Proximity effect on the general base catalysed hydrolysis of amide linkage: the role of cationic surfactant, CTABr

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 ≤ ΔSΔ (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, kH2O/kD2O+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 premicellar 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

Effect of solvent on the reactions of coordination complexes. Part 2. Kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water and ethylene glycol–water media

The kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated in aqueous methanol (MeOH) and aqueous ethylene glycol (EG) media (0-80% by weight of MeOH or EG) at 45-64.7 °C. The logarithm of the pseudo-first-order rate constants for MeOH-water media exhibits linear dependence with the reciprocal of the bulk dielectric constant (D-1s), the mole fraction of MeOH (XMeOH) and the solvent ionizing power Y(Y1-AdCl) as determined by the solvolysis rates of 1-adamantyl chloride. Similar plots (log ksobsvs.xEG or D-1s) for EG-water media are non-linear. It is evident that the solvation phenomenon plays dominant role and the rate of solvolysis is mediated by the dual solvent vectors, the overall acidity and basicity of the solvent mixtures. The relative transfer free-energy calculations indicate that the mixed solvent media exert more destabilizing effect on the transition state as compared to the initial state. The activation enthalpy and entropy vs.Xorg(where Xorg is the mole fraction of the organic solvent component) plots display maxima and minima indicating that the solvent structural changes play significant role in the activation process. The activation free energy at a given temperature, however, increases only marginally and linearly with increasing Xorg. The mutual compensatory effect of activation enthalpy and entropy on the activation free energy is in keeping with the fact that the perturbations of the reaction zone and the solvent network remain approximately proportional to each other with increasing Xorg so that the isodelphic and the lyodelphic components of ΔH± and ΔS± correlate well with each other

Base-catalysed hydrolysis of cis-(imidazolato)-, cis-(benzimidazolato)- and cis-(N-methylimidazole)-bis-(ethylenediamine)halogenocobalt(III) cations. A comparison of the reactivities of the deprotonated imidazole and benzimidazole complexes

The base hydrolysis of the complexes cis-[Co(en)2B(X)]n+ [en = ethylenediamine; B = imidazole (Him), N-methylimidazole (mim), or benzimidazole (Hbzim); X = Cl or Br] has been investigated at 20-35°C and [OH-]τ=(4.7-99.7)×10-3 mol dm-3(I= 0.10 mol dm-3) under which conditions the co-ordinated imidazole and benzimidazole undergo complete NH deprotonation. The activation parameters (85≤ΔH‡/kJ mol-1 ≤ 97, +84 ≤ ΔS‡/J K-1 mol-1 ≤ +97), and the rate dependence on the leaving groups and the non-labile amine ligands are consistent with a SN1 CB mechanism. The electron-displacement properties of the N-co-ordinated imidazolate (im) and benzimidazolate (bzim) ions appear to enhance the pKNH of the co-ordinated ethylenediamine, the effect being relatively more significant for the former anion. Analysis of the activation entropy data in terms of the dissociative activation model for the conjugate bases, cis-[Co(en)(en - H)B(X)]n+[B = im or bzim (n= 0); mim (n= 1)] indicates that the configurational rearrangement at the cobalt(III) centre most likely occurs in the transition state of the actual acta of the substitution for the less reactive chloro complexes. The presumed trigonal-bipyramidal intermediate is efficiently scavenged by azide

Kinetics and mechanism of hydrolysis of cis-chlorobis(ethylenediamine)-(imidazole)cobalt(III) and cis-bromobis(ethylenediamine)(imidazole)cobalt(III) cations

The kinetics of hydrolysis of cis-[CoX(imH)(en)2]2+(imH = imidazole; en = ethylenediamine; X = Cl or Br) cations have been investigated in perchlorate medium of I= 0.3 mol dm-3. In the range pH 0.5-5.7 the rate law for aquation takes the form -dln[CoIII]/dt=k1+k2KNH[H+]-1 where k1 and k2 are the aquation rate constants of [CoX(en)2(imH)]2+ and [CoX(im)(en)2]+ respectively and KNH is the acid dissociation constant of the co-ordinated imidazole. At 50 ° C the values of k1,k2KNH, ΔH‡, and ΔS‡ for the k1 path are (1.21 ± 0.02)× 10-5 s-1, (4.95 ± 0.11)× 10-11 mol dm-3 s-1, 92.3 ± 1.2 kJ mol-1, -54 ± 3 J K-1 mol-1 for the chloro-, and (5.52 ± 0.10)× 10-5 s-1, (33.4 ± 0.7)× 10-11 mol dm-3 s-1, 94.5 ± 0.3 kJ mol-1, and -34 ± 1 J K-1 mol-1 for the bromo-complex respectively. Values of k2 obtained from the base-hydrolysis studies are (1.28 ± 0.17)× 10-2 and (2.46 ± 0.22)× 10-2 s-1 at 31.8 °C for the chloro- and bromo-complexes respectively, and the imido-complex [CoCl(im)(en)2]+ also undergoes second-order base hydrolysis with a rate constant of 5.1 ± 1.0 dm3 mol-1 s-1 at the same temperature. The labilizing action of imidazole and its conjugate base on the Co-X bond appears to be comparable to that of pyridine and hydroxide respectively. Co-ordinated imidazole is 105 times stronger as an acid than free imidazole. The sulphate- and mercury(II)-catalysed aquations of both the substrates have also been studied. The value of kip/k1, where kip and k1 are the rate constants of aquation of [CoX(imH)(en)2]2+, [SO4]2-and [CoX(imH)(en)2]2+ species respectively, is 2.3 ± 0.2 at 60 °C for X = Cl and 4.6 ± 0.1 at 50 °C for X = Br. The mercury(II)-catalysed aquation follows second-order kinetics, -dln[CoIII]/dt=kHg[Hg2+] : at 30.5 °C the rate constant (kHg]) and activation enthalpy and entropy are (3.48 ± 0.03)× 10-2 dm3 mol-1 s-2, 68.4 ± 0.7 kJ mol-1. and -48 ± 2 J K-1 mol-1for the chtoro- and 12.4 ±0.5 dm3 mol-1s-1, 53.4 ± 0.4 kJ mol-1, and -48 ± J K-1 mol-1 for the bromo-complex respectively

