Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity.

Two oxoiron(IV) isomers (R 2a and R 2b) of general formula [FeIV (O)(R PyNMe3 )(CH3 CN)]2+ are obtained by reaction of their iron(II) precursor with NBu4 IO4 . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between R 2a and R 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that R 2a reacts one order of magnitude faster than R 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in R 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

Spin State Tunes Oxygen Atom Transfer towards FeIV O Formation in FeII Complexes.

Oxoiron(IV) complexes bearing tetradentate ligands have been extensively studied as models for the active oxidants in non-heme iron-dependent enzymes. These species are commonly generated by oxidation of their ferrous precursors. The mechanisms of these reactions have seldom been investigated. In this work, the reaction kinetics of complexes [FeII (CH3 CN)2 L](SbF6 )2 ([1](SbF6 )2 and [2](SbF6 )2 ) and [FeII (CF3 SO3 )2 L] ([1](OTf)2 and [2](OTf)2 (1, L=Me,H Pytacn; 2, L=nP,H Pytacn; R,R' Pytacn=1-[(6-R'-2-pyridyl)methyl]-4,7- di-R-1,4,7-triazacyclononane) with Bu4 NIO4 to form the corresponding [FeIV (O)(CH3 CN)L]2+ (3, L=Me,H Pytacn; 4, L=nP,H Pytacn) species was studied in acetonitrile/acetone at low temperatures. The reactions occur in a single kinetic step with activation parameters independent of the nature of the anion and similar to those obtained for the substitution reaction with Cl- as entering ligand, which indicates that formation of [FeIV (O)(CH3 CN)L]2+ is kinetically controlled by substitution in the starting complex to form [FeII (IO4 )(CH3 CN)L]+ intermediates that are converted rapidly to oxo complexes 3 and 4. The kinetics of the reaction is strongly dependent on the spin state of the starting complex. A detailed analysis of the magnetic susceptibility and kinetic data for the triflate complexes reveals that the experimental values of the activation parameters for both complexes are the result of partial compensation of the contributions from the thermodynamic parameters for the spin-crossover equilibrium and the activation parameters for substitution. The observation of these opposite and compensating effects by modifying the steric hindrance at the ligand illustrates so far unconsidered factors governing the mechanism of oxygen atom transfer leading to high-valent iron oxo species.

The Mechanism of the Intramolecular Hydrocarbyl Metathesis within a Planar Triruthenium Cluster: Combining Core Flexibility with Hydride Mobility.

The transition metal catalysed formation and cleavage of C-C bonds is of utmost importance in synthetic chemistry. While most of the existing homogeneous catalysts are mononuclear, knowledge of the behaviour of polynuclear species is much more limited. By using computational methods, here we shed light into the mechanistic details of the thermally-induced isomerization of Cp*3 Ru3 (µ-H)2 (µ3 -η2 -pentyne)(µ3 -pentylidyne) (2) into Cp*3 Ru3 (µ-H)2 (µ3 -η2 -octyne)(µ3 -ethylidyne) (3), a process that involves the migration of a C3 fragment between the hydrocarbyl ligands and across the plane formed by the three Ru centres. Our results show this to be a complex transformation that comprises of five individual rearrangements in an A→B→A→B→A order. Each so-called rearrangement A consists of the CH migration from the µ3 -η2 -alkyne into the µ3 -alkylidine ligand in the other side of the Ru3 plane. This process is facilitated by the cluster's ability to adopt open-core structures in which one Ru-Ru bond is broken and a new C-C bond is formed. In contrast, rearrangements B do not involve the formation or cleavage of C-C bonds, nor do they require the opening of the cluster core. Instead, they consist of the isomerization of the µ3 -η2 -alkyne and µ3 -alkylidyne ligands on each side of the triruthenium plane into µ3 -alkylidyne and µ3 -η2 -alkyne, respectively. Such transformation implies the migration of three H atoms within the hydrocarbyl ligands, and in this case, it is aided by the cluster's ability to behave as a H reservoir. All in all, this study highlights the plasticity of these Ru3 clusters, whereby Ru-Ru, Ru-C, Ru-H, C-C, and C-H bonds are formed and broken with surprising ease.

Cobalt(II) Pentaaza-Macrocyclic Schiff Base Complex as Catalyst for Light-Driven Hydrogen Evolution in Water: Electrochemical Generation and Theoretical Investigation of the One-Electron Reduced Species.

