| 1. |
- Ohlin, C. Andre, et al.
(author)
-
Rates of Water Exchange for Two Cobalt(II) Heteropolyoxotungstate Compounds in Aqueous Solution
- 2011
-
In: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 17:16, s. 4408-4417
-
Journal article (peer-reviewed)abstract
- Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co-II atoms in two polyoxotungstate sandwich molecules using the O-17-NMR-based Swift-Connick method. The compounds were the [Co-4(H2O)(2)(B-alpha-PW9O34)(2)](10-) and the larger alpha beta beta alpha-[Co-4(H2O)(2)(P2W15O56)(2)](16-) ions, each with two water molecules bound trans to one another in a Co-II sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal Xray crystallography, and potentiometry. For [Co-4(H2O)(2)(B-alpha-PW9O34)(2)](10-) at pH 5.4, we estimate: k(298) = 1.5(5) +/- 0.3 x 10(6) s(-1), Delta H-not equal = 39.8 +/- 0.4 kJ mol(-1), Delta S-not equal = + 7.1 +/- 1.2 J mol(-1)K(-1) and Delta V-not equal = 5.6 +/- 1.6 cm(3)mol(-1). For the Wells-Dawson sandwich cluster (alpha beta beta alpha-[Co-4(H2O)(2)(P2W15O56)(2)](16-)) at pH 5.54, we find: k(298) = 1.6(2) +/- 0.3 x 10(6)s(-1), Delta H-not equal = 27.6 +/- 0.4 kJ mol(-1) Delta S-not equal = -33 +/- 1.3 J mol(-1)K(-1) and Delta V-not equal = 2.2 +/- 1.4 cm(3)mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co-II species (such as the [Co(H2O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.
|
|
| 2. |
- Stich, Troy A., et al.
(author)
-
Structural insights into [Co4O4(C5H5N)(4)(CH3CO2)(4)](+), a rare Co(IV)-containing cuboidal complex
- 2013
-
In: Polyhedron. - : Elsevier BV. - 0277-5387 .- 1873-3719. ; 64:SI, s. 304-307
-
Journal article (peer-reviewed)abstract
- We report high-frequency (up to 219 GHz) and correspondingly high-field electron paramagnetic resonance spectra and X-ray crystal structure of [Co4O4(pyridine)(4)(acetate)(4)](+) ([Co4O4(py)(4)(OAc)(4)](+)) that serves as a structural and spectroscopic model of cobalt-oxide films that are capable of oxidizing water. These results are, in large part, consistent with those from our earlier, lower-frequency study and serve to benchmark future high-field studies on paramagnetic states of the catalyst film. (C) 2013 Published by Elsevier Ltd.
|
|
| 3. |
- McAlpin, J. Gregory, et al.
(author)
-
Electronic Structure Description of a [Co(III)(3)Co(IV)O-4] Cluster : A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation
- 2011
-
In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 133:39, s. 15444-15452
-
Journal article (peer-reviewed)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 Co-59 nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(+/- 2), 77(+/- 1), -5(+/- 15)] MHz). Mims H-1 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 N-14 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 [Co4+3.125O4] 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.
|
|
| 4. |
- Panasci, Adele F., et al.
(author)
-
Cooperation between bound waters and hydroxyls in controlling isotope-exchange rates
- 2012
-
In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 78, s. 18-27
-
Journal article (peer-reviewed)abstract
- Mineral oxides differ from aqueous ions in that the bound water molecules 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(H(2)O)(6)(3+)), 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)(H(2)O)(2)](3+) [1] and [Rh(phen)(2)(H(2)O)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 (18)O and (16)O exchange at the bound waters. The pH dependency of rates differ enormously between the two complexes. Pseudo-first-order rate coefficients at 298 K for water exchanges from the fully protonated molecules are close: k(0)(298) = 5 x 10(-8) (+/- 0.5 x 10(-8)) s(-1) for [1] and k(0)(298) = 2.5 x 10(-9)(+/- 1 x 10(-9)) for [2]. Enthalpy and entropy activation parameters (Delta H(double dagger) and Delta S(double dagger)) 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. (C) 2011 Elsevier Ltd. All rights reserved.
|
|
