Redox properties of ruthenium(III/II)–edta complexes with o-phenylenediamine and o-benzoquinone diimine ligands

The reaction of [Ru^III(edta)(H₂O)]⁻ with o-phenylenediamine (opda) in water, under aerobic conditions, affords the diamagnetic [Ru^II(edta)(bqdi)]²⁻ product (where edta stands for the ethylenediaminetetraacetate co-ligand, and bqdi represents the non-innocent o-benzoquinone α,α^′-diimine ligand). In the current communication, the redox chemistry of this system in aqueous solution is described in details on the basis of electrochemical and spectroelectrochemical studies. The electrochemical behavior of “free” opda is rather complicated with further chemical reactions following the irreversible two-proton/two-electron oxidation (opda→bqdi+2e⁻+2H⁺), whereas its complex is electrochemically well-behaved with two chemically reversible redox processes: the monoelectronic couple associated with the metal ion (Ru^III/Ru^II) and another bielectronic step centered on the coordinated ligand (bqdi/opda). The set of UV–Vis electronic spectra were obtained by electrolytical generation, in situ, of all the redox species accessible in the CV working conditions (i.e., the starting [Ru^II(edta)(bqdi)]²⁻, the fully oxidized [Ru^III(edta)(bqdi)]⁻, and the fully reduced [Ru^II(edta)(opda)]²⁻ species), which are stable and totally interconvertible. The electrochemistry and absorption spectroscopy of these complexes in water were found to be comparable with the tetraammine counterparts. A remarkable difference in redox behavior between the diimine- and the analogous dioxolene-complexes was also revealed by comparison of the system reported herein with the one derived from catechol, and rationalized in terms of the quite efficient π-accepting electronic nature of the bqdi ligand.     

Synthesis,characterization and photophysical properties of homoleptic platinum(II) complexes with 2,2′-biimidazole-based ligands

Eight pairs of cis–trans isomeric homoleptic platinum(II) complexes based on N-alkyl- or aryl-substituted 2,2′-biimidazole ligands were synthesized, and their photophysical properties were investigated. The cis and trans isomers readily interconvert at slightly elevated temperature, implying that the activation barrier for this process is low. Single crystal X-ray diffraction analysis revealed that the complexes have an ideal square-planar geometry. Their UV–Vis spectra showed lower energy absorption bands in the range of 345–378 nm, which are assigned to the typical MLCT mixed with LC transitions. In frozen glass solution at 77 K and also in the powder state, these complexes exhibit green emission ranging from 440 to 540 nm with photoluminescence quantum yields of 3.3–24.4%. The emitting excited state is dominated by ³ππ* character with some contributions from ³MLCT according to the excitation spectra.     

Effects of weak interactions on spin crossover properties of iron(II) complexes with extended π-conjugated Schiff-base ligands

The preparation and magnetic properties of two Fe(II) Schiff-base complexes, [Fe(qnal-12)₂]·2C₆H₆ (1) and [Fe(Hqsalc)₂] (2), (Hqnal-12 = N-(8′-quinolyl)-1-hydroxy-2-naphthaldimine, H₂qsalc = 4-hydroxy-3-[(8-quinolinylimino)methyl]benzoic acid) are reported. X-ray single crystal structure analysis of 1 reveals that an Fe(II) ion is coordinated by two qnal-12 ligands in a meridional fashion. Molecular packing of 1 shows that a qnal-12 interacts with neighboring two qnal-12’s through π-π interactions, which results in the formation of one-dimensional chain. Although the magnetic property of 1 shows a high-spin state at all the temperature range measured, the χT-T plot of 2 shows abrupt spin crossover behavior with a wide hysteresis of 21 K, probably due to the hydrogen-bond network originated by carboxyl groups.     

Cyclometalated platinum(II) complexes as topoisomerase IIα poisons

A platinum(II)-based major groove binder [Pt(II)(C^N)(C≡NR)(2)](+) (HC^N = 2-phenylpyridine (phpy), R = 2-naphthyl) was identified as a potent human topoisomerase IIα poison. It stabilizes the covalent TopoIIα-DNA cleavage complex and induces cancer cell death with potency significantly higher than the widely clinically used TopoIIα poison Vp-16.     

