based Double Stranded Helicates Showing

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A Family of Fe3+ based Double Stranded Helicates Showing Magnetocaloric Effect, Rhodamine-B Dye and DNA binding Activities Amit Adhikary*, Himanshu Sekhar Jena and Sanjit Konar*

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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Herein, 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), [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) (4 are reported. Complexes 1-4 have been synthesized using a hydrazide-based ligand H2L (H2L = N´1,N´4bis(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 ligand and metal ions in 2:2 ratio, where the individual Fe3+centre is lying on 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 consequence a racemic solid. Detailed magnetic studies of all the complexes reveal that Fe3+centres are magnetically isolated and isotropic in nature. Estimation of Magnetocaloric Effect (MCE) from magnetization data unveil moderate MCE at temperature of 3 K with magnetic entropy changes (-∆Sm) of 22.9, 27.7, 24.1, 26.5 J kg-1 K-1at the magnetic field of 7 T for complexes 1-4 respectively. Also variation of -∆Sm values were justified by considering the parameter of magnetization per unit mass. Stability of all the complexes in solution phase was confirmed by ESI- mass 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 isobestic points in the UV-vis study ascertain the interactions of the dye with the complexes. DNA binding study by absorption spectral titration suggests weak external intercalation of the complex 1 within the nucleotide of calf thymus DNA. Computational study supports the isotropic nature of metal centres and consequently high spins multiplicity, which assists the complexes to show significant magnetic entropy changes.

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Introduction Over the past few decades, helicates are attracting attention of chemists and biologists not only because of their aesthetically pleasing structural beauty but also considered as artificial mimics of protein helices, promising biologically active substances, and attractive molecules in materials science.1-6 However, the chemical structures of helicates can be modulated by the use of variety of multi-dentate ligands and metal centres. There has been a longstanding significant interest not only due to their fascinating structural self-assembly7 but also their potential applications in chiral catalysts8 and supramolecular devices9. Although, a large number of molecular helicates are reported using ditopic or tritopic ligand, designing molecular assembly

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with interesting diverse magnetic as well as biological properties and existence of both the properties in a single system is still a 45 challenging task. Therefore, choice of suitable metal ions and ligand system is crucial to induce different intriguing properties in a single molecular system. Mostly reported helicates contain imines, hydrazones/hydrazides, bis(bipyridine), dicatechol and 50 benzimidazole ligand derivatives with variety of metal centres.2a,2b An adequate number of iron based helicates and mesocates are 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 55 dihydrazone ligands draw much attention because of its vital role in DNA binding, cytotoxic activity on human cervix carcinoma

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Scheme 1. General schematic representation of the formation of dinuclear double helicate in complexes 1-4.

(3), [Fe2(L)2](Cl)2·2(CH3OH)·4(H2O) (4) using hydrazide based ligand H2L (H2L = N´1, N´4-bis(2-hydroxybenzylidene)2+ 45 succinohydrazide) and corresponding Fe or Fe3+ salts. Magnetic study of 1-4 reveals moderate magnetocaloric effect. DNA binding studies were inspected by UV-vis study, which suggests weak external intercalation of complex 1 within the nucleotide of calf thymus DNA. In addition, the interaction of the complexes 150 4 with the dye Rhodamine-B was examined by UV-vis and fluorescence spectroscopic study. DFT analysis shows that two Fe3+ centres are well separated from each other and experienced no interaction between themselves, which suggests the isotropic nature of each Fe3+ centres. 55

Experimental Materials and methods The reagents were used as received from Sigma Aldrich and Co. 60 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 with an applied field of 0.1 T in the 1.8 K - 300 K temperature range. The measured values were 65 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 70 out on an Elementar Micro vario Cube Elemental Analyzer. FTIR spectra (4000-400 cm-1) were recorded on KBr pellets with a Perkin Elmer Spectrum BX spectrometer. X-ray Crystallography Data collection of the complexes 1, 3 and 4 were performed on a 75 Brüker Smart Apex II CCD diffractometer using graphite monochromated MoKα (λ) (0.71073 Å) radiation and complex 2 on Bruker D8 venture CCD diffractometerusing graphite monochromated Cu-Kα (λ=1.54718Å) radiation at 120 K using a cold nitrogen stream. Data reduction and cell refinements were 24 80 performed with the SAINT program and the absorption 25 correction program SADABS was employed to correct the data for absorption effects. Crystal structures were solved by direct methods and refined with full-matrix least-squares (SHELXL14)25b and OLEX software26 with atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms. Xray crystallographic data in CIF format are available in CCDC 995297-995300.

