Publications (43)304.09 Total impact
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Article: Mechanism of the Phospha-Wittig-Horner Reaction.
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ABSTRACT: Doing the phosphate dance: The phospha-Wittig-Horner reaction proceeds through stepwise PP cleavage of an oxadiphosphetane intermediate, followed by a [2,3]-sigmatropic rearrangement that paves the way for the final E2 elimination to form 1-phosphaallenes. The mechanism is thus greatly different to that of its carbon analogue, that is, the Horner-Wadsworth-Emmons reaction.Angewandte Chemie International Edition 05/2013; · 13.45 Impact Factor -
Article: Tuning the Electronics of Bis(tridentate)ruthenium(II) Complexes with Long-Lived Excited States: Modifications to the Ligand Skeleton beyond Classical Electron Donor or Electron Withdrawing Group Decorations.
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ABSTRACT: A series of homoleptic bis(tridentate) [Ru(L)2](2+) (1, 3) and heteroleptic [Ru(L)(dqp)](2+) complexes (2, 4) [L = dqxp (1, 2) or dNinp (3, 4); dqxp = 2,6-di(quinoxalin-5-yl)pyridine, dNinp = 2,6-di(N-7-azaindol-1-yl)pyridine, dqp = 2,6-di(quinolin-8-yl)pyridine] was prepared and in the case of 2 and 4 structurally characterized. The presence of dqxp and dNinp in 1-4 result in anodically shifted oxidation potentials of the Ru(3+/2+) couple compared to that of the archetypical [Ru(dqp)2](2+) (5), most pronounced for [Ru(dqxp)2](2+) (1) with a shift of +470 mV. These experimental findings are corroborated by DFT calculations, which show contributions to the complexes' HOMOs by the polypyridine ligands, thereby stabilizing the HOMOs and impeding electron extraction. Complex 3 exhibits an unusual electronic absorption spectrum with its lowest energy maximum at 382 nm. TD-DFT calculations suggest that this high-energy transition is caused by a localization of the LUMO on the central pyridine fragments of the dNinp ligands in 3, leaving the lateral azaindole units merely spectator fragments. The opposite is the case in 1, where the LUMO experiences large stabilization by the lateral quinoxalines. Owing to the differences in LUMO energies, the complexes' reduction potentials differ by about 900 mV [E1/2(1(2+/1+)) = -1.17 V, Ec,p(3(2+/1+)) = -2.06 V vs Fc(+/0)]. As complexes 1-4 exhibit similar excited state energies of around 1.80 V, the variations of the lateral heterocycles allow the tuning of the complexes' excited state oxidation strengths over a range of 900 mV. Complex 1 is the strongest excited state oxidant of the series, exceeding even [Ru(bpy)3](2+) by more than 200 mV. At room temperature, complex 3 is nonemissive, whereas complexes 1, 2, and 4 exhibit excited state lifetimes of 255, 120, and 1570 ns, respectively. The excited state lifetimes are thus somewhat shortened compared to that of 5 (3000 ns) but still acceptable to qualify the complexes as photosensitizers in light-induced charge-transfer schemes, especially for those that require high oxidative power.Inorganic Chemistry 04/2013; · 4.60 Impact Factor -
Article: Light-driven electron transfer between a photosensitizer and a proton-reducing catalyst co-adsorbed to NiO.
