| 1. |
- Ohlin, C. Andre, et al.
(author)
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Rates of Water Exchange for Two Cobalt(II) Heteropolyoxotungstate Compounds in Aqueous Solution
- 2011
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In: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 17:16, s. 4408-4417
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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.
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| 2. |
- Wilson, Samuel, et al.
(author)
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Iron L-edge X-ray Absorption Spectroscopy of Oxy-Picket Fence Porphyrin : Electronic Structure of the Fe-O2 Bond
- 2013
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 135:3, s. 1124-1136
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Journal article (peer-reviewed)abstract
- The electronic structure of the Fe-O-2 center in oxy-hemoglobin and oxy-myoglobin is a long-standing issue in the field of bioinorganic chemistry. Spectroscopic studies have been complicated by the highly delocalized nature of the porphyrin, and calculations require interpretation of multi-determinant wave functions for a highly covalent metal site. Here, iron L-edge X-ray absorption spectroscopy, interpreted using a valence bond configuration interaction multiplet model, is applied to directly probe the electronic structure of the iron in the biomimetic Fe-O-2 heme complex [Fe(pfp)-(1-Melm)O-2] (pfp ("picket fence porphyrin") = meso-tetra(alpha,alpha,alpha,alpha-o-pivalamidophenyl)porphyrin or TpivPP). This method allows separate estimates of sigma-donor, pi-donor, and pi-acceptor interactions through ligand-to-metal charge transfer and metal-to-ligand charge transfer mixing pathways. The L-edge spectrum of [Fe(pfp)(1-MeIm)O-2] is further compared to those of [Fe-II(pfp)(1-MeIm)(2)], [Fe-II(pfp)], and [Fe-III(tpp)(ImH)(2)]Cl (tpp = meso-tetraphenylporphyrin) which have Fe-II S = 0, Fe-II S = 1, and Fe-III S = 1/2 ground states, respectively. These serve as references for the three possible contributions to the ground state of oxy-pfp. The Fe-O-2 pfp site is experimentally determined to have both significant sigma-donation and a strong pi-interaction of the O-2 with the iron, with the latter having implications with respect to the spin polarization of the ground state.
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