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In our earlier report, we have synthesized various hydrazide based molecular system using copper and lanthanide metal ions and explored their magnetic properties.21 Herein we present the synthesis, structural analysis, magnetic, Rhodamine B, 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) 2|Journal Name, [year], [vol], 00–00

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Computational Study All the theoretical calculations were performed using density functional theory (DFT) with Becke’sthree-parameter hybrid exchange functional and the Lee−Yang−Parr correlation functional (B3LYP). Dicationic structure of the complexes 1-4 was considered for geometry optimization and calculations were carried out in gaseous phase. The double-ζ basis set of Hay and Wadt (LanL2DZ) with a small core (1s2s2p3s3p3d4s4p4d) effective core potential (ECP) was used for all the atoms. A quadratic convergence method was employed in the selfconsistent field (SCF) process. High performance cluster was used for running the jobs. The orbital and spin density plots were generated by means of the Gaussian 09 package.

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DOI: 10.1039/C5DT01569C

cells, tumor cell lines etc.15Similarly, choice of metal ion also receives an equal importance for designing metal related functional properties. Recently, development of magnetic materials for magnetic 5 refrigeration is an emerging area of interest because of their potential application to replace economically the high cost conventional compressor based refrigerant for ultra-low temperature applications.16,17 The majority of the magnetic refrigerants constitutes gadolinium as metal ion. However very 10 few transition metal based magnetic refrigerants have been reported18-20 and among them very few are Fe3+ based.20 The magnetic refrigeration happens because of phenomena known as Magneto Caloric Effect (MCE) and the magnitude of the MCE of a magnetic material is characterized by ∆Sm, isothermal magnetic 15 entropy change and ∆Tad, adiabatic temperature change following a change in the applied magnetic field. Large MCE of the molecule can be achieved because of (a) large spin ground state of metal ions, which provides the largest entropy per single ion which amounts to Rln(2S + 1), (b) zero orbital momentum, which implies that the crystal field effects are extremely small, and (c) weak superexchange interactions. Fe3+ has 6S5/2 ground state term (S = 5/2, L =0) with high spin and zero orbital momentum. If the magnetic centres are separated by large distance there will be negligible magnetic exchange. More precisely, none of the iron 25 based helicates/mesocates are explored towards their application in magnetic refrigeration and hence their exploration will be exciting and timely attempt.

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Rhodamine B binding Studies Four 50 mL stock solutions of the complexes 1-4 of concentration 1 mg/mL were prepared in UV grade DMF solvent. Stock solutions of Rhodamine-B of concentration 8 µg/mL were 5 prepared in the same solvent. Different sets of solution were prepared by mixing Rhodamine-B and sample solutions varying the volume of Rhodamine-B from 50 µL to 1 mL. After keeping the solutions for 24 h without disturbing, UV-vis and Fluorescence studies were performed. 10

DNA binding Studies 2.7 x 10-6 (M) concentrated stock solution of complex 1 was prepared by dissolving it in 90% DMF/5 mM Tris-HCl/50 mM NaCl buffer at pH 7.1. Solutions of CT DNA in the buffer 5 mM 15 Tris HCl/50 mM NaCl in water provided a ratio of UV absorbance at 260 and 280 nm, A260/A280, of 1.9, indicating that the DNA was sufficiently free of 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 20 acid concentration absorption spectral titration experiments were carried out. Synthesis The ligand H2L was synthesized following previously reported 28 25 procedures. Caution! Although we encountered no problem, appropriate care should be taken in the use of the potentially explosive perchlorate salt. Synthesis of [Fe2(L)2](ClO4)(Cl)·4(CH3OH)·2(H2O) (1): 35.4 30 mg (0.1 mmol) of ligand H2L and 50.8 mg (0.2 mmol) of Fe(ClO4)2·xH2O were taken in 10 mL of methanol in a round bottomed flask and the reaction mixture was allowed to stir for 4 h. During the reaction small amount of precipitate was formed which was dissolved by the addition of few drops of 1M HCl. 35 The filtrate was kept undisturbed at room temperature for slow evaporation of the solvent. After few days dark brown colored single crystals suitable for X-ray diffraction were obtained from the solution. The crystals were washed with cold methanol and dried in air. Elemental analysis calcd. (%) for 40 C40H52Cl2Fe2N8O18: C 43.07, N 10.04, H 4.69; found C 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). Synthesis of [Fe2(L)2](BF4)2·2(H2O) (2): 35.4 mg (0.1 mmol) of 45 ligand H2L and (67.4 mg, 0.2 mmol) of Fe(BF4)2·6H2O were taken in 10 mL of methanol in a round bottomed flask and the reaction mixture was stirred for 4 h. The solution was filtered and the filtrate was kept undisturbed at room temperature for slow evaporation of the solvent. After few days dark brown single 50 crystals suitable for X-ray diffraction were obtained from the solution. The crystals were washed with cold methanol and dried in air. Elemental analysis calcd. (%) for C36H32B2F8Fe2N8O10: C 42.31, N 10.96, H 3.16; found C 42.22, N 10.81, H 3.03; Selected IR data (KBr pellet; 4000 - 400 cm-1): 3425(b), 2967(w), 1601(s), 55 1390(s), 1301(s), 1202(s), 1085(m), 903(m), 754 (m). Synthesis of [Fe2(L)2](NO3)2·3(CH3OH)·2(H2O) (3): Complex 3 was synthesized following the same procedure as for 2 by using Fe(NO3)3·9H2O (80.8 mg, 0.2 mmol) instead of Fe(ClO4)2·xH2O.


