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Recently, mixed-ligand copper(II) complexes have received much attention in searching for alternative metallodrug for cisplatin. A series of mixed ligand Cu(II) complexes of the type [Cu(L)(diimine)](ClO4) 1-6, where HL is...

A series of non-heme μ-oxo-bridged dinuclear iron(III) complexes of the type [Fe2(μ-O)(L1–L6)2Cl2]Cl2 1–6 have been isolated and their catalytic activity towards oxidative transformation of alkanes into alcohols has been studied using m-choloroperbenzoic acid (m-CPBA) as an oxidant. All the complexes were characterized by CHN, electrochemical, and UV-visible spectroscopic techniques. The molecular structures of 2 and 5 have been determined successfully by single crystal X-ray diffraction analysis and both possesses octahedral coordination geometry and each iron atom is coordinated by four nitrogen atoms of the 4N ligand and a bridging oxygen. The sixth position of each octahedron is coordinated by a chloride ion. The (μ-oxo)diiron(III) core is linear in 2 (Fe–O–Fe, 180.0°), whereas it is non-linear (Fe–O–Fe, 161°) in 5. All the diiron(III) complexes show quasi-reversible one electron transfer in the cyclic voltammagram and catalyze the hydroxylation of alkanes like cyclohexane, adamantane with m-CPBA as an oxidant. In acetonitrile solution, adding excess m-CPBA to the diiron(III) complex 2 without chloride ions leads to intramolecular hydroxylation reaction of the oxidant. Interestingly, 2 catalyzes alkane hydroxylation in the presence of chloride ions, but intramolecular hydroxylation in the absence of chloride ions. The observed selectivity for cyclohexane (A/K, 5–7) and adamantane (3°/2°, 9–18) suggests the involvement of high-valent iron–oxo species rather than freely diffusing radicals in the catalytic reaction. Moreover, 4 oxidizes (A/K, 7) cyclohexane very efficiently up to 513 TON while 5 oxidizes adamantane with good selectivity (3°/2°, 18) using m-CPBA as an oxidant. The electronic effects of ligand donors dictate the efficiency and selectivity of catalytic hydroxylation of alkanes.

Octahedral copper(ii) complexes of the type [Cu(trien)(diimine)](ClO4)2 (1-4), where trien is triethylenetetramine and diimine is 2,2'-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3), and 3,4,7,8-tetramethyl-1,10-phenanthroline (4), have been isolated. Single crystal X-ray structures of 1 and 2 reveal that the coordination geometry around Cu(ii) is tetragonally distorted octahedral. The stereochemical fluxionality of the complexes illustrates the observed trend in CuII/CuI redox potentials and DNA binding affinity (Kb: 1, 0.030 ± 0.002 < 2, 0.66 ± 0.01 < 3, 1.63 ± 0.10 < 4, 2.27± 0.20 × 105 M-1), determined using absorption spectral titration. All complexes effect oxidative DNA cleavage more efficiently than hydrolytic DNA cleavage. The bpy complex 1 with stereochemical fluxionality lower than its phen analogue 2 shows a higher cytotoxicity against both A549 lung (IC50, 3.3 μM) and MCF-7 human breast (IC50, 3.9 μM) cancer cells, and induces the generation of the highest amount of ROS in A549 cells. Complex 3 with a higher stereochemical fluxionality and higher ligand hydrophobicity exhibits a higher DNA binding and cleavage ability and higher cytotoxicity (IC50, 2.1 μM) towards MCF-7 cells. Complex 4 with a still higher stereochemical fluxionality displays the highest DNA binding and cleavage ability but a lower cytotoxicity towards both A549 and MCF-7 cell lines due to its tendency to form a five-coordinated complex with the uncoordinated amine group. Annexin V.Cy3 staining and immunoblot analysis demonstrate the mechanism of cell death caused by 1 and 2. The finding of the up-regulation of the pro-apoptotic Bax protein and down-regulation of PARP protein in western blot analysis confirms the induction of apoptosis by these complexes.

Copper amine oxidase (CAO), consisting of the topoquinone (TPQ) cofactor, catalyzes the oxidation of primary amines to aldehyde. We have successfully addressed this issue through isolation of a cop...

A series of mononuclear copper(II) complexes of the type [Cu(L)Cl2] 1–6, where L is a tridentate 3 N ligand such as N-methyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L1), N-ethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L2), N-phenyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L3), N,N-dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L4), N,N-diethyl-N′-(pyrid-2-ylmethyl)-ethylenediamine (L5) and N-methyl-N′-(pyridin-2-ylmethyl)benzene-1,2-diamine (L6), has been isolated and studied as functional models for catechol oxidase enzymes in different aqueous micellar media. The X-ray crystal structures of 2 and 5 contain a CuN3Cl2 chromophore with a trigonal bipyramidal based coordination geometry. Interestingly, all the complexes form cationic [Cu(L)(DBC)(H2O)]+ adduct species in which DBC2− acts as a monodentate ligand, and the adduct species interacts strongly with anionic SDS micelles. The rates of the oxygenase reaction follow the trend SDS > TX-100 ∼ water > CTAB, revealing that the rate of the reaction in micellar media are higher than those in aqueous solution and that the nature of biomimetic microenvironments of the micellized copper(II) catalyst and also the ligand stereoelectronic factors like donor atom basicity and steric bulk of ligand N-alkyl substituents, determine the catecholase-like activity.

Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2 (1 and 2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(II). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni–OH intermediate [Ni(L1/L2)(OH)]+ (1a and 2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO2 to produce the carbonate bridged dinickel(II) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2 (3 and 4), as characterized by elemental analysis and the IR spectral feature (∼1608 cm−1) characteristic of bridging carbonate. The single crystal X-ray structure of 3 reveals the presence of a dinickel(II) core bridged by a carbonate anion in a symmetric mode. Both the Ni(II) centers are identical to each other with each Ni(II) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by 1 (kobs, 7.7 ± 0.1 × 10−5 s−1) and 2 (kobs, 5.8 ± 0.3 × 10−4 s−1) in basic methanol solution with atmospheric CO2 has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO2. The carbonate complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO2 with excellent selectivity.

A series of non-heme μ-oxo-bridged dinuclear iron(iii) complexes of the type [Fe

Octahedral copper(ii) complexes of the type [Cu(trien)(diimine)](ClO4)2 (1-4), where trien is triethylenetetramine and diimine is 2,2'-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3), and 3,4,7,8-tetramethyl-1,10-phenanthroline (4), have been isolated. Single crystal X-ray structures of 1 and 2 reveal that the coordination geometry around Cu(ii) is tetragonally distorted octahedral. The stereochemical fluxionality of the complexes illustrates the observed trend in CuII/CuI redox potentials and DNA binding affinity (Kb: 1, 0.030 ± 0.0022, 0.66 ± 0.013, 1.63 ± 0.104, 2.27± 0.20 × 105 M-1), determined using absorption spectral titration. All complexes effect oxidative DNA cleavage more efficiently than hydrolytic DNA cleavage. The bpy complex 1 with stereochemical fluxionality lower than its phen analogue 2 shows a higher cytotoxicity against both A549 lung (IC50, 3.3 μM) and MCF-7 human breast (IC50, 3.9 μM) cancer cells, and induces the generation of the highest amount of ROS in A549 cells. Complex 3 with a higher stereochemical fluxionality and higher ligand hydrophobicity exhibits a higher DNA binding and cleavage ability and higher cytotoxicity (IC50, 2.1 μM) towards MCF-7 cells. Complex 4 with a still higher stereochemical fluxionality displays the highest DNA binding and cleavage ability but a lower cytotoxicity towards both A549 and MCF-7 cell lines due to its tendency to form a five-coordinated complex with the uncoordinated amine group. Annexin V.Cy3 staining and immunoblot analysis demonstrate the mechanism of cell death caused by 1 and 2. The finding of the up-regulation of the pro-apoptotic Bax protein and down-regulation of PARP protein in western blot analysis confirms the induction of apoptosis by these complexes.

A series of organometallic Ru(II)-arene complexes of the type [(η6-p-cymene)Ru(L)Cl](BF4) 1–6, where L is 3-phenyl-1-pyridin-2-yl-imidazo[1,5-a]pyridine (L1), dimethyl-[4-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)phenyl]-amine (L2), diphenyl-[4-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)phenyl]amine (L3), 9-[4-(1-pyridin-2-yl-imidazo-[1,5-a]pyridin-3-yl)-phenyl]-9H-carbazole (L4), 9-ethyl-3-(1-pyridin-2-yl-imidazo-[1,5-a]pyridin-3-yl)-9H-carbazole (L5), and 10-ethyl-3-(1-pyridin-2-yl-imidazo[1,5-a]pyridin-3-yl)-10H-phenothiazine (L6), has been isolated and characterised by elemental analysis, ESI-MS, NMR and cyclic voltammetry. The photophysical properties of the complexes have been studied by electronic absorption and emission spectral techniques. All the ligands exhibit tuneable photoluminescence behaviour with the emission maximum spanning through the visible region (475–670 nm) in dichloromethane while all the complexes are emissive in acetonitrile. The single crystal X-ray structures of 2, 3 and 4 reveal that the complexes have a “piano stool” coordination geometry, comprising one π-bonded arene centroid, two σ-bonded nitrogen atoms from the chelating ligand and one Cl− ion. From DNA induced EthBr emission quenching experiments the apparent DNA binding constants of the complexes (Kapp) have been evaluated, which follows the order, 2 (1.3) 100 μM), exhibit in vitro cytotoxicity against A549 small lung cancer cell lines higher than cisplatin (∼69 μM), as revealed by both MTT (11.8–18.1 μM) and crystal violet staining (12.7–23.5 μM) assays, which is in agreement with their DNA and BSA binding affinity. Also, the complexes 3–6 cause higher cell death mainly through the apoptotic mode, as revealed by the observation of a higher percentage of apoptotic cells in AO/EB (36–43%) and Annexin V-Cy3 (36–45%) stained cancer cells.

