The kinetics of oxidation of L-cysteine (cys) by trans-MnIII(salen)(OH2)2+ (H2salen = N,N '-bis(salicylidene)ethane1,2-diamine) is studied at 30.0-45.0 degrees C, 1.90 <= pH <= 7.46, I = 0.3 mol dm-3, and the same is investigated for DLhomocysteine (Hcys) at 35 degrees C for comparison. Similarly, kinetic studies are performed for glutathione (GSH) at 30.0-45. 0 degrees C, 4.35 <= pH <= 7.46. The product analysis indicated the formation of corresponding disulphides, and MnIII is reduced to MnII. The same products are also formed when the oxidation is carried out in the presence of externally added Cu2+ ions. Although the oxidation is moderately catalyzed in the presence of Cu2+ ions but it is retarded by the chelating ligand EDTA. The stoichiometric ratio Delta MnIII(salen)(H2O)+2 : Delta[X] =1:1 (X = cys, Hcys, and GSH). The reaction proceeds via fast equilibrium pre-association inner-sphere complexes between transMnIII(salen)(OH2)2 + and X, followed by very slow intramolecular electron transfer steps. The kinetic parameters for various steps for cys, Hcys, and GSH are presented. The results indicate an outer-sphere electron transfer mechanism for the reduction of MnIII to MnII. The DFT-optimized structures supported the carboxylate mode of binding and the ground state structural trans effect for all the species. In addition, it supports a proton-controlled electron transfer process (PCET).

The trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\({\hbox {OH}_{2})_{2}}^{+}\) and bioxalate (\(\hbox {HOX}^{-})\) in aqueous medium equilibrate rapidly to trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})\)(HOX) followed by the acid dissociation equilibrium to the (aqua) mono oxalato complex. The slow redox reactions of trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})\)(HOX/OX)\(^{0/-}\) with \(\hbox {H}_{2}\hbox {OX}\), \(\hbox {HOX}^{-}\),\(\hbox {OX}^{2-}\) obey second order kinetics satisfying 2:1 stoichiometry \(([\hbox {Mn}^{{\mathrm{III}}}]_{\mathrm{T}}/[\hbox {OX}]_{\mathrm{T}} = 2/1)\). The products are \(\hbox {Mn}^{{\mathrm{II}}}\) and \(\hbox {CO}_{2}\). Acrylamide monomer has no effect on the rate constant and the reaction does not induce its polymerization. The rate and activation parameters for the various rate limiting paths are reported. The intramolecular reduction of \(\hbox {Mn}^{{\mathrm{III}}}\) by the coordinated \(\hbox {HOX}^{-}\) and \(\hbox {OX}^{2-}\) in trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})(\hbox {HOX/OX})^{0/-}\) could not be detected. Contrary to our expectation, it is observed that \(\hbox {H}_{2}\hbox {OX}\) is a better reducing agent than \(\hbox {HOX}^{-}\) for trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})(\hbox {HOX})\), the slowest being the redox reaction of \(\hbox {OX}^{2-}\) with trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})(\hbox {OX})^{-}\). The molecular modelling by DFT depicts the structural trans effect in the oxalato complexes, it being maximum for trans-\(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen)(\(\hbox {OH}_{2})(\hbox {OX})^{-}\). The observed sequence of the redox activity of the oxalato complexes reflects the potential role of non-covalent interaction i.e. H-bonding, governing the proton controlled electron transfer process (PCET). The \(\hbox {Mn}^{{\mathrm{III}}}\)(Salophen/Salen) complexes may turn out to be good substitute candidates for Oxalo Oxidase (OXO) enzyme in alleviating the oxalate overload in plants and animal biochemistry.

Water exchange of trans-[MnIII(salen)(OH2)2]+ studied by line broadening 17OH2 NMR discloses its mechanism as associative interchange (Ia).

