 8.7 cm3
mol−1 K for two isolated Fe3+ ions (S = 5/2, L = 0, g = 2.0). On
The packing diagram of 2 illustrates that each discrete
double-stranded helicate is involved in various non-covalent decreasing the temperature, these values are almost constant
up to ∼10 K which indicates the isotropic nature of the Fe3+
interactions with nearby helicates and is arranged in a helical
fashion with a pitch of 15.28 Å down the a-axis (Fig. S8†). In
ions. Below 10 K they decrease rapidly to reach values of 2.3,
the noted helical self-assembly, the lattice water molecules 3.4, 5.7, 3.6 cm3 mol−1 K for complexes 1–4, respectively at
and tetrafluoroborate ions are arranged in a 1D fashion
1.8 K. These may be due to saturation effects or weak antiferro-
(Fig. S9†).
magnetic behaviour. Although the cationic structures of all of
Fig. 4 Illustration of the hydrogen bonding interactions of the hydrazido-H (NH) atoms with the lattice solvents, and anions, respectively observed
in complexes 1 (a), 2 (b), 3 (c) and 4 (d).
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Fig. 7 Field dependent isothermal magnetization of complex 1.
Fig. 5 Temperature dependence χMT plot for complexes
1
and 2
measured at 0.1 T.
26.5 J kg−1 K−1 for the complexes 1–4 at 3 K for a field change
of ΔH = 7 T (Fig. 8, S16 and S17†).The maximum theoretical
entropy change for dinuclear Fe3+ is R ln(6) = 14.9 J mol−1 K−1
.
So, the calculated entropy changes of 26.7, 29.1, 27.8, 28.6 J
kg−1 K−1 for complexes 1–4, respectively, are close to the
experimental values. The corresponding volumetric entropy
changes are 34.9, 43.5, 37.3, 39.7 mJ cm−3 −1 for 1–4, respect-
K
ively. Since the N(Fe)/mol wt ratio varies for the complexes,
their magnetization values per unit mass also vary. So com-
plexes having the highest N(Fe)/mol wt ratio show the highest
magnetocaloric effect among the four complexes. Variations of
the −ΔSm value (experimental and theoretical) with magnetiza-
tion per unit mass and N(Fe)/mol wt ratio are given in Table 3.
Mass spectrometry studies
Fig. 6 Temperature dependence χMT plot for complexes
3
and 4
In order to ensure the stability of all of the complexes in the
solution phase, ESI-MS analysis were studied for complexes
1–4 in DMF. All of the complexes show highest intensities
peak with an m/z value of 815.2 (Fig. S18–S25†). The theore-
tical mass value of 815.2 of dicationic complexes [Fe2L2]2+ of
formula C36H31N8O8Fe2 nicely match with the experimental
m/z value of 815.2 for all of the complexes. Isotropic distri-
measured at 0.1 T.
the complexes are very similar, the plots are not exactly the
same. This may be due to a sort of weak interaction through
the anions. 1/χM vs. T plots were deduced for all of the com-
plexes and fitted with the Curie–Weiss equation (Fig. S12–
S13†). From the fitting of the plot C = 8.6 cm3 mol−1 K and θ =
−1.41 K for complex 1, C = 8.33 cm3 mol−1 K and θ = −1.66 K
for complex 2, C = 8.5 cm3 mol−1 K and θ = −0.72 K for
complex 3, and C = 8.7 cm3 mol−1 K and θ = −1.67 K for
complex 4 were obtained. Low and negative values of θ further
suggest the presence of the very weak antiferromagnetic inter-
actions of the complexes. The magnetic isotherms M/(Nβ) vs. H
show a saturation value of 9.9, 9.8, 9.4 and 9.6Nβ for com-
plexes 1–4, respectively for an applied field of 7 T and at 2 K.
The saturation values match well with the theoretical value of
10Nβ for two Fe3+ ions (g = 2), as it is isotropic in nature
(Fig. 7, S14 and S15†).
