Anne K. Powell

Dissimilatory iron(III) reduction by Rhodobacter capsulatus

The photosynthetic proteobacterium Rhodobacter capsulatus was shown to be capable of dissimilatory Fe(III) reduction. Activity was expressed during anaerobic phototrophic and microaerobic growth with malate as the carbon source, but not during equivalent aerobic growth. A variety of Fe(III) complexes were demonstrated to act as substrates for intact cells and membrane fractions of strain N22DNAR+ using a ferrozine assay for Fe(II) formation. Rates of reduction appeared to be influenced by the reduction potentials of the Fe(III) complexes. However, Fe(III) complexed by citrate, which is readily reduced by Shewanella putrefaciens, was a poor substrate for dissimilation by R. capsulatus. The Fe(III)-reducing activity of R. capsulatus was located solely in the membrane fraction. The reduction of Fe(III) complexes by intact cells was inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), suggesting the involvement of ubiquinol: cytochrome c oxidoreductases in the electron transport chain. Lack of sensitivity to myxothiazol plus data from mutant strains implies that the cytochrome bc 1 complex and cytochrome c 2 are not obligatory for dissimilation of Fe(III)(maltol)3. Alternative pathways of electron transfer to Fe(III) must hence operate in R. capsulatus. Using strain N22DNAR+, the reduction rate of Fe(III) complexed by nitrilotriacetic acid (NTA) was elevated compared to that of Fe(III)(maltol)3, and moreover was sensitive to myxothiazol. However, these differences were not observed in the absence of the electron donor malate. The governing factor for the reduction rate of Fe(III)(maltol)3 thus appears to be the limited Fe(III)-reducing activity, whilst the reduction rate of Fe(III) complexed by NTA is controlled by the flux of electrons through the respiratory chain. The use of mutant strains confirmed that the role of the cytochrome bc 1 complex in Fe(III) reduction becomes apparent only with the superior substrate. The energy-conserving nature of Fe(III) reduction by R. capsulatus was demonstrated by electrochromic measurements, with the endogenous carotenoid pigments being employed as indicators of membrane potential generation in intact cells. Using Fe(III)EDTA as electron acceptor, periods of membrane potential generation were directly proportional to the quantity of complex added, and were extended in the presence of HQNO. Fe(III)-dependent carotenoid bandshifts were abolished by addition of the protonophoric uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone.

The influence of chelating agents upon the dissimilatory reduction of Fe(III) by Shewanella putrefaciens

The ability of S. putrefaciens to reduce Fe(III) complexed by a variety of ligands has been investigated. All of the ligands tested caused the cation to be more susceptible to reduction by harvested whole cells than when uncomplexed, although some complexes were more readily reduced than others. Monitoring rates of reduction by a ferrozine assay for Fe(II) formation proved inadequate using Fe(III) ligands giving Fe(II) complexes of low kinetic lability (e.g. EDTA). A more suitable assay for Fe(III) reduction in the presence of such ligands proved to be the observation of associated cytochrome oxidation and re-reduction. Where possible, an assay for Fe(III) reduction based upon the disappearance of Fe(III) complex was also employed. Reduction of all Fe(III) complexes tested was totally inhibited by the presence of O2, partially inhibited by HQNO and slower in the absence of a physiological electron donor. Upon cell fractionation, Fe(III) reductase activity was detected exclusively in the membranes. Using different physiological electron donors in assays on membranes, relative reduction rates of Fe(III) complexes complemented the data from whole cells. The differences in susceptibility to reduction of the various complexes are discussed, as is evidence for the respiratory nature of the reduction.

