Spontaneous reduction of mononuclear high-spin iron(III) complexes to mononuclear low-spin iron(II) complexes in aqueous media and nuclease activity via self-activation.

DOI: 10.1002/asia.201402954

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Tridentate Ligands

Spontaneous Reduction of Mononuclear High-Spin Iron(III) Complexes to Mononuclear Low-Spin Iron(II) Complexes in Aqueous Media and Nuclease Activity via Self-Activation Kaushik Ghosh,* Nidhi Tyagi, Ashish Kumar Dhara, and Udai P. Singh[a] Dedicated to the memory of Prof. Ivano Bertini

Abstract: Mononuclear high-spin [FeIII(Pyimpy)Cl3]·2 CH2Cl2 (1·2 CH2Cl2) and [FeIII(Me-Pyimpy)Cl3] (2), as well as low-spin FeII(Pyimpy)2](ClO4)2 (3) and [FeII(Me-Pyimpy)2](ClO4)2 (4) complexes of tridentate ligands Pyimpy and Me-Pyimpy have been synthesized and characterized by analytical techniques, spectral, and X-ray structural analyses. We observed an important type of conversion and associated spontaneous reduction of mono-chelated high-spin FeIII (1·2 CH2Cl2 and 2) complexes to low-spin bis-chelated FeII complexes 3 and 4, respectively. This process has been explored in detail by UV/

Vis, fluorescence, and 1H NMR spectroscopic measurements. The high positive potentials observed in electrochemical studies suggested a better stabilization of FeII centers in 3 and 4. Theoretical studies by density functional theory (DFT) calculations supported an increased stabilization for 3 in polar solvents. Self-activated nuclease activity of complexes 1·2CH2Cl2 and 2 during their spontaneous reduction was examined for the first time and the mechanism of nuclease activity was investigated.

Introduction

Mascharak and co-workers[7a] designed a ligand, SBPy3, which stabilizes oxidation states + 2 as well as + 3 for iron complexes. Low-spin [FeIII(SBPy3)(DMF)]3 + spontaneously reduced to low-spin [FeII(SBPy3)(MeCN)]2 + in acetonitrile. This reduction of the iron(III) center was determined by the unusual stability of the SBPy3 ligand towards FeII centers in acetonitrile (E1/2 = 1.01 V vs. SCE in MeCN). Welter and co-workers[7b,c] reported spontaneous reduction of the paramagnetic complex [FeIII(HL)2Cl] to [FeII(H2L)2Cl2] (H2L: 2-salicyloylhydrazono-1,3-dithiolane) complex with retention of the spin state; however, the mechanism of reduction was not reported. The same group also reported unprecedented spontaneous reduction of high-spin iron complex [FeIII(L(n))2Cl] to complex [FeII(HL(n))2Cl2], (with slight modification in ligand) having retention of spin state. Bruijnincx et al.[7d] also accounted for spontaneous reduction of dinuclear complex [FeIII2(m-OH)2(bik)4](NO3)4 to low-spin tris-chelated iron(II) complex [FeII(bik)3]2 + in methanol. The binuclear complex was described as high-spin complex and the mechanism of this reduction was not described (ligands are shown in Scheme 1). In this context, we would like to mention that Sima et al. also described a radical-mediated photoreduction of iron(III) complexes.[7e] Herein, we report the synthesis and spectroscopic characterization of high-spin complexes [Fe(Pyimpy)Cl3]·2 CH2Cl2 (1·2 CH2Cl2), [Fe(Me-Pyimpy)Cl3], (2) (ligands Pyimpy and MePyimpy are shown in Scheme 1) and interesting examples of conversion and spontaneous reduction of mononuclear highspin iron(III) complexes (1 and 2) to mononuclear low-spin iron(II) complexes [Fe(Pyimpy)2](ClO4)2 (3) and [Fe(Me-Pyim-

The versatile coordination chemistry of iron with a variety of chelating ligands in different oxidation states of iron has been investigated;[1] however, oxidation states II and III are two important and stable ones in ordinary aqueous and related chemistry.[2] In coordination complexes, ligands like phenolato, ammine, carboxylate and carboxamido donor(s) stabilize higher oxidation states of metal.[3] On the other hand, pyridine, imine and different p-acid ligands stabilized lower oxidation state(s) of metals center.[4] Each of the oxidation state possesses interesting chemistry in different spin states (high-spin and low-spin) of the iron center.[5] Hence, designing and synthesis of the ligands around the iron center could provide the stabilization of a particular oxidation state and ligand(s) could tune the redox and magnetic properties of resultant complexes. Oxidation of metal center during the synthesis of coordination complex in aerobic condition is a common phenomenon and in several cases oxygen and/or solvents are involved in such oxidation processes.[3g, 6] However, spontaneous reduction of metal center in aerobic condition is rare in the coordination chemistry of iron.[7] [a] Dr. K. Ghosh, Dr. N. Tyagi, A. Kumar Dhara, Dr. U. P. Singh Department of Chemistry Indian Institute of Technology Roorkee Roorkee-247667 Uttarakhand (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402954. Chem. Asian J. 2015, 10, 350 – 361

350

2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper Results and Discussion Synthesis The tridentate Schiff base ligands Pyimpy and Me-Pyimpy were synthesized according to the procedure reported earlier.[11] Synthesis of monomeric iron(II) and iron(III) complexes were carried out in air. Direct addition of (metal-to-ligand ratio 1:1) anhydrous FeCl3 to methanolic solutions of ligands Pyimpy and Me-Pyimpy readily afforded water-soluble complexes [Fe(Pyimpy)Cl3]·2 CH2Cl2 (1·2 CH2Cl2) and [Fe(Me-Pyimpy)Cl3], (2) respectively, in high yield (~ 80 %). On the other hand, complexes [Fe(Pyimpy)2](ClO4)2 (3) and [Fe(Me-Pyimpy)2](ClO4)2 (4) were synthesized by the reaction of two equivalents of respective ligands and one equivalent of Fe(ClO4)2·x H2O. Interestingly, complexes 3 and 4 could also be prepared by reacting Fe(ClO4)3·x H2O as starting material and the respective ligands and hence reduction of the metal center was observed in the latter case. During the synthesis appearance of yellow-colored precipitate clearly indicated in situ formation of [Fe(Pyimpy)2]3 + and [Fe(Me-Pyimpy)2]3 + species. We were unable to isolate the FeIII complexes, and the yellow solids became red within 2– 4 min; these red solids were characterized as complexes 3 and 4, respectively. A similar type of reduction appeared, when terpy was added to FeIII salts, and [Fe(terpy)2]2 + was formed rather than [Fe(terpy)2]3 + .[12] Investigation of the literature revealed that certain ligands could cause spontaneous reduction of FeIII (high-spin or low-spin) complexes to FeII complexes (high-spin or low-spin). Interestingly, 3 and 4 were also obtained as red solids by stirring of 1·2 CH2Cl2 and 2, respectively, in aqueous solution with excess of sodium perchlorate, and further (considering half conversion of bis-chelated complexes) we have added 2.5 equivalents of ligands in filtrate to acquired additional amount of bis-chelated complexes (see the experimental section in the Supporting Information). The synthetic procedures of all these conversions are summarized in Scheme 2. This conversion and spontaneous reduction of complexes 1·2 CH2Cl2 and 2 to 3 and 4, respectively, in water were

Scheme 1. Ligands used for spontaneous reduction (bik, HL, H2L, SBPy3) and ligands (Pyimpy, Me-Pyimpy) used in this study.

