Carmen Paradisi

Electrochemistry and spectroelectrochemistry of ruthenium(II)-bipyridine building blocks. Different behaviour of the 2,3- and 2,5-bis(2-pyridyl)pyrazine bridging ligands

We report the results of an investigation, using electrochemical and spectroelectrochemical techniques, into redox properties of uncoordinated free bis-chelating 2,3- and 2,5-bis(2-pyridyl)pyrazine ligands (2,3- and 2,5-dpp) and of the complexes of the [Ru(2,3-dpp)n(bpy)3−n]2+ and [Ru(2,5-dpp)n(bpy)3−n]2+ families (bpy=2,2′-bipyridine), which are used as building blocks for obtaining polynuclear complexes. For comparison purposes, the electrochemical behaviour of the [Ru(2,3-dpp)(DCE-bpy)2]2+complex, where DCE-bpy is 5,5′-dicarboxyethyl-2,2′-bipyridine, has also been investigated. Correlations of the E1/2 values observed for the compounds examined (genetic diagrams) have allowed us to assign all the ligand-based reduction processes as well as to discuss electronic interactions. The localisation of the first three reduction processes for each complex has also been established on the basis of the spectroelectrochemical results. Theoretical calculations (AM1 semiempirical and ab-initio level) carried out for the 2,5-dpp and 2,3-dpp ligands show that, in the uncoordinated state, the former ligand does not exhibit any substantial conformation arrangement, whereas the latter has a stable conformation for a large (56°) dihedral angle between the pyridyl and pyrazine rings. The changes in conformation upon mono- and bis-coordination of 2,3-dpp can account for its peculiar electrochemical behaviour consisting in a change of the number of redox processes with varying coordination state.

Electrochemical and spectroscopic behaviour of cyanobridged BI- and trinuclear complexes of ruthenium containing 2,2′-bipyridine and ammonia as ligands convolutive potential sweep voltammetric study of two Nernstian waves

Abstract The electrochemical and spectroscopic behaviour of the mixed valence ruthenium complexes [(NH 3 ) 5 RuCNRu(bpy) 2 CN] 3+ ([3.2]) and [(NH 3 ) 5 RuCNRu(bpy) 2 CNRu(NH 3 ) 5 ] 6+ ([3.2.3]) have been studied in dimethylformamide and in water utilizing the techniques of polarography, voltammetry with a platinum electrode with periodical renewal of the diffusion layer, cyclic voltammetry and controlled electrolysis. Both [3.2] and [3.2.3] present reduction (oxidation)processes at rather negative (positive) potentials (vs. SCE aq ), which are typical of the unit Ru(bpy) 2 (CN) 2+ 2 , as well as a reduction process at potentials near zero, which can be attributed to Ru(III) pentammine groups. In the case of [3.2], this latter process corresponds to a one-electron reversible transfer, giving rise to the fully reduced species [2.2], while for [3.2.3] two consecutive one-electron reversible transfers, characterized by very close standard potentials, occur, giving rise to the half-reduced species [3.2.2] and to [2.2.2]. The two standard potentials, and hence the conproportionation constant, have been determined from the analysis of the cyclic voltammetric curves. The reduction at controlled potential of Ru(III) pentammine groups has enabled us to obtain and characterize spectroscopically the species [2.2], [3.2.2] and [2.2.2]. Several types of electronic transitions between the various low-energy redox sites of the ions were observed, among them the intervalence transfers between adjacent and remote ruthenium atoms. Comparisons between electrochemical and spectroscopic results related to cyanide bridge formation are also reported.

Extensive redox series in dinuclear and dendritic Ru(II) complexes

Abstract The electrochemical behavior of a dinuclear and a hexanuclear dendritic ruthenium(II) bipyridine complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) as bridging ligands has been investigated in highly purified liquid SO 2 or N , N -dimethylformamide (DMF) solutions. The compounds have general formula [Ru n (bpy) n +2 (2,3-dpp) n −1 ] 2 n + , where n =2 or 6; bpy is 2,2′-bipyridine. The wide anodic and cathodic potential windows explored in liquid SO 2 and DMF, at low temperature, (up to ca. 4.3 and −3.1 V vs. SCE, respectively) have allowed the observation of several metal-centered oxidations and ligand-centered oxidations and reductions, leading to the most extensive redox series so far reported, comprising up to 26 reversible ligand-centered reductions and 14 reversible metal- or ligand-centered oxidations, for a total of 40 redox processes in the hexanuclear complex. The redox standard potentials for overlapping processes in multielectron waves have been obtained from the analysis of the voltammetric curves and their digital simulation, and the localization of all the redox processes and an evaluation of the mutual interactions between the redox centers have been obtained. These results allowed to clarify the important role played by bridging ligands in mediating the interactions between the equivalent redox sites. Furthermore, the localization of the redox sites and the determination of relative standard potentials allowed the mapping of MLCT excited states within the dendrimeric structure, through the correlation between the absorption spectroscopic data and the relevant voltammetric data.

