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The thermodynamic parameters for the complexation reaction between Zn2+ and Hg2+ ions with the mixed amino-polypyridinic ligands tris[(2-pyridyl)methyl]amine (TPA) and 6,6-bis–bis (pyridylmethyl)aminomethyl]-2,2-bipyridine (BTPA) have been obtained in anhydrous dimethylsulfoxyde (DMSO) by using spectrophotometry, polarography, potentiometry and titration calorimetry. Complexes with stoichiometry 1:1 have been observed in all cases but Hg-TPA system, for which 1:2 complex is also formed. As found in water, both $$\Updelta G_{\rm{j}}^{\circ}$$ and $$\Updelta H_{\rm{j}}^{\circ}$$ become more favorable when going down along the group. From a comparison of data with those reported for other metal ions in DMSO (Co2+, Ag+, Cd2+) and in water as solvent, it is shown that TPA is more selective towards Hg2+ in water than in DMSO and that the selectivity of BTPA in DMSO is much higher towards Cd2+ and Hg2+ ions with respect to Zn2+.

A series of electronically tuned trans- [CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2, 2′- ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis β organometallic derivative containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the −C═N– groups of the chelate. The X-ray structures of cyclized complexes, [CoIII(chelCH2)(py)(H2O)]+, show a dependence of the conformation of the seven-membered metallacycle on the different positions of substituents in the chelate. In fact, in the complex having methyls on the −C═N– groups, the conformation is characterized by having the methylene carbon atom significantly displaced (ca. 1.26 Å) from the aromatic ring plane, whereas in the complex lacking methyl groups in those positions, the atoms of the Ph–O–CH2 fragment are coplanar. The standard Gibbs energies obtained by quantum chemical calculation reveal that the different conformations observed in the solid state are mainly the result of the energetically unfavorable setup of the methyls on the −C═N– groups and of the energetically favorable displacement of the CH2 group out of plane of the aromatic ring. 1H NMR data suggest that the different conformations of the metallacycle are, at least partially, retained in solution.

Preparations of [Rh(Hdmg) 2 (H 2 O) 2 ]ClO 4 ( 1 ) (Hdmg=dimethylgyoximate) from [Rh(H 2 O) 6 ](ClO 4 ) 3 and dimethylglyoxime and of [Rh(Hdmg) 2 (PPh 3 ) 2 ]ClO 4 ( 2 ) from 1 and PPh 3 are described. 1 crystallizes in the space group with a =14.910(3), b =6.058(1), c =18.617(4), A, β =107.649(8)°, Z =4. The structure was refined up to R =0.021 for 1849 reflections. The IR and NMR spectra and electrochemical behavior of 1 and 2 are discussed and compared with other rhodoximes. Three polarographic and CV processed can be detected for 2 , the first two, respectively to Rh(II) and Rh(I), being followed by fast reactions.

A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand {[Z-Tyr(3-Ac)-OMe] 2 en} derived from the condensation of two equivalents of protected 3-acetyl- l -tyrosine with one equivalent of ethylenediamine has been synthesized. As the amino acids have l configuration, the resulting ligand is chiral. The complexes have been characterized by 1 H and 13 C NMR, ESI-MS spectrometry and UV–Vis spectroscopy. The UV–Vis spectra recorded in CH 2 Cl 2 show an absorption band at about 650 nm, characteristic of penta-coordinated alkylcobalt(III)(salen), but an evident deviation from the Lambert Beer law in the concentration range 0.20–2.0 × 10 −3 M suggests some kind of association in this solvent. In coordinating solvent the complexes are hexa-coordinated. The equilibrium constants for pyridine ligation in CH 3 OH decrease with increasing electron-donor power of the axial alkyl group. Cyclic voltammetry studies indicate that after the reduction RCo(III)/RCo(II), the complexes decompose giving a Co(I) species that is stable on the CV time scale and can be reoxidized to Co(II) and Co(III) species. Both the E pa and the E pc values show that the peak potentials are shifted to more negative values with increasing inductive effect of the axial alkyl group.