py)2](ClO4)2 (4), respectively, in aerobic condition. We have investigated the oxidation of 2,2’-azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) during spontaneous reduction. The molecular structure of 1·2 CH2Cl2 was authenticated by X-ray crystallography. The optimized structure and electronic properties of complexes will be scrutinized in the light of density functional theory. The redox properties of the metal center are described in this article. During spontaneous reduction processes, Mascharak[7a] and Walter[7b,c] reported retention of spin-state (low-spin to lowspin[7a] and high-spin to high-spin[7b,c]); by contrast, Bruijnincx et al.[7d] reported a spin change from a dinuclear high-spin iron(III) complex to a mononuclear low-spin iron(II) complex. Here, we are reporting the conversion of mononuclear highspin complexes to the mononuclear low-spin complexes and spontaneous reduction in aerobic condition. Changes in the spin state (low-spin or high-spin) of iron(III) or iron(II) have been mostly investigated for molecular magnetism or mixedvalence system and reported for designing sensors and digital memory.[8, 9] We tried to investigate the mechanism of such reduction process and speculated the role of solvent for such type of conversion. Interestingly, it has also been found that in our complexes the reduction is facile in aqueous media and this prompted us to perform nuclease activity studies during the spontaneous reduction process. As a part of our ongoing research, we have reported[10] that radicals, reactive oxygen species (ROS) and/or metal-bound transient species are responsible for oxidative DNA cleavage activity. We have also investigated the nuclease activity during regeneration of phenolato complex from phenoxyl radical complex.[10a] In fact, during nuclease activity studies an oxidizing agent like hydrogen peroxide and/or reducing agent such as 2-mercapto ethanol or ascorbic acid were utilized. We have investigated the nuclease activity during spontaneous reduction because the reduction process occurred in the absence of any reducing agent which probably could lead to DNA cleavage via self-activation.[10b,d] To the best of our knowledge, there is no report available in the literature where reactive species produced during spontaneous reduction process were utilized for nuclease activity studies.

Scheme 2. Synthesis and conversion of metal complexes. Chem. Asian J. 2015, 10, 350 – 361

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351


Full Paper an interesting reactivity that will be discussed in detail (see below).

Table 1. Electronic absorption spectral data of FeIII/FeII complexes. Complexes Medium lmax, nm (e, M1 cm1)

Spectroscopic studies

1·2 CH2Cl2

The infrared spectra of the complexes exhibited a characteristic nC=N band near 1600 cm1 for complexes 1·2 CH2Cl2, 2, 3 and 4 (Figures S1–S2, Supporting Information). The increase in nC=N afforded clear indication of binding of Nim donors to metal center; however, the nC=N for ligands Pyimpy and Me-Pyimpy were 1578 cm1 and 1584 cm1 respectively.[7a, 11] The sharp peaks around 1090 and 623 cm1 clearly showed the presence of a perchlorate ion13 in 3 and 4. The ESI mass spectra of 1·2 CH2Cl2 and 2 in dichloromethane have a mass peak at m/ z = 401.9890 and 415.0050, respectively, which corresponds to [M(Cl)] + . ESI mass spectra for 3 and 4 in dimethylformamide show a mass peak at m/z = 703.1248 and 731.1552, respectively, corresponding to [M(ClO4)] + (Figures S3–S5, in the Supporting Information). Complexes 1·2 CH2Cl2 and 2 are essentially non-electrolytic in nature, showing a molar conductance value of 25.0 and 21.0 W1 cm2 mol1, respectively, in DMF solution. However, complexes 3 and 4 exhibited (1:2) uni-bi electrolyte behavior with molar conductance values of 128.0 and 130.0 W1 cm2 mol1, respectively, in DMF solution.[14] Electronic absorption for 1·2 CH2Cl2 and 2 (in dichloromethane) provided three bands near 370, 295 and 245 nm for complex 1·2 CH2Cl2 and near 365, 290 and 245 nm for complex 2. The first two bands (wavelength above 300 nm) were tentatively assigned as chloro-to-FeIII charge-transfer transitions[15] , whereas the bands in the UV region were probably due to p–p* and n–p* transitions (wavelengths below 250 nm). For 3 and 4 (in acetonitrile), there were two moderate bands (520, 465 nm for 3 and 535, 475 nm for 4) in the visible region. The band with lmax near 500 nm could be assigned as charge-transfer transition of an electron from the 3dp orbital of metal to the LUMO of the ligand which is similar to FeII complex of bipyridine.[16] Similar type of MLCT was observed in [Fe(terpy)2]2 + with lmax at 560 nm.[17] Another band near 470 nm was observed, which was similar to iron(II) complexes described by Brook and Krumholz.[18a,b] Complex 4 showed a significant bathochromic shift (wavelength near 534, 476, 335, 275 and 243 nm) as compared to 3 probably because of the electron-donating substituent present on the ligand frame. The absorption spectra of complexes 1·2 CH2Cl2 and 2 in the solid state exhibited broad charge-transfer band in the visible region (500–650 nm), which is characteristic of FeIII located in a octahedral environment (Figure S6, Table 1). The electronic spectra of all the complexes are displayed in Figure 1. The effective magnetic moment of 1·2 CH2Cl2 and 2 were 5.56 and 5.87 BM at 296 K, respectively, which confirmed high-spin iron(III) center (d5). Low-spin nature of the iron(II) center (d6) in 3 and 4 were supported by 1H and 13C NMR spectra. During assignment of protons in 3 and 4, we observed some prominent changes in the 1H NMR spectra of complexes 3, 4 and the free ligands. In the free ligand (Pyimpy), imine proton (H-5) showed multiplet due to coupling with H-4 of pyridine. Upon coordination of imine proton to metal center, (H-5) proton in 3 moved downChem. Asian J. 2015, 10, 350 – 361

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[a]

CH2Cl2 solid [a] CH2Cl2 [b] solid [c] ACN [b]

2 3 4

[c]

ACN

370 (15,300), 295 (10,650), 246 (16,000), 230 (15,000) 570, 527, 470, 367 364 (8,600), 292 (7,900), 243 (13,050) 576, 363 519 (10,550), 463 (5,700), 383 (15,160), 332 (60,750), 272 (26,100), 240 (42,200) 534 (6,850), 476 (3,850), 335 (39,400), 248 (25,200)

[a,c] in dichloromethane and acetonitrile. [b] solid state (Nujol mull).

Figure 1. Electronic absorption spectra of 1 and 2 in dichloromethane and of 3 and 4 in acetonitrile.

field to 9.26 ppm (and showed sharp singlet due to rigidity of ligand moiety after metal binding) from 7.49 ppm in ligand (Pyimpy) (Figure 2). This may be due to the coordination of imine nitrogen to the metal center which reduces the electron density and afforded a large downfield shift in 1H NMR of the H-5 proton of 3. Similar 1H NMR shifts were observed in the complexes [Fe(PapH)2]2 + and [Zn(E-PAPHY)2]2 + .[19] However, 4 did not have any proton bonded carbon in imine function; hence no peak near 9.00 ppm was found, however methyl proton signal was moved to 2.59 ppm from 2.14 ppm after complexation with metal center. 1H and 13C NMR spectra of 3 and 4 are shown in Figures S7–S10. The molecular structure of 1·2 CH2Cl2 is depicted in Figure 3. The matrix parameters of this complex are described in Table 2, and selected bond distances and bond angles are given in Table 3. Crystals of 1·2 CH2Cl2 were obtained within a week by slow evaporation in dichloromethane: methanol (1:1) mixture. Complex 1·2 CH2Cl2 was crystallized in monoclinic space group C2/c consisting of mononuclear iron(III) complex with pyridine nitrogens (Npy) and imine nitrogen (Nim) donor atoms. The coordination geometry around FeIII centre was distorted octahedral. The high-spin FeIII center was ligated to two pyridine nitrogen (Npy), one imine nitrogen (Nim) and three chloride ions (Cl). Tridentate ligand was coordinated to the metal center in meridional fashion. The structure of 1·2 CH2Cl2 was closely related to reported complexes mer-[Fe(terpy)Cl3][20] and [Fe(DPA)Cl3].[15b] The existence of five-membered chelate rings 352


