Crystal structure of bis(acetylacetonato-κ2O,O′)(tetrahydrofuran-κO)(trifluoromethanesulfonato-κO)iron(III)

The mononuclear title complex, [Fe(CF3O3S)(C5H7O2)2(C4H8O)] or [Fe(acac)2(OTf)(THF)] (acac = acetylacetonate; OTf = trifluoromethanesulfonate; THF = tetrahydrofuran), (I), consists of one six-coordinate Fe3+ atom in a slightly distorted octahedral environment [Fe—O bond-length range = 1.9517 (11)–2.0781 (11) Å]. The triflate ligand was found to be disordered over two sets of sites, with a site-occupancy ratio of 0.622 (16):0.378 (16). Weak intermolecular C—H...O and C—H...F hydrogen-bonding interactions generate a two-dimensional supramolecular structure lying parallel to (100). This is only the second crystal structure reported of a mononuclear bis(acetylacetonato)iron(III) complex

Effects of ligands on the migratory insertion of alkenes into rhodium-oxygen bonds.

Migratory insertions of olefins into metal-oxygen bonds are elementary steps of important catalytic processes, but well characterised complexes that undergo this reaction are rare, and little information on the effects of ancillary ligands on such reactions has been gained. We report a series of alkoxo alkene complexes of rhodium(i) that contain a range of bidentate ligands and that undergo insertion of the alkene. Our results show that complexes containing less electron-donating ancillary ligands react faster than their counterparts containing more electron-donating ancillary ligands, and that complexes possessing ligands with larger bite angles react faster than those with smaller bite angles. External added ligands had several effects on the reactions, including an inhibition of olefin isomerisation in the product and acceleration of the displacement of the product from complexes of ancillary ligands with small bite angles. Complementary computational studies help elucidate the details of these insertion processes

Insights into the Hydrolytic Polymerization of Trimethoxymethylsilane. Crystal Structure of (MeO)₂MeSiONa

The commercially practiced conversion of trimethoxymethylsilane (MTM) to [OSi­(OMe)­Me)]_<i>n</i> polymers and resins is assumed to proceed via the silanol (MeO)₂MeSiOH. Access to this crucial silanol is gained via the synthesis of (MeO)₂MeSiONa, the first methoxysilanoate to be crystallographically characterized. Mild protonation of this silanoate gives (MeO)₂MeSiOH, which is shown to condense with (MeO)₂MeSiOH but not with MTM. Condensation generates reactive disiloxanols but does not produce symmetric disiloxanes. Parallel results were obtained with (MeO)₂PhSiOH

Imine-Centered Reactions in Imino-Phosphine Complexes of Iron Carbonyls

Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph₂PC₆H₄CHNC₆H₄-4-Cl (PCHNAr^Cl) complexes of iron(0), iron­(I), and iron­(II). The reaction of PCHNAr^Cl with Fe­(bda)­(CO)₃ gives Fe­(PCHNAr^Cl)­(CO)₃ (<b>1</b>), featuring an η²-imine. DNMR studies, its optical properties, and DFT calculations suggest that <b>1</b> racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The <i>N</i>-imine isomer is more stable in Fe­(PCHNAr^OMe)­(CO)₃ (<b>1</b>^<b>OMe</b>), which crystallized despite being the minor isomer in solution. Protonation of <b>1</b> by HBF₄·Et₂O gave the iminium complex [<b>1</b>H]­BF₄. The related diphosphine complex Fe­(PCHNAr^Cl)­(PMe₃)­(CO)₂ (<b>2</b>), which features an η²-imine, was shown to also undergo N protonation. Oxidation of <b>1</b> and <b>2</b> with FcBF₄ gave the Fe­(I) compounds [<b>1</b>]­BF₄ and [<b>2</b>]­BF₄. The oxidation-induced change in hapticity of the imine from η² in [<b>1</b>]⁰ to κ¹ in [<b>1</b>]⁺ was verified crystallographically. Substitution of a CO ligand in <b>1</b> with PCHNAr^Cl gave Fe­[P₂(NAr^Cl)₂]­(CO)₂ (<b>3</b>), which contains the tetradentate diamidodiphosphine ligand. This C–C coupling is reversed by chemical oxidation of <b>3</b> with FcOTf. The oxidized product of [Fe­(PCHNAr^Cl)₂(CO)₂]²⁺ ([<b>4</b>]²⁺) was prepared independently by the reaction of [<b>1</b>]⁺, PCHNAr^Cl, and Fc⁺. The C–C scission is proposed to proceed concomitantly with the reduction of Fe­(II) via an intermediate related to [<b>2</b>]⁺

