Temperature and solvent structure dependence of VO2+ complexes of pyridine-N-oxide derivatives and their interaction with human serum transferrin

The behaviour of the systems formed by VO(2+), 2-hydroxypyridine-N-oxide (Hhpo) and 2-mercaptopyridine-N-oxide (Hmpo) was studied both in solution and in the solid state through the combined application of spectroscopic (EPR and UV-Vis spectroscopy) and DFT methods. The geometry of solid bis-chelated complexes [VOL(2)], with L = hpo and mpo, is square pyramidal, but it can change to cis-[VOL(2)S], where S is a solvent molecule, when these are dissolved in a coordinating solvent. The equilibrium between the square pyramidal and cis-octahedral forms is strongly affected by solvent and temperature. At room temperature, the predominant species is [VOL(2)], which gives a pink colour to the solutions; at lower temperatures, the equilibrium is shifted--partially or completely--toward the formation of cis-[VOL(2)S], which is green. In an acidic environment and in the presence of an excess of ligand, [VOL(2)] can transform into the tris-chelated complex [VL(3)](+), in which vanadium loses the oxido ligand and adopts a hexa-coordinated geometry intermediate between octahedral and trigonal prismatic. 1-Methylimidazole (1-MeIm), which represents a model for His-N coordination, forms mixed complexes with stoichiometry cis-[VOL(2)(1-MeIm)], occupying an equatorial position. In the ternary systems VO(2+)-Hhpo-hTf and VO(2+)-Hmpo-hTf at room temperature and pH 7.4, besides (VO)hTf and (VO)(2)hTf, the mixed species cis-VO(hpo)(2)(hTf) and VO(mpo)(hTf) are observed, with the equatorial binding of an accessible histidine residue. Finally, the contribution of the N-oxide group to (51)V A(z) and A(iso) hyperfine coupling constants, which can be important in the characterisation of similar species, is discussed.     

Evaluation of the binding of oxovanadium(IV) to human serum albumin

The understanding of the biotransformations of insulin mimetic vanadium complexes in human blood and its transport to target cells is an essential issue in the development of more effective drugs. We present the study of the interaction of oxovanadium(iv) with human serum albumin (HSA) by electron paramagnetic resonance (EPR), circular dichroism (CD) and visible absorption spectroscopy. Metal competition studies were done using Cu(II) and Zn(II) as metal probes. The results show that V(IV)O occupies two types of binding sites in albumin, which compete not only with each other, but also with hydrolysis of the metal ion. In one of the sites the resulting V(IV)O-HSA complex has a weak visible CD signal and its X-band EPR spectrum may be easily measured. This was assigned to amino acid side chains of the ATCUN site. The other binding site shows stronger signals in the CD in the visible range, but has a hardly measurable EPR signal; it is assigned to the multi metal binding site (MBS) of HSA. Studies with fatted and defatted albumin show the complexity of the system since conformational changes, induced by the binding of fatty acids, decrease the ability of V(IV)O to bind albumin. The possibility and importance of ternary complex formation between V(IV)O, HSA and several drug candidates - maltol (mal), picolinic acid (pic), 2-hydroxypyridine-N-oxide (hpno) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (dhp) was also evaluated. In the presence of maltol the CD and EPR spectra significantly change, indicating the formation of ternary VO-HSA-maltol complexes. Modeling studies with amino acids and peptides were used to propose binding modes. Based on quantitative RT EPR measurements and CD data, it was concluded that in the systems with mal, pic, hpno, and dhp (V(IV)OL(2))(n)(HSA) species form, where the maximum value for n is at least 6 (mal, pic). The degree of formation of the ternary species, corresponding to the reaction V(IV)OL(2) + HSA -->/<-- V(IV)OL(2)(HSA) is hpno > pic ≥ mal > dhp. (V(IV)OL)(n)(HSA) type complexes are detected exclusively with pic. Based on the spectroscopic studies we propose that in the (V(IV)OL(2))(n)(HSA) species the protein bounds to vanadium through the histidine side chains.     

