Kinetic Screening for the Acid‐Catalyzed Hydrolysis of Some Hydrophobic Fe(II) Schiff Base Amino Acid Chelates and Reactivity Trends in the Presence of Alkali Halide and Surfactant

Kinetics of acid‐catalyzed hydrolysis of some high‐spin Fe(II) Schiff base amino acid complexes were followed spectrophotometrically at 298 K under pseudo–first‐order conditions. The studied ligands were derived from the condensation of 5‐bromosalicylaldehyde with different four amino acids (phenylalanine, aspartic acid, histidine, and arginine). The acid hydrolysis reaction was studied in aqueous media and in the presence of different concentrations of the alkali halide (KBr) and cationic surfactant (cetyl‐trimethyl ammonium bromide, CTAB). The general rate equation was suggested to be rate = k_obs[complex], where k_obs = k₂[H⁺]. The increase in [KBr] enhances the reactivity of the reaction, and the addition of CTAB to the reaction mixture accelerates the reaction reactivity. The obtained kinetic data were used to determine the values of δ_mΔG^# (the change in the activation barrier) for the studied complexes when transferred from “water to water containing different [KBr]” and from “water to water containing altered [CTAB].”     

Catalytic hydrolysis of p‐nitrophenyl picolinate by copper(II) and zinc(II) complexes of N‐(2‐deoxy‐β‐D‐glucopyranosyl‐2‐salicylaldimino)

D‐glucosamine Schiff base N‐(2‐deoxy‐β‐D‐glucopyranosyl‐2‐salicylaldimino) and its Cu(II) and Zn(II) complexes were synthesized and characterized. The hydrolysis of p‐nitrophenyl picolinate (PNPP) catalyzed by ligand and complexes was investigated kinetically by observing the rates of the release of p‐nitrophenol in the aqueous buffers at 25°C and different pHs. The scheme for reaction acting mode involving a ternary complex composed of ligand, metal ion, and substrate was established and the reaction mechanisms were discussed by metal–hydroxyl and Lewis acid mechanisms. The experimental results indicated that the complexes, especially the Cu(II) complex, efficiently catalyzed the hydrolysis of PNPP. The catalytic reactivity of the Zn(II) complex was much smaller than the Cu(II) complex. The rate constant k_N showing the catalytic reactivity of the Cu(II) complex was determined to be 0.299 s^?1 (at pH 8.02) in the buffer. The pK_a of hydroxyl group of the ternary complex was determined to be 7.86 for the Cu(II) complex. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 345–350, 2002     

Kinetics of the substitution reactions of some Pt(II) complexes with 5′‐GMP and L‐histidine

The reactions of platinum(II) complexes, [PtCl₂(dach)] (dach = (1R,2R)‐1,2‐diaminocyclohexane) and [PtCl₂(en)] (en = ethylenediamine) with biologically relevant ligands such as 5′‐GMP (guanosine‐5′‐monophosphate) and l ‐His (l ‐histidine) were studied by UV–vis spectrophotometry, ¹H NMR spectroscopy, and high‐performance liquid chromatography (HPLC). Spectrophotometrically, these reactions were investigated under pseudo‐first‐order conditions at 310 K in 25 mM Hepes buffer (pH 7.2) and 10 mM NaCl to prevent the hydrolysis of the complexes. The [PtCl₂(en)] complex reacts faster than [PtCl₂(dach)] in the reaction with studied nucleophiles. This confirms the fact that the reactivity of studied Pt(II) complexes depends on the structure of the inert bidentate ligand. Also, the substitution reactions with l ‐His are always faster than the reactions with nucleotide 5′‐GMP. The reactions of [PtCl₂(dach)] and [PtCl₂(en)] complexes with l ‐histidine are studied by ¹H NMR spectroscopy. The obtained rate constants are in agreement with those obtained by UV–vis. The same reactions were studied by HPLC comparing the obtained chromatograms during the reaction. The changes in intensity of signals of the free and coordinated ligand show that after a few days there is only one dominant product in the system. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 43: 99–106, 2011     

Kinetics and mechanism of promoted hydrolysis of 4‐nitrophenyl acetate by zinc(II)‐tripod: Different roles of mononuclear and trinuclear species