Electrochemistry of covalently linked supramolecular species: redox series of the trinuclear complex NCRu(bpy)2CNRu(DCE-bpy)2NCRu(bpy)2CN2+ (bpy = 2,2′-bipyridine; DCE-bpy = 5,5′-dicarboxyethyl-bpy)

The electrochemical behaviour of the trinuclear complex NCRuII(bpy)2RuII(DCE-bpy)2NCRuII(bpy)2CN2+, where bpy = 2,2′-bipyridine and DCE-bpy = 5,5′-dicarboxyethyl-bpy, was studied in acetonitrile and in N,N-dimethylformamide. Because of the high number of oxidizable and reducible centres, this trinuclear complex gives rise to a very extended redox series formed by 14 redox steps, three of which are due to one-electron reversible oxidations of Ru(II) to Ru(III), while the remaining 11 can be ascribed to one-electron reversible reductions of bipyridine ligands. Such behaviour, for the reduction processes, is observed only at low temperature ( −54°C) and at a sufficiently high sweep rate (10 V s−1). Comparison between the behaviour of this complex and the previously studied trinuclear species NCRuII(bpy)2CNRuII(bpy)2NCRuII(bpy)2CN2+ has allowed us to confirm the previous assignment of the localization of the electron density changes in the individual redox steps. Moreover, additional spectroscopic evidence is provided that the more easily oxidizable Ru(II) centre in such species is the central Ru(II) atom.

Ligand substitution and nucleophilic reactivity of tricarbonyl(1-5-η-cyclooctadienylium)iron complexes

Abstract Nucleophilic attack of CN − on tricarbonyl(1-5-η-cyclooctadienylium)iron leads to formation of tricarbonyl(cycloocta-1,3,5-triene)iron and dicarbonyl-(1-5-η-cyclooctadienyl)carbonitrileiron, and attack of PPh 3 leads to formation of phosphonium ions. Nucleophilic attacks on dicarbonyl(1-5-η-cyclooctadienyl)iodoiron by CN − gives dicarbonyl(1-5-η-cyclooctadienyl)cyanoiron complex, while that by PPh 3 or AsPh 3 gives dicarbonyl(1-5-η-cyclooctadienyl)triphenylphosphineiron hexafluorophosphate or dicarbonyl(1-5-η-cyclooctadienyl)triphenylarsineiron hexafluorophosphate. Treatment of the cationic compound arsine derivative with H − or OCH 3 − gives the corresponding neutral compounds.

Self-protonation mechanism in the electroreduction of quinolinols

Abstract The electroreduction of 8-, 5- and 2-hydroxyquinoline (8-, 5- and 2-QOH, respectively) was studied in DMF. Cyclic voltammetry, polarography, controlled potential coulometry and UV-visible spectroscopy were used to provide information on the stoichiometry and kinetics of the process. The electroreduction of 8- and 5-QOMe, i.e. the aprotic analogues of 8- and 5-QOH, was also investigated for comparison. The main feature of the reduction process is a fast proton transfer from the parent compounds to the electrogenerated basic intermediates (self-protonation mechanism), with the formation of the conjugate base of the former, QO − , together with the two-electron reduction product QH 2 OH. The rate constant of the self-protonation step for the primary radical anion was calculated to be 1.0x10 5 M −1 s −1 for 2-QOH, whose reduction becomes reversible at high scan rates. For 8- and 5-QOH the corresponding rate constants were evaluated to be almost diffusional, in agreement with the higher acidity of these quinolinols with respect to 2-QOH. The electrode reaction mechanism of the conjugate base QO − , reducible at very negative potentials, is discussed.

Physicochemical characterization of the 1,2-bis(cyanimido)cyclobutene-3,4-dione dianion and its twin radical anions

Controlled oxidation of the 1,2-bis(cyanoimido)cyclobutene-3,4-dione (CMCB) dianion in aprotic solvents gives electron paramagnetic resonance (EPR) spectra characteristic of the presence of two radical species in a fixed mole ratio, identified as two planar conformers of the monoanion of CMCB differing in the orientation of one of the two N—CN groups. Consideration of the nature and symmetry of the molecular orbitals, and of the IR and 13C NMR spectra, suggests that similar isomers should also be present in the solution of the dianion.Cyclic voltammograms of the dianion in N,N-dimethylformamide exhibit two oxidation peaks, the first being due to a one-electron diffusion-controlled reversible process, and the second to an irreversible process. Computer simulation of the cyclic voltammetric curves for the first process indicates that the observed behaviour is consistent with the four-member square scheme suggested by EPR, IR and NMR experiments. The irreversibility of the second peak is due to the presence of fast chemical reactions involving the product of one-electron reversible oxidation of the radical anion.