We previously reported that the tetraazamacrocyclic Schiff base complex [CoIII(CR14)(X)2]n+ (CR14 = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, X = Cl (n = 1) (1-Cl2) or H2O (n = 3) (1-(H2O)2)) is a very efficient H2-evolving catalyst (HEC) in fully aqueous solutions at pH 4.0-4.5 when used in a photocatalytic system including a photosensitizer and ascorbate as sacrificial electron donor. The excellent H2-evolving activity of this complex, compared to other cobalt and rhodium catalysts studied in the same photocatalytic conditions, can be related to the high stability of its two-electron reduced form, the putative "Co(I)" state. These very interesting results led us to investigate the H2-evolving performances of a series of compounds from a close-related family, the pentaaza-macrocyclic cobalt [CoII(CR15)(H2O)2]Cl2 complex (2, CR15 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), which comprises a larger macrocycle with five nitrogen atoms instead of four. Electrochemical as well as spectroscopic investigations in CH3CN coupled to density functional theory (DFT) calculations point to decoordination of one of the amine upon reduction of Co(II) to the low-valent "Co(I)" form. The resulting unchelated amine could potentially act as a proton relay promoting the H2 formation via proton-coupled-electron transfer (PCET) reactions. Besides, the iron, manganese, and zinc analogues, [FeII(CR15)(X)2]n+ (X = Cl (n = 0) or H2O (n = 2)) (3), [MnII(CR15)(CH3CN)2](PF6)2 (4), and {[ZnII(CR15)Cl](PF6)}n (5) were also synthesized and investigated. The photocatalytic activity of 2-5 toward proton reduction was then evaluated in a tricomponent system containing the [RuII(bpy)3]Cl2 photosensitizer and ascorbate, in fully aqueous solution. The photocatalytic activity of 2 was also compared with that of 1 in the same experimental conditions. It was found that the number of catalytic cycles versus catalyst for 2 are slightly lower than that for 1, suggesting that if the amine released upon reduction of 2 plays a role in promoting the H2-evolving catalytic activity, other factors balance this effect. Finally, photophysical and nanosecond transient absorption spectroscopies were used to investigate the photocatalytic system.

Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2-Evolving Catalysts.

We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H2 production, we performed herein a detailed electrochemical investigation of the [RhIII(dmbpy)2Cl2]+ and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

Iron(II) Complexes with Scorpiand-Like Macrocyclic Polyamines: Kinetico-Mechanistic Aspects of Complex Formation and Oxidative Dehydrogenation of Coordinated Amines.

The Fe(II) coordination chemistry of a pyridinophane tren-derived scorpiand type ligand containing a pyridine ring in the pendant arm is explored by potentiometry, X-ray, NMR, and kinetics methods. Equilibrium studies in water show the formation of a stable [FeL]2+ complex that converts to monoprotonated and monohydroxylated species when the pH is changed. A [Fe(H-2L)]2+ complex containing an hexacoordinated dehydrogenated ligand has been isolated, and its crystal structure shows the formation of an imine bond involving the aliphatic nitrogen of the pendant arm. This complex is low spin Fe(II) both in the solid state and in solution, as revealed by the Fe-N bond lengths and by the NMR spectra, respectively. The formation rate of [Fe(H-2L)]2+ in aqueous solutions containing Fe2+ and L (1:1 molar ratio) is strongly dependent on the pH, the process being completed in times that range from months in acid solutions to hours in basic conditions. However, detailed kinetic studies show that those differences are caused, at least in part, by the effect of pH on the rate of formation of the unoxidized [FeL]2+ complex. In this sense, the protonation of the donor atoms in the pendant arm of the scorpiand ligand leads to the formation of protonated species resistant to oxidative dehydrogenation. Complementary studies in acetonitrile solution indicate that the initial stage in the oxidative dehydrogenation process is the oxidation of the starting complex to form a [FeL]3+ complex, which then undergoes disproportionation into [Fe(H-2L)]2+ and [FeL]2+. Experiments starting with Fe(III) have allowed us to determine that disproportionation occurs with first order kinetics both in water and acetonitrile solutions. However, whereas a significant acceleration is observed in water when the pH is increased, no effect of the addition of acid or base on the rate of disproportionation is observed in acetonitrile. Oxidative dehydrogenation of the Fe(II) complex formed in experiments starting with an Fe(III) salt is slower than that occurring when an Fe(II) salt is used, an observation that can be explained in terms of the formation of two different Fe(III) complexes, one of them with a structure unable to evolve directly toward the product of oxidative dehydrogenation.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)](n+) Complexes in Aqueous Solution.

