Kineticomechanistic study of the redox pH cycling processes occurring on a robust water-soluble cyanido-bridged mixed-valence {CoIII/FeII}2 square

A kineticomechanistic study of reversible electron-transfer processes undergone by the water-soluble, cyanido-bridged mixed-valence [{CoIII{(Me)2(μ-ET)cyclen}}2{(μ-NC)2FeII(CN)4}2]2- square has been carried out. The oxidation reaction consists of a two-step process with the participation of a solvent-assisted outer-sphere complex, as a result of the establishment of hydrogen bonds that involve the oxo groups of the oxidant (peroxodisulfate) and the terminal cyanido ligands of the tetrametallic square. The formally endergonic reduction reaction of the fully oxidized ([{CoIII{(Me)2(μ-ET)cyclen}}2{(μ-NC)2FeIII(CN)4}2]) core by water, producing hydrogen peroxide from water even at low pH values, is also a two-step process. Each one of these processes requires a set of two preequilibria involving the association of OH- and its subsequent deprotonation by a further OH- anion. The structure of the square compound in its fully protonated form has also been determined by X-ray diffraction and shows the existence of strong hydrogen-bonding interactions, in agreement with the rather high basicity of the terminal cyanido ligands. Likewise, density functional theory calculations on the tetrametallic complex showed zones with negative electrostatic potential around the FeII centers of the square that would account for the establishment of the hydrogen bonds found in the solid state. Spectroelectrochemistry experiments demonstrated the singular stability of the {CoIII/FeII}2 2- complex, as well as that of their partially, {Co2 III/FeIIIFeII}-, and fully, {CoIII/FeIII}2, oxidized counterparts because no hysteresis was observed in these measurements.

Private Security Companies and Shared Responsibility: The Turn to Multistakeholder Standard-Setting and Monitoring Through Self-Regulation-‘Plus'

The rapid and increasing outsourcing of security services by states to Private Security Companies (PSCs) in recent years and associated human rights violations have served as one of the catalysts for long overdue regulation of the global PSC industry. As part of an ‘empirical stocktaking’, this article focuses on current multistakeholder self-regulatory developments in relation to PSCs, in particular the International Code of Conduct for Private Security Providers and the PSC1 certification standard, and considers their likely impact on the responsibility of states in this area. What is clear is that the traditional conception of interna- tional responsibility is ineffectual when applied to PSCs because of its focus on the ex post facto responsibility of states for internationally wrongful acts. Furthermore, the fact that PSCs operate in high risk and complex environments and the fact that their clients are often non-state actors, means that an alternative prophylactic approach to responsibility for human rights violations by PSCs seems to be nec- essary. As it stands, however, the ‘self-regulation-plus’ approach adopted is not the definitive solution. While endeavouring to ensure that PSCs respect human rights, this approach may allow states to evade their own obligations to protect human rights

New method for exploring deactivation kinetics in copper-catalyzed atom-transfer-radical reactions

Copper polyamine complexes are among the most utilized catalysts for controlled radical polymerization reactions. Copper(I) complexes may react reversibly with an alkyl halide to form an alkyl radical, which promotes polymerization, and a copper(II) halido complex in a step known as activation. The kinetics of the reverse reaction between the alkyl radical and higher oxidation-state copper complex (deactivation) are less studied because these reactions approach diffusion-controlled rates, and it is difficult to isolate or quantify the concentration of the alkyl radical (R(•)) in situ. Herein we report a broadly applicable electrochemical technique for simultaneously measuring the kinetics of deactivation and kinetics of activation

Redox-coupled structural changes in copper chemistry: Implications for atom transfer catalysis

The coordination chemistry of copper is dominated by its monovalent and divalent oxidation states and interplay between the two is central to the role of copper-based homogeneous catalysts. Upon reduction of most copper(II) complexes, which commonly are five coordinate, the coordination number rapidly changes to four or less. This feature is advantageous for atom transfer reactions where single electron transfer is coupled to the association or dissociation of a radical. Although this coupled change in coordination number and oxidation state is central to the mechanism of atom transfer reactions, its importance to catalysis is not widely appreciated. Herein we review the structural chemistry of copper(I) and copper(II) with a range of multidentate N-donor ligands employed in atom transfer radical polymerisation (ATRP). The remarkable resistance of copper(I) to accept more than four donor atoms is illustrated and discussed in the context of both its solid state and solution structures

Organo-copper(II) complexes as products of radical atom transfer

Copper complexes bearing polyamine chelate ligands are among the most widely used and highly active catalysts for atom transfer radical polymerization (ATRP). Copper(I) complexes of these ligands (CuL) react with an alkyl halide initiator (RX) in the atom transfer step to generate the higher oxidation state halido complex CuLX and the radical R. However, CuL present in the reaction also has the potential to react with the liberated radicals to generate the organometallic species CuLR (where R is formally a carbanion). The reversible association of radical and CuL would facilitate the operation of an alternate, competitive controlled radical polymerization pathway known as organometallic-mediated radical polymerization (OMRP). Recently this possibility has been proposed to occur for a number of different copper catalysts under ATRP conditions, but unequivocal evidence of this organometallic adduct is lacking. Herein we provide direct observation of this species, including an optical spectrum for two of the most commonly used copper catalysts. Furthermore, using cyclic voltammetry coupled to simulations, we are able to determine each of the key thermodynamic and kinetic steps involved in both the atom transfer and radical transfer pathways to assess the impact of ligand, solvent, and initiator on these

