Spin-Crossing in the ()-Selective Alkyne Semihydrogenation Mechanism Catalyzed by MoS Clusters: A Density Functional Theory Exploration.

Semihydrogenation of internal alkynes catalyzed by the air-stable imidazolyl amino [MoSCl(ImNH)] cluster selectively affords the ()-alkene under soft conditions in excellent yields. Experimental results suggest a sulfur-based mechanism with the formation of a dithiolene adduct through interaction of the alkyne with the bridging sulfur atoms. However, computational studies indicate that this mechanism is unable to explain the experimental outcome: mild reaction conditions, excellent selectivity toward the ()-isomer, and complete deuteration of the vinylic positions in the presence of CDOD and CHOD.

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

The reaction of three [Fe(TSC)] complexes, where TSC is a pyridine-substituted thiosemicarbazone of the HDpT or HBpT families, with HO in acetonitrile solution does not result in the accumulation of the corresponding [Fe(TSC)] complexes. Instead, a mixture of diamagnetic low-spin Fe species is generated. According to the MS spectra, those species result from the sequential addition of up to five oxygen atoms to the complex.

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

Two oxoiron(IV) isomers ( 2a and 2b) of general formula [Fe (O)( PyNMe )(CH CN)] are obtained by reaction of their iron(II) precursor with NBu IO . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between 2a and 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers.

Spin State Tunes Oxygen Atom Transfer towards Fe O Formation in Fe 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.

Salen‑manganese complexes for controlling ROS damage: Neuroprotective effects, antioxidant activity and kinetic studies.

A new manganese(III) complex [MnL(DCA)(HO)](HO)1 [HL is the chelating ligand N,N'-bis(2-hydroxy-3-methoxybenzylidene)-1,2-diaminopropane, and DCA is dicyanamide], has been prepared and characterized by different analytical and spectroscopic techniques. The tetragonally elongated octahedral geometry for the manganese coordination sphere was revealed by X-ray diffraction studies for 1. The antioxidant behavior of this complex and other manganese(III)-salen type complexes was tested through superoxide dismutase and catalase probes, and through the study of their neuroprotective effects in SH-SY5Y neuroblastoma cells.

Pitfalls in the ABTS Peroxidase Activity Test: Interference of Photochemical Processes.

ABTS (2,2'-azinobis(3-ethylbenzothiazoline)-6-sulfonic acid) oxidation to form its radical cation in the presence of HO is frequently used as a test for determining the peroxidase activity of enzyme mimics. Detailed studies using salen-type Mn(III) complexes show that photochemical processes involving HO, ABTS, and the complex itself can lead to erroneous results. The capability of the complexes to act as OH scavengers can be also relevant when the mechanism of their biological activity is considered.

Coordination Chemistry of Cu Complexes of Small N-Alkylated Tetra-azacyclophanes with SOD Activity.

A new tetraaza-pyridinophane macrocycle (L1) N-alkylated with two isopropyl and one methyl groups symmetrically disposed has been prepared and its behavior compared with those of the unsubstituted pyridinophane (L3) and the related compound with three methyl groups (L2). The protonation studies show that, first, a proton binds to the central methylated amine group of L1, while, second protonation leads to a reorganization of the protons that are at this stage attached to the lateral isopropylated amines. The X-ray structure of [HL1] agrees with the UV-vis and NMR studies as well as with the results of DFT calculations.

Acid-Triggered O-O Bond Heterolysis of a Nonheme Fe (OOH) Species for the Stereospecific Hydroxylation of Strong C-H Bonds.

A novel hydroperoxoiron(III) species [Fe (OOH)(MeCN)(PyNMe )] (3) has been generated by reaction of its ferrous precursor [Fe (CF SO ) (PyNMe )] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low-spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C-H bonds.

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] complex that converts to monoprotonated and monohydroxylated species when the pH is changed. A [Fe(HL)] 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.

Kinetic Analysis and Mechanism of the Hydrolytic Degradation of Squaramides and Squaramic Acids.

The hydrolytic degradation of squaramides and squaramic acids, the product of partial hydrolysis of squaramides, has been evaluated by UV spectroscopy at 37 °C in the pH range 3-10. Under these conditions, the compounds are kinetically stable over long time periods (>100 days). At pH >10, the hydrolysis of the squaramate anions shows first-order dependence on both squaramate and OH.

Kinetics Aspects of the Reversible Assembly of Copper in Heterometallic MoCuS Clusters with 4,4'-Di-tert-butyl-2,2'-bipyridine.

