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Data for complex formation of Co(II) with N-donors in dimethylsulfoxide are reviewed and rationalized on the basis of both solvational effects and ligand structural and electronic features. For the polyamine ligands, the number and stability of the species formed decreases with increasing number of N-methyl groups, due to the increase of the steric crowding and the relatively low basicity of the donor nitrogen. However also the stabilization through H-bond formation with the second-sphere solvent molecules is an important factor. The same factor causes a greater electron donation from the ligand to the metal ion stabilizing the complex as shown by the thermodynamic and structural data. When comparing the thermodynamic parameters with those available in water, it results that the stability constants are higher than those in water despite the stronger solvation of the metal ion in DMSO. This is assigned to the ligand solvation playing an important role, especially for primary and secondary amines. When ligands able to be only H-bond acceptors are considered (tertiary amines and pyridines) the stability of the complex expected on the basis of the metal ion solvation is observed. When considering the oxygen affinity of the Co(II) complexes in DMSO it results that the complexes with primary and secondary amines are able to bind O2. As O2 uptake is a redox process it strongly depends on the ligand-metal charge transfer and on the solvation of the resulting complex. Thus, the same factors affecting the stability of the Co(II) complexes with the N-donor ligands considered also influence their dioxygen affinity. A correlation between enthalpy and electrochemical data in DMSO supports this hypothesis.

Some thermodynamic and kinetic aspects of the reactions of dioxygen binding to Co(II) complexes with the two cyclic tetraamines 1,4,7,10-tetraazacyclododecane ([12]aneN4) and 1,4,8,11-tetraazacyclotridecane ([13]aneN4) are examined. Specifically, results are reported on: (1) the determination of the protonation constants and protonation enthalpies for both [12]aneN4 and [13]aneN4 ligands; (2) the study of the kinetics of the formation of the precursor CoL complexes for L=[12]aneN4 and [13]aneN4 carried out by monitoring the uptake rate of molecular dioxygen; (3) the recognition of the type of dioxygen adducts formed at various pH values; (4) the determination of the kinetic and thermodynamic data characterising the reactions of isomerization of monobridged μ-peroxo-monohydroxo (CoL)2(μ-O2)(OH) complexes into the dibridged μ-peroxo-μ-hydroxo (CoL)2(μ-O2)(μ-OH) complexes, and the reactions of conversion of the dibridged μ-peroxo-μ-hydroxo (CoL)2(μ-O2)(μ-OH) complexes into the monobridged μ-peroxo-dihydroxo (CoL)2(μ-O2)(OH)2 complexes.

The behavior of Co(NO3)2 and 1,4,7,10,13,16,19,22,25,28-decaazacyclotriacontane aqueous solutions towards molecular dioxygen has been examined over a large range of ratios R between the ligand and cobalt(II) concentrations and in the presence of different types of anions at high concentrations. Both binuclear, Co2L(O2)(OH), and mononuclear, Co(HmL)(O2), peroxo complexes are formed depending on the value of R. The formation of the binuclear species is rapid, while that of the mononuclear complexes is slow. Phosphate anions (Y) enter into the bi- and mono-nuclear peroxo complexes, giving Co2L(O2)Y and CoL(O2)Y species. The kinetics of dioxygen uptake under various experimental conditions has been studied and reaction schemes are proposed.

The kinetics and thermodynamics of O2 addition to CoII 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 CoII 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 [CoII(triamine)] complexes are discussed.

The basicity behavior and coordination properties of the ligand 2,6-bis-{[bis-(2-aminoethyl)-amino]methyl}-phenol ( L ) in aqueous solution toward Co(II), are reported and discussed together with those of the species formed with Ni(II) and Cu(II) ions. Moreover, binuclear cobalt complexes of L were investigated as possible binders for molecular dioxygen. The studies were carried out using potentiometric measurements (298.1 K, I =0.15 mol dm −3 ), UV–Vis absorption spectra, and 1 H and 13 C NMR spectra in aqueous solution. L behaves as a pentaprotic base in the range of the potentiometric measurements; the UV and NMR experiments revealed that in L , H L + , H 2 L 2+ , and H 3 L 3+ species there is the simultaneous presence of positive and one negative charge on the ligand. L forms mono and binuclear complexes with Co(II) ion. The binuclear species occur with the complete deprotonation of L ; the oxygen of the phenolate moiety bridges the two metal ions forcing them to stay close to each other, preorganizing the complexes to bind an additional molecule. This is the case of the binuclear cobalt species ([Co 2 (H −1 L )(OH) j ] (3− j )+ ( j =0–2)), which cooperate to bind one molecule of dioxygen, giving rise to mono-oxygenated species. The stoichiometry of such species ([Co 2 (H −1 L )(OH) j O 2 ] (3− j )+ ( j =0–2)), and other experimental data are in agreement with the formation of a μ-peroxocomplexes where the oxygen molecule bridges the two metal atoms. The experiments showed that the oxygenation reactions are reversible.

