Reduced Schiff Base Ligands Research Articles

Syntheses, crystal structures, and characterization of four homochiral coordination polymers based on two chiral reduced Schiff base ligands

Four homochiral coordination polymers incorporating two chiral reduced Schiff base ligands, namely, [Cu(L1)(H2O)]·H2O (1), [Zn2(L2)2] (2), [Co(L2)(H2O)] (3), and [Ni(L2)(H2O)] (4) (H2L1 = N-(4-carboxyl)benzyl-l-alanine, H2L2 = N-(4-carboxyl)benzyl-l-leucine) have been obtained by hydrothermal methods and characterized by physico-chemical and spectroscopic methods. X-ray crystallographic analysis reveals that complex 1 exhibits a chain structure with 1D channels. Complexes 2–4 all are 3D network structures with 1D channels in which the isobutyl group of the ligand points toward to the channel. Complex 2 displays strong photoluminescent emission in the purple region.

Zinc(II) and cadmium(II) coordination polymers with a tricarboxylate containing reduced Schiff base ligand: Structural characterization and luminescent property

Three new coordination polymers, [Cd(HL)(phen)]·2H2O (1), [Zn2(L)(HCOO)(bpy)2]·H2O (2) and [Zn2(L)(μ3-OH)(bpy)]·1.5H2O (3) [H3L=5-(4-carboxybenzylamino)isophthalic acid, phen=1,10-phenanthroline and bpy=2,2′-bipyridine], have been prepared and characterized by single crystal and powder X-ray diffraction, IR, elemental and thermogravimetric analyses. Complex 1 displays a 1D neutral double-stranded chain structure. Complex 2 shows the uninodal 3-connected 2D (4.82) fes network with in-situ generated [Zn2(COO)2(bpy)2] as the secondary building unit (SBU). Complex 3 exhibits the 2-nodal (3,6)-connected 2D (43)2(46.66.83) kgd net architecture containing in-situ generated [Zn4(COO)4(μ3-OH)2(bpy)2] (SBU). Significantly, concrete hexanuclear clusters (Zn4O2) exist in 3. Taking hydrogen bonding interactions into account, extended 3D supramolecular frameworks of 1–3 can be built. The influences of the coordination modes of the ligand, coordination geometries of the metal centers, steric hindrance as well as reaction temperature on the structures of the resultant complexes are discussed. Furthermore, the luminescent properties of 1–3 were preliminarily investigated.

Amino Alcohol-Derived Reduced Schiff Base VIVO and VV Compounds as Catalysts for Asymmetric Sulfoxidation of Thioanisole with Hydrogen Peroxide

We report the synthesis and characterization of several amino alcohol-derived reduced Schiff base ligands (AORSB) and the corresponding V(IV)O and V(V) complexes. Some of the related Schiff base variants (amino alcohol derived Schiff base = AOSB) were also prepared and characterized. With some exceptions, all compounds are formulated as dinuclear compounds {V(IV)O(L)}(2) in the solid state. Suitable crystals for X-ray diffraction were obtained for two of the AORSB compounds, as well as a rare X-ray structure of a chiral V(IV)O compound, which revealed a dinuclear {V(IV)O(AOSB)}(2) structure with a rather short V-V distance of 3.053(9) Å. Electron paramagnetic resonance (EPR), (51)V NMR, and density functional theory (DFT) studies were carried out to identify the intervenient species prior to and during catalytic reactions. The quantum-chemical DFT calculations were important to determine the more stable isomers in solution, to explain the EPR data, and to assign the (51)V NMR chemical shifts. The V(AORSB) and V(AOSB) complexes were tested as catalysts in the oxidation of thioanisole, with H(2)O(2) as the oxidant in organic solvents. In general, high conversions of sulfoxide were obtained. The V(AOSB) systems exhibited greater activity and enantioselectivity than their V(AORSB) counterparts. Computational and spectroscopic studies were carried out to assist in the understanding of the mechanistic aspects and the reasons behind such marked differences in activity and enantioselectivity. The quantum-chemical calculations are consistent with experimental data in the assessment of the differences in catalytic activity between V(AOSB) and V(AORSB) peroxido variants because the V(AORSB) peroxido transition states correspond to ca. 22 kJ/mol higher energy activation barriers than their V(AOSB) counterparts.

