Complexation behavior of imidazoline hydrazones towards Cu2+ and Hg2+: Structure, sensing and DNA binding studies

Polyhedron 73 (2014) 87–97

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Complexation behavior of imidazoline hydrazones towards Cu2+ and Hg2+: Structure, sensing and DNA binding studies Soma Mukherjee a,⇑, Palash Mal a, Helen Stoeckli-Evans b a b

Department of Environmental Science, University of Kalyani, Kalyani, Nadia 741235, West Bengal, India Institute of Physics, University of Neuchâtel, rue-Emile-Argand 11, CH-2000, Neuchâtel, Switzerland

a r t i c l e

i n f o

Article history: Received 9 October 2013 Accepted 11 February 2014 Available online 28 February 2014 Keywords: Chromogenic agent Ratiometric analysis Binding pattern Molecular structure DNA binding

a b s t r a c t The water soluble heterocyclic ligands pyridine-2-carboxaldehyde-2-imidazoline hydrazone L1 and its methyl derivative L2 were synthesized as the hydrobromide salts, L1HBr and L2HBr, respectively. They display different binding modes towards Hg2+ and Cu2+, respectively, as revealed by single crystal X-ray diffraction studies. The new ligand L2HBr was investigated as a chromogenic and ratiometric agent for selective detection of Cu2+ and it exhibits a characteristic absorption peak at 411 nm with bluish green coloration in presence of Cu2+. The change in color can easily be distinguished from other metal complexes by the naked eye. The binuclear Cu2+ complex 2b exhibits a strong interaction towards DNA as revealed from Kb (UV–Vis) 1.516  104 M1, K (fluorescence) 1.8145  104 M1, n (binding site size) 0.8 and Ksv (Stern–Volmer quenching constant) 2.14 values. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The scope of generating new compounds with interesting structures and technologically useful functions is important in the development of material science and supramolecular chemistry [1–8]. Control of redox and spin states, change of donor sites, insertion of bridging groups, versatile H-bonding interactions, etc. can offer diverse architecture and functions in coordination complexes. It is well known that imidazole coordination is a very common feature in the chemistry and biochemistry of transition metal ions [9–12]. In contrary, the chemistry of imidazoline function is little explored till date, only few imidazoline based systems have been reported with homogeneous catalysis and interesting biological properties. 2-Imidazoline which is present in living systems or in several drugs has been developed as antihypertensive and antiinflammatory reagents [13–16], but few works have been reported with imidazoline hydrazines on their application in biological systems. It is well known that the hydrazine function is an effective ligating group for metal cations [17–22], but till date their complexation behavior and importance in the supramolecular chemistry are not well explored [23–26]. Recently, we have reported pyridine-2-carboxaldehyde-2-imidazoline hydrazone ligand L1 as a chromogenic and ratiometric agent for Cu2+ followed by the ⇑ Corresponding author. Tel.: +91 (033) 2582 8378/2582 8750x291/292; fax: +91 33 2582 8282. E-mail address: [email protected] (S. Mukherjee). http://dx.doi.org/10.1016/j.poly.2014.02.017 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

structure determination of its mononuclear Cu2+ complex [27]. Earlier we have reported the synthesis, structure, DFT and TDDFT calculations of a sulfate bridged dimeric Cu2+ complex with the same ligand (L1) [23]. In continuation to our previous study, herein we explored the synthetic routes of mono- and binuclear Cu2+ complexes and different binding modes of acyclic hydrazone ligands towards heavy and transitional metal ions. The imidazoline function is employed in conjugation with N-pyridyl, N-imidazoline and N-imine to ensure chelate formation. Interestingly, it was found that the ligand binds in tri- and monodentate fashion towards Cu2+ and Hg2+, respectively, as authenticated from single crystal X-ray diffractometry. Notably, the imidazoline hydrazone not only exhibits different binding pattern towards Cu2+ and Hg2+ but also shows selectivity and specificity towards Cu2+, which might be helpful to develop a new water soluble chromogenic and ratiometric agent for Cu2+. Chromogenic chemosensors are receiving special attention due to their simple naked-eye applications and practical importance [28–30]. Cu2+ is an essential trace element but it is an environmental pollutant at high concentration [31,32]. Among the targeted ions, the selective signaling of Cu2+ gets more interest for the detection and treatment in various real samples [33,34]. Furthermore, binuclear Cu2+ complexes with the ability to interact with DNA or other biological systems are of great interest as they can penetrate cell barriers and perform the desired activities [35–40] and herein we also report the DNA binding activity of the resulting binuclear Cu2+ complex.

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2. Experimental 2.1. Materials All starting materials and solvents were purchased from Sigma Aldrich chemical company and used without further purification unless otherwise stated.

2.2. Physical measurements A Perkin Elmer 2400 C Elemental Analyzer was used to collect microanalytical data (C, H, N). Sartorius CP64 balance was used for weighing purpose. FTIR data were collected with the help of Shimadzu FTIR 8400 Spectrophotometer. The UV–Vis spectra of the ligand and its complexes were measured on Shimadzu UV1700 spectrophotometer and corrected for background due to solvent absorption. Fluorescence spectra were carried out with a Hitachi F 7000 luminescence spectrometer equipped with 1 cm path length at ambient temperature. The spectroscopic measurements for ligand and metal complex were carried out in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4) at room temperature. For binding constant measurements, solutions were prepared at fixed concentrations of ligands (1.0  105 M) and at a concentration of Cu2+ and Hg2+ ranging (0.1–1.0)  105 M at room temperature. Room temperature magnetic susceptibility was measured with a model 155 PAR vibrating sample magnetometer fitted with a Walker scientific L75FBAL magnet. An Orion 4 star pH. ISE Benchtop was used to measure pH values.

