Rate Law Kobs Research Articles

The effect of the chelate ligand on the rate of chloride substitution from Pt(II) complexes with picolyl and isoquinolinylcarboxamide moieties on the non-leaving ligand

Substitution reactions of four Pt(II) complexes, namely; N-(2-picolyl)picolinamide-platinum(II) chloride (Pt3), N-(8-quinolyl)pyridine-2-carboxamide platinum(II) chloride (Pt4), N-(8-quinolyl)-3-isoquinolinecarboxamide platinum(II) chloride (Pt5) and N-(8-quinolinyl)-1-isoquinolinecarboxamide platinum(II) chloride (Pt6), were studied with biorelevant nucleophiles, viz. thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) under pseudo first order conditions as a function of concentration and temperature using the stopped flow spectrophotometer. The observed pseudo first order rate constants for the substitution reactions obey the rate law kobs = k2[Nu]. The reactivity of the studied complexes depends on strength of π-backbonding of the attached ligands, which is enhanced by the strong electron withdrawing nature of the carboxamide group on the non-leaving ligands of Pt(II) complexes. Quinoline moieties of Pt5 and Pt6 lie out-of-the square plane, resulting in enhanced reactivity due to the aided entrapment of the nucleophile by the non-planar groups of the ligand. Negative activation entropies and positive enthalpies of activation support an associative mode of activation.

The role of 8-quinolinyl moieties in tuning the reactivity of palladium(II) complexes: a kinetic and mechanistic study

The rate and mechanism of chloride substitution from Pd(II) complexes, chlorobis-(2-pyridylmethyl)aminepalladium(II), 1, chloro-8-[(2-pyridylmethyl)amino]quinolinepalladium(II), 2, chloro-N-(2-pyridinylmethylene)-8-quinolinaminepalladium(II), 3, and chlorobis(8-quinolinyl)aminepalladium(II), 4, are reported. The labile chloride was substituted from the complexes by thiourea nucleophiles viz, thiourea (Tu), N,N′-dimethylthiourea (Dmtu) and N,N,N′,N′-tetramethylthiourea (Tmtu). The reactions were monitored under pseudo-first-order conditions in methanol using stopped-flow spectrophotometry as a function of concentration and temperature. All the reactions obeyed the rate law kobs = k2[Nu] following the order 1 > 3 > 2 > 4 with 4 exhibiting the slowest rate of substitution due to the stronger σ-donor effect of 8-quinolyl moiety of the coordinated ligand, which makes the Pd center more electron-rich. This slows the nucleophilic attack by the nucleophiles. The values of the thermodynamic parameters (ΔH# and ΔS#) support an associative substitution mechanism. The trends in the DFT calculated data support the experimentally observed order of the reactivity of the complexes.

Third-Order Kinetics for Interaction of Glutathione with a Dinuclear Pd(II) Complex and Their Mechanism, DNA Binding and DFT Study

A kinetics and mechanistic study of the interaction between the bridged dimer \( [{\text{Pd}}({\text{pic}})({\text{OH}})]_{2}^{2 + } \) (where pic = 2-aminomethylpyridine) and glutathione (GSH) has been performed under pseudo-first order conditions using a stopped-flow spectrophotometer. The reaction follows a distinctive third-order kinetics via two consecutive steps. The first step follows the rate law kobs = k1[GSH]2, whereas step two is independent of the GSH concentration. The activation parameters, ΔH‡ and ΔS‡ for both steps, were determined and an associative mode of activation is suggested for the substitution process. Geometry optimizations and HOMO–LUMO energy calculations were done by DFT. The UV spectra of the complexes were compared with theoretically obtained TD-DFT spectra. Complex 2 and 3 interact non-covalently with calf-thymus DNA with binding constants (Kb and KSV) of the order of 104 L·mol−1.