The kinetics of the title reactions have been investigated in aqueous perchlorate medium of 1=0.3 mol dm -3. The rate and activation parameters for the aquation reaction: cis-[CoX(imH)(en),Jn + + H20 ->cis-[Co(OH ,XimH)(en)'ll+ + x(n.31 + [X = NCS , N, (n = 2): NCSHg +, N,Hg + (n = 4)J and also for the acid-catalyzed aquation of the azido complex, cis[Co(N3)(imH)(en)2J2 + + H + + H20->cis-[Co(OH2)(imH)(en)'J + + N,H. are reported at 60-80°C. The Hg(lI)-adduct, cis[Co(N,Hg)(imH)(en),JH is _103 times thermodynamically less stable than its S-bonded analogue, cis[Co(NCSHg)(imH)(en),J4 +; but the former undergoes water substitution at the cobalt(II1) centre much faster than the latter. pH titration at 25"C (I =0.3 mol dm -.1) yields pKNIl= 10.3±O.I and 10.6±O.I for the coordinated imidazole in the case of N, and NCS complexes respectively. The overall base hydrolysis of both the complexes (0.002 .;;[OH -J .;;0.2 mol dm 3. 35-50T) reveals a two-term rate law which is interpreted in terms of the aquation and base hydrolysis of the imidazole(I-) conjugate base. cis-[CoX(im)(enl,J + . The rate and activation parameters for both the paths are reported. The rates of aquation of the complex species are found to increase in the order: [CoX(im)(en),J + > [Co(XHg)(imH)(en),]4 + > [CoX(imH)(en)'f + (X = N, or NCS -). The coordinated imidazole(I-) species is found to labilize Co X bond -10' limes stronger than imidazole.

MnIII(salen)(OH2)2+ undergoes reversible anation by HOX−via Ia mechanism followed by proton controlled electron transfer involving MnIII(salen)(HOX) and H2OX.

The kinetics of oxidation of glyoxylic acid (HGl) by MnIII(salen)(OH\(_{2})_{2}^{+}\) ((H2salen = N,N′- bis(salicylidene)ethane-1,2-diamine) is investigated at 30.0–45.0°C, 1.83 ≤ pH ≤ 6.10, I = 0.3 mol dm−3(NaClO4). The products are identified as formic acid, CO2 and MnII with the reaction stoichiometry, |Δ[MnIII]/Δ[HGl]| = 2. The overall reaction involves fast equilibrium pre-association of MnIII(salen)(OH\(_{2})_{2}^{+}\) with HGl and its conjugate base Gl− forming the corresponding inner sphere complexes (both HGl and Gl− being the monohydrate gem-diol forms) followed by the slow electron transfer steps. In addition, the second order electron transfer reactions involving the inner-sphere complexes and HGl/Gl− are also observed. The rate, equilibrium constants and activation parameters for various steps are presented. MnIII(salen)(OH2)(Gl) is virtually inert to intra molecular electron transfer while the process is facile for MnIII(salen)(OH2)(HGl)+ (105 k et = 2.8 ± 0.3 s−1 at 35.0°C) reflecting the involvement of proton coupled electron transfer mechanism in the latter case. A computational study of the structure optimization of the complexes, trans-MnIII(salen)(OH\(_{2})_{2}^{+}\), trans-MnIII(salen)(OH2)(Gl), and trans- MnIII(salen)(OH2)(HGl)+ (all high spin MnIII(d4) systems), reveals strongest axial distortion for the (aqua)(Gl) complex ; HGl bound to Mn III centre by the C=O function of the carboxyl group in the (aqua)(HGl) complex facilitates the formation of a hydrogen bond between the proton of the carboxyl group and the coordinated phenoxide moiety ((O-H …O hydrogen bond distance 1.745 Å) and the gem-diols are not involved in H-bonding in either case. A rate comparison for the second order paths: MnIII(salen)(OH2)(HGl)/Gl)+/0+ HGl/Gl−→ products, shows that HGl for the (aqua)(HGl) complex is a better reducing agent than Gl− for the (aqua)(Gl) complex (k HG ∼ 5 k Gl). The high values of activation enthalpy ( ΔH ≠= 93–119 kJ mol−1) are indicative of substantial reorganization of the bonds as expected for inner-sphere ET process.

