Abstract Rates of oxygen-isotope exchange at all structural sites in two isostructural polyoxometalates, [H_xNb₁₀O₂₈]^(6−x)− and [H_xTi₂ Nb₈O₂₈]^(8−x)−, show that small changes in structure have surprising and profound effects: a single-site substitution of Ti(IV) for Nb(V) inverts the pH dependencies for rates throughout the structures. Within a given structure, all oxygens exhibit similar pH dependencies although they react over a range greater than 10⁴, indicating that pathways involve concerted motions of the entire lattices. Profound sensitivity to changes in structure and composition suggests reaction pathways in polyoxometalate ions will be highly variable even within structural classes. The results also require new thinking about how ab initio simulations are used to understand reaction pathways involving extended structures, like the mineral−water interface. Our data indicate that reactions proceed via metastable intermediates and that the simulations must be structurally faithful or will miss the essential chemistry.

Villa, Eric M.;[1,2] Ohlin, C. André;[1,2] Rustad, James R.,[2] Casey, William H. [1,2] "Isotope-Exchange Dynamics in Isostructural Decametalates with Profound Differences in Reactivity", J. Am. Chem. Soc., 2009 , 131(45), 16488-16492. Issue cover.

1. Department of Chemistry, University of California, Davis, CA.
2. Department of Geology, University of California, Davis, CA.

Abstract The rate and activation parameters for oxygen exchange of the mu₃-oxo bridges in a manganese tetranuclear cluster with H₂¹⁸O have been measured by ESI-MS; the observed activation entropy of -146 ± 22 J K^-1.mol^-1 is consistent with the expected associative mechanism of substitution.

Ohlin, C. André;[1,2] Brimblecombe, Robin;[3] Spiccia, Leone;[3] Casey, William H. [1,2] "Oxygen isotopic exchange in a Mn^IIIMn₃^IV-oxo cubane", Dalton Trans. , 2009, 5278-5280.

1. Department of Chemistry, University of California, Davis, CA.
2. Department of Geology, University of California, Davis, CA.
3. School of Chemistry, Monash University, Victoria, Australia

Abstract The electrochemical behavior of a trinuclear ruthenium(II)-containing complex, [((phen) Ru(dpp))₂RhCl₂]⁵⁺ (where phen = 1,10-phenanthroline, dpp = 2,3-bis(2-pyridyl )pyrazine), was studied in acetonitrile (MeCN) and aqueous solutions. In MeCN containing 0.10 M tetra-n -butylammonium perchlorate (TBAP), the complex displayed a reversible, overlapping RuII/III redox process with E_1/2 = +1.21 V vs Ag/Ag+ (10 mM), an irreversible reduction of RhIII/I at −0.73 V vs Ag/Ag+, and two quasi-reversible dpp/dpp− couples with E1/2 = −1.11 and −1.36 V vs Ag/Ag+ at a Pt electrode with a scan rate of 50 mV s−1. In 0.20 M Tris buffer solution (pH 7.4), an irreversible, overlapping RuII/III oxidation at +1.48 V vs Ag /AgCl (3 M KCl), and an irreversible reduction of RhIII/II at −0.78 V vs Ag/AgCl were observed at a glassy carbon electrode with a scan rate of 50 mV/s. Investigations on the electrogenerated chemiluminescence (ECL) of the complex revealed that 2-(dibutylamino) ethanol (DBAE) was superior to tri-n-propylamine (TPrA) as an ECL coreactant within their entire concentration range of 10−100 mM in MeCN, and in aqueous media, as low as 1.0 nM of the complex can be detected using TPrA coreactant ECL. A maximum ECL emission of 640 nm, which is about 55 nm blue shift to its fluorescence, was observed in MeCN with DBAE as a coreactant. Interactions of the complex with calf thymus DNA (ctDNA) were conducted with a flow-cell based quartz-crystal microbalance, and a binding constant of 2.5 × 105 M−1 was calculated on the basis of the Langmuir isotherm equation.

Wang, Shijun;[1] Milam, Jenifer;[1] Ohlin, C. André;[+] Rambaran, Varma;[2] Clark, Eva;[1] Ward, Woodrow;[1] Seymour, Luke;[1] Casey, William H.;[+] Holder, Alvin A.;[1] Miao, Wujian [1]"Electrochemical and Electrogenerated Chemiluminescent Studies of a Trinuclear Complex, [((phen)Ru(dpp))₂RhCl₂]⁵⁺, and Its Interactions with Calf Thymus DNA", Analytical Chem. 2009, 81(10), 4068-4075

1. Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi
2. Department of Chemistry, The University of Trinidad and Tobago, Trinidad and Tobago
+. Department of Chemistry, and Department of Geology, University of California, Davis, CA.

