Outer-sphere Association Research Articles

Kinetics and mechanism of interaction of hexaaquachromium(III) with l-Dopa in aqueous medium: comparative antiparkinsonian studies

The kinetics and mechanism of the substitution reaction between [Cr(H2O)6]3+ and l-Dopa in aqueous medium has been studied over the range 1.8 ≤ pH ≤ 2.6, 1.68 × 10−2 mol dm−3 ≤ [Dopa] ≤ 5.04 × 10−2 mol dm−3, I = 0.1 mol dm−3 (KNO3) at 50 °C. The reaction takes place via an outer sphere association between Cr3+ and l-Dopa followed by chelation. The product was characterized by physicochemical and infrared spectroscopic methods. The antiparkinsonian activity of the product was found to be higher than that of l-Dopa.

Communication: Near edge x-ray absorption fine structure spectroscopy of aqueous adenosine triphosphate at the carbon and nitrogen K-edges

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen and carbon K-edges was used to study the hydration of adenosine triphosphate in liquid microjets. The total electron yield spectra were recorded as a function of concentration, pH, and the presence of sodium, magnesium, and copper ions (Na(+)/Mg(2+)/Cu(2+)). Significant spectral changes were observed upon protonation of the adenine ring, but not under conditions that promote π-stacking, such as high concentration or presence of Mg(2+), indicating that NEXAFS is insensitive to the phenomenon. Intramolecular inner-sphere association of Cu(2+) did create observable broadening of the nitrogen spectrum, whereas outer-sphere association with Mg(2+) did not.

Kinetic and mechanistic studies on the reaction of thiosemicarbazide with [(H2O)(tap)2RuORu(tap)2(H2O)]2+ {tap = 2-(m-tolylazo)pyridine}

The interaction of thiosemicarbazide with the title complex has been studied spectrophotometrically in aqueous medium as a function of [complex], [thiosemicarbazide], pH and temperature at constant ionic strength. At pH 7.4, the reaction shows two distinct paths; both of which are [thiosemicarbazide] dependent. A parallel reaction scheme fits well with the experimental findings. An associative interchange mechanism is proposed for both the paths; the activation parameters calculated from Eyring plots are ΔH 1 ≠ = 14.2 ± 0.8 kJ mol−1, ΔS 1 ≠ = −241 ± 2 JK−1 mol−1, ΔH 2 ≠ = 30.8 ± 1.4 kJ mol−1 and ΔS 2 ≠ = −236 ± 4 JK−1 mol−1. From the temperature dependence of the outer sphere association complex equilibrium constants, the thermodynamic parameters calculated are ΔH 1 ° = 34.25 ± 1.9 kJ mol−1, ΔS 1 ° = 146 ± 6 J K−1 mol−1 and ΔH 2 ° = 9.4 ± 1.1 kJ mol−1, ΔS 2 ° = 71 ± 3 JK−1 mol−1, which gives a negative ΔG° at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex.

Kinetics and mechanism of the interaction of DL-penicillamine with cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II) perchlorate in aqueous medium

The kinetics of the interaction of DL-penicillamine with cis-[Pt(cis-dach)(OH2)2]2+ has been studied spectrophotometrically as a function of [cis-[Pt(cis-dach)(OH2)2]2+], [DL-penicillamine], and temperature at a particular pH (4.0), where the complex exists predominantly as the diaqua species and DL-penicillamine as a zwitterion. The substitution reaction proceeds via rapid outer sphere association, followed by two slow consecutive steps. The first is the conversion of title complex into an inner sphere complex and the second is the slower chelation step whereby another aqua ligand is replaced. The association equilibrium constant (K E) for the outer sphere complex formation has been evaluated together with rate constant for two subsequent steps. The activation parameters for both steps have been evaluated using the Eyring equation: ( = 36 ± 4 kJ mol−1, = −175 ± 12 JK−1 mol−1 and = 44 ± 1 kJ mol−1, = −189 ± 3 JK−1 mol−1). The low enthalpy of activation and large negative value of entropy of activation are consistent with an associative mode of activation for both consecutive steps. The kinetic study has been substantiated by conductivity measurement, product isolation, IR and ESI-MS spectroscopic analysis.

Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with l-Arginine in aqueous solution

The kinetics of the aqua ligand substitution from hydroxopentaaquarhodium(III) ion, [Rh(H2O)5(OH)]2+, by l-Arginine has been studied spectrophotometrically as a function of Arginine concentration, and temperature, at pH 4.3. The reaction proceeds via a rapid outer sphere association complex formation step followed by two consecutive steps. The first of these involves ligand-assisted anation, while the second involves chelation as the second aqua ligand is displaced. The association equilibrium constant for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The activation parameters for both steps have been evaluated using Eyring’s equation. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants are also consistent with an associative mode of activation. The product of the reaction has been characterized by conductivity measurement and IR spectroscopic analysis.

Mechanistic studies of substitution reaction of trans-(diaqua)-(N,N' -ethylene-bis-salicylamide)chromium(III) ion

The kinetics of substitution of trans-[Cr(Salm)(OH 2 ) 2 ]+ with some biologically important ligands (Nu - ) viz. glycine (Gly), p-aminobenzoic acid (Paba), glycyl-glycine (Gly-gly), L -histidine (His), L -isoleucine (Ile) has been studied spectrophotometrically over the range: 25 ≤ 5 t ≤ 45 °C, 1.8 ≤ pH ≤ 5.5, 0.01 ≤ [Nucleophile] ≤ 0.30, I = 0.5 mol dm -3 (KNO 3 ). Although there are two replaceable aqua ligands in the complex, only mono substitution occurred, which is evident from Job's curve. The kinetic studies also showed one aqua ligand substitution. Unlike trans-[Cr(Salen)(OH 2 ) 2 ] + , the rates of aqua ligand substitution were found slower indicating moderate labilization of axial aqua ligand. The reaction takes place via an outersphere association between the Cr III complex and various nucleophiles followed by transformation of the outer into inner sphere complexes by slow interchange. The anation rate constants (10 4 k an /s -1 ) at 25 °C were found to be 14.98 (Gly), 8.45 (Ile), 10.52 (Paba), 14.23 (His) and 13.78 (Gly-gly) and the corresponding ΔH # (kJ mol -1 ) values are 57.7 ± 1.6, 38.3 ± 1.3, 43.0 ± 0.8, 34.2 ± 2.7 and 39.7 ± 3.1, the ΔS # (JK -1 mol -1 ) values are -105 ± 5, -175 ± 5, -157 ± 3, -65.8 ± 9 and -166 ± 11. The higher value of the k an in comparison to k ex and highly negative values of activation entropy and variation of k an by changing nucleophiles, support I a mechanism for the substitution reactions.

Molecular CoIII/FeIICyano-Bridged Mixed-Valence Compounds with High Nuclearities and Diversity of CoIIICoordination Environments: Preparative and Mechanistic Aspects

