Observed Rate Law Research Articles

Homogeneous Electrocatalytic Reduction of CO2 by a CrN3O Complex: Electronic Coupling with a Redox-Active Terpyridine Fragment Favors Selectivity for CO.

Electrocatalyst design and optimization strategies continue to be an active area of research interest for the applied use of renewable energy resources. The electrocatalytic conversion of carbon dioxide (CO2) is an attractive approach in this context because of the added potential benefit of addressing its rising atmospheric concentrations. In previous experimental and computational studies, we have described the mechanism of the first molecular Cr complex capable of electrocatalytically reducing CO2 to carbon monoxide (CO) in the presence of an added proton donor, which contained a redox-active 2,2'-bipyridine (bpy) fragment, CrN2O2. The high selectivity for CO in the bpy-based system was dependent on a delocalized CrII(bpy•-) active state. Subsequently, we became interested in exploring how expanding the polypyridyl ligand core would impact the selectivity and activity during electrocatalytic CO2 reduction. Here, we report a new CrN3O catalyst, Cr(tpytbupho)Cl2 (1), where 2-(2,2':6',2″-terpyridin-6-yl)-4,6-di-tert-butylphenolate = [tpytbupho]-, which reduces CO2 to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr(tbudhbpy)Cl(H2O) catalyst. Computational analyses indicate that, although the stoichiometry of both reactions is identical, changes in the observed rate law are the combined result of a decrease in the intrinsic ligand charge (L3X vs L2X2) and an increase in the ligand redox activity, which result in increased electronic coupling between the doubly reduced tpy fragment of the ligand and the CrII center. The strong electronic coupling enhances the rate of protonation and subsequent C-OH bond cleavage, resulting in CO2 binding becoming the rate-determining step, which is an uncommon mechanism during protic CO2 reduction.

Cobalt-Catalyzed C(sp(2))-H Borylation: Mechanistic Insights Inspire Catalyst Design.

A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B2Pin2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic isotope effects, and identification of the catalyst resting state support turnover limiting C-H activation from a fully characterized cobalt(I) boryl intermediate. Monitoring the catalytic reaction as a function of time revealed that borylation of the 4-position of the pincer in the cobalt catalyst was faster than arene borylation. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which slows the rate of C-H oxidative addition and hence overall catalytic turnover. This mechanistic insight inspired the next generation of 4-substituted PNP cobalt catalysts with electron donating and sterically blocking methyl and pyrrolidinyl substituents that exhibited increased activity for the C-H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B2Pin2. Kinetic studies on the improved catalyst, 4-(H)2BPin, established a change in turnover limiting step from C-H oxidative addition to C-B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)2BPin, was prepared and crystallographically characterized and proved inactive for C-H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(III) complexes.

How Acid-Catalyzed Decarboxylation of 2,4-Dimethoxybenzoic Acid Avoids Formation of Protonated CO2.

The decarboxylation of 2,4-dimethoxybenzoic acid (1) is accelerated in acidic solutions. The rate of reaction depends upon solution acidity in a manner that is consistent with the formation of the conjugate acid of 1 (RCO2H2(+)), with its higher energy ring-protonated tautomer allowing the requisite C-C bond cleavage. However, this would produce the conjugate acid of CO2, a species that would be too energetic to form. Considerations of mechanisms that fit the observed rate law were supplemented with DFT calculations. Those results indicate that the lowest energy pathway from the ring-protonated reactive intermediate involves early proton transfer from the carboxyl group to water along with C-C bond cleavage, producing 1,3-dimethoxybenzene and CO2 directly.

Mechanism of proton transfer to coordinated thiolates: encapsulation of acid stabilizes precursor intermediate.

Earlier kinetic studies on the protonation of the coordinated thiolate in the square-planar [Ni(SC6H4R'-4)(triphos)](+) (R' = NO2, Cl, H, Me or MeO) by lutH(+) (lut = 2,6-dimethylpyridine) indicate a two-step mechanism involving initial formation of a (kinetically detectable) precursor intermediate, {[Ni(SC6H4R'-4)(triphos)]···Hlut}(2+) (K(R)1), followed by an intramolecular proton transfer step (k(R)2). The analogous [Ni(SR)(triphos)]BPh4 {R = Et, Bu(t) or Cy; triphos = PhP(CH2CH2PPh2)2} have been prepared and characterized by spectroscopy and X-ray crystallography. Similar to the aryl thiolate complexes, [Ni(SR)(triphos)](+) are protonated by lutH(+) in an equilibrium reaction but the observed rate law is simpler. Analysis of the kinetic data for both [Ni(SR)(triphos)](+) and [Ni(SC6H4R'-4)(triphos)](+) shows that both react by the same mechanism, but that K(R)1 is largest when the thiolate is poorly basic, or the 4-R' substituent in the aryl thiolates is electron-withdrawing. These results indicate that it is both NH···S hydrogen bonding and encapsulation of the bound lutH(+) (by the phenyl groups on triphos) which stabilize the precursor intermediate.

Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Catalytic water oxidation at Ir(OH)(+) (Ir = IrCp*(Me2NHC), where Cp* = pentamethylcyclopentadienyl and Me2NHC = N,N'-dimethylimidazolin-2-ylidene) can occur through various competing channels. A potential-energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the O-O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir(V) (=O)(O(·))(+), which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential-energy-determining step. O-O bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate-limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir(OH)(+). The calculated mechanism fits well with the experimentally observed rate law: v = kobs[Ir][oxidant]. The calculated effective barrier of 24.6 kcal mol(-1) fits well with the observed turnover frequency of 0.88 s(-1). Under strongly oxidative conditions, O-O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O-O bond formation after three sequential oxidation steps becomes competitive.

Hydroxypalladation Precedes the Rate‐Determining Step in the Wacker Oxidation of Ethene

The complete reaction mechanism and kinetics of the Wacker oxidation of ethene in water under low [Cl(-)], [Pd(II)], and [Cu(II)] conditions are investigated in this work by using ab initio molecular dynamics. These extensive simulations shed light on the molecular details of the associated individual steps, along two different reaction routes, starting from a series of ligand-exchange processes in the catalyst precursor PdCl4(2-) to the final aldehyde-formation step and the reduction of Pd(II). Herein, we report that hydroxylpalladation is not the rate-determining step and is, in fact, in equilibrium. The newly proposed rate-determining step involves isomerization and follows the hydroxypalladation step. The mechanism proposed herein is shown to be in excellent agreement with the experimentally observed rate law and rate. Moreover, this mechanism is in consensus with the observed kinetic isotope effects. This report further confirms the outer-sphere (anti) hydroxypalladation mechanism. Our calculations also ratify that the final product formation proceeds through a reductive elimination, assisted by solvent molecules, rather than through β-hydride elimination.

Where Has My Acid Gone? Understanding the Self-Catalyzed Esterification of Maleic Acid in Methanol During Salt Formation

The kinetics of the esterification of maleic acid in methanol have been investigated following the contamination of a batch of an API maleate salt with the corresponding monomethyl maleate salt. The esterification was found to be catalyzed by H+ generated by the ionization of maleic acid, giving rise to an observed rate law with a one-and-a-half-order dependence upon the maleic acid concentration. On the basis of the measured rate constants over a range of temperatures a predictive model has been developed that will estimate the time taken to achieve a given degree of esterification as a function of temperature and maleic acid concentration. Rate constants have also been determined for the maleic acid-catalyzed esterification of monomethyl maleate. Neutralization of maleic acid through salt formation with triethylamine reduces the rate of esterification in a manner that is consistent with the degree of neutralization. The potential for the esterification of maleic acid in other alcohols and for the esteri...

Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O2

Metal alkylperoxides are remarkable, highly effective, yet often thermally unstable, oxidants that may react through a number of possible pathways including O-O homolytic cleavage, M-O homolytic cleavage, nucleophilic O-atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type To(M)ZnR (To(M) = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-C3H7, i-C3H7, t-Bu) that react with O2 at 25 °C to form isolable monomeric alkylperoxides To(M)ZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically characterized, and the structures show systematic variations in the Zn-O-O angle and O-O distances. The observed rate law for the reaction of To(M)ZnEt (2) and O2 is consistent with a radical chain mechanism, where the rate-limiting SH2 step involves the interaction of (•)OOR and To(M)ZnR. In contrast, To(M)ZnH and To(M)ZnMe are unchanged even to 120 °C under 100 psi of O2 and in the presence of active radical chains (e.g., (•)OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords To(M)ZnH and ROOSiR3'.

An enantioselective Claisen rearrangement catalyzed by N-heterocyclic carbenes.

In the presence of a chiral azolium salt (10 mol %), enols and ynals undergo a highly enantioselective annulation reaction to form enantiomerically enriched dihydropyranones via an N-heterocyclic carbene catalyzed variant of the Claisen rearrangement. Unlike other azolium-catalyzed reactions, this process requires no added base to generate the putative NHC-catalyst, and our investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species. Detailed kinetic studies eliminate a potential 1,4-addition as the mechanistic pathway; the observed rate law and activation parameters are consistent with a Claisen rearrangement as the rate-limiting step. This catalytic system was applied to the synthesis of enantioenriched kojic acid derivatives, a reaction of demonstrated synthetic utility for which other methods for catalytic enantioselective Claisen rearrangements have not provided a satisfactory solution.