Coordination studies on Zn(II) complexes of 1,3,5‐tri(2,5‐diazahexyl)benzene (L) show that by comparison with the non‐deprotonation of complex ZnL in a 1:1 system, the three‐dimensional complexiaton decreases the pK_a of the Zn‐bound water molecule, that is, pK_a = 7.47 for trinulclear complex Zn₃L in a 3:1 metal–ligand ratio. These two types of zinc(II) complexes have been examined as catalysts for the hydrolysis of 4‐nitrophenyl acetate (NA) in 10% (v/v) CH₃CN at 298 K, I = 0.10 mol dm^?3 KNO₃ at pH range 6.5–8.2 and 8.5–10, respectively. Kinetic studies show that the second‐order rate constants of NA‐hydrolysis catalyzed by complexes ZnL, Zn₃L, and Zn₃LH_?1 are 0.021, 0.0082, and 0.342 mol^?1 dm³ s^?1, respectively. In all the cases, the pH‐dependent observed first‐order rate constant, k_obs, shows sigmoidal pH–rate profile. The 1:1 complex ZnL–H₂O undergoes NA hydrolysis by direct rate‐determining hydrolysis to produce 4‐nitrophenol(ate) (NP^?) and ZnL(OOCCH₃); while in the 3:1 system the oxygen atom of acetic group forms a H‐bond with the Zn(II)‐bound water of the second branch of tripod indicating that the polynuclear centers are associated and bi‐functional. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 41–48 2004     

Kinetic and Mechanistic Aspects of Reactivity of the Oxidation of Some Fe(II)–Tris Schiff Base Complexes by Peroxydisulfate: Spectrophotometric Tracer and Solvent Effect on the Reactivity

The kinetics of the oxidation of some Fe(II)–Tris Schiff base complexes by peroxydisulfate was studied spectrophotometrically in the aqueous medium and in the organic–aqua binary mixture. The inspected complexes were derived from the condensation of 2‐acetylpyridine and substituted benzylamines. The oxidation reaction of the studied complexes was followed at 303 K under pseudo–first‐order conditions. It was found that the oxidation reaction by S₂O₈^2? consists of two steps. The first step is the formation of an ion pair from the reactants, and the second step is an electron transfer from the metal center to the peroxydisulfate oxidant, with an associated peroxo bond fissure. A mechanism, based on the experimental results, was proposed, and the rate law was derived. The effect of organic solvent on the reaction rate was studied in the presence of different ratios (v/v) of methanol–water and acetone–water mixtures. Moreover, the changes in the activation barrier from water to water–methanol and water–acetone mixtures were estimated from the kinetic data. The transfer chemical potentials of the initial and transition states from water into mixed solvents were determined from solubility measurements. Solvent effects on the reaction rate were discussed in terms of initial state versus transition state solvation.     

SBA‐15‐supported N‐coordinate ruthenium(II) materials bearing sulfonamide‐type ligands: Effect of ligand backbones on catalytic transfer hydrogenation of ketones and aldehydes

[RuLCl(p ‐cymene)] (L = N ‐arylsulfonylphenylenediamine) complexes ( 2 _{a – d} ) were synthesized from the reaction between [Ru(p ‐cymene)Cl₂]₂ and ligand. Additionally, SBA‐15–[RuLCl(p ‐cymene)] derived catalysts ( 3 _{a – d} ) were successfully immobilized onto mesoporous silica (SBA‐15) by an easily accessible approach. The structural elucidations of 2 _{a – d} and 3 _{a – d} were carried out with various methods such as ¹H NMR, ¹³C NMR and infrared spectroscopies, elemental analysis, thermogravimetric/differential thermal analysis, nitrogen adsorption–desorption and scanning electron microscopy/energy‐dispersive X‐ray analysis. The Ru(II) complexes and materials were found to be highly active and selective catalysts for the transfer hydrogenation (TH) reaction of aldehydes and ketones. The influence of various 1,2‐phenylenediamines on the reactivity of the catalysts (complexes or materials) was studied and the catalysts ( 2 _d and 3 _d ) with a 4,5‐dichlorophenylenediamine substituent showed the best activity (the maximum turnover frequencies are 2916 and 2154 h⁻¹ for TH of 4‐fluoroacetophenone, and 6000 and 4956 h⁻¹ for TH of 4‐chlorobenzaldehyde).     