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

An efficient Ru(II) -Rh(III) -Ru(II) polypyridyl photocatalyst for visible-light-driven hydrogen production in aqueous solution.

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.

Efficient photocatalytic hydrogen production in water using a cobalt(III) tetraaza-macrocyclic catalyst: electrochemical generation of the low-valent Co(I) species and its reactivity toward proton reduction.

A very efficient homogeneous system for visible-light driven hydrogen production in water is reported. This comprises the [Co(CR)Cl2](+) cobalt(III) tetraaza-macrocyclic complex (Cat1) as a noble metal-free catalyst, [Ru(bpy)3]Cl2 as a photosensitizer and ascorbate/ascorbic acid as a sacrificial electron donor and buffer. This system gives up to 1000 turnovers at pH 4.0 versus the catalyst with a relatively low photosensitizer/catalyst ratio (10/1) and a high concentration of catalyst (1 × 10(−4) M), thus producing a significant amount of H2 (12.3 mL for 5 mL of solution). It also exhibits long-term stability (more than 20 hours). The efficiency of Cat1 has been compared under the same experimental conditions to those of three other H2-evolving catalysts, which are known to operate in water, [Co{(DO)(DOH)pn}Br2] (Cat2), [Co(dmbpy)3]Cl2 (Cat3) and [Rh(dmbpy)2Cl2]Cl (Cat4). These comparative studies show that Cat4, although based on a noble metal, is about four times less active, while Cat2 and Cat3 produce more than one hundred times less hydrogen than Cat1. The low-valent CoI form of Cat1 has been successfully electrogenerated in CH3CN. Its high stability can be related to the high catalytic performance of the Cat1 system. We have also shown that in acidic aqueous solution (photocatalytic conditions) reduction at a slightly more negative potential than the Co(II)/Co(I) couple is needed to ensure efficient catalysis; this reduction is performed by the photogenerated [Ru(II)(bpy)2(bpy(˙−))](+) species.

Fe(II) complexes of pyridine-substituted thiosemicarbazone ligands as catalysts for oxidations with hydrogen peroxide.

The reaction of three [FeII(TSC)2] complexes, where TSC is a pyridine-substituted thiosemicarbazone of the HDpT or HBpT families, with H2O2 in acetonitrile solution does not result in the accumulation of the corresponding [FeIII(TSC)2]+ complexes. Instead, a mixture of diamagnetic low-spin FeII species is generated. According to the MS spectra, those species result from the sequential addition of up to five oxygen atoms to the complex. This capability for the addition of oxygen atoms suggested that oxygen atom transfer to external substrates may be possible, and these TSC complexes were tested in the oxidation of thioanisole and styrene with H2O2. As hypothesized, the complexes are active in both the oxidation of thioanisole to its sulfoxide and styrene to benzaldehyde, with time scales indicating the participation of the species containing added oxygen atoms. Interestingly, the free thiosemicarbazone ligands and the [Zn(Dp44mT)2] complex also catalyse the selective sulfoxidation of thioanisole, but they are ineffective in catalysing styrene oxidation to benzaldehyde. These findings open up new directions for the development of thiosemicarbazone-based metal catalysts for oxidation processes.

Participatory Design of a Clinical Trial Eligibility Criteria Simplification Method.

Clinical trial eligibility criteria are important for selecting the right participants for clinical trials. However, they are often complex and not computable. This paper presents the participatory design of a human-computer collaboration method for criteria simplification that includes natural language processing followed by user-centered eligibility criteria simplification. A case study on the ARCADIA trial shows how criteria were simplified for structured database querying by clinical researchers and identifies rules for criteria simplification and concept normalization.

Hydrogen and copper ion induced molecular reorganizations in two new scorpiand-like ligands appended with pyridine rings.

