Annette L. Nolan

X-Ray Structural Studies of K6[CoII W12O40]·~16H2O and K5[CoIII W12O40]·~16H2O and Structural Trends Along the [XW12O40]– Series, where X = PV, SiIV, CoIII and CoII

The X-ray crystal structures of K6[CoII W12O40]·~16H2O and K5[CoIIIW12O40]·~16H2O are reported. The compounds are isostructural, hexagonal, space group P 6222, and have Z 3 with a 19.118(7), c 12.383(9) A, V 3919.6(35) A3 , and a 19.111(2), c 12.509(2) A, V 3956.6(9) A3 , respectively. Both anions exhibit the standard α-Keggin structure, which consists of a central tetrahedrally coordinated CoII or CoIII , surrounded by four groups of three edge-sharing octahedra (W3O13 subunits) which are linked in turn to each other and to the central CoO4 tetrahedron by shared oxygen atoms at the vertices. Although thermogravimetric analyses show that there are c. 16 water molecules per formula unit in both compounds, only six and three water molecules for the CoII and CoIII compounds, respectively, could be crystallographically located. The others are presumably zeolitic and highly disordered in nature. Structural differences between the anions on replacement of CoII by CoIII , as well as comparisons with the related [PW12O40]3– and [SiW12O40]4– ions, which contain (formally) PV and Si IV , respectively, are discussed. This comparison indicates that the W3O13 subunits become progressively more isolated with increasing size of the central heteroatom from PV to CoII , while the bonding within an individual W3O13 subunit becomes weaker. Extended-HUckel molecular orbital calculations are used to examine stability changes in the polyoxotungstate framework, using the actual polyoxotungstate geometries of the anions, on variation in size of the central heteroatom. These are compared to that in the [H3W12O40]5– ion, which has two centrally located H+ ions, but no steric effects. The studies show that no major changes occur in the overall stability of the framework, but that there is a redistribution in the contributions of the two types of bridging oxygen atoms to the stability of the framework, which parallels the progressive isolation and weaker bonding in the W3O13 subunits.

Chemostat-controlled selective leaches of model soil phases – the hydrous manganese and iron oxides. Part 1

Selective leaching of synthetic Mn and Fe oxide phases were examined using a Chemostat to control the system temperature, pH and/or Eh. Reaction conditions for the selective dissolution of model phases of Mn oxide (birnessite, ‘MnO2’), amorphous Fe oxyhydroxide (ferrihydrite, Fe(OH)3.xH2O) and crystalline Fe oxide (hematite/goethite, Fe2O3/FeOOH) were optimized, concomitant with the behaviour of their dopant metals Cu, Pb and Zn. The pure oxide phases, as well as model compounds doped via coprecipitation with Cu, Pb and Zn, each to a concentration of 1000 ppm with respect to the mass of the oxide phase, were employed. Quantitative dissolutions of the target model phases were achieved within relatively short reaction times (within 15 min) for the purpose of high throughput field procedures. A limited study of three soil samples collected from a Pb–Zn–Ag mining province in northern Queensland, Australia was conducted employing the leach conditions found to provide optimal selectivity.

Octasodium hexatungstomanganate(IV) octadecahydrate

The title compound, Na8[MnW6O24]·18H2O, exhibits a structure with six WO6 octahedral edge-sharing units surrounding a central centrosymmetric MnO6 octahedron, with all metal atoms in a common plane. There are three unique Mn—O distances of 1.903 (7), 1.905 (7) and 1.941 (7) A [average 1.916 (17) A], while the average W—O distances are 2.17 (4), 1.938 (8) and 1.749 (12) A for three-, two- and one-coordinate O atoms, respectively.