We described the guest-induced synthesis of a rectangular host derived from methylaquacobaloxime CH3Co(dmgH)2H2O and 4,4′-biphenyldiboronic acid containing 4,4′-bipyridine (bipy) molecule as guest. The assembly, monitored by a time dependent 1H NMR experiment, showed that the guest thermodynamically drives the organization of the host. The structural characterization of the complex evidenced a centrosymmetric rectangular box having two biphenyldiboronate units and a central 4,4′-bipyridine spacer connecting the cobaloxime units (1). The spacers have coplanar rings, being negligible the tilt angle around the central C–C bond. The molecular structure of 1 and of the dinuclear complex [CH3Co(dmgH)2]2(bipy) indicates close comparable intermetallic distance for the rigid bipy bridge and a coplanar arrangement of the cobaloxime moieties that facilitates the formation of the molecular box. The other tested ditopic ligands L (α,α′-diamino-p-xylene and 1,6-diaminohexane) used in this study allowed only a partial assembly. These derivatives and the related dinuclear complexes [CH3Co(dmgH)2]2(L), which were synthesized with the aim to obtain further insights about the formation of the molecular boxes, were also characterized by X-ray structural analysis. The structures of [CH3Co(dmgH)2]2(L), beside the expected flexibility of L, indicate not facing cobaloxime moieties. CV measurements suggest the formation of dinuclear non organometallic Co(II) and Co(I) species after the first reduction for all the dinuclear compounds that are stable in solution. For 1 no electronic coupling is shown, in spite of coplanar conformation of the 4,4′-bipyridine rings.

The formation of Cd(II) and Co(II) complexes with N-methylethylenediamine (men) has been studied at 298 K in dimethylsulfoxide (dmso) in an ionic medium set to 0.1 mol dm−3 with Et4NClO4 in anaerobic conditions by means of potentiometric, UV–Vis, calorimetric and FT-IR technique. Mononuclear MLj (M=Cd, Co; j=1–3) complexes are formed in exothermic reactions, whereas the entropy changes oppose the complexes formation. The results are discussed in terms of different basicities and steric requirements and the whole of the thermodynamic data reported till now for the two ions with a number of diamines are summarized to visualize the selectivity of the ligands. The dioxygen uptake of Co(men)2 species has also been studied by means of UV–Vis and EPR techniques. The kinetic parameters and stability constants obtained for the formation of the superoxo and μ-peroxo species are discussed in terms of solvent effect and steric hindrance due to methyl group. Cyclic voltammetry was used to confirm the stability constant for the Co(dmen)2 (dmen=N,N′-dimethylethylenediamine) superoxo adduct formation but was not successful to investigate this Co(men)2–O2 system.

The electrochemical behavior of dioxygen in the presence of adenine in dimethylformamide solution on the Hg electrode is studied. Evidence for the activation of O 2 is given by the shift of 150 mV towards more positive values of the one-electron reduction, accompanied by the autooxidation of adenine, while the adsorption of O 2 is shown by the characteristics of a polarographic pre-wave and voltammetric peak. The activation of O 2 is attributed to the interaction with adenine co-adsorbed on the Hg electrode.

Rh(Hdmg)2(PPh3)Cl (Hdmg=monoanion of dimethylglyoxime) undergoes reduction with zinc amalgam to give the tetranuclear bimetallic compound [Rh(Hdmg)(ClZndmg)(PPh3)]2, characterized by elemental analysis, electronic, IR and NMR spectroscopy and X-ray crystallography. It crystallizes in the triclinic space group P1 with: a=14.262(4), b=14.675(4), c=14.698(4) A, α=82.15(4), β=87.57(4), γ=65.89(3)°, Z=2. The structure was refined up to R=0.13 for 4704 reflections. The molecule consists of two [Rh(dmg)(Hdmg)(PPh3)] subunits linked by an [Rh(II)(II)] bond of length equal to 2.852(3) A and by two Zn ions bridging the oxygens of the equatorial ligands. The binding with zinc relieves the equivalence among the atoms of each subunit.

This paper clarifies the adsorption mechanism of 1-[(2S)-3-mercapto-2-methyl-1-oxopropyl]-L-proline (CPT) on DME. Some characteristic features of CPT adsorption, previously followed by cyclic voltammetry (CV), have been investigated to give more precise information about some electrochemical constants by alternating current polarography (ACP). The first harmonic ACP was considered a suitable tool for obtaining electrochemical parameters of surface-confined reactions. The entity and sign of the molecular interaction parameter (g), the thermodynamic constant of adsorption (Kads), the extrapolated apparent electron rate constant (Kextsh), the saturation concentration (Co), maximum surface density (Γt)etc. have been determined. The values of some of these electrochemical parameters, found in a previous paper, were confirmed.