Elemental analysis calcd.(%) for C39H48Fe2N10O19: C 43.67, N 13.06, H 4.51; found C 43.55, N 13.21, H 4.63; Selected IR data (KBr pellet; 4000 - 400 cm-1): 3437(b), 2916(w), 1601(s), 1390(s), 1302(s), 1202(s), 1099(m), 903(m). 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 65 FeCl2·4H2O (39.6 mg, 0.2 mmol) instead of Fe(ClO4)2·xH2O. Elemental analysis calcd. (%) for C38H50Cl2Fe2N8O14: C 44.51, N 10.92, H 4.92; found C 43.92, N 10.81, H 4.73; Selected IR data (KBr pellet; 4000 - 400 cm-1): 3412(b), 2922(w), 1601(s), 1393(s), 1302(s), 1207(s), 1099(m), 902(m). 60

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RESULTS AND DISCUSSIONS: Synthetic aspects. Albrecht et al. proposed an empirical oddeven rule, which demonstrates that the spacer of the ligand plays an important role in the formation of helicates or mesocates.29 It 75 was suggested that an even number of C atoms or an odd number of C atoms of the alkyl linker can facilitate the formation of a helicate or a mesocate, respectively.29 Also anion-induced resolution of helicates and mesocates was reported which signifies that small spherical anion such as chloride, bromide and 80 iodide and trigonal planer nitrate ions results triple helicate formation whereas tetrahedral shaped perchlorate ion results triple mesocate formation.30 Taking the above design criteria into consideration we have synthesized a hydrazine-based ligand having two methylene 85 spacer between two amide functions (H2L) by the condensation of succinic dihydrazide and salicyldehyde in 1:2 ratio. It exhibits two tridentate (N2O) coordination sites away from each other and two amide (C(O)-NH) functionalities. The tridentate bridging unit restricts two metal centres away from each other whereas amide 90 functionalities acts as H-donor site for stabilizing anions or solvent in the lattice. The metalation of H2L with three different Fe2+ salts result complex 1, 2, 4 and with Fe(NO3)3·9H2O results complex 3 (Scheme 2) and during the formation of the complexes, Fe2+ was oxidized to Fe3+ in air.

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Scheme 2. Schematic representations for the formation of dinuclear double stranded helicates for complexes 1, 2, 4 (starting salt Fe2+) and for complex 3 (starting salt Fe3+) with both P and M helices. 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, however we did not observe any mesocate formation. Hence in all the complexes, the double stranded helicate formation is 105 independent of anions used. FT-IR spectral analysis of the

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complexes (Fig. S1-S2) reveal that bands in the range of 35003250 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 band in the 1531-1510 cm-1 region are due to the 5 –N=C(OH)-group which suggests the enolization of the ligands. So it can be inferred that the ligands are tetrabasic hexadentate and coordinate through deprotonation of all the labile protons in the enolic forms and bonding taking place through the phenolic and enolic oxygen and azomethine nitrogen. The bands occurring -1 10 in the 603-589, 530-519, 490-483, 456-430 and 371-343 cm region have been assigned to ~ (Fe - O) (phenolic), ~ (Fe - O) (enolic) and ~ (Fe - N) vibrations, respectively.

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Structural Description for 1-4 The crystal structures of 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 by single-crystal X-ray

diffraction. The crystallographic data and refinement parameters are summarized in Table 1. Complexes 1, 3 and 4 crystalizes in P-1 space group and each asymmetric unit contains one [Fe2L2]2+ cation, one anion and some lattice solvent molecules. Whereas complex 2 crystalizes in P2/c group and its assymetric unit contains one [FeL]+ cation, one tetrafluoroborate anion and one 25 water molecules. BVS calculation further supports the oxidation state of Fe3+ ion in each complexes (Table S1). The structural analysis reveals complexes 1-4 contains similar dinuclear cationic units [Fe2L2]2+ with different anions and number of solvent molecules. In all of the complexes individual 3+ 30 Fe centre is lying on C2-axis and the ligand strands wrap around it. A ball & stick representation of cationic double strand helicate [Fe2L2]2+ found in 1 is illustrated in Fig. 1 (a). The cationic structures of complexes 1, 3, 4 are distorted double helicate due to the absence of centre of symmetry. However complex 2 35 exhibits a perfect double stranded helicate structures (Fig.1 (b)). 20

Table 1.Summary of crystallographic data and refinement parameters for complexes1-4.

