Achyuta N. Acharya

Effect of solvent on the reaction of coordination complexes. Part 19.—Base hydrolysis of (αβS)-(o-methoxy benzoato)(tetraethylenepentamine)-cobalt(III) in aquo-organic solvent media

The kinetics of base hydrolysis of (αβS)-(o-methoxy benzoato)(tetren)cobalt(III) have been investigated in aquo-organic solvent media [0–70%(v/v) cosolvents] at 10 ⩽t/°C ⩽ 40 (l= 0. 01 mol dm–3) using methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, tert-butyl alcohaol, ethylene glycol, 2-methoxyethanol, acetone, acetonitrile, 1, 4-dioxane and dimethyl sulfoxide as cosolvents. The second-order base hydrolysis rate constant increased non-linearly with increasing mole fraction (Xorg) of all cosolvents, except for the ethylene glycol–water system; ethylene glycol had a rate-retarding effect. The transfer Gibbs energy of the transition state (TS) relative to that of the initial state (IS), for transfer of species from water to mixed solvent varied non-linearly with D–1s and Xorg, reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and the initial state were governed by the preferential solvation effect. The solvent stabilisation of the initial state and the transition state has been assessed for the methanol–water and ethanol–water systems by combining the solubility data of the dithionate salt of the complex with the transfer Gibbs energy data for S2O62–. The thermodynamic parameters (ΔH‡ and ΔS‡) were sensitive to the structural changes in the bulk solvent phase.

Kinetics and mechanism of reaction of trans‐(diaqua)(N,N′‐ethylene‐bis‐salicylidenimine) chromium(iii) with sulfur(iv): The labilizing effect of coordinated sulfite

The reaction of trans-[Cr(Salen)(OH2)2]+ with aqueous sulfite yields trans-[Cr(Salen)(OH2)(OSO2(SINGLEBOND)O)]− (O-bonded isomer). The rate and activation parameter data for the formation of the sulfito complex are consistent with a mechanism involving rate-limiting addition of SO2 to the CrIII(SINGLEBOND)OH bond. The complex ions, trans-[(OH2)Cr(Salen)(OSO2(SINGLEBOND)O)]−, and trans-[(OH)Cr(Salen)(OSO2(SINGLEBOND)O)]2−, undergo reversible anation by NCS−, N3−, imidazole, and pyridine resulting in the formation of trans-[XCr(Salen)(OSO2(SINGLEBOND)O)](N+1)−(n=1 for X=N3−,NCS−, and 0 for X=imidazole and pyridine) predominantly via dissociative interchange mechanism. The labilizing action of the coordinated sulfite on the trans-CrIII-X bond in trans-[XCr(Salen)(OSO2)](n+1)− follows the sequence: NCS−pyridine ca. N3− ca. imidazole. Data analysis indicated that the coordinated sulfite has little trans activating influence.

Ligand control on the reactions of oxygen- and sulphur-bonded (sulphito)pentaminecobalt(III) complexes in solution

The kinetics of formation, acid-catalysed aquation, ligand isomerisation (CoIIIOSO2+→CoIIISO3+), intramolecular electron-transfer, and base-catalysed hydrolysis and isomerisation of O-bonded sulphito complexes, cis-[Co(en)2(B)(OSO2–O)]+[B=benzimidazole (bzimH), N-methylimidazole (N-meim)] and the anation of cis-[Co(en)2(B)OH)]2+ [B=bzimH, N-meim and imH (imidazole)] by oxSO2−3 are reported. Steric acceleration is observed in the formation and acid-catalysed aquation of the O-sulphito complexes. The ligand isomerisation leads to loss of the monodentate amine with the formation of trans-[Co(en)2(SO3–S)2]− (in an excess of sulphite). Steric acceleration is more pronounced in the isomerisation and base hydrolysis than in the redox process. The results indicate cis labilisation of the coordinated O-sulphite.The [(tetraethylenepentamine)Co(OSO2–O)]+ cation undergoes base hydrolysis 103 times faster than the corresponding (en)2(B) complexes; base-catalysed ligand isomerisation for the former is not observed unlike in the latter. The anation of cis-[Co(en)2(B)OH]2+ (B=imH, bzimH, N-meim) by SO2−3 in a mild alkali pH range (pH=7.9–9.6) and in an excess of SO2−3, yields exclusively trans-[Co(en)2(SO3–S)2]− with no evidence for the formation of the cis-[Co(en)2(B)(SO3–S)]+ or its O-sulphito analogue. The intramolecularly generated amido conjugate base of the sulphite ion-pair, {cis-[Co(en)2(B)OH]2+,SO32−}.41cm{cis-[Co(en)(en-H) (B)- OH2]2+,SO32−}, is believed to generate a five-coordinate intermediate (TBP) that captures the S-end of SO2−3 selectively from a site trans-to the amine B so that the amine is labilised by the trans effect of the sulphite. The NH-deprotonated coordinated imidazolate or benzimidazolate species, cis-[Co(en)2(bzm/im)OH]+, do not undergo anation by SO2−3.