The interaction of phosphate with the mono- and binuclear cobalt(II) complexes of [30]aneN 10 (1,4,7,10,13,16,19,22,25,28-decaazacyclotriacontane) has been studied by potentiometry in 0.15 mol dm −3 NaClO 4 solution at 298.15 K under anaerobic conditions. The stable species [CoH 2 ([30]aneN 10 )PO 4 ] + , [CoH 4 ([30]aneN 10 )PO 4 ] 3+ , [Co 2 H([30]aneN 10 )PO 4 ] 2+ , [Co 2 H 2 ([30]aneN 10 )PO 4 ] 3+ and [Co 2 H 3 ([30]aneN 10 )PO 4 ] 4+ , where the phosphate anion is directly bound to the metal ions or acts as a second sphere ligand, are formed and their stability constants have been determined. The results obtained allowed for the selection of suitable conditions for the study of dioxygen uptake.

A short survey is given of the main characteristics of the reaction of uptake of molecular oxygen by biological dioxygen carriers. Co(ll) complexes able to bind dioxygen are then examined in order to determine whether the type and features of the peroxocomplexes formed are related to the nature of the ligand and the solvent. The thermodynamic and kinetic data relative to the formation in aqueous solution of peroxocompounds starting from Co(ll) complexes with ligands belonging to three families (open-chain, macrocyclic and macropolytopic) of saturated polyamines are also examined in greater detail.

Through spectrophotometric and potentiometric measurements it has been verified that in aerated aqueous solutions of CoII containing tris(2-aminoethyl)amine (tren) as ligand, the doubly bridged µ-peroxo–µ-hydroxo dicobalt ion [Co2L2(O2)(OH)]3+(L = tren) is totally formed at pH > 6.5. In aerated aqueous solutions of CoII and 3,7-diazanonane-1,9-diamine (dadn) at pH 6.5 only the singly bridged µ-peroxo dicobalt ion [Co2L2(O2)]4+(L = dadn) is present. This complex at higher pH adds OH– ions in its axial positions; at pH > 10.5 the singly bridged µ-peroxo–dihydroxo dicobalt ion [Co2L2(O2)(OH)2]2+ is totally formed. The singly bridged complex [Co2L2(O2)]4+, after several hours, converts irreversibly into the doubly bridged µ-peroxo–µ-hydroxo ion [Co2L2(O2)(OH)]3+, while the single bridged [Co2L2(O2)(OH)2]2+ ion slowly autoxidizes to CoIII. By means of calorimetric measurements, usually carried out at constant pH, the enthalpies of formation of [Co2L2(O2)]4+ and [Co2L2(O2)(OH)2]2+(L = dadn) and [Co2L2(O2)(OH)]3+(L = tren) have been obtained. These results are compared with the enthalpies relative to the oxygen addition to [Co(trien)]2+ ion (trien = triethylenetetra-amine), and the enthalpy effects, connected with the change from a linear amine (trien) to its branched isomer (tren) and from the sequence 5,5,5 (trien) to the sequence 5,6,5 (dadn) of the chelate rings, are discussed.

Aqueous solutions containing Co/sup 2 +/ and the macrocyclic ligand 1,4,8,12-tetraazacyclopentadecane (L) are able to bind reversibly molecular oxygen to form the species Co/sub 2/L/sub 2/O/sub 2//sup 4 +/ between pH 5 and 7. This species at pH greater than 8 is replaced by the complex Co/sub 2/L/sub 2/O/sub 2/(OH)/sub 2//sup 2 +/ whose concentration in the solution increases to about pH 10.5. At pH greater than 10.5, Co(II) exists totally in this ..mu..-peroxobisydroxo complex. Both the above mentioned oxygenated forms suffer a relatively slow irreversible oxidation into Co(III). From oxygen binding curves, approximate values of the equilibrium constants were calculated for the addition of oxygen to CoL/sup 2 +/ to form the ..mu..-peroxo and the ..mu..-peroxo-bishydroxo complexes. The enthalpies of formation of this latter complex starting from both Co/sup 2 +/ or CoL/sup 2 +/ have been obtained by means of calorimetric measurements. The effects of the size of the macrocyclic ligand on the affinity and the enthalpy of oxygen binding have been considered.