Jannie C. Swarts

One-Electron Oxidation of Ruthenocene : Reactions of the Ruthenocenium Ion in Gentle Electrolyte Media

The electrochemical oxidation of ruthenocene, RuCp(2) (Cp = eta(5)-C(5)H(5)), 1, has been studied in dichloromethane using a supporting electrolyte containing either the [B(C(6)F(5))(4)](-) (TFAB) or the [B(C(6)H(3)(CF(3))(2))(4)](-) (BArF(24)) counteranion. A quasi-Nernstian process was observed in both cases, with E(1/2) values of 0.41 and 0.57 V vs FeCp(2) in the respective electrolyte media. The ruthenocenium ion 1(+) equilibrates with a metal-metal bonded dimer [Ru(2)Cp(4)](2+), 2(2+), that is increasingly preferred at low temperatures. Dimerization equilibrium constants determined by digital simulation of cyclic voltammetry (CV) curves were in the range of 10(2)-10(4) M(-1) at temperatures of 256 to 298 K. Near room temperature, and particularly when BArF(24) is the counteranion, the dinuclear species [Ru(2)Cp(2)(sigma:eta(5)-C(5)H(4))(2)] (2+), 3(2+), in which each metal is sigma-bonded to a cyclopentadienyl ring, was the preferred electrolytic oxidation product. Cathodic reduction of 3(2+) regenerated ruthenocene. The two dinuclear products, 2(2+) and 3(2+), were characterized by (1)H NMR spectroscopy on anodically electrolyzed solutions of 1 at low temperatures in CD(2)Cl(2)/[NBu(4)][BArF(24)]. The variable temperature NMR behavior of these solutions showed that 3(2+) and 2(2+) take part in a thermal equilibrium, the latter being dominant at the lowest temperatures. Ruthenocene hydride, [1-H](+), was also identified as being present in the electrolysis solutions. The oxidation of ruthenocene is shown to be an inherent one-electron process, giving a ruthenocenium ion which is highly susceptible to reactions that allow it to regain an 18-electron configuration. In a dry non-donor solvent, and in the absence of nucleophiles, this electronic configuration is attained by self-reactions involving formation of Ru-Ru or Ru-C bonds. The present data offer a mechanistic explanation for the previously described results on the chemical oxidation of osmocene (Droege, M.W.; Harman, W.D.; Taube, H. Inorg. Chem. 1987, 26, 1309) and are relevant to the manner in which sigma:eta(5)-C(5)H(4)-complexes of other second and third-row metals are formed.

Synthesis and characterisation of ferrocene-containing β-diketonato complexes of rhodium(I) and rhodium(III)

Abstract The synthesis of new β-diketonato rhodium(I) complexes of the type [Rh(FcCOCHCOR)(CO) 2 ] and [Rh(FcCOCHCOR)(CO)(PPh 3 )] with Fc=ferrocenyl and R=Fc, C 6 H 5 , CH 3 and CF 3 are described. 1 H, 13 C and 31 P NMR data showed that for each of the non-symmetric β-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, K c , which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. Δ r G , Δ r H and Δ r S values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and K c exists. The relationship between RhP bond lengths, d(RhP), and 31 P NMR peak positions as well as coupling constants 1 J ( 31 P 103 Rh) has been quantified to allow calculation of approximate d(RhP) values. Variations in d(RhP) for [Rh(RCOCHCOR′)(CO)(PPh 3 )] complexes have also been related to the group electronegativities (Gordy scale) of the terminal β-diketonato R groups trans to PPh 3 . A measure of the electron density on the rhodium centre of [Rh(RCOCHCOR′)(CO)(PPh 3 )] may be expressed in terms of the IR carbonyl stretching wave number, ν (CO), the sum of the group electronegativities of the R and R′ groups, ( χ R + χ R′ ), or the observed p K a ′ values of the free β-diketones RCOCH 2 COR ′ . An empirical relationship between ν (CO) and either p K a ′ or ( χ R + χ R′ ) has also been quantified.

Electron density manipulation in rhodium(I) phosphine complexes: structure of acetylacetonatocarbonylferrocenyl- diphenylphosphinerhodium(I)

Abstract The title complex [Rh(acac)(CO)(PPh2Fc)] (acac = acetylacetonato, PPh2Fc = ferrocenyldiphenylphosphine), was obtained in acetone by reaction of the tertiary phosphine with [Rh(acac)(CO)2] and was characterised by IR spectroscopy, 1H and 31P NMR and single crystal X-ray crystallography. This is a rare example illustrating the incorporation of the ferrocenyl moiety in a monodentate tertiary phosphine complex of rhodium(I). It crystallises in the triclinic space group P 1 with two independent molecules in the asymmetric unit and refined to R=2.40% from 5215 reflections. The independent molecules, showing the usual square planar geometry, differ mainly in the rotation mode of the asymmetric phosphine around the Rh-P bond and a slight deviation in the Rh-CO bond from linearity, which was correlated with IR data.