Interaction of [VIVO(acac)2] with Human Serum Transferrin and Albumin

[VO(acac)₂] is a remarkable vanadium compound and has potential as a therapeutic drug. It is important to clarify how it is transported in blood, but the reports addressing its binding to serum proteins have been contradictory. We use several spectroscopic and mass spectrometric techniques (ESI and MALDI‐TOF), small‐angle X‐ray scattering and size exclusion chromatography (SEC) to characterize solutions containing [VO(acac)₂] and either human serum apotransferrin (apoHTF) or albumin (HSA). DFT and modeling protein calculations are carried out to disclose the type of binding to apoHTF. The measured circular dichroism spectra, SEC and MALDI‐TOF data clearly prove that at least two VO–acac moieties may bind to apoHTF, most probably forming [V^IVO(acac)(apoHTF)] complexes with residues of the HTF binding sites. No indication of binding of [VO(acac)₂] to HSA is obtained. We conclude that V^IVO–acac species may be transported in blood by transferrin. At very low complex concentrations speciation calculations suggest that [(VO)(apoHTF)] species form.     

A new family of insulin-mimetic vanadium complexes derived from 5-carboalkoxypicolinates

The reaction of 5-carboalkoxypicolinic acid (5 ROpicH, R=Me, Et, iPr, sBu; 1 a-d) with vanadyl sulfate yielded the complexes [VO(H(2)O)(5 ROpic)(2)], 2 a-d, with H(2)O and one of the picolinato ligands in the equatorial positions, and the second picolinate occupying equatorial (N) and axial (O) positions. Reaction of 1 a with [NH(4)][VO(3)] yielded [NH(4)][VO(2)(5 MeOpic)(2)], [NH(4)]-3, in which the N functions of the picolinates are trans to the doubly bonded, cis-positioned oxo groups. Complexes 1 a.H(2)O, 1 b, 1 c, 2 a.3.5 H(2)O and [NH(4)]-3.4 H(2)O have been structurally characterised. A detailed pH-potentiometric solution speciation analysis of the system VO(2+)-1 a revealed a dominance of VO(5 OMepic)(2) between pH 2 and 6, with the same coordination pattern, evidenced by EPR spectroscopy, as in the crystalline solid state. In ternary systems containing physiological concentrations of the low molecular mass biogenic binders (B) lactate, oxalate, citrate or phosphate, ternary species of general composition VO(5 MeOpic)B dominate at physiological pH, with citrate being the most effective competitor for picolinate. All of the complexes trigger glucose uptake and degradation by simian virus modified mice fibroblasts at non-toxic concentrations (<100 microM), with 2 a, [VO(2)(pic)(2)](-) and [VO(2)(dipic)](-) being at least as effective as insulin. Vanadium uptake by the cells is most effective in the case of 2 a. 2 a also effectively inhibits free fatty acid release by rat adipocytes treated with epinephrine, thus mimicking the inhibition of lipolysis by insulin.     

Coordination chemistry and insulin-enhancing behavior of vanadium complexes with maltol C6H6O3 structural isomers

Syntheses of vanadium complexes using the naturally occurring ligands isomaltol (Hima) and allomaltol (Hama), as well as a newly synthesized, potentially tetradentate diaminodipyrone [H(2)(en(ama)(2)], are reported. Complete characterization of the resulting compounds [trans-VO(ima)(2)(H(2)O), VO(ama)(2), V(ima)(3), V(ama)(3) and VO(en(ama)(2))], including X-ray crystallography analyses for trans-VO(ima)(2)(H(2)O) and V(ima)(3), are presented herein. Potentiometric titrations (25 degrees C, I = 0.16 M NaCl) were used to measure stability constants in the V(IV)-Hima system; these data were compared to previous data collected on the V(IV)-L (L = Hma, Hama) systems. The in vivo efficacy of these compounds to lower the blood glucose levels of STZ-diabetic rats was tested; all but VO(en(ama)(2)) produced significant decreases in plasma glucose levels. The results were compared to those of the benchmark compound BMOV [VO(ma)(2), bis(maltolato)oxovanadium(IV)], a known insulin-enhancing agent.     