Effect of acetonitrile–water mixtures on the reaction of dinitrochlorobenzene and dinitrochlorobenzotrifluoride with hydroxide ion

The kinetics of alkaline hydrolysis of 2‐chloro‐3,5‐dinitrobenzotrifluoride 1 and 1‐chloro‐2,4‐dinitrobenzene 2 were studied in various acetonitrile–water (AN–H₂O) mixtures (10–90% w/w) at different temperatures. Thermodynamic parameters ΔH^# and ΔS^# show great variation, whereas ΔG^# appears to vary little with the solvent composition presumably due to compensating variations. The results are discussed in terms of the solvent parameters such as preferential solvation, dielectric constant, polarity/polarizability, and hydrogen bond donor and acceptor parameters. It has been found that the factors controlling the reaction rates are the desolvation of OH^?, the solvophobicity of the medium, and free water molecules in rich AN mixed solvent. The data showed that the solvatochromic parameters of (AN–H₂O) mixed solvent are destroyed in the presence of excess OH^?. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 453–463, 2010     

Reactivity Trends and Kinetic Inspection of Hydroxide Ion Attack and DNA Interaction on Some Pharmacologically Active Agents of Fe(II) Amino Acid Schiff Base Complexes at Different Temperatures

The reactivity of few novel high‐spin Fe(II) complexes of Schiff base ligands derived from 2‐hydroxynaphthaldehyde and some variety of amino acids with the OH^? ion has been examined in an aqueous mixture at the temperature range from 10 to 40°C. Based on the kinetic investigations, the rate law and a plausible mechanism were proposed and discussed. The general rate equation was suggested as follows: rate = k_obs[complex], where k_obs. = k₁ + k₂[OH^?]. Base‐catalyzed hydrolysis kinetic measurements imply pseudo–first‐order doubly stage rates due the presence of mer‐ and fac‐isomers. The observed rate constants k_obs are correlated with the effect of substituent R in the structure of the ligands. From the effect of temperature on the rate base hydrolysis reaction, various thermodynamic parameters were evaluated. The evaluated rate constants and activation parameters are in a good agreement with the stability constants of the investigated complexes. Moreover, the reactivity of the investigated complexes toward DNA was examined and found to be in a good agreement with the reported binding constants.     

PNPP Cleavage Catalyzed by Schiff Base Mn(III) Complexes Containing Polyether Side Chains in CTAB Micellar Solutions

Two Schiff base Mn(III) complexes containing polyether side chain were synthesized and characterized. The catalytic hydrolysis of p‐nitrophenyl picolinate (PNPP) by the two complexes in the buffered CTAB micellar solution in the pH range of 6.60–8.20 was investigated kinetically in this study. The influences of acidity, temperature, and structure of complex on the catalytic cleavage of PNPP were also studied. The mechanism of PNPP hydrolysis catalyzed by Schiff base manganese(III) complexes in CTAB micellar solution was proposed. The relative kinetic and thermodynamic parameters were determined. Comparied with the pseudo‐first‐order rate constant (k ₀) of PNPP spontaneous hydrolysis in water, the pseudo‐first‐order rate constants (k _obsd) of PNPP catalytic hydrolysis are 1.93×10³ fold for MnL¹ ₂Cl and 1.06×10³ fold for MnL² ₂Cl in CTAB micellar solution at pH=7.00, T=25°C, and [S]=2.0×10^?4mol · dm^?3, respectively. Furthermore, comparing the k _obsd of PNPP catalytic hydrolysis by metallomicelles with that of PNPP hydrolysis catalyzed only by metal complexes or CTAB micelle at the above‐mentioned condition, metallomicelles of MnL₂(L=L¹, L²) Cl/CTAB exhibit notable catalytic activities for promoting PNPP hydrolysis, and MnL¹ ₂Cl/CTAB system is superior in promoting cleavage of PNPP relative to MnL² ₂Cl/CTAB system under the same experimental conditions. The results indicate that the rate of PNPP catalytic cleavage is influenced by the structures of the two complexes, the acidity of reaction systems, and the solubilization of PNPP in CTAB micelles.     

