Tuning the redox potentials of ternary cobalt(III) complexes containing various hydroxamates

Synthesis of nitrogen mustards on cobalt(III)

Bis(bidentate) and bis(tridentate) Co(III) complexes of N-(2-hydroxyethyl)ethane-1,2-diamine (heen), 2-[(2-aminoethyl)amino]ethan-1-olate (heen-H), or N-(2-chloroethyl)ethane-1,2-diamine (ceen) ligands have been synthesised, and a range of reaction conditions established for their syntheses by different routes. They can all be ultimately derived from (OC-6-12')-[Co(heen)2(NO2)2]NO3 and provide access to the trans amine trans chloride nitrogen mustard complex, (OC-6-12')-[Co(ceen)2(Cl)2]Cl. Although complex isomeric mixtures were obtained from the reaction of (OC-6-12')-[Co(heen)2(NO2)2]NO3 under different reaction conditions, ultimately, the trans amine trans chlorido configuration around the Co(III) metal centre of the (OC-6-12')-[Co(ceen)2(Cl)2]Cl complex was favoured.

Pd(II) Binding Strength of a Novel Ambidentate Dipeptide-Hydroxypyridinonate Ligand: A Solution Equilibrium Study

A novel ambidentate dipeptide conjugate (H(L1)) containing N-donor atoms of the peptide part and an (O,O) chelate at the hydroxypyridinone (HP) ring is synthesized and characterized. It is hoped that this chelating ligand can be useful to obtain multitargeted Co(III)/Pt(II) dinuclear complexes with anticancer potential. The Pd(II) (as a Pt(II) model but with faster ligand exchange reactions) binding strength of the ligand was studied in an aqueous solution with the combined use of pH-potentiometry and NMR. In an equimolar solution, (L1)⁻ was found to bind Pd(II) via the terminal amino and increasing number of peptide nitrogens of the peptide backbone over a wide pH range. At a 2:1 Pd(II) to ligand ratio, the presence of [Pd₂H_–x(L1)] (x = 1–4) species, with high stability and with the coordination of the (O,O) chelating set of the ligand, was detected. The reaction of H(L1) with [Co(tren)]³⁺ (tren = tris(2-aminoethyl)amine) indicated the exclusive binding of (L1)⁻ via its (O,O) donor atoms to the metal unit, while treatment of the resulting Co-complex with Pd(II) afforded the formation of a Co/Pd heterobimetallic complex in solution with an (NH₂, N_amide) coordination of Pd(II). Shortening the peptide backbone in H(L1) by one peptide unit compared to the structurally similar ambidentate chelator consisting of three peptide bonds resulted in the slightly more favorable formation of the N-coordinated Pd(II) species, allowing the tailoring of the coordination properties.

Donor Atom Preference of Organoruthenium and Organorhodium Cations on the Interaction with Novel Ambidentate (N,N) and (O,O) Chelating Ligands in Aqueous Solution

Two novel, pyridinone-based chelating ligands containing separated (O,O) and (N_amino,N_het) chelating sets (N_amino = secondary amine; N_het = pyrrole N for H(L3) (1-(3-(((1H-pyrrole-2-yl)methyl)-amino)propyl)-3-hydroxy-2-methylpyridin-4(1H)-one) or pyridine N for H(L5) (3-hydroxy-2-methyl-1-(3-((pyridin-2-ylmethyl)amino)propyl)pyridin-4(1H)-one)) were synthesized via reduction of the appropriate imines. Their proton dissociation processes were explored, and the molecular structures of two synthons were assessed by X-ray crystallography. These ambidentate chelating ligands are intended to develop Co(III)/PGM (PGM = platinum group metal) heterobimetallic multitargeted complexes with anticancer potential. To explore their metal ion binding ability, the interaction with Pd(II), [(η⁶-p-cym)Ru]²⁺ and [(η⁵-Cp*)Rh]²⁺ (p-cym = 1-methyl-4-isopropylbenzene, Cp* = pentamethyl-cyclopentadienyl anion) cations was studied in aqueous solution with the combined use of pH-potentiometry, NMR and HR ESI-MS. In general, organorhodium was found to form more labile complexes over ruthenium, while complexation of the (N,N) chelating set was slower than the processes of the pyridinone unit with (O,O) coordination. Formation of the organoruthenium complexes starts at lower pH (higher thermodynamic stabilities of the corresponding complexes) than for [(η⁵-Cp*)Rh]²⁺ but, due to the higher affinity of [η⁶-p-cym)Ru]²⁺ towards hydrolysis, the complexed ligands are capable of competing with hydroxide ion in a lesser extent than for the rhodium systems. As a result, under biologically relevant conditions, the rhodium binding effectivity of the ligands becomes comparable or even slightly higher than their effectivity towards ruthenium. Our results indicate that H(L3) is a less efficient (N,N) chelator for these metal ions than H(L5). Similarly, due to the relative effectivity of the (O,O) and (N,N) chelates at a 1:1 metal-ion-to-ligand ratio, H(L5) coordinates in a (N,N) manner to both cations in the whole pH range studied while, for H(L3), the complexation starts with (O,O) coordination. At a 2:1 metal-ion-to-ligand ratio, H(L3) cannot hinder the intensive hydrolysis of the second metal ion, although a small amount of 2:1 complex with [(η⁵-Cp*)Rh]²⁺ can also be detected.

