DNA-binding, catalytic oxidation, CAC coupling reactions and antibacterial activities of binuclear Ru(II) thiosemicarbazone complexes: Synthesis and spectral characterization

New hexa-coordinated binuclear Ru(II) thiosemicarbazone complexes of the type {[(B)(EPh3)(CO)ClRu]2L} (where, E = P or As; B = PPh3 or AsPh3 or pyridine; L = mononucleating NS donor of N-substituted thiosemicarbazones) have been synthesized and characterized by elemental analysis, FT-IR, UV–vis and 31P{1 H} NMR cyclic voltammetric studies. The DNA-binding studies of Ru(II) complexes with calf thymus DNA (CT-DNA) were investigated by UV–vis, viscosity measurements, gel-electrophoresis and fluorescence spectroscopy. The new complexes have been used as catalysts in CAC coupling reaction and in the oxidation of alcohols to their corresponding carbonyl compounds by using NMO as co-oxidant and molecular oxygen (O2) atmosphere at ambient temperature. Further, the new binucleating thiosemicarbazone ligands and their Ru(II) complexes were also screened for their antibacterial activity against Klebsiella pneumoniae, Shigella sp., Micrococcus luteus, Escherichia coli and Salmonella typhi.

Journal of Advanced Research (2012) 3, 233–243 Cairo University Journal of Advanced Research ORIGINAL ARTICLE DNA-binding, catalytic oxidation, CAC coupling reactions and antibacterial activities of binuclear Ru(II) thiosemicarbazone complexes: Synthesis and spectral characterization Arumugam Manimaran a, Chinnasamy Jayabalakrishnan b,* a Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamanagalam-638 401, Erode District, Tamil Nadu, India b Post Graduate and Research Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore-20, Tamil Nadu, India Received January 2011; revised July 2011; accepted 18 July 2011 Available online 31 August 2011 KEYWORDS Binuclear ruthenium(II); Thiosemicarbazone; CT-DNA; Catalytic reactions; Antibacterial activity Abstract New hexa-coordinated binuclear Ru(II) thiosemicarbazone complexes of the type {[(B)(EPh3)(CO)ClRu]2L} (where, E = P or As; B = PPh3 or AsPh3 or pyridine; L = mononucleating NS donor of N-substituted thiosemicarbazones) have been synthesized and characterized by elemental analysis, FT-IR, UV–vis and 31P{1H} NMR cyclic voltammetric studies The DNA-binding studies of Ru(II) complexes with calf thymus DNA (CT-DNA) were investigated by UV–vis, viscosity measurements, gel-electrophoresis and fluorescence spectroscopy The new complexes have been used as catalysts in CAC coupling reaction and in the oxidation of alcohols to their corresponding carbonyl compounds by using NMO as co-oxidant and molecular oxygen (O2) atmosphere at ambient temperature Further, the new binucleating thiosemicarbazone ligands and their Ru(II) complexes were also screened for their antibacterial activity against Klebsiella pneumoniae, Shigella sp., Micrococcus luteus, Escherichia coli and Salmonella typhi From this study, it was * Corresponding author Tel.: +91 4222692461/+91 9442001793; fax: +91 4222693812 E-mail address: drcjbstar@hotmail.com (C Jayabalakrishnan) 2090-1232 ª 2011 Cairo University Production and hosting by Elsevier B.V All rights reserved Peer review under responsibility of Cairo University doi:10.1016/j.jare.2011.07.005 Production and hosting by Elsevier 234 A Manimaran and C Jayabalakrishnan found out that the activity of the complexes almost reaches the effectiveness of the conventional bacteriocide ª 2011 Cairo University Production and hosting by Elsevier B.V All rights reserved Introduction Thiosemicarbazones are an important class of N, S donor ligands which have considerable interest because of their chemistry and biological activities, such as antitumor, antibacterial, antiviral, antiamoebic and antimalarial activities [1,2] They have also been used for device applications in telecommunications, optical computing, optical storage and optical information processing [3] The chemistry of complexes of ruthenium with their thiosemicarbazones, which can coordinate with the metal either in neutral thione form or in the anionic thiolate form, has received attention in recent years primarily due to its varied coordination mode, novel electrochemical and electronic properties [4–6] In particular, the use of ruthenium complexes as chemo-therapeutic agents for the treatment of cancer is also well established [7] Binding to DNA is usually accompanied by marked absorbance changes in the UV–vis frequency range, fluorescence and gel-electrophoresis Among the various mechanisms through which they carry on their action, of utmost importance is their direct interaction with DNA The capability of DNA metallointercalators is determined by several factors, such as the planarity of ligand, atom type of ligand donor and the