Palladium complexes with biological ligands—I. Equilibrium and reaction mechanism of Pd(II) complexes with ethionine

An equilibrium study has been carried out on the interaction of ethionine(eth) with Pd(II) in aqueous solution at I = 0.16 M (Cl⁻ and 25°C using potentiometic methods. It has been concluded that five complex species exist in the pH range 2.8–4.8. these species are: PdCl₃(Eth0H⁰₂, PdCl₂(Eth)⁻, PdClOH(Eth)⁻, Pd(Eth)₂(H)²⁺₂ and Pd(Eth)⁰₂. In addition, the stopped-flow method has been used to study the reaction kinetics of Pd(II) with Eth. Three kinetic steps were observed in the pH range 1–5.5. These steps are dependent on the total concentration of Eth (T_Eth) as well as the pH of the medium. The observed pseudo-first order rate constants for the three reaction kinetic steps at constant pH are expressed empirically by kⁱ_obs = m_i + m′_iT_Eth. The parameters m_i and m′_i are pH-dependent. It has been concluded that PdCl²⁻₄ and PdCl₂OH²⁻ species play an important role in the complex formation reactions with Eth. The data were interpreted in terms of the complex species obtained from the equilibrium study. cis-trans substitution reactions have been suggested to account for some kinetic steps.     

Synthesis and X-ray structure of platinum(II), palladium(II) and copper(II) complexes with pyridine–pyrazole ligands: Influence of ligands’ structure on cytotoxic activity

The new pyrazole ligand 5-(2-hydroxyphenyl)-3-methyl-1-(2-pyridylo)-1H-pyrazole-4-phosphonic acid dimethyl ester (2a) has been used to obtain a series of platinum(II), palladium(II) and copper(II) complexes (3a–7a) as potential anticancer compounds. The molecular structures of the platinum(II) and copper(II) complexes 3a and 6a have been determined by X-ray crystallography. The cytotoxicity of the phosphonic ligand 2a and its carboxylic analog 2b as well as their complexes has been evaluated on leukemia and melanoma cell lines. Copper(II) complexes were found to be more efficient in the induction of melanoma cell death than the platinum(II) or palladium(II) complexes. Cytotoxic effectiveness of compound 7b against melanoma WM-115 cells was two times better than that of cisplatin. The reaction of compound 5b with 9-methylguanine has been studied.     

Bis(imidate)palladium(II) complexes with labile ligands. Mimics of classical precursors?

A novel synthetic route to prepare palladium(II) precursor analogous of classical [Pd(Cl)(2)(solvent)(2)] has been developed. Just stirring Pd(3)(AcO)(6) in dimethyl sulfide at room temperature, in the stoichiometric presence of protic imidate ligands, resulted in the precipitation of the desired complexes [Pd(imidate)(2)(SMe(2))(2)] (imidate = succinimidate (succ) 1, phthalimidate (phthal) 2, maleimidate (mal) 3, saccharinate (sac) 4 or glutarimidate (glut) 5). The new complexes are very soluble in common solvents and have been fully characterized, including an X-ray diffraction analysis of 2. Analogous reactions with succinimide in acetonitrile or dimethylsulfoxide produced [Pd(succinimidate)(2)(solvent)(2)] (6 and 7, respectively) as off-white powders. Thermal decomposition of 6 produces a new species 6* with bridging imidate ligands that can be formulated as a trimer similar to Pd(3)(AcO)(6). The usefulness of 1-5 as precursors has been tested by reactions against monodentated neutral donor ligands, PPh(3) (a compounds), or pyridine (py, b compounds), to produce ten new derivatives of the general formula trans-[Pd(imidate)(2)(L)(2)]. The single-crystal structures of compounds 2a, 3a, 4a, 4a', 5a and 4b have also been established, allowing an interesting molecular and supramolecular structural discussion. A cis-conformation was induced when the bidentate chelate ligand 1,2-bis(diphenylphosphino)benzene (dppb, c compounds) was made to react with 1-5. Structural characterization by X-ray diffraction of complex 2c confirmed the proposed formula. Catalytic activity in Suzuki-Miyaura cross-coupling of aryl bromides and benzyl bromides with aryl boronic acids has been tested.     

