Helena Li Chum

Electrochemistry of organometallic compounds. Part III. Oxidations of methyl derivatives of organocobalt(III) complexes with delocalized electronic structure in dimethylformamide

The influence of the equatorial ligand on the electrochemical oxidation of the compounds [H3CCo(chel)B], where chel is bis (dimethylglyoximato), (DH)2; bis(salicylaldehyde)ethylenediimine, salen; bis(salicylaldehyde) o-phenylenediimine, salophen; bis(salicylaldehyde)cyclohexylenediimine, salcn; bis(acetylacetone) ethylenediimine, bae; and where B is pyridine when chel is (DH2), and dimethylformamide (DMF) when chel represents a Schiff base (salen, salcn, salophen and bae), was studied by means of cyclic voltammetry in DMF, 0.2 M in tetraethylammonium perchlorate, between 25 and −25°C, with a platinum disk working electrode. Absorption spectra in the visible and near ultraviolet regions for these compounds in DMF at 25°C were obtained. The complexes exhibit a reversible one-electron oxidation, at −20°C with scan rates >0.5 V s−; chemical reactions following electron transfer are not detected under these conditions. At slower potential or higher temperatures, the oxidized product decomposes chemically in a solvent-assisted (or nucleophile-assisted) reaction, yielding products which are electroactive in the applied potential range. The behavior of the [H3CCo (DH2)py] derivative is better described as a quasi-reversible charge transfer followed by an irreversible chemical reaction. Experimental evidence suggests that in the case of the [H3CCo(bae)] derivative at −20°C, the reactive -species is pentacoordinated and weakly adsorbed at the electrode surface. The value of E12 and the energies of the first two absorption bands in the visible spectra reveal the ability of the studied complexes to donate and to delocalize electronic charge.

Electrochemistry of organometallic compounds: Part II. Oxidations of alkyl and aryl derivatives of bis(salicylaldehyde)ethylenediimine cobalt(III) in dimethylformamide⋆

Abstract The influence of the axial organic ligand R on the electrochemical oxidation of the compounds [RCo III (salen)DMF)], where salen is bis(salicylaldehyde)ethylenediimine, and R=CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 , s-C 4 H 9 , i-C 4 H 9 , CH 2 Cl, CF 3 CH 2 , c-C 6 H 11 CH 2 , c-C 6 H 11 , C 6 H 5 , C 6 H 5 CH 2 , p -CH 3 C 6 H 4 CH 2 , and p -NO 2 C 6 H 4 CH 2 , was studied by means of cyclic voltammetry in dimethylformamide (DMF), 0.2 M in tetraethylammonium perchlorate (TEAP), at 25 and −20°C, with a platinum disc working electrode. The above-mentioned compounds can be classified according to their electrochemical behavior. (a) The complexes with R=CH 3 , C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 , c-C 6 H 11 CH 2 , and C 6 H 5 undergo a reversible one-electron oxidation in the 10–50 V s −1 potential scan range. At slower scan rates, the oxidized product decomposes chemically. At −20°C, this chemical step is slow, and a reversible one-electron electrochemical oxidation is observed. (b) The compounds with R=CH 2 Cl, C 6 H 5 CH 2 , p -CH 3 C 6 H 4 CH 2 , and p -NO 2 C 6 H 4 CH 2 undergo a quasi-reversible one-electron oxidation at room temperature. At −20°C, the electrochemical process becomes more complex. A following chemical reaction is coupled to the quasi-reversible one-electron transfer. Two reduction peaks are observed. (c) The compounds with R=i-C 4 H 9 , s-C 4 H 9 , and c-C 6 H 11 undergo a reversible one-electron oxidation at −20°C. At room temperature, the irreversible chemical reaction following the electron transfer step is too fast to allow the isolation of the electrochemical step. (d) At −20°C, the derivatives with R=C 2 H 5 , c-C 6 H 11 -CH 2 , and c-C 6 H 11 are adsorbed at the electrode surface. Evidence indicates that the reagent in these reactions is the pentacoordinated species [RCo III (salen)]. A linear free-energy relationship between E 1/2 (for reversible processes) and the Taft polar parameters σ * was obtained with a slope of ϱ * =0.25±0.03. As expected, the benzyl derivatives which present mesomeric effects do not fit this polar correlation. The rate of the electrochemical oxidation is also affected by the nature of the ligand R. For the ligands which are strong electron-withdrawing groups and for the benzyl derivatives, the rate of the electrochemical oxidation of the metal ion decreases at room temperature. At lower temperatures, it is suggested that the oxidation to the Co IV -R species is followed by a chemical reaction in which this complex is partly transformed into a Co III (R · ) species, which is reduced at a much more cathodic potential than the Co(IV) species.