Reaction kinetics and mechanism of formation of [H4Co2Mo10O38]6– by peroxomonosulfate oxidation of CoII in the presence of molybdate

Oxidation of CoII by HSO5– to CoIII in weakly acidic solution, in the presence of molybdate, resulted in the formation of (primarily) the soluble dicobalt species [H4Co2Mo10O38]6–. The kinetics of formation of this species was examined at 15–35 °C over the range pH 4.0–5.5 and found to exhibit three separate kinetically observable steps, oxidation of CoII to CoIII, followed by a ligand-breakdown reaction and a slow ligand-replacement step which produces the observed product(s). The first stage followed the expanded rate expression +d[CoIII]/dt=kox[Co2+][HSO5–]/[H+][HMoO4–]3. A value for kox of 7.05(4)× 10–14 mol3 dm–9 s–1 at 25 °C was calculated using reported formation constants describing the speciation of [MoO4]2–, [Mo7O24]6–, and their protonated forms in solution. This rate expression may be accounted for by a mechanism arising from a series of pre-equilibria involving the loss of three [HMoO4]– units and a H+ from a cobalt(II) heteropolymolybdate, most likely [H6CoMo6O24]4–, which then allows the one-electron oxidation of Co2+ to Co3+ by HSO5– to occur following co-ordination of the latter. In keeping with this proposal, [NH4]4[H6CoMo6O24]· 4H2O was crystallized from an acidic (pH 4.5) solution containing Co2+(aq) and molybdate, and its structure determined by X-ray diffraction methods. The heteropolymetalate anion exhibits a standard Anderson structure with six octahedral molybdate edge-sharing units surrounding the central cobalt, and all metal atoms effectively in a common plane. The second observable kinetic step shows no dependences on [Co3+], [oxidant], pH or [molybdate], and is interpreted as a ligand-breakdown reaction, involving loss of an OH˙ radical from the co-ordinated ‘HSO52–’(radical) present following the actual one-electron oxidation step. Dimerization is then assumed to occur, followed by slow [H4Co2Mo10O38]6– formation stemming from further reaction of the immediate product of the dimerization step, involving loss of co-ordinated SO42–.

Chemostat-controlled selective leaches of model soil phases – the hydrous manganese and iron oxides. Part 2: re-adsorption studies

Selective leaching of synthetic Mn and Fe oxide phases was examined using a Chemostat to control the system temperature, pH and reductant concentration or Eh. Reaction conditions for the selective dissolution of model phases of Mn oxide (birnessite, ‘MnO2’), amorphous Fe oxyhydroxide (ferrihydrite, Fe(OH)3.xH2O) and crystalline Fe oxide (hematite/goethite, Fe2O3/FeOOH) were examined, concomitant with the behaviour of dopant metals Cu, Pb and Zn, present at concentrations of 1000 mg kg−1 each. Part 1 of this study focused on achieving quantitative dissolutions of the target model phases within short reaction times (≤15 min) for the purpose of high-throughput field procedures. In this study, the behaviour of the doped trace metals during the selective extractions was examined. Selective extraction protocols were developed that attempt to optimize both the bulk phase dissolution and the selectivity of the doped trace metals hosted by particular phases. The behaviour of the doped trace metals suggests that trace metal selectivity is limited with respect to both re-adsorption onto and extraction from residual phases. Significant re-adsorption of extracted trace metals occurred under the necessarily milder conditions required for selective dissolution of Mn oxide (pH >3). At pH 5, 95% of Pb, 52% of Cu and 6% of Zn extracted from the Mn oxide phase re-adsorbed onto the residual amorphous Fe oxide phase. Partial extractions attempting to achieve selective dissolution of the Mn oxide phase and its associated trace metals must therefore be treated with caution. Selective extractions of soil samples from four different mineralized areas, employing the extraction conditions found to provide optimal selectivity, exhibited much slower dissolution kinetics for the bulk oxide phases, particularly for amorphous Fe oxide, in comparison to the model phases. Reaction times of ≤2 h are unlikely to be quantitative for amorphous Fe oxide extractions under these conditions; however, this leach is considered to be more robust than Mn oxide extractions.