coordination geometry [8] Herein, in connection with our ongoing interest [9,10], we describe the synthesis and characterization of a series of new class of binuclear ruthenium(II) thiosemicarbazone complexes along with their CT-DNA binding studies, CAC coupling reaction and oxidation of alcohols Further, the new ligands and their ruthenium(II) thiosemicarbazone complexes were also screened for their antibacterial activity against Klebsiella pneumoniae, Shigella species, Micrococcus luteus, Escherichia coli, and Salmonella typhi Experimental Material and methods All the chemicals and solvents used were purified and dried by standard methods RuCl3Ỉ3H2O was purchased from Lobachemie Pvt Ltd Bombay, India and was used without further purification FT-IR spectra were recorded as KBr pellets with a Nicolet FT-IR spectrometer in 4000–400 cmÀ1 range Electronic spectra of the complexes were recorded in DMSO solutions using Systronics double beam UV–vis spectrophotometer-2202 in the range 800–200 nm Microanalyses were carried out with a Vario El AMX-400 elemental analyzer 31 P{1H} NMR spectra were monitored on a Bruker AMX400 NMR spectrophotometer using DMSd6 as solvent and tetramethylsilane (1H) and orthophosphoric acid (31P) as internal standards at Indian Institute of Science (IISc), Bangalore, India Cyclic voltammetric studies were carried out with a BAS CV-27 model electrochemical analyzer in acetonitrile using glassy carbon working electrode and the potentials were referenced to Ag–AgCl electrode Melting points were recorded with a Veego VMP-DS model heating table and were uncorrected The starting complexes [RuHCl(CO)(PPh3)3] [11], [RuHCl(CO)(AsPh3)3] [12] and [RuHCl(CO)(PPh3)2(py)] [13] were prepared by the reported literature methods Preparation of binucleating thiosemicarbazone ligands The binucleating thiosemicarbazone ligands have been prepared by adding ethanolic solution of thiosemicarbazide, (H2L1) (0.1820 g, mmol)/N(4)-methylthiosemicarbazide, (H2L2) (0.2101 g, mmol)/N(4)-phenylthiosemicarbazide, (H2L3) (0.3341 g, mmol)/N(4)-(2-chlorophenyl)thiosemicarbazide, (H2L4) (0.4020 g, mmol) with ethanolic solution of terephthalaldehyde (0.134 g, mmol) in 2:1 molar ratio The reaction mixture was then refluxed on a water bath for about h The condensation product was filtered, thoroughly washed with ethanol and ether, recrystallized with ethanol and dried under reduced pressure over anhydrous CaCl2 Purity of the synthesized thiosemicarbazone ligands was monitored by TLC using silica gel The analytical data, FT-IR, 1H NMR spectral data confirm the proposed molecular formula and the structure of the thiosemicarbazone ligands (Scheme 1) (Yield: 76–85%) Synthesis of binuclear ruthenium(II) thiosemicarbazone complexes To a solution of [RuHCl(CO)(PPh3)3] (0.1904 g, 0.2 mmol)/ [RuHCl(CO)(AsPh3)3] (0.2170 g, 0.2 mmol)/[RuHCl(CO)(PPh3)2(py)] (0.1540 g, 0.2 mmol) in equimolar ratio of DMSO/benzene (20 cm3), an appropriate thiosemicarbazone ligand, H2L1AH2L4 (0.0280–0.0500 g, 0.1 mmol) (Scheme 2) was added (2:1 ratio) and heated under reflux for 4–6 h and then it was concentrated to cm3 The new complex was separated by the addition of 10 cm3 of petroleum ether (60– 80 °C) The product was filtered, washed with petroleum ether and recrystallized from methanol/CH2Cl2 mixture and dried in vacuo The purity of the complexes was checked by TLC (Yield %80%) Catalytic oxidation reactions by binuclear Ru(II) thiosemicarbazone complexes Catalytic oxidation of primary and secondary alcohols to corresponding aldehydes and ketones, respectively, by ruthenium(II) thiosemicarbazone complexes was studied in the presence of NMO as co-oxidant and O2 atmosphere at N HN RHN S NHR NH N N N S RHN HS 'Keto' form NHR N N 'Enol' form Where, R=H (H2L1) or CH3 (H2L2) or C6H5 (H2L3) or o-Cl-C6H4 Scheme SH Structure of thiosemicarbazone ligands Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes N HN RHN S S [RuHCl(CO)(EPh3)2(B)] + NHR NH N DMSO/Benzene Reflux 4-6 h OC Cl Ph3E Ru B N N S RHN Antibacterial studies Where R= H / CH3 / phenyl / o-Cl-phenyl; E= P / As; B=PPh / AsPh / pyridine Scheme Formation of binuclear Ru(II) thiosemicarbazone complexes MN Ru 2+ + NMO R1 R1 + Ru 4+ O CH OH R2 R1 H CH O Ru 4+ O R2 Ru 4+ O R2 H + C OH R2 + NM Ru3+ O R1 R1 C O C OH Ru 3+ O + H+ R2 R2 H Ru 4+ O R1 H CH O + added with stirring and the mixture was heated under reflux The remaining PhBr in Et2O (5 cm3) was added dropwise and the mixture was refluxed for 40 To this mixture, 1.03 cm3 (0.01 mol) of PhBr in anhydrous Et2O (5 cm3) and the Ru(II) thiosemicarbazone complex (0.05 mmol) chosen for investigation were added and heated under reflux for h The reaction mixture was cooled and hydrolyzed with a saturated solution of aqueous NH4Cl and the ether