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ABSTRACT: While inter-molecular hole-hopping along the surface of semiconductors is known, there are no previous examples of electron-hopping between molecules on a surface. Herein, we present the first evidence of electron transfer from the bound photosensitizer Coumarin-343 (C343) to a bound complex (1) on the surface of NiO. In solution, 1 has been shown to be a mononuclear Fe-based proton-reducing catalyst. The reduction of 1 is reversible and occurs within 50 ns after excitation of C343. Interfacial recombination between the reduced 1(-) and NiO hole occurs on a 100 μs timescale by non-exponential kinetics. The observed process is the first essential step in the photosensitized generation of H2 from a molecular catalyst in the absence of a sacrificial reagent.Journal of the American Chemical Society 11/2012; · 9.91 Impact Factor -
Article: Alternative Synthesis and Structures of C-monoacetylenic Phosphaalkenes
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ABSTRACT: An alternative synthesis of C-monoacetylenic phosphaalkenes trans-Mes*P=C(Me)(C≡CR) (Mes* = 2, 4, 6-tBu3Ph, R = Ph, SiMe3) from C-bromophosphaalkenes cis-Mes*P=C(Me)Br using standard Sonogashira coupling conditions is described. Crystallographic studies confirm cis-trans isomerization of the P=C double bond during Pd-catalyzed cross coupling, leading exclusively to trans-acetylenic phosphaalkenes. Crystallographic studies of all synthesized compounds reveal the extend of π-conjugation over the acetylene and P=C π-systems.Zeitschrift für anorganische und allgemeine Chemie 11/2012; 638(14):2219-2224. · 1.25 Impact Factor -
Article: Mixed-valence [Fe(I)Fe(II)] hydrogenase active site model complexes stabilized by a bidentate carborane bis-phosphine ligand.
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ABSTRACT: A series of [FeFe]-hydrogenase active site analogues, with the general formula [Fe(2)(dt)(CO)(4)(BC)] (dt = dithiolate, pdt = propyl-1,3-dt (), bdt = benzene-1,2-dt (), edt = ethyl-1,2-dt (); BC = 1,2-bisdiphenylphosphine-1,2-o-carborane), has been prepared and structurally characterized. While the electrochemical reductions of are largely invariant to the different nature of their dt bridges, the oxidations differ by more than 120 mV in between the series. Remarkably, all three compounds are reversibly oxidized, with complex that contains the most electron-donating pdt ligand at the mildest potential of -0.09 V vs. Fc/Fc(+). The one-electron oxidized state (ox) is stable for several minutes and was spectroscopically characterized by FTIR and EPR. EPR spectroscopy provided evidence that in the mixed-valence [Fe(I)Fe(II)] state most of the spin density is located on the iron with the BC-ligand. This is monitored through the strong (31)P hyperfine coupling of the phenyl groups of the BC ligand, while further delocalization into the o-carborane unit is negligible.Dalton Transactions 09/2012; 41(40):12468-77. · 3.84 Impact Factor -
Article: Electronic structure of an [FeFe] hydrogenase model complex in solution revealed by X-ray absorption spectroscopy using narrow-band emission detection.
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ABSTRACT: High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe(2)(μ-adt)(CO)(4)(PMe(3))(2)] (1, adt = S-CH(2)-(NCH(2)Ph)-CH(2)-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,Ph), 1HHy(CO,Ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (Kß(2,5) emission) electronic transitions, and of Kα RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by ~1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of ~2.3 eV for 1(powder) and ~2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2)-y(2)) levels of ~0.5 eV for the nonhydride and ~0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,Ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.Journal of the American Chemical Society 08/2012; 134(34):14142-57. · 9.91 Impact Factor -
Article: Cascade reactions forming highly substituted, conjugated phospholes and 1,2-oxaphospholes.
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ABSTRACT: More than just a carbon copy: The reaction of a phospha-Wittig-Horner reagent with diacetylenic ketones (see scheme) results in a cascade of reactions that can lead to both an oxaphosphole-terminated cumulene system and an alkene-bridged bis-phosphole. The reaction outcome is determined by the nature of the acetylene termini, with phenyl groups stabilizing a carbene intermediate that dimerizes to give the bis-phosphole product.Angewandte Chemie International Edition 06/2012; 51(31):7776-80. · 13.45 Impact Factor -
Article: Structural and spectroscopic characterization of tetranuclear iron complexes containing a bridge
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ABSTRACT: A pair of tetranuclear iron complexes consisting of two Fe2(Cl2bdt)(CO)5 subunits (Cl2bdt = 3,6-dicholorobenzene-1,2-dithiolate) bridged by different cyclic 1,5-diaza-3,7-diphosphacyclooctane (P2N2) ligands were prepared and structurally characterized. In the solid state, the P2N2 ligands adopt a boat conformation, which results in rather short distances between the two Fe2(Cl2bdt)(CO)5 clusters that promotes electronic communication across the diphosphine ligand.Journal of Coordination Chemistry 06/2012; 65. · 1.55 Impact Factor -
Article: Site-selective X-ray spectroscopy on an asymmetric model complex of the [FeFe] hydrogenase active site.
