- Direct Determination of the Number of Electrons Needed to Reduce Coenzyme F430 Pentamethyl Ester to the Ni(I) Species Exhibiting the Electron Paramagnetic Resonance and Ultraviolet-Visible Spectra Characteristic for the MCRredl State of Methyl-coenzyme M Reductase
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The UV-visible and electron paramagnetic resonance (EPR) spectra of MCRred1, the catalytically active state of methyl-coenzyme M reductase, are almost identical to those observed when free coenzyme F430 or its pentamethyl ester (F430M) are reduced to the Ni(I) valence state. Investigations and proposals concerning the catalytic mechanism of MCR were therefore based on MCRred1 containing NiIF430 until, in a recent report, Tang et al. (J. Am. Chem. Soc. 2002, 124, 13242) interpreted their resonance Raman data and titration experiments as indicating that, in MCRred1, coenzyme F430 is not only reduced at the nickel center but at one of the C=N double bonds of the hydrocorphinoid macrocycle as well. To resolve this contradiction, we have investigated the stoichiometry of the reduction of coenzyme F430 pentamethyl ester (F430M) by three independent methods. Spectroelectrochemistry showed clean reduction to a single product that exhibits the UV-vis spectrum typical for MCRred1. In three bulk electrolysis experiments, 0.96 · ± 0.1 F/mol was required to generate the reduced species. Reduction with decamethylcobaltocene in tetrahydrofuran (THF) consumed 1 mol of (Cp*)2Co/mol of F430M, and the stoichiometry of the reoxidation of the reduced form with the two-electron oxidant methylene blue was 0.46 ± 0.05 mol of methylene blue/mol of reduced F430M. These experiments demonstrate that the reduction of coenzyme F430M to the species having almost identical UV-vis and EPR spectra as MCRred1 is a one-electron process and therefore inconsistent with a reduction of the macrocycle chromophore.
- Piskorski, Rafal,Jaun, Bernhard
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- Synthesis, reactivity, and electronic structure of a bioinspired heterobimetallic [Ni(μ-S2)Fe] complex with disulfur monoradical character
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The first synthesis of a monoradical Ni(μ-S2)Fe core in the [(Nacnac)Ni(μ-S2)Fe(dmpe)2] complex 3 could be accomplished in good yields by PMe3 elimination from the zerovalent iron complex [(dmpe)2(PMe3)Fe] (2; dmpe =1,2-bis(dimethylphosphine)ethane) upon reaction with the supersulfido nickel(II) complex [(Nacnac)Ni(S2)] (1; Nacnac = CH{(CMe)(2,6- iPr2C6H3N)}2). Complex 3 bears Ni(II) and Fe(II) centers, both of which are in a low-spin state. A single electron is located in the HOMO and is somewhat delocalized over the Ni(μ-S2)Fe core, so that the bridging disulfur subunit exhibits some subsulfide S23- character. Compound 3 represents a bioinspired example of a monoradical with a Ni(μ-S 2)Fe structural motif, reminiscent of the Ni(μ-S2)Fe core structure of the active site in [NiFe] hydrogenases. Its oxidation with [Fe(η5-C5H5)2][B(C 6H3(CF3)2)4] affords the product [(Nacnac)Ni(μ-S)2Fe(dmpe)2][B(C 6H3(CF3)2)4] (4), and complex 3 can alternatively be prepared via a reductive route upon reaction of [Co(η5-C5Me5)2][(Nacnac)NiS 2] (6) with the Fe(0) precursor 2. All synthesized complexes were fully characterized, including in some cases single-crystal X-ray diffraction analysis, magnetometry, EPR, NMR, and 57Fe Moessbauer spectroscopy. DFT calculations were used to compute the spectroscopic parameters and to establish the electronic structure of 3 and its oxidized and reduced forms and related complexes.
- Rudolph, Robert,Blom, Burgert,Yao, Shenglai,Meier, Florian,Bill, Eckhard,Van Gastel, Maurice,Lindenmaier, Nils,Kaupp, Martin,Driess, Matthias
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- Synthesis, structure, and bonding of stable complexes of pentavalent uranyl
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Stable complexes of pentavalent uranyl [UO2(salan- tBu2)(py)K]n (3), [UO2(salan- tBu2)(py)K(18C6)] (4), and [UO2(salophen- tBu2)(thf)]K(thf)2}n (8) have been synthesized from the reaction of the complex {[UO2py 5][KI2py2]}n (1) with the bulky amine-phenolate ligand potassium salt K2(salan-tBu 2) or the Schiff base ligand potassium salt K2(salophen- tBu2) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [UVO2(salan- tBu2)(py)][Cp*2Co], 6, were also isolated from the reduction of [UVIO2(salan-tBu 2)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO2py5]I ?py (2). The measured redox potentials E1/2 (U VI/UV) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give μeff of 2.20 and 2.23 μB at 300 K respectively, which is compared with a μeff of 2.6(1) μB (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g x = 1.98; gy = 1.25; gz = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the πu*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO 2(salan-tBu2)(py)K] and [UO 2(salophen-tBu2)( py)K] complexes with respect to disproportionation and hydrolysis.