Crystal and molecular structures of tetracarbonyl(3,3′-dimethyl-2,2′-bipyridine)-chromium(0) and -molybdenum(0) benzene solvates and the unsolvated tungsten(0) analogue

Abstract The crystal structures of the complexes of 3,3-dimethyl- 2,2′-bipyridine (L), cis-[W(CO)4L], cis-[MO(CO)4L] · C4H6 and cis-[Cr(CO)4L] · C6H6 have been determined by X-ray diffraction. These structures show that repulsion between the methyl groups is principally responsible for the geometric distortion in each complex. Within the ligand, L, the two pyridine rings suffer distortion, in opposite directions, to pseudo-boat conformations so as to optimize metal—nitrogen binding. The benzene solvent molecules are arranged in a layer structure alternating with units of the metal complex. The metal complex molecule is beyond van der Waals contact with the benzene solvent molecule in each case.

Crystal and molecular structure of a new µ-oxo-bridged iron(III) dimer formed with the nitrilotriacetate ligand

The molecular structure of the oxo-bridged dimer barium µ-oxo-bis{aqua[nitrilo-κN-triacetato(3–)-κ3O,O′,O″]ferrate(1–)}–water(1/4), Ba[{Fe(nta)(H2O)}2O]·4H2O, which has an Fe–O–Fe angle of 153.2(6)° has been determined by X-ray crystallography. The iron ions are six-co-ordinate with one tetradentate nta ligand and one water molecule completing the co-ordination sphere. The relationship of this compound to other iron–nta complexes is discussed in the context of the hydrolysis reactions of iron(III) in aqueous solutions.

X-Ray crystal structure of a mesogenic octa-substituted phthalocyanine

Crystals of 1,4,8,11,15,18,22,25-octahexylphthalocyanine are composed of tilted stacks of molecules with an unexpectedly large separation between the aromatic cores.

Crystal structure and isomerism of a tumour targeting radiopharmaceutical: [ReO(dmsa)2]–(H2dmsa =meso-2,3-dimercaptosuccinic acid)

A crystal structure determination shows that one isomer of the complex anion [ReO(dmsa)2]–(H2dmsa =meso-2,3-dimercaptosuccinic acid), a compound which in radiopharmaceutical form has been shown to localise in certain human tumours, has a square-pyramidal structure with all carboxylic acid groups uncoordinated and oriented endo relative to the oxo ligand.

Terminal water ligand exchange and substitution by isonicotinamide on the oxo-centred triruthenium(III) complex [Ru3(μ3-O)(μ-CH3CO2)6(OH2)3]+. Crystal structure of [Ru3(μ3-O)(μ-CH3CO2)6(OH2)3]ClO4·HClO4·H2O

Abstract A study or water exchange and substitution by isonicotinamide on the triruthenium carboxylate complex [Ru 3 (μ 3 -O)(μ-CH 3 CO 2 ) 6 (OH 2 ) 3 ] + using 17 O NMR and UVVis spectrophotometry has been carried out in aqueous perchlorate media, I =1.0 M. The present results represent the first such kinetic studies carried out on this complex in aqueous media and provide support for a general I d mechanism for substitution/exchange reactions at the terminal L ligands in [Ru 3 (μ 3 -O)(μ-CH 3 CO 2 ) 6 L 3 ] + complexes. The X-ray crystal structure reported for [Ru 3 (μ 3 -O)(μ-CH 3 CO 2 ) 6 (OH 2 ) 3 ]ClO 4 ·HClo 4 ·H 2 O is the first such structure on a triruthenium(III) carboxylate complex and provides evidence of a significant elongation in the RuO(OH 2 ) bond length (av. 2.091 A) when compared to that, for example, in [Ru(OH 2 ) 6 ] 3+ (2.029 A). It is concluded that a significant trans labilisation effect from the planar μ 3 -oxo ligand is relevant for [Ru 3 (μ3-O)(μ-CH 3 CO 2 ) 6 (OH 2 ) 3 ] + and responsible for a water exchange rate constant (25 °C) some 10 3 × larger than that on the hexaaqua ion.

