A series of mononuclear Co(III) complexes using tridentate N,O-donor ligands: Chemical properties and cytotoxicity activity

Investigation of cobalt(III)-phenylalanine complexes for hypoxia-activated drug delivery

Four cobalt(iii)-phenylalanine complexes, [Co(Phe)(py2en)](ClO4)2·H2O (1), [Co(Phe)(TPA)](ClO4)2·H2O (2), [Co(Phe)(py2enMe2)](ClO4)2·H2O (3) and [Co(bipy)2(Phe)](ClO4)2·H2O (4), were investigated as prototype models for hypoxia-activated delivery of melphalan - a phenylalanine derivative anticancer drug of the class of nitrogen mustards. Single crystal X-ray diffraction analysis provided the molecular structures of 1-4, as a single isomer/conformer. According with NMR and theoretical calculations, the solid-state structures of 2 and 4 are maintained in solutions. For complexes 1 and 3, though, a mixture of isomers was found in DMSO solutions: Λ-cisα(exo,exo) and Δ-cisβ1(exo,exo) for 1 (3 : 2 ratio), and Λ-cisα(exo,exo) and Δ-cisα(exo,exo) for 3 (5 : 1 ratio). Theoretical calculations point to a re-equilibration reaction of the solid-state Λ-cisβ1 isomer of 1 in solution. Electrochemical analysis revealed a correlation between the electron-donor capacity of the ancillary ligands and the redox potentials of the complexes. The potentials varied from +0.01 for 1 to +0.31 V vs. SHE for 4 in aqueous media and indicate that reduction should be achieved in biological media. The integrity of the complexes in pH 5.5 and 7.4 buffered solutions was confirmed by UV-Vis monitoring up to 24 h at 25 °C. Reduction by ascorbic acid (AA) shows an O2-dependent dissociation of the l-Phe for complexes 1-3, with higher conversion rates at pH 7.4. For complex 4, a fast dissociation of l-Phe was observed, with conversion rates unaffected by the pH and presence of O2.

Evaluation of cobalt(III) complexes as potential hypoxia-responsive carriers of esculetin

Differentiation between hypoxic and normoxic tissues have been exploited for the development of selective chemotherapeutic agents. In this context, cobalt(III)-based coordination compounds have been designed and investigated as prospective hypoxia-responsive drug delivery systems. Three cobalt(III) complexes, namely [Co^III(esc)(py₂en)]ClO₄·(CH₃OH)₂ (1) [Co^III(esc)(TPA)]ClO₄·3H₂O (2) and [Co^III(bipy)₂(esc)]ClO₄·2.5H₂O (3) (py₂en = N,N'-bis(pyridin-2-ylmethyl)ethylenediamine, TPA = tris(2-pyridylmethyl)amine, bipy = 2,2'-bipyridine and esc = 6,7-dihydroxycoumarin or esculetin), were prepared and investigated as potential carriers of esculetin. The spectroscopic and electrochemical properties of 1-3 were investigated and compared. Reactions of the complexes with biologically relevant reducing agents, viz. ascorbic acid, cysteine and glutathione, were monitored spectroscopically for 24 h, in pH 6.2 and 7.4 PBS phosphate buffer saline (PBS) solutions at 37 °C, under air, argon and dioxygen atmospheres. Dissociation of esculetin was observed upon Co³⁺/Co²⁺ reduction preferably under hypoxic conditions, with more effective conversion rates for 3 > 2 > 1. These results illustrate the importance to modulate the Co³⁺/Co²⁺ redox potential through the donor-acceptor properties of the ancillary ligands. Complex 3 is cytotoxic against HCT-116 but not against HT-29 and HEK-293 cells. In addition, DNA-binding studies indicate that interactions of 1 and 3 with the biomolecule are electrostatic.

Crystal structure of tetra-kis-(μ2-(E)-2,4-di-bromo-6-{[2-(pyridin-2-yl)eth-yl]imino-meth-yl}phen-olato)trizinc bis-(perchlorate) aceto-nitrile disolvate

The title compound, [Zn3(C14H11Br2N2O)4](ClO4)2·2CH3CN, crystallizes as a symmetrical trinuclear cation with all three metal atoms being located on a twofold rotation axis. It contains a tetra-hedral ZnII atom that bridges two six-coordinate ZnII atoms. The complex contains N- and O-donor atoms of four tridentate 2,4-di-bromo-6-{[2-(pyridin-2-yl)eth-yl]imino-meth-yl}phenolate ligands. The ratio of ZnII atoms to ligands is 3:4. The two terminal ZnII cations adopt distorted octa-hedral geometries and the central ZnII cation adopts a distorted tetra-hedral geometry. In the cation there are π-π inter-actions between the di-bromo-phenyl rings, as well as halogen-bonding inter-actions between the di-bromo-phenyl rings in the cation, which stabilize its conformation. In addition, there are C-H⋯O inter-actions between the anions and both the cations and solvent mol-ecules as well as C-H⋯N inter-actions between the cation and solvent mol-ecules. These inter-species inter-actions link the cations, anions and solvent mol-ecules into a complex three-dimensional array.

Investigation of cobalt(iii)–TPA complexes as potential bioreductively activated carriers for naphthoquinone-based drugs

Differently from CH₃, halogens as substituents foster incorporation of methoxide into NQs upon complexation.