A series of non-heme (μ-oxo)bis(μ-dicarboxylato)-bridged diiron(iii) complexes, [Fe2(O)(OOCH)2(L)2](2+)1, [Fe2(O)(OAc)2(L)2](2+)2, [Fe2(O)(Me3AcO)2(L)2](2+)3, [Fe2(O)(OBz)2(L)2](2+)4, [Fe2(O)(Ph2AcO)2(L)2](2+)5 and [Fe2(O)(Ph3AcO)3(L)2](2+)6, where L = N,N-dimethyl-N'-(pyrid-2-ylmethyl)ethylenediamine, OAc(-) = acetate, Me3AcO(-) = trimethylacetate, OBz(-) = benzoate, Ph2AcO(-) = diphenylacetate and Ph3AcO(-) = triphenylacetate, have been isolated and characterized using elemental analysis and spectral and electrochemical techniques. They have been studied as catalysts for the selective oxidation of alkanes using m-chloroperbenzoic acid (m-CPBA) as the oxidant. Complexes 2, 3, and 4 possess a distorted bioctahedral geometry in which each iron atom is coordinated to an oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-carboxylate bridge and three nitrogen atoms of the 3N ligand. In an acetonitrile/dichloromethane solvent mixture all the complexes display a d-d band characteristic of the triply bridged diiron(iii) core, revealing that they retain their identity in solution. Upon replacing electron-donating substituents on the bridging carboxylates by electron-withdrawing ones the E1/2 value of the one-electron Fe(III)Fe(III)→ Fe(III)Fe(II) reduction becomes less negative. On adding one equivalent of Et3N to a mixture of one equivalent of the complex and an excess of m-CPBA in the acetonitrile/dichloromethane solvent mixture an intense absorption band (λmax, 680-720 nm) appears, which corresponds to the formation of a mixture of complex species. All the complexes act as efficient catalysts for the hydroxylation of cyclohexane with 380-500 total turnover numbers and good alcohol selectivity (A/K, 6.0-10.1). Adamantane is selectively oxidized to 1-adamantanol and 2-adamantanol (3°/2°, 12.9-17.1) along with a small amount of 2-adamantanone (total TON, 381-476), and interestingly, the sterically demanding trimethylacetate bridge around the diiron(iii) centre leads to high 3°/2° bond selectivity; on the other hand, the sterically demanding triphenylacetate bridge gives a lower 3°/2° bond selectivity. A remarkable linear correlation between the pKa of the bridging carboxylate and TON for both cyclohexane and adamantane oxidation is observed, illustrating the highest catalytic activity for 3 with strongly electron-releasing trimethylacetate bridges.

Aspects of inorganic medicinal chemistry aim at developing metal coordinated complexes that are efficient therapeutic agents for cancer. However, intrinsic toxicity of these complexes impedes their effective clinical use. In order to reduce the toxicity and improve the pharmacokinetics and biodistribution, we developed a nanovector candidate and encapsulated in it a metal complex (mononuclear copper(II) complex [Cu(L)(dppz)]+ (LH = 2-[(2-dimethylaminoethylimino)methyl]phenol, coordinated ligand, dppz = dipyrido[3,2-a:2′,3′-c]phenazine) which is an efficient targeted delivery platform based on folate-receptor. The in-vitro cytotoxicity of (FA@MSNPs-[Cu(L)(dppz)]+) to the breast cancer cells MCF-7 and MDA-MB-231, and normal breast epithelial cell HBL-100 have been evaluated. The complex (FA@MSNPs-[Cu(L)(dppz)]+), compared to free [Cu(L)(dppz)]+, is highly cytotoxic to the breast cancer cells, in view of its efficient cellular uptake, and the delivery is mediated by FA receptor which produced only negligible cytotoxicity to the normal breast epithelial cell, HBL-100. Also, the mechanism of breast cancer cell death brought about by [Cu(L)(dppz)]+ loaded mesoporous silica nanoparticles (MSNPs) is primarily apoptosis whereas they are not toxic to normal breast epithelial cells. Hence, folate-conjugated MSNPs encapsulated with [Cu(L)(dppz)]+ has potential to be developed as a target-specific drug for breast cancer. This metal complex-loaded platform opens up newer opportunities to accommodate multiple metal-based drugs for cell-specific cancer therapy.

The new complexes [Cu(nal)2]∙2H2O 1 and [Cu(en)2(nal)2] 2, where H(nal) is the antibacterial drug nalidixic acid, have been isolated and their X-ray structures successfully determined. The complex 1 possesses a square planar CuO4 chromophore constituted by the pyridone and carboxylate oxygen atoms of nalidixate anion. Complex 2 contains a CuN4O2 chromophore with a tetragonally distorted octahedral geometry, and the trans-axial coordination of two nalidixate anions at longer distances to the CuN4 plane constituted by ethylenediamine is stabilized by a network of H-bonding. DFT studies illustrate the structures and stabilities of complexes. The DNA binding affinity of 2 (Kb, 9.2 (±0.2) × 104 M−1) is higher than that of [Cu(en)2]2+ [Kb, 2.0 (±0.2) × 104 M−1], illustrating that bound nal¯ enhances the DNA binding affinity. DNA docking studies reveal that 2 bound to DNA undergoes interesting coordinative distortions more than 1, causing more significant changes in DNA host. Thus, 2 displays oxidative DNA cleavage (ascorbic acid) more prominent than [Cu(en)2]2+, by generating OH radicals, and shows poor cytotoxicity towards A549 lung cancer cell lines.

Targeting selective hydroxylation under mild conditions of saturated hydrocarbons has become a challenging and exciting scientific area that has drawn larger attention in recent years because high temperatures and high pressures are required for industrial hydroxylation processes. In contrast, enzymes such as cytochrome P450, methane monooxygenases, and bleomycin catalyze the selective hydroxylation reactions using molecular oxygen as green oxidant under mild conditions. Inspired by these natural enzymes, noteworthy attempts have been made to replicate their functional features and several bio-mimetic and bio-inspired metal complexes have been investigated as catalysts for the hydroxylation of alkanes. Although iron complexes are considered as one of the most proficient transition metal catalysts for this industrially important reaction, in recent years nickel complexes have attracted greater attention as catalysts for alkane hydroxylation, epoxidation and benzene oxidation reactions. The objective of this review is to enumerate the versatile uses of nickel(II) complexes as catalysts for various oxidation reactions in the last few years. The review will summarize the important role of ligand stereoelectronic factors in dictating the selectivity and efficiency of the nickel(II) catalyzed C H and C C bond oxidation reactions.

The interaction of simple and ternary Cu(II) complexes of 1,10-phenanthrolines with DNA has been studied extensively because of their various interesting and important functions such as DNA cleavage activity, cytotoxicity towards cancer cells, and DNA based asymmetric catalysis. Such functions are closely related to the DNA binding modes of the complexes such as intercalation, groove binding, and electrostatic surface binding. A variety of spectroscopic methods have been used to study the DNA binding mode of the Cu(II) complexes. Of all these methods, DNA-fiber electron paramagnetic resonance (EPR) spectroscopy affords unique information on the DNA binding structures of the complexes. In this review we summarize the results of our DNA-fiber EPR studies on the DNA binding structure of the complexes and discuss them together with the data accumulated by using other measurements.

A series of Ni(II) complexes of the types [Ni(L)(CH3CN)2](BPh4)21–3, 5 and [Ni(L4)](BPh4)24, where L = N,N′-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N,N′-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L3), N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethylenediamine (L5) and L4 = N,N′-bis((1-methyl-1H-imidazole-2-yl)methyl)-1,4-diazepane, have been isolated and characterized. The complex cations of 1 and 4 possess, respectively, distorted octahedral and low-spin square planar coordination geometries in which nickel(II) is meridionally coordinated to all four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with the ethylenediamine backbone coordinates in the cis-β mode in [Ni(L5)(CH3CN)2]2+5, but in the cis-α mode in [Ni(L5)(H2O)2]2+. Also, they illustrate the role of ligand donor atom type, diazacyclo backbone and steric hindrance to coordination of pyridyl nitrogen in conferring novel coordination geometries on Ni(II). All these complexes catalyse the oxidation of cyclohexane in the presence of m-CPBA as the oxidant up to 600 turnover numbers (TON) with relatively good alcohol selectivity (A/K, 5.6–7.2). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with high bond selectivity (3°/2°, 8.7–11.7). The incorporation of methyl substituent(s) on one (2) or both (3) of the pyridyl rings and the replacement of both the pyridylmethyl arms in 1 by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with the cis-β mode of coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and product/bond selectivity.