Molecule Formula

1 C40H52Cl2Fe2N8O18

2 C36H36B2F8Fe2N8O10

3 C39H48Fe2N10O18.75

4 C38H50Cl2Fe2N8O14

Mr

1115.50

1026.05

1068.57

1025.44

Temperature/K

120

120

120

120

Crystal System

Triclinic

Monoclinic

Triclinic

Triclinic

P2/c

Space Group a /Å

10.7106(11)

12.280(4)

12.1418(13)

12.8832(6)

b/ Å

12.6687(14)

11.817(4)

12.4085(13)

13.5040(6)

c/ Å

19.429(2)

15.788(5)

15.8390(18)

15.2650(7)

α/◦

81.790(3)

90

74.689(4)

106.9900(17)

β/◦

83.552(3)

108.274(8)

89.892(4)

92.5610(17)

γ /◦

67.758(3)

90

85.089(4)

113.645(2)

V/ Å

2410.3(4)

2175.5(12)

2292.6(4)

2286.53(19)

Z

2

2

2

2

0.71073

1.54178

0.71073

0.71073

Dc/g cm

1.537

1.566

1.548

1.489

-1

µ/mm

0.794

6.229

0.721

0.826

Reflection Measured

11005

4989

8326

9336

Unique Refelctions

11000

4217

5952

6862

GOF

1.187

1.162

1.062

1.031

0.0693, 0.2032

0.0794, 0.2136

0.0664, 0.1847

0.0419, 0.1251

λ (Å) -3

a

R1 , wR2

b

a

2

2

R1 = Σ||Fo| - |Fc||/Σ|Fo|, bwR2 = [Σw(Fo2- Fc )/Σ( Fo )2]

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(a)

(b) 2+

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Fig. 1 Ball-stick model of cationic units [Fe2(L)2] found (a) in complex 1 and (b) in complex 2 illustrating double stranded helicates formation. Anions and solvents are omitted for clarity. Colour codes: C (grey), N (blue), O (red), Fe (orange).

In all the cases, ligand L posses two identical tridentate chelating units (N2O) (homotopic = same denticity, same connectivity, same donor atoms) disposed along the helical strand in 10 meriodional manner to saturate the stereochemical requirements around the Fe3+centres. In all the complexes, Fe3+centres feature the typical octahedral coordination environment where pairs of carbonyl-O and phenoxo-O are in cis to each other whereas the hydrazide-N are trans to each other. In ligand L, the C=O bond 15 distances are in the range of 1.249(5) – 1.272(6) Å and (O)C-NH bond distances are in the range of 1.319(4) – 1.337(6) Å which suggest that it is in the dianionic keto form during the coordination. In addition, the symmetrical ligand L, becomes unsymmerical 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+centres in complexes 1-4 are listed in Table 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 25 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 30 helicates are independent of the anion present. The mechanism of the formation of double stranded helicates is a difficult task to analyse. However the observed helicate self-assembly might be the due to the flexibility of the ligand and presence of even number of flexible (-CH2-CH2-) spacer in it. 35 In all the cases, the Fe···Fe distances are in the range of 6.2 - 6.6 Å (Table 2) and the C2 axis pass through the Fe···Fe vector (Fig. 2). In all the complexes, both the meridional ligand strands are bent at -C(O)-CH2-CH2-C(O)- moieties to have the “pseudo-C” conformation and the two -C(O)-NH- functions in each ligand 40 strand are orientated at an angle in the range of ~ 69° - 90° (Table 2).

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M 50

20


Fig. 2 A view down the Fe···Fe vector of 1 emphasizing the double stranded helical conformation of the molecule.

P

Fig. 3 Space-filling representations of the two independent cationic 2+ complexes [Fe2L2] found in complex 1 with M- and P-helicity. The two strands are differently coloured and hydrogen atoms are omitted for clarity.

