Binding of antitumor ruthenium complexes to DNA and proteins: a theoretical approach

The thermodynamics of the binding of the antitumor ammine, amine, and immine complexes of ruthenium(II) and ruthenium(III) to DNA and peptides was studied computationally using model molecules. We performed density functional calculations on several monofunctional ruthenium complexes of the formula [Ru(NH3)5B]z+, where B is an adenine, guanine, or cytosine nucleobase or an 4-methylimidazole, a dimethylthioether, or a dimethylphosphate anion and z = 2 and 3. The pentammineruthenium fragment has been intensively studied and also constitutes a good model for a wide class of antitumor ammine, amine, and imine complexes of Ru(II) and Ru(III), while the considered bases/ligands have been chosen as models for the main binding sites of DNA, nucleobases, and phosphate backbone and proteins, histidyl, and sulfur-containing residue such as methionine or cysteine. Bond dissociation enthalpies and free energies have been calculated for all the considered metal binding sites both in the gas phase and in solution and allow building a binding affinity order for the considered nucleic acid or protein binding sites. The binding of guanine to some bifunctional complexes, [Ru(NH3)(4)Cl2], [cis-RuCl(2)(bpy)2], and [cis-RuCl(2)(azpy)2], has also been considered to evaluate the effect of a second labile chloro or aquo ligand and more realistic polypyridyl and arylazopyridine ligands.     

Iron(II), manganese(II) and cobalt(II) complexes containing tetradentate biphenyl-bridged ligands and their application in alkane oxidation catalysis

A series of manganese(II), iron(II) and cobalt(II) bis(triflate) complexes containing linear tetradentate bis(imine) and bis(amine) ligands with a biphenyl bridge have been synthesized. The twist in the ligand backbone due to the biphenyl unit leads in the case of the bis(imine) ligands (1 and 2) containing sp2 hybridised N donors, to a distorted cis-alpha coordination geometry, whereas in the case of the biphenyl- and biphenylether-bridged bis(amine) ligands (7 - 9 and 12), a trans coordination geometry is observed. The catalytic properties of the complexes for the oxidation of cyclohexane, using H2O2 as the oxidant, have been evaluated. Only the iron complexes show any catalytic activity under the conditions used, but the low conversions and selectivies observed indicate that these catalysts lead predominantly to free radical auto-oxidation.     

Trans ruthenium(II) complexes with NH-bridged tetradentate symmetric and asymmetric polypyridyl ligands

NH-Bridged tetradentate ligands were synthesized to achieve stable trans Ru(II) bis(polypyridyl) complexes. The polypyridyl part of the ligand was either symmetric, as in N,N-bis(1,10-phenanthroline-2-yl)amine (phen-NH-phen), or asymmetric, as in N-(1,10-phenanthroline-2-yl)-N-(6-yl-dipyridyl[2,3-a:2',3'-c]phenazine)amine (dppz-NH-phen). Protonation of phen-NH-phen with trifluoroacetic acid and the subsequent reaction with RuCl3 yield trans-[Ru(phen-NH-phen)Cl2]. The chloro ligands in this compound can easily be replaced by stronger ligands, such as CH3CN and DMSO. In this way, complexes trans-[Ru(phen-NH-phen)(CH3CN)(DMSO)](PF6)2 (1), trans-[Ru(phen-NH-phen)(DMSO)2](PF6)2 (2), and trans-[Ru(phen-NH-phen)(CH3CN)2](PF6)2 (3) were obtained. X-ray structures were determined for 1 and 3. Following a procedure similar to that used with phen-NH-phen, the complex trans-[Ru(dppz-NH-phen)(CH3CN)2](PF6)2 (4) was obtained. To our knowledge, this is the first reported trans ruthenium(II) bis(polypyridyl) complex with two different polypyridyl ligands in the equatorial plane.     

