Remarkably high catalytic activity of the Ru(III)(edta)/H2O2 system towards degradation of the azo-dye Orange II

The Ru(III)(edta)/H(2)O(2) system (edta(4-) = ethylenediaminetretaacetate) was found to degrade the azo-dye Orange II at remarkably high efficiency under ambient conditions. Catalytic degradation of the dye was studied by using rapid-scan spectrophotometry as a function of [H(2)O(2)], [Orange II] and pH. Spectral analyses and kinetic data point towards a catalytic pathway involving the rapid formation of [Ru(III)(edta)(OOH)](2-) followed by the immediate subsequent degradation of Orange II prior to the conversion of [Ru(III)(edta)(OOH)](2-) to [Ru(IV)(edta)(OH)](-) and [Ru(V)(edta)(O)](-)via homolysis and heterolysis of the O-O bond, respectively. The higher oxidation state Ru(IV) and Ru(V) complexes react three orders of magnitude slower with Orange II than the Ru(III)-hydroperoxo complex. In comparison to biological oxygen transfer reactions, the Ru(edta) complexes show the reactivity order Compound 0 ? Compounds I and II.     

Kinetics and mechanism of the [Ru(III)(edta)(H2O)](-)-mediated oxidation of cysteine by H2O2

The kinetics and mechanism of the [Ru(III)(edta)(H(2)O)](-)-mediated oxidation of cysteine (RSH) by hydrogen peroxide (edta(4-) = ethylenediaminetetraacetate), were studied in detail as a function of both the hydrogen peroxide and cysteine concentrations at pH 5.1 and room temperature. The kinetic traces reveal clear evidence for a catalytic process in which hydrogen peroxide reacts directly with cysteine coordinated to the Ru(III)(edta) complex in the form of [Ru(III)(edta)SR](2-). A parallel process in which [Ru(III)(edta)(H(2)O)](-) first reacts with H(2)O(2) to produce [Ru(V)(edta)O](-) and subsequently oxidizes cysteine, is orders of magnitude slower than the [Ru(III)(edta)(H(2)O)](-)-mediated oxidation in which cysteine rapidly coordinates to [Ru(III)(edta)(H(2)O)](-) prior to the reaction with H(2)O(2). HPLC product analyses revealed the formation of cystine (RSSR) as major product along with cysteine sulfinic acid (RSO(2)H) in the reaction system, and established the catalytic role of [Ru(III)(edta)(H(2)O)](-). Simulations were performed to account for the rather complex kinetic traces in terms of the suggested reaction mechanism. The results of the simulations support the proposed reaction mechanism that involves the oxidation of coordinated cysteine to cysteine sulfenic acid (RSOH), which subsequently rapidly reacts with H(2)O(2) and RSH to form RSO(2)H and RSSR, respectively.     

Reaction of [RuIII(edta)(H2O)]- with H2O2 in aqueous solution. Kinetic and mechanistic investigation

The reaction of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with hydrogen peroxide was studied kinetically as a function of [H(2)O(2)], temperature (5-35 degrees C) and pressure (1-1300 atm) at a fixed pH of 5.1 using stopped-flow techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOH)](2-) intermediate which subsequently undergoes parallel heterolytic and homolytic cleavage to produce [(edta)Ru(V)=O](-) (45%) and [(edta)Ru(IV)(OH)](-) (55%), respectively. The water soluble trap, 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first conditions for [ABTS] > [HOBr] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS* (+). A detailed mechanism in agreement with the rate and activation parameters is presented, and the results are discussed with reference to data reported for the corresponding [Fe(III)(edta)(H(2)O)](-)/H(2)O(2) system.     

Kinetics and mechanism of O-O bond cleavage in the reaction of [RuIII(edta)(H2O)]- with hydroperoxides in aqueous solution

The reactions of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ((t)BuOOH) and potassium hydrogenpersulfate (KHSO(5)) were studied kinetically as a function of oxidant concentration and temperature (10-30 degrees C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOR)](2-) intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru(V)=O](-). Since [(edta)Ru(V)=O](-) was produced almost quantitatively in the reaction of 1 with the hydroperoxides (t)BuOOH and KHSO(5), the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] > [hydroperoxide] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS (*+). The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru(II)(edta)(H(2)O)](-) with H(2)O(2).     