Moreover, in 1-4, each of the independent cationic complexes [Fe2L2]2+ are inherently chiral and possesses P for right-hand and M for left-hand helicity (Fig. 3 and S3-S5). Based on the fact that all the complexes crystalized in achiral space group, the unit cell accommodates equal amount of P and M enantiomers of the 2+ 60 cationic complexes [Fe2L2] , thus forming a racemic solid in overall. In all the complexes, four hydrazide-H atoms (=N-NHC(O)-) are involved in strong H-bonding interactions with oxygen atoms of lattice water as well as with the respective anions (Fig. 4). The H-bond parameters are listed in Table S6. Besides the 65 above H-bonding interactions, various weak interactions such as C-H···O, C-H···N, C-H···F, π···π and anion···π interactions are 31 also perceptible. Packing diagram of 1 illustrates that the hydrophilic region consists of anions and solvent molecules and those are arranged in 1D zigzag manner down the b axis (Fig. 70 S6). The de-solvated de-anionic framework of 1 consists of small channels of dimensions 4.5 x 5.0 Å2 (Fig. S7). 55


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(a)

(b)

5

(c)

(d)

Fig. 4. Illustration of hydrogen bonding interaction of hydrazido-H (NH) atoms with the lattice solvents, and anions respectively observed in complexes 1(a), 2 (b), 3 (c) and 4(d). 10

Table 2. A comparative structural analysis among complexes 1-4. Complex

Fe···Fe separation (Å)

Cage dimensions (Å2)

Dihedral angles between two meriodional plane(°)

Dihedral angle (°)[-C(O)CH2-CH2-C(O)-] -70.2(8), -69.3 (7)

79.00, 85.15

75.9(5)

88.15

1

6.264(1)

4.5 x 5.0

83.09, 83.70

2

6.614(1)

4.81 x 4.89

88.49

3

6.252(1)

4.6 x 4.9

83.12, 83.71

4

6. 250(1)

4.32 x 4.19

67.32, 83.98

Packing diagram of 2, illustrates that each discrete double 15 stranded helicates are involved in various non-covalent interactions with nearby helicates and arranged in a helical fashion with a pitch of 15.28 Å down the a- axis (Fig. S8 ). In the noted helical self-assembly the lattice water molecules and tetrafluoroborate ions are arranged in 1D fashion (Fig. S9). 20 Packing diagram of 3 illustrates that the the phenoxo groups of each discrete double stranded helicates are stacked through weak π···π interactions (4.494 and 5.026 Å) down the c-axis (Fig. S10). Similarly, packing diagram of 4 illustrates that each discrete double stranded helicates are involved in above noted 25 non-covalent interactions to form a 3D self-assembly. It was 6|Journal Name, [year], [vol], 00–00

-69.2 (7), -70.2(8) 60.8(4), 72.8(4)°

Angle between (°) [-C(O)-NH] functions

81.60, 89.86 72.91, 73.78

found that in the packing solvent molecules and two anions are arranged in a continuous distorted hexagonal shape down the c axis (Fig. S11). 30

Magnetic Study

DC susceptibility data of complexes 1-4 were collected on 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 magnetic 35 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 well agreed with the theoretical values of 8.7 cm3 mol-1 K for two This journal is © The Royal Society of Chemistry [year]



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isolated Fe3+ ions (S = 5/2, L = 0, g = 2.0). On decreasing temperature, these values are almost constant up to ~ 10 K which indicates isotropic nature of Fe3+ ions. Below 10 K it decreases rapidly to reach the value of 2.3, 3.4, 5.7, 3.6 cm3 mol-1 K for the 5 complexes 1-4 respectively at 1.8 K. These may be due to saturation effect or weak antiferromagnetic behaviour. Although cationic structures of all the complexes are very similar, the plots are not exactly same. This may be due to sort of weak interactions through the anions. 1/χMvs T plots were deduced for 10 all the complexes 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, 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 presence of very weak antiferromagnetic interactions of the complexes. The magnetic isotherms M/(Nβ) vs. H show a saturation value of 9.9, 9.8, 9.4 and 9.6 Nβ of complexes 1-4 respectively for an applied field of 7 T and at 2 K. The saturation values match well with the theoretical value of 10 Nβ for two Fe3+ ions (g = 2) as it is isotropic in nature (Fig. 7, Fig. S14 and Fig. S15).

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Fig.7 Field dependent isothermal magnetization of complex 1.

35

for the complexes 1-4 at 3 K for the field change of ∆H = 7 T (Fig. 8, Fig. S16 and Fig. S17).The maximum theoretical entropy changes for dinuclear Fe3+ is Rln(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 40 complex 1-4 respectively are close to the experimental values. Corresponding volumetric entropy changes are 34.9, 43.5, 37.3, 39.7 mJ cm-3 K-1 for 1-4 respectively. SinceN(Fe)/mol. wt. ratio vary for the complexes, their magnetization values per unit mass also vary. So complexes having highest N(Fe)/mol. wt. ratio 45 show highest magneto caloric effect among the four complexes. Variation of -∆Smvalue (experimental and theoretical) with magnetization per unit mass and N(Fe)/mol. wt. ratio are given in Table 3.