- Nocton, Gregory,Horeglad, Pawel,Vetere, Valentina,Pecaut, Jacques,Dubois, Lionel,Maldivi, Pascale,Edelstein, Norman M.,Mazzanti, Marinella
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p. 495 - 508
(2010/03/25)
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- Synthesis of tungsten complexes that contain hexaisopropylterphenyl- substituted triamidoamine ligands, and reactions relevant to the reduction of dinitrogen to ammonia
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[HIPTN3N]WCl (WCl) can be synthesized readily by adding H 3[HIPTN3N] to WCl4(DME) followed by LiN(SiMe3)2 ([HIPTN3N]3- = [(HIPTNCH2CH2)3N]3- where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C 6H3 = HexalsoPropylTerphenyl). Reduction of WCl with KC8 in benzene under N2 yields WN=NK. WN=NK is readily oxidized in THF by ZnCl2 to yield zinc metal and WN2. Reduction of WN2 to [WN2]- is reversible at -2.27 V vs. FeCp2+/0 in 0.1 mol/L [Bu4N] [BAr′4]/PhF electrolyte (Ar′ = 3,5-(CF3) 2C6H3), while oxidation of WN2 to [WN2]+ is also reversible at -0.66 V. Protonation of WN=NK by [Et3NH][OTf] in benzene yields WN=NH essentially quantitatively. Protonation of WN=NH at Nβ with [H(OEt)2] [BAr′4] in ether affords [W=NNH2] [BAr′4] quantitatively. Electrochemical reduction of [W=NNH2][BAr′4] in 0.1 mol/L [Bu4N] [BAr′4]/PhF is irreversible at scan rates of up to 1 V/s. Addition of NaBAr′4 and NH3 to WCl in PhF yields [W(NH3)][BAr′4]. Electrochemical reduction of [W(NH3)][BAr′4] in 0.1 mol/L [Bu4N]- [BAr′4]/PhF is irreversible at -2.06 V vs. FeCp 2+/0 at a scan rate of 0.5 V/s. Treatment of [W(NH 3)][BAr′4] with triethylamine and [FeCp 2][PF6] in C6D6, followed by LiN(SiMe3)2, yielded W≡N. Treatment of [W(NH 3)][BAr′4] with LiBHEt3 (1 mol/L in THF) results in formation of WH, which is converted to WH3 upon exposure to an atmosphere of H2. Attempts to prepare WN=NH by treating WN 2 with [2,6-LutH][BAr′4] and CoCp2 yielded only [W=NNH2][BAr′4]. [W=NNH 2][BAr′4] is reduced to W=NNH2 by CoCp2*, but this species disproportionates to yield WN=NH, W≡N, and ammonia. Reduction of [W(NH3)][BAr′4] with CoCp2* does not yield any observable W(NH3). Attempted catalytic reduction of dinitrogen using WN2 as the catalyst under conditions identical or similar to those employed for catalytic reduction of dinitrogen by MoN2 and related Mo complexes failed. Single crystal X-ray studies were carried out on W-N=NK, WN2, W-N=NH, [W=NNH2][BAr′4], and [W(NH3)] [BAr′4].
- Yandulov, Dmitry V.,Schrock, Richard R.
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p. 341 - 357
(2007/10/03)
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- Organometallic crystal engineering with multidentate building blocks and template guest size effect. Supra-anionic organic framework obtained from cyclobutane-1,2,3,4-tetracarboxylic and trans-acootinic acids
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The organic acids cyclobutane-1,2,3,4-tetracarboxylic C4H4(CO2H)4, and trans-acotinic acid C3H3(CO2H)3, have been treated in thf with the organometallic hydroxides [Co(η5-C5H5)2]+[OH]-, [Co(η5-C5Me5)2]+[OH]-, and [Cr(η6-C6H6)2]+[OH]- prepared in situ from the oxidation of the corresponding neutral complexes to yield the novel organic-organometallic crystalline materials [Co(η5-C5H5)2]+[C4H4(CO2H)3COO]- 1, [Cr(η6-C6H6)2][C4H4(CO2H)3(CO2]-*H2O 2, and [Co(η5-C5Me5)2]+[C3H3(CO2H)2CO2(H)*C3H3(CO2H)2CO2]-*H2O 3. Self-assembly of the monodeproptonated organic acid C4H4(CO2H)4 generates supra-anionic framework structures held together by O-H...O and O-H...O- hydrogen bonds which accomodate the diamagnetic [Co(η5-C5H5)2]+ and paramagnetic [Cr(η6-C6H6)2]+ cations, respectively. Crystalline 1 does not form single crystals with defined shapes but rather an enamel like material which grows parallel to the crystallographic bc plane. The same reaction between trans-acotinic acid and [Co(η5-C5Me5)2]+[OH]- generates a large honeycomb-type structure in [Co(η5-C5Me5)2]+[C3H3(CO2H)2(CO2(H)*C3H3(CO2H)2CO2]-*H2O 3. The effect of the size of the templating units [Co(η5-C5H5)2]+ and [Co(η5-C5Me5)2]+ is discussed. The previously unknown structure of the starting material [Co(η5-C5Me5)2]+[PF6]- 4, used for the preparation of [Co(η5-C5Me5)2]+[OH]-, has also been determined.