A new family of nickel(II) complexes of the type [Ni(L)(CH(3)CN)](BPh(4))(2), where L=N-methyl-N,N',N'-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N',N'-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L(3))(CH(3)CN)](BPh(4))(2) reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni(II) /Ni(III) redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH(3)CN)](BPh(4))(2) by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH(3)CN)](BPh(4))(2) or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH(3)CN)](BPh(4))(2) and the quinolylmethyl arms ([Ni(L4)(CH(3)CN)](BPh(4))(2), both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni(II)-O(.)] intermediate (ts1(hs), ts2(doublet)), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni(II) (L5)(OOCOC(6)H(4)Cl)](+) (5 a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni(II) leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.

A series of Mn(II) complexes of the types [MnII(L)](ClO4)2 1–4 and [MnII(L)Cl2] 1a–4a, where L = N,N′-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N-(1-methyl-1H-imidazol-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L3) and N-(quinol-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L4), has been isolated and characterized both in the solid state and in solution. In the X-ray crystal structure of 4, Mn(II) adopts a distorted octahedral coordination geometry. The peroxo adducts [MnIII(O2)(L)]+ have been generated by adding H2O2 (30%) and triethylamine to 1–4 in CH3CN solution at −30 °C and have been characterized by UV-Vis and mass spectral and electrochemical techniques. The role of the nucleophilic nature of the peroxo adducts in the aldehyde deformylation reaction has been studied by using cyclohexanecarboxaldehyde (CCA) and para-substituted benzaldehydes as substrates. The rate constant (k2) for the deformylation reaction of CCA follows the trend 3b > 1b > 2b > 4b, which illustrates the dependence of reaction rate upon the Lewis basicity of the ligand donor atoms, as reflected by the MnII/MnIII redox potential of the complexes. Also, a linear correlation between the rate of deformylation catalysed by 3b and the substituent constant (σ) has been observed for the deformylation of various para-substituted benzaldehydes. The complexes catalyse the epoxidation of cis-cyclooctene, cyclohexene and styrene with H2O2 as the oxygen source in the presence of acetic acid with high conversion and selectivity, and the product yields are dependent upon the ligand stereoelectronic factors.

A series of non-heme (μ-oxo)bis(μ-benzoato)-bridged diiron(III) complexes of the type [Fe 2 (O)(OBz) 2 (L) 2 ] 2+ ( 1 – 6 ), where OBz = benzoate, L = bis(pyridin-2-ylmethyl)amine (L1), N -((6-methylpyridin-2-yl-)methyl)(pyridin-2-yl)methanamine (L2), N,N -dimethyl- N ′-(pyrid-2-ylmethyl)ethylenediamine (L3), (1-methyl-1 H -imidazol-2-yl)- N- (pyridin-2-ylmethyl)-methanamine (L4), (1 H -benzo[o]imidazol-2-yl)- N- (pyridin-2-ylmethyl)methanamine (L5) and bis((1 H -benzo[o]imidazol-2-yl)methyl)amine (L6), have been isolated and characterized by means of elemental analysis, spectral and electrochemical methods. They have been studied as catalysts for selective hydroxylation of alkanes using m -choloroperbenzoic acid ( m -CPBA) as the oxidant. In acetonitrile/dichloromethane mixed solvent all the complexes display a d–d band characteristic of a triply bridged diiron(III) core, revealing that they retain their identity in solution. Upon replacing a donor atom on the capping ligand by a stronger donor, the E 1/2 value of the one-electron Fe III Fe III → Fe III Fe II reduction becomes more negative. All the complexes function as efficient catalysts for hydroxylation of cyclohexane, with 390–410 total turnover numbers and good alcohol selectivity (A/K, 9.3–12.8). Adamantane is selectively oxidized (3°/2°, 15.7–28.1) to 1-adamantanol and 2-adamantanol, along with a small amount of 2-adamantanone (Total TON, 336–437), and interestingly the 3N capping ligands with the pyridyl donor around the diiron(III) center lead to high 3°/2° bond selectivity.

A new family of nickel(II) complexes of the type [Ni(PA)(L)(CH3CN)n]BPh4 1–5, where n = 1, 2, H(PA) is 2-picolinic acid and L is N,N′-tetramethylethylenediamine (L1) 1, N,N′,N″-pentamethyldiethylenetriamine (L2) 2, 2,2′-bipyridine (L3) 3, 1,10-phenanthroline (L4) 4 or 2,9-dimethyl-1,10-phenanthroline (L5) 5, has been isolated and characterized using CHN analysis, UV–Vis spectroscopy and ESI-MS. The complex [Ni(PA)(L2)(CH3CN)](BPh4) 2 possesses a distorted octahedral coordination geometry in which Ni(II) is chelated to 2-picolinate anion and L2. DFT calculations show that trans isomers of 3–5 are more stable than cis isomers by ca. 4.0 kJ/mol. In contrast, cis-1 is more stable than trans-1 by 15.8 kJ/mol. The complexes catalyze the hydroxylation of cyclohexane efficiently in presence of m-CPBA as oxidant with 244–569 turnover numbers and good alcohol selectivity (A/K, 3.4–7.0). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with varying bond selectivity (3°/2°, 9.3–14.2) while cumene is selectively oxidized to 2-phenyl-2-propanol. Upon replacing bidentate L1 by tridentate L2 or strongly π-back bonding phen the catalytic activity increases. In contrast, phen is replaced by non-planar bpy or 2,9-dmp with sterically hindering methyl groups the catalytic activity decreases. Thus ligand denticity, Lewis acidity of Ni(II) center and π-back bonding determine the catalytic activity.

A series of mononuclear mixed ligand copper(II) complexes of the type [Cu(pmdt)(diimine)](ClO4)2 1–4, where pmdt is N,N,N′,N″,N″-pentamethyldiethylenetriamine and diimine is 2,2′-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3) and dipyrido-[3,2-d:2′,3′-f]-quinoxaline (4), have been isolated. All the complexes exhibit square-based pyramidal coordination geometry. The DNA binding affinity (Kb) of the complexes varies as 3 > 4 > 2 > 1 depending on the diimine co-ligand. The complex 3, which strongly binds to both DNA and protein and performs both DNA cleavage and protein cleavage, exhibits a higher cytotoxicity than the other complexes due to enhanced hydrophobicity of both pmdt and the diimine co-ligand. Both 3 and 4 induce cell death through both apoptosis and necrosis and 3 shows more effective ROS generation in cancer cells.

The water soluble mixed ligand complexes [Cu(nal)(diimine)(H2O)](ClO4) 1-4, where H(nal) is nalidixic acid and diimine is 2,2'-bipyridine (1), 1,10-phenanthroline (2), 5,6-dimethyl-1,10-phenanthroline (3), and 3,4,7,8-tetramethyl-1,10-phenanthroline (4), have been isolated. The coordination geometry around Cu(II) in 1 and that in the Density Functional Theory optimized structures of 1-4 has been assessed as square pyramidal. The trend in DNA binding constants (Kb) determined using absorption spectral titration (Kb: 1, 0.79±0.1

A series of Mn(II) complexes [Mn(L)Cl2] 1–5, where L is a tetradentate 4N ligand such as N,N-bis(2-pyridylmethyl)-1,2-diaminoethane (L1A), 1,4-bis(2-pyridylmethyl)piperazine (L2), N,N-bis(2-pyridyl-methyl)hexahydropyrimidine (L3), N,N-bis(2-pyridylmethyl)-1,4-diazepane (L4) and N,N-bis(2-pyridylmethyl)-1,5-diazocane (L5), has been isolated, characterized by using electronic and ESI-MS spectral techniques and screened for catalytic olefin oxidation with a representative set of olefins. Interestingly, when the ligand N,N-bis(2-pyridylmethyl)imidazolidine (L1) is treated with MnCl2·6H2O in methanol it undergoes imidazolidine ring hydrolysis to form the complex [Mn(L1A)Cl2] possessing a distorted octahedral coordination geometry around Mn(II). The complex [Mn(L3)(OTf)2(H2O)] contains Mn(II) with a distorted pentagonal bipyramidal coordination geometry while [Mn(L4)Cl2] contains Mn(II) with an octahedral coordination geometry. The complex [Mn(L5)Cl2] adopts a rare trigonal prismatic coordination geometry, presumably because of steric interactions imposed by the ligand backbone. The catalytic ability of the solvent coordinated complex species [Mn(L)(ACN)2]2+ show significant activity towards olefin epoxidation using iodosylbenzene (PhIO) as oxygen source and addition of N-methylimidazole to the reaction mixture increases the epoxide yield. The epoxidation of cis-cyclooctene catalyzed by the complexes proceeds with high conversion (22–65%) and selectivity (100%). The epoxide yield and product selectivity increase upon increasing the Lewis acidity of the Mn(II) center, as modified by the variation in the diazacycloalkane ligand backbone.