extract on evaporation gave a crude product which was chromatographed to get pure biphenyl and compared well with an authentic sample [14,15] NHR N S N Ph3E Ru B Cl CO MN 235 H+ Ru2+ + H 2O (where, R1 = aryl / alkyl; R2 = alkyl / H) Scheme Proposed mechanism for the oxidation of alcohols using ruthenium/NMO ambient temperature separately A typical reaction using the ruthenium complexes as catalyst and benzyl alcohol, cinnamyl alcohol and cyclohexanol as substrates at 1:100 molar ratio is described as follows A solution of ruthenium complex (0.01 mmol) in 20 cm3 CH2Cl2 was added to the solution of substrate (1 mmol) and NMO (3 mmol) and O2 atmosphere at ambient temperature The solution mixture was stirred for h and the solvent was then evaporated from the mother liquor under reduced pressure The solid residue was then extracted with petroleum ether (60–80 °C) (20 cm3) and the ether extracts were evaporated to give corresponding aldehydes/ketone, which were then isolated and quantified as their 2,4-dinitrophenylhydrazone derivative [14] The binucleating thiosemicarbazone ligands and their ruthenium(II) complexes have been tested in vitro to asses their growth inhibitory activity against K pneumoniae, Shigella species, M luteus, E coli, and S typhi by Kirby Bauer method [16] The test organisms were grown on nutrient agar medium in petri plates The compounds to be tested were dissolved in DMSO and soaked in a filter paper disc of mm diameter and mm thickness The concentrations used in this study were 0.5%, 1.0%, 1.5%, 2.0% and 2.5% The discs were placed on the previously seeded plates and incubated at 37 °C for 24 h.Amoxycilin, ampicillin, erythromycin and streptomycin were used as standards with different concentrations A control test with the solvent was also carried out under identical conditions DNA-binding experiments Stock solutions of disodium salt of CT-DNA solutions were prepared by DNA in buffer, 50 mmol NaCl/5 mmol Tris– HCl in water DNA concentration was determined by UV absorbance at 260 nm after 1:100 dilutions Stock solutions were kept at °C and used within days Doubly distilled water was used to prepare the buffer All the titrations were done using DNA stock solutions pretreated with metal complex to take care of the dilution effects Solution of DNA gave the ratio of UV absorbance at 260 and 280 nm, A260/ A280 $ 1.9 indicating that the DNA was sufficiently free of protein [17] Absorption spectra were recorded on a Systronics double beam UV–vis spectrophotometer-2202 using cuvettes of cm path length For UV–vis spectral titrations, · 10À5 mol concentration of ruthenium solutions were used and calf thymus DNA (10.5 mmol) was added in steps till R = 40 (Retention Time) Intense MLCT bands were monitored to follow the interaction of the complex with CT-DNA The intrinsic binding constant Kb of the complex to CT-DNA was determined from Eq (1), through a plot of [DNA]/ea–ef vs [DNA], where [DNA] represents the concentration of DNA and ea, ef and eb, the apparent extinction coefficient (Aobs/[M]), the extinction coefficient for free metal [M] complex and the extinction coefficient for the free metal [M] complex in the fully bound form, respectively In plots of [DNA]/ea–ef vs [DNA], Kb is given by the ratio of slope to intercept [18] CAC coupling reaction by binuclear ruthenium(II) thiosemicarbazone complexes ½DNAŠ ½DNAŠ ẳ ỵ ea ef eb ef Kb eb À ef Þ Magnesium turnings (0.320 g) were placed in a flask equipped with a CaCl2 guard tube A crystal of iodine was added PhBr (0.75 cm3 of total 1.88 cm3) in anhydrous Et2O (5 cm3) was For viscosity measurements, the Ubbelohde viscometer was thermostated at 25 °C in a constant temperature bath The concentration of DNA was 160 lM and the flow-times were ð1Þ 486 486 484 490 402, 368, 403, 369, 369, 368, 372, 368, 370, 402, 368, 550 424, 424, 420, 425, 1558 1559 1575 1560 1558 1564 1560 1578 1582 1580 1559 1575 1602 1608 1612 1609 3.87 (3.83) 3.81 (3.82) 3.55 (3.50) 3.42 (3.39) 3.50 (3.53) 3.45 (3.49) 3.23 (3.20) 3.12 (3.06) 4.65 (4.66) 4.56 (4.51) 4.19 (4.17) 4.01 (3.97) 22.87 (22.83) 20.79 (20.80) 14.83 (14.81) 12.79 (12.76) determined with a digital timer (1/R = [Ru]/[DNA] = 0.5) Emission spectra were determined with Hitachi F-2500 fluorescence spectrophotometer at room temperature Fluorescence experiments were conducted by adding aliquots of 0–10 · 10À5 mol solutions of the complexes C9AC12 to samples containing · 10À5 mol ethidiumbromide and · 10À4 mol CT-DNA in Tris–HCl buffer The DNA cleavage activity of ruthenium(II) thiosemicarbazone complexes were monitored by agarose gel electrophoresis on CT-DNA The tests were performed under aerobic conditions with H2O2 as an oxidant Each reaction mixture (20 lL total volume) containing 240 ng of CT-DNA, 50 