2.3. Synthesis of ligand and complexes 2.3.1. Ligands The ligand L1 was synthesized and characterized following the reported method [27] and L2 was prepared by dropwise addition of a methanolic solution (5 ml) of 2-acetylpyridine (0.0242 g, 0.2 mmol) to a methanolic solution (5 ml) of 2-hydrazino-2-imidazoline hydrobromide (0.0362 g, 0.2 mmol) (Scheme 1). The reaction mixture was stirred for 1 h at room temperature and the light yellow solution was filtered off, the solvent was evaporated by rotary evaporator and recrystallized from methanol to give yellow crystals: Yield 0.055 g (92%). X-ray diffraction analysis indicated that in the solid state the ligand exists as the hydrobromide salt (L2HBr). Elemental Anal. Calc. For C10H14N5Br2(H2O): C, 37.51; H, 5.67; N, 21.87. Found: C, 37.54; H, 5.61; N, 21.95%. FTIR (KBr pellets, cm1): 3213, 2910, 1699, 1654, 1622, 1581, 1475, 1433, 1375, 1319, 1153, 1130, 1064, 995, 925, 781, 742, 646, 603. UV–Vis (aqueous methanol 1% v/v), kmax (nm) (e, M1 cm1): 319(169 100).

2.3.2. [CuBr2(L2)]H2O (2a) A methanolic solution of copper(II) bromide (0.2 mmol, 0.0446 g) was added drop-wise to a solution containing L2HBr (0.2 mmol, 0.0595 g) in methanol (5 ml) and the resulting green solution was stirred for 0.5 h (Scheme 2). The solvent was evaporated by rotary evaporation and the solid obtained was recrystallized from methanol giving green plate-like crystals: Yield 0.082 g (78.77%). Elemental Anal. Calc. for C10H13Br2CuN5H2O (2a): C, 27.08; H, 3.18; N, 15.79. Found: C, 26.64; H, 3.02; N, 15.22%. FTIR (KBr pellets, cm1): 3398, 3296, 3020, 2929, 1633, 1533, 1494, 1458, 1365, 1284, 1170, 1024, 933, 730, 669, 430. UV–Vis (aqueous methanol 1% v/v), kmax (nm) (e, M1 cm1): 298(14 010), 411(8370), 680(310). 2.3.3. [Cu2Br2(L2)2(l-SO4)2]0.5H2O (2b) 2b can be synthesized by the reaction between the ligand L2HBr, (0.2 mmol, 0.099 g) and copper(II) sulfate pentahydrate (0.2 mmol, 0.05 g) in methanol following the procedure for 2a. The same compound can be obtained by stirring an alkaline methanolic solution (10 ml) of L2HBr (0.2 mmol, 0.059 g) and CuBr2 (0.2 mmol, 0.044 g) for 0.5 h followed by the addition of Na2SO4. The reaction mixture was stirred for 0.5 h (Scheme 2). The solvent was evaporated by the rotary evaporator and the solid obtained was recrystallized from methanol giving green plate-like crystals: Yield 0.115 g (77.18%). Elemental Anal. Calc. for C20H26Br2Cu2N10 O4S0.5(H2O) (2b): C, 30.51; H, 3.07; N, 17.79. Found: C, 30.52; H, 3.11; N, 17.76%. FTIR (KBr pellets, cm1): 3325, 3196, 3074, 1639, 1529, 1452, 1380, 1284, 1145, 1091, 883, 784, 736, 607, 557, 493, 460. UV–Vis (aqueous methanol 1% v/v), kmax (nm) (e, M1 cm1): 298(15 300), 411(9045), 680(442). 2.3.4. [HgBr1.67Cl1.33 (L1)] (1a) A methanolic solution of Hg2+ chloride (0.2 mmol, 0.054 g) was added to a solution of L1HBr (in the solid state ligand L1 also exists as the hydrobromide salt [27]) (0.2 mmol, 0.053 g) in methanol (5 ml) and the solution was stirred for 2.0 h (Scheme 2). The solvent was evaporated by rotary evaporation and the solid obtained was recrystallized from methanol giving pale-orange rod-like crystals: Yield 0.079 g (74%). Elemental Anal. Calc. for C9H12N5Br1.67 Cl1.33HgH2O (1a) (by crystal) C, 18.34; H, 2.39; N, 11.88. Found: C, 18.52; H, 2.21; N, 11.94%. FTIR (KBr pellets, cm1): 3537, 3241, 3070, 2896, 1660, 1621, 1585, 1526, 1478, 1435, 1371, 1292, 1240, 1207, 1161, 1139, 1071, 1006, 1023, 928, 781, 742, 696, 662, 585, 520. UV–Vis (aqueous methanol 1% v/v), kmax (nm) (e, M1 cm1): 396sh(2497), 298 (11 250). 2.4. X-ray crystallography The intensity data for L2HBr were collected at 293 K on a Stoe Mark I-Image Plate Diffraction System [41], while for 2a, 2b and 1a

Scheme 1. Synthetic route of ligands.