Treatment of the triangular [MoSCl(dbbpy)]Cl cluster ([1]Cl) with CuCl produces a novel tetrametallic cuboidal cluster [Mo(CuCl)SCl(dbbpy)][CuCl] ([2][CuCl]), whose crystal structure was determined by X-ray diffraction (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This species, which contains two distinct types of Cu(I), is the first example of a diimine-functionalized heterometallic MM'S cluster. Kinetics studies on both the formation of the cubane from the parent trinuclear cluster and its dissociation after treatment with halides, supported by NMR, electrospray ionization mass spectrometry, cyclic voltammetry, and density functional theory calculations, are provided.

Exceedingly Fast Oxygen Atom Transfer to Olefins via a Catalytically Competent Nonheme Iron Species.

The reaction of [Fe(CF3 SO3 )2 (PyNMe3 )] with excess peracetic acid at -40 °C leads to the accumulation of a metastable compound that exists as a pair of electromeric species, [Fe(III) (OOAc)(PyNMe3 )](2+) and [Fe(V) (O)(OAc)(PyNMe3 )](2+) , in fast equilibrium. Stopped-flow UV/Vis analysis confirmed that oxygen atom transfer (OAT) from these electromeric species to olefinic substrates is exceedingly fast, forming epoxides with stereoretention. The impact of the electronic and steric properties of the substrate on the reaction rate could be elucidated, and the relative reactivities determined for the catalytic oxidations could be reproduced by kinetic studies.

Trapping a Highly Reactive Nonheme Iron Intermediate That Oxygenates Strong C-H Bonds with Stereoretention.

An unprecedentedly reactive iron species (2) has been generated by reaction of excess peracetic acid with a mononuclear iron complex [Fe(II)(CF3SO3)2(PyNMe3)] (1) at cryogenic temperatures, and characterized spectroscopically. Compound 2 is kinetically competent for breaking strong C-H bonds of alkanes (BDE ≈ 100 kcal·mol(-1)) through a hydrogen-atom transfer mechanism, and the transformations proceed with stereoretention and regioselectively, responding to bond strength, as well as to steric and polar effects. Bimolecular reaction rates are at least an order of magnitude faster than those of the most reactive synthetic high-valent nonheme oxoiron species described to date.

On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M3 S4 Clusters with Alkynes: Insights from Experiment and Theory.

Whereas the cluster [Mo3 S4 (acac)3 (py)3 ](+) ([1](+) , acac=acetylacetonate, py=pyridine) reacts with a variety of alkynes, the cluster [W3 S4 (acac)3 (py)3 ](+) ([2](+) ) remains unaffected under the same conditions. The reactions of cluster [1](+) show polyphasic kinetics, and in all cases clusters bearing a bridging dithiolene moiety are formed in the first step through the concerted [3+2] cycloaddition between the C≡C atoms of the alkyne and a Mo(μ-S)2 moiety of the cluster. A computational study has been conducted to analyze the effect of the metal on these concerted [3+2] cycloaddition reactions.

Correlation between the molecular structure and the kinetics of decomposition of azamacrocyclic copper(ii) complexes.

The formation of copper(ii) complexes with symmetrical dinucleating macrocyclic ligands containing two either monomethylated () or trimethylated () diethylenetriamine (Medien or Me3dien) subunits linked by pyridine spacers has been studied by potentiometry. Potentiometric studies show that has larger basicity than as well as higher stability of its mono- and binuclear complexes. The crystal structures of ·6HCl (), [Cu2(L1)Cl2](CF3SO3)2 (), [Cu2(L1)(OH)](ClO4)3·3H2O () and [Cu(L1)](ClO4)2 () show that adopts different coordination modes when bound to copper(ii).

Synthesis and structure of trinuclear W3S4 clusters bearing aminophosphine ligands and their reactivity toward halides and pseudohalides.

The aminophosphine ligand (2-aminoethyl)diphenylphosphine (edpp) has been coordinated to the W3(μ-S)(μ-S)3 cluster unit to afford trimetallic complex [W3S4Br3(edpp)3](+) (1(+)) in a one-step synthesis process with high yields. Related [W3S4X3(edpp)3](+) clusters (X = F(-), Cl(-), NCS(-); 2(+)-4(+)) have been isolated by treating 1(+) with the corresponding halide or pseudohalide salt. The structure of complexes 1(+) to 4(+) contains an incomplete W3S4 cubane-type cluster unit, and only one of the possible isomers is formed: the one with the phosphorus atoms trans to the capping sulfur and the amino groups trans to the bridging sulphurs.

Mechanism of [3+2] cycloaddition of alkynes to the [Mo3 S4 (acac)3 (py)3 ][PF6 ] cluster.