Mechanism of the formation of carbon oxides under conditions of the oxidative chlorination of methane: IV. Kinetics of the reaction of CCl4 with oxygen on copper-containing salt catalysts for methane oxychlorination at reduced partial pressures of Cl2 in the reaction mixture

The kinetics of the oxidative dechlorination of CCl4 on two supported salt catalysts for methane oxychlorination (CuCl2-KCl and CuCl2-KCl-LaCl3) in the presence of methane at 350–450°C was studied using a gradientless method. It was found that, at \( P_{Cl_2 } \) > 0.15 kPa, the presence of methane in the reaction mixture had no effect on the kinetics of CCl4 oxidation but decreased the activation energy E to ∼22 kcal/mol. At \( P_{Cl_2 } \) < 0.15 kPa, the kinetic orders with respect to oxygen and chlorine changed and E decreased to ∼17 kcal/mol. The rate of formation of CO2 as a by-product of the reaction of methane oxychlorination was described by an exponential equation in combination with rate equations for the oxidation of other chloromethanes and methane. An equation consistent with the observed rate laws of this reaction in the presence of methane was derived from the previously proposed reaction scheme for the mechanism of the oxidative dechlorination of CCl4.

Kinetics and mechanism of RuIII catalysed oxidation of 1,2,3,4-tetrahydronaphthalene (tetralin ) by CeIV in aqueous nitric acid medium

The kinetics and mechanism of Ru III catalysed oxidation of 1,2,3,4-tetrahydronaphthalene (tetralin) by Ce IV in aqueous nitric acid to tetralone under the conditions [TL] > > [Ce IV ] at different temperatures (30-50 °C) have been studied in 3.0 mol dm -3 nitric acid medium. The experimentally observed rate law conforms to.

Mechanism of the Intramolecular Hydroamination of Alkenes Catalyzed by Neutral Indenyltitanium Complexes: A DFT Study

Three mechanistic pathways for the [Ind(2)TiMe(2)]-catalyzed intramolecular hydroamination of alkenes have been investigated by employing density functional theory calculations on the possible intermediates and transition states. The results indicate that the reaction cycle proceeds via a Ti-imido-amido complex as the catalytically active species. However, at the moment, the question as to whether this imido-amido complex is involved in a [2+2]-cycloaddition with the alkene or a newly proposed insertion of the alkene into a Ti--N single bond cannot be answered; the calculated barriers of both the insertion mechanism and the [2+2]-cycloaddition mechanism are similar (143 vs. 136 kJ mol(-1)), and both pathways are in accordance with the experimentally observed rate law (first-order dependence on the aminoalkene concentration). Interestingly, the newly proposed insertion mechanism that takes place by an insertion of the alkene moiety into the Ti--N single bond of an imido-amido complex seems to be much more likely than a mechanism that involves an alkene insertion into a Ti--N single bond of a corresponding trisamide. The latter mechanism, which has been proposed in analogy to rare-earth-metal-catalyzed hydroamination reactions, can be ruled out for two reasons: a surprisingly high activation barrier (164 kJ mol(-1)) and the fact that the rate-limiting insertion step is independent of the aminoalkene concentration. This is in sharp contrast to the experimental findings for indenyltitanium catalysts.

Reactive Sulfur Species: Kinetics and Mechanism of the Equilibrium between Cysteine Sulfenyl Thiocyanate and Cysteine Thiosulfinate Ester in Acidic Aqueous Solution

The kinetics and mechanism of the hydrolysis of cysteine sulfenyl thiocyanate (CySSCN) to give cysteine thiosulfinate ester (CyS(=O)SCy) have been investigated between pH 0 and 4. The reaction is reversible. The hydrolysis of CySSCN is second-order in [CySSCN] and inverse first-order in [H+] and [SCN-]. The following mechanism is proposed for the hydrolysis of CySSCN (where the charge depends upon the pH): CySSCN0/+ + H2O <==>CySOH0/+ + SCN- + H+, CySOH0/+ + CySSCN0/+ --> CyS(=O)SCy0/+/2+ + SCN- + H+; k1 = 3.36 +/- 0.01 x 10-3 s-1, K1k2 = 0.13 +/- 0.05 Ms-1 (which yields k2/k-1 = 39 M). The observed rate law rules out alternative mechanisms for 1 <or= pH <or= 4, including the condensation of two molecules of cysteine sulfenic acid (CySOH) to give CyS(=O)SCy. The equilibrium can be approached from the opposite direction. The reaction of CyS(=O)SCy with SCN- to give CySSCN is first-order in [CyS(=O)SCy], [SCN-], and [H+] (for [H+] > 0.4 M). The following mechanism is proposed: CyS(=O)SCy2+ + H+ <==> CyS(OH)=SCy3+, Ka; CyS(OH)SCy3+ + SCN- --> CySOH+ + CySSCN+, k-2 = 0.239 +/- 0.007 M-2s-1/Ka M-1. Since cysteine sulfenic acids are known to play an important function in many enzymes, and SCN- exists in abundance in physiologic fluids, we discuss the possible role of sulfenyl thiocyanates in vivo.