The role of annular nitrogen in tuning the reactivity of bifunctional platinum(II) complexes appended to pyridyl spacers; A kinetic and mechanistic investigation

A kinetic and mechanistic study on ligand substitution reactions of pyridine functionalized dinuclear Pt(II) complexes visa-viz., N,N,N’,N’-tetrakis(2-pyridyl)-2,3 pyridinediaminetetraquaplatinum(II) (PtL1); N,N,N’,N’-tetrakis(2-pyridyl)-2,4-pyridinediaminetetraquaplatinum(II) (PtL2); N,N,N’,N’-tetrakis(2-pyridyl)-2,6-pyridinediaminetetraquaplatinum(II) (PtL3); N,N,N’,N’-tetrakis(2-pyridyl)-2,5-pyridinediaminetetraquaplatinum(II) (PtL4) and N,N,N’,N’-tetrakis(2-pyridylaminomethyl)-2,6-pyridinediaminetetraquaplatinum(II) (PtL5) complexes with thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’N’-tetramethylthiourea (TMTU) as the entering nucleophiles was carried out under pseudo-first-order conditions by stopped-flow and UV–Vis spectrophotometric techniques. Substitution reactions proceeds via a two-step pathway with the rate law kobs(1,2) = k2(1st/2nd)[Nu]. The first substitution step is attributed to simultaneous displacement of one water molecule at each platinum(II) center. The second step is attributed to the slow phase substitution of the second water molecule occurring concurrently with the slow displacement of the bridging linker. The steps were confirmed by 1H and 195Pt NMR spectroscopy. The reactivity of the complexes is governed by the position of the annular nitrogen in the pyridine spacer. The Low positive values of enthalpy of activation and significantly large negative values of entropy of activation parameters for both steps indicate an associative mechanism of substitution.

Synthesis and Characterization of 2-Pyridinylmethylene-2-quinolyl Hydrazone Cobalt(III) Complexes. Reactivity Trends and Solvent Effect on the Initial and Transition States of Base Catalyzed Hydrolysis

The complexes of pyridine-2-aldehyde-2-quinolylhydrazone Co(III) nitrate [Co(paqh)2](NO3)2, methyl-2-pyridylketone-2-quinolinhyrazone Co(III) nitrate [Co(mpkqh)2](NO3)2, and phenyl-2-pyridylketon-2-quinolinhyrazone Co(III) nitrate [Co(ppkqh)2](NO3)2 were prepared and characterized. Solubilities of Co(III)–hydrazone complexes were measured. Transfer chemical potentials were calculated from the measured solubilities of the Co(III) complexes in aqueous methanol mixtures at 25 °C. The reactivity trends in the transfer chemical potentials are discussed in terms of the nature of the bonded ligands. Kinetics of the base hydrolysis of Co(III)–hydrazone complexes in the aqueous methanol mixtures have been studied at 25 °C, and follow the rate law k obs = k 2[OH−]. The solvent effects on the reactivity trends of Co(III) complexes are analyzed into initial state (is) and transition states (ts) components. The reaction rates are reduced by the increase of methanol content. The destabilization of the transition state is remarkable compared to the initial state in the aqueous methanol mixtures. The initial state is more hydrophobic in nature than the transition state for Co(III) complex reactions.

Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

TAML complex is oxidized by H2O2 or tBuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer 2, which oxidizes readily ring-substituted thioanisoles p-XC6H4SMe (X=H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of 1. The oxidation studied under pseudo-first-order conditions using the stopped-flow technique by monitoring the fading of the 420-nm band of 2 follows hyperbolic kinetics according to the rate law kobs = ab[p-XC6H4SMe]/(1 + b[p-XC6H4SMe]) at pH 8 and 25 °C. Parameters a, b, and ab all decrease for electron-poorer thioanisoles and the Hammett value ρ ~ 1 has been found for ab, which can be associated with the second-order rate constants for oxidation of thioanisoles by 2. The kinetics of oxidation of p-NO2C6H4SMe by H2O2 catalyzed by 1 has been studied under steady-state conditions. Covering the concentration of 1 in a 100-fold range has revealed that though first-order kinetics in 1 is observed at low catalyst concentrations (below 10−6 M), there is a significant negative deviation from linearity at [1] > 10−6 M. The latter was rationalized by the equilibrium between the monomeric and dimeric FeIV species 2 M ⇌ M–M (Kd), both being able to oxidize p-NO2C6H4SMe with rate constants km and kd which were found to be (13 ± 1) × 104 and (0.32 ± 0.01) × 104 M−1 s−1, respectively. The difference in the rate constants is the key for resolving the dilemma of faster catalysis versus slower single-turnover reactivity of TAML activators in water.