A few iron(III) 3,5-di-tert-butylcatecholate (DBC2−) adducts of the type [Fe(L)(DBC)(CH3OH)], where L is a tridentate substituted monophenolate ligand such as 2-((N-benzylpyrid-2-ylmethylamino)methyl)phenol (H(L1)), 2-((N-benzylpyrid-2-ylmethylamino)-methyl)-4,6-dimethylphenol (H(L2)), 2-((N-benzylpyrid-2-ylmethylamino)methyl)-4,6-di-tert-butylphenol (H(L3)) and 2-((N-benzylpyrid-2-ylmethylamino)methyl)-4-nitrophenol (H(L4)), have been isolated and characterized by elemental and ESI-MS analysis. The spectral and electrochemical properties and dioxygenase activities of the adducts have been studied in methanol solution. Upon varying the substituents on the phenolate ring from electron-releasing to electron-withdrawing, the redox potential of DBSQ/DBC2− couple is shifted to a more positive value indicating an increase in covalency of iron(III)–catecholate bonds. All the complexes elicit cleavage of DBC2− using molecular oxygen to afford both intra- (I) and extradiol (E) cleavage products with the product selectivity (E/I) varying in the range 0.3–1.9. Interestingly, the incorporation of electron-withdrawing substituents facilitates the regioselective extradiol cleavage of catechol while that of electron-releasing substituents facilitate the regioselective intradiol cleavage.

The spectral and redox behavior of bis(diimine)copper(II) complexes, where diimine is bipyridine, 1,10-phenanthroline, 4-methyl-1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 5,6-dimethyl-1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline and dipyrido-[3,2-d:2',3'-f]-quinoxaline, are significantly different in aqueous and in aqueous SDS, CTAB and Triton X-100 micellar solutions. The (1)H NMR spectral study in aqueous (D(2)O) and aqueous micelles reveals that the Cu(II) complexes interact more strongly with SDS than with CTAB and Triton X-100 micelles and at sites on SDS micelles different from those on the latter. Ligand Field spectral studies reveal that the complexes exist as the dicationic aquated species [Cu(diimine)(2)(H(2)O)(2)](2+), which interacts strongly with the anionic SDS micelles through columbic forces. However, they exist as [Cu(diimine)(2)(H(2)O)Cl](+) and/or [Cu(diimine)(2)H(2)] located in the hydrophobic microenvironments in Triton X-100 and CTAB micelles. The attainment of reversibility of the redox systems in the micellar microenvironments is remarkable and this illustrates that the Cu(II) and Cu(I) species undergo stereochemical changes suitable for reversible electron-transfer. The remarkable differences in spectral and electrochemical properties of Cu(II) complexes in aqueous and aqueous micellar solutions illustrate that the complexes are nestled largely within the micellar environments and imply that the accessibilities of the complexes to electron-transfer are different and are dependent on the nature of micelles as well as the nature and hydrophobicity of the diimine ligands.

When the complexes [Cu(L1)(H2O)](ClO4)2 1, where L1 = 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane, and [Cu(L2)Cl2] 2, where L2 = 4-methyl-1-(quinol-2-ylmethyl)-1,4-diazacycloheptane are interacted with one/two equivalents of bis(p-nitrophenylphosphate, (p-NO2Ph)2PO2, BNP), no hydrolysis of BNP is observed. From the solution the adducts of copper(II) complexes [Cu2(L1)2((p-NO2Ph)2PO2)2]-(ClO4)2 3 and [Cu(L2)((p-NO2Ph)2PO2)2]·H2O 4 have been isolated and structurally characterised. The X-ray crystal structure of 3 contains two Cu(L1) units bridged by two BNP molecules. The Cu···Cu distance (5.1 A) reveals no Cu–Cu interaction. On the other hand, the complex 4 is mononuclear with Cu(II) coordinated to the 3N ligand as well as BNP molecules through phosphate oxygen. The trigonality index (τ, 0.37) observed for 4 is high suggesting the presence of significant trigonal distortion in the coordination geometry around copper(II). The complexes are further characterized by spectral and electrochemical studies.

Catechol dioxygenases are responsible for the last step in the biodegradation of aromatic molecules in the environment. The iron(II) active site in the extradiol-cleaving enzymes cleaves the C–C bond adjacent to the hydroxyl group, while the iron(III) active site in the intradiol-cleaving enzymes cleaves the C–C bond in between two hydroxyl groups. A series of mononuclear iron(III) complexes of the type [Fe(L)Cl3], where L is the linear N-alkyl substituted bis(pyrid-2-ylmethyl)amine, N-alkyl substituted N-(pyrid-2-ylmethyl)ethylenediamine, linear tridentate 3N ligands containing imidazolyl moieties and tripodal ligands containing pyrazolyl moieties have been isolated and studied as structural and functional models for catechol dioxygenase enzymes. All the complexes catalyse the cleavage of catechols using molecular oxygen to afford both intra- and extradiol cleavage products. The rate of oxygenation depends on the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding N-alkyl groups. Also, our studies reveal that stereo-electronic factors like the Lewis acidity of the iron(III) center and the steric demand of ligands, as regulated by the N-alkyl substituents, determine the regioselectivity and the rate of dioxygenation. In sharp contrast to all these complexes, the pyrazole-containing tripodal ligand complexes yield mainly the oxidized product benzoquinone.

A series of new copper(II) complexes of four sterically hindering linear tridentate 3N ligands N′-ethyl-N′-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L1), N′-benzyl-N′-(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L2), N′-benzyl-N′-(6-methylpyrid-2-yl-methyl)-N,N-dimethylethylenediamine (L3) and N′-benzyl-N′-(quinol-2-ylmethyl)-N,N-dimethylethylenediamine (L4) have been isolated and examined as catalysts for olefin aziridination. The complexes [Cu(L1)Cl2]·CH3OH 1, [Cu(L2)Cl2]·CH3OH 2, [Cu(L3)Cl2]·0.5 H2O 3 and [Cu(L4)Cl2] 4 have been structurally characterized by X-ray crystallography. In all of them copper(II) adopts a slightly distorted square pyramidal geometry as inferred from the values of trigonality index (τ) for them (τ: 1, 0.02; 2, 0.01; 3, 0.07; 4, 0.01). Electronic and EPR spectral studies reveal that the complexes retain square-based geometry in solution also. The complexes undergo quasireversible Cu(II)/Cu(I) redox behavior (E1/2, −0.272 − −0.454 V) in acetonitrile solution. The ability of the complexes to mediate nitrene transfer from PhINTs and chloramine-T trihydrate to olefins to form N-tosylaziridines has been studied. The complexes 3 and 4 catalyze the aziridination of styrene very slowly yielding above 80% of the desired product. They also catalyze the aziridination of the less reactive olefins like cyclooctene and n-hexene but with lower yields (30–50%). In contrast to these two complexes, 1 and 2 fail to catalyze the aziridination of olefins in the presence of both the nitrene sources. All these observations have been rationalized based on the Cu(II)/Cu(I) redox potentials of the catalysts.

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration. The Mo-O(oxo) bond lengths differ only by ≈0.01 Å. Complexes 1, 2, 5, and 6 exhibit two successive Mo(VI)/Mo(V) (E(1/2), -1.141 to -1.848 V) and Mo(V)/Mo(IV) (E(1/2), -1.531 to -2.114 V) redox processes. However, only the Mo(VI)/Mo(V) redox couple was observed for 3 and 4, suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1, 2, 5, and 6 elicit efficient catalytic oxygen-atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe(3) at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe(3) at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.

The iron(III) complexes of the tridentate N 3 ligands pyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L1), 3,5-dimethylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L2), 3- iso -propylpyrazol-1-ylmethyl(pyrid-2-ylmethyl)amine (L3) and (1-methyl-1 H -imidazol-2-ylmethyl)pyrid-2-ylmethylamine (L4) have been isolated and studied as functional models for catechol dioxygenases. They have been characterized by elemental analysis and spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L1)Cl 3 ] 1 has been successfully determined. The complex possesses a distorted octahedral coordination geometry in which the tridentate ligand facially engages iron(III) and the Cl − ions occupy the remaining coordination sites. The Fe–N pz bond distance (2.126(5) A) is shorter than the Fe–N py bond (2.199(5) A). The systematic variation in the ligand donor substituent significantly influences the Lewis acidity of the iron(III) center and hence the interaction of the present complexes with a series of catechols. The catecholate adducts [Fe(L)(DBC)Cl], where H 2 DBC = 3,5-di- tert -butylcatechol, have been generated in situ and their spectral and redox properties and dioxygenase activities have been studied in N , N -dimethylformamide solution. The adducts [Fe(L)(DBC)Cl] undergo cleavage of DBC 2− in the presence of dioxygen to afford major amounts of intradiol and smaller amounts extradiol cleavage products. In dichloromethane solution the [Fe(L)(DBC)Cl] adducts afford higher amounts of extradiol products (64.1–22.2%; extradiol-to-intradiol product selectivity E / I , 2.6:1–4.5:1) than in DMF (2.5–6.6%; E / I , 0.1:1–0.4:1). The results are in line with the recent understanding of the function of intra- and extradiol-cleaving catechol dioxygenases.