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.     

Monohydroxamic acids and bridging dihydroxamic acids as chelators to ruthenium(III) and as nitric oxide donors: syntheses, speciation studies and nitric oxide releasing investigations

The synthesis and spectroscopic characterisation of novel mononuclear Ru(III)(edta)(hydroxamato) complexes of general formula [Ru(H2edta)(monoha)] (where monoha = 3- or 4-NH2, 2-, 3- or 4-C1 and 3-Me-phenylhydroxamato), as well as the first example of a Ru(III)-N-aryl aromatic hydroxamate, [Ru(H2edta)(N-Me-bha)].H2O (N-Me-bha = N-methylbenzohydroxamato) are reported. Three dinuclear Ru(III) complexes with bridging dihydroxamato ligands of general formula [{Ru(H2edta)}2(mu-diha)] where diha = 2,6-pyridinedihydroxamato and 1,3- or 1,4-benzodihydroxamato, the first of their kind with Ru(III), are also described. The speciation of all of these systems (with the exception of the Ru-1,4-benzodihydroxamic acid and Ru-N-methylbenzohydroxamic systems) in aqueous solution was investigated. We previously proposed that nitrosyl abstraction from hydroxamic acids by Ru(III) involves initial formation of Ru(III)-hydroxamates. Yet, until now, no data on the rate of nitric oxide (NO) release from hydroxamic acids has been published. We now describe a UV-VIS spectroscopic study, where we monitored the decrease in the ligand-to-metal charge-transfer band of a series of Ru(III)-monohydroxamates with time, with a view to gaining an insight into the NO-releasing properties of hydroxamic acids.     

Bis(amido)ruthenium(IV) Complexes with 2,3-Diamino-2,3-dimethylbutane. Crystal Structure and Reversible Ru(IV)-Amide/Ru(III)-Amine and Ru(IV)-Amide/Ru(II)- Amine Redox Couples in Aqueous Solution

Two bis(amido)ruthenium(IV) complexes, [Ru(IV)(bpy)(L-H)(2)](2+) and [Ru(IV)(L)(L-H)(2)](2+) (bpy = 2,2'-bipyridine, L = 2,3-diamino-2,3-dimethylbutane, L-H = (H(2)NCMe(2)CMe(2)NH)(-)), were prepared by chemical oxidation of [Ru(II)(bpy)(L)(2)](2+) and the reaction of [(n-Bu)(4)N][Ru(VI)NCl(4)] with L, respectively. The structures of [Ru(bpy)(L-H)(2)][ZnBr(4)].CH(3)CN and [Ru(L)(L-H)(2)]Cl(2).2H(2)O were determined by X-ray crystal analysis. [Ru(bpy)(L-H)(2)][ZnBr(4)].CH(3)CN crystallizes in the monoclinic space group P2(1)/n with a = 12.597(2) ?, b = 15.909(2) ?, c = 16.785(2) ?, beta = 91.74(1) degrees, and Z = 4. [Ru(L)(L-H)(2)]Cl(2).2H(2)O crystallizes in the tetragonal space group I4(1)/a with a = 31.892(6) ?, c = 10.819(3) ?, and Z = 16. In both complexes, the two Ru-N(amide) bonds are cis to each other with bond distances ranging from 1.835(7) to 1.856(7) ?. The N(amide)-Ru-N(amide) angles are about 110 degrees. The two Ru(IV) complexes are diamagnetic, and the chemical shifts of the amide protons occur at around 13 ppm. Both complexes display reversible metal-amide/metal-amine redox couples in aqueous solution with a pyrolytic graphite electrode. Depending on the pH of the media, reversible/quasireversible 1e(-)-2H(+) Ru(IV)-amide/Ru(III)-amine and 2e(-)-2H(+) Ru(IV)-amide/Ru(II)-amine redox couples have been observed. At pH = 1.0, the E degrees is 0.46 V for [Ru(IV)(bpy)(L-H)(2)](2+)/[Ru(III)(bpy)(L)(2)](3+) and 0.29 V vs SCE for [Ru(IV)(L)(L-H)(2)](2+)/[Ru(III)(L)(3)](3+). The difference in the E degrees values for the two Ru(IV)-amide complexes has been attributed to the fact that the chelating saturated diamine ligand is a better sigma-donor than 2,2'-bipyridine.     