Solution equilibria and antitumor activities of pentamethylcyclopentadienyl rhodium complexes of picolinic acid and deferiprone

Complex formation processes of rhodium(III)-η⁵-pentamethylcyclopentadienyl cation [RhCp*(H₂O)₃]²⁺ with 1,2-dimethyl-3-hydroxy-pyridin-4(1H)-one (deferiprone, dhp) and pyridine-2-carboxylic acid (pic) were studied with the aid of pH-potentiometry, ¹H NMR, and UV–Visible spectrophotometry in aqueous solution in the presence and absence of chloride ions. Stoichiometry and overall stability constants of the complexes formed were determined. Formation of mononuclear, monoligand complexes such as [RhCp*(L)Z] (where L = dhp or pic; Z = Cl^? or H₂O) and mixed hydroxido species [RhCp*(L)(OH)] were found. Relatively high pK_a values (9.32–11.90) were determined for the hydrolysis of the [RhCp*(L)Z] complexes. [RhCp*(L)Z] species predominate at physiological pH and negligible decomposition is probable only at low micromolar concentrations. More favored complex formation was found in the case of pic. Stability of the studied organorhodium complexes was compared with analogous Ru(II)(η⁶-p-cymene) compounds. In addition, the aqua/chlorido ligand replacement reaction in [RhCp*(L)(H₂O)]⁺ of dhp and pic was monitored to provide equilibrium constants with which the extent of aquation at various chloride concentrations can be estimated. Single crystals of [RhCp*(dhp)Cl] suitable for X-ray diffraction analysis were also obtained. The [RhCp*(L)Cl] complexes of dhp and pic were tested for cytotoxicity in various human cancer cell lines where they showed activity depending on the attached ligand scaffold.     

Synthesis and photoluminescence of iridium complexes with benzothiazol-2-yl carbazole derivative ligand

Two new iridium(III) complexes containing benzothiazol-2-yl carbazole derivative as a cyclometalated ligand (L) and picolinate (pic) or acetylacetonate (acac) as the ancillary ligand, Ir(III) bis(3-(benzothiazol-2-yl)-9-butyl-carbazole)(picolinate) [Ir(L)₂(pic)] and Ir(III) bis(3-(benzothiazol-2-yl)-9-butyl-carbazole)(acetylacetonate) [Ir(L)₂(acac)], were synthesized and characterized by elemental analysis, ¹H NMR, FT-IR, and UV–Vis absorption spectra. Both the iridium(III) complexes emit intense green–yellow emissions, indicating that they are useful for the fabrication of organic light-emitting diodes.     

Structural models of vanadate-dependent haloperoxidases and their reactivity

Vanadium(V) complexes with hydrazone-based ONO and ONN donor ligands that partly model active-site structures of vanadate-dependent haloperoxidases have been reported. On reaction with [VO(acac)₂] (Hacac = acetylacetone) under nitrogen, these ligands generally provide oxovanadium(IV) complexes [VO(ONO)X] (X = solvent or nothing) and [VO(acac)(ONN)], respectively. Under aerobic conditions, these oxovanadium(IV) species undergo oxidation to give oxovanadium(V), dioxovanadium (V) or μ-oxobisoxovanadium(V) species depending upon the nature of the ligand. Anionic and neutral dioxovanadium(V) complexes slowly deoxygenate in methanol to give monooxo complexes [VO(OMe)(MeOH)(ONO)]. The anionic complexes [VO₂(ONO)]^- can also be convertedin situ on acidification to oxohydroxo complexes [VO(OH)(HONO)]⁺ and to peroxo complexes [VO(O₂)(ONO)]^-, and thus to the species assumed to be intermediates in the haloperoxidases activity of the enzymes. In the presence of catechol (H₂cat) and benzohydroxamic acid (H₂bha), oxovanadium (IV) complexes, [VO (acac)(ONN)] gave mixed-chelate oxovanadium(V) complexes [VO(cat)(ONN)] and [VO(bha)(ONN)] respectively. These complexes are not very stable in solution and slowly convert to the corresponding dioxo species [VO₂(ONN)] as observed by⁵¹V NMR and electronic absorption spectroscopic studies.     

Linkage effects of chromium(III) acetylacetonato units on chiral induction of liquid crystal phases