Electron transfer. Part 165: Oxidations of Ti(II)(aq) with ligated iron(III) and ruthenium(III)

Titanium(II) solutions, prepared by dissolving titanium wire in triflic acid + HF, contain equimolar quantities of Ti(IV). Treatment of such solutions with excess Fe(III) or Ru(III) complexes yield Ti(IV), but reactions with Ti(II) in excess give Ti(III). Oxidations by (NH 3) 5Ru(III) complexes, but not by Fe(III) species, are catalyzed by titanium(IV) and by fluoride. Stoichiometry is unchanged. The observed rate law for the Ru(III)–Ti(II)–Ti(IV) reactions in fluoride media points to competing reaction paths differing by a single F −, with both routes involving a Ti(II)–Ti(IV) complex which is activated by deprotonation. It is suggested that coordination of Ti(IV) to Ti II(aq) minimizes the mismatch of Jahn–Teller distortions which would be expected to lower the Ti(II,III) self-exchange rate.

Aquachlororuthenium(III) complex catalysis in the oxidation of malonic and methylmalonic acids by bromate in perchloric acid medium. Study of induction period and evaluation of individual kinetic parameters

The kinetics and mechanism of an aquachlororuthenium(III) complex catalyzed oxidation of malonic and methylmalonic acids by bromate in aqueous acetic acid medium containing perchloric acid and mercury(II) is reported here. The reaction shows an induction period (τ0), whose duration is inversely proportional to [substrate]0 and [catalyst]0, and temperature; however, independent of [bromate]0. After induction, the reaction exhibits first order each in [bromate] and [catalyst], less than unit order in [substrate] and an inverse fractional order in [HClO4]. The reaction also shows an inverse solvent isotope effect \({(k_{D_{2}O}/k_{H_{2}O} = 0.72 \pm 0.05.)}\) The observed rate law is interpreted by a mechanism involving the oxidation of RuIII to RuV by BrV (Kf), followed by complex formation (Kc) between the RuV and enol form (Ken) of the substrate in 1:1 ratio, which decomposes (kd) into products in the rate-determining step with regeneration of RuIII. The observed induction period is interpreted as the time-lag in building-up of the reactive intermediate(s). The reaction constants (Kf, Kc, Ken and kd) involved in the mechanism proposed have been evaluated and τ0 has been correlated in terms of initial concentrations of substrate and catalyst as: 1/τ0 = Q(X-1)[Sa]0(x-1) = Q′(Z-1)[Ru(III)]0(z-1).

D abstraction by H on Si( [formula omitted]) surfaces

We studied H-induced abstraction (ABS) and adsorption-induced-desorption (AID) on the D/Si(1 1 1) surface for various initial D coverages θ 0 D and surface temperatures T s. As θ 0 D was increased, the kinetics of HD ABS was found to switch from the first- to second-order rate law with respect to θ D. D 2 AID appeared to obey a nearly third-order rate law in θ D. The hot atom mechanism was ruled out for both ABS and AID. We found that AID is associated with β 2 temperature-programmed-desorption, i.e. D 2 AID proceeds as a spontaneous desorption of deuterium from a di-deuteride or di-hydride phase formed by H atoms. Rate equations for HD and D 2 desorptions at 620 K were proposed on the basis of the observed rate laws. The apparent activation energy of D 2 AID was evaluated to be ∼0.16 eV for T s⩽650 K. Taking steady state adsorption and desorption into consideration, the origin of the third-order rate law as well as the low activation energy for AID was ascribed to the process of sticking by H atoms rather than the process of D 2 desorption. An isotope effect on both ABS and AID was recognized in the rate curve analysis. The hot complex-mediated reaction model was invoked to explain the complicated kinetic phenomena appeared in ABS and AID on the Si(1 1 1) surface.