Effects of solvent on the reactions of coordination complexes: Part 17. Kinetics and mechanism of base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt (III) in methanol + water and dimethylsulphoxide + water media

The base hydrolysis of (αβS) (salicylato) (tetraethylenepentamine)cobalt(III) has been investigated in MeOH + water and DMSO + water media (0–70% (v/v) cosolvents) at 20.0 ? t°C ? 35.0 and I = 0.10 mol dm^?3 (ClO₄^?). The phenoxide species [(tetren)CoO₂CC₆H₄O]⁺ undergoes both OH^?-independent and OH^?-catalyzed hydrolysis via SN₁ICB and SN₁CB mechanism, respectively. The OH^?-independent hydrolysis of the phenoxide species is catalyzed by both DMSO + water and MeOH + water media, the former exerting a much stronger rate accelerating effect than the latter. The OH^?-catalyzed reaction is strongly accelerated by DMSO + water medium but insensitive to the composition of MeOH + water medium up to 40% (v/v) MeOH beyond which it was not detectable under the experimental conditions. Data analysis has been attempted on the basis of the solvent stabilizing and destabilizing effects on the initial state and transition state of the concerned reactions. The nonlinear variation of the activation parameters, ΔH^≠ and ΔS^≠, with solvent compositions presumably indicates that the solvent structural effects mediate the energetics of solvation of the initial state and transition state of the concerned reactions. The linearity in ΔH^≠ vs. ΔS^≠ plot accomodating all data for k₁ and k₂ paths in DMSO + water and MeOH + water further suggests that the solvent effects on these parameters are mutually compensatory.     

Medium effects on the initial and the transition states upon the solvolysis of trans‐dichlorobis(N‐methyl‐ethylenediamine)cobalt(III) complex in water‐mixed solvents

The kinetics of solvolysis of trans‐dichlorobis(N‐methylethylenediamine)cobalt(III) complex have been investigated in aqua‐organic solvent media (0–60% (v/v) cosolvent) at 25 ≤ t°C ≤ 60, using n‐propanol and tert‐butyl alcohol as cosolvents. The first‐order rate constant increased nonlinearly with the reciprocal of the dielectric constant D_s^?1, and x_org, reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and initial state were governed by the preferential solvation effect. The thermodynamic parameters (ΔH^≠ and ΔS^≠) were sensitive to the structural changes in the bulk solvent phase. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 495–499, 2002     

The Substitution Reactions of the Small Biomolecules and Dinuclear Pt(II) Complexes with Alkanediamine Linker

The substitution reactions of the complexes [{trans‐Pt(NH₃)₂H₂O}₂(μ‐1,4‐diaminobutane)]⁴⁺ ( I ), [{trans‐Pt(NH₃)₂H₂O}₂(μ‐1,6‐diaminohexane)]⁴⁺ ( II ), and [{trans‐Pt(NH₃)₂H2O}₂(μ‐1,8‐diaminooctane)]⁴⁺ ( III ), with nucleophiles L‐cysteine (L‐Cys), glutathione (GSH), guanosine‐5′‐monophosphate (5′‐GMP), L‐histidine (L‐His), and pyridine were studied in 0.1 M NaClO₄ aqueous solutions at pH = 2.5. The substitutions were studied under pseudo‐first‐order conditions as a function of concentration and temperature using UV–vis spectrophotometry. At three different temperatures (288, 298, and 308 K) the reactions of the II and III complexes and 5′‐GMP were studied. The order of reactivity of study ligands is L‐Cys > GSH > 5′‐GMP > L‐His > pyridine and the order of reactivity of the complexes is I < II ≈ III . The obtained results indicate that the structure of the alkanediamine linker in the dinuclear Pt(II) complexes controls the substitution process. The negative values reported for entropy of activation confirmed the associative substitution mode. These results are discussed in order to find the connection between structure and reactivity of the dinuclear Pt(II) complexes.     

A study on the hydrolysis of bis(nitrophenyl)phosphate catalyzed by N‐methyldiethanolamine‐Ce(III) complex

The hydrolysis of bis(p‐nitrophenyl)phosphate (BNPP) catalyzed by N‐methyldiethanolamine‐Ce(III) complex in the presence and absence of cetyltrimethylammonium bromide (CTAB) and Brij35 surfactants at pH 7.20 and 303 K has been studied. The experimental results indicate that N‐methyldiethanolamine‐Ce(III) complex remarkably accelerates the hydrolysis of BNPP. The observed first‐order rate constant of the hydrolysis of BNPP catalyzed by N‐methyldiethanolamine‐Ce(III) complex at pH 7.20 and 303 K is 1.22 × 10^?2 s^?1, which is 1.09 × 10⁹ times of that of spontaneous hydrolysis of BNPP at pH 7. It is close to the activity of natural enzyme. A general quantitative treatment of the catalytic reaction involved a ternary complex as M_mL_lS has also been proposed in this paper. Applying this method to the catalytic hydrolysis of BNPP, we have obtained its thermodynamic and kinetic parameters. CTAB and Brij35 surfactant micelles obviously influence the rate constants of the catalytic hydrolysis of BNPP. Brij35 micelles promote the catalytic hydrolysis of BNPP, while CTAB micelles inhibit it. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 687–692, 2004     