The linkage effects of polynuclear metal complexes on chiral induction have been studied by application of the chiral oligomers of acetylacetonato chromium(III) units as a dopant, inducing chiral nematic phases. The compounds were prepared by reacting [Cr(acac)(3)] (acac = acetylacetonato) and 1,1,2,2-tetraacetylethane (taetH(2)) in solid phase at 160 degrees C. Binuclear diastereomers were separated on a silica gel column, followed by chromatographic resolution on a chiral column packed with an ion-exchange adduct of Delta-[Ru(phen)(3)](2+) (phen = 1,10-phenanthroline) and synthetic hectorite. An enantiomeric pair (DeltaDelta- and LambdaLambda-[Cr(acac)(2)(taet)Cr-(acac)(2)]) and a meso species (DeltaLambda-[Cr(acac)(2)(taet)Cr(acac)(2)]) were identified. The binuclear enantiomers were doped into a room-temperature nematic liquid crystal, N-methoxybenzylidene-4-n-butylaniline. Helical twisting power (beta(M)) was found to be +97.9 and -88.9 microm(-1) for LambdaLambda- and DeltaDelta-[Cr(acac)(2)(taet)Cr(acac)(2)], respectively. The values were compared with beta(M) for the monomeric enantiomers (+99.5 and -91.0 microm(-1) for Lambda- and Delta-[Cr(acac)(3)], respectively). The results are interpreted on the basis of the surface chirality model. DeltaDelta-[Cr(acac)(2)(taet)Cr(acac)(2)] was found to photoisomerize both in a hexane solution and in a liquid crystal phase of ZLI-1132. The quantum yield of photoisomerization in a liquid crystal phase was lowered to ca. 30% of that in a hexane solution.     

Can a meso-type dinuclear complex be chiral?: dinuclear β-diketonato Ru(III) complexes

Dinuclear Ru(III) complexes, [Ru(III)(acac)(2)(dabe)Ru(III)(acac)(2)] (acacH = acetylacetone; dabeH(2) = 1, 2-diacetyl-1,2-dibenzoylethane) and [Ru(III)(acac)(2)(tbet)Ru(III)(acac)(2)] (tbetH(2) = 1,1,2,2-tetrabenzoylethane) were synthesized by reacting [Ru(acac)(2)(CH(3)CN)(2)]PF(6) with dabeH(2) and tbetH(2) respectively, in toluene. The X-ray structural analysis of a meso-type dinuclear Ru(III) complex, ΔΛ-[Ru(III)(acac)(2)(dabe)Ru(III)(acac)(2)], showed that the bridging part became chiral due to the orthogonal twisting of two non-symmetrical β-diketonato moieties. To confirm this conclusion, the complex was resolved chromatographically to provide a pair of optical antipodes. Such chirality in the bridging part was not generated for [Ru(III)(acac)(2)(tbet)Ru(III)(acac)(2)], because the β-diketonato moieties in tbet(2-) are symmetrical.     

Racemic monoperoxovanadium(V) complexes with achiral OO and ON donor set heteroligands: synthesis, crystal structure and stereochemistry of [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O and [NH3(CH2)2NH3][VO(O2)(ox)(pca)

Monoperoxovanadium(V) complexes, [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O (1) and [NH3(CH2)2NH3][VO(O2)(ox)(pca)] (2) [NH3(CH2)2NH3 = ethane-1,2-diammonium(2+), ox=oxalate(2-), pic=pyridine-2-carboxylate(1-), pca=pyrazine-2-carboxylate(1-)], were synthesized and characterized by X-ray analysis, IR and Raman spectroscopies. The five equatorial positions of the pentagonal bipyramid around the vanadium atoms are occupied by the eta2-peroxo ligand, two oxygen atoms of the ox, and the nitrogen atom of the pic or pca ligands, respectively. The oxo ligand and the oxygen atom of pic or pca are in the axial positions. Networks of X-HO (X=C, N or O) hydrogen bonds, and pi-pi interactions between aromatic rings in and anion-pi interactions in , determine the molecular packings and build up the supramolecular architecture. Three stereochemical rules for occupation of the donor sites in two-heteroligand [VO(O2)(L1)(L2)] complexes (L1, L2 are bidentate neutral or differently charged anionic heteroligands providing an OO, NN or ON donor set) are discussed. and crystallize as racemic compounds. The 51V NMR spectra proved that the parent complex anions of and partially decompose on dissolution in water to the monoperoxo-ox, -pic or -pca complexes.     

Acid-induced degradation of phosphorescent dopants for OLEDs and its application to the synthesis of tris-heteroleptic iridium(III) bis-cyclometalated complexes

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir(2-(2,4-difluorophenyl)pyridine)(2)(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Br?nsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir(C^N)(2)(X^O)] complex (C^N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (diFppy) and X^O = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium(III) dimer [{Ir(C^N)(2)(μ-Cl)}(2)]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium(III) complexes [Ir(C^N(1))(C^N(2))(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium(I) complex, [{Ir(COD)(μ-Cl)}(2)], and a stoichiometric amount of two different C^N ligands (C^N(1) = ppy; C^N(2) = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir(ppy)(diFppy)(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir(ppy)(2)(acac)] and [Ir(diFppy)(2)(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir(ppy)(diFppy)(μ-Cl)}(2)], which can be used as starting material for the preparation of a new tris-heteroleptic iridium(III) complex based on these two C^N ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C^N ligands on the observed photophysical and electrochemical properties.     