S. Mukherjee et al. / Polyhedron 73 (2014) 87–97

89

Scheme 2. Syntheses of 1a, 2a and 2b.

the intensity data were collected at 173 K on a Stoe Mark II-Image Plate Diffraction System [42] equipped with a two-circle goniometer, both using Mo Ka graphite monochromated radiation. The structures were solved by direct methods with SHELXS-97 [43]. The refinement and all further calculations were carried out with SHELXL-2013. In all compounds the NH and water H-atoms could be located in difference electron-density maps but were refined with distance restraints: N–H = 0.88(2) Å with Uiso(H) = 1.2Ueq(N) and O–H = 0.84(2) Å with Uiso(H) = 1.5Ueq(O). The C-bound Hatoms were included in calculated positions and treated as riding atoms: C–H = 0.93, 0.97 and 0.96 Å for CH, CH2, and CH3 H-atoms, respectively, with Uiso(H) = 1.5Ueq(C-methyl) and =1.2Ueq(C) for other H atoms. For 2b a region of disordered electron density was treated with the SQUEEZE routine in PLATON [44] and finally equated to one half molecule of water of crystallization per binuclear complex. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. A semi-empirical absorption correction was applied using the MULscanABS routine in PLATON. The molecular structure and crystallographic numbering scheme and crystal packing are illustrated in Figs. 1–4. Further crystallographic data and details of the refinement are given in Table 1. 2.5. DNA binding experiments The DNA binding experiments were performed at 25.0 ± 0.2 °C. The concentration of CT-DNA was determined from its absorption intensity using the known molar extinction coefficient value of 6600 M1 cm1 at 260 nm [45]. The UV–Vis titration of 2b was

performed in buffer (50 mM NaCl–50 mM Tris–HCl, pH 7.2) medium using a fixed complex concentration to which increments of the DNA stock solution (1.0  105–1.0  104 M) was added. The resulting solution was incubated for 10 min before absorption spectra were recorded. During the fluorescence quenching experiment, the DNA was pretreated with ethidium bromide (EB) for 0.5 h and 2b was added to this mixture. On excitation at 490 nm, emission peak was observed between 500 and 700 nm. For all fluorescence measurements, both the entrance and exit slits were maintained at 10 nm. The fluorescence intensity was measured until a 50% reduction of the intensity had occurred. The experiments of DNA competitive binding with EB were carried out in the buffer by keeping [DNA]/[EB] = 2.5 ([DNA] = 50 lM, [EB] = 20 lM) and varying the concentrations of the metal complexes (0–36 lM). The fluorescence Scatchard plot was performed to study the binding constant determination from the luminescence titration method. Binding data were cast into the form of a Scatchard plot [46]. The binding constant K was calculated using the Eq. (1) [47].

r=C f ¼ kðn  rÞ

ð1Þ

where Cf is the concentration of free ethidium bromide, r is the ratio of bound complex to total DNA concentration [DNA], and n is the number of binding sites per nucleic acid. The concentration of the free ethidium was calculated using the Eq. (2) [48].

C f ¼ C total  ½ðF max  FÞ=ðF max  F 0 Þ

ð2Þ

Where, Ctotal is the total concentration of EB, F is the observed fluorescence emission intensity at a given EB concentration, Fmax is the

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Fig. 1. (a) Molecular structure of L2HBr2(H2O) showing the atomic numbering scheme and the displacement ellipsoids drawn at the 50% probability level. (b) A view along the a axis of the crystal packing of L2HBr2(H2O), showing the various hydrogen bonds as dashed lines (Br green). (Color online.)

Fig. 2. (a) Molecular structure of 2a (showing the atomic numbering scheme and the thermal displacement ellipsoids drawn at the 50% probability level. (b) A view along a axis of the crystal packing of 2a, showing the various hydrogen bonds as dashed lines (Cu yellow; Br green; C bound H atoms have been omitted for clarity). (Color online.)

Fig. 3. (a) Molecular structure of 2b, showing the atomic numbering scheme and the thermal displacement ellipsoids drawn at the 50% probability level. Symmetry code: (a) = x + 1, y, z + 1/2. (b) A view along the b axis of the crystal packing of 2b, showing the various hydrogen bonds as dashed lines (Cu yellow; Br green; S brown; C bound H atoms have been omitted for clarity). (Color online.)

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Fig. 4. (a) Perspective view and atom labeling scheme for 1a. All atoms are represented by their 50% thermal probability ellipsoids. (b) The crystal packing of 1a, viewed along the a axis [the hydrogen bonds are shown as dashed lines (Hg green, Br/Cl brown; C bound H atoms have been omitted for clarity). (Color online.)

Table 1 Crystallographic data and structure refinement for compounds L2HBr, 2a, 2b and 1a.

Formula Formula weight T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) Crystal size (mm3) h range (°) Index range

Reflections collected/unique Reflections independent Reflections observed [I > 2r(I)] Refinement Data/restraints/parameters Goodness-of-fit on F2 a R1, wR2b [I > 2r(I)] a R1, wR2b (all data) Largest difference in peak and hole (e Å3) a b

L2HBr

2a

2b

1a

C10H14N5Br2(H2O) 320.0 293 monoclinic P21/n (No. 14) 7.0816(5) 8.9828(4) 16.7559(10) 90 100.741(8) 90 1379.92(16) 4 1.541 2.98 656 0.19  0.20  0.42 2.12, 25.97 8 < h < 8 14 < k < 14 20 < l < 20 10 777, 2624 0.038 1791 full-matrix least-squares on F2 2624/8/176 0.87 0.027, 0.057 0.048, 0.061 0.39, 0.29