The reduction of the octahedral cobalt(III) complex CoIII(HL)·9H2O, H4L = 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane by glutathione (GSH) has been studied by conventional spectrophotometry at 25.0 ≤ t/°C ≤ 45.0, 0.02 ≤ [H+]/mol dm−3 ≤ 0.20 and I = 0.3 mol dm−3 (NaClO4). The reaction is biphasic. The fast initial phase is attributed to the H+-induced formation of the mixed ligand complex, [CoIII(H2L)GSH]+, for which the rate-limiting step is the chelate ring opening via CoIII–NH (amide–N) bond cleavage of the protonated species, [CoIII(H2L)]+. Outer-sphere association equilibria between GSH/GSH2 + and [CoIII(H2L)]+ substantially retard the ring opening process and consequently the mixed ligand complex formation. This is then followed by a slow phase involving reduction of [CoIII(H2L)GSH]+ by both GSH and GSH2 +. The final products are the corresponding Co(II) complex and the oxidized form of GSH, GS–SG. The kinetic data and activation parameters for the redox process are interpreted in terms of an outer-sphere electron transfer mechanism.

The protonated and unprotonated N,N'-ethylene-bissalicylamidat6iron(III) ions, Fe (SALMH)+ and Fe(SALMt, undergo fast complexation with L-ascorbic acid to form ternary complexes, [Fe(SALM)(HASc)], and [Fe(SALM)(ASc)r in which the ascorbate ion is chelated to Fe centre. These ternary complexes further undergo intramolecular electron transfer via innersphere mechanism in the stopped flow time scale producing Fe and dehydroascorbic acid; the order of reactivity is kFe(SALMH) > kFc(SALM ).

The bis phenoxide forms of (1,2)bis(2-hydroxybenzamido)ethane(I), (1,5)bis(2-hydroxybenzamido)3-azapentane(II), (1,3)bis(2-hydroxybenzamido)propane(III), and (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane(IV) undergo facile hydrolysis of one of the amide groups (0.02 ≤ [OH − ]T (mol dm − 3) ≤ 0.5, 10% MeOH (v/v) + H2O medium) without exhibiting [OH − ] dependence. The reactivity trend follows I ~ II > > III ~ IV with low activation enthalpy {25.7 ± 2.8 ≤ ΔH≠ (kJ mol − 1) ≤ 64.8 ± 7.0}. The high negative and comparable values of activation entropy {− 234 ± 8 ≤ ΔS≠ (J K − 1 mol − 1) ≤ −127 ± 20} are consistent with closely similar, and ordered transition states which can be assembled by favourably oriented phenoxide groups. The solvent kinetic isotope effect for I, k H2O/k D2O + H2O ~1 (20 and 50 volume% D2O), indicates that proton transfer is not involved as a part of the rate controlling process. The observed slowing down of the rate of this reaction for I in the micellar pseudo phase of CTABr also supports the proposed mechanism. Under pre-micellar conditions, however, rate acceleration is observed, a consequence believed to be associated with the capping effect of the hydrophobic tail of the surfactant cation forming the reactive ion-pair, CTA + , (I-2H)2 −  exclusively in the aqueous pseudo phase.

The oxidation of glyoxylic acid (HGl) by MnIVL {L4−=tetra deprotonated 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane} was investigated in the pH range 1.67–10.18, at 25–45°C and 0.5M ionic strength. The reaction exhibited biphasic kinetics with MnIIIL− as the reactive intermediate. MnIV was reduced to MnII. The products of oxidation of HGl were identified as formic acid and CO2 in acidic medium, and oxalate in basic medium, consistent with the stoichiometry: −Δ[MnIV]/−Δ[HGl]=1. In acidic medium, both MnIVL and MnIIIL− formed outer-sphere adducts with the neutral HGl {HC(OH)2COOH} molecule, with an association constant Qav of 28 and 70M−1, respectively. A similar adduct formation was not observed for the glyoxylate mono anion {Gl−, CH(OH)2(CO2−)} and glyoxylate dianion {Gl2−, CH(OH)(O−)CO2−}. The rate and activation parameters for the various paths are reported and an outer-sphere electron transfer mechanism is suggested.