Effective and Catalytic Reduction of Perchlorate by Atom Transfer–Reaction Kinetics and Mechanisms

The reaction kinetics and mechanisms of catalytic reduction of perchlorate and chlorate ions by organic thioethers are described. Oxazoline and thiazoline oxorhenium complexes serve as efficient catalysts for the reduction of perchlorate under mild conditions by a pure oxygen atom transfer mechanism. The observed rate law features saturation kinetics with respect to oxo donor (perchlorate or chlorate) concentration. The factors that make this new family of catalytsts effective and viable have been delineated including different pathways of catalyst deactivation and inhibition. Comparisons to other transition metal complexes that do react controllably with perchlorate are presented.

Substitution behaviour of an unusual seven-coordinate iron(III) complex in different solvents

In order to clarify the solution chemistry of 3d metal hepta-coordinate complexes, a detailed mechanistic study of the two-step substitution reaction between the “tweezer-like” iron(III) complex, [Fe(dapsox)(H 2O) 2] + (H 2dapsox=2,6-diacetylpyridine-bis-(semioxamazide)), and SCN − was performed in water and methanol as solvents. Based on the observed rate law and reported activation parameters ( ΔH #, ΔS # and ΔV #) for both reaction steps in aqueous solution, the operation of an I a mechanism for the diaqua, an I d mechanism for the aquathiocyanato, and an I d or D mechanism for the aquahydroxo forms of the Fe(III) complex, is proposed. The operation of a limiting D mechanism is proposed for both reaction steps in methanol as solvent.

Formation of singlet molecular oxygen by the Radziszewski reaction between acetonitrile and hydrogen peroxide in the absence and presence of ketones

Measurements of the infrared phosphorescence of singlet molecular oxygen (1O2) at 1270 nm have been used to demonstrate the formation of 1O2 by the Radziszewski reaction between acetonitrile and hydrogen peroxide. The kinetics of the Radziszewski reaction either alone or in the presence of ketones have been studied by this technique. The rate-determining step of the 1O2 formation of the reaction in the absence of ketones was found to be independent of both the concentration of acetonitrile and that of hydrogen peroxide. The kinetic data, the results of the volumetric measurements of the oxygen liberated and the results of the determination of the amount of 1O2 generated by the reaction are consistent with the assumption that the reaction between acetonitrile and hydrogen peroxide occurs via the heterolytic decomposition of the intermediate, peroxyacetimidic acid (PAIA), forming 1 mol acetamide and 0.5 mol 1O2 according to the stoichiometric equation: CH3CN + H2O2 → CH3C(O)NH2 + 0.5 1O2. The rate constant of the heterolytic decomposition of PAIA was determined to be k8 = 1.2 × 10−3 dm3 mol−1 s−1 at T = 30 °C. From the measurements at different pH values in the range 9.1 < pH < 12.0 the pKa(PAIA) value was estimated to be 11.1 at T = 30 °C. The investigation of the reaction between acetonitrile and hydrogen peroxide by using N,N-dimethyl-4-oxopiperidinium nitrate as catalyst, has unequivocally shown that the rate of 1O2 formation is considerably enhanced by this ketone. For the ketone-catalysed decomposition of PAIA a rate law can be derived showing a first order dependence on the concentration of acetonitrile and hydrogen peroxide at a given pH. In accordance with the observed rate law are the results with acetonitrile in 50% acetone containing a tenfold excess of hydrogen peroxide at pH 8.2 and T = 60 °C.

Mechanistic Aspects of Samarium-Mediated σ-Bond Activations of Arene C−H and Arylsilane Si−C Bonds

To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH3 to Ph2SiH2 and SiH4, the samarium phenyl complex [Cp*2SmPh]2 (1) was prepared by oxidation of Cp2*Sm (2) with HgPh2. Compound 1 thermally decomposes to yield benzene and the phenylene-bridged disamarium complex Cp*2Sm(mu-1,4-C6H4)SmCp*2 (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp*2SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate = k1[1] + k2[1]2. Complex 1 efficiently transfers its phenyl group to PhSiH3, with formation of Ph2SiH2 and [Cp*2Sm(mu-H)]2 (4). Quantitative Si-C bond cleavage of C6F5SiH3 is effected by the samarium hydride complex 4, yielding silane and [Cp*2Sm(mu-C6F5)]2 (5). In contrast, Si-H activation takes place upon reaction of 4 with o-MeOC6H4SiH3, affording the samarium silyl species [structure: see text] Cp*2SmSiH2(o-MeOC6H4) (7). Complex 7 rapidly decomposes to [Cp*2Sm(mu-o-MeOC6H4)]2 (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C6F5)2 and Hg(o-MeOC6H4)2, respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in sigma-bond metathesis reactions, are discussed.