Heterogeneous electron transfer rate constants were determined as a function of electrode potential for one-electron oxidation in acetonitrile (AN) at O °C of a series of organocobaloximes [R-Co(DH)2L] bearing widely different organic groups. Reaction entropies were determined by voltammetric half-wave potential (Er 1 2 ) measurements in a non-isothermal cell. The electron transfer coefficients and reorganization parameters were calculated following the Marcus theory. The reaction free energies relative to a reference couple ΔG∘ are linearly correlated with the polar Taft constant of the organic substituent R. The steric effects on ΔG∘ are shown by the correlation of Er sol1 2 with the CoC bond distance. Assuming constancy of double layer effects along the series in the given solution composition, the trends of the apparent rate constants kapp were considered in order to evaluate the effects of the nature of the organic ligand on the activation energy ΔG‡ of the electron transfer. The steric effects on ΔG‡ are pointed out i.a. by consideration of the relationship between ΔG‡ and ΔG∘.

The electrochemical reduction of alkylcobaloximes at the dropping mercury electrode involves an initial one electron transfer process to give a radical anion with the cobalt in the CoII formal oxidation state. The CoC bond cleavage in the radical anion is an homolytic process yielding the CoI cobaloxime and competing with a second electron transfer. The nature of the organic group and of the solvent coordinated in the trans position to the Co atom influences the thermodynamics of the electron transfer and the stability of the radical anion. Electronic and steric effects on the thermodynamics of the first electron transfer can be distinguished. These effects have only a small influence on the kinetics of the first electron transfer.

Diaquocobaloxime and three other chelating ligands obtained by substitution of the O ⋯ H ⋯ O bridge with (CH2)3 or BF2 groups have been examined in order to compare the effects of changes in the in-plane ligand on the relative stability of different oxidation states of the cobalt atom and on the cis-effect of the equatorial structure with those observed in the corresponding organometallic derivatives. The cis-effect of the in-plane ligand was evaluated from the electrochemistry of the CoIII/CoII electron transfer which is dependent upon the Co-solvent bond strength changes in the axial positions. The axial interaction depends mainly on the nature of the axial ligand. The introduction of either one (CH2)3 group or two BF2 groups or both leads to a displacement of the stability range of the CoII oxidation state towards more anodic redox potentials. The (CH2)3 group appears to be more effective both in the displacement of the redox potential and in teh bathochromic shift of the visible band at 450–500 nm of the CoII chelates.

Some new derivatives of organocobaloximes containing para -substituted diphenylboryl groups, RCo(DH) 2− n (DB( p- XPh) 2 ) n L (R=alkyl or aryl group, L= N -MeIm, Py or H 2 O, X=OCH 3 , CH 3 or Cl, n =1 or 2) have been synthesized. The X-ray structures and the 1 H-NMR spectra are compared with those of the corresponding RCo(DH) 2− n (DBPh 2 ) n L and RCo(DBF 2 ) 2 L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial CoN distances, whereas the CoPy distances increase slightly in the order (DB( p- OCH 3 Ph) 2 ) 2 p- ClPh) 2 ) 2 2 ) 2 . 1 H-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh 2 derivatives. Electrochemical studies on the corresponding MeCo(DB( p- XPh) 2 ) 2 H 2 O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the CoC bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands.

Electrocatalysis of dioxygen reduction to superoxide ion in dimethylformamide is afforded by a reversible interaction with thiol or hydroxo forms of pyrimidine which takes place only on the mercury surface electrode even in the absence of transition metal ions. The interaction is revealed by a polarographic wave and by a voltammetric peak which can be recorded only in the presence of dioxygen in the pyrimidines solution. The polarographic wave and voltammetric peak are attributed to the reduction of the oxygenated pyrimidine. In the range of concentration where the polarographic limiting currents due to the reduction of the oxygenated and unoxygenated pyrimidine are of the same order of magnitude, the stoichiometric ratio dioxygen/pyrimidine is determined to be 1. The electrode process probably involves the formation of a complex of mercury with the thiol or the hydroxo form of the pyrimidine on the electrode surface. Adsorption or coadsorption of the reactants seems to be involved.