Spectroscopic and Electrochemical Studies of the Interaction of Some Gold(III) Complexes with Biologically Relevant Thiones

The interaction of gold(III) complexes, [Au(cis‐DACH)Cl₂]Cl and [Au(cis‐DACH)₂]Cl₃ complexes (DACH = cis‐1,2‐diaminocyclohexane), with ¹³C, ¹⁵N‐enriched thiourea (Tu) and 1,3‐diazinane‐2‐thione ligands was investigated. The progress of these reactions was monitored by NMR (¹H, ¹³C, and ¹⁵N) and UV–vis spectroscopy as well as square wave stripping voltammetry. The kinetic studies of the substitution reactions between the above‐mentioned complexes with thiones in aqueous solutions containing 30 mM KCl, which is used to suppress the hydrolysis of the chloride complexes, were conducted. These reactions were followed under pseudo–first‐order conditions as functions of ligand concentration, pH, and temperature. The activation parameters (ΔH^#, ΔS^#) were calculated from Eyring plots, and the negative values of ΔS^≠ lend support for an associative mechanism. The kinetic data also indicated a relatively higher reactivity of [Au(cis‐DACH)Cl₂]Cl than that of [Au(cis‐DACH)₂]Cl₃ toward the thiones.     

Simple HPLC‐UV for the quantification of a new leishmanicidal candidate (E)‐1‐4(trifluoromethyl) benzylidene)‐5‐(2‐4‐dichlorozoyl) carbonylhydrazine (LASSBio‐1736) in rat plasma for pharmacokinetics assessment

In this study, a sensitive HPLC‐UV assay was developed and validated for the determination of LASSBio‐1736 in rat plasma with sodium diclofenac as internal standard (IS). Liquid–liquid extraction using acetonitrile was employed to extract LASSBio‐1736 and IS from 100 μL of plasma previously basified with NaOH 0.1 M. Chromatographic separation was carried on Waters Spherisorb^®S5 ODS2 C₁₈ column (150 × 4.6 mm, 5 μm) using an isocratic mobile phase composed by water with triethylamine 0.3% (pH 4), methanol and acetonitrile grade (45:15:40, v/v/v) at a flow rate of 1 mL/min. Both LASSBio‐1736 and IS were eluted at 4.2 and 5 min, respectively, with a total run time of 8 min only. The lower limit of quantification was 0.2 μg/mL and linearity between 0.2 and 4 μg/mL was obtained, with an R² > 0.99. The accuracy of the method was >90.5%. The relative standard deviations intra and interday were <6.19 and <7.83%, respectively. The method showed the sensitivity, linearity, precision, accuracy and selectivity required to quantify LASSBio‐1736 in preclinical pharmacokinetic studies according to the criteria established by the US Food and Drug Administration and European Medicines Agency. Copyright © 2016 John Wiley & Sons, Ltd.     

Kinetics of base hydrolysis of α‐amino acid esters catalyzed by the copper(II) complex of N,N,N′,N′‐tetramethylethylenediamine (Me4en)

The kinetics of base hydrolysis of glycine‐, histidine‐, and methionine methyl esters in the presence of [Cu‐Me₄en]²⁺ complex is studied in aqueous solutions and in dioxane–water solutions of different compositions at T = 25°C and I = 0.1 mol dm^?1. The kinetics of base hydrolysis of glycine and methionine methyl esters is studied at different temperatures. The kinetic data fits assuming that the hydrolysis proceeds in one step. The activation parameters for the base hydrolysis of the complexes are evaluated. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 737–745, 2006     

Synthesis and characterization of new complexes of the type [Ru(CO)2Cl2 (2‐phenyl‐1,8‐naphthyridine‐kN) (2‐phenyl‐1,8‐naphthyridine‐kN′)]. Preliminary applications in homogeneous catalysis