The kinetico-mechanistic study of the formation of discrete tri- and tetranuclear mixed-valence cyano-bridged (Co(III))(2)/Fe(II) and (Co(III))(3)/Fe(II) complexes has been carried out from their already described parent dinuclear Co(III)/Fe(II) and mononuclear Co(III) complexes. Different Co(III) encapsulating units have been used in order to tune both the redox potential on the mono- and dinuclear complexes and the lability of the mononuclear building blocks. The importance of outer-sphere precursor complex formation involving the oppositely charged reactants has been established by experiments conducted at high ionic strength where electrostatic effects are nullified. The influence of the pH is also crucial, and this has been linked to the pH dependence of the precursor Co(III/)(II) redox potentials in terms of enabling a redox-assisted association (at low pH) between the ferricyanide analogue and the labile Co(II) partner. A new asymmetric Co(III)L/Fe(II)/Co(III)L' complex has been fully characterized, and a series of putative (Co(III)L)(2)/Fe(II)/Co(III)L' and (Co(III)L)(3)/Fe(II) forms have been spectroscopically detected. A comparison with the mechanistic reaction pathways established for the formation of the parent dinuclear Co(III)/Fe(II) complexes indicates that an important tuning of the direct substitution and redox-catalyzed mechanism is applicable. For the formation of the trinuclear complexes, only in the most favorable conditions is a redox-assisted sequence observed. The rate and activation parameters for the reactions have been determined and are indicative of an essential outer-sphere precursor formation. Similarly, for reactions where redox-assisted mechanisms are unfavorable, only direct substitution processes have been found to be applicable, with their rate coefficients and activation parameters also agreeing with the expectations, once an outer-sphere precursor complex is formed. Formation of the tetranuclear (Co(III)L)(3)/Fe(II) complexes has only been detected on decomposition of the parent trinuclear (Co(III)L)(2)/Fe(II) complexes following reduction to their Co(II) form. The overall processes seem to be based on the outer-sphere association between the respective building blocks in such a way that both unfavorable redox potentials and Lewis basicities are overcome.

Kinetic and Mechanistic Studies on the Reaction of DL-Methionine with in Aqueous Medium at Physiological pH

The reaction has been studied spectrophotometrically; the reaction shows two steps, both of which are dependent on ligand concentration and show a limiting nature. An associative interchange mechanism is proposed. Kinetic and activation parameters ( and ) and (, , , and ) have been calculated. From the temperature dependence of the outer sphere association equilibrium constant, thermodynamic parameters ( and ; and ) have also been calculated.

Kinetic and mechanistic studies on the substitution of aqua ligands from cis-diaqua-bis-(bypyridyl)-ruthenium(II) ion by vic-dioximes

Kinetics of aqua ligand substitution from cis-[Ru(bpy)2(H2O)2]2+ by three vicinal dioximes, namely dimethylglyoxime (L1H), 1,2-cyclohexane dionedioxime (L2H) and α-furil dioxime (L3H) have been studied spectrophotometrically in the 45–60 °C temperature range. The rate constants increase with increasing dioxime concentration and approach a limiting condition. We propose the following rate law for the reaction in the 3.5–5.5 pH range: \( {\text{Rate }} = k_{ 2} K_{\text{E}} \left[ {{\text{Ru}}\left( {\text{bpy}} \right)_{ 2} \left( {{\text{H}}_{ 2} {\text{O}}} \right)_{2}{}^{2+} } \right]{{\left[ {\text{LH}} \right]} \mathord{\left/ {\vphantom {{\left[ {\text{LH}} \right]} {\left( { 1+ K_{\text{E}} \left[ {\text{LH}} \right]} \right)}}} \right. \kern-\nulldelimiterspace} {\left( { 1+ K_{\text{E}} \left[ {\text{LH}} \right]} \right)}}, \) where k 2 is the interchange rate constant from outer sphere to inner sphere complex and K E is the outer sphere association equilibrium constant. Activation parameters were calculated from the Eyring plots for all three systems: ΔH ≠ = 59.2 ± 8.8, 63.1 ± 6.8 and 69.7 ± 8.5 kJ mol−1, ΔS ≠ = −122 ± 27, −117 ± 21 and −99 ± 26 J K−1 mol−1 for L1H, L2H and L3H, respectively. An associative interchange mechanism is proposed for the substitution process. Thermodynamic parameters calculated from the temperature dependence of the outer sphere association equilibrium constants give negative ΔG 0 values for all the systems studied at all the temperatures (ΔH 0 = 30.05 ± 2.5, 18.9 ± 1.1 and 11.8 ± 0.2 kJ mol−1; ΔS 0 = 123 ± 8, 94 ± 3 and 74 ± 1 J K−1 mol−1 for L1H, L2H and L3H, respectively), which also support our proposition.