Bowl adamanzanes--bicyclic tetraamines: syntheses and crystal structures of complexes with cobalt(III) and chelating coordinated oxo-anions

Seven cobalt(III) complexes of the macrobicyclic tetraamine ligand [2(4).3(1)]adamanzane ([2(4).3(1)]adz) are reported along with the crystal structure of six of these complexes. The solid state and solution structures are discussed, and a detailed assignment of the NMR spectra of the sulfato complex is provided. Four of the seven complexes contain a chelate coordinating oxo-anion (sulfate, formiate, nitrate, carbonate). Equilibration of these species with the corresponding diaqua complex is generally slow. The rates of equilibration in 5 mol dm(-3) perchloric acid at 25 degrees C have been measured, yielding half lives of 20 min, 10 min and 3 h for the sulfato, formiato and carbonato species respectively. The corresponding reaction for the nitrato complex occurs with a half life of less than 3 min. The concentration acid dissociation constant for the Co([2(4).3(1)]adz)(HCO(3))(2+) ion has been measured to K(a) = 0.33 mol dm(-3) [25 degrees C, I = 2 mol dm(-3)] and K(a) = 0.15 mol dm(-3) [25 degrees C, I = 5 mol dm(-3)]. The propensity for coordination of sulfate was found to be large enough for a quantitative conversion of the carbonato complex to the sulfato complex to occur in 3 mol dm(-3) triflic acid containing a small sulfate contamination. On this basis the decarboxylation in 5 mol dm(-3) triflic acid of the corresponding cobalt(III) carbonato complex of the larger macrobicyclic tetraamine ligand [3(5)]adz was reinvestigated and found to lead to the sulfato complex as well. The difference in exchange rate of the oxo-anion ligands for the cobalt(III) complexes of the two adamanzane ligands is discussed and attributed to fundamental differences in the molecular structure where an inverted configuration of the secondary non-bridged amine groups is seen for the complexes of the larger [3(5)]adz ligand. The high affinity for chelating coordination of oxo-anions for these two cobalt(iii)-adamanzane-moieties is rationalised on basis of the N-Co-N angles. N-Co-N angles are compared for a series of adamanzane complexes, and the structural consequences are discussed.     

Reactivity trends of cobalt(III) complexes towards various amino acids based on the properties of the amino acid alkyl chains

The reactivity of the cobalt(III) complexes dichlorido[tris(2‐aminoethyl)amine]cobalt(III) chloride, [CoCl₂(tren)]Cl, and dichlorido(triethylenetetramine)cobalt(III) chloride, [CoCl₂(trien)]Cl, towards different amino acids (l ‐proline, l ‐asparagine, l ‐histidine and l ‐aspartic acid) was explored in detail. This study presents the crystal structures of three amino acidate cobalt(III) complexes, namely, (l ‐prolinato‐κ²N,O)[tris(2‐aminoethyl)amine‐κ⁴N,N′,N′′,N′′′]cobalt(III) diiodide monohydrate, [Co(C₅H₈NO₂)(C₆H₁₈N₄)]I₂·H₂O, I , (l ‐asparaginato‐κ²N,O)[tris(2‐aminoethyl)amine‐κ⁴N,N′,N′′,N′′′]cobalt(III) chloride perchlorate, [Co(C₄H₇N₂O₃)(C₆H₁₈N₄)](Cl)(ClO₄), II , and (l ‐prolinato‐κ²N,O)(triethylenetetramine‐κ⁴N,N′,N′′,N′′′)cobalt(III) chloride perchlorate, [Co(C₄H₇N₂O₃)(C₆H₁₈N₄)](Cl)(ClO₄), V . The syntheses of the complexes were followed by characterization using UV–Vis spectroscopy of the reaction mixtures and the initial rates of reaction were obtained by calculating the slopes of absorbance versus time plots. The initial rates suggest a stronger reactivity and hence greater affinity of the cobalt(III) complexes towards basic amino acids. The biocompatibility of the complexes was also assessed by evaluating the cytotoxicity of the complexes on cultured normal human fibroblast cells (WS1) in vitro. The compounds were found to be nontoxic after 24 h of incubation at concentrations up to 25 mM.     