Kinetics and mechanism of the reaction of [RuIII(edta)(H2O)]- with HOBr to form an intermediate RuV=O complex in aqueous solution

The interaction of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with the oxygen transfer agent HOBr, was studied kinetically as a function of [HOBr] and temperature (5-35 degrees C) at a fixed pH of 6.2. Spectroscopic evidence is reported for the formation of a high valent intermediate (edta)Ru(V)=O complex. Water soluble 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate) (ABTS) was employed as a trap for this intermediate in order to gain further mechanistic information. Reactions were carried out under pseudo-first conditions for [ABTS] > [HOBr] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS(*+). The reported kinetic data are interpreted in terms of a suggested reaction mechanism and discussed in reference to data reported before.     

Mechanistic implications of proton transfer coupled to electron transfer.

The kinetics of electron transfer for the reactions cis-[Ru(IV)(bpy)2(py)(O)]2+ + H+ + [Os(II)(bpy)3]2+ <==> cis-[Ru(III)(bpy)2(py)(OH)]2+ + [Os(III)(bpy)3]3+ and cis-[Ru(III)(bpy)2(py)(OH)]2+ + H+ + [Os(II)(bpy)3]2+ <==> cis-[Ru(II)(bpy)2(py)(H2O)]2+ + [Os(III)(bpy)3]3+ have been studied in both directions by varying the pH from 1 to 8. The kinetics are complex but can be fit to a double "square scheme" involving stepwise electron and proton transfer by including the disproportionation equilibrium, 2cis-[Ru(III)(bpy)2(py)(OH)]2+ <==> (3 x 10(3) M(-1) x s(-1) forward, 2.1 x 10(5) M(-1) x s(-1) reverse) cis-[Ru(IV)(bpy)2(py)(O)]2+ + cis-[Ru(II)(bpy)2(py)(H2O)]2+. Electron transfer is outer-sphere and uncoupled from proton transfer. The kinetic study has revealed (1) pH-dependent reactions where the pH dependence arises from the distribution between acid and base forms and not from variations in the driving force; (2) competing pathways involving initial electron transfer or initial proton transfer whose relative importance depends on pH; (3) a significant inhibition to outer-sphere electron transfer for the Ru(IV)=O2+/Ru(III)-OH2+ couple because of the large difference in pK(a) values between Ru(IV)=OH3+ (pK(a) < 0) and Ru(III)-OH2+ (pK(a) > 14); and (4) regions where proton loss from cis-[Ru(II)(bpy)2(py)(H2O)]2+ or cis-[Ru(III)(bpy)2(py)(OH)]2+ is rate limiting. The difference in pK(a) values favors more complex pathways such as proton-coupled electron transfer.     

Immobilization of the [RuII(edta)NO+] Ion on the surface of functionalized silica gel

The reaction of NO and the immobilized dimer complex (edta)(2)Ru(2)(III(1/2),III(1/2)) on silica gel chemically modified with [3-(2-aminoethyl)aminopropyl]trimethoxysilane (AEATS) produces the corresponding immobilized nitrosyl complex AEATS/Ru(II)NO(+). This compound, a monomer, was obtained by reducing the immobilized ruthenium dimer either electrochemically or with Eu(II) and reacting this species with NO(2)(-) ions. The properties of [Ru(edta)NO](-) in solution and anchored (AEATS/Ru(II)NO(+)) on silica were compared using electrochemical (DPV, CV) and spectroscopic (IR, UV-vis, and ESR) techniques. The results indicate that immobilization does not alter the reactivity of the ruthenium complex and confirm that [Ru(edta)(H(2)O)](2)(-) may be used, either in solution or immobilized, as a catalyst for the conversion of NO(2)(-) to NO(+). Both the anchored nitrosyl complex AEATS/Ru(II)NO(+) and the [Ru(edta)NO](-) species in solution, upon one-electron reduction, liberate NO at comparable rates.     