Structural, thermodynamic and kinetic consequences of a spectroscopic study of the equilibrium between isomeric forms of ferrocene-containing β-diketones

Abstract Proton NMR showed that, at equilibrium and at 25°C, asymmetric enolization in the direction furthest from the ferrocenyl group is dominant for the β-diketones 1-ferrocenyl-4,4,4-trifluoro-1,3-butanedione (ferrocenoyltrifluoroacetone, Hfctfa), 1-ferrocenyl-4,4,4-trichloro-1,3-butanedione (ferrocenoyltrichloroacetone, Hfctca), 1-ferrocenyl-1,3-butanedione (ferrocenoylacetone, Hfca), 1,3-diferrocenyl-1,3-propanedione (diferrocenoylmethane, Hdfcm) and 1-ferrocenyl-3-phenyl-1,3-propanedione (benzoylferrocenoylmethane, Hbfcm). This finding is considered to be the result of resonance driving forces rather than inductive electronic effects of substituents on the pseudo-aromatic β-diketone core. Lowering of the concentration of the β-diketones from 35 to below 1 mmol dm−3 only slightly shifts the equilibrium more towards the keto side. By increasing the temperature of the solvent (CDCl3) from 20 to 60°C, the percentage keto isomer at equilibrium of Hdfcm (32.9–34.2%), Hfca (22.5–28.4%), Hbfcm (8.8–11.9%) and Hfctca (4.9–9.8%) increased but it decreased the keto percentage of Hfctfa from 3.2 to below 0.7%. Slow conversion kinetics from the keto to the dominant enol isomer explains why, directly after isolation of newly prepared compounds, higher percentages of the keto isomer are observed.

Comparison of the lipid properties of captive, healthy wild, and pansteatitis-affected wild Nile crocodiles (Crocodylus niloticus).

The results presented describe and compare the fatty acid composition and melting properties of captive, healthy wild, and pansteatitis-affected wild crocodiles (Crocodylus niloticus). Differences in fatty acid composition between intramuscular and adipose fat is noted in captive crocodiles, and the latter differs from wild crocodiles as a result of different diets. Adipose fat of healthy wild crocodiles differs minimally from diseased ones, respectively with 37.3+/-2.6% vs. 43.2+/-2.3% monounsaturated fatty acids, and 43.2+/-2.9% in dead crocodiles, while polyunsaturated fatty acids decrease from 27.3+/-1.9% to as low as 21.9+/-3.6% respectively. Of the unsaturated fatty acids 18:2n-6 decreased from 6.5+/-2.6% in unaffected crocodiles to 3.5+/-0.6% in highly affected and 3.2+/-0.4% in dead crocodiles, and 22:5n-3 from 2.8+/-0.6% to 1.8+/-0.3% and 2.2+/-0.3% respectively. The melting properties as determined by differential scanning calorimetry show that extracted adipose fat is a small degree softer in pansteatitis-affected tissue, specifically in the temperature range 7-36 degrees C, and does not contribute to the hard texture noted for adipose fat tissue of pansteatitis-affected animals. A high moisture content of 51.0+/-19.7% of the fat tissue of pansteatitis-affected animals vs.17.1+/-8.0% of healthy ones, suggests that physiological changes due to interstitial inflammation may contribute to the hard texture.

Structure and electrochemistry of a tetrakis(ferrocenecarboxylato)diruthenium(II,III) diadduct: evidence for ferrocenyl–ferrocenyl communication

Abstract 1-Propanol and ethanol adducts of tetrakis(ferrocenecarboxylato)diruthenium(II,III) hexafluorophosphate have been synthesized and the 1-propanol diadduct has been characterized by X-ray crystallography. The cyclopentadienyl rings of all four ferrocenyl groups showed on average a 13° deviation from the eclipsed conformation. Cyclic voltammetry measurements in 1,2-dichloroethane show an irreversible RuIIIRuII/RuIIRuII reduction and four partially superimposed one-electron ferrocenyl-centred electron transfer processes. The existence of ferrocenyl-based mixed-valent intermediate states posessing FeIII and FeII sites are implied by the splitting of the ferrocenyl wave into two observable cathodic and anodic peaks. The first cathodic wave shows typical desorption behaviour. Osteryoung Square Wave Voltammetry (OSWV) supports the above findings.