Synthesis, characterisation, reactivity and in vitro antiamoebic activity of hydrazone based oxovanadium(IV), oxovanadium(V) and mu-bis(oxo)bis{oxovanadium(V)} complexes

Binuclear, mu-bis(oxo)bis{oxovanadium(V)} complexes [(VOL)2(mu-O)2](2 and 7)(where HL are the hydrazones Hacpy-nah I or Hacpy-fah II; acpy = 2-acetylpyridine, nah = nicotinic acid hydrazide and fah = 2-furoic acid hydrazide) were prepared by the reaction of [VO(acac)2] and the ligands in methanol followed by aerial oxidation. The paramagnetic intermediate complexes [VO(acac)(acpy-nah)](1) and [VO(acac)(acpy-fah)](6) have also been isolated. Treatment of [VO(acac)(acpy-nah)] and [VO(acac)(acpy-fah)] with aqueous H2O2 yields the oxoperoxovanadium(V) complexes [VO(O2)(acpy-nah)](3) and [VO(O2)(acpy-fah)](8). In the presence of catechol (H2cat) or benzohydroxamic acid (H2bha), 1 and 6 give the mixed chelate complexes [VO(cat)L](HL =I: 4, HL =II: 9) or [VO(bha)L](HL =I: 5, HL =II: 10). Complexes 4, 5, 9 and 10 slowly convert to the corresponding oxo-mu-oxo species 2 and 7 in DMF solution. Ascorbic acid enhances this conversion under aerobic conditions, possibly through reduction of these complexes with concomitant removal of coordinated catecholate or benzohydroxamate. Acidification of 7 with HCl dissolved in methanol afforded a hydroxo(oxo) complex. The crystal and molecular structure of 2.1.5H2O has been determined, and the structure of 7 re-determined, by single crystal X-ray diffraction. Both of these binuclear complexes contain the uncommon asymmetrical {VO(mu-O)}2 diamond core. The in vitro tests of the antiamoebic activity of ligands I and II and their binuclear complexes 2 and 7 against the protozoan parasite Entamoeba histolytica show that the ligands have no amoebicidal activity while their vanadium complexes 2 and 7 display more effective amoebicidal activity than the most commonly used drug metronidazole (IC50 values are 1.68 and 0.45 microM, respectively vs 1.81 microM for metronidazole). Complexes 2 and 7 catalyse the oxidation of styrene and ethyl benzene effectively. Oxidation of styrene, using H2O2 as an oxidant, gives styrene epoxide, 2-phenylacetaldehyde, benzaldehyde, benzoic acid and 1-phenyl-ethane-1,2-diol, while ethyl benzene yields benzyl alcohol, benzaldehyde and 1-phenyl-ethane-1,2-diol.     

Reduction of an eta2-iminoacyl ligand to eta2-iminium enabled by adjacent carbon monoxide ligand replacement with a variable electron donor alkyne ligand in a cationic Tungsten(II) bis(acetylacetonate) complex