Coordination interactions of alkyl-substituted 2,2′-dipyrrolylmethene derivatives with copper(II) aminoacid complexes

Using the method of electron spectroscopy we showed that reaction of alkyl-substituted 2,2′-dipyrrolylmethene derivatives with copper(II) aminoacid complexes led to the formation of heteroligand complexes with two chelated metallocycles forming their coordination sphere. Formation constants of the heteroligand complexes were established and their interrelations with the structure of the aminoacid residue side group were elucidated. It was found that alongside the ability to the primary solvolytic dissociation of aminoacid complex, the main effect on the formation of combined coordination sphere of the chelate is defined by the steric factor depending on the structure of substituent in the aminoacid.     

Heteroleptic ruthenium(II) polypyridine complexes with a series of substituted 2,2′-bipyridine ligands

Five substituted-2,2′-bipyridine ligands L, (4-(p-methylphenyl)-6-phenyl-2,2′-bipyridine (L1), 4-(p-bromophenyl)-6-(p-bromophenyl)-2,2′-bipyridine (L2), 4-(p-bromophenyl)-6-phenyl-2,2′-bipyridine (L3), 4-phenyl-6-(p-bromophenyl)-2,2′-bipyridine (L4), and 4-(p-fluorophenyl)-6-(p-fluorophenyl)-2,2′-bipyridine (L5) were synthesized by stepwise formation. Reaction of cis-[RuCl₂(bpy)₂]?2H₂O or cis-[RuCl₂(phen)₂]?2H₂O and the substituted-2,2′-bipyridine ligands in the presence of KPF₆ afforded the corresponding cationic polypyridine-ruthenium complexes of the type [(bpy)₂Ru(L)](PF₆)₂ (bpy = 2,2′-bipyridine, 1–5) or [(phen)₂Ru(L)](PF₆)₂ (phen = 1,10-phenanthroline, 6–10), respectively. All complexes have been spectroscopically characterized by UV–vis, luminescence, and electrogenerated chemiluminescence. The structures of 1?CH₃COCH₃, 3?CH₃COCH₃, 5?2CH₃COCH₃, 6, 8, 9, and 10 have been determined by single-crystal X-ray diffraction.     

Crystal structures and spectroscopic properties of copper(II)–dipicolinate complexes with benzimidazole ligands

The syntheses, crystal structures and spectroscopic properties of three Cu(II)–dipicolinate complexes with benzimidazole ligands, namely [Cu(bzim)(dipic)(MeOH)] (1), [Cu₂(2-Etbzim)₂(dipic)₂]_n·0.5nH₂O (2) and [Cu₂(2-ⁱPrbzim)₂(dipic)₂]_n (3), where dipic?=?dipicolinate, bzim?=?1-H-benzimidazole, 2-Etbzim?=?2-ethyl-1-H-benzimidazole and 2-iPrbzim?=?2-isopropyl-1-H-benzimidazole, are reported. Crystal structure studies revealed different coordination modes of the dipicolinate ligands; tridentate chelating for monomeric complex 1, and both tridentate chelating and bridging for similar polymeric complexes 2 and 3. Polymers 2 and 3 both contain two units, in which the Cu(II) central atoms Cu1 and Cu2 have different coordination polyhedra. The first unit {Cu(dipic)₂} with Cu1 is connected to the second via two bidentate carboxylate groups of an μ₃-bridging dipicolinate. In the second unit, Cu2 is coordinated by two imidazole nitrogen atoms from 2-ethyl-1-H-benzimidazole (2) or 2-isopropyl-1-H-benzimidazole (3) ligands. Complex 2 is of higher symmetry and has a localized Cu(II) atom Cu2 in a special position on the twofold axis. EPR spectra of all three Cu(II) complexes, which were measured at both room temperature and 98 K, indicate distorted tetragonal coordination spheres for all the Cu(II) atoms. The g-factor relation (g_//>?g_┴?>?2.0023) is consistent with a \(d_{{x^{2} - y^{2} }}\) ground electronic state in each case.     