Magnetic entropy changes (−ΔSm) were calculated using
Ð
the Maxwell relationship: ΔSmðT; ΔHÞ ¼ ½@MðT; HÞ=@TꢀHdH
from magnetization data (Fig. 7, S14–S15†). On lowering the
temperature, the −ΔSm value gradually increases and the
maximum entropy changes are obtained as 22.9, 27.7, 24.1,
Fig. 8 Temperature dependencies (3
entropy change (−ΔSm) of complex 1 as obtained from magnetization
data.
K to 10 K) of the magnetic
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Table 3 Theoretical and experimental values of −ΔSm and their variation with N(Fe)/mol wt and magnetization/mol wt
Magnetiztion/mol wt
Experimental −ΔSm
Theoretical −ΔSm
Complex
N(Fe)/mol wt
(Nβ kg−1
)
(J kg−1 K−1
)
(J kg−1 K−1
)
1
2
3
4
1.79 × 10−3
1.95 × 10−3
1.89 × 10−3
1.92 × 10−3
8.96
9.75
9.46
9.60
22.9
27.7
24.1
26.5
26.7
29.1
27.8
28.6
bution patterns are in good agreement with the theoretical metabolic processes, such as activation, transport or detoxifi-
expectation, which indicates the existence of cationic helicate cation of molecular oxygen.32 Several dinuclear non-heme iron
structures in the solution phase. For further confirmation of proteins have been characterized by crystallographic studies
the stability of each of the complexes in solution phase, and their specific function and electronic fingerprints have
IR spectroscopy studies were performed on solids as well as on also been elucidated by biochemical, site directed muta-
samples in DMF solvent. Both solid and solution phase genesis, spectroscopic and theoretical studies.
spectra have similar IR patterns in the fingerprint region for
Additionally, it was evidenced that the groove of the double
all of the complexes (Fig. S1 and S2†). Also, the carbonyl helicate can interact with the dye through covalent or weak
stretching frequency at 1662 cm−1 remains the same in both interactions.33 Therefore, interaction of the dye Rhodamine B
solid and solution phase. So from FT-IR spectra it is evident with the complexes 1–4 was examined by UV-vis and fluo-
that all of the structures remain stable in the solution phase.
rescence spectroscopic analysis. Rhodamine B was chosen as a
dye because of its spectral-luminescence properties and an
ability to vary in the relationship between the fluorescence
quantum yield and intersystem crossing upon the formation
of associations,34 thus sharp maxima of the absorbance
spectra and emission spectra can be detected easily. These
experiments were performed because it is a very sensitive tech-
nique to study the changes in microenvironment resulting due
to the interaction of helicates with the dye molecules. In
general, the spectra of the octahedral Fe3+ complexes show one
absorption band in the visible region at around 436 nm due to
d–d transitions and Rhodamine B shows an absorption band
at 544 nm in DMF (Fig. 10). The electronic absorption spectra
of the complexes in the presence of Rhodamine B are given in
Fig. 11, S26 and S27.†
UV-Vis studies
In order to examine the dynamics of the helicates toward
different anions in solution, UV-vis studies were performed in
DMF solvent at room temperature (Fig. 9). Complexes 1–3
show two absorption maxima at around 278 nm and 356 nm.
However, the intensities of both of the maxima are much
higher for complex 2 than for 1 and 3. Interestingly, for
complex 4, the maximum at 356 nm almost disappears and
only one maximum is observed at 278 nm with the highest
intensity among the four complexes. The distinct behaviour
might be mainly due to different grooving of the helicates and
their interaction with different anions. Besides, the different
behaviour of complex 4 may be due to the spherical geometry
of the chloride anion rather than trigonal (nitrate ion) or tetra-
hedral (perchlorate and tetrafluoroborate) ions.
One sharp maximum at 560 nm and one broad maximum
at around 440 nm were observed for all of the complexes. The
concentration of the dye was varied from 2 × 10–5 (M) to 7 ×
10−5 (M). Upon increasing the concentration of the dye, the
Rhodamine B binding studies
It has been reported that dinuclear non-heme iron systems
comprise active components of a variety of enzymes in several
Fig. 10 UV-vis spectra of complexes 1–4 and Rhodamine B in DMF in
Fig. 9 UV spectra of complexes 1–4 in DMF.
the visible region.
