Metallorganische Aspekte der Fischer-Tropsch-Synthese

Potential Precursors for Terminal Methylidene Rare-Earth-Metal Complexes Supported by a Superbulky Tris(pyrazolyl)borato Ligand

A series of solvent-free heteroleptic terminal rare-earth-metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [TptBu,Me LnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,Me LnMe(AlMe4 )] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3 X (X=Cl, I) gave [TptBu,Me LnX2 ] in high yields. The addition of only one equivalent of SiMe3 Cl to [TptBu,Me LuMe(AlMe4 )] selectively afforded the desired mixed methyl/chloride complex [TptBu,Me LuMeCl]. Further reactivity studies of [TptBu,Me LuMeCl] with LiR or KR (R=CH2 Ph, CH2 SiMe3 ) through salt metathesis led to the monomeric mixed-alkyl derivatives [TptBu,Me LuMe(CH2 SiMe3 )] and [TptBu,Me LuMe(CH2 Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3 X featuring more weakly coordinating moieties (here X=OTf, NTf2 ). X-ray structure analyses of this diverse set of new [TptBu,Me LnMeR/X] compounds were performed to reveal any electronic and steric effects of the varying monoanionic ligands R and X, including exact cone-angle calculations of the tridentate tris(pyrazolyl)borato ligand. Deeper insights into the reactivity of these potential precursors for terminal alkylidene rare-earth-metal complexes were gained through NMR spectroscopic studies.

Reactions in the Photocatalytic Conversion of Tertiary Alcohols on Rutile TiO2 (110)

According to textbooks, tertiary alcohols are inert towards oxidation. The photocatalysis of tertiary alcohols under highly defined vacuum conditions on a titania single crystal reveals unexpected and new reactions, which can be described as disproportionation into an alkane and the respective ketone. In contrast to primary and secondary alcohols, in tertiary alcohols the absence of an α‐H leads to a C−C‐bond cleavage instead of the common abstraction of hydrogen. Surprisingly, bonds to methyl groups are not cleaved when the alcohol exhibits longer alkyl chains in the α‐position to the hydroxyl group. The presence of platinum loadings not only increases the reaction rate but also opens up a new reaction channel: the formation of molecular hydrogen and a long‐chain alkane resulting from recombination of two alkyl moieties. This work demonstrates that new synthetic routes may become possible by introducing photocatalytic reaction steps in which the co‐catalysts may also play a decisive role.

Gallium Methylene

Despite the eminent importance of metal alkylidene species for organic synthesis and industrial catalytic processes, molecular homoleptic metal methylene compounds [M(CH₂ )_n ] as the simplest representatives, have remained elusive. Reports on this topic date back to 1955 when polymeric [Li₂ (CH₂ )]_n and [Mg(CH₂ )]_n were accessed by pyrolysis of methyllithium and dimethylmagnesium, respectively. However, the insoluble salt-like composition of these compounds has impeded their application as valuable reagents. We report that rare-earth metallocene methyl complexes [(C₅ Me₅ )₂ Ln{(μ-Me)₂ GaMe₂ }] (Ln=Lu, Y) trigger the formation of homoleptic gallium methylene [Ga₈ (μ-CH₂ )₁₂ ] from trimethylgallium [GaMe₃ ] (Me=methyl) via a cascade C-H bond activation involving the dodecametallic clusters [(C₅ Me₅ )₆ Ln₃ (μ₃ -CH₂ )₆ Ga₉ (μ-CH₂ )₉ ] as crucial intermediates. Such gallium methylene compounds feature a reversible [Ga₈ (μ-CH₂ )₁₂ ]/[Ga₆ (μ-CH₂ )₉ ] oligomer switch in donor solvents and act as Schrock-type methylene-transfer reagents.

Valence-to-core X-ray emission spectroscopy of iron-carbonyl complexes: implications for the examination of catalytic intermediates

Valence-to-core X-ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely [Fe(CO)5], [Fe2(CO)9], [Fe3(CO)12], [Fe(CO)3(cod)] (cod=cyclo-octadienyl), [Fe2Cp2(CO)4] (Cp=cyclo-pentadienyl), [Fe2Cp*2(CO)4] (Cp*=tetramethylcyclopentadienyl), and [FeCp(CO)2(thf)][B(ArF)4] (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.

CH Activation and CC Double Bond Formation Reactions in Iridiumortho-Methyl Arylphosphane Complexes

The Vaska-type iridium(I) complex [IrCl(CO){PPh(2)(2-MeC(6)H(4))}(2)] (1), characterized by an X-ray diffraction study, was obtained from iridium(III) chloride hydrate and PPh(2)(2,6-MeRC(6)H(3)) with R=H in DMF, whereas for R=Me, activation of two ortho-methyl groups resulted in the biscyclometalated iridium(III) compound [IrCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)] (2). Conversely, for R=Me the iridium(I) compound [IrCl(CO){PPh(2)(2,6-Me(2)C(6)H(3))}(2)] (3) can be obtained by treatment of [IrCl(COE)(2)](2) (COE=cyclooctene) with carbon monoxide and the phosphane in acetonitrile. Compound 3 in CH(2)Cl(2) undergoes intramolecular C-H oxidative addition, affording the cyclometalated hydride iridium(III) species [IrHCl(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}] (4). Treatment of 2 with Na[BAr(f) (4)] (Ar(f)=3,5-C(6)H(3)(CF(3))(2)) gives the fluxional cationic 16-electron complex [Ir(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}(2)][BAr(f) (4)] (5), which reversibly reacts with dihydrogen to afford the delta-agostic complex [IrH(CO){PPh(2)(2,6-CH(2)MeC(6)H(3))}{PPh(2)(2,6-Me(2)C(6)H(3))}][BAr(f)(4)] (6), through cleavage of an Ir-C bond. This species can also be formed by treatment of 4 with Na[BAr(f)(4)] or of 2 with Na[BAr(f)(4)] through C-H oxidative addition of one ortho-methyl group, via a transient 14-electron iridium(I) complex. Heating of the coordinatively unsaturated biscyclometalated species 5 in toluene gives the trans-dihydride iridium(III) complex [IrH(2)(CO){PPh(2)(2,6-MeC(6)H(3)CH=CHC(6)H(3)Me-2,6)PPh(2)}][BAr(f) (4)] (7), containing a trans-stilbene-type terdentate ligand, as result of a dehydrogenative carbon-carbon double bond coupling reaction, possibly through an iridium carbene species.