- Braga, Dario,Benedi, Oscar,Maini, Lucia,Grepioni, Fabrizia
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p. 2611 - 2618
(2007/10/03)
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- Interaction between cobaltocenium and decamethylcobaltocenium salts and Ph3ELi (E = Si, Ge, Sn)
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Reactions of cobaltocenium salts (C5R5)2Co]PF6 (R = H, Me) with PhjELi (E = Si, Ge, Sn) and with Pf-4SbLi mainly follow two pathways (nucleophilic addition and oneelectron reduction), yielding cobalt cyclopentadiene-cyclopentadienyl complexes (
- Maryin,Petrovskii,Krasilmkova
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p. 2427 - 2430
(2007/10/03)
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- Pyridine as an C-Orthometalated and ?-N-Complexed Heteroarene Ligand on the surface of a Co3 Cluster
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Reaction of 5-Cp*)(acac-O,O')Co> (1) with potassium metal in pyridine results in reductive ligand transfer of the acac ligand to potassium and formation of a mixture of two organocobalt complexes which can be separated from each other by chromatography.Besides a mononuclear complex containing no pyridine unit the trinuclear cobalt cluster 5-Cp*)Co>3-μ-pyridine> (3) is formed in a cyclometalation reaction in 57 percent yield.The pyridine ligand in 3 is located on one edge of the Co3 core according to the spectroscopic data and a preliminary X-ray crystal structure analysis. - Key Words: Pyridine, trinuclear cobalt cluster / Heteroarene ligand, ?-N-complexed
- Schneider, Joerg J.
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p. 321 - 322
(2007/10/02)
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- Crystal and Molecular Structure of the 2:1 Charge-Transfer Salt of Decamethylferrocene and Perfluoro-7,7,8,8-tetracyano-p-quinodimethane: .+>22-. Electronic Structure of n (n = 0, 1-, 2-)
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The reaction of decamethylferrocene, Fe(C5Me5)2, and perfluoro-7,7,8,8-tetracyano-p-quinodimethane, TCNQF4, leads to the isolation of two phases of 1:1 and 2:1 stoichiometry.The crystal and molecular structure of the black 2:1 substance has been determine
- Dixon, David A.,Calabrese, Joseph C.,Miller, Joel S.
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p. 2284 - 2291
(2007/10/02)
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- Solvent Dynamical Effects in Electron Transfer: Comparisons of Self-Exchange Kinetics for Cobaltocenium-Cobaltocene and Related Redox Couples with Theoretical Predictions
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Rate constants, Kex, and activation parameters for the self-exchange of cobaltocenium-cobaltocene Cp2Co+/0, and the decamethyl derivative Cp'2Co+/0, in 13 organic solvents have been evaluated by using the proton NMR line-broadening technique with the objective of probing the influence of solvent dynamics upon the electron-transfer kinetics.Together with some corresponding measurements reported earlier for ferrocenium-ferrocene Cp2Fe+/0, additional measurements for the decamethyl derivative, Cp'2Fe+/0, and with corresponding data for Cp2Co+/0 electrochemical exchange, these results enable a systematic comparative examination to be made of the effects of solvent dielectric relaxation on the barrier-crossing frequency for such simple outer-sphere reactions.For the facile Cp'2Co+/0 couple the solvent dependence of the observed frequency factors, νn(obsd), extracted from the kex values by correcting for the solvent-dependent barrier height, ΔG*, is in approximate accordance with the relative frequency factors νos(calcd), predicted from the continuum model of overdamped solvent relaxation.The subunity (ca. 0.7-0.8) slope of the logarithmic νn(obsd) - ??os(calcd) plot for Cp'2C0+/0 self-exchange is consistent with a recent theoretical prediction of the combined effect of overdamped solvent motion and reactant vibrations (ref 2g).In a given solvent, the sequence of kex values is Cp'2Co+/0 > Cp2Co+/0 ca.Cp'2Fe+/0 > Cp2Fe+/0, with Cp2Co+/0 and Cp2Fe+/0 being about 10- and 100-folds lower, respectively, than Cp'2Co+/0 self-exchange.While these reactivity differences can be traced to variations in donor-acceptor orbital overlap, the solvent dependencies of kex for Cp2Co+/0 and Cp2Fe+/0 electron exchange nevertheless exhibit a strong influence from overdamped solvent relaxation.Marked deviations from the dielectric continuum predictions are seen, however, in several solvents.Thus the barrier-crossing frequencies in propylene carbonate, N-methylformamide, and especially methanol are substantially (4-100-fold) larger than expected from νos(calcd), implicating the presence of surprisingly rapid relaxation modes in these solvents.The solvent-dependent activation parameters also differ significantly from the expectations of conventional theoretical modes.