Effects of solvent on the reactions of coordination complexes. Part 24. Kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylene-pentamine)cobalt(III) complexes in methanol+water media: The role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of some (aminomonocarboxylato)(tetraethylenepentamine)cobalt(III) complexes, [(tetren)CoO2CR]2+ (R=NH2CH2, pyridine-2, NH2CH2CH2, NH2CH(CH3) (αβS isomer); R=NH2CH(CH3) (αβR isomer)), have been investigated in methanol–water media (0–80 vol % MeOH) at 15.0≤t°C≤40.0 (0.02 mol dm−3 NaOH). The second-order rate constant at zero ionic strength, k2°, increases nonlinearly with XMeOH. The transfer free energy of the initial state and the transition state of the amido conjugate base ([ΔtG (i)](sw)) for the glycinato- and pyridine-2–carboxylato complexes have been calculated using the solubility data of their picrate salts, pKNH date of their N-protonated forms, and the k2° values in mixed solvent media. The kinetic solvent effects have been interpreted in terms of preferential solvation of the initial state, transition state, and the solvent structure. The activation enthalpies and entropies varied nonlinearly with XMeOH displaying extrema, which is attributable to the solvent structural effects on these thermodynamic parameters. It is also evident that the mutation process, αβRαβS isomer for the α-alaninato complex, where this isomerisation refers to the arrangement of the tetren skeleton around the planar secondary NH is sensitive to the nature of the cosolvent molecules and solvent structure. The mutation process is generally more favorable for the five coordinate amido conjugate bases than the initial state.

STERIC EFFECTS ON COMPLEX FORMATION BETWEEN NICKEL(II) AND (2-IMIDAZOLEAZO)BENZENE, 2,2'-BIIMIDAZOLE AND 2,2'-BIBENZIMIDAZOLE

SummaryThe kinetics and mechanism of reversible complexation of NiII with (2-imidazoleazo)benzene (IAB), 2,2′-biimidazole (Biim) and 2,2′-bibenzimidazole (Bibzm) have been investigated at 15–35 °C, I = 0.30 mol dm−3. The stability constants, KM, of the [NiL]2+ species vary in the sequence: [Ni(IAB)]2+ < [Ni(Bibzm)]2+ < [Ni(Biim)]2+. The values of the spontaneous dissociation rate constant (kr) at 25 °C decrease in the sequence: [Ni(IAB)]+ > [Ni(Biim)]2+ > [Ni(Bibzm)]2+. The aquation of [Ni(IAB)]2+ is insensitive to acid catalysis, whilst [Ni(Biim)]2+ is relatively more susceptible towards acid-catalysed aquation than [Ni(Bibzm)]2+. The chelate ring in [NiL]2+ (L = IAB, Biim or Bibzm) is sterically strained. The formation of [Ni(IAB)]2+ and [Ni(Bibzm)]2+ may be chelation controlled while the normal Id mechanism is supported by our data for [Ni(Biim)]2+.