The effect of pyridinecarboxylate chelating groups on the stability and electronic relaxation of gadolinium complexes

The ligand N,N'-bis[(6-carboxy-2-pyridylmethyl]ethylenediamine-N,N'-diacetic acid (H(4)bpeda) was synthesised using an improved procedure which requires a reduced number of steps and leads to a higher yield with respect to the published procedure. It was obtained in three steps from diethylpyridine-2,6-dicarboxylate and commercially available ethylenediamine-N,N[prime or minute]-diacetic acid with a total yield of approximately 20%. The crystal structure of the hexa-protonated form of the ligand which was determined by X-ray diffraction shows that the four carboxylates and the two amines are protonated. The crystal structure of the polynuclear complex [Gd(bpeda)(H(2)O)(2)](3)[Gd(H(2)O)(6)](2)Cl(3)(2), isolated by slow evaporation of a 1:1 mixture of GdCl(3) and H(4)bpeda at pH approximately 1, was determined by X-ray diffraction. In complex three [Gd(bpeda)(H(2)O)(2)] units, containing a Gd(III) ion ten-coordinated by the octadentate bpeda and two water molecules, are connected in a pentametallic structure by two hexa-aquo Gd(3+) cations through four carboxylato bridges. The protonation constants (pK(a1)= 2.9(1), pK(a2)= 3.5(1), pK(a3)= 5.2(2), and pK(a4)= 8.5(1)) and the stability constants of the complexes formed between Gd(III) and Ca(II) ions and H(4)bpeda (log beta(GdL)= 15.1(3); log beta(CaL)= 9.4(1)) were determined by potentiometric titration. The unexpected decrease in the stability of the gadolinium complex and of the calcium complex of the octadentate ligand bpeda(4-) with respect to the hexadentate ligand edta(4-) has been interpreted in terms of an overall lower contribution to stability of the metal-nitrogen interactions. The EPR spectra display very broad lines (apparent DeltaH(pp) approximately 800-1200 G at X-band and 90-110 G at Q-band depending on the temperature), indicating a rapid transverse electron spin relaxation. At X-band, Gd(bpeda) is among the fastest relaxing Gd(3+) complexes to date suggesting that the presence of pyridinecarboxylate chelating groups in itself does not lead to slow electron relaxation.     

Acid-base equilibria of various oxidation states of aqua-ruthenium complexes with 1,10-phenanthroline-5,6-dione in aqueous media

Syntheses and pH dependent electrochemical properties of aqua-ruthenium(II) complexes, [Ru(trpy)(PDA-N,N')(OH2)](ClO4)2 ([1](ClO4)2) and [Ru(trpy)(PD-N,N')(OH2)](ClO4)2 ([2](ClO4)2) (trpy = 2,2':6',2'-terpyridine, PDA = 6-acetonyl-6-hydroxy-1,10-phenanthroline-5-one, PD = 1,10-phenanthroline-5,6-dione) are presented. Treatment of [Ru(trpy)(PD-N,N')Cl](PF6) with AgClO4 in a mixed solvent of acetone and H2O selectively produced the acetonyl-PD complex [1](ClO4)2, and the similar treatment in a mixed solvent of 2-methoxyethanol and H2O gave the PD complex [2](ClO4)2. The molecular structures of both complexes were determined by X-ray structural analysis. The proton dissociation constants of various oxidations state of [1]2+ and [2]2+ were evaluated by simulation of E(1/2) values of those redox potentials depending on pH. The simulation revealed that the acetonyl-PD complex [1]2+ underwent successive Ru(II)/Ru(III) and Ru(III)/Ru(IV) redox couples though the two redox reactions were not separated in the cyclic voltammograms. The redox behavior of [2]2+ in H2O is reasonably explained by not only the similar successive metal-centered redox reactions but also simultaneous two-electron quinone/catechol redox couple of the PD ligand including the contribution of hydration on a carbonyl carbon.     

Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands

Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) mol(-1), respectively. For [Ni(II)(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, DeltaH(double dagger) = 22 +/- 4 kJ mol(-1), and DeltaS(double dagger) = -59 +/- 5 J K(-1) mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.     

Study of the ligand substitution reactions of cis,cis-[(bpy)(2)(L)RuORu(L')(bpy)(2)](n+) (L,L' = H(2)O,OH(-), NH(3)) using electrospray ionization mass spectrometry and (1)H NMR

We have successfully applied electrospray ionization mass spectrometry (ESI-MS) and (1)H NMR analyses to study ligand substitution reactions of mu-oxo ruthenium bipyridine dimers cis,cis-[(bpy)(2)(L)RuORu(L')(bpy)(2)](n+) (bpy = 2,2'-bipyridine; L and L' = NH(3), H(2)O, and HO(-)) with solvent molecules, that is, acetonitrile, methanol, and acetone. The results clearly show that the ammine ligand is very stable and was not substituted by any solvents, while the aqua ligand was rapidly substituted by all the solvents. In acetonitrile and acetone solutions, the substitution reaction of the aqua ligand(s) competed with a deprotonation reaction from the ligand. The hydroxyl ligand was not substituted by acetonitrile or acetone, but it exchanged slowly with CH(3)O(-) in methanol. The substitution reaction of the aqua ligands in [(bpy)(2)(H(2)O)Ru(III)ORu(III)(H(2)O)(bpy)(2)](4+) was more rapid than that of the hydroxyl ligand in [(bpy)(2)(H(2)O)Ru(III)ORu(IV)(OH)(bpy)(2)](4+). In methanol, slow reduction of Ru(III) to Ru(II) was observed in all the mu-oxo dimers, and the Ru-O-Ru bridge was then cleaved to give mononuclear Ru(II) complexes.     

Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.     

Rates of water exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 molecule and its implications for geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ～10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.     

Synthesis and physicochemical characterization of two gadolinium(III) TTDA-like complexes and their interaction with human serum albumin