Electrochemistry of organometallic compounds: I. Cyclic voltammetry of organocobaloximes in aqueous acid solutions

This paper presents a systematic investigation on effects of the nature of the organic axial ligand on the primary electrochemical oxidation steps of organoaquobis(dimethylglyoximato)cobalt(III). Evidence is presented to support a one electron reversible process, yielding a cobalt(III) compound attached to the organic radical. Studies of p-substituted benzyl and phenyl derivatives support further the proposed process. The following step is a pseudo-first order irreversible dissociation of the oxidized species, yielding the trans-Co(DH)2(H2O)+ and the organic radical that can be further oxidized at the electrode. Linear free energy correlations obtained between E1/2 and Taft or Hammett parameters, depending on the nature of the organic substituent in axial position, strongly favor that Co-alkyl(aryl) bonding electrons are involved in the electron transfer.

Cyclic voltammetry of iron dimine complexes in acetonitrile

Abstract The electrochemical behavior of iron diimine complexes, (H3C−N=C(R)−C(R′)=N−CH3)3Fe(II) (R, R′=H,H;H, CH3; CH3, CH3), and (C5H4N−C(R1)=N(R2))3Fe(II) (R1, R2=H, CH3; CH3, CH3) on a platinum working electrode in acetonitrile is described, and compared to that of the parent aromatic complex, tris-(2,2′-bipyridine)Fe(II). One-electron reversible oxidations were found for all the compounds studied. The electrochemical reductions show 2–3 reduction waves in the potential range studied. Only for the complexes of mixed diimine ligands or 2,2′-bipyridine, a pre-adsorption wave is also observed. It is possible to stabilize low valence states with all ligands studied. A formal iron(I) state is described for the first time for all aliphatic diimine complexes, thus showing that the acceptor properties of the diimine complexes do not depend on the presence of the aromatic rings, but on the iron-diimine chromophore.

Electrochemistry of organometallic compounds. Part IV. Oxidations of benzylic derivatives and p-substituted benzylic derivatives of bis(dimethylglyoximato) and bis(salicylaldehyde)o-phenylenediimine cobalt(III) in dimethylformamide

The electrochemical oxidation of some p-substituted benzylic derivatives of Co(III) dimethylglyoximato and Co(III)bis(salicylaldehyde)o-phenylenediimine, in dimethylformamide, 0.2 M in tetraethylammonium perchlorate, on a platinum electrode, at several temperatures, is described as an ECE type, the first electrochemical step being a quasi-reversible one-electron charge transfer at room temperature. At temperatures around −20°C, or lower, the influence of the irreversible chemical decomposition of the oxidized species, via a solvent or other nucleophilic-assisted reaction, is negligible. It is suggested that at low temperatures the oxidation to the formally CoIV-R species is followed by an isomerization reaction in which this complex is partially transformed in a CoIII-(R) species or a π-complex, which undergoes an electroreduction at less positive potentials than those corresponding to the reduction of the CoIV-R species.

Low valence states of iron complexes of aliphatic and mixed aliphatic-aromatic diimine ligands

Abstract Electrochemical reductions of iron(II) diimine complexes [FeL 3 2+ , where L = CH 3 NC(R)C(R′) NCH 3 , aliphatic diimine series with R,R′ = H,H; H,CH 3 and CH 3 ,CH 3 and L = C 5 H 4 NC(R)N(R′), mixed diimine series with R,R′ = H,CH 3 and CH 3 ] were investigated through polarography and cyclic voltammetry in acetonitrile, with tetraethylammonium perchlorate supporting electrolyte (0.2 M) as a function of temperature. In the 0 to −2.4 V vs. Ag/AgCl potential range two to four polarographic waves were observed for the aliphatic series. The first two waves can be described as one-electron reversible reduction processes. They indicate that low valence states iron(I) and iron(0) are stabilized in acetonitrile. In the mixed ligand series three one-electron reversible reduction waves were observed, indicating that in addition to the low valence states stabilized in the aliphatic diimine series the formal reduction state Fe(–I) is also stabilized. The stabilization of the low oxidation states is due to the electron acceptor properties of the diimine ligands, inherent to the presence of the chromophoric iron diimine group. The half-wave potential data and the stabilization of the low valence states point to the importance of analyzing both σ and π effects. The molecular electronegativity values for the series of iron diimine complexes investigated evidences a synergistic interaction between the metal-ligand σ and π bonds. Diffusion coefficients, temperature effects on the heterogeneous electron transfer step, and electrocapillary curves were obtained for these complexes. No evidence for adsorption of the complexes on mercury electrodes was found for the one-electron reversible steps. When comparing polarographic data with those obtained on platinum disk working electrodes employed in the cyclic voltammetric experiments, we observed that for the symmetric aliphatic diimine ligands the observed cathodic currents are larger than expected on the basis of the previously calculated diffusion coefficients. In addition, the reduction waves are shifted 0.14 V to more negative potentials. The symmetric aliphatic diimine complexes exhibit adsorption of the electroactive species on the surface of the platinum electrodes in this potential range.