Full Paper 1·2 CH2Cl2 to 3, four isosbestic points near 435, 340, 305 and 273 nm (Figure 4 a) and shifting of 357 peak to 334 nm (show in inset of Figure 4 a) suggested the formation of 3. Complex 3 provided visible spectra which were similar to the spectra obtained at the end of the conversion in water (see Figure 1). In case of 2, only one clear isosbestic point at 348 nm was observed along with a blue-shift of 349 nm peak to 331 nm (inset of Figure 4 b) indicating the formation of 4. We have mentioned earlier that Fe(ClO4)3·xH2O start-

Figure 2. 1H NMR spectra of (a) ligand Pyimpy and (b) complex 3 in (CD3)2SO at room temperature.

Table 2. Selected crystallographic data for 1·2CH2Cl2. C19H18N4Cl7Fe

Empirical formula 1

Formula weight [g mol ] T [K] l [] (MoKa) Crystal system Space group a [] b [] c [] a [8] g [8] b [8] V (3) Z 1calc [g cm3] Crystal size [mm] F(000) q range for data collection Index ranges

Figure 3. ORTEP diagram (50 % probability level) of complex 1·2 CH2Cl2. Solvent molecules and hydrogen atoms are omitted for clarity.

causes a distortion of the octahedron towards the imine group. Distortion at the iron(III) center was also described by the angles cis-N-Fe1-N, 73.82(9)8 and 73.40(9)8; trans-N1-Fe1N3, 147.22(10)8. Similar distortions were also observed in complexes [Fe(tptz)Cl3] and mer-[Fe(terpy)Cl3] with cis-N-Fe-N angles at 74.898 and 75.008, however for trans-N-Fe-N angle of 149.84(5)8.[20] Packing diagram of 1·2 CH2Cl2 showed non-covalent interactions with solvent molecule and axial chloride atom of 1·2 CH2Cl2 (3.423 , 2.767 ) (Figure S11, Supporting Information). Weak p–p interaction between pyridine rings of two adjacent molecules (3.854 ) was consistent with reported values.[21]

Refinement method Data/restraints/ parameters GOFa on F2 R1b [I > 2s(I)] R1[all data] wR2c [I > 2s(I)] wR2 [all data]

1.123 0.0459 0.0790 0.1430 0.1767

[a] GOF = [S[w(Fo2-Fc2)2]/M-N)]1/2 (M = number of reflections, N = number [c] wR2 = of parameters refined). [b] R1 = S j j Fo j- j Fc j j /S j Fo j , [S[w(Fo2Fc2)2]/S[w(Fo2)2]]1/2.

Spontaneous reduction We have mentioned in the previous section that complexes 3 and 4 could also be prepared by spontaneous reduction of 1·2 CH2Cl2 and 2, respectively, in aqueous media. This conversion and spontaneous reduction of 1·2 CH2Cl2 and 2 to complexes 3 and 4, respectively in water media have been monitored by UV/Vis and fluorescence spectrophotometry. Figure 4 describes the reduction of high-spin FeIII center of 1·2 CH2Cl2 and 2 and formation of low-spin FeII complexes 3 and 4, respectively, and this was indicated by slow growth of two distinct peaks near 525 and 465 nm. During the conversion of Chem. Asian J. 2015, 10, 350 – 361

606.5 296(2) 0.71073 Monoclinic C 2/c 25.789(5) 14.376(3) 16.099(3) 90.00 90.00 127.581(6) 4730.1(17) 4 1.584 0.329 0.129x 0.102 2272.0 0.980–28.28 33 < h < 25, 19 < k < 15, 14 < l < 21 Full matrix least squares on F2 5753/0/267

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ing material in presence of Pyimpy and Me-Pyimpy in 1:2 metal to ligand ratio afforded [Fe(Pyimpy)2]3 + and [Fe(MePyimpy)2]3 + , respectively (Scheme 2). However, these two species undergo spontaneous reduction, and [Fe(Pyimpy)2]3 + species gave rise to 3 whereas [Fe(Me-Pyimpy)2]3 + species gave rise to 4. The formation of [Fe(Pyimpy)2]3 + , [Fe(Me-Pyimpy)2]3 + and concomitant spontaneous reduction event could be monitored by UV-visible spectral studies (in acetonitrile) and are dis353


Full Paper ed the FeII complex [(N4Py)Fe(MeCN)]2 + as the single product. We want to mention here that Fe(ClO4)3 xH2O and Pyimpy, MePyimpy (1:1) also afforded low-spin FeII complexes 3 and 4, reBond distances [] spectively, in acetonitrile. We have also tried to determine the percent conversion of complex 1 to complex 2 (by monitoring Fe1–N1 2.163(2) Fe1-N2 2.164(2) Fe1–N3 2.139(3) Fe1-Cl1 2.2352(10) the OD at 520 nm). We found the percent conversion in acetoFe1–Cl2 2.3300(9) Fe1-Cl3 2.3651(10) nitrile to be low (11 %); however, percent conversion in water is about 35 %. Bond angles [deg] It is already known that high-spin iron(III) (d5-high-spin) comN1-Fe1-N3 147.22(10) N1-Fe1-N2 73.82(9) plexes are kinetically labile, while low-spin iron(II) complexes N2-Fe1-N3 73.40(9) N2 Fe1 Cl1 85.63(6) N1 Fe1 Cl1 89.86(6) N2-Fe1-Cl2 176.54(7) are exchange inert. As mentioned above, FeIII salts such as N3 Fe1 Cl2 109.96(7) N1 Fe1 Cl2 102.82(7) FeCl3 or Fe(ClO4)3 xH2O were treated with an excess amount of N3 Fe1 Cl3 87.06(6) N1 Fe1 Cl3 90.90(6) terpyridine (tpy) to produce a green-colored [Fe(tpy)2]3 + comN2 Fe1 Cl3 86.52(7) N3 Fe1 Cl1 87.77(6) plex immediately. It has previously been reported that the deCl2 Fe1 Cl3 94.39(4) Cl3-Fe1-Cl1 171.57(4) Cl2-Fe1-Cl1 93.62(3) velopment of the 1:1 complex is substituted complexes are easily produced the rate determining step of a similar process and higher as a major product due to the spatial arrangement of two tpy ligands (meridional). Based on above facts and titration of the complex [Fe(Pyimpy)Cl3], 1, (after 15 h, in water) with various equivalents of chelate ligand (Pyimpy), the characteristic peak at 520 nm increases with increase in ligand equivalent (Figure S12 c–e). We propose first [Fe(Pyimpy)Cl3] (1) (highspin, mono-chelated) in water or acetonitrile was converted to [Fe(Pyimpy)2]3 + (3) (low-spin, bischelated). After conversion to bis-chelated low-spin [Fe(Pyimpy)2]3 + species, a subsequent reduction to [Fe(Pyimpy)2]2 + species happened. A similar mechanism was also proposed for the conversion of complex 2 to 4. The E1/2 value for FeII/FeIII couple was found to be 1.095 V vs. Ag/ Figure 4. Electronic absorption and fluorescence spectra showing conversion and spontaneous reduction: AbsorpAgCl. This data clearly indicated tion changes of (a) 1 to 3 (b) 2 to 4 were taken at 15 min intervals in water (insets show the initial and final spechighly stable FeII low-spin contra after 12 h). (c,d) Fluorescence spectra showing increase in emission intensity in water (lex = 310 nm) at room figuration in bis-chelated comtemperature; (c) 1, (1.5 mm) (a!s), (d) 2, (1.5 mm) (a!m). plex, that is, [Fe(Pyimpy)2]2 + (see below). Fluorescence spectra of complexes 1·2 CH2Cl2 and 2 in water played in Figure S12 in the Supporting Information. These results prompted us to investigate two different aspects for comalso support spontaneous reduction to diamagnetic FeII center. plexes 1·2 CH2Cl2 and 2. First, conversion of 1·2 CH2Cl2 and 2 to Initially paramagnetic iron complexes 1·2 CH2Cl2 and 2 did not 3 and 4, respectively, in aqueous solution because in the preshow any fluorescence, however, free ligand Pyimpy and Mecursor complexes 1·2 CH2Cl2 and 2 iron are in + 3 oxidation pyimpy also displayed no fluorescence[23] in the same experistates. Second, bis- chelation of ligand and spontaneous reducmental conditions. As of reduction of metal center paramagtion with change in spin state (high-spin to low-spin). Morenetic metal center (1, 2) changes to diamagnetic complexes 3 over, the above information was supported by a different synand 4, their emission intensity enhanced at 430–440 nm (lex ~ thetic route for the synthesis of 2 starting from Fe(ClO4)3 xH2O 310 nm) (Figure 4 (c), (d)). The above events prompted us to study NMR spectra during or Fe(ClO4)2 xH2O with two equivalents of ligands Pyimpy and this conversion and concomitant reduction processes. 1H NMR Me-Pyimpy (shown in Scheme 2). Roelfes et al.[22] also reported that reaction of N4Py with both FeII as well as FeIII salts affordspectra of complex 1·2 CH2Cl2 were taken in a mixture of Table 3. Selected bond lengths () and angles (deg) of complexes 1·2 CH2Cl2.

