On the electrochemical reduction of some Cr(III) complexes inDMSO investigated by cyclic voltammetry

The compounds investigated were: [Cr(en)₃]³⁺, [Cr(ur)₆]³⁺, [Cr(DMSO)₆]³⁺, [Cr(dien)₂]³⁺, [Cr(en)₂(acac)]²⁺, [Cr(en) (acac)₂]⁺ and Cr(acac)₃.A distinctly different behaviour is caused by the introduction of one or moreacac ligands into the molecule. The first step is much more cathodic and quite irreversible, while it is reversible or quasi reversible for the first group of ions. This is due to a -type interaction between theacac ligand and the central ion. This interaction is responsible for a third peak occurring for the second group of compounds and may be attributed to the reduction of the Cr(I) ion.Some correlations were found e.g. between the extinction of thed-d band of the first group of ions and the potential of the first peak, and the number ofacac groups introduced in the second group of compounds and the shift of the potential of the first peak.The determined electrochemical data are tabulated.

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Untersuchungen zur elektrochemischen Reduktion einiger Cr(III)-Komplexe in DMSO mittels zyklischer Voltammetrie

Zusammenfassung Die elektrochemischen Daten einer Reihe von Cr(III)-Komplexen miten-, ur-, DMSO-, dien- undacac-Liganden werden präsentiert und einige Möglichkeiten der Korrelation der physikalischen Eigenschaften der Komplexe werden aufgezeigt.Die erste Stufe bei der Einführung vonacac-Liganden ist auf Grund einer -Wechselwirkung zwischen Ligand und Zentralion irreversibel.

     

Photoredox reactions of Cr(III) mixed-ligand complexes

Irradiation of chromium(III) complexes with oxalate and pyridinedicarboxylate ligands (pda = 2,3-, 2,4-, or 2,5-dicarboxylate) leads to diverse behaviors, dictated by light energy, presence of oxygen and the ligand nature. Irradiation within the MC bands is unaffected by O₂ and results in ligand substitution. The LMCT excitation is effective only when oxalate is coordinated to Cr(III); then electron transfer from oxalate to central ion generates an intermediate, consisted of a Cr(II)species and the C₂O₄^? radicals. The species undergo fast redox reactions dependent on the presence of O₂ and the pda ligand.(1) In anoxic medium the fast outersphere electron transfer from Cr(II) to solvent, generates hydrated electrons and re-oxidizes the chromium centre to Cr^III. Then geminate recombination regenerates substrate, whereas competitive release of the C₂O₄^? radical leads to substitution of one oxalate ligand by two water molecules (aquation induced by the LMCT excitation). In the presence of the pda ligand the outersphere electron transfer is accompanied by the innersphere CT, generating Cr(III) coordinated to two radical ligands: C₂O₄^? and pda^3?; the intermediate releases also e_aq^?, but this reaction is slower than that of the homoleptic oxalate complex. Hydrated electrons are scavenged also by the released radicals. All these processes are completed within microseconds and in consequence, the Cr(III) complexes irradiated in deoxygenated solutions are insensitive to subsequent oxygenation.(2) When UV-irradiation is carried out in oxygenated medium reaction of Cr(II) species with molecular oxygen competes with the outer- or inner electron transfer observed in anoxic medium. Both these pathways result in generation of chromate(VI). Quantum yield of the Cr(VI) production is sensitive to the presence and structure of pda ligand, decreasing within the series 2,3-pda > 2,4-pda > 2,5-pda.     

Axial Ligand exchange in chiral macrocyclic ytterbium(III) complexes

We investigate the role of axial ligands on the near-IR-optical and paramagnetic NMR spectra of the complex [YbL](+3) where L is the stereodefined enantiopure chiral macrocycle (L = hexaazapentacyclo[25.3.1.1(12,24).0(4,9).0(19,24)]dotriaconta-1(31),2,10,12,14,16(32),17,25,27,29-decaene). The conformation in solution of the lanthanide complex is characterized by analyzing the pseudocontact 1H NMR shifts and is consistent with X-ray data of single crystal of analogue systems. The macrocycle is confined within a thin equatorial disk, leaving the cation open to at least two axial sites, on the opposite hemispheres. We recorded, assigned, and analyzed the 1H NMR spectra of several species upon changing the anion in solution, calculating the magnetic susceptibility anisotropy tensor for each. Near-IR circular dichroism is used to investigate the solution equilibria involving the competing ligands and to derive a spectroscopic series for Yb.     