- Nielson, Roger M.,McManis, George E.,Golovin, Neal M.,Weaver, Michael J.
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p. 3441 - 3450
(2007/10/02)
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- Kinetics and mechanism of the reduction of protons to hydrogen by cobaltocene
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The rate constant kobsd for the overall reaction 2CoCp2 + 2H+ → 2CoCp2+ + H2 (eq i) was determined by pulse radiolysis in an aqueous sulfuric acid medium for cobaltocene (1), 1,2,3,4,5-pentamethylcobaltocene (2), and decamethylcobaltocene (3). The reaction is first order in 1 and in protons with k2,obsd = 42 ± 1.5 s-1 M-1 at cH+ = 8 × 10-3-4.7 × 10-1 M. Possible mechanistic pathways for eq i are discussed, and a second-order disproportionation of protonated 1 or a mechanism that involves reduction of protonated 1 by unprotonated 1 is best compatible both with the observed kinetics and with chemical evidence. Deuteriation experiments show protonation to occur exclusively at the metal.
- Koelle,Infelta,Gr?tzel
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p. 879 - 883
(2008/10/08)
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- The Orbital-Overlap Factor in Electron Transfer: Sensitivity of Homogeneous Self-Exchange Kinetics for Some Metallocenes to Electronic Structure
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Rate constants, khex, for the electron self-exchange of cobaltocenium-cobaltocene, Cp2Co+/0, and for the decamethyl derivative, (Cp-Me5)2Co+/0, obtained by using the NMR line-broadening technique in acetonitrile and dimethyl sulfoxide are compared with corresponding data for the ferrocene couples Cp2Fe+/0 and (Cp-Me5)2Fe+/0 and for bis(benzene)chromium(I)/(0).The rate constants in a given solvent display a marked sensitivity to the reactant structure, the khex values being about tenfold larger for Cp2Co+/0 relative to Cp2Fe+/0; decamethyl substitution yields tenfold increases in khex for both these couples.A relationship is established between these ca. 100-fold rate variations and the nature of the donor and acceptor orbitals.In particular, the markedly slower self-exchange kinetics observed for Cp2Fe+/0 relative to Cp2Co+/0 are consistent with the much greater ligand-delocalized character of the 4elg orbital involved in the latter electron transfer as compared with the 4e2 or 8alg orbital for the former reaction.The same argument is likely to account for the similar relative rates for (Cp-Me5)2Fe+/0 versus (Cp-Me5)2Co+/0.These rate differences are very unlikely to be due to variations in nuclear reorganization factors since the molecular structures of these couples are virtually identical, and they feature only small differences (ca. 0.3 kcal mol-1) in the inner-shell barriers.The results therefore provide unusually clear evidence for the influence of donor-acceptor electronic coupling in outer-sphere redox reactivity.
- Nielson, Roger M.,Golovin, M. Neal,McManis, George E.,Weaver, Michael J.
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p. 1745 - 1749
(2007/10/02)
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- Permethylmetallocene, II. Decamethylcobaltocen: Synthese und Umwandlung in methylierte (Aren)(cyclopentadienyl)cobalt-Kationen
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Decamethylcobaltocene (1b) is obtained from dibromo(1,2-dimethoxyethane)cobalt and pentamethylcyclopentadienyllithium. 1b shows enhanced reactivity in oxidative addition reactions with electrophiles RX.This reaction, when performed with chloroiodoalkanes, leads to exo-chloroalkyl complexes, which undergo a solvolytic ring expansion reaction, again more smoothly than the analogous derivatives of cobaltocene (1a), to form permethylated (cyclohexadienyl)(cyclopentadienyl)cobalt cations +.The latter are oxidized with nitric acid to yield for the first time (arene)(cyclopentadienyl)cobalt dications, 2+, methylated in both ligand rings.
- Koelle, Ulrich,Khouzami, Fouad
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p. 2929 - 2937
(2007/10/02)
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