Novel ruthenium (II) complexes were prepared containing 2‐phenyl‐1,8‐naphthyridine derivatives. The coordination modes of these ligands were modified by addition of coordinating solvents such as water into the ethanolic reaction media. Under these conditions 1,8‐naphthyridine (napy) moieties act as monodentade ligands forming unusual [Ru(CO)₂Cl₂(η¹‐2‐phenyl‐1,8‐naphthyridine‐ kN )(η¹‐2‐phenyl‐1,8‐naphthyridine‐kN′)] complexes. The reaction was reproducible when different 2‐phenyl‐1,8‐naphthyridine derivatives were used. On the other hand, when dry ethanol was used as the solvent we obtained complexes with napy moieties acting as a chelating ligand. The structures proposed for these complexes were supported by NMR spectra, and the presence of two ligands in the [Ru(CO)₂Cl₂(η¹‐2‐phenyl‐1,8‐naphthyridine‐ kN )(η¹‐2‐phenyl‐1,8‐naphthyridine‐kN′)] type complexes was confirmed using elemental analysis. All complexes were tested as catalysts in the hydroformylation of styrene showing moderate activity in N,N′‐dimethylformamide. Copyright © 2008 John Wiley & Sons, Ltd.     

The different kinetic and mechanistic behaviors of molybdenum and tungsten in the reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes by ascorbic acid in aqueous media

The mono‐electronic reduction of tris(benzene‐1,2‐dithiolato)Mo(VI) and W(VI) complexes (ML₃: M = Mo, W; L = S₂C₆H^2?₄, S₂C₆H₃CH^2?₃) to their anionic forms ML^?₃ by L (+)‐ascorbic acid (H₂A) has been studied in tetrahydrofurane (THF):water and THF:methanol by means of diode‐array, stopped‐flow, and mass spectrometry–electrospray ionization (MS‐ESI) spectroscopy. The kinetic study in methanol demonstrates that the reaction is first order in each reactant, the electron transfer being rate limiting. This fact was assessed by the absence of a primary saline effect and by the correlation observed between the activation free enthalpy (ΔG^≠) and the reduction potentials measured by cyclic voltamperometry. In aqueous media, Mo(VI)‐tris(dithiolenes) also reduce to their Mo(V) anionic forms. The reaction obeys the rate law ? d[ML₃]/dt = (k_S+k_A[H₂A]_T)[ML₃] (M = Mo), in agreement with a parallel kinetic scheme involving the reduction of complexes by ascorbic acid (k_A) and by interaction with the solvent (k_S). Unexpectedly, the W(VI) complexes were not reduced by excess hydrogen ascorbate in the presence of water. These compounds underwent an extremely rapid autoreduction, which initially yielded an oxo W(VI)‐dithiolene and [W(S₂C₆H₄)₃]^?, as assessed by the MS‐ESI spectra. This observation suggests that tungsten tris(dithiolenes) are capable of coordinating water efficiently, undergoing further reduction after ligand displacement. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 279–291, 2011     

Solvent Role on Cobalt(II) Dioxygen Carriers Based on Simple Polyamine Ligands

The kinetics and thermodynamics of O₂ addition to Co^II complexes containing the simple triamine ligand (L) diethylenetriamine (=N‐(2‐aminoethyl)ethane‐1,2‐diamine; dien) or N,N″‐dimethyldiethylenetriamine (=N‐methyl‐N′‐[2‐(methylamino)ethyl]ethane‐1,2‐diamine; dmdien) in the aprotic solvent dimethyl sulfoxide (DMSO) were studied by UV/VIS spectrophotometry, potentiometry, and O₂ absorption measurements. A parallel investigation on the anaerobic formation of Co^II complexes with dmdien, as well as on their reactivity towards O₂, was carried out in aqueous 0.1M NaClO₄ solution. [CoL]²⁺ and [CoL₂]²⁺ were the common species formed under anaerobic conditions in both aqueous and DMSO solutions. Under aerobic conditions, O₂ adducts of different stoichiometry were formed: a superoxo complex [CoL₂O₂]²⁺ in DMSO and dimeric species in H₂O. The role of the reaction medium as well as effects of N‐alkylation of the triamine ligand in the formation and reactivity of the [Co^II(triamine)] complexes are discussed.