A series of mixed ligand ruthenium(II) complexes [Ru(Hdpa)2(diimine)](ClO4)2, 1-5 where Hdpa is 2,2'-dipyridylamine and diimine is 1,10-phenanthroline (phen) and a modified/extended 1,10-phenanthroline such as, 5,6-dimethyl-1,10-phenanthroline (5,6-dmp), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), 5-methyldipyrido[3,2-d:2',3'-f]quinoxaline (mdpq) and dipyrido[3,2-a:2',3'-c]phenazine (dppz) have been isolated and characterized by analytical and spectral methods. The complex [Ru(Hdpa)2(phen)](PF6)2 1 has been structurally characterized and the coordination geometry around Ru(II) in it is described as distorted octahedral. 1H NMR spectral data reveal that 1-5 should have a C2 symmetry lying on the diimine plane due to the rapid flapping of the coordinated Hdpa ligands. The interaction of the complexes with calf thymus (CT) DNA has been explored by using absorption and emission spectral and viscometry and electrochemical techniques and the mode of DNA binding of the complexes has been proposed. The DNA binding affinity of the complexes decreases with decrease in number of planar aromatic rings in the co-ligand supporting the intercalation of the diimine co-ligands in between the DNA base pairs. Circular dichroic spectral studies reveal that the complexes 3-5 exhibit induced circular dichroism upon binding to CT DNA. Interestingly, upon interaction with CT DNA all the complexes show an increase in anodic current in the cyclic voltammograms suggesting that they are involved in electrocatalytic guanine oxidation. Interestingly, of all the complexes, only 5 alters the DNA superhelicity upon binding with supercoiled pBR322 DNA, which is consistent with its higher DNA binding affinity. Further, the cytotoxicities of the complexes against human cervical epidermoid carcinoma cell line (ME180) have been examined. Interestingly, 5 exhibits a cytotoxicity against ME180 higher than other complexes with potency approximately 8 times more than cisplatin for 24 h incubation but 4 times lower than cisplatin for 48 h incubation.

A series of mixed ligand ruthenium(II) complexes [Ru(pdto)(diimine)](ClO4)2/(PF6)2 1-3 and [Ru(bbdo)(diimine)](ClO4), 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)](PF6)2 la, [Ru(pdto)(dpq)(Cl](PF6) 2a, [Ru(bbdo)(phen)](PF6)2 4a and [Ru(bbdo)(dpq)](ClO4)2 5 have been structurally characterized and their coordination geometries around ruthenium(II) are described as distorted octahedral. In la, 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-alpha" configuration. (I)H NMR spectral data of the complexes support this configuration in solution. In [Ru(pdto)(dpq)Cl](PF6) 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 +/- 0.02 x 10(4) M(-1), s = 1.0; 3, 3.00 +/- 0.01 x 10(6) 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.

The aerobic reaction of the Schiff-base ligand N-(benzimidazol-2-yl)salicylaldimine (Hbisi) with iron(II) perchlorate in methanol leads to the formation of the remarkable coordination compound [Fe(4)(mu(4)-O)(mu-MeO)(4)(bisi)(4)](ClO(4))(2) x 4 MeOH (1), whose single-crystal X-ray structure reveals the presence of a discrete Fe(III)(4)(mu(4)-O) core. Magnetic and Mossbauer studies both show that the exchange interaction within the square tetranuclear iron(III) unit is dominated by the central bridging mu(4)-oxido ligand, the involvement of the mu-methoxido bridges being negligible.

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