Insertion of a Hydroxido Bridge into a Diphenoxido Dinuclear Copper(II) Complex: Drastic Change of the Magnetic Property from Strong Antiferromagnetic to Ferromagnetic and Enhancement in the Catecholase Activity

A diphenoxido-bridged dinuclear copper(II) complex, [Cu(2)L(2)(ClO(4))(2)] (1), has been synthesized using a tridentate reduced Schiff base ligand, 2-[[2-(diethylamino)ethylamino]methyl]phenol (HL). The addition of triethylamine to the methanolic solution of this complex produced a novel triple bridged (double phenoxido and single hydroxido) dinuclear copper(II) complex, [Cu(2)L(2)(OH)]ClO(4) (2). Both complexes 1 and 2 were characterized by X-ray structural analyses, variable-temperature magnetic susceptibility measurements, and spectroscopic methods. In 1, the two phenoxido bridges are equatorial-equatorial and the species shows strong antiferromagnetic coupling with J = -615.6(6.1) cm(-1). The inclusion of the equatorial-equatorial hydroxido bridge in 2 changes the Cu···Cu distance from 3.018 Å (avg.) to 2.798 Å (avg.), the positions of the phenoxido bridges to axial-equatorial, and the magnetic coupling to ferromagnetic with J = 50.1(1.4) cm(-1). Using 3,5-di-tert-butylcatechol as the substrate, the catecholase activity of the complexes has been studied in a methanol solution; compound 2 shows higher catecholase activity (k(cat) = 233.4 h(-1)) than compound 1 (k(cat) = 93.6 h(-1)). Both complexes generate identical species in solution, and they are interconvertible simply by changing the pH of their solutions. The higher catecholase activity of 2 seems to be due to the presence of the OH group, which increases the pH of its solution.

Synthesis, Crystal Structures, Magnetic Properties and Catecholase Activity of Double Phenoxido-Bridged Penta-Coordinated Dinuclear Nickel(II) Complexes Derived from Reduced Schiff-Base Ligands: Mechanistic Inference of Catecholase Activity

Three double phenoxido-bridged dinuclear nickel(II) complexes, namely [Ni(2)(L(1))(2)(NCS)(2)] (1), [Ni(2)(L(2))(2)(NCS)(2)] (2), and [Ni(2)(L(3))(2)(NCS)(2)] (3) have been synthesized using the reduced tridentate Schiff-base ligands 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL(1)), 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL(2)), and 2-[1-(3-dimethylamino-propylamino)-ethyl]-phenol (HL(3)), respectively. The coordination compounds have been characterized by X-ray structural analyses, magnetic-susceptibility measurements, and various spectroscopic methods. In all complexes, the nickel(II) ions are penta-coordinated in a square-pyramidal environment, which is severely distorted in the case of 1 (Addison parameter τ = 0.47) and 3 (τ = 0.29), while it is almost perfect for 2 (τ = 0.03). This arrangement leads to relatively strong antiferromagnetic interactions between the Ni(II) (S = 1) metal centers as mediated by double phenoxido bridges (with J values of -23.32 (1), -35.45 (2), and -34.02 (3) cm(3) K mol(-1), in the convention H = -2JS(1)S(2)). The catalytic activity of these Ni compounds has been investigated for the aerial oxidation of 3,5-di-tert-butylcatechol. Kinetic data analysis following Michaelis-Menten treatment reveals that the catecholase activity of the complexes is influenced by the flexibility of the ligand and also by the geometry around the metal ion. Electrospray ionization mass spectroscopy (ESI-MS) studies (in the positive mode) have been performed for all the coordination compounds in the presence of 3,5-DTBC to characterize potential complex-substrate intermediates. The mass-spectrometry data, corroborated by electron paramagnetic resonance (EPR) measurements, suggest that the metal centers are involved in the catecholase activity exhibited by the complexes.