The new mononuclear RuIII complexes [Ru(ntb)Cl2]Cl (1), [Ru(ntb)Cl2]ClO4 (1a), and [Ru(mntb)Cl2]Cl (2) (ntb = tris(benzimidazol-2-ylmethyl)amine, mntb = tris(N-methylbenzimidazol-2-ylmethyl)amine) have been synthesized and characterized. The X-ray crystal structure of 1 reveals that the coordination geometry around the RuIII center is distorted octahedral in which four sites are occupied by the tetradentate ligand ntb and the remaining cis positions by two chloride ions. The stronger Ru–Nbzim bonds elongate the Ru–Cl bonds thereby labilizing the coordinated chloride ions. In the electronic absorption spectra the RuIII complexes show two bands corresponding to π(bzim) t2g(Ru) and pπ(Cl–) t2g(Ru) ligand-to-metal charge transfer (LMCT) transitions along with intraligand transitions in the UV region. Complex 1 shows rhombic EPR spectral features (g1, 2.238; g2, 2.071; g3, 1.790). The RuIII complexes display both RuIII RuII reduction and RuIII RuIV oxidation processes. Complexes 1 and 2 catalyze the allylic oxidation of cyclohexene, selective and higher epoxidation of cyclooctene, and hydroxylation of alkanes in the presence of the peroxide TBHP and the peracid m-CPBA as cooxidants illustrating that the electronic and steric effects of tripodal 4N ligands can be tuned to catalyze the effective oxidative transformation of organic compounds. ESI-MS studies reveal the formation of ruthenium peroxido species in these catalytic reactions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The Schiff base ligands 2-(2,6-diisopropylphenyliminomethyl)phenol H(L1), 5-diethylamino-2-(2,6-diisopropylphenyliminomethyl)phenol H(L2), 2,4-di-tert-butyl-6-(2,6-diisopropylphenyliminomethyl)phenol H(L3), 3-(2,6-diisopropylphenyliminomethyl)naphthalen-2-ol H(L4) and 4-(2,6-diisopropylphenyliminomethyl)-5-hydroxymethyl-2-methylpyridin-3-ol H(L5) have been synthesized by the condensation, respectively, of salicylaldehyde, 4-(diethylamino)salicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, 2-hydroxy-1-napthaldehyde and pyridoxal with 2,6-diisopropylaniline. The copper(II) bis-ligand complexes [Cu(L1)2] 1, [Cu(L2)2] 2, [Cu(L3)2] 3, [Cu(L4)2] 4 and [Cu(L5)2] · CH3OH 5 of these ligands have been isolated and characterized. The X-ray crystal structures of two of the complexes [Cu(L1)2] 1 and [Cu(L5)2] · CH3OH 5 have been successfully determined, and the centrosymmetric complexes possess a CuN2O2 chromophore with square planar coordination geometry. The frozen solution EPR spectra of the complexes reveal a square-based CuN2O2 chromophore, and the values of g‖ and g‖/A‖ index reveal enhanced electron delocalization by incorporating the strongly electron-releasing –NEt2 group (2) and fusing a benzene ring on sal-ring (4). The Cu(II)/Cu(I) redox potentials of the Cu(II) complexes reveal that the incorporation of electron-releasing –NEt2 group and fusion of a benzene ring lead to enhanced stabilization of Cu(II) oxidation state supporting the EPR spectral results. The hydrogen bonding interactions between the two molecules present in the unit cell of 5a generate an interesting two-dimensional hydrogen-bonded network topology.

The iron(III) complexes of the 4N ligands 1,4-bis(2-pyridylmethyl)-1,4-diazepane (L1), 1,4-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L2), and 1,4-bis(2-quinolylmethyl)-1,4-diazepane (L3) have been generated in situ in CH 3CN solution, characterized as [Fe(L1)Cl 2] (+) 1, [Fe(L2)Cl 2] (+) 2, and [Fe(L3)Cl 2] (+) 3 by using ESI-MS, absorption and EPR spectral and electrochemical methods and studied as functional models for the extradiol cleaving catechol dioxygenase enzymes. The tetrachlorocatecholate (TCC (2-)) adducts [Fe(L1)(TCC)](ClO 4) 1a, [Fe(L2)(TCC)](ClO 4) 2a, and [Fe(L3)(TCC)](ClO 4) 3a have been isolated and characterized by elemental analysis, absorption spectral and electrochemical methods. The molecular structure of [Fe(L1)(TCC)](ClO 4) 1a has been successfully determined by single crystal X-ray diffraction. The complex 1a possesses a distorted octahedral coordination geometry around iron(III). The two tertiary amine (Fe-N amine, 2.245, 2.145 A) and two pyridyl nitrogen (Fe-N py, 2.104, 2.249 A) atoms of the tetradentate 4N ligand are coordinated to iron(III) in a cis-beta configuration, and the two catecholate oxygen atoms of TCC (2-) occupy the remaining cis positions. The Fe-O cat bond lengths (1.940, 1.967 A) are slightly asymmetric and differ by 0.027 A only. On adding catecholate anion to all the [Fe(L)Cl 2] (+) complexes the linear tetradentate ligand rearranges itself to provide cis-coordination positions for bidentate coordination of the catechol. Upon adding 3,5-di- tert-butylcatechol (H 2DBC) pretreated with 1 equiv of Et 3N to 1- 3, only one catecholate-to-iron(III) LMCT band (648-800 nm) is observed revealing the formation of [Fe(L)(HDBC)] (2+) involving bidentate coordination of the monoanion HDBC (-). On the other hand, when H 2DBC pretreated with 2 equiv of Et 3N or 1 or 2 equiv of piperidine is added to 1- 3, two intense catecholate-to-iron(III) LMCT bands appear suggesting the formation of [Fe(L)(DBC)] (+) with bidentate coordination of DBC (2-). The appearance of the DBSQ/H 2DBC couple for [Fe(L)Cl 2] (+) at positive potentials (-0.079 to 0.165 V) upon treatment with DBC (2-) reveals that chelated DBC (2-) in the former is stabilized toward oxidation more than the uncoordinated H 2DBC. It is remarkable that the [Fe(L)(HDBC)] (2+) complexes elicit fast regioselective extradiol cleavage (34.6-85.5%) in the presence of O 2 unlike the iron(III) complexes of the analogous linear 4N ligands known so far to yield intradiol cleavage products exclusively. Also, the adduct [Fe(L2)(HDBC)] (2+) shows a higher extradiol to intradiol cleavage product selectivity ( E/ I, 181:1) than the other adducts [Fe(L3)(HDBC)] (2+) ( E/ I, 57:1) and [Fe(L1)(HDBC)] (2+) ( E/ I, 9:1). It is proposed that the coordinated pyridyl nitrogen abstracts the proton from chelated HDBC (-) in the substrate-bound complex and then gets displaced to facilitate O 2 attack on the iron(III) center to yield the extradiol cleavage product. In contrast, when the cleavage reaction is performed in the presence of a stronger base like piperidine or 2 equiv of Et 3N a faster intradiol cleavage is favored over extradiol cleavage suggesting the importance of bidentate coordination of DBC (2-) in facilitating intradiol cleavage.

A series of mixed ligand ruthenium(II) complexes [Ru(pdto)(diimine)]$(ClO_4)_2/(PF_6)_2 $ 1–3 and [Ru(bbdo)(diimine)]$(ClO_4)_2$ 4–6, where pdto is 1,8-bis(pyrid-2-yl)-3,6-dithiooctane, bbdo is 1,8-bis(benzimidazol-2-yl)-3,6-dithiooctane and diimine is 1,10-phenanthroline (phen), dipyrido-[3,2-d:2,3-f]-quinoxaline (dpq) and dipyrido[3,2-a:2,3-c]phenazine (dppz), have been isolated and characterised by analytical and spectral methods. The complexes [Ru(pdto)(phen)]$(PF_6)_2$ 1a, [Ru(pdto)(dpq)(Cl)]$(PF_6)$ 2a, [Ru(bbdo)(phen)]$(PF_6)_2$ 4a and [Ru(bbdo)(dpq)]$(ClO_4)_2$ 5 have been structurally characterized and their coordination geometries around ruthenium(II) are described as distorted octahedral. In 1a, 4a and 5 the two thioether sulfur and two py/bzim nitrogen atoms of the tetradentate pdto/bbdo ligand are folded around Ru(II) to give predominantly a cis- configuration. IH NMR spectral data of the complexes support this configuration in solution. In [Ru(pdto)(dpq)Cl]$(PF_6)$ 2a with a distorted octahedral coordination geometry, one of the two py nitrogens of pdto is not coordinated. The DNA binding constants $(Kb: 2, 2.00 \pm 0.02 X 104 M^{-1}, s = 1.0; 3, 3.00 \pm 0.01 X 106 M^{-1}, s = 1.3)$ determined by absorption spectral titrations of the complexes with CT DNA reveal that 3 interacts with DNA more tightly than 2 through partial intercalation of the extended planar ring of coordinated dppz with the DNA base stack. The DNA binding affinities of the complexes increase with increase in the number of planar aromatic rings in the co-ligand, and on replacing both the py moieties in pdto complexes (1–3) by bzim moieties to give bbdo complexes (4–6). Upon interaction with CT DNA the complexes 1, 2, 5 and 6 show a decrease in anodic current in the cyclic voltammograms. On the other hand, interestingly, 3 and 4 show an increase in anodic current suggesting their involvement in electrocatalytic guanine oxidation. Interestingly, of all the complexes, only 6 alters the superhelicity of DNA upon binding with supercoiled pBR322 DNA. The cytotoxicities of the dppz complexes 3 and 6, which avidly bind to DNA, have been examined by screening them against cell lines of different cancer origins. It is noteworthy that 6 exhibits selectivity with higher cytotoxicity against the melanoma cancer cell line (A375) than other cell lines, potency approximately twice that of cisplatin and toxicity to normal cells 3 and 90 times less than cisplatin and adriamycin respectively.