C10H13Br2CuN5H2O 444.63 173 triclinic  (No. 2) P1 8.0581(6) 9.0455(6) 11.2098(8) 89.218(6) 87.267(6) 64.361(5) 735.77(10) 2 2.007 6.914 434 0.12  0.23  0.40 1.8, 25.6 9 < h < 9 10 < k < 11 13 < l < 13 10 201, 2766 0.047 2438 full-matrix least-squares on F2 2766/4/189 1.08 0.028, 0.061 0.036, 0.063 0.88, 0.49

C20H26Br2Cu2N10SO40.5(H2O) 798.48 173 monoclinic C2/c (No. 15) 12.3123(6) 15.5644(8) 14.4979(7) 90 105.616(4) 90 2675.7(2) 4 1.960 4.706 1568 0.10  0.27  0.45 2.2, 25.6 14 < h < 13 18 < k < 18 17 < l < 17 12 041, 2522 0.072 2209 full-matrix least-squares on F2 2522/2/178 1.06 0.036, 0.084 0.044, 0.087 0.66, 0.61

C9H12N5Br1.67Cl1.33HgH2O 589.30 173(2) monoclinic P21/c 6.7269(3) 14.1153(7) 16.6621(9) 90 92.743(4) 90 1580.29(14) 4 2.477 15.158 1256 0.15  0.24  0.4 1.44, 26.09 7 < h < 8 17 < k < 17 20 < l < 20 9744, 2974 0.073 2344 full-matrix squares on F2 2974/3/190 1.02 0.040, 0.087 0.057, 0.092 1.23, 0.95

P P R1 = [ jjFoj  jFc||]/ jFoj (based on F1=2 o). P P 2 2 wR2 ¼ ½½ wðjF 2o  F 2c jÞ =½ wðF 2o Þ  (based on F2o).

intensity in the absence of complex and F0 is the fluorescence of the totally bound complex. The plot of r/Cf versus r gives the association constant and the binding site size for the agents. DNA-melting experiments were carried out by monitoring the absorbance of CT-DNA (100 lM) at 260 nm in the absence and presence of 2b, in a 2:1 ratio of DNA with an increase in the temperature of the solution by 0.25 °C per min in Tris buffer (pH 7.2) using a peltier system attached to the UV–Vis spectrophotometer. For viscosity measurements, the Ubbelohde viscometer was thermostated in a water-bath maintained at 25 °C. The flow time for each sample was measured three times using digital stopwatch and an average flow time was calculated. The rate of flow of the Tris buffer (pH 7.2), DNA (100 lM) and DNA with 2b at various

concentrations were measured. The relative specific viscosity was calculated using the equation g = (t  t0)/t0, where t0 is the flow time for the buffer and t is the observed flow time for DNA in the absence and presence of 2b. Data are presented as (g/g0)1/3 versus [complex]/[DNA]), g is the viscosity of DNA in the presence of the complex and g0 is the viscosity of DNA alone [49,50]. 3. Results and discussion 3.1. Synthesis and characterization The ligand L2HBr binds with Cu2+ in a tridentate manner utilizing imidazoline-N, pyridyl-N and imine-N as potential donor sites

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whereas it binds with Hg2+ in a monodentate fashion using the pyridyl-N as shown in Scheme 2. The ligand and complexes are soluble in water and methanol. The room temperature magnetic moments of Cu2+ complexes [CuBr2(L2)]H2O (2a) and [Cu2Br2(L2)2(lSO4)2]Br20.5H2O (2b) are 1.82 lb and 1.84 lb, respectively. These values are consistent with an S = 1/2 spin state as expected for lowspin d9(t62e3) systems. The complex 1a is diamagnetic. 3.2. Crystal structures of compounds L2HBr, 2a, 2b and 1a Molecular structures of the ligand, L2HBr and complexes [CuBr2(L2)]H2O (2a), [Cu2Br2(L2)2(l-SO4)2]Br20.5H2O (2b) and [HgBr1.67Cl1.33(HL1)]H2O (1a) are shown in Figs. 1–4, respectively. Selected bond lengths and bond angles for the complexes 2a, 2b and 1a are listed in Tables 2–4, respectively. Details of hydrogen bonding of the ligand (L2HBR) and the three complexes are listed in Table 5. 3.2.1. L2HBr The ligand (L2) crystallized as the dihydrate of the hydrobromide salt (Fig. 1) and is relatively planar with the pyridine ring being inclined to the imidazoline ring by only 2.70(14)°. In the crystal, the various units are linked via N–H  O, N–H  Br, O– H  O, O–H  Br, and O–H  N hydrogen bonds involving the NH group of the ligand, the Br ion and the water molecules forming a two-dimensional network lying parallel to plane (1 0 –1). 3.2.2. [CuBr2(L2)]H2O (2a) The molecular structure of 2a clearly shows that it is a monomeric Cu2+ complex with CuN3Br2 coordination (Fig. 2a). The ligand binds in a tridentate manner utilizing imidazoline-N, pyridyl-N and imine-N as potential donor sites. The complex 2a has a square pyramidal geometry, the s value being 0.004 (s = 0 for perfect square pyramidal; s = 1 for perfect trigonal bypyramidal) [51]. Atoms N1, N2, N5 and Br2 occupy the basal plane while atom Br1 occupies the apical position. The bond distances of the coordinating atoms with the central metal atoms are Cu(1)–N(1) 2.031(2) Å, Cu(1)–N(2) 2.002(2) Å, Cu(1)–N(5) 1.952(2) Å, Cu(1)– Br(1) 2.6978(5) Å and Cu(1)–Br(2) 2.4031(5) Å. The angles at Cu1 between the cis- positioned donor pairs span the range 77.95(10)–104.35(8)° and those between the trans-positioned pairs are156.16(8)–156.38(10)° (Table 2). In the crystal, molecules are linked via O–H  Br, N–H  O, and N–H  Br hydrogen bonds which leads to the formation of a polymer chain propagating along the b axis direction (Fig. 2b and Table 5). 3.2.3. [Cu2Br2(L2)2(l-SO4)2]0.5H2O (2b) The molecular structure of 2b (Fig. 3a) clearly shows that the molecule possesses twofold rotational symmetry with the twofold axis bisecting atom S1. The Cu2+ atoms are connected by means of the sulfate group, bridging in an end-to-end fashion. Each copper atom has a slightly distorted square planar geometry (s = 0.10) and a pentacoordinate N3OBr environment (three nitrogen atoms Table 2 Selected bond lengths [Å] and bond angles [°] for 2a. Complex 2a Cu(1)–Br(1) Cu(1)–N(1) Cu(1)–N(5) Br(1)–Cu(1)–Br(2) Br(1)–Cu(1)–N(2) Br(2)–Cu(1)–N(1) Br(2)–Cu(1)-N(5) N(1)–Cu(1)–N(5)