A study, involving kinetic measurements on the stopped-flow and conventional UV/Vis timescales, ESI-MS, NMR spectroscopy and DFT calculations, has been carried out to understand the mechanism of the reaction of [Mo3 S4 (acac)3 (py)3 ][PF6 ] ([1]PF6 ; acac=acetylacetonate, py=pyridine) with two RCCR alkynes (R=CH2 OH (btd), COOH (adc)) in CH3 CN. Both reactions show polyphasic kinetics, but experimental and computational data indicate that alkyne activation occurs in a single kinetic step through a concerted mechanism similar to that of organic [3+2] cycloaddition reactions, in this case through the interaction with one Mo(μ-S)2 moiety of [1](+) . The rate of this step is three orders of magnitude faster for adc than that for btd, and the products initially formed evolve in subsequent steps into compounds that result from substitution of py ligands or from reorganization to give species with different structures.

The role of hydroxo-bridged dinuclear species and the influence of "innocent" buffers in the reactivity of cis-[Co(III)(cyclen)(H₂O)₂]³⁺ and [Co(III)(tren)(H₂O)₂]³⁺ complexes with biologically relevant ligands at physiological pH.

In view of the relevance of the reactivity of inert tetraamine Co(III) complexes having two substitutionally active cis positions capable of interact with biologically relevant ligands, the study of the reaction of cis-[Co(cyclen)(H2O)2](3+) and [Co(tren)(H2O)2](3+) with chlorides, inorganic phosphate and 5'-CMP (5'-cytidinemonophosphate) has been pursued at physiological pH. The results indicate that, in addition to the actuation of the expected labilising conjugate-base mechanism, the formation of mono and inert bis hydroxo-bridged species is relevant for understanding their speciation and reactivity. The reactivity pattern observed also indicates the key role played by the "innocent" buffers frequently used in most in vitro studies, which can make the results unreliable in many cases.

Kinetic and DFT studies on the mechanism of C-S bond formation by alkyne addition to the [Mo3S4(H2O)9]4+ cluster.

Reaction of [Mo3(μ3-S)(μ-S)3] clusters with alkynes usually leads to formation of two C-S bonds between the alkyne and two of the bridging sulfides. The resulting compounds contain a bridging alkenedithiolate ligand, and the metal centers appear to play a passive role despite reactions at those sites being well illustrated for this kind of cluster. A detailed study including kinetic measurements and DFT calculations has been carried out to understand the mechanism of reaction of the [Mo3(μ3-S)(μ-S)3(H2O)9](4+) (1) cluster with two different alkynes, 2-butyne-1,4-diol and acetylenedicarboxylic acid.

Influence of the ligand alkyl chain length on the solubility, aqueous speciation, and kinetics of substitution reactions of water-soluble M3S4 (M = Mo, W) clusters bearing hydroxyalkyl diphosphines.

Water-soluble [M3S4X3(dhbupe)3](+) diphosphino complexes (dhbupe = 1,2-bis(bis(hydroxybutyl)phosphino)ethane), 1(+) (M = Mo, X = Cl) and 2(+) (M = W; X = Br), have been synthesized by extending the procedure used for the preparation of their hydroxypropyl analogues by reaction of the M3S4(PPh3)3X4(solvent)x molecular clusters with the corresponding 1,2-bis(bishydroxyalkyl)diphosphine. The solid state structure of the [M3S4X3(dhbupe)3](+) cation possesses a C3 symmetry with a cuboidal M3S4 unit, and the outer positions are occupied by one halogen and two phosphorus atoms of the diphosphine ligand. At a basic pH, the halide ligands are substituted by hydroxo groups to afford the corresponding [Mo3S4(OH)3(dhbupe)3](+) (1OH(+)) and [W3S4(OH)3(dhbupe)3](+) (2OH(+)) complexes.

Equilibrium and kinetic studies on complex formation and decomposition and the movement of Cu(2+)metal ions within polytopic receptors.

Potentiometric studies carried out on the interaction of two tritopic double-scorpiand receptors in which two equivalent 5-(2-aminoethyl)-2,5,8-triaza[9]-(2,6)-pyridinophane moieties are linked with 2,9-dimethylphenanthroline (L1) and 2,6-dimethylpyridine (L2) establish the formation of mono-, bi- and trinuclear Cu(2+) complexes. The values of the stability constants and paramagnetic (1)H NMR studies permit one to infer the most likely coordination modes of the various complexes formed. Kinetic studies on complex formation and decomposition have also been carried out.

Water-soluble Mo3S4 clusters bearing hydroxypropyl diphosphine ligands: synthesis, crystal structure, aqueous speciation, and kinetics of substitution reactions.

The [Mo(3)S(4)Cl(3)(dhprpe)(3)](+) (1(+)) cluster cation has been prepared by reaction between Mo(3)S(4)Cl(4)(PPh(3))(3) (solvent)(2) and the water-soluble 1,2-bis(bis(hydroxypropyl)phosphino)ethane (dhprpe, L) ligand. The crystal structure of [1](2)[Mo(6)Cl(14)] has been determined by X-ray diffraction methods and shows the typical incomplete cuboidal structure with a capping and three bridging sulfides. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of the diphosphine ligand.