The synthesis of two new ligands constituted of a tris(2-aminoethyl)amine moiety linked to the 2,6 positions of a pyridine spacer through methylene groups in which the hanging arm is further functionalized with a 2-pycolyl (L1) or 3-pycolyl (L2) group is presented. The protonation of L1 and L2 and formation of Cu(2+) complexes have been studied using potentiometric, NMR, X-ray, and kinetic experiments. The results provide new information about the relevance of molecular movements in the chemistry of this kind of so-called scorpiand ligand. The comparison between these two ligands that only differ in the position of the substituent at the arm reveals important differences in both thermodynamic and kinetic properties. The Cu(2+) complex with L1 is several orders of magnitude more stable than that with L2, surely because in the latter case the pyridine nitrogen at the pendant arm is unable to coordinate to the metal ion with the ligand acting as hexadentate, a possibility that occurs in the case of [CuL1](2+), as demonstrated by its crystal structure. Significant differences are also found between both ligands in the kinetic studies of complex formation and decomposition. For L1, those processes occur in a single kinetic step, whereas for L2 they occur with the formation of a detectable reaction intermediate whose structure corresponds to that resulting from the movement typical of scorpiands. Another interesting conclusion derived from kinetic studies on complex formation is that the reactive form of the ligand is H(3)L(3+) for L1 and H(2)L(2+) for L2. DFT calculations are also reported, and they allow a rationalization of the kinetic results relative to the reactive forms of the ligands in the process of complex formation. In addition, they provide a full picture of the mechanistic pathway leading to the formation of the first Cu-N bond, including outer-sphere complexation, water dissociation, and reorganization of the outer-sphere complex.

Geometric isomerism in pentacoordinate Cu2+ complexes: equilibrium, kinetic, and density functional theory studies reveal the existence of equilibrium between square pyramidal and trigonal bipyramidal forms for a tren-derived ligand.

A ligand (L1) (bis(aminoethyl)[2-(4-quinolylmethyl)aminoethyl]amine) containing a 4-quinolylmethyl group attached to one of the terminal amino groups of tris(2-aminoethyl)amine (tren) has been prepared, and its protonation constants and stability constants for the formation of Cu(2+) complexes have been determined. Kinetic studies on the formation of Cu(2+) complexes in slightly acidic solutions and on the acid-promoted complex decomposition strongly suggest that the Cu(2+)-L1 complex exists in solution as a mixture of two species, one of them showing a trigonal bipyramidal (tbp) coordination environment with an absorption maximum at 890 nm in the electronic spectrum, and the other one being square pyramidal (sp) with a maximum at 660 nm. In acidic solution only a species with tbp geometry is formed, whereas in neutral and basic solutions a mixture of species with tbp and sp geometries is formed. The results of density functional theory (DFT) calculations indicate that these results can be rationalized by invoking the existence of an equilibrium of hydrolysis of the Cu-N bond with the amino group supporting the quinoline ring so that CuL1(2+) would be actually a mixture of tbp [CuL1(H(2)O)](2+) and sp [CuL1(H(2)O)(2)](2+). As there are many Cu(2+)-polyamine complexes with electronic spectra that show two overlapping bands at wavelengths close to those observed for the Cu(2+)-L1 complex, the existence of this kind of equilibrium between species with two different geometries can be quite common in the chemistry of these compounds. A correlation found between the position of the absorption maximum and the tau parameter measuring the distortion from the idealized tbp and sp geometries can be used to estimate the actual geometry in solution of this kind of complex.

Synthesis, structure, spectroscopy and reactivity of new heterotrinuclear water oxidation catalysts.

Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp-) and 2,2':6',2''-terpyridine (trpy) of the general formula {[RuII(trpy)]2(µ-[M(X)2(bpp)2])}(PF6)2, where M = CoII, MnII and X = Cl-, AcO- (M = CoII, X = Cl-: Ru2Co-Cl2 ; M = MnII, X = Cl-: Ru2Mn-Cl2 ; M = CoII, X = AcO-: Ru2Co-OAc2 ; M = MnII, X = AcO-: Ru2Mn-OAc2 ), have been prepared for the first time. The complexes have been characterized using different spectroscopic techniques such as UV-vis, IR, and mass spectrometry. X-Ray diffraction analyses have been used to characterize the Ru2Mn-Cl2 and Ru2Mn-OAc2 complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CH3CN or CH2Cl2) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexes Ru2Co-OAc2 and Ru2Mn-OAc2 are further characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all complexes in the temperature range of 2-300 K reveal paramagnetic properties due to the presence of high spin Co(ii) and Mn(ii) centers. The complexes Ru2Co-OAc2 and Ru2Mn-OAc2 act as precatalysts for the water oxidation reaction, since the acetato groups are easily replaced by water at pH = 7 generating the active catalysts, {[Ru(H2O)(trpy)]2(µ-[M(H2O)2(bpp)2])}4+ (M = CoII: Ru2Co-(H2O)4 ; M = MnII: Ru2Mn-(H2O)4 ). The photochemical water oxidation reaction is studied using [Ru(bpy)3]2+ as the photosensitizer and Na2S2O8 as a sacrificial electron acceptor at pH = 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOFi = 0.21 s-1), whereas the Mn containing complex only generates a TON of 8. The water oxidation reaction of Ru2Co-(H2O)4 is further investigated using oxone as a sacrificial chemical oxidant at pH = 7. Labelled water oxidation experiments suggest that a nucleophilic attack mechanism is occurring at the Co site of the trinuclear complex with cooperative involvement of the two Ru sites, via electronic coupling through the bpp- bridging ligand and via neighboring hydrogen bonding.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight.

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2](+) (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru(II)(bpy)3](2+) (Ru) as photosensitizer and ascorbate/ascorbic acid (HA(-)/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co(III)(CR)(H2O)2](3+) (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn(II)(CR)Cl](+) (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni(II)(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA(-)/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×10(7) M(-1) s(-1)) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×10(9) M(-1) s(-1)). The reduced catalyst can then enter into the cycle of hydrogen evolution.

Luminescent iridium complexes for detection of molybdate.

Reactions of [Ir(C^N)(2)Cl](2) [HC^N = 2-(3-R-phenyl)pyridine, 2-(3-R-phenylpyrazole) R = H, Me] with Me(2)-phencat give luminescent complexes [Ir(C^N)(2)(Me(2)-phencat)][PF(6)] (Me(2)-2a, b, c)[PF(6)]. Deprotection of the methoxy groups with BBr(3) is problematic as simultaneous bromination of the cyclometallated phenyl groups occurs. However, deprotection of Me(2)-phencat with BBr(3) followed by complexation with [Ir(C^N)(2)Cl](2) gives luminescent complexes [Ir(C^N)(2)(H(2)-phencat)][PF(6)] (H(2)-3a, c)[PF(6)], which are luminescent sensors for molybdate.

Copper(II) complexes of quinoline polyazamacrocyclic scorpiand-type ligands: X-ray, equilibrium and kinetic studies.

The formation of Cu(II) complexes with two isomeric quinoline-containing scorpiand-type ligands has been studied. The ligands have a tetraazapyridinophane core appended with an ethylamino tail including 2-quinoline (L1) or 4-quinoline (L2) functionalities. Potentiometric studies indicate the formation of stable CuL(2+) species with both ligands, the L1 complex being 3-4 log units more stable than the L2 complex. The crystal structure of [Cu(L1)](ClO(4))(2)·H(2)O shows that the coordination geometry around the Cu(2+) ions is distorted octahedral with significant axial elongation; the four Cu-N distances in the equatorial plane vary from 1.976 to 2.183 Å, while the axial distances are of 2.276 and 2.309 Å. The lower stability of the CuL2(2+) complex and its capability of forming protonated and hydroxo complexes suggest a penta-dentate coordination of the ligand, in agreement with the type of substitution at the quinoline ring. Kinetic studies on complex formation can be interpreted by considering that initial coordination of L1 and L2 takes place through the nitrogen atom in the quinoline ring. This is followed by coordination of the remaining nitrogen atoms, in a process that is faster in the L1 complex probably because substitution at the quinoline ring facilitates the reorganization. Kinetic studies on complex decomposition provide clear evidence on the occurrence of the molecular motion typical of scorpiands in the case of the L2 complex, for which decomposition starts with a very fast process (sub-millisecond timescale) that involves a shift in the absorption band from 643 to 690 nm.

Inhibition of xanthine oxidase by thiosemicarbazones, hydrazones and dithiocarbazates derived from hydroxy-substituted benzaldehydes.