A series of iron(III) complexes of the type [Fe(L)Cl3], where L is the variously N-alkyl-substituted bis(pyrid-2-ylmethyl)amine ligand such as bis(pyrid-2-ylmethyl)amine (L1), N,N-bis(pyrid-2-ylmethyl)methylamine (L2), N,N-bis(pyrid-2-ylmethyl)-n-propylamine (L3), N,N-bis(pyrid-2-ylmethyl)-iso-butylamine (L4), N,N-bis(pyrid-2-ylmethyl)-iso-propylamine (L5), N,N-bis(pyrid-2-ylmethyl)cyclohexylamine (L6), and N,N-bis(pyrid-2-ylmethyl)-tert-butylamine (L7), have been isolated and characterized by elemental analysis and spectral and electrochemical methods. The crystal structures of the complexes [Fe(L2)Cl3] 2, [Fe(L3)Cl3] 3, and the complex-substrate adduct [Fe(L5)(TCC)(NO3)] 5a, where TCC2- is the tetrachlorocatecholate dianion, have been determined by single-crystal X-ray crystallography. The complexes [Fe(L2)Cl3] 2 and [Fe(L3)Cl3] 3 possess a distorted octahedral geometry, in which the linear tridentate 3N ligands are cis-facially coordinated to the iron(III) center, and three chloride ions occupy the remaining coordination sites. The replacement of the N-methyl group in 2 by N-n-propyl group as in 3 leads to the formation of the Fe-Npy bonds and also the Fe-Cl bonds located trans to them of different lengths. The catecholate adduct 5a also possesses a distorted octahedral geometry, in which the ligand is cis-facially coordinated to iron(III) center, TCC2- is asymmetrically chelated trans to the two pyridyl moieties of the ligand, and one of the oxygen atoms of the nitrate ion occupies the sixth coordination site. All of the present complexes have been interacted with simple and substituted catechols. The catecholate adducts [Fe(L)(DBC)Cl] and [Fe(L)(DBC)(Sol)]+, where H2DBC is 3,5-di-tert-butylcatechol and Sol=H2O/CH3CN, have been generated in situ, and their spectral and redox properties and dioxygenase activities have been studied in dimethylformamide and dichloromethane solutions. All of the complexes catalyze the cleavage of H2DBC using molecular oxygen to afford both intra- and extradiol cleavage products. The formation of extradiol cleavage products is facilitated by cis-facial coordination of the 3N ligands and availability of vacant coordination site on iron(III) center for dioxygen binding. It is remarkable that the nature of the N-alkyl substituent in 3N ligands controls the regioselectivity of cleavage, with the n-propyl, iso-butyl, iso-propyl, and cyclohexyl groups enhancing the yield of extradiol products (46-68%) in dichloromethane. The rate of oxygenation depends upon the solvent and the Lewis acidity of iron(III) center as modified by the sterically demanding N-alkyl groups-length and degree of substitution. The plot of log (kO2) versus energy of the low-energy DBC2--to-iron(III) LMCT band is linear, demonstrating the importance of the Lewis acidity of the iron(III) center in dictating the rate of the dioxygenase reaction.

Two new mixed-ligand Ru(II) complexes [Ru(pdto)(dppt)](ClO4)2 (1) and [Ru(bbdo)(dppt)](ClO4)2 (2), where pdto = 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane, bbdo = 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane and dppt = 3-(pyridin-2-yl)-5,6-diphenyl-1,2,4-triazine, have been isolated and characterised by elemental analysis. NMR and electronic absorption and emission spectral and electrochemical techniques have been used to investigate the solution structures and electronic properties of the complexes. The 1H and 13C spectra of the complexes in solution reveal that the N2S2 donor set of the pdto and bbdo ligands is “cis-α” coordinated and the dppt ligand is chelated to Ru(II) through both triazine N2 and pyridine nitrogen atoms. The proton chemical shifts of the phenyl rings of dppt are not affected much upon coordination, supporting the triazine N2 rather than N4 coordination. The anomalous upfield shifts of the H61 and H62 (1) and H72 and H81 (2) protons are caused by the shielding magnetic anisotropy due to the ring currents of the py and tra rings of dppt, which are forced to be coplanar by coordination. The py and bzim rings of pdto and bbdo are obliged to rotate away from dppt and the Ru–Npy and Ru–Nbzim bonds lengthen in order to minimise the steric clashes with dppt. The c.i.s values for 1 are less positive than those for 2 suggesting that the ligand bzim nitrogens of bbdo rather than the py nitrogens of pdto are involved in stronger σ-bonding with Ru(II). Both the complexes display a strong MLCT transition (1, 470; 2, 515 nm) along with intense intraligand transitions in the UV region, and when excited in the MLCT band an emission band (650 nm) is observed for both 1 and 2. In acetonitrile solution they show a quasi-reversible Ru(II)/Ru(III) redox couple (E1/2, 1, 1.18; 2, 0.90 V). Two more redox processes (E1/2, 1, −0.97, −1.09; 2, −1.06, −1.42 V) involving the coordinated dppt ligand are also observed. A plot of the difference between the metal oxidation and ligand reduction potentials of the complexes versus the absorption or emission maxima is linear, illustrating that the lowest π∗ orbitals of dppt are involved in the redox, absorption and emission processes in the complexes. Electrochemical parameterisation of the Ru(II)/Ru(III) redox potentials of the present complexes has been carried out using Lever’s method and the calculated ligand reduction potential EL(L) correlates well with the observed Ru(II)/Ru(III) redox potentials.

This article provides an overview of our work on mononuclear iron(III) complexes of phenolate and non-phenolate ligands as structural and functional models for the intradiol-cleaving non-heme catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) enzymes. All the complexes are cis-facially coordinated to iron(III) with a distorted octahedral geometry. The iron(III) complexes of linear tridentate 3N ligands and tetradentate tripodal phenolate ligands possess octahedral geometries with cis-coordination positions available for bidentate coordination of catechols. In two of these complexes with sterically demanding –NMe2 pendant, the Fe–O–C bond angle is around 135.7°, which is close to those (Fe–O–C, 133°, 148°) in 3,4-PCD enzyme. Also, interestingly, one of the bis-phenolate complexes displays trigonal bipyramidal coordination geometry as in the enzymes. The efficiency of the complexes to catalyze the intradiol-cleavage of 3,5-di-tert-butylcatechol (H2DBC) could be illustrated not only on the basis of Lewis acidity of the iron(III) center alone, but also by assuming that product release is the rate-determining phase of the catalytic reaction.

A few water soluble mixed ligand copper(II) complexes of the type [Cu(bimda)(diimine)] 1-5, where bimda is N-benzyliminodiacetic acid and diimine is 2,2'-bipyridine (bpy, 1) or 1,10-phenanthroline (phen, 2) or 5,6-dimethyl-1,10-phenanthroline (5,6-dmp, 3) or 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-tmp, 4) and dipyrido[3,2-d: 2', 3'-f] quinoxaline (dpq, 5), have been successfully isolated and characterized by elemental analysis and other spectral techniques. The coordination geometry around copper(II) in 2 is described as distorted square based pyramidal while that in 3 is described as square pyramidal. Absorption spectral titrations and competitive DNA binding studies reveal that the intrinsic DNA binding affinity of the complexes depends upon the diimine co-ligand, dpq (5) > 3,4,7,8-tmp (4) > 5,6-dmp (3) > phen (2) > bpy (1). The phen and dpq co-ligands are involved in the p-stacking interaction with DNA base pairs while the 3,4,7,8-tmp/5,6-dmp and bpy co-ligands are involved in respectively hydrophobic and surface mode of binding with DNA. The small enhancement in the relative viscosity of DNA upon binding to 1-5 supports the DNA binding modes proposed. Interestingly, 3 and 4 are selective in exhibiting a positive induced CD band (ICD) upon binding to DNA suggesting that they induce B to A conformational change. In contrast, 2 and 5 show CD responses which reveal their involvement in strong DNA binding. The complexes 2-4 are unique in displaying prominent double-strand DNA cleavage while 1 effects only single-strand DNA cleavage, and their ability to cleave DNA in the absence of an activator varies as 5 > 4 > 3 > 2 > 1. Also, all the complexes exhibit oxidative double-strand DNA cleavage activity in the presence of ascorbic acid, which varies as 5 > 4 > 3 > 2 > 1. The ability of the complexes to bind and cleave the protein BSA varies in the order 4 > 3 > 5 > 2 > 1. Interestingly, 3 and 4 cleave the protein non-specifically in the presence of H2O2 as an activator suggesting that they can act also as chemical proteases. It is remarkable that 2-5 exhibit cytotoxicity against human breast cancer cell lines (MCF-7) with potency higher than the widely used drug cisplatin indicating that they have the potential to act as effective anticancer drugs in a time dependent manner. The morphological assessment data obtained by using Hoechst 33258 staining reveal that 3 and 4 induce apoptosis much more effectively than other complexes. Also, the alkaline single-cell gel electrophoresis study (comet assay) suggests that the same complexes induce DNA fragmentation more efficiently than others.