Cationic iminoacyl-carbonyl tungsten complexes of the type [W(CO) (eta (2)-MeNCR)(acac) 2] (+) (acac = acetylacetonate; R = Ph ( 1a), Me ( 1b)) easily undergo thermal substitution of CO with two-electron donors to yield [W(L)(eta (2)-MeNCR)(acac) 2] (+) (L = tert-butylisonitrile [R = Ph ( 2a), Me ( 2b)], 2,6-dimethylphenylisonitrile [R = Me ( 2c)], triphenylphosphine [R = Ph ( 3a), Me ( 3c)], and tricyclohexylphosphine [R = Ph ( 3b)]). Tricyclohexylphosphine complex 3b exhibits rapid, reversible phosphine ligand exchange at room temperature on the NMR time scale. Photolytic replacement of carbon monoxide with either phenylacetylene or 2-butyne occurs efficiently to form [W(eta (2)-alkyne)(eta (2)-MeNCR)(acac) 2] (+) complexes ( 5a- d) with a variable electron donor eta (2)-alkyne paired with the eta (2)-iminoacyl ligand in the W(II) coordination sphere. PMe 3 adds to 1a or 5b to form [W(L)(eta (2)-MeNC(PMe 3)Ph)(acac) 2] (+) [L = CO ( 4), MeCCMe ( 6)] via nucleophilic attack at the iminoacyl carbon. Addition of Na[HB(OMe) 3] to 5b yields W(eta (2)-MeCCMe)(eta (2)-MeNCHPh)(acac) 2, 8, which exhibits alkyne rotation on the NMR time scale. Addition of MeOTf to 8 places a second methyl group on the nitrogen atom to form an unusual cationic eta (2)-iminium complex [W(eta (2)-MeCCMe)(eta (2)-Me 2NCHPh)(acac) 2][OTf] ( 9[OTf], OTf = SO 3CF 3). X-ray structures of 2,6-dimethylphenylisonitrile complex 2c[BAr' 4 ], tricyclohexylphosphine complex 3b[BAr' 4 ], and phenylacetylene complex 5a[BAr' 4 ] confirm replacement of CO by these ligands in the [W(L)(eta (2)-MeNCR)(acac) 2] (+) products. X-ray structures of alkyne-imine complexes 6[BAr' 4 ] and 8 show products resulting from nucleophilic addition at the iminoacyl carbon, and the X-ray structure of 9[BAr' 4 ] reflects methylation at the imine nitrogen to form a rare eta (2)-iminium ligand.     

Designing molecular bistability in ruthenium dimethyl sulfoxide complexes

Compounds of the type [Ru(tpy)(L2)(dmso)](z+) (tpy is 2,2':6',2' '-terpyridine; L2 can be 2,2'-bipyridine (bpy), N,N,N',N'-tetramethylethylenediamine (tmen), 2-pyridine carboxylate (pic), acetylacetonate (acac), malonate (mal), or oxalate (ox)) have been studied by X-ray crystallography, electrochemistry, NMR, IR, and UV-vis spectroscopy. When L2 is bpy, tmen, or pic, the dmso ligand can be intramolecularly isomerized either electrochemically or photochemically. Isomerization is not observed when L2 is acac, mal, or ox. Isomerization results in a drastic change in the absorption spectrum, as well as in the voltammetry. Absorption maxima shift by 3470 (419-490 nm), 4775 (421-527 nm), and 4440 cm(-)(1) (429-530 nm) for the bpy, pic, and tmen complexes, respectively. Reduction potentials for S-bonded and O-bonded complexes differ by 0.57, 0.75, and 0.62 V for the bpy, pic, and tmen complexes, respectively. Quantum yields of isomerization (phi(S)(-->)(O)) were determined for the bpy (0.024 +/- 1), pic (0.25 +/- 1), and tmen (0.007 +/- 1) complexes. In comparison of these data to photosubstitution quantum yields, it appears that the isomerization mechanism does not involve the ligand field states. This result is surprising given the importance of these states in the photochemistry of ruthenium and osmium polypyridine complexes. These results and details of the mechanism are discussed.     

Coordination and Redox Chemistry of Substituted-Polypyridyl Complexes of Ruthenium