2.6978(5) 2.031(2) 1.952(2) 99.48(2) 104.36(7) 100.24(7) 98.17(8) 156.39(10)

Cu(1)–Br(2) Cu(1)–N(2) Br(1)–Cu(1)–N(1) Br(1)–Cu(1)–N(5) Br(2)–Cu(1)–N(2) N(1)–Cu(1)–N(2) N(2)–Cu(1)–N(5)

2.4031(5) 2.002(2) 92.80(7) 98.63(8) 156.14(7) 77.96(10) 79.21(10)

Table 3 Selected bond lengths [Å] and angles [°] for 2b. Complex 2b Br(1)–Cu(1) Cu(1)–N(1) Cu(1)–N(5) Br(1)–Cu(1)–O(1) Br(1)–Cu(1)–N(2) O(1)–Cu(1)–N(1) O(1)–Cu(1)–N(5) N(1)–Cu(1)–N(5)

2.3921(6) 2.044(3) 1.943(3) 98.51(7) 161.58(9) 90.53(11) 103.84(12) 155.08(12)

Cu(1)–O(1) Cu(1)–N(2)

2.258(3) 2.002(3)

Br(1)–Cu(1)–N(1) Br(1)–Cu(1)–N(5) O(1)–Cu(1)–N(2) N(1)–Cu(1)–N(2) N(2)–Cu(1)–N(5)

100.42(10) 97.49(10) 99.87(11) 78.19(12) 79.33(12)

Table 4 Selected bond lengths [Å] and angles [°] for 1a. Complex 1a Hg(1)–Br(1) Hg(1)– Cl(1) Hg(1)–N(1) Br(1)–Hg(1)–Br(2) Br(1)–Hg(1)–N(1) Br(2)–Hg(1)–Cl(1) Cl(1)–Hg(1)–N(1) Cl(2)–Hg(1)–N(1)

2.5209(9) 2.614(2) 2.409(6) 131.0(2) 101.05(13) 102.3(2) 95.14(15) 111.9(8)

Hg(1)–Br(2) Hg(1)–Cl(2) Br(1)–Hg(1)–Cl(1) Br(1)–Hg(1)–Cl(2) Br(2)–Hg(1)–N(1) Cl(1)–Hg(1)–Cl(2)

2.468(8) 2.67(3) 107.05(5) 129.9(7) 114.5(2) 106.5(8)

Table 5 Hydrogen bonding [Å] in compounds HL2HBr, 2a, 2b and 1a. D–H  A

D–H

H  A

D   A

D–H  A

Ligand L2HBra N(3)–H(3N)  O(1Wi) N(4)–H(4N)  Br(1) N(5)–H(5N)  O(1Wi) O(1W)–H(1WA)  O(2W) O(1W)–H(1WA)  Br(1ii) O(2W)–H(2WA)  Br(1iii) O(2W)–H(2WB)  N(1iv)

0.86(2) 0.85(2) 0.83(2) 0.82(2) 0.82(2) 0.83(2) 0.83(2)

2.08(2) 2.56(2) 2.20(2) 1.90(2) 2.52(2) 2.48(2) 2.11(2)

2.840(2) 3.3212(19) 2.880(3) 2.714(3) 3.334(2) 3.304(2) 2.885(3)

147(2) 149(2) 139(2) 169(3) 170(3) 173(3) 156(3)

Complex 2ab N(3)–H(3N)  O(1W) N(4)–H(4N)  Br(1i) O(1W)–H(1A)  Br(1ii) O(1W)–H(1B)  Br(1i)

0.87(2) 0.88(2) 0.83(2) 0.83(2)

1.88(2) 2.55(2) 2.65(3) 2.49(2)

2.715(4) 3.422(3) 3.439(3) 3.295(2)

163(3) 173(3) 160(5) 162(5)

Complex 2bc N(3)–H(3N)  O(2i) N(4)–H(4N)  O(1ii)

0.87(2) 0.87(2)

1.82(2) 2.03(2)

2.680(4) 2.900(4)

170(4) 172(4)

Complex 1ad N(3)–H(3N)  O(1W) N(4)–H(4N)  Cl(1i) N(5)–H(5N)  Cl(1ii) O(1W)–H(1WA)  Br(2) O(1W)–H(1WB)  Br(1iii) O(1W)–H(1WB)  Cl(1ii)