A kinetic and mechanistic study of dinuclear Pt(II) 2,2′:6′,2″-terpyridine compounds bridged with polyethyleneglycol ether flexible linkers

A series of dinuclear Pt(II) complexes bridged with polyethyleneglycol ether of the type [ClPt(tpy)O(CH2CH2O)n(tpy)PtCl]Cl2 where n = 1 (Ptdteg), 2 (Ptdtdeg), 3 (Ptdtteg), 4 (Ptdttteg), and linker-free complex, (Ptdt) (where tpy = 2,2′:6′,2″-terpyridine), were synthesized and characterized to investigate the role of bridging polyethyleneglycol ether linker on the substitution reactivity of dinuclear Pt(II) complexes. Substitution reactions were studied using thiourea nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU) under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow reaction analyzer. The reactions gave single exponential fits following the rate law kobs = k2[Nu]. Introduction of polyethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The results obtained indicate that the rate of substitution is controlled by both electronic and steric hindrance which increases with the length of the linker. Experimental results are supported by density functional theory calculations and structures obtained at B3LYP/LANL2DZ level. The order of the reactivity of the nucleophiles is TU > DMTU > TMTU. The magnitude and the size of the enthalpy of activation and entropy of activation support an associative mode of mechanism, where bond formation in the transition state is favored.

Equilibrium and dissociation kinetics of the [Al(NOTA)] complex (NOTA = 1,4,7-triazacyclononane-1,4,7-triacetate)

A detailed investigation of the equilibria and dissociation kinetics for the [Al(NOTA)] complex has been carried out. This complex and its derivatives are known as very good carriers for 18F-isotope in positron emission tomography. The thermodynamic stability of [Al(NOTA)] has been studied by “out of cell” pH-potentiometric technique since the formation rate of the complex is very low in acidic medium. 1H- and 27Al-NMR spectra have been recorded to check the time course of equilibration and to validate the equilibrium model consisting of [Al(NOTA)] with lgK = 17.9(1) and [Al(HNOTA)]+ with lgK H = 1.9(3). A metastable mixed hydroxido complex [Al(NOTA)(OH)]− with lgK Al(NOTA) OH = −12.2(1) was detected in alkaline solution by direct pH-potentiometry, which transforms slowly to [Al(OH)4]−. The decomplexation reactions of [Al(NOTA)] have been investigated in both acidic and basic conditions. The rate of dissociation is extremely low in acidic medium, while in alkaline solution, it can be characterized by the rate law kobs = k 0 + k 1[OH−], where k 0 = (2.0 ± 0.1) × 10−6 s−1 and k 1 = (6.8 ± 0.5) × 10−6 M−1s−1. The formation of the ternary [Al(NOTA)(F)]− complex via direct reaction of [Al(NOTA)] and F− cannot be detected by either fluoride selective electrode or by 19F-NMR spectroscopy. However, by applying solvent mixture (1:1 ethanol:water) and heating, the ternary [Al(NOTA)(F)]– complex was found to form quantitatively within 15 minutes.