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1-6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C˙ at formal reduction potentials <-1.7 V vs. FcH/FcH(+). A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent Cr=C species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the Cr=C centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

Isolation and physicochemical characterisation of starch from cocoyam (Colocasia esculenta) grown in Malawi

BACKGROUND The aim of this study was to determine the physicochemical properties of starches isolated from Malawian cocoyams and compare them with those of cassava and corn starches. RESULTS The purity of the isolated starches varied from 851 to 947 g kg(-1) and pH from 4.93 to 6.95. Moisture, ash, protein, fat and amylose contents ranged from 104 to 132, 0.3 to 1.5, 3.5 to 8.4, 0.9 to 1.6, and 111 to 237 g kg(-1), respectively. Cocoyam starches gave higher potassium and phosphorus but lower calcium levels than the other starches. The shape of starch granules varied from spherical to polygonal with cocoyam starches displaying smaller-sized granules than cassava and corn starches. Cocoyam starches gave a higher wavelength of maximum iodine absorption and blue value but lower reducing capacity values than cassava and corn starches. The extent of acid hydrolysis of the starches also differed. Cocoyam starches exhibited amylopectin molecules of higher molecular weights but amylose molecules of lower molecular weights than cassava and corn starches. Cocoyam starches exhibited lower water absorption capacity and swelling power, paste clarity and viscosity but higher solubility, gelatinisation temperatures and retrogradation tendencies than cassava and corn starches. CONCLUSIONS The physicochemical properties of native Malawian cocoyam starches vary among the different accessions and differ from those of cassava and corn starches.

Kinetic studies on the reduction of the tyrosyl radical of the R2 subunit of E. coli ribonucleotide reductase

Kinetic studies at 25 degrees C, I = 0.100 M (NaCl), on the reduction of the tyrosyl radical of the R2 protein of E. coli ribonucleotide reductase with hydroxyurea (HU), N-methylhydroxylamine, catechol, and seven hydroxamic acid derivatives are reported. There are no pH-dependences in the range 6.2-8.6 investigated except that introduced with N-methylhydroxylamine which itself protonates in this range. At pH 7.6 the rate constant (0.46 M-1 s-1) for the HU reaction is in agreement with earlier values. Slower reactions are observed with the bulkier acetohydroxamic (0.020 M-1 s-1) and benzohydroxamic acids (0.040 M-1 s-1). In the case of N-methylhydroxylamine the rate constant (0.41 M-1 s-1 at pH 7.6) decreases with pH, and it is concluded that the protonated form CH3NH2+OH(pKa = 6.2) has little or no reactivity with Tyr. For this reaction under air-free conditions a second-stage (0.027 M-1 s-1) corresponding to reduction of Fe(III)2 is observed. Mid-point redox potentials for the reductants and estimates of reduction potentials applying in the case of the protein are considered. The reactions with 1,2-dihydroxybenzene (catechol) and 3,4-dihydroxybenzohydroxamic acid (Didox) also have two stages, when the initial Tyr reduction, rate constants/M-1 s-1 for catechol (3.2) and Didox (0.010), is followed by removal of the Fe(III) to give catechol and catechol like Fe(III)-complexed products. The single stage reactions of the hydroxamic acid derivatives which incorporate charged amino-acid groups L-glutamic acid, L-histidine, L-glycine and L-lysine, are slow, and saturation kinetics are observed consistent with association (small K values) prior to redox. The mechanism of reduction of R2-Tyr by all of the reagents studied is discussed.

Intramolecular communication and electrochemical observation of the 17-electron ruthenocenium cation in fluorinated ruthenocene-containing β-diketones; polymorphism of C10H21 and C10F21 derivatives

Ruthenocene-containing β-diketones, RcCOCH2COR with Rc = ruthenocenyl and R = C10F21 (1), CF3 (2), C6F5 (3), C10H21 (4), CH3 (5) and C6H5 (6), were synthesised by Claisen condensation of the appropriate methyl ester with acetylruthenocene, and their spectroscopic, electrochemical and thermal properties were compared. A new synthetic route utilising 1,2,3-benzotriazol-1-ylethanone (9) or 1,2,3-benzotriazol-1-yl(ruthenocenyl)methanone (10) as a reactant, rather than the conventional esters, was found to be more efficient for β-diketone synthesis. The apparent acid dissociation constants, pKa′, of the new ruthenocene-containing β-diketones are 7.14(4) (1, R = C10F21), 9.92(3) (3, R = C6F5) and 10.06(2) (4, R = C10H21). Peak anodic potentials of the ruthenocenyl group of 1–6, pKa′ values and the FTIR ν(CO) stretching frequencies of the precursor esters, RCOOCH3, correlated linearly with the Gordy scale group electronegativity, χR, of the C10F21 (χC10F21 = 3.04), C6F5 (χC6F5 = 2.46), C10H21 (χC10H21 = 2.43) and other R-groups. An electrochemical study in the non-interacting solvent and electrolyte system CH2Cl2/0.1 mol dm−3 [N(nBu4)][B(C6F5)4] revealed electrochemically irreversible one-electron transfer Rc/Rc+ couples in the potential range 650 90% abundance under the conditions of study and the first order rate constant of enol to keto conversion varied between 220 and 50 000 s−1 depending on solvent (CDCl3 or CD3CN) and R-groups. Thermal analysis (DSC) of 1 and 4 showed no liquid crystalline mesophase behaviour but definite polymorphism was observed. β-diketones 1 and 4 exist as low temperature polymorphs below 42 °C or 12 °C respectively. The high temperature polymorphs converted to isotropic liquids at 83 °C (compound 1) or 52 °C (compound 4).