mmol phosphate buffer (pH 7.8), in the presence or absence of 120 lM of H2O2, and varying concentrations of ruthenium(II) complexes C9AC12 in the range 1–50 lM, were incubated at 37 °C for different periods of time After incubation, the solution was subjected to electrophoresis on a 1% agarose gel in 1· TAE buffer (40 mmol Tris–acetate, mmol EDTA) at 100 V, for h After electrophoresis the gel was stained with lg/mL ethidiumbromide (EB) for 30 prior to being photographed under UV light The elemental analysis (C, H, N, S) of all thiosemicarbazone ligands and their Ru(II) complexes are in good agreement with the calculated values thus confirming the proposed binuclear composition for all the complexes (Table 1) The complexes were obtained in powder form Various attempts have been made to obtain the single crystals of the complexes but it has been unsuccessful So for all the ligands and their complexes are stable at room temperature, non-hygroscopic and insoluble in water, methanol, ethanol and soluble in CH2Cl2, CHCl3, DMF, DMSO and CH3CN IR spectra C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 L1 L2 L3 L4 {[(PPh3)2(CO)ClRu]2L1} C84H70Cl2N6O2P4Ru2S2 {[(PPh3)2(CO)ClRu]2L2} C86H74Cl2N6O2P4Ru2S2 {[(PPh3)2(CO)ClRu]2L3} C96H78Cl2N6O2P4Ru2S2 {[(PPh3)2(CO)ClRu]2L4} C96H76Cl4N6O2P4Ru2S2 {[(AsPh3)2(CO)ClRu]2L1} C84H70As4Cl2N6O2Ru2S2 {[(AsPh3)2(CO)ClRu]2L2} C86H74As4Cl2N6O2Ru2S2 {[(AsPh3)2(CO)ClRu]2L3} C96H78As4Cl2N6O2Ru2S2 {[(AsPh3)2(CO)ClRu]2L4} C96H76As4Cl4N6O2Ru2S2 {[(py)(PPh3)(CO)ClRu]2L1} C58H50P2Cl2N8O2Ru2S2 {[(py)(PPh3)(CO)ClRu]2L2} C60H54P2Cl2N8O2Ru2S2 {[(py)(PPh3)(CO)ClRu]2L3} C70H58P2Cl2N8O2Ru2S2 {[(py)(PPh3)(CO)ClRu]2L4} C70H56P2Cl4N8O2Ru2S2 H2L1 C10H12N6S2 H2L2 C12H16N6S2 H2L3 C22H20N6S2 H2L4 C22H18Cl2N6S2 128 170 120 118 56 61 64 110 58 >250 70 >250 183 130 220 195 60.90 61.31 63.75 61.41 55.06 55.52 58.10 56.15 50.55 51.25 54.94 52.57 42.84 46.73 61.09 52.69 (4.25) (4.41) (4.32) (4.05) (3.90) (3.97) (4.00) (3.70) (3.70) (3.85) (3.86) (3.53) (4.30) (5.20) (4.70) (3.60) 5.07 (5.01) 4.99 (5.00) 4.65 (4.68) 4.48 (4.50) 4.59 (4.60) 4.52 (4.48) 4.23 (4.23) 4.09 (4.05) 8.13 (8.10) 7.97 (8.01) 7.32 (7.30) 7.01 (7.05) 29.97 (30.01) 27.25 (27.21) 19.43 (19.34) 16.76 (16.80) 4.26 4.43 4.35 4.08 3.85 4.01 3.96 3.73 3.66 3.87 3.82 3.53 4.31 5.23 4.66 3.62 Results and discussion (61.00) (61.30) (63.72) (61.39) (55.01) (55.48) (58.06) (56.12) (50.51) (51.23) (55.00) (52.53) (42.86) (46.69) (61.06) (52.65) 1535 1532 1540 1538 1532 1539 1535 1536 1540 1538 1539 1533 – – – – 750 745 746 740 752 750 753 758 754 748 744 747 848 856 810 832 1926 1927 1933 1932 1927 1928 1930 1942 1924 1926 1928 1927 – – – – 254, 254, 255, 254, 254, 255, 254, 254, 254, 254, 254, 250, 232, 235, 230, 235, 369, 302, 369, 303, 304, 300, 294, 297, 297, 297, 298, 360, 280, 287, 282, 288, mC‚N mN‚N‚C mC„S mC„O kmax (nm) (cmÀ1) (cmÀ1) (cmÀ1) (cmÀ1) S Melting Elemental analysis found (Calcd.)% point C H N (°C) S No Ligand/complexes Table Analytical, FT-IR and UV–vis spectral data of binuclear ruthenium(II) thiosemicarbazone complexes FT-IR spectral data UV–vis 429, 403, 434, 403, 403, 403, 403, 402, 391, 430, 403, 537 433, 540, 434, 434, 433, 609 530 528 461, 433, 606 464, 618 A Manimaran and C Jayabalakrishnan 576 628 464, 639 468, 609 465, 615 236 The FT-IR spectra of the free thiosemicarbazone ligands were compared with those of the new complexes in order to confirm the coordination of ligand to the ruthenium metal (Table 1) The bands present around 3100 cmÀ1 in the spectrum of the ligand (H2L1) are assigned to masym and msym vibration of the terminal NH2 group This band is also present in the spectra of the complexes C1, C5 and C9 indicating the non-involvement of this group in coordination The absorption due to C‚N group of the free ligand present around 1602–1612 cmÀ1 region undergoes a negative shift by 20–53 cmÀ1 (1558–1582 cmÀ1) in the spectra of the complexes indicating the coordination of azomethine nitrogen to the metal The m(NAN) bands of the ligands are present around 1058 cmÀ1 The increase in frequency of this band 1060–1080 cmÀ1 in the spectra of the complexes provides evidence for the co-ordination via the azomethine nitrogen [19] A strong band in the 2988–3020 cmÀ1 region attributed to m(NAH) group of ANHAN‚CA found in the spectra of free ligands (H2L2, H2L3 and H2L4) is not present in the spectra of the complexes The band present around 810–856 cmÀ1 in the spectra of thiosemicarbazone ligands is assigned to AHNAC‚S group This band is also not present in the spectra of the complexes But new bands are present at 1532–1540 cmÀ1 and 740–758 cmÀ1 which are assigned to the new azomethine group AC‚NANA and ACAS, respectively These Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes Table S No L1 L2 L3 L4 