0.88(2) 0.88(2) 0.87(2) 0.84(2) 0.84(2) 0.84(2)

1.89(3) 2.51(6) 2.39(3) 2.62(5) 2.99(10) 2.62(10)

2.753(9) 3.221(7) 3.236(7) 3.416(11) 3.573(7) 3.207(7)

170(8) 139(7) 166(8) 158(11) 129(10) 128(10)

a Symmetry codes: (i) x + 1/2, y + 1/2, z  1/2; (ii) x  1, y, z; (iii) x + 3/2, y  1/2, z + 1/2; (iv) x + 1, y + 1, z. b Symmetry codes: (i) x + 1, y + 1, z + 1; (ii) x, y  1, z. c Symmetry codes: (i) x + 1, y + 1, z; (ii) x, y + 1, z1/2. d Symmetry codes: (i) x, y + 1, z  1/2; (ii) x + 1, y + 1, z; x + 1, y + 1/2, z + 1/2.

from the ligand, one bromide ion and one oxygen atom from the sulfate ion) and forms a binuclear [Cu2(l-SO4)] core unit. The Cu(1)  Cu(1a) distance is 6.07 Å [symmetry code: (a) = x + 1, y, z + 1/2]. For the coordination sphere of Cu1, atoms N1, N2, N5 and Br1 define the basal plane while sulfate atom O1 occupies the axial position [Cu(1)–N(1) 2.044(3) Å, Cu(1)–N(2) 2.002(3) Å, Cu(1)–N(5) 1.943(3) Å, Cu(1)–Br(1) 2.3921(7) Å, and Cu(1)–O(1) 2.287(3) Å]. The angles at Cu1 between the cis-positioned donor pairs span the range 78.19(12)–103.84(12)° and those between the trans-positioned pairs are 155.08(12)° and 161.58(9)° (Table 3). In the crystal structure of 2b, symmetry-related molecules are

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connected by N–H  O, hydrogen bonds, which leads to the formation of a polymer chain propagating along the c axis direction (Fig. 3b and Table 5). Upon comparing the structures of the mono- and binuclear Cu2+ complexes of L2HBr with our previously reported Cu2+ complexes of ligand L1 [23,27], it was found that 2a has a distorted square pyramidal geometry with bond distances and angles very close to those reported for the mononuclear Cu2+ complex of ligand L1 [Cu1–N1 2.037(2) Å, Cu1–N2 2.013(2) Å, Cu1–N4 1.964(2) Å, Br1– Cu1 2.447(5) Å and Br2–Cu1 2.605(5) Å) [27]]. The only difference is that the new complex 2a crystallized with a water molecule in the crystal lattice. In addition the binuclear Cu2+ complex, 2b exhibits structural similarity with the previously reported binuclear Cu2+ complex of ligand L1 [Cu(1)–N(1) 2.058(11) Å, Cu(1)– N(2) 2.000(11) Å, Cu(1)–N(4) 1.956(11) Å, Cu(1)–Br(1) 2.386(2) Å and Cu(1)–O(1) 2.274(10) Å [23]].

3.2.4. [HgBr1.67Cl1.33(HL1)]H2O (1a) Interestingly, in the compound 1a the ligand L1 is protonated and binds Hg2+ in a monodentate fashion using one pyridyl-N atom with a binding mode that has rarely been reported [52]. The Hg–N bond distance is 2.409(6) Å which is close to the value reported for other Hg2+ complexes [52,53]. The other coordination sites are satisfied by three halide ions to afford a four coordinate tetrahedral geometry of the type HgN3X (Fig. 4a and Table 4). This is in contrast to the situation in complexes 2a and 2b where ligand L2HBr binds in a tridentate manner utilizing the pyridyl, imine and imidazoline N atoms. Interestingly one of the halide sites (Br2/Cl2) is disordered with the position occupied by 2/3 of a bromine ion and 1/3 of a chloride ion. In the crystal, a combination of O–H  Cl, O–H  Br, N–H  O, N–H  N and N–H  Cl hydrogen bonds leads to the formation of a three-dimensional structure (Fig. 4b and Table 5).

Fig. 5. Absorbance Spectra of L2HBr (1.0  105 M) upon addition of Cu2+(1.0  106–1.0  10–5 M) in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4) at 298 K (Inset: Absorption spectra of 1, (1.0  103 M) upon addition of Cu2+ (1.0  104–1.0  103 M)).

3.3. Spectroscopic studies The ligand L2HBr exhibits a characteristic absorption band in the UV region (kmax = 319 nm). Upon titration of L2HBr (1.0  105 M) with Cu2+ (1.0  106–1.0  105 M) in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4), this absorption band is red-shifted and exhibits two absorption peaks at 411 nm and 680 nm (weak) with successive increase in the absorption intensity at new peaks and a well-defined isosbestic point at 360 nm (Fig. 5). The spectral positions have only slight changes in the e values compared to those for the Cu2+ complexes of ligand L1 reported in our previous works [23,27]. When L2HBr is titrated with Hg2+ (0.5  105–5.0  105 M) in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4) the absorption intensity of L2HBr (kmax = 319 nm) decreases with a blue shift at 298 nm (Fig. 6). The association constants (Kass) of complexes 2a, 2b and 1a were estimated spectrophotometrically according to the following equation (Eq. (3)) [27,54] and were found to be (5.01 ± 0.006)  104, (5.95 ± 0.01)  104 and (5.35 ± 0.004)  104, respectively.

n X ¼ X 0 þ ðX lim  X 0 Þ= 2C 0 C H þ C G þ 1=K ass  ½ðC H þ C G þ 1=K ass Þ2 o  4C H C G 1=2 ð3Þ where, X represents the absorption intensity, Xlim represents the absorption intensity at full complexation, C0 is the initial concentration of the ligand, CH and CG are the corresponding concentrations of the ligand and metal ion during the titration.