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Full Paper H2O:CD3CN (1:5) and these spectra were monitored for 6 days (Figure S13). We would like to mention here that in deuterated solvent this conversion and associated reduction was very slow that is why minimum amount of water was added. Complex 1·2 CH2Cl2 was dissolved in H2O:CD3CN (1:5) and after 2 h, spectra were recorded (Figure 5, the control experiment

days which also authenticated the conversion (Figure S16 in the Supporting Information). This type of spontaneous reduction and spin state change is known for iron(III) porphyrins in the presence of strong field ligands like pyridine, piperidine, cyanide, n-hexanethiol, phosphine, alkoxides etc. In porphyrins this reduction is either through base catalyzed (by one electron oxidation of the substrates) auto reduction via piperidine, alkoxide, cyanides etc. or strong Lewis acidity of iron(III) porphyrins causes the reduction to iron(II) porphyrins in presence of an axial ligand such as pyridine, imidazoles. Patra et al.[24] also reported high-spin [FeIII(tnOEP)(L)2.X] (X: Cl, ClO4) porphyrins spontaneously autoreduced to low-spin [FeII(tn-OEP)(L)2] complexes depending upon axially bounded strong field ligand L (L: pyridine/substituted pyridine). Dioxoporphodimethene iron(III) complex also spontaneously reduced to low-spin bis(pyridine) iron(II) complex in the presence of pyridine.[25] This conversion of 1 and 2 and associated spontaneous reduction are probably driven by the stability gained by ligands (Pyimpy, Me-Pyimpy) and lability of FeIII (high-spin) centres. This reduction process was also supported by cyclic voltammetric studies.

Figure 5. 1H NMR spectra of complex 1·2 CH2Cl2 in H2O/CD3CN (1:5) taken after every 24 h. Images of 1·2 CH2Cl2 (left) and complex 2 (right) at day 1 and day 6 in H2O/CD3CN.

Electrochemistry We have investigated the redox properties of all four complexes 1–4 by cyclic voltammetry (Figure 6). Mono complexes 1·2 CH2Cl2 and 2 stabilized high-spin FeIII center with E1/2 values of 0.270 V and 0.264 V vs Ag/AgCl electrode respectively (Table 4). The E1/2 values for 1·2 CH2Cl2 and 2 were slightly

(H2O:CD3CN, 1:5) was also done and is shown in Figures S14S15 in the Supporting Information). A sharp peak at 12.70 ppm, probably for solvent or water coordination to the metal coordination, was observed. NMR spectra of same solution were taken after every 24 h interval, for 6 days. After 6 days, the peak at 12.07 ppm disappeared completely and sharp peak at around 9.0 ppm appeared. This peak was probably due to imine proton (H-5). Color of solution was also changed from orange-yellow to dark red after 6 days in H2O:CD3CN (1:5) as shown in Figure 5. Slight shift in solvent peaks (Figure S14–S15) was also observed. We were unable to obtain similar data for complex 2 because of overlap of CH3 protons with CD3CN and water peaks. The color change from orange-brown (first day) to red after six days in H2O:CD3CN (1:5) for complex 2 is shown in Figure 6. ESI-MS spectra of complexes 1 and 2 in CD3CN: H2O (1:5) were recorded after six

Table 4. Electrochemical data for FeIII/FeII redox couple 298 Ka vs Ag/ AgCl. Complex 1·2CH2Cl2 2 3 4

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Epc/V

E1/2b,V (DEpc, mV)

d

0.373 0.403 1.238 1.164

0.168 0.126 1.157 1.025

0.270 0.264 1.197 1.095

(205) (277) (81) (139)

n = iPa/iPc

0.86 0.75 1.18 0.25

[a] Measured in acetonitrile for 1·2CH2Cl2, 2, 3 and in dichloromethane for 4 with 0.1 m tetrabutylammonium perchlorate (TBAP). [b] Data from cyclic voltammetric measurements; E1/2 is calculated as average of anodic (Epa) and cathodic (Epc) peak potentials E1/2 = 1/2(Epa + Epc); and [c] DEp = EpaEpc at scan rate 0.1 V s1. dConstant-potential coulometric data n = ipa/ ipc calculated for 1e transfer.

smaller than the values reported for complex [Fe(tpy)Cl3] by Dhanalakshmi et al.[26] E1/2 values of 3 and 4 (1.197 V and 1.095 V vs Ag/AgCl respectively; Table 3) clearly indicated the stabilization of FeII center in 3 and 4. The E1/2 values for 3 and 4 were in the range of E1/2 values for FeII complexes reported in literature having ligands with soft donors (pyridine or imine donors)[7a, 22] and showed Pyimpy or Me-Pyimpy stabilized FeII metal center. Due to the presence of electron donating methyl (-Me) substituent in ligand frame the E1/2 values shifts to less positive in case of 2 and 4 as compared to 1·2 CH2Cl2 and 3 respectively.[13] Cyclic voltammetric data clearly showed one e

Figure 6. Cyclic voltammograms of 1·2 CH2Cl2, 2, 3 (in acetonitrile), and 4 (in dichloromethane) at 298 K vs. Ag/AgCl. Chem. Asian J. 2015, 10, 350 – 361

FeIII/FeII Epa/V

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Full Paper transfer process. High positive potential for 3 and 4 easily described the facile reduction of 1·2CH2Cl2 and 2 to 3 and 4 respectively.

Table 5. Energies of optimized structure in gas phase and in water using PCM.