A series of mixed ligand copper(II) complexes of the type [Cu(L)(phen)(ACN)](ClO4)(2) 1-5, where L is a bidentate Schiff base ligand (N-1-(anthracen-10-ylmethylene)-N-2-methylethane-1,2-diamine (L1), N1-(anthracen-10-ylmethylene)-N-2,N-2-dimethylethane-1,2-diamine (L2), N-1-(anthracen-10-yl-methylene)-N-2-ethylethane-1,2-diamine (L3), N-1-(anthracen-10-ylmethylene)-N-2,N-2-diethylethane-1,2-diamine (L4) and N-1-(anthracen-10-ylmethylene)-N-3-methylpropane-1,3-diamine (L5)) and phen is 1,10-phenanthroline, have been synthesized and characterized by spectral and analytical methods. The X-ray crystal structure of 5 reveals that the coordination geometry around Cu(II) is square pyramidal distorted trigonal bipyramidal (tau, 0.76). The corners of the trigonal plane of the geometry are occupied by the N-2 nitrogen atom of phen, the N4 nitrogen atom of L5 and the N5 nitrogen of acetonitrile while the N1 nitrogen of phen and the N3 nitrogen of L5 occupy the axial positions with an N1-Cu1-N3 bond angle of 176.0(3)degrees. All the complexes display a ligand field band (600-705 nm) and three less intense anthracene-based bands (345-395 nm) in solution. The K-b values calculated from absorption spectral titration of the complexes (pi ->pi*, 250-265 nm) with Calf Thymus (CT) DNA vary in the order 5 > 4 > 3 > 2 > 1. The fluorescence intensity of the complexes (520-525 nm) decreases upon incremental addition of CT DNA, which reveals the involvement of phen rather than the appended anthracene ring in partial DNA intercalation with the DNA base stack. The extent of quenching is in agreement with the DNA binding affinities and the relative increase in the viscosity of DNA upon binding to the complexes as well. Thus 5 interacts with DNA more strongly than 4 on account of the stronger involvement in hydrophobic DNA interaction of the anthracenyl moiety, which is facilitated by the propylene ligand backbone with chair conformation. The ability of complexes (100 mu M) to cleave DNA (pUC19 DNA) in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.1 in the absence of a reducing agent or light varies in the order 5 > 4 > 3 > 2 > 1, which is in conformity with their DNA binding affinities. Interestingly, cytotoxicity studies on the MCF-7 human breast cancer cell line show that the IC50 value of 5 is less than that of cisplatin for the same cell line, revealing that it can act as an effective cytotoxic drug in a time-dependent manner.

A series of mixed ligand Ru(II) complexes of 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) as primary ligand and 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), pyridine (py) and NH3 as co-ligands have been prepared and characterized by X-ray crystallography, elemental analysis and 1H NMR and electronic absorption spectroscopy. The X-ray crystal structure of the complex [Ru(phen)2(bpy)]Cl2 reveals a distorted octahedral coordination geometry for the RuN6 coordination sphere. The DNA binding constants obtained from the absorption spectral titrations decrease in the order, tris(5,6-dmp)Ru(II) > bis(5,6-dmp)Ru(II) > mono(5,6-dmp)Ru(II), which is consistent with the trend in apparent emission enhancement of the complexes on binding to DNA. These observations reveal that the DNA binding affinity of the complexes depend upon the number of 5,6-dmp ligands and hence the hydrophobic interaction of 5,6-dimethyl groups on the DNA surface, which is critical in determining the DNA binding affinity and the solvent accessibility of the exciplex. Among the bis(5,6-dmp)Ru(II) complexes, those with monodentate py (4) or NH3 (5) co-ligands show DNA binding affinities slightly higher than the bpy and phen analogues. This reveals that they interact with DNA through the co-ligands while both the 5,6-dmp ligands interact with the exterior of the DNA surface. All these observations are supported by thermal denaturation and viscosity measurements. Two DNA binding modes – surface/electrostatic and strong hydrophobic/partial intercalative DNA interaction – are suggested for the mixed ligand complexes on the basis of time-resolved emission measurements. Interestingly, the 5,6-dmp ligands promote aggregation of the complexes on the DNA helix as a helical nanotemplate, as evidenced by induced CD signals in the UV region. The ionic strength variation experiments and competitive DNA binding studies on bis(5,6-dmp)Ru(II) complexes reveal that EthBr and the partially intercalated and kinetically inert [Ru(phen)2(dppz)]2+ (dppz = dipyrido[3,2-a:2′,3′-c]phenazine) complexes revert the CD signals induced by exciton coupling of the DNA-bound complexes with the free complexes in solution.

Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) are bacterial non-heme iron enzymes, which catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incorporation of molecular oxygen via a mechanism involving a high-spin ferric centre. The iron(III) complexes of tripodal phenolate ligands containing N3O and N2O2 donor sets represent the metal binding region of the iron proteins. In our laboratory iron(III) complexes of mono- and bisphenolate ligands have been studied successfully as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The single crystal X-ray crystal structures of four of the complexes have been determined. One of thebis-phenolato complexes contains a FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. The Fe-O-C bond angle of 136.1‡ observed for one of the iron(III) complex of a monophenolate ligand is very similar to that in the enzymes. The importance of the nearby sterically demanding coordinated -NMe2 group has been established and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133, 148‡). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulate the electronic spectral features, FeIII/FeII redox potentials and catechol cleavage activity of the complexes. Upon interacting with catecholate anions, two catecholate to iron(III) charge transfer bands appear and the low energy band is similar to that of catechol dioxygenase-substrate complex. Four of the complexes catalyze the oxidative cleavage of H2DBC by molecular oxygen to yield intradiol cleavage products. Remarkably, the more basic N-methylimidazole ring in one of the complexes facilitates the rate-determining productreleasing phase of the catalytic reaction. The present study provides support to the novel substrate activation mechanism proposed for the intradiol-cleavage enzymes.

The synthesis, structure and spectral and redox behaviour of copper(II) complexes of sterically constrained N3, N3S2 and N2S2 ligands are described. The ligands 2,6-bis(2,4,6-trimethylphenyliminomethyl)pyridine (L1), 2,6-bis(2,6-diisopropylphenyliminomethyl)pyridine (L2), 2,6-bis(2-methylthiophenyliminomethyl)pyridine (L3), 1,3-bis(2-methylthiophenyliminomethyl)benzene (L4), 1,3-bis(2,4,6-trimethylphenyliminomethyl)benzene (L5) and 1,3-bis(2,6-diisopropyl-phenyliminomethyl)benzene (L6) have been synthesized. The 1:1 copper(II) complexes of L1–L5 have been isolated and characterized by spectral and electrochemical techniques. The X-ray crystal structure of the complex [Cu(L2)Cl2] (2) has been successfully determined and is found to possess a distorted square pyramidal coordination geometry. The replacement of isopropyl groups in this complex by the less sterically demanding methyl groups as in [Cu(L1)Cl2] leads to a decreased geometric distortion. The complexes show relatively high positive redox potentials with a low reorganizational energy barrier for electron transfer involving the Cu(II)/Cu(I) couple. The redox cycle of the complexes 1 and 2 has been explained by assuming a square scheme. The spectral and electrochemical properties of CuN3S2 and CuN2S2 complexes generated by the incorporation of thioether sulfurs into L1/L2 and L4/L5, respectively, are also discussed. Both the complexes show positive redox potentials suggesting the coordination of at least one thioether sulfur to copper(II). The incorporation of a pyridine nitrogen donor into the CuN2S2 complexes leads to higher LF field strengths but with lower absorptivities. The coordination of Cl− ions to the CuN3S2 and CuN2S2 complexes significantly alter the spectral and redox properties of the complexes.

The synthesis, structure and spectral and redox properties of the copper(II) complexes [Cu(pmtpm)Cl2] (1) and [Cu(pmtpm)2](ClO4)2 (6), where pmtpm is the linear tridentate ligand 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine containing a thioether and two pyridine donors, are described. Also, the mixed ligand complexes [Cu(pmtpm)(diimine)](ClO4)2 (2–5), where the diimine is 2,2′-bipyridine (bpy) (2), 1,10-phenanthroline (phen) (3), 2,9-dimethyl-1,10-phenanthroline (2,9-dmp) (4) or dipyrido-[3,2-d:2′,3′-f]-quinoxaline (dpq) (5), have been isolated and studied. The X-ray crystal structures of the complexes 1, [Cu(pmtpm)(2,9-dmp)](ClO4)2 4 and 6 have been successfully determined. The complex 1 possesses a trigonal bipyramidal distorted square based pyramidal (TBDSBP) coordination geometry in which three corners of the square plane are occupied by two nitrogens and thioether s of pmtpm ligand and the remaining equatorial and the axial positions by two chloride ions. The complex 4 contains a CuN4S chromophore also with a TBDSBP coordination geometry in which two nitrogens and the thioether sulfur of pmtpm ligand occupy three corners of the square plane. One of the two nitrogens of 2,9-dmp ligand is equatorially coordinated and the other axially to copper. On the other hand, the complex 6 is found to possess a square based pyramidal distorted trigonal bipyramidal (SPDTBP) coordination geometry. The CuN2S trigonal plane in it is comprised of the pyridine and imine nitrogens and the thioether sulfur of the pmtpm ligand. The pyridine nitrogens of the ligand occupy the axial positions and the second thioether sulfur remains uncoordinated. On long standing in acetonitrile solution the mixed ligand complexes 2 and 3 undergo ligand disproportionation to provide the corresponding bis-complexes of bpy and phen, respectively, and 6. The electronic and EPR spectral parameters and the positive redox potential of complex 4 are consistent with the equatorial location of the thioether sulfur in the square-based coordination geometry around copper(II). On the other hand, the higher g and lower A values and lower E1/2 values for the complexes 2, 3 and 5 are consistent with the axial coordination of the thioether sulfur. Also, the Cu(II)/Cu(I) redox potentials increase with increase in number of aromatic rings of the diimine ligand. The steric and electronic effects of the bidentate diimine ligands in orienting the thioether coordination to axial or equatorial position are discussed.