Pd(ii)-Complexes of a novel pyridinone based tripeptide conjugate: solution and solid state studies

A novel peptide conjugate (H(L2)) incorporating N-donors of the peptide backbone and an (O,O) donor set of a hydroxypyridinone moiety is synthesized and characterized. This ambidentate chelating ligand is intended to develop Co(iii)/Pt(ii) heterobimetallic multitargeted complexes with anticancer potential. To explore its metal ion binding ability the interaction with Pd(ii) (as a Pt(ii) model but with faster ligand exchange reactions) was studied in aqueous solution by the combined use of pH-potentiometry, NMR and HR MS. In an equimolar solution H(L2) was found to bind Pd(ii) via the terminal amino group and increasing number of peptide nitrogens of the peptide backbone over a wide pH range. Around physiological pH an (N,N) and (O,O) chelated 2 : 2 minor species was also identified. At a 2 : 1 Pd(ii) to ligand ratio the formation of dinuclear species, [Pd2H-x(L2)] (x = 1-4), with high stability and with the involvement of the (O,O) chelating set of the ligand too, was demonstrated. Reaction of H3(L2)2+ with Pd(ii) in the presence of chloride ions at pH ∼ 2.0 afforded [PdH(L2)Cl2]·2H2O (3) in a solid state whose molecular structure was assessed by single crystal X-ray diffraction. The structure of 3 revealed that Pd(ii) is coordinated by a (NH2, Namide) chelate of the ligand in a square planar fashion. It also indicates that under suitable conditions a 2N coordinated Pd(ii) complex can also be obtained even in the presence of four available nitrogen donors in the chelatable position in the ligand most likely due to its neutral charge and the decreased conditional stability of the amide-involved chelate(s) under acidic conditions. Reaction of H(L2) with [Co(tren)]3+ (tren = tris(2-aminoethyl)amine) revealed the exclusive coordination of (L2)-via its (O,O) chelate to the metal core while treatment of the Co-complex with Pd(ii) resulted in the formation of a Co/Pd heterobimetallic complex in solution with (NH2, Namide) chelated Pd(ii). Reaction of 3 with 9-methylguanine indicated the N7 coordination of this simple DNA model to Pd(ii) in a 1 : 1 ratio.

Synthesis, characterization and cytotoxicity studies of Co(III)-flavonolato complexes

Hypoxia activated Co(III) complexes as prodrugs may provide with a selective delivery of cytotoxic or antibacterial compounds. Whithin this field sixteen novel Co(III) ternary complexes with the general formula [Co(4N)(flav)](ClO₄)₂, where 4N = tris(2-aminoethyl)amine (tren) or tris(2-pyridylmethyl)amine (tpa) and flav = deprotonated form of differently substituted flavonols have been synthesized, characterized, and their cytotoxicity assayed under both normoxic and hypoxic conditions. Molecular structures of two free flavonols and seven complexes are also reported. In all the complexes the bioligands exhibited the expected (O,O) coordination mode and the complexes showed a slightly distorted octahedral geometry. Cyclic voltammetric studies revealed that both the substituents of the flavonoles and the type of 4N donor ligands had an impact on the reduction potential of the complex. The ones containing tren demonstrated significantly higher stability than the tpa analogues, making these former compounds promising candidates for the development of hypoxia-activated prodrug complexes. Tpa complexes showed higher activity against both selected human cancer cell lines (A549, A431) than their free ligand flavonols, indicating that the anticancer activity of the bioligand can be enhanced upon complexation. However, slight hypoxia-selectivity was found only for a tren complex (11) with moderate cytotoxicity.

Effect of the replacement of tripodal 4N donors by two 2N chelators on the redox and cytotoxic activity of maltolato and deferipronato containing Co(III) complexes

Fourteen novel Co^III ternary complexes with the general formula [Co(4N)(2O)]X₂ or [Co(2N)₂(2O)]X₂ where 4N = tris(2-aminoethyl)amine (tren) or tris(2-pyridylmethyl)amine (tpa); 2N = 1,10-phenantroline (phen), 2,2'-bipyridine (bipy), 1,2-diaminoethane (en) or 2-(aminomethyl)pyridine (ampy) and 2O = 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (dhpH), 3-hydroxy-2-methyl-4-pyrone (maltH) or 2-ethyl-3-hydroxy-4H-pyran-4-one (etmaltH) were synthesized, characterized and their redox features explored. Molecular structure of some selected [Co(2N)₂(2O)](ClO₄)₂ (2N = phen, bipy, en; 2O = dhp, malt) or [Co(4N)(2O)](ClO₄)₂ (4N = tpa; 2O = etmalt) type complexes were assessed by X-ray diffraction and showed the expected octahedral geometry. Replacement of the 4N donor ligands by two 2N donor ligands resulted in the decrease of the cathodic peak potential of the complexes indicating easier reduction and allowing therefore the tailoring of the redox properties of the complexes. Screening of selected compounds against a human derived cancer cell line, HeLa, showed that, unlike the [Co(4N)(2O)]X₂ derivatives, the complexes containing 2N = bipy or phen ligands have better anticancer activity than cisplatin or carboplatin.