Decarboxylation of Hydrogencarbonatopentamminecobalt(III) in Aquo-organic solvent media

The decarboxylation of hydrogencarbonatopentaamminecobalt(III) has been investigated in aqueous, 99% D2O and aquo-organic solvent media (0-70 wt.% of cosolvent) at 15 ≤ t/°C ≤ 40 (I = 0.02 mol dm-3), using methanol, propan-2-01, tert-butyl alcohol, ethylene glycol, acetone, acetonitrile, DMSO and ethylene carbonate as cosolvents. The solvent isotope effects on rate (kH2O/kD2O=O 1.0 at 1535°C) and activation parameters (ΔH≠ = 77.7 ± 1.0, 77.8 ± 0.9 kJ mol-1 and ΔS≠ = 16 ± 3, 16 ± 3 J K-1 mol-1 for aqueous and 99% D,O media, respectively) were negligible. The decarboxylation rate constant increased with increasing mole fraction (Xorg) of the cosolvent and the effect was pronounced at relatively high values of Xorg for the dipolar aprotic cosolvents. This was attributed to a greater degree of destabilisation of the initial state as compared to the transition state with increasing mole fraction of the cosolvent. The In ks vs. 1/εs plots (ks is the rate constant and εs, the bulk relative permittivity) showed marked dependence on the nature of the cosolvents; the gradients of such plots generally increased with increasing dipole moment of the cosolvent molecules, indicating thereby that the solvation of the initial state and the transition state of the substrate is governed by the ion-dipole interactions between the water and cosolvent molecules. The relative transfer free energy of activation, [ΔΔGt≠](s←w), decreased linearly with Xorg for all mixed-solvent media, indicating that the preferential solvation effect is not significant. The activation enthalpy and entropy vs. Xorg plots displayed extrema suggesting that these thermodynamic parameters are sensitive to the structural changes in the bulk solvent phase. The solvent effects on ΔH≠ and ΔS≠ are mutually compensatory

Kinetics and mechanism of complex formation of some bivalent and trivalent metal ions with pentaammine-(nitrilotriacetato)cobalt(III) in aqueous medium

The kinetics of reversible complex formation of NiII, CoII and CuII with the pentaammine-(nitrilotriacetato)cobalt(III) ion, [Co(NH3)5(H2nta)]2+(H3nta = nitrilotriacetic acid) have been investigated at 0.0025 ≤[M2+] ≤ 0.04, 0.004 ≤ [H+] ≤ 0.05 mol dm−3, 10.0 ≤ T ≤ 40.0 °C and I= 0.3 mol dm−3. The rate constants for the formation of the binuclear species are at least 103 times less than the water exchange rate constants of [M(OH2)6]2+ under comparable conditions. General base catalysis indicated that proton transfer from the NH+ site of the co-ordinated ligand (nta) is involved in the rate determining step. The binuclear species undergo dissociation via spontaneous and acid-catalysed paths. The low values of spontaneous dissociation rate constants also support the chelate nature of the binuclear species. It is likely that the nta moiety of (NH3)5Co(nta) acts at least as a tridentate ligand and the chelate ring closure/opening via N-MII bond formation/dissociation is rate limiting. Complex formation with FeIII and AlIII has been investigated at 15–35 °C (I= 1.0 mol dm−3) and 25 °C (I= 0.3 mol dm-3), respectively. General base catalysis was not observed for these trivalent metal ions. The [M(OH2)5(OH)]2+ species reacted faster than [M(OH2)6]3+. The reaction of [M(OH2)6]3+ may involve an associative interchange mechanism while that for [M(OH2)5(OH)]2+ involves dissociative interchange