Photophysical and electrochemical properties of the outer-sphere associate of [Ru(bipy)3]2+ with p-sulfonatothiacalix[4]arene

1H NMR titration and X-ray diffraction analysis revealed that [Ru(bipy)3]2+ forms an outer-sphere inclusion complex with p-sulfonatothiacalix[4]arene in a ratio of 1: 1 in both aqueous solutions and the solid state. According to cyclic voltammograms and fluorimetric data, the outer-sphere association of [Ru(bipy)3]2+ with p-sulfonatothiacalix[4]arene changes the reversible character of the electrochemical oxidation of [Ru(bipy)3]2+ and lowers its emission intensity.

Kinetics and mechanism of aqua ligand substitution from cis-diaqua(cis-1,2-diaminocyclohexane)platinum(II)perchlorate by diethyldithiocarbamate anion in aqueous medium

The kinetics of the interaction of diethyldithiocarbamate (Et2DTC) with [Pt(dach)(H2O)2]2+ (dach = cis-1,2-diaminocyclohexane) have been studied spectrophotometrically as a function of [Pt(dach)(H2O)22+], [Et2DTC] and temperature at a particular pH (4.0). The reaction proceeds via rapid outer sphere association complex formation followed by two slow consecutive steps. The first step involves the transformation of the outer sphere complex into an inner sphere complex containing a Pt–S bond and one aqua ligand, while the second step involves chelation when the second aqua ligand is replaced. The association equilibrium constant KE and two rate constants k1 and k2 have been evaluated. Activation parameters for both the steps have been calculated (∆H1# = 66.8 ± 3.7 kJ mol−1, ∆S1# = −81 ± 12 JK−1 mol−1 and ∆H2# = 95.1 ± 2.8 kJ mol−1, ∆S2# = −34.4 ± 9.1 JK−1 mol−1). The low enthalpy of activation and negative entropy of activation indicate an associative mode of activation for both the steps.

The outer-sphere association of p-sulfonatothiacalix[4]arene with some Co(III) complexes: the effect on their redox activity in aqueous solutions

The effect of the ion-pairing of Co(III) complexes with p-sulfonatothiacalix[4]arene (STCA) on Fe(II)–Co(III) electron transfer rate was evaluated from the analysis and comparison of kinetic data in double Co(III)–Fe(II) and triple Co(III)–Fe(II)—STCA systems at various concentration conditions. Complexes [Co(en)3]3+(1), [Co(en)2ox]+(2), [Co(dipy)3]3+ (3), [Co(His)2]+(4) and [Fe(CN)6]4− were chosen as Co(III) and Fe(II) compounds. The effect of STCA was found to correlate with the association mode. The outer-sphere association with STCA was found to exhibit the insignificant effect on Fe(II)–Co(III) electron transfer ket constants for complexes 3 and 4 with bulky and rigid chelate rings, while more sufficient inclusion of flexible ethylendiaminate rings of 1 and 2 into the cavity of STCA results in the unusual increase of ket.

Kinetics and mechanism of the reaction of hydroxopentaaquarhodium(III) ion with L-proline in aqueous solution

The kinetics of the reaction of L-proline (H 2 L + ) with hydroxopentaaquarhodium(III) has been studied over the range 2.48 ≤ 10 3 [H 2 L + ] ≤ 7.44, 3.5 ≤ pH ≤ 4.5, 50° ≤ t ≤ 70° at l = 0.1 mol dm -3 (NaClO 4 ). The reaction proceeds through an outersphere association beween [Rh(OH)] 2+ and HL (conjugate base of H 2 L + ) followed by transformation of the outer into an inner sphere complex (chelate is formed) by slow interchange. The anation rate constant k an of RhOH 2+ at 25 °C and I = 0.1 mol dm -3 (NaClO 4 ) is estimated to be 1.56 x 10 -4 s -1 . ΔH≠ and ΔS≠ for k an path are found to be 54.5 ± 3.5 kJ mol -1 and -135.7 ± 10.5 JK -1 mol -1 respectively. The product of the reaction has been isolated and characterised. The substitution reaction follows an Ia path.