Density Functional Theory (DFT) Calculations. Geometry optimization for complexes 1, 3 have been carried out at DFT level (Figure 7). The geometrical parameters as bond lengths and bond angles in gas phase as well as in solvent (water) were calculated (Table S1, Supporting Information)

Energies

[Fe(Pyimpy)Cl3] 1

[Fe(Pyimpy)2]2 + 3

Egas (Hartree) Ewater (Hartree) EDCM (Hartree) EACN (Hartree) EgasEwater [kcal mol1]

1043.5409091 1043.583473 1043.5754438 1043.5806012 0.0425639 [26.71]

EgasEACN [kcal mol1]

0.0396921 [24.91] 0.0345347 [5.04]

1873.2710255 1873.454533 1873.4354247 1873.4515367 0.1835081 [115.15] 0.1805112 [113.27] 0.1643992 [11.99]

EgasEDCM [kcal mol1]

water, acetonitrile (ACN) and dichloromethane (DCM) are shown in Table 5. We observed, polar solvent has more stabilizing effect for example, in case of complex 1 water has stabilizing effect of 26 kcal mol1 whereas for complexes 3 it is 115.15 kcal mol1. Interestingly, water has more stabilizing effect in complex 3 than other solvents. This may explain the formation complex 3 in water via spontaneous reduction.

Figure 7. Ground state optimized geometry of complexes 1 (a) and 3 (b). Hydrogen atoms are omitted for clarity.

Oxidation of ABTS to ABTS· + by complexes 1·2 CH2Cl2 and 2

using Gaussian 09 package.[27] Contour plots of molecular orbitals of complexes 1 and 3 were generated using Gauss view 5.0 and shown in Figure 8. TD-DFT was performed on both the complexes and significant transitions along with their orbital contribution are given in Table S2. For complex 3, two transi-

Oxidation of the diammonium salt of 2,2’-azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) generally used for peroxidase-like activity, where colorless ABTS is readily reacts with hydrogen peroxide in presence of a peroxidase catalyst to yield a stable green colored radical cation (ABTS· + ) (see Scheme S1, Supporting Information), which absorbs light at 415, 650, 735 and 815 nm.[28] To confirm the reduction of complexes 1·2 CH2Cl2 and 2 and generation of reactive species we have mixed aqueous solution of ABTS to complexes 1·2 CH2Cl2 and 2. Oxidation of ABTS starts immediately by generation of green color (ABTS· + ) solution after addition of complexes 1·2 CH2Cl2 and 2. The characteristic absorption bands of this species were depicted in Figure 9 (see Figure S17, Supporting Information, Figure 8. Frontier orbitals diagram for the HOMO, HOMO-1, HOMO-2, HOMO-3 and LUMO, LUMO + 1, LUMO + 2, LUMO + 3 of complex 1 (a) and complex 3 (b). Hydrogen atoms are removed for clarity. for 2). UV-vis spectra showed peak at 415, 650, 735 and 815 nm and confirmed the fortions observed near 500 nm which corresponds to HOMO! mation of radical cation ABTS· + .[28] In literature it is already LUMO transition (f = 0.0032) and HOMO!LUMO + 1 (f = known, that reactive intermediates or reactive oxygen species 0.0026) having orbital contribution of 84 % in both. Another (ROS) are responsible of DNA nuclease via nucleobase modifitransition near 450 nm is mostly due to HOMO-1!LUMO, cation or sugar-hydrogen (C’1-H, C’2-H, C’3-H, C’4-H and C’5-H) LUMO + 1. A strong peak at 388 nm (f = 0.0903) is also obabstraction.[29] As we already elucidate that, complex 1·2CH2Cl2 served corresponding to HOMO-1!LUMO + 1. Energies of the and 2 generated reactive oxygen species in aqueous solution optimized structures in gas phase and using PCM model for and we have utilized this property for nuclease activity. Chem. Asian J. 2015, 10, 350 – 361

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Full Paper 1·2CH2Cl2 and 2 through NC and LC form of pBR322 plasmid. Mechanism of nuclease activity during spontaneous reduction was investigated in presence of NaN3 (singlet oxygen scavangers) and KI (OH . radical scavangers). Inhibition of nuclease in presence of KI (Figure 10 (a), lane: 8, Figure 10 (b), lane: 8) indicated the possible role of OH . during nuclease of 1·2CH2Cl2 and 2. To the best of our knowledge this is the first example of nuclease activity of complexes during spontaneous reduction. We have found that, this nuclease activity was enhanced in presence of oxidizing or reducing agent. As oxidizing or reducing agents also help in generation of reactive species in presence of redox active metal complexes. The oxidative cleavage of complexes 1·2 CH2Cl2 and 2 was studied in the presence of H2O2 and BME (Figure 11, S18). It should be noted that, we did

Figure 9. Absorption spectra of complex 1 showing oxidation of ABTS.

Nuclease Activity Nuclease during spontaneous reduction. During spontaneous reduction process possible role of solvent was suggested and generation of reactive species in solution could cause DNA cleavage activity.[29] This prompted us to study the nuclease activity of 1·2 CH2Cl2 and 2 in different experimental conditions. Nuclease activity was investigated in presence of 1·2 CH2Cl2 and 2 only (without addition of any oxidizing or reducing agent) and we did not observe any (or negligible) chemical nuclease activity within 3 h of incubation (Figure S18, Supporting Information), however oxidative nuclease was observed in presence of H2O2 and BME within that period of time. In another experiment, keeping in mind the time period of conversion and concomitant spontaneous reduction of 1·2 CH2Cl2, 2 to 3, 4 respectively, we investigated DNA cleavage activity with 12 h on incubation. For that, we incubated 1·2 CH2Cl2 and 2 with pBR322 DNA (without addition of any oxidizing or reducing agent) for 12 h and estimated the extent of nuclease activity by gel electrophoresis diagram (Figure 10). In

Figure 11. Gel electrophoresis separations showing cleavage of supercoiled pBR322 DNA (60 ng) by 1·2 CH2Cl2 and 2 after incubation at 37 8C for 3 h. (a) lane 1: DNA, lane 2: DNA + 1 (15 mm) + H2O2 (200 mm), lane 3: DNA + 1 (25 mm) + H2O2 (200 mm), lane 4: DNA + 1 (50 mm) + H2O2 (200 mm), lane 5: DNA + 1 (25 mm) + H2O2 (15 mm), lane 6: DNA + 1 (25 mm) + H2O2 (25 mm), lane 7: DNA + 1 (25 mm) + H2O2 (50 mm), lane 8: DNA + 1 (25 mm) + H2O2 (100 mm), lane 9: DNA + H2O2 (200 mm). (b) lane 1: DNA, lane 2: DNA + 2 (1 mm) + H2O2 (50 mm), lane 3: DNA + 2 (5 mm) + H2O2 (50 mm), lane 4: DNA + 2 (10 mm) + H2O2 (50 mm), lane 5: DNA + 2 (15 mm) + H2O2 (50 mm), lane 6: DNA + 2 (15 mm) + H2O2 (100 mm), lane 7: DNA + 2 (15 mm) + H2O2 (150 mm), lane 8: DNA + 2 (15 mm) + H2O2 (200 mm), lane 9: DNA + H2O2 (200 mm).

not incubate the DNA-metal-complex mixture more than 3 h just to avoid the formation of bis-chelated complexes 3 and 4. We want to mention here that although the reaction starts (bis complex formation and concomitant reduction) however, we did not observe significant nuclease activity in that time scale. Control experiments using H2O2 (Figure 11, lane: 9) and FeCl3 salt (Figure 12 b), lane: 9) alone did not show any cleavage of pBR322 DNA under similar experimental conditions. A 15 mm

Figure 10. Gel electrophoresis separations showing cleavage of supercoiled pBR322 DNA (20 ng) by 1·2CH2Cl2 and 2 after incubation at 37 8C for 15 h. (a) lane 1: DNA, lane 2: DNA + 1 (50 mm), lane 3: DNA + 1 (100 mm), lane 4: DNA + 1 (200 mm), lane 5: DNA + 1 (400 mm), lane 6: DNA + 1 (400 mm) incubated for 3 h, lane 7: DNA + 1 (400 mm) + NaN3 (20 mm), lane 8: DNA + 1 (400 mm) + KI (20 mm). (b) lane 1: DNA, lane 2: DNA + 2 (400 mm), lane 3: DNA + 2 (200 mm), lane 4: DNA + 2 (150 mm), lane 5: DNA + 2 (50 mm), lane 6: DNA + 2 (400 mm) incubated for 3 h, lane 7: DNA + 2 (400 mm) + NaN3 (20 mm), lane 8: DNA + 2 (400 mm) + KI (20 mm).