Fig. 5 Temperature dependence χMT plot for complexes 1 and 2 measured at 0.1 T.

50

Fig. 8 Temperature dependencies (3 K to 10 K) of magnetic entropy change (-∆Sm) of complex 1 as obtained from magnetization data.

Table 3. Theoretical and experimental value of -∆Sm and their variation with N(Fe)/mol.Wt. and magnetization/ mol. Wt. 55

Fig. 6 Temperature dependence χMT plot for complexes 3 and 4 measured at 0.1 T.

30

Magnetic entropy changes (-∆Sm) were calculated using the Maxwell relation: ∆Sm(T, ∆H) = ∫[∂M(T,H)/∂T]HdH from magnetization data (Fig.7, Fig.S14-S15). On lowering the temperature, -∆Sm value gradually increases and the maximum entropy changes are obtained as 22.9, 27.7, 24.1, 26.5 J kg-1 K-1 This journal is © The Royal Society of Chemistry [year]

Complex

N(Fe)/mol. Wt.

Magnetiztion /mol. Wt. (Nβ kg-1)

Experimental -∆Sm (J kg-1 K-1)

Theoretical -∆Sm (J kg-1 K-1)

1

1.79 x10-3

8.96

22.9

26.7

2

1.95 x10-3

9.75

27.7

29.1

3

1.89 x10-3

9.46

24.1

27.8

4

1.92 x10-3

9.60

26.5

28.6



DOI: 10.1039/C5DT01569C


Dalton Transactions

Dalton Transactions


UV-Vis Studies In order to examine dynamic 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 25 intensities of both the maxima are much higher for complex 2 than 1 and 3. Interestingly for complex 4, the maximum at 356 nm is almost disappearing and only one maximum is observed at 278 nm with highest intensity among the four complexes. The distinct behaviour might be mainly due to different grooving of 30 the helicates and their interaction with different anions. Besides, the different behaviour of complex 4 may be due to spherical geometry of the chloride anion rather than trigonal (nitrate ion) or tetrahedral (perchlorate and tetrafluroborate) ions. 20

35

Rhodamine B Binding Studies

It has been reported that, dinuclear non-heme iron systems comprise a active components of variety of enzymes in several metabolic processes, such as activation, transport or detoxification of molecular oxygen.32 Several dinuclear non40 heme iron proteins have been characterized by crystallographic studies and their specific function and the electronic fingerprints have also been elucidated by biochemical, site directed mutagenesis, spectroscopic and theoretical studies.

Fig. 10 UV-vis spectra of complexes 1-4 and Rhodamine B in DMF in the visible region.

Additionally, it was evidenced that groove of the double helicate can interact with the dye through covalent or weak interactions.33 Therefore, interaction of the dye Rhodamine-B with the complexes 1-4 was examined by UV-vis and Fluorescence spectroscopic analysis. Rhodamine-B was chosen as a dye 55 because of it its spectral-luminescent properties and an ability to vary the relation between the fluorescence quantum yield and intersystem crossing upon formation of associations,34 thus sharp maxima of absorbance spectra and emission spectra can be detected easily. These experiments were performed because it is 60 a very sensitive technique 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 around 436 nm due to d-d transition and Rhodamine-B show 65 absorption band at 544 nm in DMF (Fig. 10). Electronic absorption spectra of the complexes in presence of Rhodamine-B are given in Fig. 11, Fig. S26 and Fig. S27. One sharp maximum at 560 nm and one broad maximum around 440 nm were observed for all the complexes. Concentration of -5 70 the dye was varied from 2 x 10 (M) to 7 x 10-5 (M). Upon increasing the concentration of the dye, the intensity of the absorption maxima at 560 nm were increased and broad absorption maxima around 440 nm were decreased. This hyperchromicity of one band and hypochrmicity of another band 75 generates two isobestic points at 595 nm and 495 nm. These changes are typical for complexes binding to the dye through non-covalent interactions. The weak interaction is usually characterized by positive cooperativity. So that the binding of one dye molecule facilitates the binding of the second dye 80 molecule and so on. 50

45

Fig. 9 UV spectra of complexes 1-4 in DMF. Fig. 11 UV spectra of Rhodamine-B encapsulated complex 1 in DMF.