Reaction of L-cystine with cis-diaqua(ethylenediamine)platinum(II) cation A kinetic and mechanistic study

Substitution reaction of cis-diaqua(ethylenediamine)platinum(II) cation by L-cystine (H 4 L 2+ ) has been studied spectrophotometrically over the range 7.5 ≤ 10 4 [H 4 L 2+ ] T ≤ 50.0, 3.04 ≤ pH S 4.43, 40 S t ≤ 60 °C and I = 0.1 mol dm -3 (NaClO 4 ). The reaction takes place via an outer sphere association between the aquaplatinum(II) cation and H 4 L 2+ followed by transformation of the ouler into inner-sphere complex by slow interchange. The anation rate constant at 25 °C and I = 0.1 mol dm -3 (NaClO 4 ) is found to be 3.39 x 10 -4 s -1 , ΔH # and ΔS # for k an path are found to be 28.9 kJ mol -1 and -214.4 JK -1 mol -1 . The anation reaction at platinum(ll) center follows an la path.

A Kinetic and Mechanistic Study on Substitution of Aqua Ligands from cis-diaqua-bis-(bipyridyl)-ruthenium(II) Complex by Thiosemicarbazide in Aqueous Medium

The kinetics of the interaction of thiosemicarbazide with cis-[Ru(bipy)2(H2O)2]2+ (bipy = α α′-bipyridyl) have been studied spectrophotometrically as a function of [Ru(bipy)2(H2O) 2 2+ ], [bipyridyl] and temperature, at a particular pH (4.8), where the substrate complex exists predominantly as the diaqua species and thiosemicarbazide as the neutral ligand. The reaction proceeds via an outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex, and the second step involves the entrance of another thiosemicarbazide molecule in the coordination zone of Ru(II) whereby, in each step, an aqua ligand is replaced. The association equilibrium constant (KE) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (ΔH 1 # = 25.37±1.6 kJ mol−1, ΔS 1 # = −215.48 ± 4.5 J K−1 mol−1, ΔH 2 # = 24.24 ± 1.1 kJ mol−1, ΔS 2 # = −207.14 ± 3.0 J K−1 mol−1). The low enthalpy of activation and large negative value of entropy of activation indicate an associative mode of activation for both aqua ligand substitution processes. From the temperature dependence of KE, the thermodynamic parameters calculated are: ΔH0 = 10.75±0.54 kJ mol−1 and ΔS0 = 84.67 ± 1.75 J K−1 mol−1, which give a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex prior to the first step.

Outer-Sphere Association of p-Sulfonatothiacalix[4]arene and Tetrasulfonatomethylated Calix[4]resorcinarene with Cobalt(III) Tris(dipyridyl): The Effect on the Spectral and Electrochemical Properties of the Latter

According to the 1H NMR and X-ray data, the outer-sphere association of [Co(dipy)3]3+ with cyclophanic anions p-sulfonatothiacalix[4]arene and tetrasulfonatomethylated calix[4]resorcinarene is accompanied by the inclusion of the dipyridyl moiety of the former into the cavities of the cyclophanic anions in aqueous and aqueous-DMSO solutions and in the solid state. The first-step dissociation of p-sulfonatothiacalix[4]arene does not change the inclusion mode, but the increase in DMSO content leads to a shallower inclusion of [Co(dipy)3]3+. The electrochemical property changes of [Co(dipy)3]3+ with both receptors undergoing ion pairing indicate the stabilization of the tris(dipyridyl) cobalt complexes in lower oxidation states. The effect of the cyclophanic anions on the cyclic voltammograms of [Co(dipy)3]3+ correlates with the efficiency of binding of the latter with tetrasulfonatomethylated calix[4]resorcinarene and p-sulfonatothiacalix[4]arene.