Cytotoxic properties of platinum(IV) and dinuclear platinum(II) complexes and their ligand substitution reactions with guanosine-5′-monophosphate

The substitution reaction of the Pt(IV) complex [PtCl₄(bipy)] with guanosine-5??-monophosphate (5??-GMP) was studied by UV?CVis spectrophotometry. This reaction was investigated under pseudo-first-order conditions at 37?°C in 25?mM Hepes buffer (pH?=?7.2) in the presence of 10?mM NaCl to prevent the hydrolysis of the complex. The substitution of chlorides in [{trans-Pt(NH₃)₂Cl}₂(??-1,2-bis(4-pyridyl)ethane)](ClO₄)₂ (Pt3) complex by 5??-GMP was followed by ¹H NMR spectroscopy under second-order conditions. Very similar values for the rate constants of both substitution steps were obtained. The Pt(IV) complexes, [PtCl₄(bipy)] and [PtCl₄(dach)], as well as dinuclaer Pt(II) [{trans-Pt(NH₃)₂Cl}₂(??-pyrazine)](ClO₄)₂ (Pt1), [{trans-Pt(NH₃)₂Cl}₂(??-4,4??-bipyridyl)](ClO₄)₂?·?DMF (Pt2) and [{trans-Pt(NH₃)₂Cl}₂(??-1,2-bis(4-pyridyl)ethane)](ClO₄)₂ (Pt3) complexes, displayed potent cytotoxic activity against human ovarium carcinoma cell line TOV21G and lower activity toward human colon carcinoma HCT116 cell line at the same concentrations. Our data indicate that these platinum complexes could be explored further, as potential therapeutic agents for ovarian cancer.     

Nickel(II) complexes of some quadridentate Schiff-base ligands—II. Infrared spectra

The infrared spectra (4000-200cm⁻¹) of a series of nickel(II) complexes of Schiff bases formed by the condensation of 5-chloro-2-hydroxybenzophenone and some alkanediamines, H₂N(CH₂)_nNH₂ (n = 2–12), has been investigated. The nickel⁶² isotope substitution method has been used to help identify the metal sensitive vibrations.     

Synthesis and magnetic characterization of Ln(III)–Co(II) complexes with 1,10-phenanthroline ligands

Four heteronuclear complexes, [Ln₂Co₂L₁₀(H₂O)(phen)₂] · n(H₂O) (Ln = La 1, n = 2; Ln = Nd 2, Sm 3, Gd 4, n = 0; HL = α-methylacrylic acid, phen = 1,10-phenanthroline), have been synthesized and characterized by elemental analysis, IR and X-ray diffraction. The complexes with a discrete Co–Ln–Ln–Co tetranuclear molecule are isomorphous in the triclinic space group P 1 and Z = 1, in which all metal ions are bridged by bidentate α-methylacrylato groups. Magnetic measurements of 1, 2 and 3 show antiferromagnetic exchange interaction between paramagnetic centers.     

Transition metal complexes with the thiosemicarbazide-based ligands—I. Nickel(II) complexes with the quadridentate ligands based on S-methylisothiosemicarbazide; X-ray crystal structure of (acetyl-acetone N(1)-salicylidene-S-methylizothiosemicarbazonato) nickel(II)

The reaction of a warm ethanolic solution of [Ni(SALSMeTSC-H)Py]Cl·0.5Py (SALSMeTSC = salicylaldehyde S-methylisothiosemicarbazone) with salicylaldehyde and acetylacetone (HACAC), yielded the corresponding square-planar complexes [Ni(SAL₂SMeTSC-2H)] (A) and [Ni(SALACACSMeTSC-2H)] (B) (SAL₂SMeTSC, and SALACACSMeTSC = quadridentate 2O2N ligand: N(1),N(3)-bis(salicylidene)-S-methylisothiosemicarbazide, and acetylacetone N(1)-salicylidene-S-methylisothiosemicarbazone, respectively). An X-ray analysis of complex B showed that in the reaction of the starting complex with HACAC a rearrangement of the salicylaldehyde moiety takes place (while the CN(3) bond of the azomethine group is ruptured) and its binding to the N(1) nitrogen. At the same time, HACAC is simultaneously bonded to the liberated N(3)-nitrogen of the hydrazine fragment. Crystal data for the complex B (NiC₁₄H₁₅N₃O₂S) are: M_r = 348.0, orthorhombic, space group Pna2₁, a = 7.484(3), b = 21.995(8), c = 8.866(3) Å; V = 1459.44 ų, Z = 4, F(000) = 720, Dc = 1.58 g cm⁻¹, Do = 1.56 g cm⁻¹, μ(MoKα) = 14.45 cm⁻¹. The structure was solved by the heavy-atom method and refined anisotropically to an R value of 0.078 for 1174 non-zero reflections. The complex molecules are planar, the NiNi distance of nearest molecules being about 3.76 Å. The compounds have been characterized by elemental analysis as well as by IR and electronic spectra.     