Kinetics of the base hydrolysis of iron (II) complexes with pyridyl–quinolyl Schiff base ligands in aqueous and aqueous/methanol binary mixtures

Kinetics of the reactivity against OH− ion of some novel bis-N,N′,N″-(pyridin-2-ylmethylene-quinolin-8-yl) Schiff base iron (II) iodide (1), bis-N,N′,N″-[(1-pyridin-2-yl-ethylidene)-quinolin-8-yl] Schiff base iron (II) iodide (2) and bis-N,N′,N″-[(phenyl-pyridin-2-ylmethylene)-quinolin-8-yl] Schiff base iron (II) iodide (3) have been studied spectrophotometrically. A relevant tentative mechanism was suggested with respect to the rate law of the base hydrolysis in aqueous media and aqueous/methanol binary mixtures. The reaction revealed pseudo first-order kinetics with a rate law k obs = k 2[OH−]. The rate trends and activation parameters are consistent with the complexes’ stability discussed according to the hydrophobicity/hydrophilicity of the substituent groups (R) in the complexes. The co-organic solvent, i.e., methanol, has a positive impact on the rate of base hydrolysis.

PdII–PdII bonding interaction in dinuclear PdII complex with non-macrocyclic (O&N) chelates: Characterization, kinetics and DFT study

A unique PdII complex with metal–metal (d8–d8) bonding interaction, [Pd(pic)(cbdca)]2·H2O 4 (where, pic=2-aminomethylpyridine and cbdca2−=cyclobutane-1,1-dicarboxylate) has been synthesised and characterized by X-ray structure, FT-IR, ESI Mass, Raman and NMR spectroscopy. Density Functional Theory (DFT) study shows PdII–PdII bond distance of 3.18Å in complex 4 which is close to the observed distance of 3.1790(1)Å found from X-ray crystal structure. Powder X-ray structure of [Pd(pic)Cl2] 1 has been solved and polymeric chain structure is found without any supporting ligand. At pH 5.5, complex [Pd(pic)(OH2)2](ClO4)22, the hydrolyzed product of 1 exists as dimeric complex [Pd(pic)(OH)]2(ClO4)23. Third order kinetics is followed for the reaction of cbdca with the di-μ-hydroxo bridged dimer 3. The substitution process is a ligand-dependent second order reaction, found to take place in two consecutive steps in accordance with the rate law kobs=k1[cbdca]2. The reaction follows an associative mechanistic path and the activation parameters (ΔH‡ and ΔS‡) for both the steps were evaluated using Eyring equation.

Tuning the π-backbonding and σ-trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study

The nucleophilic substitution reaction of cyclometallated complexes; [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea (TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M LiCF3SO3 and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function of nucleophile concentration and temperature using UV–visible and stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate law kobs = k2[Nu] + k−2. The reactivity of the investigated complexes when [PtL1Cl] is used as a reference follows the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. The lability of the chloro group is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL2Cl] and [PtL3Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving group, have a higher reaction rate compared to [PtL4Cl], which has an electron-donating methyl group attached to the ligand backbone. The position of the substituent on the phenyl group trans to the leaving group also influences the overlap of frontier molecular orbitals which result in controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the substituent, either electron withdrawing or electron donating, results in an increase in the rate of substitution. Second-order kinetics and large negative activation entropies (ΔS#) support an associative substitution mechanism. The experimental data are supported by DFT calculations.

Kinetic and mechanistic investigation into the influence of substituents on the substitution reactions of Pt(II) NCN-donor complexes

The substitution kinetics of cyclometallated platinum(II) complexes [PtL1Cl] (L1 = 1,3-di(2-pyridyl)benzene), [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene) and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with neutral nitrogen-donor nucleophiles imidazole, 1-methylimidazole, 1,2-dimethylimidazole, pyrazole and 1,2,4-triazole were investigated under pseudo-first-order conditions in methanol solution with an ionic strength of 0.1 M. The rate of substitution of the chloride ligand was studied as a function of nucleophile concentration and temperature using stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants, kobs, for the substitution reactions obeyed the rate law kobs = k2[Nu]. The reactivity of these complexes follows the order PtL2Cl > PtL3Cl > PtL4Cl > PtL1Cl. The lability of the chloride ligand is influenced by the extent of π-backbonding and also by the σ-trans effect. The reactivity of the nucleophiles depends on their basicity, inductive effect and steric hindrance. Second-order kinetics and negative activation entropies support an associative substitution mechanism. The experimental data are supported by the results of DFT calculations.