Geometrical isomerism in octahedral complexes arising from the presence of a fused ring on a triaza macrocycle

The chiral 2,5,8-triazabicyclo[7.4.01,9]tridecane (L1), an analogue of 1,4,7-triazacyclononane, (L2) that has a trans-cyclohexane ring fused to the tacn framework, forms bis complexes with a wide range of metal ions where two geometric forms may exist depending on the relative locations of the substituent cyclohexane units. For the crystal structure of bis(RR-2,5,8-triazabicyclo[7.4.01,9]tridecane)nickel(II) nitrate, two macrocyclic ligands each occupy a face on opposite sides of the metal-centred octahedron, with the cyclohexane rings, when viewed down an axis passing through the centre of these faces and the metal, arranged in an anti disposition. However, the crystal structure of bis(RR-2,5,8-triazabicyclo[7.4.01,9]tridecane)chromium(III) perchlorate shows the cyclohexane rings disposed in the alternate syn arrangement. Isomerism is defined simply by the spatial disposition of macrocycle substituents. Isomeric preference has been probed by a molecular mechanics analysis, including an energy profile analysis as one macrocycle is rotated relative to the other. The force field calculations predict very small differences between isomers, but a large barrier to interconversion by a twist mechanism. Complexation of L1 with a range of labile metal(II) ions has been probed by determination of apparent stability constants via spectrophotometric titrations. In general, formation constants of L1 complexes are similar to or even slightly higher than the unsubstituted 1,4,7-triazacyclononane (L2) analogues, consistent with no strong steric influence of the fused cyclohexane rings in L1 and with differences in properties reflecting minor electronic effects and ligand flexibility differences resulting from substitution.

Reaction kinetics and mechanism of formation of MnW6O24 (8-) by hypochlorous acid oxidation of Mn(II)(aq) in the presence of tungstate

Oxidation of Mn(II) by HOCl in weakly acidic solution, in the presence of tungstate, results in the formation of the soluble heteropolyoxotungstate [MnW6O24]. The kinetics of formation of this species was studied from 15-35°C over the pH range 4.9-5.4 and found to exhibit solution autocatalytic behaviour. The oxidation kinetics was found to follow an expanded rate expression consisting of two terms:

Kinetics and mechanism of the oxidation of Mn(II)aq by bromate and peroxomonosulfate in the presence of molybdate to form [MnIVMo9O32]6−

The oxidation of Mn(II) by both BrO3− and HSO5− in the presence of MoO42−, in weakly acidic solution over the pH ranges 3.9–5.5 and 4.4–5.5, respectively, results in the formation of the heteropolyoxomolybdate [MnMo9O32]6− in each case. The kinetics of oxidation were studied at 40.0 °C for BrO3− and 30.0 °C for HSO5−, along with temperature dependence studies, and for each oxidant were found to exhibit solution autocatalytic behaviour. For BrO3− the oxidation kinetics followed the expanded rate expression +d[MnMo9O326−]/dt = kAC(1)[Mn2+][MnMo9O326−][HMoO4−]2[BrO3−] based on an examination of the individual [BrO3−], pH and actual [MoO42−] dependences, with a value for kAC(1) of 9.09(34) × 106 dm12 mol−4 s−1. For HSO5− the oxidation kinetics followed the expanded two-term rate expression +d[MnMo9O326−]/dt = kAC(2a)[Mn2+][MnMo9O326−][HMoO4−]−5[SO5−] + kAC(2b)[Mn2+][MnMo9O326−][HMoO4−]−5[HSO5−] based on [HSO5−], pH and [MoO42−] dependences. The values of kAC(2a) and kAC(2b) are 4.6(4) × 10−11 dm−9 mol3 s−1 and 2.6(4) × 10−15 dm−9 mol3 s−1. For BrO3− oxidation, from the composition of the transition state, it is proposed that the product [MnMo9O32]6− species combines with Mn(II) and two HMoO4− ions to generate a Mn(II)-substituted lacunary [MnIIMnIVMo11O39]6− anion based on a Keggin structure, with the Mn(IV) located at the centre of the polyoxomolybdate framework. Extended-Huckel molecular orbital calculations have been used to investigate the stability of the proposed Mn(II)-substituted lacunary species, based on an α-Keggin structure, relative to the unassembled components. For HSO5− oxidation, the two parallel pathways indicate oxidation by both HSO5− and its deprotonated form SO5−. The two mechanisms reflect the differences in how BrO3− and HSO5− operate oxidatively and have been highlighted by the facile nature of polyoxomolybdate polymerization. In each case following oxidation of Mn(II) to Mn(IV) fast separation of the two Mn(IV) centres must subsequently occur, along with rapid assembly of the polyoxomolybdate frameworks around each centre to yield the product species.