A new Cr III –Schiff base complex of N,N ′-ethylene-bis-salicylamide (Salm), supported on acidic alumina, was characterized by chemical and spectral (UV–Vis-DRS and FT-IR) analyses. The comparative catalytic activity of the neat and supported metal complex was evaluated using H 2 O 2 decomposition as the model reaction at varying H 2 O 2 concentration and amount of catalyst which showed a higher activity for the supported metal complex. A preliminary study showed that the supported Cr III (Salm) was also active for the oxidation of phenol using H 2 O 2 as oxidant leading to catechol (selectivity ~62%) and hydroquinone (selectivity ~32%) as the major products.

The substitution reactions of [Cr(SALM)(OH2)(2)](+) (SALM =N,N'-ethylene-bis-salicylamide) and Cr(H2O)(6)(3+) with ranitidine hydrochloride, an antacid have been studied spectrophotometrically over the range 0.5 x 10(4) <= [Cr(III)complex](T) <= 3 x 10(-4), 3.6 <= pH <= 4.2, 25 degrees C <= t <= 45 degrees C and I = 0.5 mol dm(-3) [Cr(III)complex] = ([Cr (III)(SALM)(OH2)(2)](+) or [Cr(H2O)(6)](3+)]). This reaction follows a mechanism which takes place via an outer sphere association between [Cr (III)(SALM)(OH2)(2)](+), [Cr(H2O)(6)](3+) and ranitidine hydrochloride followed by transformation of outer into inner sphere complex by slow interchange. The anation rate constant at 25 degrees C and I = 0.5 mol dm(-3) (NaClO4) is found to be 4.43 x 10(-4) s(-1) for [Cr(III)(SALM) (OH2)(2)](+) and 3.05x10(-4) s(-1) for Cr(H2O)(6)(3+). The Delta H-# for k(an) path are found to be 48.2 +/- 0.6, 41.2 +/- 0.2 kJmol(-1) and Delta S-# -147.6 +/- 2.0, -174.2 +/- 0.6 JK(-1) mol(-1) for Cr(III)SALM(OH2)(2)(+) and Cr(H2O)(6)(3+) ion respectively.

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.

Article Kinetic and Mechanistic Study of the Interaction of L-cysteine and N-acetylcysteine with cis-[Pt(en)(H2O)2](ClO4)2 in Aqueous Medium was published on December 1, 2008 in the journal BioInorganic Reaction Mechanisms (volume 6, issue 4).

The kinetics of the reactions of L-cysteine and N-acetylcysteine with cis[Pt(en)(OH2)(2)](2+) have been studied spectrophotometrically as a function of [Pt(en) (OH2)(2)](2+)(T) [L-cysteine](T)/[N-acetylcysteine](T) and temperature at pH (4.0) where the Pt(II) complex exists predominantly as a diaqua species and L-cysteine and N-acetylcysteine as zwitterions. The substitution reaction shows two consecutive processes; the first step is the ligand assisted anation and the second one is the chelation step. The anation rate constants for N-acetylcysteine were found to be greater than that for L-cysteine. Delta H-1(#)(61.6 +/- 1.8 kJ mol(-1)) Delta H-2(#) (47.5 +/- 2.0 kJ mol(-1)), Delta S-1(#) (-54.4 +/- 5.3 J.K-1 mol(-1)) and Delta S-2(#) (-164.8 +/- 6.5 J.K-1 mol(-1)) for 1 2 L-cysteine and Delta H-1(#) (23.2 +/- 0.8 kJ mol(-1)), Delta H-2(#) (61.5 +/- 0.6 kJ mol(-1)) and Delta S-1(#) 1 2 1 (-178.6 +/- 2.6 J.K-1 mol(-1)) Delta S-2(#) (-115.6 +/- 2.0 J.K-1 mol(-1)) for N-acetylcysteine indicate an associative mode of activation for both the ligand Substitution processes in the two consecutive steps.