DOI: 10.1039/C5DT01569C

Mass Spectral Studies In order to ensure stability of all the complexes in solution phase, ESI-MS analysis were studied for complexes 1-4 in DMF. All the complexes show highest intensities peak with m/z value of 5 815.2 (Fig.S18-S25). The theoretical mass value of 815.2 of dicationic complexes [Fe2L2]2+ of formula C36H31N8O8Fe2 nicely matches with the experimental m/z value of 815.2 for all the complexes. Isotropic distribution patterns are in well agreement with the theoretical expectation, which indicates existence of 10 cationic helicate structure in the solution phase. For further confirmation of the stability of the each complexes in solution phase, IR spectral studies were performed in solid as well as in DMF solvent. Both solid and solution phase spectra have similar IR pattern in the finger print region for all the complexes (Fig. S1 -1 15 and Fig. S2). Also the carbonyl stretching frequency at 1662 cm remains same both solid and solution phase. So from FT-IR spectra it is evident that all the structures remain stable in solution phase.

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DOI: 10.1039/C5DT01569C

Upon addition of CT DNA, decrease in molar absorptivity (hypochromism) and 2- 9 nm shifts in wavelength (blue shift) were observed and the hypochromism of the band at 278 nm reached upto 8.2%. The above observance suggests the binding of complex 1 with DNA through intercalative interaction.36 50 Hypochromism originates from the contraction of DNA helix axes as well as the conformational changes on molecule of DNA, while hyperchromism results from the secondary damage ofDNA double helix structure.

Fig. 12 Fluorescence spectra of Rhodamine-B encapsulated complex 1 in DMF.

As a result, associations of the dye molecules occur on the exterior surface of the double helix.35 Thus the noted change in microenvironment reflects in the UV-Vis spectra. All the complexes show similar absorption spectra, which indicates that binding of Rhodamine-B is almost independent of anions. To ensure the interactions of the complex with Rhodamine B 10 only, similar experiment was performed only with the ligand and Rhodamine B (Fig. S28). The UV-vis studies show that with increasing the concentration of the dye, absorbance of peak at 560 nm increases. However there is no appearance of isobestic point and hypochromicity. These suggest that the ligand plays a 15 vital role for the binding of 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 increasing concentration of the dye, emission 20 maxima around 585 nm increases and blue shifts have been observed (Fig. 12, Fig. S29 and Fig. S30). The bathochromic shifts are in the range 3- 11 nm for all the complexes. These further confirm the interaction of the dye with the complexes. It can also be pointed out that, as there is no drastic change in both 25 UV-Vis and fluorescence spectra, indicating no significant structural change of the complexes upon binding with Rhodamine-B. 5

30

35

40

DNA Binding Studies Further we aimed to explore the DNA binding ability of the complexes as they contain groove of the helicates, which can bind with DNA through various non-covalent interactions32. UVvis 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 cationic part of the helicates are only stable. UV-Vis and fluorescence studies also reveal the interaction of complexes 1-4 with the Rhodamine B dye are independent of the presence of corresponding anions which further support the stability of only cationic part of the complexes in solution state. Therefore, in the present investigation only complex 1 was only chosen for the DNA binding study with calf thymus DNA (CT DNA). Intense metal to ligand charge transfer band (MLCT ) for the complex at 278 nm was monitored against different concentration of CT DNA.


55

Fig. 13 Absorption spectra of complex 1 in the absence (-- -) presence (-) of increasing amounts of CT-DNA.

and

Although complete intercalation of dihydrazone type ligands between a set of adjacent base pairs (A..T or G…C) of DNA is 60 sterically difficult since the bis(bidentate) ligand strands wrap around the metal centres, but some partial intercalation can be envisaged. To enable quantitative estimation of DNA binding affinities, intrinsic binding constant (Kb) was determined by using the equation37: 65

[DNA]/(εa-εf)) = [DNA]/(εb-εf)+1/Kb(εb-εf)

70

75

80

85

where [DNA] is the concentration of DNA in base pairs, εa is the extinction coefficient at a given DNA concentration obtained by calculating Aobsd/[complex], εf corresponds to the extinction coefficient of the complex in its free form, and εb refers to the extinction coefficient of the complex in the fully bound form. Linear fitting of the data with the above equation gives a slope of 1/(εb-εf) = 0.855 x 10-6 and intercept 1/Kb(εb-εf) = 3.62 x 10-10. -1 These give the value of Kb = 2.3 x 104 mol which lies in the normal range for characteristics binding by intercalation.38 The value of Kb is lower than classical intercalator which indicates that the complex interact with DNA through weak external intercalation and did not insert completely within the base pairs. This is mainly due to steric clash generating from long flexible ligand of the complex. This also suggests affinity of binding the complex to host DNA binding is less than classical intercalators. So the interaction of CT-DNA with the complex may lead to novel therapeutic agents for various treatment like tumor cell, cancer cell etc.




45

Dalton Transactions


45

Fig. 15 Spin density plot computed for the UB3LYP level DFT optimized di-cationic structures of 1-4. Hydrogen atoms are omitted for clarity.

Fig. 14 Typical plots of [DNA]/(εa-εf) vs [DNA] for absorption titration of CT-DNA with complex 1.