The thermal decomposition of some Cr(III) complexes

The thermal decomposition reactions of the following chromium(III) complexes were investigated: Cr(CH₃COO)₃·2 H₂O, [C_r3O(CH₃COO)₆(H₂O)₃]Cl·2 H₂O and [Cr₃O(CH₂ClCOO)₆(H₂O)₃]Cl·6H₂O. Simultaneous TG/DTG/DTA were applied nonisothermal conditions. From the recorded curves, the activation energiesE _a were calculated for all the thermal decomposition steps. Appropriate chemical reactions were attributed to the thermal effects, with consideration to the X-ray diffraction and IR spectra results.

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Zusammenfassung Für die Untersuchung der thermischen Zersetzungsreaktion der Chrom(III)-Komplexe Cr(CH₃COO)₃·2H₂O [C_r3O(CH₃COO)₆·(H₂O)₃]Cl2H₂O und [C_r3O(CH₂ClCOO)₆·(H₂O)₃]C₁₆H₂O wurde simultane TG/DTG/DTA unter nichtisothermen Bedingungen eingesetzt. Ausgehend von den aufgezeichneten Kurven wurden für alle Schritte der Zersetzungsreaktion die E_a-Werte berechnet. In Übereinstimmung mit röntgenographischen und IR-spektroskopischen Ergebnissen wurden den thermischen Effekten passende chemische Reaktionen zugeordnet.

     

Ligand exchange and ligand migration reactions involving dicyclopentadienylniobium(III) carbonyl derivatives

The methylniobocene carbonyl (C₅H₅)₂Nb(CH₃)(CO) shows an unexpected lack of reactivity with respect to ligand migration. Whereas (C₅H₅)₂V(CH₃) has been reported to react with CO to yield the acetyl derivative (C₅H₅)₂V(OC-CH₃)(CO) immediately, we find that the niobocene analogue (C₅H₅)₂Nb(CH₃) reacts with CO only to regenerate (C₅H₅)₂Nb(CH₃)(CO), from which it was obtained by photolysis. This resistance of the methylniobocene carbonyl derivative towards ligand migration is interpreted in terms of the bonding properties of the acetyl intermediate (C₅H₅)₂Nb(OCCH₃).     

Redox reactions of V(III) and Cr(III)picolinate complexes in aqueous solutions

Reactions of e_aq⁻, H-atoms, OH, (CH₃)₂COH, and CO₂⁻ radicals with V(III)picolinate and Cr(III)picolinate have been studied by the pulse radiolysis technique. The spectra of V(II)picolinate, V(IV)picolinate, Cr(II)picolinate, OH adduct of Cr(III)picolinate and Cr(IV)picolinate have been obtained and the rate constants of the reactions of various radicals with V(III) and Cr(III)picolinate have been determined. The implications of these results to the chemical decontamination of nuclear reactor systems are discussed.     

The nephelauxetic effect and ortho- to para-positronium conversion reactions promoted by Cr(III) complexes

The ortho- (o-Ps) to para-positronium (p-Ps) conversion reactions promoted by ten Cr(III) complexes were investigated at several temperatures. The results obtained are discussed together with those of three other Cr(III) complexes measured previously.It was found that all the reactions studied are diffusion controlled and that their rate constants,k _SE, increased as the corresponding ratiosB/B _o between the Racah interelectronic repulsion parameters of Cr(III) complexes and Cr(III) gaseous ion decrease; that is,K _SE increases as 3d electron cloud expansion promoted by ligands, increases. Therefore the o-Ps to p-Ps conversion reactions can be considered a complementary probe for the spectroscopic method in investigating the ligand capabilities to cause 3d cloud expansion. The effect is called nephelauxetic which means expanding (electron) cloud.Finally an empirical average environmental rule, similar to that hypothesized for B/B_o ratios, is proposed for estimating the rate constants,K _SE, of complexes formed with mixed ligand sets.This work was supported by Ministero Universitá e Ricerca Scientifica e Tecnologica (M.U.R.S.T.) and by Consiglio Nazionale Ricerche (C.N.R.).     