Figure 12. Time-dependence of cleavage of supercoiled pBR322 plasmid DNA (60 ng) by 1·2 CH2Cl2 and 2 in the presence of H2O2 (200 mm) incubated at 37 8C. (a) lane 1: DNA, lane 2: DNA + 1 (15 mm) + H2O2 (200 mm) (35 min), lane 3: DNA + 1 (15 mm) + H2O2 (200 mm) (15 min), lane 4: DNA + 1 (15 mm) + H2O2 (200 mm) (7 min), lane 5: DNA + 1 (1 mm) + H2O2 (200 mm) (35 min), lane 6: DNA + 1 (1 mm) + H2O2 (200 mm) (15 min), lane 7: DNA + H2O2 (200 mm) (35 min). (b) lane 1: DNA, lane 2: DNA + 2 (15 mm) + H2O2 (200 mm) (35 min), lane 3: DNA + 2 (15 mm) + H2O2 (200 mm) (15 min), lane 4: DNA + 2 (15 mm) + H2O2 (200 mm) (7 min), lane 5: DNA + 2 (1 mm) + H2O2 (200 mm) (35 min), lane 6: DNA + 2 (1 mm) + H2O2 (200 mm) (15 min), lane 7: DNA + 2 (1 mm) + H2O2 (200 mm) (7 min), lane 8: DNA + H2O2 (200 mm) (35 min), lane 9: DNA + FeIII salt (15 mm).

case of 1·2 CH2Cl2, we found double strand cleavage with generation of LC (linear-circular form) form (Figure 10 (a), lane: 4– 5), however, complex 2 was able to produce single strand cleavage (nicked-circular form, NC) as shown in Figure 10 (b), lane: 2-4. These observation suggested 1·2 CH2Cl2 is more efficient DNA cleavage agent than complex 2. Figure 10, clearly indicated the self-activated DNA cleavage of complexes Chem. Asian J. 2015, 10, 350 – 361

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Full Paper Conclusions

solution of 1·2 CH2Cl2 cleaved SC (supercoiled-circular form) DNA to LC (linearized DNA) form on treatment with 200 mm H2O2 (Figure 11 (a), lane: 2) with ~ 80 % of total DNA cleavage (Table S3). On further increasing the complex concentration (15-50 mm) more than 80 % total DNA cleavage was observed (Figure 11 (a), lane: 3–8 and Table S3). A 1–15 mm of 2 with 50 mm H2O2 showed double strand cleavage (Figure 11 (b), lane: 2–8) (< 85 % total DNA cleavage, Table S4). A 50 mm solution of 1·2CH2Cl2 and 2 showed ~ 80 % of total DNA cleavage in presence of BME (Figure S19, Table S5, S6). Time dependent cleavage was also monitored in presence of H2O2 and BME with complexes 1·2CH2Cl2 and 2. Both complexes showed double strand scission in ~ 7 min (Figure 12 (a),(b), lane: 4). Total DNA cleavage using both 1·2 CH2Cl2 and 2, we obtained more than 80 % in 35 min (Table S7, S8, Supporting Information). Complexes 1·2 CH2Cl2, 2 were also capable of inducing strand scission in presence of BME however, only single strand scission appeared in this case (Figure S19). The above observation suggested that both complexes were efficiently cleaved DNA with fast and enhanced activity in presence of oxidizing or reducing agent. Interestingly, the nuclease activities for 3 and 4 have been shown in Figure S19, Supporting Information. Both the complexes did not show cleavage in presence of 3, 4 or even in presence of H2O2 (Figure S20, Supporting Information). Mechanistic aspects of activity of nuclease of complexes 1·2CH2Cl2 and 2 were studied using different quenchers: singlet oxygen scavengers like NaN3 or l-histidine and hydroxyl radical scavengers like DMSO, KI, urea, ethanol, D2O. Complex 1·2CH2Cl2 showed significant inhibitory effect in the chemical nuclease activity in presence of DMSO, KI and urea (Figure 13 (a), lane: 4–6). Similarly, 2 did not show any apparent inhibition in presence of NaN3 while showed inhibitory effect in presence of DMSO, KI, D2O and urea (Figure 13 (b), lane: 5–8). This mechanistic data suggested that the DNA cleavage reaction for 1·2 CH2Cl2 and 2 in the presence of H2O2 proceeds probably via hydroxyl radical pathway following the Fenton type reaction.[30]

In conclusion, mononuclear FeIII high-spin complexes 1·2 CH2Cl2 and 2 and FeII low-spin complexes 3 and 4 were synthesized and characterized spectroscopically. The molecular structure of 1·2 CH2Cl2 was determined by X-ray crystallography. The optimized DFT geometry of complex 1 in the gas phase and in different solvents agreed well with the crystal structure, and solvent (water) effect in complexes 1 and 3 suggested better stabilization for complex 3 in water. The important results are highlighted as follows: (a) An important example of spontaneous reduction after their conversion from high-spin mono-chelated FeIII complexes (1·CH2Cl2, 2) to bis-chelated complexes in aqueous solution under aerobic condition was described. These processes were monitored by UV/Vis, fluorescence and 1H NMR spectral studies. (b) Oxidation of ABTS clearly expressed the reduction of metal center and generation of reactive species in water. The mechanism of this conversion is still unknown; however, we speculate the formation of FeIII low-spin intermediates, that is, [[Fe(Pyimpy)2]3 + for 1 and [Fe(Me-Pyimpy)2]3 + for 2 in solution, and a concomitant reduction gave rise to complexes 3 and 4, respectively. Conversion in acetonitrile and water indicated a possible role of the solvent in the spontaneous reduction. (c) The generation of reactive species during the spontaneous reduction process in aqueous solution is probably responsible for nuclease activity. Hence, there was no need of any external agent (H2O2 or BME, etc.) for DNA cleavage by complexes 1 and 2. Thus, we found DNA cleavage via self-activation. To the best of our knowledge, this is the first report on self-activated DNA cleavage activity observed during spontaneous reduction of FeIII to FeII. (d) Complexes 1·2 CH2Cl2 and 2 also showed enhanced nuclease activity in the presence of H2O2 and BME via a hydroxyl radical pathway.