Binary and ternary complexes of nickel(II) with 2-aminomethylbenzimidazole and salicylaldehyde: kinetic and equilibrium studies

The complexation of NiII with 2-aminomethylbenzimidazole (L) has been investigated at 20–40 °C, I= 0.30 mol dm-3. Both monoprotonated and unprotonated ligands bind the metal ion to form [NiL]2+. and the rate and activation parameters for the formation and acid-catalysed dissociation of this chelate are calculated. In the presence of Salicylaldehyde (Hsal), a mixed-ligand complex. [NiL(sal)]+, is also formed as an intermediate which further condenses to the Schiff-base complex, [NiL']+[HL'=N-(benzimidazol-2-ylmethyl)salicylideneimine]. The kinetics of the fast ternary complex formation and its slow intramolecular transformation to the Schifi base complex have been investigated at 25 °C. The presence of L in the co-ordination sphere of Ni2+ enhances the dissociation of [NiL(sal)]+ to [NiL]2+ and sal– with respect to [Ni(Sal)]+, as evidenced by the stability constants of [NiL(sal)]+ and [Ni(sal)]+. Calculations based on the values of ΔS° for the ionisation of H2L2+ and the formation of [NiL]2+combined with S [> with combining macron]aq°(H+) and S[> with combining macron]aq°(Ni2+) data yielded the values S[> with combining macron]aq°(H2L2+)–S[> with combining macron]aq°(L)= 110, S[> with combining macron]aq°(L)–S[> with combining macron]aq°(HL+)=–116 and S[> with combining macron]aq°([NiL]2+)–S[> with combining macron]aq°(L)=–294 J K−1 mol−1, which presumably reflect the varying solvent-ordering effects of L, HL+, H2L2+ and [NiL]2+

Complex formation between nickel(II) and some pentamine (substituted salicylato)cobalt(III) ions

The kinetics of reversible complexation of NiII with pentamine(substituted salicylato)cobalt(III) ions, [Co(N5){O2CC6H3(X)OH}]2+[N5= 5NH3, (en)2(NH3)(cis isomer, en = ethane-1,2-diamine) or tetren (tetraethylenepentamine), X = 3-NO2; N5= 5NH3, X = 5-NO2], was investigated by the stopped-flow technique at 15–35 °C, pH 5.70–6.90 and I= 0.30 mol dm−3(ClO4−). The formation of [(Co(N5){O2CC6H3(X)O}Ni]3+ occurs via the reaction of [Ni(OH2)6]2+ with the phenoxide form of the cobalt(III) substrates. The rate and activation parameters have been determined for the formation and dissociation of the binuclear species in which nickel(II) is chelated by the salicylate moiety. The data are consistent with and Id mechanism. The rate constant for spontaneous dissociation of the binuclear species to the reacting partners is sensitive to the nature of the pentamine moiety and decreases in the sequence tetren > (en)2(NH3)− 5NH3. The acid-catalysed dissociation of cis-[(en)2(NH3)Co{O2CC6H3(NO2−3)O}Ni]3+ conforms to a two-step process

Glyoxylate as a reducing agent for manganese(III) in salen scaffold: a kinetics and mechanistic study

The kinetics of oxidation of glyoxylic acid (HGl) by MnIII(salen)(OH2)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)(OH2)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)+ (105ket = 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)(OH2)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 MnIII 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 (kHG ~ 5 kGl). 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

Micellar effects upon the reactions of complex ions in solution. Part 4. Kinetics of aquation and base hydrolysis of some cis-(chloro)(amine)bis(ethylenediamine)cobalt(III) complexes in the presence of neutral and anionic surfactants in an aqueous medium

The binding of the substrates cis-[Co(en)2BCl]2+(en = 1,2-diaminoethane, B = alkylamines, imidazole, N-methylimidazole) to the micellar surface of sodium dodecyl sulphate resulted in the retardation of their dissociative aquation rates, the effect being sensitive to the hydrophobicity of the nonlabile amine ligand B. A contrastingly small rate acceleration for the corresponding ethanolamine and propan-2-ol amine complexes was observed under similar conditions. Triton X-100 (0.02 ≤ [Triton X]T/mol dm-3 ≤ 0.1) had virtually no effect on the aquation rates of such complexes except for cis-[Co(en)2(C6H11NH2)Cl]2+, in which case a small rate retardation was also observed. The rates of base hydrolysis of the cobalt(III) substrates were strongly retarded by the anionic micelles of SDS; the neutral micelles of Triton X-100 were effective in decelerating the rate of base hydrolysis of the cyclohexylamine complex cis-[Co(en)2(C6H11NH2)Cl]2+ only. The pseudo-phase ion-exchange equilibrium model satisfactorily explained the binding of the cationic substrates to the anionic micellar pseudo-phase of SDS. The values of the ion-exchange equilibrium constant and the relative base hydrolysis rates (kW/kM) indicated that both micellar binding and retardation of hydrolysis are governed by hydrophobic and electrostatic interactions