Fig. 6. Absorbance spectra of L2HBr (0.8  105 M)upon addition of Hg2+ (0.5  105–0.8  105 M) in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4) at 298 K.

3.4. Chromogenic and ratiometric sensing of Cu2+ The absorption peaks of Cu2+ complexes (2a and 2b) of HL2HBR generated in the visible region with a distinct color change from light yellow to deep green can easily be read through naked eyes without any spectroscopic instrument (Fig. 7). The selectivity of L2HBR towards Cu2+ over other metal ions was investigated in the present study. In presence of one equivalent of other metal ions such as Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Zn2+, Ag+, Cd2+, Sn2+, Hg2+ and Pb2+ to the solution of L2HBr, it was observed that only Cu2+ gives a characteristic absorption peak at 411 nm due to the formation of 2a. This may be due to the small ionic radii, the high LFSE and the structural preference of Cu2+ towards five coordinate square pyramidal geometry in the presence of acyclic tridentate NNN ligand systems [55,56]. Moreover in the competition experiment no significant variation in absorbance was found when Cu2+ was added to the solution of L2HBr in the presence of 5.0 equivalents of other metal ions (Fig. 8). Addition of Cu2+ to L2HBr shifted the UV–Vis wavelength from 319 to 411 nm due to the formation of 2a and the red-shift of the absorption spectrum allowed a ratiometric analysis of the

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Fig. 7. The photograph of the ligand L2HBr in water after the addition of different metalions.

signaling behavior. A correlation between absorption ratios (A411/ A319) of L2HBr at 411 and 319 nm versus Cu2+ concentration (Fig. S1) was studied. No observable changes in the absorbance ratios were found, even though equivalents of other cations were added to L2HBr (Fig. S2). The detection of a metal or complex formation are often pHdependent [57], therefore we have also investigated the role of pH on the determination of Cu2+ in aqueous solution spectrophotometrically. The absorbance of the complex solution containing 1.0  105 M of Cu2+ were measured at 411 nm within a pH range of 4.0–12.0 by adjusting the pH using 10 mM HEPES buffer (Fig. S3). At low pH (<6.5), absorbance was lower indicating that the protonated donor atoms coordinates Cu2+ less effectively in acidic solution whereas at pH 7.0 the absorbance was significantly higher and it remains almost constant at higher pH as the non-protonated donor atoms more readily forms a complex with Cu2+. Within dynamic range, the concentration of L2HBr has no significant effect in the Cu2+ determination. The concentration of L2HBr used was fixed to 1.0  105 M. Thus the optimized condition for Cu2+ determination was selected as 1.0  105 M L2HBr, HEPESbuffered aqueous solution (methanol 1% v/v, pH 7.4). The relationship between absorbance at 411 nm and Cu2+ concentration was obtained from the calibration curve for the determination of Cu2+ by L2HBr and that was A = 8220.9C + 0.0014, where A was the absorbance at 411 nm and C was the concentration of Cu2+. The linear range of the method was found to be at least 1.0  106–1.0  105 M Cu2+ with a correlation coefficient of R2 = 0.9989 (n = 15). The limit of detection (LOD) was evaluated as 1.5  106 M using 3r/s [27,58], where r is the standard deviation of the blank signals and s is the slope of the linear calibration plot (Fig. S4). This method for Cu2+ determination can be applied successfully for the real sample analysis as reported earlier [27].

3.5. DNA binding studies UV–Vis spectroscopy is usually employed to determine the binding modes of ligands and complexes with the DNA helix. A complex bound to DNA through intercalation is characterized by the spectral changes (hypochromism or hyperchromism) that reflect the corresponding alteration of DNA in its conformation and structure after the ligands or metal complexes bound to DNA [59]. Herein, we have also studied the DNA binding activities of L2HBr, 2a and 2b. In the case of L2HBr and 2a, no significant interaction was observed, whereas in 2b, the decrease in absorption intensity (hypochromism) with increasing concentration of CT-DNA (0.0–4.0  105 M) was observed (Fig. 9a). In order to further investigate the intensity of interactions between 2b and CT-DNA, we have calculated the intrinsic binding constant, Kb, from the plot of [DNA]/(ea–ef) versus [DNA] using the following Eq. (4), [60–62].

½DNA=ðea  ef Þ ¼ ½DNA=ðeb  ef Þ þ 1=K b ðeb  ef Þ

ð4Þ

where, [DNA] is the concentration of DNA in base pairs, the molar absorption coefficients ea, eb and ef represent the apparent absorption coefficient for the complex and the extinction coefficient for the complex in the fully bound form and the extinction coefficient for the free complex, respectively. The Kb is obtained by the ratio of the slope to the intercept and is found to be 1.516  104 M1 for 2b and this value is very close and has the same order of 104 M1 as reported for other binuclear Cu2+ complexes (Fig. 9b) [37,40,63,64]. Competitive ethidium bromide (EB) binding study was carried out by fluorescence spectral method to understand the mode of DNA interaction of 2b in Tris buffer (pH 7.2). EB was non-emissive in Tris-buffer medium due to fluorescence quenching by the sol-

Fig. 8. The absorbance change profile of L2HBr (1.0  10–5 M) with Cu2+ (1.0  10–5 M) at 411 nm in the presence of selected metal ions at ratio of 5:1 (competing ion/Cu2+) in HEPES-buffered aqueous solution (methanol 1% v/v, pH 7.4).