Experimental Section Materials and measurements All the chemicals were purchased from Sigma, S. D. Fine, Alfa Aesar, Acros, Himedia, Merck and SRL. Anhydrous FeCl3 was purchased from Rankem, Delhi, India and Fe(ClO4)2 xH2O and Fe(ClO4)3 xH2O were purchased from Sigma Aldrich, Steinheim, Germany and used without further purification. Agarose was purchased from Himedia Laboratories Pvt. Ltd., Mumbai, India. The supercoiled pBR322 DNA was purchased from Bangalore Genei (India). Tris(hydroxymethyl)aminomethane-HCl (Tris-HCl) buffer was prepared in deionised water. Solvent used for spectroscopic studies were HPLC grade and purified by standard procedure before use.[31] Ligand Pyimpy and Me-Pyimpy were prepared according to reported procedures.[11] Elemental analyses were carried out microanalytically at Elementar Vario EL III. The infrared spectra were recorded with Thermo Nikolet Nexus FT-IR spectrometer after preparing KBr pellets with complexes. Electronic absorption spectra were recorded with an Evolution 600, Thermo Scientific UV-visible spectrophotometer. Fluorescence spectra were recorded on a RF-5301 PC Shimazu spectro-

Figure 13. Gel electrophoresis separations showing cleavage of supercoiled pBR322 DNA (60 ng) by 1·2CH2Cl2 and 2 in water incubated at 37 8C for 3 h. (a) lane 1: DNA, lane 2: DNA + 1 (15 mm) + H2O2 (200 mm), lane 3: DNA + 1 (15 mm) + H2O2 (200 mm) + NaN3 (50 mm), lane 4: DNA + 1 (15 mm) + H2O2 (200 mm) + DMSO (50 mm), lane 5: DNA + 1 (15 mm) + H2O2 (200 mm) + urea (50 mm), lane 6: DNA + 1 (15 mm) + H2O2 (200 mm) + KI (25 mm), lane 7: DNA + 1 (15 mm) + H2O2 (200 mm) + l-his (50 mm), lane 8: DNA + 1 (15 mm) + H2O2 (200 mm) + D2O (50 mm), lane 9: DNA + H2O2 (200 mm), lane 10: DNA + FeCl3 (15 mm). (b) lane 1: DNA, lane 2: DNA + H2O2 (100 mm), lane 3: DNA + 2 (15 mm) + H2O2 (100 mm), lane 4: DNA + 2 (15 mm) + H2O2 (100 mm) + NaN3 (50 mm), lane 5: DNA + 2 (15 mm) + H2O2 (100 mm) + DMSO (50 mm), lane 6: DNA + 2 (15 mm) + H2O2 (100 mm) + urea (50 mm), lane 7: DNA + 2 (15 mm) + H2O2 (100 mm) + KI (25 mm), lane 8: DNA + 2 (15 mm) + H2O2 (100 mm) + D2O (50 mm). Chem. Asian J. 2015, 10, 350 – 361

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Full Paper fluorimeter. ESI-MS spectra were measured on a Thermo Scientific Exactive. Molar conductivities were determined in DMF at 103 m at 25 8C with a Systronics 304 conductometer. 1H and 13C NMR were recorded on Bruker AVANCE, 500.13 MHz spectrometer in the deuteriated solvents. Magnetic susceptibilities were measured at 296 K with Vibrating Sample Magnetometer model 155, using nickel as a standard. Diamagnetic corrections were carried out with Pascal’s increments.[32] Cyclic voltammetry measurements were carried out using a CHI-600C electroanalyzer in solvents like dichloromethane and acetonitrile. A conventional three-electrode arrangement consisting of platinum wire as auxiliary electrode, glassy-carbon as working electrode and Ag(s)/AgCl electrode as reference electrode, was used. These measurements were performed in the presence of 0.1 m tetrabutyl ammonium perchlorate (TBAP) as the supporting electrolyte, using complexes concentration 103 m. The ferrocene/ ferrocenium couple was found at E1/2 = + 0.42 (72) V vs. Ag/AgCl under the same experimental conditions. All experiments were performed at room temperature and solutions were thoroughly degassed with nitrogen prior to beginning the experiments and during the measurements nitrogen atmosphere was maintained.

hybrid exchange functional with the Lee–Yang–Parr (LYP) non-local correlation functional was used throughout the computational study.[27b,c] A LANL2DZ basis set was used in the calculation. The Gauss View-5 program was used for pictorial representation of frontier molecular orbitals. Time dependent density functional theory (TD-DFT) calculations were also employed on the optimised geometries to evaluate the electronic transitions. Solvent effect was studied using Polarisable Continnum Model (PCM) and water as a solvent.

Oxidation of ABTS to ABTS· + Oxidation of 2,2’-azinobis(3-ethylbenzothiazoline)sulfonic (ABTS) acid with complexes 1 and 2 was tested in the following manner.[28] An aqueous solution of ABTS (50 mL; 0.009 m) and aqueous solution of the both complexes (10 mL; 103 m) were added to water (3 mL).

Synthesis of metal complexes Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive. Only a small amount of material should be prepared and handled with caution.

X-ray crystallography Red crystals of 1·2CH2Cl2 were obtained by slow evaporation of solution of complex in dichloromethane: methanol (1:1). The X-ray data collection and processing for 1·2 CH2Cl2 was performed on Bruker Kappa Apex-II CCD diffractometer by using graphite monochromated MoKa radiation (l = 0.71070 ) at 296 K. Crystal structure was solved by direct methods. Structure solution, refinement and data output were carried out with the SHELXTL program.[33] All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in geometrically calculated positions and refined using a riding model. Image was created with the DIAMOND program.[34]

Synthesis of [Fe(Pyimpy)Cl3]·2 CH2Cl2] (1·2 CH2Cl2) A solution of anhydrous FeCl3 (110 mg, 0.68 mmol) in 5 mL of methanol was slowly added to a stirred solution of ligand (Pyimpy) (187 mg, 0.68 mmol) in 15 mL of methanol. This reaction mixture was stirred for 1 h and green solid obtained was filtered and washed with methanol, dried in vacuum. Shiny red crystal were grown by dichloromethane: methanol (1:1)/diethyl ether layering at 20 8C for data collection. Yield: 220 mg, (53 %). Selected IR data (KBr): nmax = 1597 cm1, nC=N. UV-visible [CH3CN, lmax/nm (e/ M1 cm1)]: 370 (15,300), 295 (10,650), 246 (16,000), 230 (15,000). meff (296 K): 5.56 BM. ESI-MS in dichloromethane: m/z = 401.9890, [M-(Cl)] + . LM/W1 cm2 mol1 (in DMF): 25. Anal. Calc. For C19H18N4Cl7Fe: C, 37.63; H, 2.99; N, 9.24. Found C, 37.69; H, 2.89; N, 9.53.

DNA cleavage experiments DNA cleavage was measured by the conversion of supercoiled pBR322 plasmid DNA to nicked circular (NC-form) and linear DNA forms (LC-form). Supercoiled pBR322 DNA (100–200 ng) in (TBE) Tris-boric acid-EDTA buffer (pH 8.2) was treated with iron complexes 1·CH2Cl2, 2 (in water) and 3, 4 (in dimethylformamide 10 %). The chemical nuclease activity of the complexes was studied using hydrogen peroxide (H2O2) as an oxidizing agent and 2-mercaptoethanol (BME) as the reducing agent. The samples were incubated at 37 8C, added loading buffer (25 % bromophenol blue and 30 % glycerol). The agarose gel (0.8 %) containing 0.4 ng mL1 of ethidium bromide (EB) was prepared and the electrophoresis of the DNA cleavage products was performed on it. The mechanistic studies were carried out using different additives as quenchers of singlet oxygen NaN3, l-his and scavengers of hydroxyl radicals DMSO, KI, Urea, D2O prior to addition of the complex. The quantitative parameter of total DNA cleavage was calculated using formula DNA cleavage = DII/(DI + DII) or (DII + 2 x DIII)/(DI + DII + 2 x DIII), where DI, DII, DIII : integrated density of SC-form, NC-form and LCform respectively.[3g] The gel was run at 60 V in TBE buffer and the bands were identified by placing the stained gel under an illuminated UV lamp. The fragments were visualized by a UV illuminator (BIO RAD).