Synthesis and Investigation of Geometrical Isomerism of Trinitro(aminoacidato)amminecobaltate(III) Complex Salts

Synthesis of a new class of coordination compounds of cobalt(III) with amino acids (glycine, alanine, α-amino-butyric acid and norvaline), i.e., trinitro(aminoacidato)-amminecobaltate(III) salts, M[CoNH₃Am(NO₂)₃], is described. The synthesis consists in the action of amino acid alkali salts on the Erdmann salt or peripheral isomer of trinitrotriamminecobalt(III). The same compounds were also obtained by direct synthesis, i.e., by hydrogen peroxide oxidation of cobalt(II) to cobalt(III) in the presence of the corresponding ligands. The peripheral configuration of the complexes obtained was determined by chemical and physical methods (electronic and PMR spectroscopy). The compounds are intermediates formed in obtaining of dinitrobis(aminoacidato)cobaltate(III) salts by the reaction of amino acids with the Erdmann salt or peripheral isomer of trinitrotriamminecobalt(III).     

X-ray photoelectron spectra and structure of cobalt compounds with N-heterocyclic ligands

The electronic structure of cobalt complexes with bi-, tri-, and tetradentate ligands and the mutual influence of ligands in them have been studied by X-ray photoelectron spectroscopy. The Co2p, N1s, and O1s photoelectron spectra have been studied. Unlike low-spin Co(III) complexes, the high-spin Co(II) compound exhibits a strong satellite line in the Co2p spectrum. For the high-spin Co(II) compound having unpaired 3d electrons, the Co2p _1/2-Co2p _3/2 spin-orbit splitting is larger than that in the low-spin Co(III) complexes. All cobalt complexes under consideration contain strongly bound dioxygen, which can be considered an inherent structural unit.     

Synthesis, structure, and physical properties for a series of monomeric iron(III) hydroxo complexes with varying hydrogen-bond networks

Mononuclear iron(III) complexes with terminal hydroxo ligands are proposed to be important species in several metalloproteins, but they have been difficult to isolate in synthetic systems. Using a series of amidate/ureido tripodal ligands, we have prepared and characterized monomeric Fe (III)OH complexes with similar trigonal-bipyramidal primary coordination spheres. Three anionic nitrogen donors define the trigonal plane, and the hydroxo oxygen atom is trans to an apical amine nitrogen atom. The complexes have varied secondary coordination spheres that are defined by intramolecular hydrogen bonds between the Fe (III)OH unit and the urea NH groups. Structural trends were observed between the number of hydrogen bonds and the Fe-O hydroxo bond distances: the more intramolecular hydrogen bonds there were, the longer the Fe-O bond became. Spectroscopic trends were also found, including an increase in the energy of the O-H vibrations with a decrease in the number of hydrogen bonds. However, the Fe (III/II) reduction potentials were constant throughout the series ( approximately 2.0 V vs [Cp 2Fe] (0/+1)), which is ascribed to a balancing of the primary and secondary coordination-sphere effects.     

Cobalt(III) complexes of formyl- and acetylpyrazine N(4)-substituted thiosemicarbazones

Cobalt(III) complexes of acetyl- and formylpyrazine N(4)-substituted thiosemicarbazones have been synthesized and characterized by spectral and physical techniques. The crystal structure of the bis{formylpyrazine N(4)-methylthiosemicarbazone}cobalt(III) complex shows the two ligands coordinated in a mer-configuration, and the orientation of the thiosemicarbazone moiety's N(4)-methyl is Z with respect to the thiolato sulfur in both ligands. Included is the crystal structure of the bis- {2-acetylpyridineN(4)-propylthiosemicarbazone}cobalt(III) complex in which the two thiosemicarbazone ligands are also in the mer-configuration, but one ligand has the propyl group oriented Z with respect to the thiolato sulfur and the other E. Both complexes' bond lengths and bond angles are compared with other cobalt(III) thiosemicarbazone complexes.     