Synthesis, crystal structure, DNA-binding properties, cytotoxic and antioxidation activities of several ternary copper(II) complexes with a new reduced Schiff base ligand

Three new ternary copper(II) complexes, [Cu2(phen)2(PDIMAla)(H2O)2](ClO4)2·CH3OH (1), [Cu2(dpq)2(PDIMAla)(H2O)2](ClO4)2·CH3OH (2) and [Cu2(dppq)2(PDIMAla)(H2O)2](ClO4)2·CH3OH (3) (phen=1,10-phenanthroline, dpq=dipyrido[3,2:2′,3′-f]quinoxaline, dppz=dipyrido[3,2-a:2′,3′-c]phenazine, H2PDIMAla=N,N′-(p-xylylene)di-alanine acid) have been synthesized and the complex 1 has been structurally characterized by single-crystal X-ray crystallography, spectrometric titrations, ethidium bromide displacement experiments, CD (circular dichroism) spectral analysis and viscosity measurements. The results indicate that the three compounds, especially the complex 3, can strongly bind to calf-thymus DNA (CT–DNA). The intrinsic binding constants Kb of the ternary copper(II) complexes with CT–DNA are 0.38×105, 1.4×105 and 7.8×105 for 1, 2 and 3, respectively. Comparative cytotoxic activities of the copper(II) complexes are also determined by acid phosphatase assay. The results show that the ternary copper(II) complexes have significant cytotoxic activity against the human hepatic (HepG2), human promyelocytic leukemia (HL60) and human prostate (PC3) cell lines. Investigation of antioxidation property show that all the copper(II) complexes have strong scavenging effects for hydroxyl radicals.

Synthesis, crystal structure and nonlinear optical property of a Zinc(II) complex base on the reduced Schiff-base ligand

Abstract By hydrothermal reactions of Zn(II) nitrate with N-(4-carboxyl)benzyl- l -alanine acid (H2L), a novel Zn(II) complex, namely, [Zn(HL)2(H2O)2] (1), have been obtained. Single crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the C2 space group and displays a 0D dimensional structure. The Zn(II) ions are four-coordinated and each Zn(II) ion coordinated with two HL anions forming a [Zn(HL)2(H2O)2] unit. The stacking of such units by hydrogen bonds results in its supramolecular structure. Complex 1 shows a second harmonic generation response that is ~ 1.5 times that of KDP (KH2PO4).

Syntheses, crystal structures and characterizations of six coordination polymers from reduced Schiff base ligands

Six coordination polymers from reduced Schiff base ligands, namely, [Zn(L1)2]·2H2O (1), [Ni(L1)2]·2H2O (2), [Ni(L1)2(H2O)2] (3), [Zn(L2)2]·H2O (4), [Ni(L2)2]·0.5H2O (5), and [Co(L2)2(HL2)] (6) (HL1=4-((4-pyridinylmethyl)amino)benzoic acid, HL2=3-((4-pyridinylmethyl)amino)benzoic acid) have been obtained by hydrothermal method and studied by single crystal X-ray diffraction, elemental analyses, powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. Compounds 1 and 2 are isostructural while compounds 4 and 5 also are isostructural. All these four compounds exhibit a 2D layer structure. Compound 2 is isomeric to 3 but has a chain structure. Compound 6 also exhibits a chain structure. The photoluminescence properties of compounds 1 and 4 were studied.

Cis–trans isomerism in diphenoxido bridged dicopper complexes: role of crystallized water to stabilize the cis isomer, variation in magnetic properties and conversion of both into a trinuclear species

The trans-[Cu(2)L(2)Cl(2)] (1), and cis-[Cu(2)L(2)Cl(2)]·H(2)O (2) isomers of a diphenoxido bridged Cu(2)O(2) core have been synthesized using a tridentate reduced Schiff base ligand 2-[(2-dimethylamino-ethylamino)-methyl]-phenol. The geometry around Cu(II) is intermediate between square pyramid and trigonal bipyramid (Addison parameter, τ = 0.463) in 1 but nearly square pyramidal (τ = 0.049) in 2. The chloride ions are coordinated to Cu(II) and are trans oriented in 1 but cis oriented in 2. Both isomers have been optimized using density functional theory (DFT) calculations and it is found that the trans isomer is 7.2 kcal mol(-1) more favorable than the cis isomer. However, the hydrogen bonding interaction of crystallized water molecule with chloride ions compensates for the energy difference and stabilizes the cis isomer. Both complexes have been converted to a very rare phenoxido-azido bridged trinuclear species, [Cu(3)L(2)(μ(1,1)-N(3))(2)(H(2)O)(2)(ClO(4))(2)] (3) which has also been characterized structurally. All the complexes are antiferromagnetically coupled but the magnitude of the coupling constants are significantly different (J = -156.60, -652.31, and -31.54 cm(-1) for 1, 2, 3 and respectively). Density functional theory (DFT) calculations have also been performed to gain further insight into the qualitative theoretical interpretation on the overall magnetic behavior of the complexes.