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Fig. 9. (a) Variation of UV–Vis absorption spectra of 2b (1.0  105 M) in presence of CT-DNA (1.0  104 M) in buffer (Tris–HCl–NaCl, pH 7.2) medium at room temperature. (b) Plot of [DNA]/(ea  ef) vs. [DNA].

Fig. 10. Variations of emission spectra of EB (2.0  105 M) with CT-DNA (5.0  105 M) on addition of 2b (0–3.6  105 M), Inset: Plot of I0/I vs. r.

I0 =I ¼ 1 þ K sv r

ð5Þ

The Ksv value obtained from the ratio of slope to intercept of I0/I versus r linear plot is found to be 2.14 indicating a strong interaction of 2b with CT-DNA. The value is very close to Stern–Volmer quenching constant values of other reported dinuclear Cu2+ complexes [37,40,67–69]. Further studies to obtain the DNA binding constant (K) and binding site size (n) of 2b from the fluorescence study the Scatchard plot was performed (Fig. 11). From the fluorescence experiment the values were found to be 1.8145  104 M1 and 0.8, respectively. The binding constant value obtained from the Scatchard plot is quite similar as the value obtained for Kb from UV– Vis spectral study (1.516  104 M1). Both the values have the Fig. 11. Scatchard Plot.

vent molecules. In the presence of CT DNA, it showed enhanced emission intensity due to its strong intercalation between the adjacent DNA base pairs. A competitive binding of 2b to CT DNA resulted in the reduction of the emission intensity due to displacement of bound EB and/or quenching of the fluorescence of EB by paramagnetic 2b [65,66]. Here, upon addition of 2b, in Tris–HCl–NaCl buffer (pH 7.2) to CT-DNA (4.0  105 M) pretreated with EB the emission intensity decreases almost upto 50% (Fig. 10). The quenching of the EB bound to DNA by the complex 2b is in agreement with the linear Stern–Volmer equation, Eq. (5), [35,65] where, I0 and I represent the fluorescence intensities in the absence and presence of 1b, r is the concentration ratio of 2b to DNA. Ksv is the linear Stern–Volmer quenching constant.

Fig. 12. Thermal melting curves of CT-DNA (blue) and CT-DNA + 2b (brown) (R = Me). (Color online.)

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Fig. 13. Effect of the increasing amount of 2b (R = Me) on the relative viscosity of CT-DNA at 25 °C, [DNA] = 100 lM.

same order (104 M1) [70] and lower magnitude than that of classical intercalator (EB-DNA 1.4  106 M1 in 25 mM Tris–HCl/ 40 mM NaCl buffer at pH 7.9, 3.0  106 M–1 in 5 mM Tris–HCl/ 50 mM NaCl buffer at pH 7.2) [71–74]. From the literature, it is revealed that the mode of DNA interaction of the complex can be interpreted from the thermal denaturation study. The denaturation of DNA from double-strand to single strand results in absorption hyperchromism at 260 nm [75]. The melting temperature Tm, which is defined as the temperature where half of the total base pairs gets non-bonded, is an important parameter to identify this transition process. The binding of metal complexes with double-stranded DNA generally stabilizes the duplex structure to some extent, depending on the strength of the interaction with the nucleic acid. Thus the binding should lead to an increase in the melting temperature (DTm) of DNA as compared to DNA itself [76]. Herein we have performed the thermal denaturation studies of CT-DNA with 2b. The melting curves of CT-DNA in the absence and presence of 2b is presented in Fig. 12. A moderate positive increase of melting temperature (3 °C) of CT-DNA in the presence of 2b indicates the possibility of electrostatic and/or groove binding nature of 2b in preference to an intercalative mode of binding to DNA [77,78]. The DNA binding mode of 2b was further investigated by viscosity measurements. Herein the relative specific viscosity of DNA was examined by varying the concentration of 2b. It was found that the viscosity of DNA remain almost unchanged upon addition of 2b (Fig. 13), suggesting a non-intercalative, and probably groove binding mode of the Cu2+ complex [79]. 4. Conclusion In summary, the imidazoline hydrazone based ligands L1HBr, L2HBr, and their Hg2+ and Cu2+complexes, respectively, were synthesized. Single crystal X-ray diffraction studies clearly authenticate the different coordination modes of the two ligands towards metal ions. The ligand L2HBr was investigated as a new water soluble colorimetric and ratiometric chemosensor for Cu2+. The binuclear Cu2+ complex (2b) shows DNA binding activity as revealed from absorption and emission characteristics, thermal denaturation and viscosity studies probably due to groove binding of DNA. Acknowledgments Financial support received from CSIR, UGC, DST-FIST, DSTPURSE, New Delhi, India and Indo Swiss Joint Research Programme (ISJRP) for Joint Utilisation of Advanced Facilities (JUAF) are gratefully acknowledged. We are grateful to the XRD Application Laboratory, CSEM, Neuchâtel, Switzerland for access to the X-ray

diffractometer. P. Mal is grateful to CSIR, New Delhi, for Senior Research Fellowship. We are grateful to the University of Kalyani for providing infrastructural facilities.

Appendix A. Supplementary data CCDC 800432, 800429, 800431, and 849681 contains the supplementary crystallographic data for L2HBr, 2a, 2b and 1a, respectively. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.02.017. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

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