Complete assignments of NMR data of salens and their cobalt (III) complexes

Salens, derived from 1,2‐ethylenediamine and salicylaldehydes, have been widely used as ligands for metal complexes which have been showing enormous potential in chemical properties of asymmetric catalysts as well as biological properties such as anticancer agents. Almost all of the salen–metal complexes with their corresponding metal (II)‐complexes show the evidences of chelation of two oxygens in salens. However, several metal (II) complexes, especially cobalt (II) complexes, could not show NMR spectra due to their paramagnetism. Recently, it has been reported that one of the cobalt (III) complexes was used for NMR spectroscopy to evaluate its stereoselectivity as a catalyst. Even though many salen ligands are known, their NMR data are not assigned completely. It was possible that modification in northern part of salen with 2‐hydroxyphenyl group afforded another oxygen chelation site in salen ligand. Here we report that synthesis and full NMR assignment of new salen ligands, which form meso 1,2‐bis(2‐hydroxyphenyl)ethylenediamine) and their cobalt (III) complexes. The assignments of ¹H and ¹³C NMR data obtained in this experiment can help us to predict the NMR data of other salen ligands. Copyright © 2008 John Wiley & Sons, Ltd.     

Bioreductive activation and drug chaperoning in cobalt pharmaceuticals

The potential for cobalt(III) complexes in medicine, as chaperones of bioactive ligands, and to target tumours through bioreductive activation, has been examined over the past 20 years. Despite this, chemical properties such as reduction potential and carrier ligands required for optimal tumour targeting and drug delivery have not been optimised. Here we review the chemistry of cobalt(III) drug design, and recent developments in the understanding of the cellular fate of these drugs.     

Über Metall-Stickoxid-Komplexe,XXVII. Nitrosyl-Komplexe von Metallen der Eisengruppe mit Aminosäuren und Aldoximen als Chelatbildnern

In addition to the previously described dinitrosyl complexes of iron and cobalt with anions of 1,3-diketones, salicylic acid etc. nitrosyl complexes which contain the anions of amino acids and aldoximes as chelate ligands have been prepared. Corresponding mononitrosyl chelate complexes of nickel are obtained from the dimeric [Ni(NO)P(C₆H₁₁)₃Br]₂. The infra red spectra and magnetic properties of these compounds are reported.     

Stability constants: a new twist in transition metal bispidine chemistry

Transition metal complexes with 2,4-substituted tetradentate, 2,3,4- and 2,4,7-substituted pentadentate, and 2,3,4,7-substituted hexadentate bispidine ligands (bispidine = 3,7-diazabicyclo[3.3.1]nonane) with two tertiary amine and two, three, or four pyridine donors are relatively stable (10 < log K(CuL) < 18). Interestingly, the two isomeric pentadentate ligands have very different stabilities with a variety of metal ions and, depending on the metal ion, one of the isomers leads to more stable complexes than the hexadentate and the other to less stable complexes than the tetradentate ligand. Another interesting observation is that the complex stabilities of all bispidine ligands reported here do not follow the Iriving-Williams series since the stability constants of the cobalt(II) complexes are up to 4 log units larger than those of the corresponding nickel(II) complexes. All these observations are analyzed on the basis of subtle distortions of the coordination geometries, and these have been related previously to Jahn-Teller-derived distortions for the copper(II) complexes. However, similar but less pronounced structural properties are observed with other metal centers, as shown, e.g., with the experimental structures of the two zinc(II) complexes with the isomeric pentadentate ligands reported here. The structural properties and the related stabilities are also discussed on the basis of force field calculations.     

Syntheses,Structures and Characterization of Cobalt(II) and Cobalt(III) Complexes with N-Benzylated Polyamines and a Terminal Azido Ligand

Mononuclear cobalt(II) and cobalt(III) complexes, [Co(trenb)(N₃)]Cl (1) and [Co(dienb)(N₃)₂(OAc)] (2) (trenb = tris[2-(benzylamino)ethyl]amine, dienb = 1,9-diphenyl-2,5,8-triazanonane) were synthesized and characterized by elemental analyses, IR and electronic spectra. Their crystal structures were also determined by X-ray diffraction analyses. In Complex 1, cobalt(II) is five-coordinate trigonal bipyramidal with one azido nitrogen atom and four nitrogen donors of the tripodal ligand; the chloride interacts weakly with one of the secondary amino groups of trenb via a hydrogen bond. In Complex 2, cobalt(III) is in a distorted octahedral coordination environment, consisting of three nitrogen atoms of the amine ligand, two azide nitrogen atoms and an oxygen atom of the acetate ion; a six-membered ring involving the hydrogen bond may stabilize the complex, which maintains its solid geometry in DMF as indicated by the electronic spectrum.     