Nickel(II), copper(II), palladium(II) and platinum(II) complexes of bidentate SN ligands derived from S-alkyldithiocarbazates and the X-ray crystal structures of the [Ni(tasbz)2] and [Cu(tasbz)2] · CHCl3 complexes

New complexes of general empirical formula, [M(NS)₂] · nCHCl₃ (M = Ni^II, Cu^II, Pd^II or Pt^II; NS = anionic form of the thiophene-2-aldehyde Schiff bases of S-methyl- and S-benzyldithiocarbazate; n = 0, 1) have been synthesized and characterized by physico-chemical techniques. Magnetic and spectroscopic evidence support a square-planar structure for these complexes. The crystal structures of the [Ni(tasbz)₂] and [Cu(tasbz)₂] · CHCl₃ complexes (tasbz = anionic form of the thiophene-2-aldehyde Schiff base of S-benzyldithiocarbazate) have been determined by X-ray diffraction. Both complexes have a trans-planar structure in which the two Schiff base ligands are coordinated to the metal(II) ion as uninegatively charged bidentate ligands via the thiolate sulfur and the azomethine nitrogen atoms. This revised version was published online in June 2006 with corrections to the Cover Date.     

A 3-dimensional spiro-functionalized platinum(II) complex to suppress intermolecular π-π and Pt···Pt supramolecular interactions for a high-performance electrophosphorescent device

A 3-D platinum(II)-based spirometal complex has been designed and synthesized to suppress aggregation and excimer emission. A prototype phosphorescent organic light-emitting device exhibits the high-performance orange emission with an external quantum efficiency of up to 5.2%.     

Electrochemical properties of 2,2":6",2″-terpyridine complexes of platinum(ii) with arenethiolate ligands. Alkylation of reduced forms of the complexes

Three mixed-ligand terpyridine-thiolate Pt^II complexes containing 2,2":6",2-terpyridine (tpy) and para-substituted thiophenolate ions, [(tpy)Pt(SC₆H₄-4-X)]BF₄ (X = NMe₂, H, NO₂) were synthesized. The complexes were isolated as chlorides or tetrafluoroborates and characterized by electrochemistry, visible electronic spectroscopy and ¹H NMR spectroscopy, and elemental analysis. Remarkable influence of the electron-donating/withdrawing properties of the substituent in the thiolate ligand on the physicochemical properties of the complexes, in particular, on the electrochemical reduction potentials, was found. The reduced forms of the complexes undergo electrochemically initiated alkylation with alkyl halides, resulting in the formation of Pt alkyl compounds.     

Mixed neutral compounds of palladium(II) and platinum (II) chelated by diolato(2−) and di-imine ligands

The synthesis and characterization are described for compounds abbreviated (a) 1–5: [Pd(phen)(OO)], where OO = the dianion from 1,2-ethanediol (1), (+)-1,2-propanediol (2), (±)-2,3-butanediol (3), (−)-1,2-butanediol (4), catechol (5); (b) the sulphur analogue (6) [Pd(phen)(SCH₂CH₂S)], from ethane-1,2-dithiol; (c) the platinum analogue (7) [Pt(phen)(OCH₂CH₂O)]; (d) the 2,2′-bipyridyl analogue (8), [Pd(bipy)(OCH₂CH₂O)] (phen = 1,10-phenanthroline and bipy = 2,2′-bipyridyl).