Ligand substitution and nucleophilic reactivity of tricarbonyl(1–3:5,6-η-cyclooctadienylium)ruthenium cation and its derivatives

The tricarbonyl(1-3:5,6-η-cyclooctadienylium)iron cation (I) undergoes attack by triphenylphosphine at two different sites, on the organic group or at the metal; the first type of attack leads to a phosphonium ion and the second case to a dicarbonyl(1-3:5,6-η-cyclooctadienylium)triphenylphosphineiron cation. The two products have been isolated and identified by IR spectroscopy. The reaction between cation I and iodide ion can also involve attack on the organic group or the metal, giving rise to tricarbonyl[4-6-η,1-σ-(2-iodocyclooctenyl)]iron and dicarbonyl(1-3:5,6-η-cyclooctadienylium)iodoiron, respectively. Only the latter has been isolated and characterized; for the former indirect evidence is available, involving its reactions with CN− and PPh3. The reactions between cation I and the various nucleophiles OMe−, N−3, CN−, and AsPh3 and those between its iodo derivative and PPh3 and AsPh3 were also studied.

Voltammetric behaviour of tris(8-quinolinolato) iridium(III) complex in dimethylformamide

Abstract The polarographic and cyclic voltammetric behaviour of Ir(QO) 3 (QO − =8-quinolinolato anion) has been investigated in N,N -dimethylformamide. In cyclic voltammetry, at sufficiently high sweep rates, v , the complex exhibits three one-electron reversible reduction peaks. By decreasing v the first peak becomes irreversible and two other cathodic peaks appear. Concomitantly, the second and third peaks, observed at high v , disappear. The irreversibility of the first peak is attributed to a first order chemical reaction, involving the product of the first one-electron transfer, and in which the formation of Ir(QO) 2 and QO − takes place. The second and third peaks observed at low v , are attributed, respectively, to the reduction of Ir(QO) 2 and QO − formed in the first process. As to the polarographic behaviour, the processes responsible for the three waves oberved could be similar to those described for the three reduction peaks observed in cyclic voltammetry at low v . A qualitative MO discussion of the character of the molecular levels involved in the reduction processes is also reported.

Mechanism of octahedral substitution in non aqueous media. I. Isotopic exchange reactions of chloride-36 in Trans-chlorocyanobis(ethylenediamine)cobalt(III) cation in methanol, dimethylsulfoxide, ethylene glycol and N,N-dimethylformamide

Abstract The first order rate constants of chloride-36 exchange in trans -[Coen 2 CNCl] + are independent on LiCl concentration in the solvents methanol, dimethylsulfoxide (DMSO), and ethylene glycol, while in dimethylformamide (DMF) the first order rate constants for isotopic exchange are higher than those obtained with other studied and dependent on LiCl concentration in the range 10 −3 -10 −2 M. The activation parameters are E a = 21.32 Kcal/mol, ΔS≠ = −10.94 cal K −1 mol −1 , A = 2 × 10 10 sec −1 in methanol; E a = 20.13 Kcal/mol, ΔS≠ = −12.94 cal K −1 mol −1 , A = 8.6 × 10 9 sec −1 in DMSO; E a = 21.96 Kcal/mol, ΔS≠ = −3.98 cal K −1 mol −1 , A = 7 × 10 11 in ethylene glycol; E a = 32.5 Kcal/mol, ΔS≠ = +35.5 cal K −1 mol −1 , A = 3 × 10 20 sec −1 in DMF. Bearing in mind that methanol and DMSO are good solvating agents, while ethylene glycol is not, we propose an associative interchange I a stoichiometric mechanism, via solvation reaction, in methanol and DMSO, and a dissociative interchange stoichiometric in ethylene glycol. In DMF the results are consistent with the formation of a strong ion-pair between chloride and trans -chlorocyanobis(ethylenediamine) cobalt(III) ions K(P) = 1870 mol −1 and seems to indicate the presence of a stoichiometric mechanism of intrchange I, where the ion-pair is the activated complex in the transition state. Within the ion-pair the intimate mechanism of isotopic exchange is probably of I d type. Thus we have found that for the complex trans -[Coen 2 LCl] + the role of L = CN is not solvent independent.