Mechanism of H₂ Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

The intermediacy of a reduced nickel–iron hydride in hydrogen evolution catalyzed by Ni–Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)­Ni­(μ-pdt)­Fe­(CO)­(dppv) ([<b>1</b>]⁰; dppv = <i>cis</i>-C₂H₂­(PPh₂)₂) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [<b>1</b>]⁰ initially produces <i>unsym</i>-[H<b>1</b>]⁺, which converts by a first-order pathway to <i>sym</i>-[H<b>1</b>]⁺. These species have <i>C</i>₁ (unsym) and <i>C</i>_<i>s</i> (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H<b>1</b>]⁺ protonates at sulfur. The <i>S</i> = 1/2 hydride [H<b>1</b>]⁰ was generated by reduction of [H<b>1</b>]⁺ with Cp*₂Co. Density functional theory (DFT) calculations indicate that [H<b>1</b>]⁰ is best described as a Ni­(I)–Fe­(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H<b>1</b>]⁰. Whereas [H<b>1</b>]⁺ does not evolve H₂ upon protonation, treatment of [H<b>1</b>]⁰ with acids gives H₂. The redox state of the “remote” metal (Ni) modulates the hydridic character of the Fe­(II)–H center. As supported by DFT calculations, H₂ evolution proceeds either directly from [H<b>1</b>]⁰ and external acid or from protonation of the Fe–H bond in [H<b>1</b>]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H<b>1</b>]⁰ initially produces [<b>1</b>]⁺, which is reduced by [H<b>1</b>]⁰. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [<i>t</i>-HFe₂(Me₂pdt)­(CO)₂(dppv)₂]⁺ ([<i>t</i>-H<b>1</b>]⁺) (Me₂pdt^2– = Me₂C­(CH₂S^–)₂; dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) and related derivatives were prepared with ⁵⁷Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental results were rationalized using DFT molecular modeling and spectral simulations. The spectroscopic analysis was aimed at supporting assignments of Fe–H vibrational spectra as they relate to recent measurements on [FeFe]-hydrogenase enzymes. The combination of bulky Me₂pdt^2– and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5–16 kcal/mol more stable bridging hydride ([μ-H<b>1</b>]⁺) with <i>t</i>_1/2(313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [<i>t</i>-H<b>1</b>]⁺ interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [<i>t</i>-H⁵⁷<b>1</b>]⁺ exhibits multiple NRVS features in the 720–800 cm^–1 region containing the bending Fe–H modes, the deuterated [<i>t</i>-D⁵⁷<b>1</b>]⁺ sample exhibits a unique Fe-D/CO band at ∼600 cm^–1. In contrast, the NRVS spectra for [μ-H⁵⁷<b>1</b>]⁺ exhibit weaker bands near 670–700 cm^–1 produced by the Fe–H–Fe wagging modes coupled to Me₂pdt^2– and dppv motions

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [<i>t</i>-HFe₂(Me₂pdt)­(CO)₂(dppv)₂]⁺ ([<i>t</i>-H<b>1</b>]⁺) (Me₂pdt^2– = Me₂C­(CH₂S^–)₂; dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) and related derivatives were prepared with ⁵⁷Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental results were rationalized using DFT molecular modeling and spectral simulations. The spectroscopic analysis was aimed at supporting assignments of Fe–H vibrational spectra as they relate to recent measurements on [FeFe]-hydrogenase enzymes. The combination of bulky Me₂pdt^2– and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5–16 kcal/mol more stable bridging hydride ([μ-H<b>1</b>]⁺) with <i>t</i>_1/2(313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [<i>t</i>-H<b>1</b>]⁺ interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [<i>t</i>-H⁵⁷<b>1</b>]⁺ exhibits multiple NRVS features in the 720–800 cm^–1 region containing the bending Fe–H modes, the deuterated [<i>t</i>-D⁵⁷<b>1</b>]⁺ sample exhibits a unique Fe-D/CO band at ∼600 cm^–1. In contrast, the NRVS spectra for [μ-H⁵⁷<b>1</b>]⁺ exhibit weaker bands near 670–700 cm^–1 produced by the Fe–H–Fe wagging modes coupled to Me₂pdt^2– and dppv motions

Reaction Coordinate Leading to H₂ Production in [FeFe]-Hydrogenase Identified by Nuclear Resonance Vibrational Spectroscopy and Density Functional Theory

[FeFe]-hydrogenases are metalloenzymes that reversibly reduce protons to molecular hydrogen at exceptionally high rates. We have characterized the catalytically competent hydride state (H_hyd) in the [FeFe]-hydrogenases from both <i>Chlamydomonas reinhardtii</i> and <i>Desulfovibrio desulfuricans</i> using ⁵⁷Fe nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT). H/D exchange identified two Fe–H bending modes originating from the binuclear iron cofactor. DFT calculations show that these spectral features result from an iron-bound terminal hydride, and the Fe–H vibrational frequencies being highly dependent on interactions between the amine base of the catalytic cofactor with both hydride and the conserved cysteine terminating the proton transfer chain to the active site. The results indicate that H_hyd is the catalytic state one step prior to H₂ formation. The observed vibrational spectrum, therefore, provides mechanistic insight into the reaction coordinate for H₂ bond formation by [FeFe]-hydrogenases