Synthesis, crystal structures and magnetic properties of two bis(μ-phenoxido)dicopper(II) complexes derived from reduced Schiff base ligands

Two phenoxido bridged dinuclear Cu(II) complexes, [Cu 2(L 1) 2(NCNCN) 2] ( 1) and [Cu 2(L 2) 2(NCNCN) 2]·2H 2O ( 2) have been synthesized using the tridentate reduced Schiff-base ligands 2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol (HL 1) and 2-[1-(3-methylamino-propylamino)-ethyl]-phenol (HL 2), respectively. The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. Both the complexes present a diphenoxido bridging Cu 2O 2 core. The geometries around metal atoms are intermediate between trigonal bipyramid and square pyramid with the Addison parameters ( τ) = 0.57 and 0.49 for 1 and 2, respectively. Within the core the Cu–O–Cu angles are 99.15° and 103.51° and average Cu–O bond distances are 2.036 and 1.978 Å for compounds 1 and 2, respectively. These differences have marked effect on the magnetic properties of two compounds. Although both are antiferromagnetically coupled, the coupling constants ( J = −184.3 and −478.4 cm −1 for 1 and 2, respectively) differ appreciably.

Syntheses and characterizations of zinc(II) and copper(II) complexes with reduced Schiff base derived from salicylaldehyde and D,L-selenomethionine

New zinc(II) and copper(II) complexes with a reduced Schiff-base ligand derived from D,L-selenomethionine and salicylaldehyde have been synthesized and characterized by elemental analysis, magnetic susceptibility, IR, and NMR measurements. The single-crystal X-ray structure of the Cu(II) complex reveals that this complex is a carboxylate-bridged dimer of dinuclear copper(II) subunits and all metal centers are five-coordinate with O4N donor sets in distorted square-pyramidal geometries. The Cu(II) complex consists of a 1-D coordination polymer.

Synthesis, characterization and structures of copper(II) complexes containing carboxylate and sulfonate groups

Four copper(II) complexes containing Schiff base and reduced Schiff base ligands derived from pyridine-2-aldehyde and amino acid containing carboxylate and sulfonate functional groups (N-(2-pyridylmethylene)-amino acid and N-(2-pyridylmethyl)-amino acid, (amino acids = β-alanine and aminoethanesulfonic acid) namely, [Cu(Pbals)(H2O)2]ClO4·H2O 1, [Cu(Pbal)(ClO4)(H2O)] 2, [Cu2(Paes)2(ClO4)2]·2H2O 3, and [Cu(Pae)(H2O)]·ClO4·H2O 4 have been synthesized and characterized. The structural features of carboxylate and sulfonate donor groups have been elucidated. These copper(II) complexes demonstrate different coordination behaviour of the carboxylate and sulfonate groups. Carboxylate groups in complexes 1 and 2 bridge the metal centers and facilitate the formation of 1D helical coordination polymeric structures. In compound 3, the sulfonate groups bridge the metal centers to form a discrete dinuclear complex. In 4, the sulfonate groups link the neighbouring metal centers to form a 1D coordination polymeric structure.

Synthesis and structural characterization of a novel seven-coordinate cobalt(II) complex: 2,9-Bis(ethanolamine)-1,10-phenanthrolinechlorocobalt(II) chloride

Condensation reaction of 2,9-dicarboxaldehyde-1,10-phenanthroline with 2-aminoethanol followed by NaBH 4 reduction yielded the polydentate Schiff base ligand 2,9-bis(ethanolamine)-1,10-phenanthroline in its reduced form. This ligand was characterized by elemental analysis, LC–MS, IR, UV–Vis and NMR spectroscopy. Reaction of the reduced Schiff base ligand with aqueous solution of cobalt(II) chloride affords 2,9-bis(ethanolamine)-1,10-phenanthrolinechlorocobalt(II) chloride in high yield. Single crystals of the cobalt(II) complex were obtained from the crystallization in ethanol and its structure was elucidated by X-ray structural analysis. The cobalt(II) complex ion was found to be seven-coordinated in a pentagonal bipyramidal geometry, whereby cobalt(II) ion is surrounded by the six donor atoms in the ligand molecule and a chloride ion.