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 237 31 P{1H} NMR spectra of binucleating thiosemicarbazone ligands and their ruthenium(II) complexes Ligand/complex H2L H2L2 H2L3 H2L4 {[(PPh3)2(CO)ClRu]2L1} {[(PPh3)2(CO)ClRu]2L2} {[(PPh3)2(CO)ClRu]2L3} {[(PPh3)2(CO)ClRu]2L4} {[(AsPh3)2(CO)ClRu]2L1} {[(AsPh3)2(CO)ClRu]2L2} {[(AsPh3)2(CO)ClRu]2L3} {[(AsPh3)2(CO)ClRu]2L4} {[(py)(PPh3)(CO)ClRu]2L1} {[(py)(PPh3)(CO)ClRu]2L2} {[(py)(PPh3)(CO)ClRu]2L3} {[(py)(PPh3)(CO)ClRu]2L4} H NMR (d ppm) dACH‚NA daromatic dASH dANH/NHA dACH3 9.1 8.2 9.8 10.4 10.5 9.6 10.0 9.5 10.4 9.2 8.8 9.8 10.0 10.0 9.9 9.4 6.2–6.8 7.8–8.1 6.4–7.0 6.6–7.3 5.6–7.0 6.6–7.4 6.8–8.2 6.8–7.4 5.8–7.2 6.7–7.6 6.2–7.2 6.2–7.0 6.4–8.2 6.4–7.2 6.2–7.0 6.4–7.2 11.9 11.5 10.4 12.4 – – – – – – – – – – – – 4.6 3.3 4.4 3.8 3.8 5.0 3.4 3.4 3.8 4.0 4.4 4.4 2.5 3.4 4.4 3.4 – 2.4 – – – 3.2 – – – 2.8 – – – 1.8 – – 31 P NMR (dppm) – – – – 27.4, 27.0, 27.5, 26.9, – – – – 57.3 57.3 57.7 58.2 44.3 38.3 38.3 38.7 observations indicate the enolization of the ANHAC‚S group and subsequent deprotonation before coordination to the metal In all the complexes, the band due to free m(C„O) group is present at 1924–1942 cmÀ1 The absorption due to coordinated pyridine is seen in the complexes, C9AC12 around 1000 cmÀ1 The characteristic bands due to PPh3/AsPh3 are also present around 1417–1456 cmÀ1, 1091–1093 cmÀ1, 736–796 cmÀ1 and 516–518 cmÀ1 in all the complexes The replacement of the hydride ion in the starting complexes by the ligands has been confirmed by the absence of a band around 2020 cmÀ1 in all the complexes [20] UV–vis spectra Electronic spectra of ruthenium(II) thiosemicarbazone complexes in DMSO showed absorption in the range of 250– 639 nm (Table 1) which can be assigned, metal to ligand charge transfer followed by intra-ligand transitions, respectively The ground state of Ru(II) in an octahedral environment is 1A1g, arising from the t2g6 configuration The excited state terms are 3T1g, 3T2g, 1T1g and 1T2g Hence, four bands corresponding to the transitions 1A1g fi 3T1g, 1A1g fi 3T2g, 1A1g fi 1T1g and 1A1g fi 1T2g are possible in the order of increasing energy The electronic spectral bands around 528–639 nm are assigned to 1A1g fi 1T2g The other high intensity band in the visible region around 402–468 nm has been assigned to charge transfer transitions arising from the metal t2g level to the unfilled P* molecular orbital of the ligand The high intensity bands around 300–372 and 250–298 nm has been designated as n fi P* and fi P fi P* transitions, respectively The pattern of the electronic spectra for all the complexes indicates the presence of an octahedral environment around ruthenium(II) similar to that of other ruthenium octahedral complexes [21] H NMR spectra of thiosemicarbazone ligands and their Ru(II) complexes The ligand to metal bonding is further supported by 1H NMR spectra The nuclear magnetic resonance spectrum of the thiosemicarbazone ligands (Table and Fig 1) and ruthenium(II) diamagnetic compounds (Table and Fig 1) taken in Fig 1H NMR spectra of binucleating H2L1 ligand and {[(PPh3)2(CO)ClRu]2L1} complex DMSd6 solution confirm the complex formation In the spectra of all the complexes a sharp singlet appeared at 8.8– 10.5 ppm has been assigned to azomethine proton (AHC‚N) The positions of azomethine signal in the complexes are up field/down field compared to that of the free ligands observed at 8.2–10.4 ppm, indicating coordination through the azomethine nitrogen atom Enolization of thiocarbonyl group is indicated by the singlet present at 10.4–12.0 ppm in the spectra of the ligands, which are attributed to ACASH protons of thioamide group The absence of thionyl group in the complexes indicates deprotonation of this group of the thiosemicarbazone ligands on complexation and coordination to ruthenium through thionyl sulfur The terminal ANH2 protons in the complexes C1, C5 and C9 and ANH protons in the complexes C2AC4, C6AC8 and 238 Fig A Manimaran and C Jayabalakrishnan 31 P NMR spectra of binuclear C1 and C9 complexes C10AC12 are seen in the positions with slight deviation as in the ligands spectrum around 2.5–3.8 and 3.4–5.0 ppm confirming the non-involvement of this group in coordination with the metal Multiplets are observed around 5.6–8.2 ppm in all the complexes and have been assigned to the aromatic protons of triphenylphosphine, triphenylarsine, pyridine and phenyl of the thiosemicarbazone ligands Methyl protons appear as singlets in the region of 1.8–3.2 ppm in the complexes C2, C6 and C10 Furthermore, in all the complexes, largest Dd are observed for the protons that are located close to the coordinating atoms So, the deshielding effect of the metal is apparent to such protons [22] 31 P NMR spectra of Ru(II) complexes The 31P NMR