Casey Laboratory

Abstract: Water molecules bound to mineral oxides differ from aqueous ions in that they are usually attached to different metal centers, or vicinal, and thus separated from one another. In contrast, for most monomeric ions used to establish kinetic reactivity trends, such as octahedral aquo ions [e.g., Al(H₂O)₆³⁺], the bound waters are closely packed, or geminal. Because of this structural difference, the existing literature about ligand substitution in monomer ions may be a poor guide to the reactions of geochemical interest. To understand how coordination of the reactive functional groups might affect the rates of simple water-exchange reactions, we synthesized two structurally similar Rh(III) complexes, [Rh(phen)₂(H₂O)₂]³⁺[1] and [Rh(phen)₂(H₂O)Cl]²⁺[2] where (phen)=1,10-phenanthroline. Complex [1] has two adjacent, geminal, bound waters in the inner-coordination sphere and [2] has a single bound water adjacent to a bound chloride ion. We employed Rh(III) as a trivalent metal rather than a more geochemically relevant metal like Fe(III) or Al(III) to slow the rate of reaction, which makes possible measurement of the rates of isotopic substitution by simple mass spectrometry. We prepared isotopically pure versions of the molecules, dissolved them into isotopically dissimilar water, and measured the rates of exchange from the extents of ¹⁸O and ¹⁶O exchange at the bound waters.

The pH dependency of rates differ enormously between the two complexes. Pseudo-first-order rates coefficients at 298 K for water exchanges from the fully protonated molecules are close: = 5·10^-8(± 0.5·10^-8) s^-1 for [1] and = 2.5·10^-9(± 1·10^-9) for [2]. Enthalpy and entropy activation parameters (ΔH^‡ and ΔS^‡) were measured to be 119(± 3) kJ mol^-1, and 14(±1) J mol^-1 K^-1, respectively for [1]. The corresponding parameters for the mono-aquo complex, [2], are 132(±3) kJ mol^-1 and 41.5(±2) J mol^-1 K^-1. Rates increase by many orders of magnitude upon deprotonation of one of the bound waters in complex [1] because of the close proximity of a transferable proton that can convert the bound hydroxyl to a bound water. This interconversion allows the oxygen to exchange as a bound water, rather than as a bound hydroxyl, which is slow at near-neutral pH conditions.

Panasci, A. F., McAlpin, J. G., Ohlin, C. A., Christensen, S., Fettinger, J. C., Britt, R. D., Rustad, J. R., Casey W. H. Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates. Geochim. Cosmochim. Acta, 2011, In Press.

Abstract: N/A

Johnson, R. L., Harley, S. J., Ohlin, C. A., Panasci, A., Casey W. H. Multinuclear NMR study of the pressure
dependence for carbonate exchange in the UO₂(CO₃)₃^4-(aq) ion. ChemPhysChem, 2011, 12, 2903-2906.

Abstract: Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co₄O₄(C₅H₅N)₄(CH₃CO₂)₄]⁺ (1⁺), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt–phosphate (Co–Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled ⁵⁹Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims ¹H ENDOR spectra of 1⁺ with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency ¹⁴N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d_xz,yz-based molecular orbital for this essentially [Co^+3.125₄O₄] system. Comparing the EPR spectrum of 1⁺ with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.

J. Gregory McAlpin, Ohlin, C. André; Surendranath, Yogesh; Nocera, Daniel G.; Casey, William H.; Britt, R. David "Electronic structure description of a [Co(III)₃Co(IV)O₄ cluster: A model for the paramagnetic intermediate in cobalt-catalyzed water oxidation" J. Am. Chem. Soc., 2011, 133, 15444-15452.

Abstract: Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO_x^cfs) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO_x^cfs provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O₂ to homogeneous catalysis yielding H₂O₂ is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.

James B. Gerkent, J. Gregory McAlpin, Jaime Y. C. Chent, Matthew L. Rigsby, William H. Casey, R. David Britt, Shannon S. Stahl Electrochemical water oxidation with cobalt-based electrocatalysts from pH 0-14: The thermodynamic basis for catalyst structure, stability and activity J. Am. Chem. Soc., 2011, 133, 14431-14442.