Synthesis, crystal structure, and urease inhibition studies of copper(II) and cobalt(III) complexes with bi(2-fluorobenzylaminoethyl)amine

Three new mononuclear Cu(II) and Co(III) complexes [Cu(L)Cl]ClO4 (1), [Cu(L)Cl(SCN)] (2), and [Co(L)(N3)3] (3), where L is a reduced Schiff-base ligand bi(2-fluorobenzylaminoethyl)amine, were synthesized and characterized. X-ray crystallographical analysis reveals that the Cu(II) atom adopts a square-planar environment in complex 1, while the geometry in 2 can be described as distorted square-pyramidal. The Co(III) atom in 3 is in a distorted octahedral geometry. Three complexes were investigated for their inhibitory activities in vitro against jack bean urease. The Cu(II) complexes 1 and 2 were found to have excellent inhibitory activities. The Co(III) complex 3 was also shown to have activity comparable to that of acetohydroxamic acid.

Two unusual mixed-valent trinuclear CuII2CuI complexes containing copper(i) tribromide dianion as bridging ligand: Identification of an unprecedented doubly hydrogen-bonded water dimer

Two very rare mixed-valent CuII2CuI trinuclear complexes, [Cu2(Br–L1)2CuBr3]·2H2O (1) and [Cu2(L2)2CuBr3]·2CH3OH (2) have been synthesized using CuBr2 and the tridentate reduced Schiff-base ligands HL1 (2-[(2-dimethylamino-ethylamino)-methyl]-phenol) and HL2 (2-[1-(2-dimethylamino-ethylamino)-ethyl]-phenol), respectively. The complexes have been characterized by X-ray structural analyses. In both complexes, the deprotonated tridentate reduced Schiff base forms a di-μ-phenoxo bridge between the two CuII centers and the CuBr32− anion acts as an additional bridge between them to result in the trinuclear structures. During complex formation, ligand HL1 has been brominated at the five position of the aromatic ring but HL2 did not undergo any such reaction. In complex 1 an unprecedented doubly hydrogen-bonded water dimer, which is considered as an important transition state for the interchange of hydrogen atoms within the water dimer, has been identified. The dimensions of this water dimer agree quite well to those obtained from theoretical calculations.

Use of a Reduced Schiff-Base Ligand to Prepare Novel Chloro-Bridged Chains of Rare Cu(II) Trinuclear Complexes with Mixed Azido/Oxo and Chloro/Oxo Bridges

Two mixed bridged one-dimensional (1D) polynuclear complexes, [Cu(3)L(2)(mu(1,1)-N(3))(2)(mu-Cl)Cl](n) (1) and {[Cu(3)L(2)(mu-Cl)(3)Cl].0.46CH(3)OH}(n) (2), have been synthesized using the tridentate reduced Schiff-base ligand HL (2-[(2-dimethylamino-ethylamino)-methyl]-phenol). The complexes have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. In both complexes the basic trinuclear angular units are joined together by weak chloro bridges to form a 1D chain. The trinuclear structure of 1 is composed of two terminal square planar [Cu(L)(mu(1,1)-N(3))] units connected by a central Cu(II) atom through bridging nitrogen atoms of end-on azido ligands and the phenoxo oxygen atom of the tridentate ligand. These four coordinating atoms along with a chloride ion form a distorted trigonal bipyramidal geometry around the central Cu(II). The structure of 2 is similar; the only difference being a Cl bridge replacing the mu(1,1)-N(3) bridge in the trinuclear unit. The magnetic properties of both trinuclear complexes can be very well reproduced with a simple linear symmetrical trimer model (H = -JS(i)S(i+1)) with only one intracluster exchange coupling (J) including a weak intertrimer interaction (j) reproduced with the molecular field approximation. This model provides very satisfactory fits for both complexes in the whole temperature range with the following parameters: g = 2.136(3), J = -93.9(3) cm(-1) and zj = -0.90(3) cm(-1) (z = 2) for 1 and g = 2.073(7), J = -44.9(4) cm(-1) and zJ' = -1.26(6) cm(-1) (z = 2) for 2.