Electrophilic substitution in phenanthroline coordinated to transition metal ions. Rates of nitration of some rhodium(III) and cobalt(III) complexes

Summary The rates of nitration in sulphuric acid of 1,10-phenanthroline coordinated to rhodium(III) and cobalt(Ill) in a number of complexes have been measured and found to be much greater in these complexes than in free phenanthroline under the same conditions. Relative rates are generally higher in those complexes which bear the smaller overall positive charges, but these charges may be modified by protonation of the other ligands in the complexes.Contrary to earlier reports, some other electrophilic substitutions do not occur in a variety of phenanthroline complexes.     

Cobalt complexes with tripodal ligands: implications for the design of drug chaperones

Extensive research is currently being conducted into metal complexes that can selectively deliver cytotoxins to hypoxic regions in tumours. The development of pharmacologically suitable agents requires an understanding of appropriate ligand-metal systems for chaperoning cytotoxins. In this study, cobalt complexes with tripodal tren (tris-(2-aminoethyl)amine) and tpa (tris-(2-pyridylmethyl)amine) ligands were prepared with ancillary hydroxamic acid, β-diketone and catechol ligands and several parameters, including: pK(a), reduction potential and cytotoxicity were investigated. Fluorescence studies demonstrated that only tpa complexes with β-diketones showed any reduction by ascorbate in situ and similarly, cellular cytotoxicity results demonstrated that ligation to cobalt masked the cytotoxicity of the ancillary groups in all complexes except the tpa diketone derivative [Co(naac)tpa](ClO(4))(2) (naac = 1-methyl-3-(2-naphthyl)propane-1,3-dione). Additionally, it was shown that the hydroxamic acid complexes could be isolated in both the hydroxamate and hydroximate form and the pK(a) values (5.3-8.5) reveal that the reversible protonation/deprotonation of the complexes occurs at physiologically relevant pHs. These results have clear implications for the future design of prodrugs using cobalt moieties as chaperones, providing a basis for the design of cobalt complexes that are both more readily reduced and more readily taken up by cells in hypoxic and acidic environments.     

Dinuclear chromium(V) amino acid complexes from the reduction of chromium(VI) in the presence of amino acid ligands: XAFS characterization of a chromium(V) amino acid complex

The first synthesis and characterization of Cr(V) complexes of non-sulfur-containing amino acids are reported. The reduction of Cr(VI) in methanol in the presence of amino acids glycine, alanine, and 2-amino-2-methylpropanoic acid (alpha-aminoisobutyric acid, Aib) yielded several Cr(V) EPR signals. For the reaction involving glycine, the only Cr(V) EPR signals detected were those of the Cr(V)-intermediate methanol complexes, which were also observed in the absence of amino acids. The reaction involving alanine yielded one Cr(V) signal with a g(iso) value of 1.9754 (a(iso) = 4.88 x 10(-4) cm(-1) and A(iso)(53Cr) = 17.89 x 10(-4) cm(-1)). However, a solid product isolated from the reaction solution was EPR silent and was characterized as a dioxo-bridged dimeric species, [Cr(V)2(mu-O)2(O)2(Ala)2(OCH3)2](2-), by multiple-scattering XAFS analysis and electrospray mass spectrometry. The EPR spectrum of the reduction reaction of Cr(VI) in the presence of Aib showed several different Cr(V) signals. Those observed at lower g(iso) values (1.9765, 1.9806) were assigned to Cr(V)-methanol intermediates, while the relatively broad six-line signal at g(iso) = 2.0058 was assigned as being due to a Cr(V) complex with coupling to a single deprotonated amine group of the amino acid. This was confirmed by simplification of the superhyperfine coupling lines from six to three when the deuterated ligand was substituted in the reaction. The reduction of Cr(VI) with excess alanine or Aib ligands resulted in the formation of tris-chelate Cr(III) complexes, which were analytically identical to complexes formed via Cr(III) synthesis methods. The fac-[Cr(Aib)3] complex was characterized by single-crystal X-ray diffraction.     