spectra were recorded for C1AC4 and C9AC12 to confirm the presence of triphenylphophine group and heterocyclic nitrogen base in the binuclear ruthenium(II) thiosemicarbazone complexes (Table and Fig 2) The signal appeared at 26.9–27.5 and 38.3–44.3 ppm in the spectrum of C1AC4 attributed to the two phophine ligands are cis to each other in these complexes However, complexes C9AC12 exhibited only one signal at 57.3–58.2 ppm consistent with the presence of one triphenylphosphine group This observation indicates the retention of coordinated pyridine in the complexes even after the coordination of bidentate thiosemicarbazones The 31P NMR spectral studies clearly indicate a more labile RuAP bond compared to RuAN bond in these complexes, which is a reflection of better r-donating ability of the nitrogen bases compared to that of phosphorus in PPh3 [23] Catalytic oxidation reactions The catalytic activity of the newly synthesized binuclear ruthenium(II) thiosemicarbazone complexes was examined in the presence of N-methylmorpholine-N-oxide (NMO) and molecular oxygen (Table 3) as co-oxidants for the oxidation of primary and secondary alcohols in CH2Cl2 Results of the present investigation suggest that the complexes are able to react more efficiently with NMO than with molecular oxygen as indicated by the low product yield when molecular oxygen is employed as the co-oxidant This is in accordance with a previous observation [24] Formation of a high valent ruthenium-oxo-species, RuIV‚O, is proposed as the intermediate in ruthenium(II) complexes catalyzed oxidation with NMO Ruthenium compounds are known to be good hydride ion abstracting agents N-methylmorpholine and water are the by products during the course of the reaction (Scheme 3) This was further supported by spectral changes that occur by addition of NMO to a dichloromethane solution of the ruthenium(II) complexes The appearance of a peak at 390 nm is attributed to the formation of RuIV‚O species which is in conformity with other oxo ruthenium complexes [25–27] The proposed mechanism in the presence of molecular oxygen includes the oxidation of alcohol by a Run+ complex to form aldehyde and Ru(nÀ2)+, followed by the oxidation of Ru(nÀ2)+ to Run+ with O2 [28] However, one cannot rule out a concerted mechanism, for example one including the oxidation of alcohol by O2 in the coordination sphere of Run+ Metal complexes can act as an oxygen atom transfer reagent resulting in the formation of metal-oxo species [29] An important characteristic of ruthenium NMO or O2 system results in the selective oxidation at the alcoholic group of ACH2 benzyl alcohol to benzaldehyde while the ACH2 group remains unaffected Saturated aliphatic alcohols such as cyclohexanol, Propan-1-ol and 2-methylpropyl alcohol are converted into the corresponding aldehydes/ketones with high conversion All the synthesized ruthenium complexes were found to catalyze the oxidation of alcohols to aldehydes/ketones, but the yields and the turnover were found to vary with different catalysts The relatively higher product yield obtained for the oxidation of benzyl alcohol than for cyclohexanol, Propan-1-ol and 2-methylpropyl alcohol was due to the fact that the aCH moiety of benzyl alcohol is more acidic than that of cyclohexanol [28], Propan-1-ol and 2-methyl propyl alcohol It has also been found that triphenylphosphine complexes possess higher catalytic activity than the triphenylarsine complexes [29] and pyridine substituted complexes This may be due to the higher donor ability of the arsine and pyridine ligand compared with that of phosphine It has also been found that ruthenium(II) complexes, C2, C6 and C10 derived from N(4)-methyl thiosemicarbazone ligands showed less catalytic activity when compared with other complexes, which could be attributed to the presence of the electron-releasing methyl group in these complexes, which decreases the catalytic activity by increasing the electron density on the metal ion [30] The catalytic activity study reveals that the ligand system influences the catalytic behavior of the complexes As expected, the ligands containing electron donating group increases the catalytic activity of the complexes CAC coupling reactions The CAC coupling reactions by the synthesized binuclear ruthenium(II) thiosemicarbzone complexes {[(B)(EPh3)Cl(CO)Ru]2L} (where, L = binucleating thiosemicarbazone ligands; E = P/As; B = PPh3/pyridine) were carried out and the results of this study are listed in Table The system chosen for our study is the coupling of phenylmagnesium bromide with bromobenzene that was first converted into the corresponding Grignard reagent [31] Then bromobenzene, followed by the complex chosen for the investigation, was added to the above reagent and