Phosphorus donor chemistry of [W(CO)(Ph2PCCPPh2)(S2CNEt2)2]

Reactions of [W(CO)(Ph2PCCPPh2)(S2CNEt2)2] with [M(CO)4(pip)2](M = Cr, Mo or W; pip = piperidine) afford [W(CO){(Ph2PCCPPh2)M(CO)4}(S2CNEt2)2]. The molecular structures of all three compounds have been determined by X-ray diffraction studies and found to be isostructural. In all three cases the acetylenic bond of Ph2PCCPPh2 is co-ordinated to the tungsten(II) centre, which is also ligated by carbonyl and dithiocarbamate ligands, while both phosphorus atoms are co-ordinated to the metal tetracarbonyl fragment to form a five-membered chelate ring. The 13C-{1H} NMR data are discussed in relation to the molecular structures, and the barriers to alkyne rotation in these complexes have been established by variable-temperature 31P-{1H} NMR spectroscopy. Reactions of [W(CO)(Ph2PCCPPh2)(S2CNEt2)2] with [Co2(CO)8] and [Fe2(CO)9] afford [W(CO){(Ph2PCCPPh2)Co2(µ-CO)2(CO)4}(S2CNEt2)2] and [W(CO)(x)4Fe(Ph2PCCPPh2)Fe(CO)4}(S2CNEt2)2] respectively, which have been characterised by IR, NMR and mass spectroscopy.

Synthesis and coordination chemistry of 1-phenyl-1-phospha-4,7-dithiacyclononane

Abstract Reaction of PhP(CH2CH2SH)2 with 1,2-dichloroethane and Cs2CO3 under high dilution conditions affords 1-phenyl-1-phospha-4,7-dithiacyclononane (L) as an oil in 52% yield. The geometry of L has been probed by 1H NMR spectroscopy, which indicates an equilibrium mixture containing gauche S–C–C–S and S–C–C–P segments, and molecular mechanics calculations which find a lowest energy endodentate conformation minimum and also energetically accessible exodentate minima. Endo- and exodentate conformations are observed in the crystal structure of [Cu(η1-L)(η3-L)][PF6] (1) formed by the reaction of L with [Cu(NCMe)4][PF6]. Reaction of Hg(ClO4)2 with L affords [HgL2][ClO4]2 (2) whose structure has also been established by X-ray diffraction. The mercury ion is in a very distorted octahedral environment with two short Hg–P bonds (average 2.404 A) and four long Hg–S bonds (average 3.092 A). Reaction of L with AgBF4 or Ni(BF4)2 or CoBr2 or Fe(BF4)2 affords [AgL2][BF4] (3), [NiL2][BF4]2 (4), [CoL2]Br2 (5) and [FeL2][BF4]2 (6), respectively, which were characterised by FAB mass spectroscopy and elemental analysis. Cyclic voltammetry studies were used to compare the redox properties of 4, 5 and 6 with their trithiacyclononane (9S3) analogues and in compound 4 the ligand L was found to stabilise Ni in the oxidation states 0, I, II and III. UV–Vis spectroscopic measurements show that L is a stronger field ligand than 9S3. EXAFS studies were also performed on 4, 5 and 6, which established that all three complexes are six-coordinate.

Oxidation of 1,10-phenanthroline by tetraoxomanganate(VI) and (VII). Preparation, structure and properties of 1H-cyclopenta[2,1-b:3,4-b′]dipyridine-2,5-dione

Oxidation of 1,10-phenanthroline with tetraoxomanganate(VI) gave good yields of ketone 3 and the previously unknown dione 5, formed by the unusual further oxidation of 3 at the 2 position of a pyridine ring. In contrast, use of the tetraoxomanganate(VII) gave the bipyridine diacid 2(69%), ketone 3(20%) and only a trace of the dione 5. The X-ray crystal structure of the anion of 5 indicates that the negative charge is located mainly on the 2-O rather than 5-O atom, with some delocalisation into the pyridine ring.