Computational Study In order to verify isotropic nature of the Fe3+ and high magnetic entropy change, DFT study was performed. Geometry optimization was carried out considering spin multiplicity of 11 electrons i.e. high spin Fe3+. The optimized structures obtained 10 from DFT calculation are very similar with the X-ray crystal structure. The Fe···Fe distance obtained from X-ray structure and geometry optimized structure are comparable with each other (Table 4). So this verifies the helical arrangement of the complexes. 5

15

Table 4. Fe···Fe distance obtained from X-ray structure and DFT calculation. X-ray DFT

Complex 1 6.264(1) 6.345

Complex 2 6.612 6.345

Complex 3 6.252(1) 6.345

Complex 4 6.2496(7) 6.345

Spin distribution analysis shows that there is no magnetic coupling occurring between the paramagnetic Fe3+centres (Fig. 15). Also the spin density mainly concentrates on the metal centres only -CH2-CH2- group of the ligands have zero spin density. In one side amide conjugated part of the ligand contains some spin density and other side has negligible spin density. 25 These suggest weak transmit spin density from the metal centres. Mulliken atomic spin densities of individual atoms are given in Table S7 and the calculation results are summarized in Table S8. HOMO and LUMO diagram of the dicationic structures reveal existence of isolated d-orbital (Fig. 16). In both HOMO and 3+ 30 LUMO cases the magnetic orbitals of Fe promote weak delocalization and do not interact with the neighbouring Fe3+ ion. As a result there is no magnetic coupling within two magnetic centres. The contour surface diagram of the spin density suggests there is clear separation of two wave functions indicating no 35 transmission of magnetic exchange occurring between the metal centres (Fig. S31). Also this suggests isotropic nature of the Fe3+ ion. So the evaluation of magnetic coupling constant from broken-symmetry calculation was not performed. As Fe3+centres (L = 0) have no orbital contribution, it was expected to be 40 isotropic in nature. DFT calculation further confirms this isotropic nature and hence the system will have high spin multiplicity with theoretically magnetic entropy change as (-∆Sm) = nRln (2S+1) = 2Rln6 (S = 5/2). 20


50

Fig. 16 HOMO (top) and LUMO (bottom) computed for the UB3LYP level DFT optimized di-cationic structures of 1-4. Hydrogen atoms are omitted for clarity.

CONCLUSION We have successfully designed and synthesised a hydrazide based ligand by incorporating (-(CH2)2-) chain in between two amide (-C(O)NH-) functions and shown to be capable of forming dinuclear double stranded helicates [Fe2(L)2]2+ complexes. 60 Structural analysis shows that in complexes 1-4, ligand L adopts “pseudo-C” conformations and forms a double-stranded dinuclear helicate. 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. Magnetic study of all the 65 complexes shows that they show significant MCE and can be potential candidates to use in magnetic refrigerant. Stability of all the complexes in solution phase was confirmed by ESI- mass analysis and liquid phase FT-IR spectroscopy. Further, the interaction of the complexes 1-4 with Rhodamine-B dye were 70 examined by UV-vis and fluorescence spectroscopic study and 55




DOI: 10.1039/C5DT01569C

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the result ascertains the weak interaction of dye with the complexes. Computational study supports the isotropic nature of metal centres and consequently high spins multiplicity, which assists the complexes to show significant magnetic entropy 5 changes. We believe this to be the first report exploring the MCE effect in Fe3+ based metallo-helicate systems. We are currently extending this work to incorporate other isotropic metal ions in conjunction with other hydrazide derivatives to see whether these species might able to act as potential candidate for magnetic 10 refrigeration. Works along these lines are under progress. AA acknowledges CSIR for his SRF fellowships. HSJ thanks IISER Bhopal for post-doctoral fellowship. SK thanks DST, Government of India (Project No. SR/FT/CS-016/2010) and 15 IISER Bhopal for generous financial and infrastructural support.

Notes and references Department of Chemistry, IISER Bhopal, Bhopal 462066, MP, INDIA, Fax: +91-755-6692392; Tel: +91-755-6692339 E-mail: [email protected], [email protected] 20 † Electronic Supplementary Information (ESI) available: X-ray crystallographic data for complexes 1-4 in CIF format, synthesis of ligand, structural figures, tables of bond distance and bond angle, hydrogen bonding table and table of significant magnetic entropy changes.See DOI: 10.1039/b000000x/ 25

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DOI: 10.1039/C5DT01569C

Dalton Transactions


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DOI: 10.1039/C5DT01569C

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A series of Fe3+ based double stranded helicates unveil significant magnetocaloric effect as well as binding activities with Rhodamine-B dye and CT-DNA



DOI: 10.1039/C5DT01569C

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