Two novel derivatives of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), TTDA-BOM and TTDA-N'-BOM, each having a benzyloxymethyl group, were synthesized. (17)O NMR longitudinal and transverse relaxation rates and chemical shifts of aqueous solutions of their Gd(III) complexes were measured at variable temperature with a magnetic field strength of 9.4 T. The water exchange rate (k(ex)(298)) values for [Gd(TTDA-BOM)(H(2)O)](2-) (117 x 10(6) s(-1)) and [Gd(TTDA-N'-BOM)(H(2)O)](2-) (131 x 10(6) s(-1)) are significantly higher than those of [Gd(DTPA)(H(2)O)](2-) (4.1 x 10(6) s(-1)) and [Gd(BOPTA)(H(2)O)](2-) (3.45 x 10(6) s(-1)). The rotational correlation time (tau) values for [Gd(TTDA-BOM)(H(2)O)](2-) (119 ps) and [Gd(TTDA-N'-BOM)(H(2)O)](2-) (125 ps) are higher than those of [Gd(DTPA)(H(2)O)](2-) (103 ps) and [Gd(TTDA)(H(2)O)](2-) (104 ps). The stepwise stoichiometric binding constants of [Gd(TTDA-BOM)(H(2)O)](2)(-) and [Gd(TTDA-N'-BOM)(H(2)O)](2)(-) bound to HSA are obtained by ultrafiltration studies. Fluorescent probe displacement studies exhibit that [Gd(TTDA-BOM)(H(2)O)](2-) and [Gd(TTDA-N'-BOM)(H(2)O)](2-) can displace dansylsarcosine from HSA with inhibition constants (K(i)) of 1900 and 1600 microM, respectively; however, they are not able to displace warfarin. These results indicate that [Gd(TTDA-BOM)(H(2)O)](2-) and [Gd(TTDA-N'-BOM)(H(2)O)](2-) have a weak binding to site II on HSA. In addition, the mean bound relaxivity (r(1b)) and bound relaxivity (r(1)(b)) values for the [Gd(TTDA-BOM)(H(2)O)](2-)/HSA and [Gd(TTDA-N'-BOM)(H(2)O)](2-)/HSA adducts are obtained by ultrafiltration and relaxivity studies, respectively. The bound relaxivity of these adducts values are significantly higher than those of [Gd(BOPTA)(H(2)O)](2-)/HSA and [Gd(DTPA-BOM(3))(H(2)O)](2-)/HSA. These results also suggest that bound relaxivity is site dependent. In binding sites studies of Gd(III) chelates to HSA, a significant decrease of the relaxation rates (R(1obs)) was observed for the [Eu(TTDA-BOM)(H(2)O)](2-) complex which was added to the [Gd(TTDA-N'-BOM)(H(2)O)](2-)/HSA solution, and this indicated that these Gd(III) complexes share the same HSA binding site. Finally, as measured by the Zn(II) transmetalation process, the kinetic stability of these Gd(III) complexes are significantly higher than that of [Gd(DTPA-BMA)(H(2)O)].     

Hydrolytic behaviour and chloride ion binding capability of [Ru(η6-p-cym)(H2O)3]2+: a solution equilibrium study

Hydrolysis of an organometallic cation, [Ru(η(6)-p-cym)(H(2)O)(3)](2+) (p-cym = 1-isopropyl-4-methylbenzene), in the presence of 0.20 M KNO(3) or KCl as supporting electrolyte was studied in detail with the combined use of pH-potentiometry, (1)H-NMR, UV-VIS and ESI-TOF-MS. Stoichiometry and stability constants of chlorido, hydroxido and mixed chlorido-hydroxido complexes formed in aqueous solution have been determined. At pH < 4.0 where hydrolysis of [Ru(η(6)-p-cym)(H(2)O)(3)](2+) is negligible with increasing chloride ion concentration two chlorido complexes, [Ru(η(6)-p-cym)(H(2)O)(2)Cl](+) and [{Ru(η(6)-p-cym)}(2)(μ(2)-Cl)(3)](+), are detectable. At pH > 5.0, in chloride ion free samples the exclusive formation of [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(3)](+) is found. However, if chloride ion is present (in the range 0-3.50 M) novel mixed chlorido-hydroxido species, [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(2)(μ(2)-Cl)](+) and [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(μ(2)-Cl)(2)](+) can also be identified at pH > 4.0. The results obtained in this study may help in rationalizing the solution behaviour of half-sandwich [Ru(η(6)-p-cym)(XY)Z] type complexes which, after dissociation of both the monodentate Z and the chelating XY, are capable of yielding the free aqua species [Ru(η(6)-p-cym)(H(2)O)(3)](2+). Our results demonstrate that different chloride ion concentrations can influence the speciation in the acidic pH range but at biologically relevant conditions (pH = 7.4, c(Cl(-)) = 0.16 M) and at c(M) = 1 μM [{Ru(η(6)-p-cym)}(2)(μ(2)-OH)(3)](+) is predominant in the absence of any coordinating ligands.     