The octahedral complex, [CoIII(HL)]·9H2O (H4L = (1,8)-bis(2-hydroxybenzamido)-3,6-diazaoctane) incorporating bis carboxamido-N-, bis sec-NH, phenolate, and phenol coordination has been synthesized and characterized by analytical, NMR (1H, 13C), e.s.i.-Mass, UV–vis, i.r., and Raman spectroscopy. The formation of the complex has also been confirmed by its single crystal X-ray structure. The cyclic voltammetry of the sample in DMF ([TEAP] = 0.1 mol dm−3, TEAP = tetraethylammonium perchlorate) displayed irreversible redox processes, [CoIII(HL)] → [CoIV(HL)]+ and [CoIII(HL)] → [CoII(HL)]− at 0.41 and −1.09 V (versus SCE), respectively. A slow and H+ mediated isomerisation was observed for the protonated complex, [CoIII(H2L)]+ (pK = 3.5, 25 °C, I = 0.5 mol dm−3). H2Asc was an efficient reductant for the complex and the reaction involved outer sphere mechanism; the propensity of different species for intra molecular reduction followed the sequence: [{[CoIII(HL)],(H2Asc)}–H]− <<< {[CoIII(H2L)],(H2Asc)}+ < {[CoIII(HL)],(H2Asc)}. A low value (ca. 3.7 × 10−10 dm3 mol−1 s−1, 25 °C, I = 0.5 mol dm−3) for the self exchange rate constant of the couple [CoIII(HL)]/[CoII(HL)]− indicated that the ligand HL3− with amido (N-) donor offers substantial stability to the CoIII state. HSO 3 − and [CoIII(HL)] formed an outer sphere complex {[CoIII(HL)],(HSO 3 − )}, which was slowly transformed to an inner sphere S-bonded sulfito complex, [CoIII(H2L)(HSO3)] and the latter was inert to reduction by external sulfite but underwent intramolecular SIV → CoIII electron transfer very slowly.

The kinetics of reversible complexation between gallium(III) and title cobalt(III) complex, trans-bis[(en)(2) Co(malH)(2)](+), have been investigated using stopped flow technique at 25 degrees C, 0.1 <= [H+], <= 0.3, 0.001 <= [Ga3+], <= 0.005 and I = 1.0 mol dm(-3). A general mechanism is proposed which accounts for the available data on the formation of binuclear species involving mainly the reaction or Ga3+ with dissociated form or the Co-III complex. The pseudo-first order rate constants increased linearly with the increase of [Ga3+](T) at a fixed acidity indicating the formation of 1 : 1 binuclear complex formed between Ga-III and conjugate base of Co-III complex. Comparing the derived rate parameters with water exchange rate constant of Ga(OH)(2))(6)(3+) and with several analogous systems, it is concluded that the mechanism of substitution reaction at Ga3+ centre is more likely to be I-a. The rate of spontaneous dissociation of the binuclear complex has also been reported.

The MnIV complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane (MnIVL) with phenolate-amido-amine coordination is reduced by l-ascorbic acid and oxalic acid obeying overall 1:1 stoichiometry. The reactions are biphasic and MnIIIL− is the reactive intermediate. The product of oxidation of ascorbic acid (H2Asc) is dehydroascorbic acid and that of oxalic acid (H2OX) is CO2, while MnII is the end product from MnIV. Both MnIVL and MnIIIL− form outer sphere adducts with H2Asc and H2OX with high values of equilibrium constants of formation (Q>102 dm3 mol−1, I = 0.5 mol dm−3, 25.8 °C, 1.5% v/v MeOH+H2O). The adduct formation is diffusion controlled and is attributed to hydrogen bonding interactions between the reactants. The rate constants for the electron transfer in (MnIV/IIIL, H2A), (MnIV/IIIL, HA−) (H2A = H2Asc, H2OX) and for (MnIVL, H2Asc)+H2Asc, (MnIIIL−, HAsc−)+HAsc− are reported. There was no evidence of direct coordination of the reductants to the MnIV/III center indicating an outer sphere (ET) mechanism.