the mixture was heated under reflux for h After work up, the mixture yielded biphenyl Only a very little amount of biphenyl is formed when the Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes Table 239 Catalytic activity data of new binuclear Ru(II) thiosemicarbazones complexes Complexes Catalytic oxidation of alcohols Substrate C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 a Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol Benzyl alcohol 2-methyl propyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methylpropyl alcohol Benzylalcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Benzyl alcohol Cyclohexanol Propan-1-ol 2-methyl propyl alcohol Product Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde Benzaldehyde 2-Methyl-propionaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde Benzaldehyde Cyclohexanone Propionaldehyde 2-Methyl-propionaldehyde CAC coupling reaction In presence of NMO In presence of O2 atm Yield (%) Turnover numbera Yield (%) Turnover numbera 80 93 60 65 75 85 59 63 86 97 68 72 89 98 73 77 73 79 58 58 63 74 55 73 59 79 61 64 78 80 67 69 71 74 55 56 64 74 67 49 70 74 57 58 72 77 63 67 88 96 79 84 77 89 78 82 90 101 87 93 90 103 92 96 77 82 78 77 66 78 75 75 79 82 80 83 80 83 84 88 74 75 74 76 67 72 65 78 72 76 77 78 76 81 80 88 75 85 56 61 66 74 50 56 71 81 59 65 78 83 64 68 668 53 55 57 60 45 62 51 70 53 55 64 70 54 64 62 66 47 50 55 59 40 45 59 60 41 45 61 66 50 54 78 89 72 80 70 76 68 72 73 84 77 83 80 86 82 88 67 70 71 75 58 64 62 65 70 72 70 73 67 73 70 79 64 98 64 69 57 72 55 62 62 63 57 62 64 69 68 72 Yield (mg) Yield (%) 0.420 44 0.429 45 0.285 45 0.269 28 0.368 38 0.312 32 0.225 23 0.220 23 0.305 32 0.226 24 0.250 26 0.268 28 Moles of product per mole of catalyst reaction is carried out without the catalyst This is an insignificant amount compared to the yields of biphenyl that have been obtained from the reactions catalyzed by binuclear Ru(II) complexes The catalytic properties of the new binuclear complexes have also been compared with those already reported binuclear complexes [32] It has been observed that the binuclear ruthenium(II) thiosemicarbazone complexes are moderate active catalysts 3.7 Antibacterial activities The variation in the effectiveness of the different compounds against different organisms depends on their impermeability of the microbial cells or on the difference in the ribosome of the microbial cells In general, the complexes (Table 4) are more active than that of parent ligands and ruthenium(II) starting complexes The increase in the antibacterial activity 240 Table Ligand/ complex Antimicrobial activities of new thiosemicarbazone ligands and their binuclear Ru(II) complexes Inhibition zone concentration in mm Shigella sp K Pneumoniae E coli S typhi 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 0.5% 1.0% 1.5% 2.0% 2.5% 9 10 11 10 10 12 11 12 11 11 10 12 11 12 D – 9 10 10 11 12 13 12 12 12 12 11 10 13 12 12 E 12 10 10 10 12 12 13 14 13 11 11 10 11 13 12 12 Ax 11 10 11 12 10 12 12 12 10 12 11 12 12 12 13 Ak 23 10 10 11 12 11 12 12 11 11 11 11 12 13 13 13 S 17 8 10 11 10 12 10 10 10 11 12 14 14 14 D – 8 8 10 12 12 12 10 11 10 14 12 14 14 15 E 16 9 10 12 12 10 10 11 14 14 13 13 15 15 Ax 10 9 11 12 14 11 12 11 14 14 14 15 15 16 Ak 21 10 12 13 14 11 14 11 14 15 14 15 15 16 S 10 7 11 10 11 12 12 10 10 11 11 11 D – Where, D, DMSO, E, erythromycin, Ax, amoxycilin, Ak, ampicillin; and S, streptomycin 8 11 10 11 12 12 9 10 12 12 11 Ak 15 8 10 12 10 11 12 13 11 10 11 12 13 12 Ax – 8 10 12 13 12 11 13 11 10 12 12 14 14 S 9 11 12 13 12 11 11 12 12 12 10 14 14 E 16 7 10 13 12 10 10 11 12 12 11 12 11 12 D – 7 8 10 13 12 10 12 11 12 12 11 12 12 12 E – 8 10 12 14 10 12 11 12 14 13 14 14 14 Ax – 8 11 14 14 12 14 12 13 14 13 14 14 14 Ak – 9 11 14 14 12 14 10 13 14 12 12 15 10 S – 9 11 12 12 10 11 11 11 12 12 12 12 12 Ak 16 10 10 10 11 12 12 13 12 12 12 12 14 12 12 12 Ax – 10 10 10 10 13 14 13 12 12 10 12 14 11 14 13 E 10 11 11 10 12 14 14 12 14 14 10 10 15 13 14 13 S – 11 11 10 12 14 12 12 14 14 10 11 12 13 14 14 D A Manimaran and C Jayabalakrishnan H2L1 H2L2 H2L3 H2L4 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 Standards M Luteus Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes 241 1.20 a b c d e η/η0, Relative specific viscosity 1.18 1.16 1.14 1.12 1.10 1.08 1.06 1.04 1.02 1.00 Fig UV–vis spectra of C11-DNA complexes 10 20 30 40 50 -3 x 10 1/R = {[Ru-complex]/[DNA]} 1.8 Fig On the effect of C9 (a), C10 (b), C11 (c), C12 (d) and CTDNA (e) on the viscosity of CT-DNA at 1/R = 0.5 relative viscosity vs 1/R (Eo-Ef)/(Eb-Ef) 1.6 1.4 1.2 0.8 0.6 0.4 0.2 0 0.001 0.002 0.003 0.004 0.005 0.006 [DNA], M Fig Plot of (eo–ef)/(eb–ef) vs [DNA] for C11 complex of the metal chelates with increase in concentration is due