Spectral studies, cyclic voltammetry and synthesis of cobalt(II) and ruthenium(III) complexes with symmetric and asymmetric ring containing membered N2S2, N4, and N5 donor macrocyclic ligands

Reaction of divalent cobalt(II) and trivalent ruthenium(III) salts (NO3, SCN and SO4) with macrocyclic ligands L1, L2 and L3 having N2S2, N4 and N5 core, have been designed and carry out. All these three macrocyclic ligands and their complexes were obtained in pure form. Their structures were investigated by using microanalytical analyses, IR, mass, magnetic moments, electronic and EPR spectral studies. The redox properties of the complexes were also examined by cyclic voltammetry. An interesting feature of complexes is that the relatively large rings of macrocyclic ligands prevent the macrocyclic rings from approaching the metal center as closely as they would, if they were not constrained. So the Ru-N distances are longer than expected due to ring size. Electrochemical studies show that the macrocyclic ligand L1 is more effective electron donors to ruthenium than of L2 and L3. Electronic spectral properties also show that the sulphur donor atom of L1 weakens the ligand field with respect to ligand-to-metal charge-transfer band. However it is expected that second-row transition metal-ligand bonds tend to be weaker than third-row transition metal-ligand bonds. There are well-established examples of reactions in which decreased of reactivity down a triad of transition metals is not observed. These novelties are usually attributed to pi-bonding effects for ligands such as carbon monoxide, solvent effects, or a change in mechanism.     

Polynuclear lanthanide complexes of a series of bridging ligands containing two tridentate N,N',O-donor units: structures and luminescence properties

A set of three potentially bridging ligands containing two tridentate chelating N,N',O-donor (pyrazole-pyridine-amide) donors separated by an o, m, or p-phenylene spacer has been prepared and their coordination chemistry with lanthanide(III) ions investigated. Ligand L(1) (p-phenylene spacer) forms complexes with a 2:3 M:L ratio according to the proportions used in the reaction mixture; the Ln(2)(L(1))(3) complexes contain two 9-coordinate Ln(III) centres with all three bridging ligands spanning both metal ions, and have a cylindrical (non-helical) 'mesocate' architecture. The 1:1 complexes display a range of structural types depending on the conditions used, including a cyclic Ln(4)(L(1))(4) tetranuclear helicate, a Ln(2)(L(1))(2) dinuclear mesocate, and an infinite one-dimensional coordination polymer in which metal ions and bridging ligands alternate along the sequence. ESMS studies indicate that the 1:1 complexes form a mixture of oligonuclear species {Ln(L(1))}(n) in solution (n up to 5) which are likely to be cyclic helicates. In contrast, ligands L(2) and L(3) (with o- and m-phenylene spacers, respectively) generally form dinuclear Ln(2)L(2) Ln(III) complexes in which the two ligands may be arranged in a helical or non-helical architecture about the two metal ions. These complexes also contain an additional exogenous bidentate bridging ligand, either acetate or formate, which has arisen from hydrolysis of solvent molecules promoted by the Lewis-acidity of the Ln(III) ions. Luminescence studies on some of the Nd(III) complexes showed that excitation into ligand-centred pi-pi* transitions result in the characteristic near-infrared luminescence from Nd(III) at 1060 nm.     

Phosphorescence imaging of living cells with amino acid-functionalized tris(2-phenylpyridine)iridium(III) complexes

A series of nine luminescent cyclometalated octahedral iridium(III) tris(2-phenylpyridine) complexes has been synthesized, functionalized with three different amino acids (glycine, alanine, and lysine), on one, two, or all three of the phenylpyridine ligands. All starting complexes and final compounds have been fully analyzed by one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy, and photophysical data have been obtained for all the mono-, bis-, and tri- substituted iridium(III) complexes. Cellular uptake and localization have been studied with flow cytometry and confocal microscopy, respectively. Confocal experiments demonstrate that all nine substituted iridium(III) complexes show variable uptake in the tumor cells. The monosubstituted iridium(III) complexes give the highest cellular uptake, and the series substituted with lysines shows the highest toxicity. This systematic study of amino acid-functionalized Ir(ppy)(3) complexes provides guidelines for further functionalization and possible implementation of luminescent iridium complexes, for example, in (automated) peptide synthesis or biomarker specific targeting.