Understanding the role of flexible 4′-functionalized polyethylene glycoxy chains on the behavior of platinum(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine: a kinetic and a mechanistic study

The ligand substitution kinetics of 4′-functionalized mononuclear Pt(II) (4′-(ethylene glycoxy)-2,2′:6′,2′′-terpyridine complexes, [Pt(nY-tpy)Cl)Cl] (where Y = ethylene glycoxy, n = number of ethylene, glycoxy units = 1, 2, 3, and 4, and tpy = 2,2′:6′,2′′-terpyridine), with thiourea, 1,3-dimethyl-2-thiourea, 1,1,3,3-tetramethyl-2-thiourea, and iodide were investigated under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow technique. The observed first-order rate constants followed the simple rate law kobs = k2[Nu]. The data obtained show that the ethylene glycoxy pendant, trans to the leaving group, acts as a σ-donor into the terpyridine ligand and is effective only up to n = 1, beyond which the substitution reactivity of the complexes are controlled by the steric influence of the appended ethylene glycoxy pendant units, which decreases with increase in the number of ethylene glycoxy units. The activation parameters obtained support an associative mechanism, where bond formation in the transition state is favored. The observed reactivity trends were supported by density functional theory calculations.

Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid (H2ODO2A)

The dissociation kinetics of selected trivalent lanthanide (Ln3+, Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H2ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A+, were studied in the [H+] range (0.1–2.4) × 10−4 M in the temperature range 15–45 °C. Excess Cu2+ ions were used as the scavenger for the ligand in acetate–acetic acid buffer medium. The dissociation reactions are independent of [Cu2+] and follow the rate law kobs = kd + kAC[Acetate] + K′klim[H+]/(1 + K′[H+]), where kd, kAC, and klim are the respective dissociation rate constants for the [H+]-independent, acetate-assisted, and the [H+]-dependent limiting pathways; K′ is the equilibrium constant for the protonation reaction LnODO2A+ + H+ LnODO2AH2+. The dissociation rates of LnODO2A+ complexes are all faster than those of the corresponding LnDO2A+ complexes (DO2A2− is the fully deprotonated dianion of the ligand H2DO2A, 1,4,7,10-tetrazacyclo-dodecane-1,7-diacetic acid), consistent with the notion that LnODO2A+ complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A+ complexes, and H2ODO2A is not pre-organized for Ln3+ ion complexation but H2DO2A is.

Kinetic and mechanistic studies of 1,3-bis(2-pyridylimino)isoindolate Pt(ii) derivatives. Experimental and new computational approach

The rate of substitution of the chloride ligand by three bio-relevant nucleophiles, thiourea (Tu), N,N-dimethylthiourea (Dmtu) and N,N,N,N-tetramethylthiourea (Tmtu), in the complexes: 1,3-bis(2-pyridylimino)isoindoline platinum(II) chloride (Pt2), 1,3-bis(2-pyridylimino)benz(f)isoindoline platinum(II) chloride (Pt3) and 1,3-bis(1-isoquinolylimino)isoindoline platinum(II) complex (Pt4) was investigated under pseudo first-order conditions as a function of concentration and temperature using stopped-flow and UV-Visible spectrophotometry. Computational modeled data of bis(pyridylimino)3,4-pyrrolate platinum(II) chloride (Pt1) were incorporated in the study for comparison. The observed pseudo first-order rate constants for substitution reactions obey the rate law kobs = k2[Nu]. High negative activation entropies and second-order kinetics for the displacement reactions all support an associative mode of activation. The reactivity is dependent on stabilization of the LUMO energy and inversely proportional to the number of phenyl rings added irrespective of the site of attachment. The electron density on the ligand moiety plays a significant role in the substitution behavior of the platinum(II) complexes, as supported by DFT descriptors {electrophilicity index (ω) and chemical hardness (η)}.