Abstract The aluminum Keggin polycation (Al₁₃) has been identified as an effective specie for neutralization and coagulation of anionic contaminants in water. In this study, we compare efficacy of the aluminum Keggin-ion to the analogues containing a single Ga-atom or single Ge-atom (GaAl₁₂ and GeAl₁₂, respectively) substituted into the center of the polycation in water-treatment studies. We investigated removal of bacteriophage (model viruses), Cryptosporidium, dissolved organic carbon (DOC), and turbidity. In every study, the order of contaminant removal efficacy trends GaAl₁₂ > Al₁₃ > GeAl₁₂. By ESI MS (electrospray ionization mass spectrometry), we noted the GaAl₁₂ deprotonates least of the three aluminum polycations, and thus probably carries the highest charge, and also optimal contaminant-neutralization ability. The ESI MS studies of the aluminum polycation solutions, as well as solid-state characterization of their resulting precipitates both reveal some conversion of Al₁₃ to larger polycations, Al₃₀ for instance. The GaAl12 does not show any evidence for this alteration that is responsible for poor shelf life of commercial prehydrolyzed aluminum coagulants such as polyaluminum chloride. Based on these studies, we conclude that substitution of a single Ga-atom in the center of the aluminum Keggin polycation produces an optimal water-treatment product due to enhanced shelf life and efficacy in neutralization of anionic contaminants.

Stewart, Tom;[1] Trudell, Daniel;[1] Alam, Todd;[1] Ohlin, André;[2,3] Lawler, Christian;[2,3] Casey, William;[2,3] Jett, Stephen;[4] Nyman, May [1] Enhanced water purification: a single atom makes a difference, Environ. Sci. Techn. 2009, 43(14), 5416-5422. Link

This was featured in C&EN 2009, 87(25),35. Link

1. Sandia National Laboratories, P.O. Box 5800 MS-0750; Albuquerque, New Mexico
2. Department of Chemistry, University of California, Davis, California
3. Department of Geology, University of California, Davis, California
4. Department of Cell Biology and Physiology, Electron Microscopy Facility, University of New Mexico, Albuquerque, New Mexico

Abstract A new water-soluble polyoxometalate, [TiNb₉O₂₈]^7- , completes an isostructural and homologous decametalate series from the decaniobate to the dititano-octaniobate ion.

Ohlin, C. A.,[1,2] Villa, E. M.,[1,2] Fettinger, J. C.,[1] Casey, William H. [1,2] "A new titanoniobate ion - Completing the series [Nb₁₀O₂₈]^6-,[TiNb₉O₂₈]^7- and [Ti₂ Nb₈O₂₈]^8-" Dalton Trans., 2009 , 2677-2678.

1. Department of Chemistry, University of California, Davis, California
2. Department of Geology, University of California, Davis, California

Abstract Rates of oxygen-isotope exchange at all structural sites in two isostructural polyoxometalates, [H_xNb₁₀O₂₈]^(6−x)− and [H_xTi₂ Nb₈O₂₈]^(8−x)−, show that small changes in structure have surprising and profound effects: a single-site substitution of Ti(IV) for Nb(V) inverts the pH dependencies for rates throughout the structures. Within a given structure, all oxygens exhibit similar pH dependencies although they react over a range greater than 10⁴, indicating that pathways involve concerted motions of the entire lattices. Profound sensitivity to changes in structure and composition suggests reaction pathways in polyoxometalate ions will be highly variable even within structural classes. The results also require new thinking about how ab initio simulations are used to understand reaction pathways involving extended structures, like the mineral−water interface. Our data indicate that reactions proceed via metastable intermediates and that the simulations must be structurally faithful or will miss the essential chemistry.

Ohlin, C.A., Villa, E.M., Casey, W. H. One-pot synthesis of the decaniobate salt [N(CH₃)₄]₆[Nb₁₀O₂₈]^.6[H₂O] from hydrous niobium oxide., Inorg. Chim. Acta, 2009, 362, 1391-1392.

Department of Chemistry, and Department of Geology, University of California, Davis, CA