Synthesis of [Fe(Me-Pyimpy)Cl3] (2) A batch of (162 mg, 1.0 mmol) anhydrous FeCl3 in 10 mL of methanol was added dropwise to stirred solution of (288 mg, 1.0 mmol) ligand (Me-Pyimpy) in 10 mL of dichloromethane. The color of solution was changed to deep red and after 1=2 h green solid was separated out, however, this green solid on dissolution gives red color. The green solid obtained was filtered and washed with small amount of methanol and diethylether. Yield: 360 mg, (80 %). Selected IR data (KBr): nmax = 1598 cm1, nC=N. UV-visible [CH3CN, lmax/nm (e/M1 cm1)]: 364 (8,600), 292 (7,900), 243 (13,050). meff (296 K): 5.87 BM. ESI-MS in dichloromethane: m/z = 415.0050, [M-(Cl)] + . LM/ W1 cm2 mol1 (in DMF): 21. Anal. Calc. For C18H16N4Cl3Fe: C, 47.98; H, 3.58; N, 12.44. Found C, 47.80; H, 3.92; N, 12.23.

Synthesis of [Fe(Pyimpy)2](ClO4)2 (3) Method A A solution of Fe(ClO4)2.xH2O (66 mg, 0.26 mmol) in 1 mL of methanol was slowly added to a stirred solution of ligand (Pyimpy) (145 mg, 0.53 mmol) in 2 mL of methanol and stirred for 15 min at room temperature. The color of solution was immediately changed to red and within 1=2 h deep red solid separated out. Red solid was filtered, washed with methanol and dried in vacuum. Yield:

DFT study and computational details The DFT calculation for complexes 1 and 3 were carried out using Gaussian 09 program package.[27] The Becke’s three parameters Chem. Asian J. 2015, 10, 350 – 361

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Full Paper 299 mg, (70 %). Selected IR data (KBr): nmax = 1602, n-C=N, 1090, 622 cm1, nClO4-. UV-visible [CH3CN, lmax/nm (e/M1 cm1)]: 519 (10,550), 465 (5,700), 383 (15,160), 332 (60,750), 272 (26,100), 240 (42,200). ESI-MS in dimethylformamide: m/z = 703.12482, [M(ClO4)] + . LM/W1 cm2 mol1 (in DMF): 128. 1H NMR ([D6]DMSO, 400 MHz): d 9.26 [s, 1H], 8.33–8.29 [d, J = 12.8 Hz, 2H], 8.12–8.03 [m, 4H], 7.98–7.92 [m, 3H], 7.77–7.73 [t, J = 8.0 Hz, J = 7.6 Hz, 1H], 7.38–7.35 [t, J = 6.8 Hz, 6.0 Hz, 1H], 7.11–7.08 [t, J = 6.4 Hz, 1H], 6.59–6.56[d, J = 8.8 Hz, 1H]. 13C NMR ([D6]DMSO, 400 MHz): 159.58, 158.54, 153.38, 150.89, 145.83, 142.09, 139.56, 134.78, 133.30, 132.96, 130.88, 127.98, 127.14, 122.22, 109.51. Anal. Calc. For C34H28N8O8Cl2Fe: C, 50.83; H, 3.51; N, 13.95. Found C, 50.80; H, 3.12; N, 13.23.

red solid was washed with excess of water and methanol and dried in vacuum. (Yield: 31 %) (ii) Conversion of complex 2 to 4. A batch of 2 (132 mg, 0.29 mm) in 5 mL of distilled water was stirred for overnight. Next day excess of sodium perchlorate (NaClO4.xH2O) was added to this mixture red solid separated out immediately. The reaction mixture was stirred for another 1 h and precipitate was filtered out. The purple-red solid was washed with excess of water, methanol and dried in vacuum. (Yield: 34 %)

Acknowledgements KG is also thankful to DST-SERB, India for financial assistance no. SR/S1/IC-47/2012 dated 21-OCT-2013. NT is thankful to CSIR and AKD is thankful to UGC for fellowship. We are thankful to DST-FIST program for providing us ESI-MS facility in our department.

Method B A solution of Fe(ClO4)3.xH2O (60.1 mg, 0.17 mm) in 1 mL of methanol was slowly added to a stirred solution of ligand (Pyimpy) (95.8 mg, 0.35 mmol) in 2 mL of methanol and stirred. Initially yellow solid separated from solution but after 5–10 min stirring the solid color changed to red. This mixture was further stirred for 1 h and red solid was separated out by filtration washed with methanol dried in vacuum. Yield: 150.2 mg, (53 %).

Keywords: ABTS oxidation · density functional calculations · iron complexes · self-activated nuclease · spontaneous reduction

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Method A A solution of Fe(ClO4)2.xH2O (66 mg, 0.26 mmol) in 1 mL of methanol was slowly added to a stirred solution of ligand (Me-Pyimpy) (145 mg, 0.53 mmol) in 2 mL of methanol and stirred for 15 min at room temperature. The color of solution immediately changed to purple-red within 1=2 h purple-red solid separated out. This mixture was further stirred for 1 h. Purple-red solid was obtained after filtration and washed with methanol. Yield: 100 mg, (82 %). Selected IR data (KBr): nmax = 1600, n-C=N, 1090, 622 cm1, nClO4-. UV-visible [CH3CN, lmax/nm (e/M1 cm1)]: 534 (7,750), 476 (4,100), 335 (49,750), 275 (22,400), 243 (32,000). ESI-MS in dimethylformamide: m/z = 731.1552, [M-(ClO4)] + . LM/W1 cm2 mol1 (in DMF): 130. 1 H NMR ([D6]DMSO, 400 MHz): d = 8.28–7.93 [m, 9H], 7.72–7.68 [t, J = 8.4 Hz, 7.6 Hz, 1H], 7.49–7.46 [t, J = 6.4 Hz, 6.4 Hz, 1H], 7.13–7.10 [t, J = 6.8 Hz, 6.4 Hz, 1H], 6.42–6.40 [d, J = 8.4 Hz, 1H], 2.42 ppm [s, 3H]. 13C NMR ([D6]DMSO, 400 MHz): d = 161.46, 160.44, 156.56, 153.52, 152.67, 150.52, 141.50, 139.75, 139.37, 132.68, 131.95, 131.60, 128.45, 125.83, 121.93, 116.08, 112.92, 18.39 ppm. Anal. Calc. For C36H32N8O8Cl2Fe: C, 52.00; H, 3.88; N, 13.48. Found C, 52.90; H, 3.12; N, 13.23 %.

Method B A solution of Fe(ClO4)3.xH2O (35.4 mg, 0.10 mm) in 1 mL of methanol was slowly added to a stirred solution of ligand (Me-Pyimpy) (57.6 mg, 0.20 mmol) in 2 mL of methanol. Initially yellow solid separated from solution but after 5–10 min stirring the solid color changed to red. This mixture was further stirred for 1 h and purple-red solid was separated by filtration washed with methanol and dried in vacuum. Yield: 102 mg, (61 %).

Conversion of complexes 1·2CH2Cl2 and 2 (i) Conversion of complex 1·2 CH2Cl2 to 3. A batch of 1 (102 mg, 0.23 mm) in 5 mL of distilled water was stirred for overnight. Next day excess of sodium perchlorate (NaClO4.xH2O) was added to this mixture red solid separated out immediately. The reaction mixture was stirred for another 1 h and precipitate was filtered out. The Chem. Asian J. 2015, 10, 350 – 361

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