Surprising Condensation Reactions of the Azadithiolate Cofactor.

Azadithiolate, a cofactor found in all [FeFe]-hydrogenases, is shown to undergo acid-catalyzed rearrangement. Fe [(SCH ) NH](CO) self-condenses to give Fe [(SCH ) N] (CO) . The reaction, which is driven by loss of NH , illustrates the exchange of the amine group.

Mechanism of molybdate insertion into pterin-based molybdenum cofactors.

The molybdenum cofactor (Moco) is found in the active site of numerous important enzymes that are critical to biological processes. The bidentate ligand that chelates molybdenum in Moco is the pyranopterin dithiolene (molybdopterin, MPT). However, neither the mechanism of molybdate insertion into MPT nor the structure of Moco prior to its insertion into pyranopterin molybdenum enzymes is known.

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.

Vibrational Perturbation of the [FeFe] Hydrogenase H-Cluster Revealed by CH-ADT Labeling.

[FeFe] hydrogenases are highly active catalysts for the interconversion of molecular hydrogen with protons and electrons. Here, we use a combination of isotopic labeling, Fe nuclear resonance vibrational spectroscopy (NRVS), and density functional theory (DFT) calculations to observe and characterize the vibrational modes involving motion of the 2-azapropane-1,3-dithiolate (ADT) ligand bridging the two iron sites in the [2Fe] subcluster. A -CH- ADT labeling in the synthetic diiron precursor of [2Fe] produced isotope effects observed throughout the NRVS spectrum.

Asymmetry in the Ligand Coordination Sphere of the [FeFe] Hydrogenase Active Site Is Reflected in the Magnetic Spin Interactions of the Aza-propanedithiolate Ligand.

[FeFe] hydrogenases are very active enzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. Their active site, the H-cluster, contains a unique binuclear iron complex, [2Fe], with CN and CO ligands as well as an aza-propane-dithiolate (ADT) moiety featuring a central amine functionality that mediates proton transfer during catalysis. We present a pulsed C-ENDOR investigation of the H-cluster in which the two methylene carbons of ADT are isotope labeled with C.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates.

Described are the syntheses of several Ni(μ-SR)Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS) and Ni(NS) (PS = tris(phenyl-2-thiolato)phosphine, NS = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(CF). The following new nickel precursor complexes were characterized: PPh[Ni(NS)] and the dimeric imino/amino-phosphine complexes [NiCl(PCH═N)] and [NiCl(PCHNH)] (P = PhPCH-2-).

Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes.

Dichalcogenolene platinum(II) diimine complexes, (L')Pt(bpy), are characterized by charge-separated dichalcogenolene donor (L') → diimine acceptor (bpy) ligand-to-ligand charge transfer (LL'CT) excited states that lead to their interesting photophysics and potential use in solar energy conversion applications. Despite the intense interest in these complexes, the chalcogen dependence on the lifetime of the triplet LL'CT excited state remains unexplained. Three new (L')Pt(bpy) complexes with mixed chalcogen donors exhibit decay rates that are dominated by a spin-orbit mediated nonradiative pathway, the magnitude of which is proportional to the anisotropic covalency provided by the mixed-chalcogen donor ligand environment.

A [RuRu] Analogue of an [FeFe]-Hydrogenase Traps the Key Hydride Intermediate of the Catalytic Cycle.

The active site of the [FeFe]-hydrogenases features a binuclear [2Fe] sub-cluster that contains a unique bridging amine moiety close to an exposed iron center. Heterolytic splitting of H results in the formation of a transient terminal hydride at this iron site, which, however is difficult to stabilize. We show that the hydride intermediate forms immediately when [2Fe] is replaced with [2Ru] analogues through artificial maturation.

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate.

The kinetically robust hydride [t-HFe(Mepdt)(CO)(dppv)] ([t-H1]) (Mepdt = MeC(CHS); dppv = cis-1,2-CH(PPh)) and related derivatives were prepared with Fe 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.

Reaction Coordinate Leading to H 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) in the [FeFe]-hydrogenases from both Chlamydomonas reinhardtii and Desulfovibrio desulfuricans using Fe 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.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands.

The reaction of Fe(pdt)(CO) with two equivalents of PhPCHNH (PNH) affords the amido hydride HFe(pdt)(CO)(PNH)(PNH) {[HH], pdt = CH(CHS)}. Isolated intermediates in this conversion include Fe(pdt)(CO)-(κ-PNH) and Fe(pdt)(CO)(κ-PNH). X-ray crystallographic analysis of [HH] shows that the chelating amino/amido-phosphine ligands occupy -dibasal positions.

Syntheses of transition metal methoxysiloxides.

The paper describes three methods for the preparation of methoxysiloxide complexes, a rare class of complexes of relevance to room temperature vulcanization (RTV) of polysiloxanes. The salt metathesis reaction involves the use of the recently described reagent NaOSi(OMe)Me with various metal chlorides to give Cp*Ti[OSi(OMe)Me](OMe), (NN)NiOSi(OMe)Me, (IPr)CuOSi(OMe)Me, and (triphos)CoOSi(OMe)Me (Cp* = CMe, triphos = Me(CHPPh)). Several attempted reactions gave methoxide complexes instead, a pathway that is attributed to the intermediacy of κ-OSi(OMe)Me species.

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) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+).

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

The commercially practiced conversion of trimethoxymethylsilane (MTM) to [OSi(OMe)Me)]n 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.

Crystal structure of bis-(acetyl-acetonato-κ(2) O,O')(tetra-hydro-furan-κO)(tri-fluoro-methane-sulfonato-κO)iron(III).

The mononuclear title complex, [Fe(CF3O3S)(C5H7O2)2(C4H8O)] or [Fe(acac)2(OTf)(THF)] (acac = acetyl-acetonate; OTf = tri-fluoro-methane-sulfon-ate; THF = tetrahydrofuran), (I), consists of one six-coordinate Fe(3+) atom in a slightly distorted octa-hedral environment [Fe-O bond-length range = 1.9517 (11)-2.0781 (11) Å].

Crystal structure of di-μ-hydroxido-κ(4) O:O-bis[bis(acetyl-acetonato-κ(2) O,O')cobalt(III)].

The dinuclear title complex, [Co2(C5H7O2)4(μ-OH)2] or [Co(acac)2(μ-OH)]2, where acac is acetyl-acetonate, is centrosymmetric with half of the mol-ecule per asymmetric unit. The mol-ecular structure is a dimer of octa-hedrally coordinated Co(III) atoms with four O atoms from two chelating acac ligands and two O atoms from bridging hydroxide ligands. The crystal packing features weak C-H⋯O inter-actions between neighboring mol-ecules, leading to the formation of chains normal to the ac plane.

Crystal structure of tetrakis-(acetyl-acetonato)di-chloridodi-μ3-methano-lato-tetra-μ2-methano-lato-tetra-iron(III).

The title complex, [Fe4(C5H7O2)4(CH3O)6Cl2] or [Fe4(acac)4(μ2-OMe)4(μ3-OMe)2Cl2] (acac = acetyl-acetonate), crystallizes in the ortho-rhom-bic Pbca space group with one half of the mol-ecule per asymmetric unit, the other half being completed by inversion symmetry. The core structure consists of a face-sharing double pseudo-cubane entity with two opposite corners missing. Weak C-H⋯Cl inter-molecular inter-actions result in a two-dimensional layered structure parallel to the ac plane.