Synthesis and characterization of the novel monoamide derivatives of Gd-TTDA

For this study, the N'-monoamide derivatives of TTDA (3,6,10-tri(carboxymethyl)-3,6,10-triazadodecanedioic acid), N'-methylamide (TTDA-MA), N'-benzylamide (TTDA-BA), and N'-2-methoxybenzylamide (TTDA-MOBA), were synthesized. Their protonation constants and stability constants (log K(ML)'s) formed with Ca(2+), Zn(2+), Cu(2+), and Gd(3+) were determined by potentiometric titration in 0.10 M Me(4)NCl at 25.0 +/- 0.1 degrees C. The relaxivity values of [Gd(TTDA-MA)](-), [Gd(TTDA-BA)](-), and [Gd(TTDA-MOBA)](-) remained constant with respect to pH changes over the range 4.5-12.0. The (17)O NMR chemical shift of H(2)O induced by [Dy(TTDA-MA)(H(2)O)](-) at pH 6.80 showed 0.9 inner-sphere water molecules. Water proton relaxivity values for [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) at 37.0 +/- 0.1 degrees C and 20 MHz are 3.89, 4.21, and 4.25, respectively. The water-exchange lifetime (tau(M)) and rotational correlation time (tau(R)) of [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) are obtained from reduced the (17)O relaxation rate and chemical shifts of H(2)(17)O. The (2)H NMR longitudinal relaxation rates of the deuterated diamagnetic lanthanum complexes for the rotational correlation time were also thoroughly investigated. The water-exchange rates (K(298)(ex) for [Gd(TTDA-MA)(H(2)O)](-), [Gd(TTDA-BA)(H(2)O)](-), and [Gd(TTDA-MOBA)(H(2)O)](-) are lower than that of [Gd(TTDA)(H(2)O)](2)(-) but significantly higher than those of [Gd(DTPA)(H(2)O)](2)(-) and [Gd(DTPA-BMA)(H(2)O)]. The rotational correlation times for [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) are significantly longer than those of [Gd(TTDA)(H(2)O)](2)(-) and [Gd(DTPA)(H(2)O)](2)(-) complexes. The marked increase of the relaxivity of [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) results mainly from their longer rotational correlation time. The noncovalent interaction between human serum albumin (HSA) and [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) complexes containing a hydrophobic substituent was investigated by measuring the water proton relaxation rate of the aqueous solutions. The binding association constant (K(A)) values are 1.0 +/- 0.2 x 10(3) and 1.3 +/- 0.2 x 10(3) M(-1) for [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-), which indicates a stronger interaction of [Gd(TTDA-BA)(H(2)O)](-) and [Gd(TTDA-MOBA)(H(2)O)](-) with HSA.     

Nitrosyl induces phosphorous-acid dissociation in ruthenium(II)

The trans-[Ru(NO)(NH(3))(4)(P(OH)(3))]Cl(3) complex was synthesized by reacting [Ru(H(2)O)(NH(3))(5)](2+) with H(3)PO(3) and characterized by spectroscopic ((31)P-NMR, δ = 68 ppm) and spectrophotometric techniques (λ = 525 nm, ε = 20 L mol(-1) cm(-1); λ = 319 nm, ε = 773 L mol(-1) cm(-1); λ = 241 nm, ε = 1385 L mol(-1) cm(-1); ν(NO(+)) = 1879 cm(-1)). A pK(a) of 0.74 was determined from infrared measurements as a function of pH for the reaction: trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) + H(2)O ? trans-[Ru(NO)(NH(3))(4)(P(O(-))(OH)(2))](2+) + H(3)O(+). According to (31)P-NMR, IR, UV-vis, cyclic voltammetry and ab initio calculation data, upon deprotonation, trans-[Ru(NO)(NH(3))(4)(P(OH)(3))](3+) yields the O-bonded linkage isomer trans- [Ru(NO)(NH(3))(4)(OP(OH)(2))](2+), then the trans-[Ru(NO)(NH(3))(4)(OP(H)(OH)(2))](3+) decays to give the final products H(3)PO(3) and trans-[Ru(NO)(NH(3))(4)(H(2)O)](3+). The dissociation of phosphorous acid from the [Ru(NO)(NH(3))(4)](3+) moiety is pH dependent (k(obs) = 2.1 × 10(-4) s(-1) at pH 3.0, 25 °C).