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ABSTRACT: The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe(I)2(CO)(Ph(2)P-CH(2)-NCH(3)-CH(2)-PPh(2))] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. Kβ emission spectra (Kβ(1,3), Kβ') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the Kβ(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (Kβ(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d → 1s emission and 1s → 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. Kβ-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various Kβ emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.Inorganic Chemistry 03/2012; 51(8):4546-59. · 4.60 Impact Factor -
Article: Photoelectrochemical hydrogen generation by an [FeFe] hydrogenase active site mimic at a p-type silicon/molecular electrocatalyst junction.
Chemistry 01/2012; 18(5):1295-8. · 5.93 Impact Factor -
Article: Chemistry. Ironing out hydrogen storage.
Science 09/2011; 333(6050):1714-5. · 31.20 Impact Factor -
Article: Pentacoordinate iron complexes as functional models of the distal iron in [FeFe] hydrogenases.
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ABSTRACT: Mononuclear pentacoordinate iron complexes with a free coordination site were prepared as mimics of the distal Fe (Fe(d)) in the active site of [FeFe] hydrogenases. The complexes catalyze the electrochemical reduction of protons at mild overpotential.Chemical Communications 09/2011; 47(42):11662-4. · 6.17 Impact Factor -
Article: Acetylene-expanded dendralene segments with exotopic phosphaalkene units.
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ABSTRACT: Bis-TMS protected C,C-diacetylenic phosphaalkene (A(2)PA) 1 (Mes*P=C(C≡CTMS)(2); Mes* = 2,4,6-tBu(3)Ph) has been used as a building block for the construction of butadiyne-expanded dendralene fragments in which phosphaalkenes feature as exotopic double bonds. Treatment of 1 with CuCl gives rise to a Cu(I) acetylide that is selectively formed at the acetylene trans to the Mes* group. The cis-TMS-acetylene engages in similar chemistry, albeit at higher temperatures and longer reaction times. The differentiation between the two acetylene termini of 1 allows for the controlled synthesis of the title compounds by a variety of different Cu- and Pd-catalyzed oxidative acetylene homo- and heterocoupling protocols. Crystallographic characterization of A(2)PA 1 and dimeric Mes*P=C(C≡CR(1))C(4)(R(2) C≡C)C=PMes* (3b, R(1) = R(2) = Ph; 6, R(1) = R(2) = TMS), and 10 (R(1) = R(2) = C≡CPh) verifies that the stereochemistry across the P=C bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis/trans isomerization in an iodo-substituted A(2)PA intermediate 4 (Mes*P=C(C≡CTMS)(C≡CI). UV/Vis spectroscopic and electrochemical studies reveal that efficient π conjugation operates through the entire acetylenic phosphaalkene framework, even in the cross-conjugated dimeric structures. The P centers contribute considerably to the frontier molecular orbitals of the compounds, thereby leading to smaller HOMO-LUMO gaps than in all-carbon-based congeners. Phenyl- and/or ethynylphenyl substituents at the A(2)PA framework influence the HOMO and LUMO to a varying degree depending on their relationship to the Mes* group, thus enabling a fine-tuning of the frontier molecular orbitals of the compounds.Chemistry 09/2011; 17(43):12153-62. · 5.93 Impact Factor -
Article: C,C-diacetylenic phosphaalkenes in palladium-catalyzed cross-coupling reactions.