Ligand exchange processes in some iron-sulphur-carbonyl and -nitrosyl complexes

The complex [Fe₂(SMe)₂(CO)₆] undergoes stepwise exchange with Et₂S₂ to yield successively [Fe₂(SMe)(SEt)(CO)₆] and [Fe₂(SEt)₂(CO)₆]. Carbonyl complexes [Fe₂(SR)₂(CO)₆] are efficiently converted to the nitrosyls [Fe₂(SR)₂(NO)₄] by the action either of NO gas or of methanolic sodium nitrite: the analogous species [Fe₂S₂(CO)₆], [Fe₂S₂(CO)₆]^2?, and [Fe₃S₂(CO)₉] all, with methanolic nitrite, yield [Fe₄S₃(NO)₇]^?. This anion, [Fe₄S₃(NO)₇]^?, reacts with sulphur to give the cubane-like [Fe₄S₄(NO)₄]: the synthesis of its selenium analogue, [Fe₄Se₃(NO)₇]^? is described. The complexes [Fe₂(SR)₂(NO)₄] (R = Me, Et, Prⁿ, Prⁱ, Bu^t, PhCH₂) all consist of two isomers in solution, presumed to have structures of C_2h and C_2v, symmetry: activation parameters for the C_2h?C_2v reaction are reported.     

Biotechnology of Cr(VI) transformation into Cr(III) complexes

The dose-dependent formation of Cr(III) complexes and uptake of chromium by Arthrobacter oxydans — a Gram-positive bacterium from contaminated Columbian basalt rocks (USA) — were studied along with the testing under aerobic conditions of two bacterial strains of Arthrobacter genera isolated from the polluted basalts from the Republic of Georgia. Instrumental neutron activation analysis (INAA) was used to track the accumulation of chromium in the bacterial cells. To monitor and identify Cr(III) complexes in these bacteria, electron spin resonance (ESR) spectrometry was employed.     

Determination of Cr(VI) in the presence of Cr(III) and humic acid by cathodic stripping voltammetry

A voltammetric procedure in the flow system for determination of traces of Cr(VI) in the presence of Cr(III) and humic acid is presented. The calibration graph is linear from 5×10⁻¹⁰ to 1×10⁻⁷ mol l⁻¹ for an accumulation time of 120 s. The R.S.D. for 1×10⁻⁸ mol l⁻¹ Cr(VI) is 5.3% (n=5). The detection limit estimated from 3σ for a low concentration of Cr(VI) and accumulation time of 120 s is 2×10⁻¹⁰ mol l⁻¹. The method can be used for Cr(VI) determination in the presence of up to 50 mg l⁻¹ of humic acid. The validation of the method was carried out by studying the recovery of Cr(VI) from spiked river water and by the comparison of the results of determination of Cr(VI) in a soil sample. The method cannot be used for analysis of samples containing high concentrations of chloride ions such as seawater and estuarine water.     

Ligand control on the reactions of oxygen- and sulphur-bonded (sulphito)pentaminecobalt(III) complexes in solution