The complexes [Ru(tpy)(acac)(Cl)], [Ru(tpy)(acac)(H(2)O)](PF(6)) (tpy = 2,2',2"-terpyridine, acacH = 2,4 pentanedione) [Ru(tpy)(C(2)O(4))(H(2)O)] (C(2)O(4)(2)(-) = oxalato dianion), [Ru(tpy)(dppene)(Cl)](PF(6)) (dppene = cis-1,2-bis(diphenylphosphino)ethylene), [Ru(tpy)(dppene)(H(2)O)](PF(6))(2), [Ru(tpy)(C(2)O(4))(py)], [Ru(tpy)(acac)(py)](ClO(4)), [Ru(tpy)(acac)(NO(2))], [Ru(tpy)(acac)(NO)](PF(6))(2), and [Ru(tpy)(PSCS)Cl] (PSCS = 1-pyrrolidinedithiocarbamate anion) have been prepared and characterized by cyclic voltammetry and UV-visible and FTIR spectroscopy. [Ru(tpy)(acac)(NO(2))](+) is stable with respect to oxidation of coordinated NO(2)(-) on the cyclic voltammetric time scale. The nitrosyl [Ru(tpy)(acac)(NO)](2+) falls on an earlier correlation between nu(NO) (1914 cm(-)(1) in KBr) and E(1/2) for the first nitrosyl-based reduction 0.02 V vs SSCE. Oxalate ligand is lost from [Ru(II)(tpy)(C(2)O(4))(H(2)O)] to give [Ru(tpy)(H(2)O)(3)](2+). The Ru(III/II) and Ru(IV/III) couples of the aqua complexes are pH dependent. At pH 7.0, E(1/2) values are 0.43 V vs NHE for [Ru(III)(tpy)(acac)(OH)](+)/[Ru(II)(tpy)(acac)(H(2)O)](+), 0.80 V for [Ru(IV)(tpy)(acac)(O)](+)/[Ru(III)(tpy)(acac)(OH)](+), 0.16 V for [Ru(III)(tpy)(C(2)O(4))(OH)]/[Ru(II)(tpy)(C(2)O(4))(H(2)O)], and 0.45 V for [Ru(IV)(tpy)(C(2)O(4))(O)]/[Ru(III)(tpy)(C(2)O(4))(OH)]. Plots of E(1/2) vs pH define regions of stability for the various oxidation states and the pK(a) values of aqua and hydroxo forms. These measurements reveal that C(2)O(4)(2)(-) and acac(-) are electron donating to Ru(III) relative to bpy. Comparisons with redox potentials for 21 related polypyridyl couples reveal the influence of ligand changes on the potentials of the Ru(IV/III) and Ru(III/II) couples and the difference between them, DeltaE(1/2). The majority of the effect appears in the Ru(III/II) couple. ()A linear correlation exists between DeltaE(1/2) and the sum of a set of ligand parameters defined by Lever et al., SigmaE(i)(L(i)), for the series of complexes, but there is a dramatic change in slope at DeltaE(1/2) approximately -0.11 V and SigmaE(i)(L(i)) = 1.06 V. Extrapolation of the plot of DeltaE(1/2) vs SigmaE(i)(L(i)) suggests that there may be ligand environments in which Ru(III) is unstable with respect to disproportionation into Ru(IV) and Ru(II). This would make the two-electron Ru(IV)O/Ru(II)OH(2) couple more strongly oxidizing than the one-electron Ru(IV)O/Ru(III)OH couple.     

Peroxydisulfate activation by [RuII(tpy)(pic)(H2O)]+. Kinetic, mechanistic and anti-microbial activity studies

The oxidation of [Ru(II)(tpy)(pic)H(2)O](+) (tpy = 2,2',6',2'-terpyridine; pic(-) = picolinate) by peroxidisulfate (S(2)O(8)(2-)) as precursor oxidant has been investigated kinetically by UV-VIS, IR and EPR spectroscopy. The overall oxidation of Ru(II)- to Ru(IV)-species takes place in a consecutive manner involving oxidation of [Ru(II)(tpy)(pic)H(2)O](+) to [Ru(III)(tpy)(pic)(OH)](+), and its further oxidation of to the ultimate product [Ru(IV)(tpy)(pic)(O)](+) complex. The time course of the reaction was followed as a function of [S(2)O(8)(2-)], ionic strength (I) and temperature. Kinetic data and activation parameters are interpreted in terms of an outer-sphere electron transfer mechanism. Anti-microbial activity of Ru(II)(tpy)(pic)H(2)O](+) complex by inhibiting the growth of Escherichia coli DH5α in presence of peroxydisulfate has been explored, and the results of the biological studies have been discussed in terms of the [Ru(IV)(tpy)(pic)(O)](+) mediated cleavage of chromosomal DNA of the bacteria.     

Synthesis and Characterization of Six-Coordinate Nitrido Complexes of Vanadium(V), Chromium(V), and Manganese(V). Isolation of a Dinuclear, Mixed-Valent &mgr;-Nitrido Chromium(III)/Chromium(V) Species