A kinetic and mechanistic study into the substitution behaviour of platinum(ii) polypyridyl complexes with a series of azole ligands

The kinetics of chloride substitution from a series of square-planar platinum(II) complexes, viz. [Pt(terpyridine)Cl](+), (Pt1), [Pt{2-(2'-pyridyl)-1,10-phenanthroline}Cl]Cl, (Pt2), [Pt{4'-(2'''-CH3-phenyl)-2,2':6',2''-terpyridine}Cl]CF3SO3 (Pt3) and [Pt{4'-(2'''-CH3-phenyl)-6-(3''-isoquinoyl)-2,2'bipyridine}Cl]SbF6 (Pt4) were studied using a series of five-membered heterocyclic neutral nitrogen donor nucleophiles, viz. pyrazole (Pz), triazole (Tz), imidazole (Im), 1-methylimidazole (MIm) and 1,2-dimethylimidazole (DMIm) under pseudo-first-order conditions in methanol using UV/Visible spectrophotometry and conventional stopped-flow techniques. The observed second-order rate constants, k2, followed a two term rate law k(obs) = k2[nucleophile] + k(s) except for DMIm with Pt1, Pt3, Pt4 and Pz, Tz with Pt1. Increasing the π-conjugation in the cis position decreases the rate of chloride substitution by decreasing the π-acceptor property of the terpy moiety. However, increasing the π-conjugation in the cis/trans position increases the rate of substitution by enhancing the π-acceptor property within the ligand framework whereby increasing the reactivity of the metal centre. The observed trend for the reactivity was Pt2 > Pt1 > Pt3 > Pt4. The substitution kinetics was influenced by the basicity of the incoming nucleophiles except for the sterically hindered nucleophile, DMIm. The general trend observed for the reactivity of the nucleophiles is MIm > Im > DMIm > Pz > Tz.

Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

AbstractThe substitution kinetics of the complexes [Pt{4′‐(o‐CH3‐Ph)‐terpy} Cl]SbF6 (CH3PhPtCl(Sb)), [Pt{4′‐(o‐CH3‐Ph)‐terpy}Cl]CF3SO3 (CH3PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF6 (PhPtCl), [Pt(terpy)Cl]Cl·2H2O (PtCl), [Pt{4′‐(o‐Cl‐Ph)‐terpy}Cl]SbF6 (ClPhPtCl), and [Pt{4′‐(o‐CF3‐Ph)‐terpy}Cl]SbF6 (CF3PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N,N′‐dimethylthiourea (DMTU), and N,N,N′,N′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law kobs = k2[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF3PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH3PhPtCl. Thiourea was found to be the best nucleophile with N,N,N′,N′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008

Does increased chelation enhance the rate of ligand substitution at PtII (N,N,N) centres? A detailed kinetic &amp; mechanistic study