Domestic violence and maternal reports of young children's functioning

Maternal reports of 60 preschool-aged children were used to investigate trauma responses to living in households where domestic violence was present. Post-trauma symptoms were measured using developmentally modified criteria based on the Child Behavior Checklist (CBCL). Mothers’ level of anxiety, depression, somatization, and self-reported parenting stress were also assessed. Results suggested that, in addition to clinical levels of Internalizing, Externalizing and Total Problem scores on the CBCL, young children displayed a range of post-trauma symptoms. Mothers reported a high level of violence, and a significant relationship was found between self-reported levels of distress and parenting stress. Parenting stress was found to be the strongest predictor of children’s scores on the CBCL. Although domestic violence alone was not significantly correlated to child outcomes, results indicated that maternal distress adversely impacted on the parent-child relationship. The importance of the child’s relationship with the primary caregiver was discussed. Implications regarding further research, developmentally appropriate diagnostic criteria, and early interventions with the primary caregiver are highlighted.

A kinetico-mechanistic study on CuII deactivators employed in atom transfer radical polymerization

Copper complexes of tertiary amine ligands have emerged as the catalysts of choice in the extensively employed atom transfer radical polymerization (ATRP) protocol. The halide ligand substitution reactions of five-coordinate copper(II) complexes of tris[2-(dimethylamino)ethyl]amine (Me6tren), one of the most active ATRP catalysts, has been studied in a range of organic solvents using stopped-flow techniques. The kinetic and activation parameters indicate that substitution reactions on [CuII(Me6tren)X]+ (X- = Cl- and Br-) and [CuII(Me6tren)(Solv)]2+ (Solv = MeCN, DMF, DMSO, MeOH, EtOH) are dissociatively activated; this behavior is independent of the solvent used. Adjusting the effective concentration of the solvent by addition of an olefinic monomer to the solution does not affect the kinetics of the halide binding (kon) but can alter the outer-sphere association equilibrium constant (KOS) between reactants prior to the formal ligand substitution. Halide (X-/Y-) exchange reactions (X = Br and Y = Cl) involving the complex [Cu(Me6tren)X]+ and Y- reveal that the substitution is thermodynamically favored. The influence of solvent on the substitution reactions of [Cu(Me6tren)X]+ is complex; the more polar DMF confers a greater entropic driving force but larger enthalpic demands than MeCN. These substitution reactions are compared with those for copper(II) complexes bearing the tris[2-(diethylamino)ethyl]amine (Et6tren) and tris[2-(pyridyl)methyl]amine (tpa) ligands, which have also been used as catalysts for ATRP. Changing the ligand has a significant impact on the kinetics of X-/Y- exchange. These correlations are discussed in relation to the ability of five-coordinate [CuLX]+ complexes to deactivate radicals in ATRP

SET-LRP of NIPAM in water via in situ reduction of Cu(II) to Cu(0) with NaBH4

The direct and quantitative reduction of the air-stable Cu(II) Br-2/Me6TREN to Cu(0) by NaBH4 represents a new method for the aqueous single electron transfer-living radical polymerization (SET-LRP) of water soluble polymers. By changing the stoichiometry of NaBH4 to Cu(II)Br-2, any desired ratio of Cu(II)Br-2 to Cu(0) could be obtained with no evidence of Cu(I) species. This quantitative reduction to Cu(0) in combination with rapid disproportionation of Cu(I) Br/Me6TREN in water resulted in activation by Cu(0) initially and during the polymerization. Polymerizations of NIPAM produced polymer within minutes and with controlled and narrow molecular weight distributions in agreement with ideal 'living' radical behavior. The direct in situ thio-bromo 'click' reaction produce polymers with stable chain-end functionality, eliminating the concern of hydrolysis of the halide end-group (i.e. Br). It was found that the end-group functionality was greater than 95%, and for the very rapid polymerizations close to 100%

N-Oxides rescue Ru(v) in catalytic Griffith-Ley (TPAP) alcohol oxidations

The redox and ligand exchange reactions of oxido-ruthenium complexes are central to the function of the Sharpless and Griffith-Ley one-step alcohol oxidation protocols. However, their mechanisms have not been elucidated. Cyclic voltammetry and UV-vis spectroelectrochemical analysis of [RuO4](-) has provided new insight into the key ruthenium oxidation states involved in catalysis. Furthermore, the oxidation states sensitive to the presence of the N-oxide co-oxidant N-methylmorpholine N-oxide (NMO), and its role in catalysis, have been determined

The fate of copper catalysts in atom transfer radical chemistry

Recent experimental observations of the role played by copper polyamine catalysts in atom transfer radical polymerisation (ATRP) have indicated that important side reactions in competition with conventional radical activation by halogen atom transfer may occur and that these can potentially affect reaction control. Specifically the copper(i) catalyst may recapture the organic radical and even possibly abstract H-atoms from the radical to form copper(ii) alkyl or hydrido complexes. Employing an established electrochemical methodology and supported by Density Functional Theory calculations, we report a systematic experimental and theoretical investigation of the ATRP catalyst [Cu(Me tren)] (Me tren = tris(2-dimethylaminoethyl)amine) and its reactions with a homologous series of bromo-ester initiators that yield tertiary, secondary and primary radicals. The results reveal that organocopper(ii) complexes are preferred to hydrido complexes and that increasing steric bulk at the initiator steers the reaction pathway away from the organocopper complex and toward the desired ATRP product