The new mixed ligand complex [Ru(5,6-dmp) 2 (dppz)]Cl 2 [5,6-dmp = 5,6-dimethyl-1,10-phenanthroline, dppz = dipyrido[3,2- a :2′,3′- c ]phenazine] has been isolated and its DNA-binding properties studied by employing UV–visible (UV–Vis), steady-state and time-resolved emission and circular dichroism spectral methods, viscometry, thermal denaturation and cyclic/differential pulse voltammetric techniques. The complex acts as a ‘molecular light-switch’ on binding to DNA, but the enhancement in emission intensity is only 75% of that of the parent complex [Ru(phen) 2 (dppz)] 2+ (phen = 1,10-phenanthroline). The emission decay curves and quenching studies suggest two different DNA-binding modes both involving intercalation of the dppz ligand of [Ru(5,6-dmp) 2 (dppz)]Cl 2 . The characteristic red-shift of the induced CD signal, which is not observed for the phen analogue, arises from exciton coupling. The hydrophobicity and polarizability of 5,6-dmp co-ligand strongly favour the formation of a stable structural and electronic scaffold on the DNA surface for the unbound molecules to couple with the DNA-bound complexes facilitating spontaneous assembly of novel extended molecular aggregates using DNA as a helical nanotemplate. This observation is consistent with the shift in Ru(II)/Ru(III) redox potential to more positive values with a dramatic drop in peak current on binding of the 5,6-dmp complex to calf thymus (CT) DNA. Equilibrium dialysis experiments monitored by CD spectroscopy unambiquously reveal the preferential binding of the Δ-enantiomer to the right-handed calf thymus (CT) DNA. The 5,6-dmp complex exhibits preferential binding to [d(AT) 6 ] 2 over [d(GC) 6 ] 2 and the complex aggregates formed consist of six [Ru(5,6-dmp) 2 (dppz)] 2+ cations per base pair of [d(AT) 6 ] 2 ; however, only one [Ru(phen) 2 (dppz)] 2+ cation per base pair is involved in DNA binding.

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2''-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi--pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.

Copper(II) complexes of three linear unsymmetrical tridentate ligands viz. N-methyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L1), N,N-dimethyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L2) and N,N-dimethyl-N'-((6-methyl)pyrid-2-ylmethyl)ethylenediamine (L3) have been isolated and characterized by elemental analysis, electronic absorption and EPR spectroscopy and cyclic and differential pulse voltammetry. Of these complexes [Cu(L2)Cl2] and [Cu(L3)Cl2] have been structurally characterized by X-ray crystallography. The [Cu(L2)Cl2] complex crystallizes in the monoclinic space group P2(1)/n with a=11.566(2) A, b=7.369(1) A, c=15.703(3) A, alpha=90 degrees , beta=109.68(8) degrees , gamma=90 degrees and Z=4 while [Cu(L3)Cl2] crystallizes in the triclinic space group P1 with a=9.191(2) A, b=12.359(3) A, c=14.880(3) A, alpha=79.61(13) degrees , beta=86.64(13) degrees , gamma=87.28(8) degrees and Z=2. The coordination geometries around copper (II) in these two complexes are best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). The distorted CuN3Cl basal plane in them is comprised of three nitrogen atoms of the meridionally coordinated ligand and a chloride ion and the axial position is occupied by the other chloride ion. The interaction of these complexes with Calf Thymus DNA (CT DNA) has been studied by using absorption, emission and circular dichroic spectral methods, thermal denaturation studies, viscometry and cyclic and differential pulse voltammetry. A strong blueshift in the ligand field band and a redshift in the ligand based bands of the copper(II) complexes on binding to DNA imply a covalent mode of DNA binding of the complexes, which involves coordination of most possibly guanine N7 nitrogen of DNA to form a CuN4 chromophore. This is supported by studying the interaction of the complexes with N-methylimidazole (N-meim), guanosine monophosphate (GMP), adenosine monophosphate (AMP) and cytidine (cytd) by ligand field and EPR spectral methods, which indicate the formation of a CuN4 chromophore only in the case of the more basic N-meim and GMP. The DNA melting curves obtained in the presence of copper(II) complexes reveal a monophasic and irreversible melting of the DNA strands and the high positive DeltaTm values (12-21 degrees C) also support the formation of strong Cu-N bonds by the complexes with DNA, leading to intra- and/or interstrand crosslinking of DNA. Competitive ethidium bromide (EthBr) binding studies show that the L2 and L3 complexes are less efficient than the L1 complex in quenching EthBr emission, which is consistent with their forming DNA crosslinking preventing the displacement of the DNA-bound EthBr. A very slight decrease in relative viscosity of DNA is observed on treating the L1 and L2 complexes with CT DNA; however, a relatively significant decrease is observed for the L3 complex suggesting that the length of the DNA fiber is shortened. DNA cleavage experiments show that all the complexes induce the cleavage of pBR322 plasmid DNA, the complex of L1 being more efficient than those of sterically hindered L2 and L3 ligands.

A series of copper(II) complexes of the type [Cu(L)]2+, where L = N,N'-dialkyl-1,10-phenanthroline-2,9-dimethanamine and R = methyl (L1), n-propyl (L2), isopropyl (L3), sec-butyl (L4), or tert-butyl (L5) group, have been synthesized. The interaction of the complexes with DNA has been studied by DNA fiber electron paramagnetic resonance (EPR) spectroscopy, emission, viscosity and electrochemical measurements and agarose gel electrophoresis. In the X-ray crystal structure of [Cu(HL2)Cl2]NO3, copper(II) is coordinated to two ring nitrogens and one of the two secondary amine nitrogens of the side chains and two chloride ions as well and the coordination geometry is best described as trigonal bipyramidal distorted square based pyramidal (TBDSBP). Electronic and EPR spectral studies reveal that all the complexes in aqueous solution around pH 7 possess CuN3O2 rather than CuN4O chromophore with one of the alkylamino side chain not involved in coordination. The structures of the complexes in aqueous solution around pH 7 change from distorted tetragonal to trigonal bipyramidal as the size of the alkyl group is increased. The observed changes in the physicochemical features of the complexes on binding to DNA suggest that the complexes, except [Cu(L5)]2+, bind to DNA with partial intercalation of the derivatised phen ring in between the DNA base pairs. Electrochemical studies reveal that the complexes prefer to bind to DNA in Cu(II) rather than Cu(I) oxidation state. Interestingly, [Cu(L5)]2+ shows the highest DNA cleavage activity among all the present copper(II) complexes suggesting that the bulky N-tert-butyl group plays an important role in modifying the coordination environment around the copper(II) center, the Cu(II)/Cu(I) redox potential and hence the formation of activated oxidant responsible for the cleavage. These results were compared with those for bis(1,10-phenanthroline)copper(II), [Cu(phen)2]2+.

A series of 1:1 iron(III) complexes of simple and sterically hindered tridentate 3N donor ligands have been synthesized and studied as functional models for catechol dioxygenases. All of them are of the type [FeLCl3], where L is bis(pyrid-2-yl-methyl)amine (L1), N,N-bis(benzimidazol-2-ylmethyl)amine (L2), N-methyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L3), N,N-dimethyl-N'-(pyrid-2-ylmethyl)-ethylenediamine (L4) and N-phenyl-N'-(pyrid-2-ylmethyl)ethylenediamine (L5). They have been characterised by spectral and electrochemical methods. The X-ray crystal structure of the complex [Fe(L4)Cl3] has been successfully determined. The complex crystallizes in the triclinic space group P1 with a = 7.250(6), b = 8.284(3), c = 12.409(4) angstroms, alpha = 80.84(3) degrees, beta = 86.76(6) degrees, gamma = 72.09(7) degrees and Z = 2. It possesses a distorted octahedral geometry in which the L4 ligand is cis-facially coordinated to iron(III) and the chloride ions occupy the remaining coordination sites. The systematic variation in the ligand donor atom type significantly influences the Lewis acidity of the iron(III) center and hence the binding interaction of the complexes with simple and substituted catechols. The spectroscopic and electrochemical properties of the catecholate complexes generated in situ have been investigated. All the complexes catalyze mainly the oxidative intradiol cleavage of 3,5-di-tert-butylcatechol (H2DBC) in the presence of dioxygen, which is unexpected of the cis-facial coordination of the ligands. The rate of intradiol catechol cleavage reaction depends upon the Lewis acidity of iron(III) center and steric demand and hydrogen-bonding functionalities of the ligands. Interestingly, the electron-sink property of N-phenyl substituent in [Fe(L5)Cl3] complex leads to enhancement in rate of cleavage. All these observations provide support to the substrate activation mechanism proposed for intradiol-cleaving enzymes.