The kinetics of formation, acid-catalysed aquation, ligand isomerisation (CoIIIOSO2+→CoIIISO3+), intramolecular electron-transfer, and base-catalysed hydrolysis and isomerisation of O-bonded sulphito complexes, cis-[Co(en)2(B)(OSO2–O)]+[B=benzimidazole (bzimH), N-methylimidazole (N-meim)] and the anation of cis-[Co(en)2(B)OH)]2+ [B=bzimH, N-meim and imH (imidazole)] by oxSO2−3 are reported. Steric acceleration is observed in the formation and acid-catalysed aquation of the O-sulphito complexes. The ligand isomerisation leads to loss of the monodentate amine with the formation of trans-[Co(en)2(SO3–S)2]− (in an excess of sulphite). Steric acceleration is more pronounced in the isomerisation and base hydrolysis than in the redox process. The results indicate cis labilisation of the coordinated O-sulphite. The [(tetraethylenepentamine)Co(OSO2–O)]+ cation undergoes base hydrolysis 103 times faster than the corresponding (en)2(B) complexes; base-catalysed ligand isomerisation for the former is not observed unlike in the latter. The anation of cis-[Co(en)2(B)OH]2+ (B=imH, bzimH, N-meim) by SO2−3 in a mild alkali pH range (pH=7.9–9.6) and in an excess of SO2−3, yields exclusively trans-[Co(en)2(SO3–S)2]− with no evidence for the formation of the cis-[Co(en)2(B)(SO3–S)]+ or its O-sulphito analogue. The intramolecularly generated amido conjugate base of the sulphite ion-pair, {cis-[Co(en)2(B)OH]2+,SO32−}.41cm{cis-[Co(en)(en-H) (B)- OH2]2+,SO32−}, is believed to generate a five-coordinate intermediate (TBP) that captures the S-end of SO2−3 selectively from a site trans-to the amine B so that the amine is labilised by the trans effect of the sulphite. The NH-deprotonated coordinated imidazolate or benzimidazolate species, cis-[Co(en)2(bzm/im)OH]+, do not undergo anation by SO2−3. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adduct formation of some tris(N,N dialkyldithiocarbamato)Cr(III) complexes with iodine

Spectrophotometric studies of dichloromethane solutions containing iodine and some Cr(S₂CNR₂)₃ complexes (where R₂NCS^?₂ are: OC₄H₈NCS^?₂ = modtc; (C₂H₅)₂NCS^?₂ = Et₂dtc; (C₆H₅CH₂)₂NCS^?₂ = Bz₂dtc) have shown the formation of 1:1 adducts whose spectral features were studied and which suggested that charge-transfer interaction involving iodine and a sulphur atom of a coordinated dithiocarbamato ligand occurred.A new BASIC non-linear least-squares program which performs an optimization of the absorbance data taken at several wavelengths was devised to evaluate reliable formation constants.The thermodynamic parameters have been calculated and compared with those obtained in the same conditions as above by a reinvestigation of the known 1:1 complex formation of iodine with the corresponding tetraalkylthiuram-disulphides, where the donor site is also the sulphur atom of the thiocarbonyl group. The ?ΔH° values related to the Cr(dtc)₃ · I₂ complexes are about 2 kcal lower than those of thiuram disulphides, showing a lowering of the donor strength of the thiocarbonyl sulphur as a consequence of the coordination to the metal.     

Study of inclusion complexes of cyclodextrin by cyclic voltammetry

The inclusion complexes of a series of organometallic compound-cyclodextrin and aromatic compound-cyclodextrin have been studied by cyclic voltammetry using glassy carbon electrode. The variations of peak potential and peak current are showed on cyclic voltammogram when the electroactive guest molecules are complexed by cyclodextrins. Dissociation constants of cyclodextrin inclusion complexes have been calculated on the basis of this variation by both potential and current methods. According to the magnitude of dissociation constants the relationship between the stability of cyclodextrin inclusion complex and the degree of matching host molecule with guest molecule has been discussed.     

Enantioselective glyoxylate-ene reactions catalysed by (salen)chromium(III) complexes

An enantioselective carbonyl-ene reaction of alkyl glyoxylates with various 1,1-disubstituted olefins, catalysed by chiral (salen)Cr(III)BF₄ complexes, has been studied. We found that a chromium complex bearing adamantyl substituents at the 3,3′-positions of the salicylidene moiety catalysed the reaction with much greater selectively than the classic Jacobsen-type catalyst. The reaction proceeded effectively under undemanding conditions in the presence of 2 mol % of the catalyst in an acceptable yield and with 59-92% ee.     

Thermal decomposition of dinitrogen complexes of Cr(III)

The thermal behaviour of two dinitrogen complexes of chromium with EDTA and CDTA have been studied using DTA and TG. IR spectroscopy is used for the characterization of intermediate products. The loss of dinitrogen occurs at 240°C in both cases.