Kinetic and mechanistic studies on the interaction of thymidine with the hydroxopentaaquarhodium(III) ion

The interaction of thymidine, a nucleoside, with hydroxopentaaquarhodium(III), [Rh(H2O)5(OH)]2+ ion in aqueous medium is reported and the possible mode of binding is discussed. The kinetics of interaction between thymidine and [Rh(H2O)5OH]2+ has been studied spectrophotometrically as a function of [Rh(H2O)5OH2+], [thymidine], pH and temperature. The reaction has been monitored at 298 nm, the λmax of the substituted complex, and where the spectral difference between the reactant and product is a maximum. The reaction rate increases with [thymidine] and reaches a limiting value at a higher ligand concentration. From the experimental findings an associative interchange mechanism for the substitution process is suggested. The activation parameters (ΔH‡=47.8 ± 5.7 kJ mol−1, ΔS‡=−173 ± 17 J K−1 mol−1) supports our proposition. The negative ΔG0 (−13.8 kJ mol−1) for the first equilibrium step also supports the spontaneous formation of the outer sphere association complex.

Outer-sphere interactions between octahedral chiral cobalt(iii) complexes and water-soluble calixarenes

The structures and stability of outer-sphere associates of sulfonate derivatives of thiacalix[4]arene and calix[4]resorcinarene with coordinatively saturated cobalt(iii) bis- and tris-chelates ([Co(L-His)2]+, [Co(en)2ox]+, [Co(en)3]3+, and [Co(dipy)3]3+) were compared based on the data from UV, CD, 1D 1H NMR, and 2D (2D NOESY) 1H NMR spectroscopy and conductometry. Outer-sphere association is accompanied by partial penetration of the chelate rings of the complexes into the hydrophobic cavity of calixarene, which induces changes in the spectroscopic and spectropolarimetric properties of the cobalt(iii) complexes.

Kinetics and mechanism of the interaction of azide with [(H2O)(tap)2RuORu(tap)2-(H2O)]2+ion at physiological pH

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N3−; both processes are [ligand]-dependent. The rate constant for the processes are: k1∼10−3 s−1 and k2∼10−5 s−1. The activation parameters calculated from Eyring plots are: ΔH1‡ = 14.8 ± 1 kJ mol−1, ΔS1‡ = −240 ± 3 J K−1 mol−1, ΔH2‡ = 44.0 ± 1.5 kJ mol−1 and ΔS2‡ = −190 ± 4 J K−1 mol−1. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: ΔH10 = 4.4 ± 0.9 kJ mol−1, ΔS10 = 64 ± 3 J K−1 mol−1 and ΔH20 = 14.2 ± 2.9 kJ mol−1, ΔS20 = 90 ± 9 J K−1 mol−1, which gives a negative ΔG0 value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.

Kinetic effect of cosolvents on the metalation reaction of porphyrin with bis(β-diketonato)copper(II) in supercritical carbon dioxide

The kinetic effect of cosolvents on the Cu(II) ion incorporation into 5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin (H 2tpfpp) with bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)copper(II) (Cu(hfac) 2) in supercritical carbon dioxide has been investigated using methanol, ethanol, and 1-propanol as the cosolvent. The conditional first-order rate constants showed a saturation dependence versus the mole fraction of a large excess Cu(hfac) 2. The metalation reaction is postulated to consist of a fast pre-equilibrium of the outer-sphere association between Cu(hfac) 2 and H 2tpfpp and a rate-determining Cu(II) ion incorporation into the porphyrin core. The apparent reaction rates are accelerated by the addition of cosolvents and the effects will be discussed in relation to the solvation of the cosolvents.