Photolysis of a series of octahedral monoazido complexes of the type [LM(III)(didentate ligand)(N(3))](n)(+)X(n) of vanadium(III), chromium(III), and manganese(III) in the solid state or in solution yields quantitatively the corresponding six-coordinate nitrido complexes [LM(V)(didentate ligand)(N)](n)(+)X(n) and 1 equiv of dinitrogen. L represents the macrocycle 1,4,7-triazacyclononane or its N-methylated derivative (L'), the didentate ligands are pentane-2,4-dionate (acac), 2,2,6,6-tetramethylheptane-3,5-dionate (tacac), picolinate (pic), phenanthroline (phen), and oxalate (ox), and X(-) represents perchlorate or hexafluorophosphate. The following nitrido complexes were prepared: [LV(V)(N)(acac)](ClO(4)) (6), [LCr(V)(N)(acac)](ClO(4)) (13), [LCr(V)(N)(tacac)](ClO(4)) (14), [LCr(V)(N)(pic)](ClO(4)) (15), [LCr(V)(N)(phen)](ClO(4))(2) (16), [LCr(V)(N)(ox)] (19), [L'Mn(V)(N)(acac)]PF(6) (21). Photolysis of [LCr(III)(N(3))(ox)] (17) in the solid state produces the &mgr;-nitrido-bridged mixed-valent species [L(2)Cr(2)(ox)(2)(&mgr;-N)](N(3)) (18). The structures of the precursor complex [L'Mn(acac)(N(3))]BPh(4) (20), of 13, and of [L'Mn(V)(N)(acac)]BPh(4) (21) have been determined by X-ray crystallography. Complex 13 crystallizes in the orthorhombic space group Pnma, with cell constants a = 27.187(5) ?, b = 9.228(2) ?, c = 7.070(1) ?, V = 1773.7(6) ?(3), and Z = 4; complex 20 crystallizes in the triclinic space group P&onemacr; with a = 14.769(5) ?, b = 16.83(1) ?, c = 16.96(1) ?, alpha = 108.19(5) degrees, beta = 105.06(4) degrees, gamma = 99.78(4) degrees, V = 3719(2) ?(3), and Z = 4; and complex 21 crystallizes in the monoclinic space group P2(1)/n with a = 10.443(3) ?, b = 16.035(4) ?, c = 21.463(5) ?, beta = 95.76(1) degrees, V = 3575.9(14) ?(3), and Z = 4. The Cr(V)&tbd1;N and Mn(V)&tbd1;N distances are short at 1.575(9) and 1.518(4) ?, respectively, and indicate a metal-to-nitrogen triple bond.     

Kinetics of oxidation of vanadyl acetylacetonate by oxygen in methanolic solution

The kinetics of oxidation of vanadyl acetylacetonate to VO(OH)(OMe)(acac) by molecular O2 in MeOH have been studied spectrophotometrically. The reaction, which is pseudo first-order with respect to [VO(acac)2] and [O2], is inhibited by Hacac and a vanadium(V) complex. The rate data were used to calculate the thermodynamic activation parameters. A mechanism for the reaction is discussed.     

Kinetics and mechanistic studies of anticarcinogenic bisperoxovanadium(V) compounds: ligand substitution reactions at physiological pH and relevance to DNA interactions

Bisperoxovanadium(V) compounds with bidentate ligands have shown tumor growth inhibition by cleaving DNA. The kinetics and mechanisms of ligand substitution reactions of two bisperoxovanadium(V) compounds [VO(O(2))(2)(bpy)](-) (bpVbpy) and [VO(O(2))(2)(phen)](-) (bpVphen) with entering ligands picolinic acid (pic) and dipicolinic acid (dipic) at physiological pH are reported, and its relevance to their DNA-cleavage activities are discussed. The products of the ligand substitution reactions with pic and dipic are the monoperoxo complexes [VO(O(2))(pic)(2)](-) and [VO(O(2))(dipic)(H(2)O)](-), respectively. (51)V NMR experiments indicate that bpVphen is substantially more inert in aqueous solution than bpVbpy. As a result, bpVbpy is more prone to ligand substitution and subsequent conversion to monoperoxo species. The rate of reaction for bpVbpy was faster than that of bpVphen by an order of magnitude, indicating that the ancillary ligand plays an important role in ligand substitution reactions. The ligand substitution reactions of bpVbpy feature first-order dependence on both [pic](T) and [dipic](T) whereas the substitution kinetics of bpVphen feature saturation behavior with dipic. The substitution reactions of both bpVbpy and bpVphen with pic showed first-order dependence on [H(+)] whereas no acid dependence was observed for the reactions with dipic. Hydrogen peroxide was determined to be a competitive inhibitor with respect to dipic. The ligand substitution reaction mechanisms and the rate laws consistent with these results are presented. The substitution reactions with pic and dipic proceed through different mechanisms; the substitution reactions with dipic proceed via solvolysis as the first step in the mechanisms whereas the reactions with pic bypass solvolysis to go through a mixed ligand monoperoxo vanadium intermediate.