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Fig. 11 UV spectra of Rhodamine B encapsulated complex 1 in DMF.
Fig. 12 Fluorescence spectra of Rhodamine B encapsulated complex 1
in DMF.
intensity of the absorption maxima at 560 nm increased and
the broad absorption maxima at around 440 nm decreased.
This hyperchromicity of one band and hypochromicity of
another band generates two isosbestic points at 595 nm and
495 nm. These changes are typical for complexes binding to
the dye through non-covalent interactions. The weak inter-
action is usually characterized by positive cooperativity, so that
the binding of one dye molecule facilitates the binding of the
second dye molecule, and so on.
actions.32 The UV-vis spectroscopic method was employed to
characterize the binding of the complexes with DNA, as it was
evidenced from the solution state ESI-MS analysis that only the
cationic part of the helicates are stable. UV-Vis and fluo-
rescence studies also reveal that the interaction of complexes
1–4 with the Rhodamine B dye are independent of the pres-
ence of corresponding anions which further support the stabi-
lity of only the cationic part of the complexes in the solution
state. Therefore, in the present investigation, only complex 1
was chosen for the DNA binding study with calf thymus DNA
(CT-DNA). An intense metal-to-ligand charge transfer (MLCT)
band for the complex at 278 nm was monitored against
different concentrations of CT-DNA.
Upon the addition of CT-DNA, a decrease in molar absorp-
tivity (hypochromism) and 2–9 nm shifts in the wavelength
(blue-shift) were observed and the hypochromism of the band
at 278 nm reached up to 8.2% (Fig. 13). The above observation
suggests the binding of complex 1 with DNA through an inter-
calative interaction.36 Hypochromism originates from the con-
traction of the DNA helix axes as well as the conformational
changes on the molecule of DNA, while hyperchromism
results from the secondary damage of the DNA double helix
structure.
As a result, associations of the dye molecules occur on the
exterior surface of the double helix.35 Thus, the noted change
in the microenvironment is reflected in the UV-Vis spectra. All
of the complexes show similar absorption spectra, which indi-
cates that the binding of Rhodamine B is almost independent
of the anions.
To ensure the interactions of the complex with Rhodamine
B only, a similar experiment was performed with only the
ligand and Rhodamine B (Fig. S28†). The UV-vis studies show
that upon an increase in the concentration of the dye, the
absorbance of the peak at 560 nm increases. However, there is
no appearance of an isosbestic point and hypochromicity. This
suggests that the ligand plays a vital role in the binding of the
complex with the dye. Overall, it confirms that the interaction
of the complex with Rhodamine B originates not completely
because of the ligand itself.
Further, the fluorescence properties study of complexes 1–4
revealed that with an increase in the concentration of the dye,
the emission maxima at around 585 nm increases and blue-
shifts were observed (Fig. 12, S29 and S30†). The bathochromic
shifts are in the range of 3–11 nm for all of the complexes.
These further confirm the interaction of the dye with the com-
plexes. It can also be pointed out that as there is no drastic
change in both the UV-Vis and fluorescence spectra, this indi-
cates no significant structural change of the complexes upon
binding with Rhodamine B.
Although complete intercalation of dihydrazone type
ligands between a set of adjacent base pairs (A⋯T or G⋯C) of
DNA is sterically difficult since the bis(bidentate) ligand
strands wrap around the metal centres, some partial intercala-
tion can be envisaged. To enable the quantitative estimation of
DNA binding affinities, the intrinsic binding constant (Kb) was
determined by using the equation37:
½DNAꢀ=ðεa ꢁ εf Þ ¼ f½DNAꢀ=ðεb ꢁ εf Þg þ f1=Kbðεb ꢁ εf Þg
where [DNA] is the concentration of DNA in base pairs, εa is
DNA binding studies
the extinction coefficient at
a given DNA concentration
Further, we aimed to explore the DNA binding ability of the obtained by calculating Aobsd/[complex], εf corresponds to the
complexes as they contain the grooves of the helicates, which extinction coefficient of the complex in its free form, and εb
can bind with DNA through various non-covalent inter- refers to the extinction coefficient of the complex in the fully
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Table 4 Fe⋯Fe distances obtained from the X-ray structures and DFT
calculations
Complex 1
Complex 2
Complex 3
Complex 4
X-ray
DFT
6.264(1)
6.345
6.612
6.345
6.252(1)
6.345
6.2496(7)
6.345
structures obtained from the DFT calculation are very similar
to the X-ray crystal structures. The Fe⋯Fe distances obtained
from the X-ray structures and geometry optimized structures
are comparable with each other (Table 4). So, this verifies the
helical arrangement of the complexes.