Abstract: Signal analysis in Nuclear Magnetic Resonance spectroscopy is among the most powerful methods to quantify reaction rates in aqueous solutions. To this end, the Swift- Connick approximations to the Bloch-McConnell equations have been used extensively to estimate rate parameters for elementary reactions. The method is primarily used for ¹⁷O-NMR in aqueous solutions, but the list of geochemically relevant nuclei that can be used is long, and includes ²⁹Si, ²⁷Al, ¹⁹F, ¹³C and many others of particular interest to geochemists. Here we review the derivation of both the Swift-Connick and Bloch-McConnell equations and emphasize assumptions and quirks. For example, the equations were derived for CW-NMR, but are used with modern pulse FT-NMR and can be applied to systems that have exchange rates that are shorter than the lifetime of a typical pulse. The method requires a dilute solution where the minor reacting species contributes a neglible amount of total magnetization. We evaluate the sensitivity of results to this dilute-solution requirement and also highlight the need for chemically well- defined systems if reliable data are to be obtained. The limitations in using longitudinal relaxation to estimate reaction rate parameters are discussed. Finally, we provide examples of the application of the method, including ligand exchanges from aqua ions and hydrolysis complexes, that emphasize its flexibility. Once the basic requirements of the Swift-Connick method are met, it allows geochemists to establish rates of elementary reactions. Reactions at this scale lend themselves well to methods of computational simulation and could provide key tests of accuracy.

Harley, Stephen J.; Ohlin, C. André; Casey, William H. Geochemical kinetics via the Swift-Connick equations and solution NMR Geochim. Cosmochim. Acta , 2011, 75, 3711-3725

Abstract: Water oxidation in all oxygenic photosynthetic organisms is catalysed by the Mn₄CaO₄ cluster of Photosystem II. This cluster has inspired the development of synthetic manganese catalysts for solar energy production. A photoelectrochemical device, made by impregnating a synthetic tetranuclear-manganese cluster into a Nafion matrix, has been shown to achieve efficient water oxidation catalysis. We report here in situ X-ray absorption spectroscopy and transmission electron microscopy studies that demonstrate that this cluster dissociates into Mn(II) compounds in the Nafion, which are then reoxidized to form dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase. Cycling between the photoreduced product and this mineral-like solid is responsible for the observed photochemical water-oxidation catalysis. The original manganese cluster serves only as a precursor to the catalytically active material. The behaviour of Mn in Nafion therefore parallels its broader biogeochemistry, which is also dominated by cycles of oxidation into solid Mn(III/IV) oxides followed by photoreduction to Mn²⁺.

Hocking, Rosalie K.; Brimblecombe, Robin; Chang, Shery L. Y.; Singh, Archana; Cheah, Mun Hoh; Glover, Chris; Casey, William H.; Spiccia, Leone Water oxidation catalysis by manganese in a geochemical-like cycle Nature Chem. , 2011, 3, 461-466

Abstract: Rates of oxygen-isotope exchange were measured in a tetrasiliconiobate ion in order to better understand how large oxide ions interact with water. The molecule has 19 non-equivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygens react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over ca 10⁴ across the molecule between pH 6 and 13 but with similar pH dependencies. There is NMR evidence of an intra- or intermolecular reaction at pH 7 where new peaks began to slowly form without losing the ¹⁷O isotopic tag and at pH less-or-equal-to 6 these new peaks formed rapidly. The oxygens bonded to silicon atoms began to isotopically exchange at pH 9 and below. The ¹⁷O NMR peak positions also vary considerably with pH for some, but not all, non-equivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygens should shift similarly upon protonation. Instead, we see that some sites change enormously with pH while other, similarly coordinated oxygens are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the ffect of the coordinated protons.

Johnson, Rene L.; Villa, Eric M.; Ohlin, C. André; Rustad, James R.; Casey, William H. ¹⁷O NMR and computational study of a tetrasiliconiobate ion, [H_2+xSi₄Nb₁₆O₅₆]^(14-x)- Chem. Eur. J., 2011 , 17, 9359-9367.