Nonpurine xanthine oxidoreductase (XOR) inhibitors represent important alternatives to the purine analogue allopurinol, which is still the most widely used drug in the treatment of conditions associated with elevated uric acid levels in the blood. By condensing mono-, di- and trihydroxybenzaldehydes with aromatic thiosemicarbazides, aryl hydrazides and dithiocarbazates, three series of structurally related Schiff bases were synthesised, characterised and tested for XOR inhibitory activity. Hydroxy substitution in the para-position of the benzaldehyde component was found to confer high inhibitory activities. Acyl hydrazones were generally less potent than thiocarbonyl-containing Schiff bases. Within the thiosemicarbazone series, chloro and cyano substituents in the para-position of the thiosemicarbazide unit increased activities further, up to potencies approximately four-times higher than that of the benchmark allopurinol, as measured under the same assay conditions. In order to illustrate the potential of the Schiff bases to bind directly to the molybdenum centre in the active site of the enzyme, a representative example (H2L) of each inhibitor series was co-ordinated to a cis-dioxomolybdenum(VI) unit, and the resulting complexes, [MoO2(L)MeOH], were structurally characterised. Subsequent steady-state kinetic investigations, however, indicated mixed-type inhibition, similar to that observed for inhibitors known to bind within the substrate access channel of the enzyme, remote from the Mo centre. Enzyme co-crystallisation studies are thus required to determine the exact binding mode. Finally, the coordination of representative inhibitors to copper(II) gave rise to significantly decreased IC50 values, revealing an additive effect that merits further investigation.

Structural reorganisation in polytopic receptors revealed by kinetic studies.

One of the first kinetic studies of metal ion reorganisation between the different sites of a tritopic polyaza ligand reveals well defined pathways for the movement of the metal ion.

Mechanistic aspects of the chemistry of mononuclear Cr(III) complexes with pendant-arm macrocyclic ligands and formation of discrete Cr(III)/Fe(II) and Cr(III)/Fe(II)/Co(III) cyano-bridged mixed valence compounds.

The kinetics and mechanism of the redox reaction between [Fe(II)(CN)(6)](4-) and the macrocyclic ligand complex [CrClL(15)](2+) (L(15) = 6-methyl-1,4,8,12-tetraazacyclopentadecane-6-amine) has been studied at different pH values. In acidic solution, the expected redox process occurs with no formation of any of the possible Cr(III)/Fe(II) mixed valence complexes, as those seen for the Co(III) species of the same family, due to the enhanced lability of the Cr(II) species formed on Fe(II) to Fe(III) oxidation. In alkaline conditions, the formation of the complex [Cr(L(15))(OH)(2)](+) takes place as an initial step that precedes a simple substitution process producing the expected cyano-bridged Cr(III)/Fe(II) complex. In this species the potentially pentadentate ligand, L(15), has a tetradentate coordination mode with a protonated exocyclic primary amine group and the redox potential is shifted to more negative values, thus disfavouring a redox driven reaction; the equivalent complex [CrCl(HL(14))(H(2)O)](3+) (L(14) = 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) has been prepared by the same method and characterized by X-ray crystallography. The final [Fe(II)(CN)(6)](4-) substituted complex, [{(HL(15))(OH)Cr(III)NC}Fe(II)(CN)(5)](-) shows pK(a) values of 3.8 and 7.4, as expected for the aqua and amino ligands, respectively. Its characterization indicated its Class II mixed valence character with a very intense MMCT band at 350 nm showing a much larger extinction coefficient than that observed for the Co(III) complexes of the same family. This fact is in good agreement with the much larger Cr(III)-Fe(II) (t(2g)-t(2g)) coupling through cyanide bridging ligands expected for these complexes. The fully mixed metal/valence/ligand trimetallic complex [{(HL(15))(OH)Cr(III)NC}{L(13)Co(III)NC}Fe(II)(CN)(4)](2+) has been prepared following the same procedures and the results are comparable. The final complex has the same Class II mixed valence character and its electronic spectrum shows the characteristics of both the Fe(II)-to-Cr(III) and Fe(II)-to-Co(III) CT bands. The study allows the application of the redox and/or substitutionally driven methodology, already described and kinetically designed, for the preparation of discrete mixed valence complexes of inert centres, so avoiding the unpredictable nature of other approaches seen in the past.