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ABSTRACT: The reactivity of bis-TMS-substituted C,C-diacetylenic phosphaalkene (A(2)PA) 1 in Sonogashira-Hagihara cross-coupling reactions has been examined. The selective and successive deprotection of the two silyl groups in 1 is enabled by the steric bulk of the Mes* group which renders the acetylene trans to Mes* more reactive and thereby facilitates selective and consecutive couplings with iodoarenes. In situ transformation of the TMS-protected acetylenes into Cu(i)acetylides is the key step in the synthetic sequence and enables the preparation of the first dimeric A(2)PA linked by a phenylene spacer. cis-trans Isomerization across the P[double bond, length as m-dash]C bond is triggered by a tertiary amine and exclusively observed in the case of nitrophenyl-substituted A(2)PAs. The introduced aryl groups are integral parts of the entire π-system as evidenced by spectroscopic and electrochemical studies.Organic & Biomolecular Chemistry 07/2011; 9(18):6246-55. · 3.70 Impact Factor -
Article: A model of the [FeFe] hydrogenase active site with a biologically relevant azadithiolate bridge: a spectroscopic and theoretical investigation.
Angewandte Chemie International Edition 02/2011; 50(6):1439-43. · 13.45 Impact Factor -
Chapter: Photosynthesis, Artificial: Progress in Swedish Consortium
01/2011; , ISBN: 9780470862100 -
Article: Spectroscopically characterized intermediates of catalytic H2 formation by [FeFe] hydrogenase models
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ABSTRACT: This review compiles species that are known or potential intermediates in the catalytic formation of H2 by diiron dithiolate complexes inspired by the active site of the [FeFe] hydrogenases. The data collection emphasizes spectroscopic characteristics (NMR, IR, UV-Vis, EPR) of protonated and reduced derivatives of the iron complexes that could provide reference data to the identification of intermediates in mechanistic studies.Energy & Environmental Science 01/2011; 4(7):2340-2352. · 9.61 Impact Factor -
Article: In vitro hydrogen production--using energy from the sun.
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ABSTRACT: Using solar energy to produce molecular hydrogen is a promising way to supply the civilization with clean energy. Nature provides the key components to collect solar energy as well as to reduce protons, scientists have developed mimics of these enzymatic centers and also found new ways to catalyze the same reactions. This perspective article surveys the different components and in particular the various coupling possibilities of a light sensitizer and catalyst. Pros and cons are discussed.Physical Chemistry Chemical Physics 01/2011; 13(1):47-57. · 3.57 Impact Factor -
Article: Catalytic hydrogen evolution from mononuclear iron(II) carbonyl complexes as minimal functional models of the [FeFe] hydrogenase active site.
Angewandte Chemie International Edition 10/2010; 49(43):8033-6. · 13.45 Impact Factor -
Article: Directing protonation in [FeFe] hydrogenase active site models by modifications in their second coordination sphere.
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ABSTRACT: Subtle changes in the second coordination sphere of [Cl(2)bdtFe(2)(CO)(4)(Ph(2)P-CH(2)-X-CH(2)-PPh(2))] (bdt = benzene-1,2-dithiolate, X = NCH(3), NCH(2)CF(3), CH(2)) that do not influence the electronic character of the Fe(2) center can however direct protonation to three different sites: the N in the bis-phosphane, the Fe-Fe bond or the bdt-S.Chemical Communications 08/2010; 46(31):5775-7. · 6.17 Impact Factor
Top co-authors
Top Journals
Institutions
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2006–2013
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Uppsala University
- Department of Chemistry - Ångström Laboratory
Uppsala, Uppsala, Sweden
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2012
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University of California, San Diego
San Diego, CA, USA
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2008–2012
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Freie Universität Berlin
- Institute of Experimental Physics
Berlin, Land Berlin, Germany
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2003–2006
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Stockholm University
- Department of Organic Chemistry
Stockholm, Stockholm, Sweden
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