to the effect of metal ion on normal cell process Such increase in the activity of the metal chelates can be explained on the basis of Overtone’s concept [33] and Chelation theory [34,35] 3.8 DNA binding studies The binuclear ruthenium(II) thiosemicarbazone complexes, C1AC4 exhibit intense MLCT as well as ligand based P–P* transitions with very high molar absorptivities (Fig 3) When titrated with a solution of calf thymus DNA (R = [DNA]/[Ru complex] = 1–40), all of them show hypochromism and redshifts in the MLCT band [36] The complexes C1AC8 show small or no redshift This is supported by lower values of R at which maximum hypochromism was observed for C9AC12 (R = 8–15 for C1AC4; R = 25 for C5AC8; R = 30 for C9AC12 The complexes C9 and C10 with only thiosemicarbazide and N(4)-methylthiosemicarbazide ligands show initially hypochromism (R = 5), then hyperchromism (R = 15) and again hypochromism (R = 40) with a red-shift during titration with CT-DNA revealing the presence of two or more DNA binding regimes As the titration progressed, a response curve was plotted from the experimental data (Fig 4) as the measured emission as a function of DNA concentration, allowing the progress of the experiment to be monitored It is assumed that when no further significant increase in fluorescence is detected, i.e Fig Fluorescence spectrum C11-DNA complex the response curve plateaus, that the metal complex has reached saturation Saturation is defined as the point where all binding sites on the DNA are occupied Corrected emission is defined as the observed emission minus emission of the metal complex alone The DNA binding constants obtained reveal several interesting trends in DNA binding The complexes C9 and C10 have been shown to exhibit a DNA binding affinity higher than its N(4)-phenylthiosemicarbazone complex, C11 and N(4)-o-chlorophenylthiosemicarbazone complex, C12, which is well known to exhibit partial intercalation with DNA This is interesting as phenyl substitution on N(4) is expected to hinder the partial insertion of phenyl ring in between the DNA base pairs So it is obvious that hydrophobic interaction of the phenyl groups with DNA contributes to the stabilization of the bound complex This is supported by the observed decrease in values of K in the order, H2L4 > H2L3 > H2L2 > H2L1 of Ru(II) complexes It is clear that 242 A Manimaran and C Jayabalakrishnan References Fig Agarose gel-electrophoresis photograph of C11-DNA complex the C11 and C12 complexes fit most closely against the DNA helical structure with van der Waals interactions between complex and DNA being maximum These ligands favor hydrophobic interaction with DNA rather than with water molecules leading to the release of water molecules from DNA on binding, enhancement in entropy and stabilization of the DNA-bound complex All the complexes possess a binding site size of 4–6 base pairs, which is consistent with earlier reports [37] When CT DNA (50 lM) was titrated with ruthenium(II) complexes (1/R = 0–0.5 = [Ru]/[DNA]), the relative viscosity of the CT DNA increases for most of the complexes (Fig 5) The increase in relative viscosity of C11 and C12 is higher than that of C9 and C10, which is due to the aggregation of C9 and C10 on the helical DNA as nano template via hydrophobic interaction [38] The mixed ligand complexes C11 and C12 also show an increase in relative viscosity of CT DNA but less than C9 and C10, obviously due to partially intercalating phenyl ring in them The fluorescence titration spectra have been confirmed to be effective for characterizing the binding mode of the metal complexes to DNA [39] The enhancements in the emission intensity of the ligand and its ruthenium(II) complexes with increasing CT-DNA concentrations are shown in Fig The ligand and its ruthenium(II) complexes could emit luminescence in Tris–HCl buffer with a maximum appearing at 510 nm This phenomenon was related to the extent to which the compounds penetrate into the hydrophobic environment inside the DNA, thereby avoiding the quenching effect of solvent water molecules The marked increase in emission intensity of these three compounds were also in accordance with those observed in the fluorescence titration spectra studies of other intercalators [40] Consequently, it might be concluded that the 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(H2L2) or C6H5 (H2L3) or o-Cl-C6H4 Scheme SH Structure of thiosemicarbazone ligands Synthesis and spectral characterization of binuclear Ru(II) thiosemicarbazone complexes N HN RHN S S [RuHCl(CO)(EPh3)2(B)]... expected, the ligands containing electron donating group increases the catalytic activity of the complexes CAC coupling reactions The CAC coupling reactions by the synthesized binuclear ruthenium(II)