Fig. 13 Absorption spectra of complex 1 in the absence (- - -) and
Spin distribution analysis shows that there no magnetic
coupling occurs between the paramagnetic Fe3+ centres
(Fig. 15). Also, the spin density mainly concentrates on the
metal centres only, the –CH2–CH2– group of the ligands has
zero spin density. On one side, the amide conjugated part of
the ligand contains some spin density and the other side has
negligible spin density. This suggests weak transmission of
the spin density from the metal centres. Mulliken atomic spin
densities of the individual atoms are given in Table S7† and
the calculation results are summarized in Table S8.† The
HOMO and LUMO diagram of the dicationic structures reveals
the existence of isolated d-orbitals (Fig. 16). In both the
HOMO and LUMO cases the magnetic orbitals of Fe3+ promote
weak delocalization and do not interact with those of neigh-
bouring Fe3+ ions. As a result, there is no magnetic coupling
between the two magnetic centres. The contour surface
diagram of the spin density suggests there is clear separation
of two wave functions indicating no transmission of magnetic
exchange occurs between the metal centres (Fig. S31†). Also,
this suggests the isotropic nature of the Fe3+ ion. So, the evalu-
ation of the magnetic coupling constant from broken-sym-
metry calculations was not performed. As the Fe3+centres (L =
0) have no orbital contribution, they were expected to be isotro-
pic in nature. DFT calculations further confirm this isotropic
nature and hence, the system will have high spin multiplicity
with a theoretical magnetic entropy change of (−ΔSm) = nR ln
(2S + 1) = 2R ln 6(S = 5/2).
presence (-) of increasing amounts of CT-DNA.
bound form. Linear fitting of the data with the above equation
gives a slope of 1/(εb − εf) = 0.855 × 10−6 and an intercept of
1/Kb(εb − εf) = 3.62 × 10−10 (Fig. 14). These give a value of Kb =
2.3 × 104 mol−1 which lies in the normal range for character-
istic binding by intercalation.38 The value of Kb is lower than
that of a classical intercalator which indicates that the
complex interacts with DNA through weak external intercala-
tion and did not insert completely within the base pairs. This
is mainly due to steric clash generating from the long flexible
ligand of the complex. This also suggests that the affinity of
binding the complex to host DNA is less than for classical
intercalators. So, the interaction of CT-DNA with the complex
may lead to novel therapeutic agents for various treatments of
tumor cells, cancer cells, etc.
Fig. 14 Typical plots of [DNA]/(εa − εf) vs. [DNA] for the absorption
titration of CT-DNA with complex 1.
Computational study
In order to verify the isotropic nature of Fe3+ and the high
magnetic entropy change, a DFT study was performed. Geo-
metry optimization was carried out considering a spin multi-
plicity of 11 electrons, i.e. high spin Fe3+. The optimized
Fig. 15 Spin density plot computed for the UB3LYP level DFT optimized
di-cationic structures of 1–4. Hydrogen atoms are omitted for clarity.
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AA acknowledges CSIR for his SRF fellowships. HSJ thanks
IISER Bhopal for a postdoctoral fellowship. SK thanks DST,
Government of India (Project no. SR/FT/CS-016/2010) and
IISER Bhopal for generous financial and infrastructural
support.
Notes and references
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Conclusion
We have successfully designed and synthesised a hydrazide
based ligand by incorporating a (–(CH2)2–) chain in between
two amide (–C(O)NH–) functions and have shown it to be
capable of forming dinuclear double-stranded helicate
[Fe2(L)2]2+ complexes. Structural analysis shows that in com-
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derivatives to see whether these species might able to act as
potential candidates for magnetic refrigeration. Work along
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