Ligand oxidation in iron diimine complexes. V. Kinetics of the oxidation of tris[2-pyridinal-α-methyl-(methylimine)] iron(II) by cerium(IV)

Abstract At low acid concentration, e.g., M H 2 SO 4 , the cerium(IV) oxidation of the iron(II) complex of the mixed-diimine ligand 2-pyridinal-α-methyl-(methylimine), PMM = C 5 H 4 NC(CH 3 )NCH 3 , is a very complex reaction in which ligand-oxidized iron(II) and iron(III) complexes are formed and partly dissociated, with a total consumption of 10–11 equivalents of Ce(IV) per mole of Fe(PMM) 2+ 3 . The major oxidation product in ∼50% yield is Fe(PO) 3+ 3 , where PO = C 5 H 4 NC(CHO)NCH 3 . The oxisation product has the α-methyl group oxidized to an aldehyde group. At the beginning of the reaction ( 2 Po 2+ was identified as the major reaction product: Ce(IV) + Fe(PMM) 2+ 3 = Ce(III) + Fe(PMM) 3+ 3 → 0.75 Fe- (PMM) 2+ 3 + 0.25 Fe(PMM) 2 PO 2+ . At this stage the kinetics of the reaction was analyzed by two potentiometric techniques. From experiments of addition of Fe(PMM) 3+ 3 , prepared in 5 M H 2 SO 4 , to Fe(PMM) 2+ 3 , to a final 1 M H 2 SO 4 concentration, the variation of the potential of the Fe(PMM) 3+ 3 /Fe(PMM) 2+ 3 couple is recorded with time. This variation is associated with the rate of disproportionation of Fe(PMM) 3+ 3 yielding Fe(PMM) 2 PO 2+ and the original iron(II) complex. the rate law from these experiments is -d[Fe(PMM) 3+ 3 ]/dt = k[Fe(PMM) 3+ 3 ] 1.5 . From automatic titration experiments, in which CE(IV) is added to Fe(PMM) 2+ 3 solutions at constant rate (ϱ), at high Fe(PMM) 3+ 3 concentration, the rate law is ϱ = k′[Fe(PMM) 3+ 3 ]. A mechanism, similar to that proposed for the oxidation of the aliphatic diimine complexes, is proposed. The first step is the formation of Fe(PMM) 3+ 3 and Ce(III) with an equilibrium constant of 4.4 × 10 7 . The iron(III) complex undergoes an intramolecular reduction to the iron(II) state with concomitant radical formation at the ligand, in a step assisted by solvent water and retarded by acid. Further oxidation of the iron(II) complex-ligand-radical by Fe(PMM) 3+ 3 to the final production of Fe(PMM) 2 PO 2+ leads to the derivation on the following rate law, (ϱ) - (d[Fe(PMM) 3+ 3 ]/dt) = (3k 3 K 7 [Fe(PMM) 3+ 3 ] 2 )/ (k 4 + k 7 [Fe(PMM) 3+ 3 ]), which at high [Fe(PMM) 3+ 3 ] (automatic titrations) reduces to a first order reaction [Fe(PMM) 3+ 3 ] with k 3 = 0.02 s -1 . Analysis of the potential variation experiments with time (low [Fe(PMM) 3+ 3 ]) with the whole kinetic expression above leads to k 3 = 0.02 s 1- .

Substituent effects on iron diimine complexes II. Correlations with thermodynamic and spectral properties in acetonitrile

Abstract The electrochemical oxidation of nineteen iron(II) complexes, FeL 2+ 3 , with aliphatic diimine ligands, L = H 3 CNC(R)C(R′)NCH 3 , where R, R′ = H, H; H, CH 3 ; CH 3 , CH 3 ; CH 3 , C 2 H 5 ; CH 2 CH 2 CH 2 CH 2 ; CH 2 CH(CH 3 )CH 2 CH 2 ; H, C 6 H 5 ; CH 3 , C 6 H 5 , and mixed diimine ligands, L = C 5 H 4 NC(R 1 )N(R 2 ), where R 1 , R 2 = H, CH 3 ; H, C 2 H 5 ; CH 3 , CH 3 ; CH 3 , C 2 H 5 ; CH 3 , C 6 H 5 ; CH 3 , m - or p -NH 2 C 6 H 4 ; C 6 H 5 ; CH 3 , m - or p -NH 2 C 6 H 4 ; C 6 H 5 , C 6 H 5 ; C 6 H 5 , m - or p -NH 2 C 6 H 4 was studied by means of cyclic voltammetry in acetonitrile, 0.2 M tetra-ethylammonium perchlorate at 25.0 °C. Except for compounds R, R′ = H, C 6 H 5 ; CH 3 , C 6 H 5 , and for the compounds with free amino groups, reversible one-electron oxidation processes were found. For the amino containing compounds the electrochemical oxidation is more complex due to concurrent oxidation of the amino group. For the phenyl derivatives of the aliphatic series, chemical and electrochemical steps follow the primary reversible one-electron reaction. For the compounds which exhibit reversible electrochemical oxidation, the half-wave potentials are identical to the standard electrode potentials. These potentials can be correlated with the sum of the polar Taft parameters for the substituents. Substituents presenting large steric or mesomeric effects do not fit this correlation, since these effects are excluded from the polar Taft parameters. A better correlation can be obtained by including steric effects. The frequencies of the intense absorption band in the visible region, assigned to an inverse charge transfer, are correlated with the half-wave potentials of the reversible oxidation processes. A correlation between the square-foot of the molar absorpivities at the absorbance maximum of the inverse charge transfer band with the sum of the polar Taft parameters of the substituents is also obtained.