ChemInform Abstract: Diphenoxo‐Bridged Copper(II) Complexes of Reduced Schiff Base Ligands as Functional Models for Catechol Oxidase

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Nickel(II) and copper(II) complexes of unsymmetrical tetradentate reduced Schiff base ligands

Two new reduced Schiff base ligands, [HL 1 = 4-{2-[(pyridin-2-ylmethyl)-amino]-ethylimino}-pentan-2-one and HL 2 = 4-[2-(1-pyridin-2-yl-ethylamino)-ethylimino]-pentan-2-one] have been prepared by reduction of the corresponding tetradentate unsymmetrical Schiff bases derived from 1:1: 1 condensation of 1,2-ethanediamine, acetylacetone and pyridine-2-carboxaldehyde/2-acetyl pyridine. Four complexes, [Ni(L 1)]ClO 4 ( 1), [Cu(L 1)]ClO 4 ( 2), [Ni(L 2)]ClO 4 ( 3), and [Cu(L 2)]ClO 4 ( 4) with these two reduced Schiff base ligands have been synthesized and structurally characterized by X-ray crystallography. The mono-negative ligands L 1 and L 2 are chelated in all four complexes through the four donor atoms to form square planar nickel(II) and copper(II) complexes. Structures of 3 and 4 reveal that enantiomeric pairs are crystallized together with opposite chirality in the nitrogen and carbon atoms. The two Cu II complexes ( 2 and 4) exhibit both irreversible reductive (Cu II/Cu I; E pc, −1.00 and −1.04 V) and oxidative (Cu II/Cu III; E pa, +1.22 and +1.17 V, respectively) responses in cyclic voltammetry. The electrochemically generated Cu I species for both the complexes are unstable and undergo disproportionation.

Di-μ-perchlorato-bis{μ-2-[(2-pyridyl)methylaminomethyl]phenolato)dicopper(II) acetonitrile disolvate

In the crystal of the dinuclear title compound, [Cu2(C13H13N2O)2(ClO4)2]·2CH3CN, the two bridging perchlorate ions chelate to the two CuII atoms in a μ-O:O′ fashion on opposite sides of the equatorial plane. The CuII ions display a distorted octa­hedral coordination geometry (in the usual 4 + 2 Jahn–Teller arrangement), each being coordinated by two O atoms from the two perchlorate ligands, and two N and O atoms from the reduced Schiff base ligand. The asymmetric unit contains two acetonitrile solvent mol­ecules. In the crystal structure, in addition to N—H⋯O hydrogen bonds, there are weak C—H⋯O inter­actions between the perchlorate O atoms and the reduced Schiff base ligand. C—H⋯N inter­actions are also present.

DNA binding and cleavage activity of reduced amino-acid Schiff base complexes of cobalt(II), copper(II), and cadmium(II)

Three new reduced amino-acid Schiff base complexes, [Co(HL)2(H2O)2] · 4H2O (1), [Cu(HL)2(H2O)2] · 2H2O (2), and [Cd(HL)2(H2O)3] · 2H2O (3), where H2L is the reduced Schiff-base ligand derived from the condensation of N-(4-hydroxybenzaldehyde) with L-glycine, have been synthesized and characterized by physico-chemical and spectroscopic methods. In these complexes, the two bidentate monoanionic Schiff base ligands coordinate the metal center through the secondary amine N atom and the carboxylate O atom. Water ligands complete a distorted octahedral (1, 2) or a pentagonal bipyramidal coordination geometry (3) around each metal center. The binding interactions of the complexes with CT-DNA have been investigated by UV–visible spectrophotometry and fluorescence quenching methods. The results show that these complexes bind to CT-DNA with an intercalative mode. In addition, DNA cleavage experiments have been also investigated by agarose gel electrophoresis. Complexes 1–3 show oxidative DNA cleavage activity in the presence of H2O2/sodium ascorbate and the reactive oxygen species responsible for the DNA cleavage is most likely singlet oxygen.