Effect of solvent on the reactions of coordination complexes. Part 21.—Base hydrolysis of some cis-(halogeno)(amine)-bis(ethylenediamine)cobalt(III) complexes in methanol–water media: the role of substrate hydrophobicity and solvent structure

The kinetics of base hydrolysis of a series of (halogeno)(amine)cobalt(III) complexes, cis-[(en)2Co(B)X]2+(B = NH3, CH3NH2, CH3CH2NH2, C6H5CH2NH2, C6H11NH2, imidazole, N-methylimidazole, benzimidazole, CH3CH(OH)CH2NH2, X = Cl, Br) have been investigated in methanol–water media (0–90 vol.% MeOH) at 15.0 ⩽t/°C ⩽ 40.0 (0.02 mol dm–3 NaOH). The relative second-order rate constants at ionic strength, I= 0, [(kOH2)0 s/k0 w2], generally increased non-linearly with increasing mole fraction (XMeOH) of MeOH; the effect was sensitive to the bulkiness and hydrophobicity of the non-labile amine ligands. The solvent effects on rate have been interpreted in terms of the role of solvent structure and preferential solvation of the initial state and transition state, presumably due to the hydrophobic interaction. This is evident from the linear plot of log(kOH2)0 sversustext-decoration:overlineV°1/3 at 25 °C where text-decoration:overlineV° is the standard partial molar volume of the complex ion in aqueous medium at 25 °C. The activation enthalpies and entropies also varied non-linearly with XMeOH and the observed extrema in the plots of ΔH#(ΔS#)vs. XMeOH presumably reflected the solvent structural effect on the solvation component of these thermodynamic parameters.

Kinetics and mechanism of the interaction of pentaammine(pyridine-2-carboxylato)cobalt(III) ions with hexaaquanickel(II) and nickel(II) complexes: the role of chelating ligands

The reversible complexation of the pentaammine(pyridine-2-carboxylato)cobalt(III) ion [N5Co{O2C-(2)-C5H4 N}]2+ [N5=5HN3 and tetraethylenepentaammine (tetren)] with NiIIL(OH2)6-n [L=H2O (N5=tetren); L=bipy, ida2- (iminodiacetate) and nta3- (nitrilotriacetate), N5=5NH3 and tetren] has been investigated by the stopped-flow technique at 20-40 degC, and I= 0.3mol dm-3. At 25degC, the rate constants, kf(dm3 mol-1s-1), DeltaH(kJmol-1) and DeltaS(JK-1mol-1) for the formation of the ternary complexes [(tetren)-CoIII{O2C-(2)-C5H4N} NiIIL(OH2)6-n] are as follows: L=H2O, 530+9, 53+2, -15+7, respectively; L=bipy, 640+30, 37+3, -65+9; L=ida2-, 3900+100, 47+3, -18+11; L=nta3-, 10200+400, 49+1, −2+2. Nickel(II), in the ternary complexes, is chelated by the free pyridyl-N and the carboxylato moiety of the pyridine-2-carboxylate bound to the cobalt centre. The formation rate constant (kf) and the associated activation parameters are relatively insensitive to the N5 moieties for a given ligand L; kf increased in the order: Ni(OH2)62+Ni(bipy)(OH2)42+ Ni(ida)(OH2)3 (nta)(OH2)2-. Data analysis indicated that the mechanism shifted from the dissociative interchange (Id) to the chelation-controlled one, with the decrease of the available sites for coordination in NiIIL(OH2)6−n. The rate constants (kr) for the dissociation of [N5CoIII{O2C-(2)-C5 H4N}NiIIL(OH2) 6-(n+2)] to the parent reactants indicated steric acceleration [krL(5NH3) kr Ni(ida) >krNi(bipy)2+ for both pentaammine substrates. The chelate ring opening rate constants for the ternary complexes were estimated, from which it was apparent that the tetren envelope of cobalt(III) exerted relatively greater steric pressure as compared with 5NH3 in favouring opening up of the chelate ring.

Kinetics and mechanistic study of the reduction of \(\hbox {Mn}^{\mathrm{III}}\) by oxalate in Salophen scaffold: relevance to oxalate oxidase

The trans-MnIII\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}

Kinetics and mechanism of complex formation between beryllium(II) and the 3-nitrosalicylatopentaamminecobalt(III) ion

\hbox {Mn}^{{\mathrm{III}}}