The rate of substitution of the chloride and aqua moieties from the platinum(II)-amine complexes, viz. [Pt(dien)Cl]Cl(Pt1-Chloro) and [Pt(en)(NH3)Cl]Cl (Pt2-Chloro) and their corresponding aqua analogues, viz. [Pt(dien)(OH2)] (ClO4)2 (Pt1-Aqua) and [Pt(en)(NH3)(OH2)](ClO4)2 (Pt2-Aqua), by a series of neutral and anionic nucleophiles,viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU), iodide (I−) and thiocyanate (SCN−), was determined under pseudo first-order conditions as a function of concentration and temperature using UV/Visible spectrophotometry and standard stopped-flow techniques. The observed pseudo first-order rate constants for the substitution reactions obeyed the simple rate law k obs = k 2[Nucleophile]. Second-order kinetics and negative activation entropies, ca. −93 J K−1 mol−1 and −71 J K−1 mol−1, for the chloro and aqua complexes respectively, support an associative mode of activation. The rate of substitution of both the chloro and aqua moieties are observed to decrease with an increase in the steric bulk of the neutral nucleophiles, whilst rate of substitution by SCN− was observed to be faster than that of I−, in correlation with the observed nucleophilicities of the two nucleophiles. A comparison of the second-order rate constants, k 2, at 298 K, obtained for the substitution reactions of Pt1and Pt2 shows that an increase in chelation in moving from Pt2 to Pt1 results in a corresponding increase in the reactivity, by a factor of ca. 3, (28.31 ± 0.15 and 8.02 ± 0.13 m −1 s−1 for Pt1 and Pt2 respectively, in the case of substitution of the aqua species by TU). Computational analysis of the chloro complexes, viz. Pt1-Chloro, Pt2-Chloro and [Pt(NH3)3Cl]Cl (Pt3) support this conclusion by demonstrating that the Pt–N bond trans to the leaving group is shortened and that the Pt–Cl bond is lengthened when chelation is increased from Pt3 to Pt1. Consequently, these results suggest that the increase in reactivity of Pt1 over Pt2, promoted by increased chelation, is as a result of ground state destabilization.

Preparation, Characterization, and Aquation Kinetics of Pyridine N‐Oxide Complexes of Chromium(III)

AbstractA series of (H2O)5Cr(X‐pyO)3+ ions (pyO = pyridine N‐oxide, X = H, 3‐CH3, 4‐CH3, 4‐OCH3, 4‐NO2) were prepared by the reduction of the corresponding pyridine N‐oxide adducts of diperoxochromium(VI) species with acidic ferrous perchlorate. The (H2O)5Cr(X‐pyO)3+ complexes undergo aquation to yield Cr(H2O)63+ and X‐pyO according to the rate law kobs = ko + k–1[H+]–1. The values of the rate constants extrapolated to 298 K at 1.0 M ionic strength are: k0 = 2.80 × 10–6 s–1, k–1 = 1.86 × 10–8 M s–1 (X = 4‐NO2); 7.80 × 10–8, 6.27 × 10–10 (H);4.80 × 10–8, 3.20 × 10–10 (3‐CH3); 3.05 × 10–8, 1.60 × 10–10 (4‐CH3); and 2.37 × 10–9, 4.76 × 10–11 (4‐OCH3). The reaction of the 4‐OCH3 complex exhibits two additional terms in the rate law, k1[H+] + k–2[H+]–2. The binding of 4‐OCH3–pyO to chromium is suggested to take place through the methoxy group. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Oxidation of Nitrite by Peroxynitrous Acid

The kinetics of the oxidation of nitrite to nitrate by peroxynitrous acid at pH 5.2 is best described by the rate law kobs = kiso + k‘[NO2-] + k‘ ‘[NO2-]2, in which the peroxynitrous acid isomerization rate constant kiso = (1.10 ± 0.05) s-1, k‘ = (3.2 ± 0.1) M-1 s-1, and k‘ ‘ = (4.2 ± 0.3) M-2s-1, at 25 °C. The ternary reaction may involve initial formation of an adduct between nitrite and peroxynitrite, followed by reaction with a second nitrite to form two nitrite and a nitrate. Ab initio calculations indicate that there is only a small intrinsic barrier to the net transfer of HO+ from peroxynitrous acid to the nitrogen atom of nitrite. A similar transfer to either of the two oxygens of nitrite produces the reactants, and would not lead to an increase in the rate of disappearance of peroxynitrous acid, as observed. The low rate constant is most likely due to stringent orientational constraints. Formal transfer of HO+ to 15NO2- results in formation of 15NO3-, as experimentally observed. HO+ transfer is common to the chemistry of peracids, a family of compounds to which peroxynitrous acid belongs.