Robert Müller

From silicon(II)-based dioxygen activation to adducts of elusive dioxasiliranes and sila-ureas stable at room temperature

Dioxygen activation for the subsequent oxygenation of organic substrates that involves cheap and environmentally friendly chemical elements is at the cutting edge of chemical research. As silicon is a non-toxic and highly oxophilic element, the use of silylenes could be attractive for facile dioxygen activation to give dioxasiliranes with a SiO(2)-peroxo ring as versatile oxo-transfer reagents. However, the latter are elusive species, and have been generated and studied only in argon matrices at -233 degrees C. Recently, it was demonstrated that unstable silicon species can be isolated by applying the concept of donor-acceptor stabilization. We now report the first synthesis and crystallographic characterization of dioxasiliranes stabilized by N-heterocyclic carbenes that feature a three-membered SiO(2)-peroxide ring, isolable at room temperature. Unexpectedly, these can undergo internal oxygen transfer in toluene solution at ambient temperature to give a unique complex of cyclic sila-urea with C=O --> Si=O interaction and the shortest Si=O double-bond distance reported to date.

Activation of Ammonia by a Si═O Double Bond and Formation of a Unique Pair of Sila-Hemiaminal and Silanoic Amide Tautomers

The new silanone complex 3 is accessible in 82% yield and is capable of undergoing addition of ammonia under mild conditions, yielding the sila-hemiaminal 4 and, at the same time, its unique tautomer 5, the first silanoic amide. The unexpected formation of 3 is due to the presence of the basic exocyclic methylene group in the C(3)N(2) ligand backbone. Strikingly, the tautomers 4 and 5 are in equilibrium in solution and can be cocrystallized in benzene or THF solutions having a SiOH...O=Si hydrogen bond as confirmed by single-crystal X-ray diffraction analysis.

A neutral, monomeric germanium(I) radical

Stoichiometric reduction of the bulky β-diketiminato germanium(II) chloride complex [((But)Nacnac)GeCl] ((But)Nacnac = [{N(Dip)C(Bu(t))}(2)CH](-), Dip = C(6)H(3)Pr(i)(2)-2,6) with either sodium naphthalenide or the magnesium(I) dimer [{((Mes)Nacnac)Mg}(2)] ((Mes)Nacnac = [(MesNCMe)(2)CH](-), Mes = mesityl) afforded the radical complex [((But)Nacnac)Ge:](•) in moderate yields. X-ray crystallographic, EPR/ENDOR spectroscopic, computational, and reactivity studies revealed this to be the first authenticated monomeric, neutral germanium(I) radical.

A Trimetallic Gold Boride Complex with a Fluxional Gold–Boron Bond

Stable compounds featuring gold–boron bonds are rare, and until 2006, were limited to those containing hypercoordinate boron ligands, for example, polymetallic monoboron clusters and complexes in which a {LAu} fragment formally bridges an apical boron atom of a carborane cluster and another metal atom. Linking low-coordinate boron to gold has proved a challenging task, but this has recently been shown to reflect little more than a synthetic roadblock. Traditional methods for constructing transition-metal–boron bonds (based on B/M polarity) were found to be unsuitable: Au appears to lack sufficient reducing strength or nucleophilicity to perturb boron–halide bonds or to donate to an nonbridging Lewis acidic borane. Synthetic strategies based on the opposite polarity (B /M) were unthinkable until very recently, because of the absence of boron-based nucleophiles. In the last three years, success has been found with both of these strategies, and two new boron–gold coordination modes have been discovered, namely the borane gold complexes (Au!B), and the boryl gold complexes (Au !B), by the groups of Bourissou, and Yamashita and Nozaki, respectively. Unlike the earlier complexes, these two new bonding patterns are considered to be classical two-center-two-electron interactions, adding to their novelty. The borane gold complexes feature one or more tethered phosphine groups binding orthogonally to the boron–gold axis (B-Au-L: 79–848, L = chelating phosphino group), while in the boryl gold complexes, the B-Au-L (L = phosphine, Nheterocyclic carbene) axis is effectively linear. 5] There is a correspondingly large difference between the Au B distances of borane gold complexes (2.32–2.66 ) and those of boryl gold complexes (2.08–2.09 ). These structural patterns correlate well with the generally accepted understanding of the boryl ligand as a pure s-donor, and the borane ligand as a pure s-acceptor. Herein we experimentally and theoretically examine the structure of an unusual dimanganese boryl gold complex with a bonding situation that is not described correctly by either of the abovementioned patterns. The anionic complex [Li(dme)3] [{(h C5H4Me)(OC)2Mn}2B] (1, dme = 1,2-dimethoxyethane; Scheme 1), and its nucleophilic reactivity with methyl

Synthesis, Structure, and Reactivity of Borole‐Functionalized Ferrocenes

Herein, we report on the synthesis of ferrocenylborole [Fc(BC(4)Ph(4))(2)] featuring two borole moieties in the 1,1-positions. The results of NMR and UV/Vis spectroscopy and X-ray diffraction studies provided conclusive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc(BC(4)Ph(4))(2)] and the 4-Me-NC(5)H(4) adduct of monoborole [Fc(BC(4)Ph(4))], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in general was nicely demonstrated by the reaction of [Fc(BC(4)Ph(4))] with 4,4-bipyridine. Unlike common borane derivatives such as [FcBMe(2)], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4-bipyridine remaining uncoordinated. In addition, the reduction chemistry of ferrocenylboroles [Fc(BC(4)Ph(4))] and [Fc(BC(4)Ph(4))(2)] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC(4)Ph(4))] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either η(5)-coordinated C(5)H(4) or η(5)-coordinated BC(4)Ph(4) moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC(4)Ph(4))(2)], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to further enhance the borole ratio in ferrocenylboroles aiming at the synthesis of [Fc(BC(4)Ph(4))(4)] failed and, instead, generated an uncommon ansa-ferrocene containing two borole fragments in the 1,1-positions and a B(2)C(4) ansa-bridge.

From Silylone to an Isolable Monomeric Silicon Disulfide Complex.

The synthesis and characterization of the first bis-N-heterocyclic carbene stabilized monomeric silicon disulfide (bis-NHC)SiS2 2 (bis-NHC=H2 C[{NC(H)C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) is reported. Compound 2 is prepared in 89u2009% yield from the reaction of the zero-valent silicon complex (silylone) 1 [(bis-NHC)Si] with elemental sulfur. Compound 2 can react with GaCl3 in acetonitrile to give the corresponding (bis-NHC)Si(S)S→GaCl3 Lewis acid-base adduct 3 in 91u2009% yield. Compound 3 is also accessible through the reaction of the unprecedented silylone-GaCl3 adduct [(bis-NHC)Si→GaCl3 ] 4 with elemental sulfur. Compounds 2, 3, and 4 could be isolated and characterized by elemental analyses, HR-MS, IR, (13) C- and (29) Si-NMR spectroscopy. The structures of 3 and 4 could be determined by single-crystal X-ray diffraction analyses. DFT-derived bonding analyses of 2 and 3 exhibited highly polar Si-S bonds with moderate pπ -pπ bonding character.

Elimination of interfering compounds during GC determination of toxaphene residues by photodechlorination reactions

SummaryThe HPGC determination of Toxaphene in the presence of other chlorinated hydrocarbons with similar retention times is often difficult. This problem can be satisfactorily overcome by including photodehalogenation reactions of interfering substances. Irradiation of the extracts after clean-up procedures in protonated solvents with wavelengths at 254 nm lead to photodechlorinated products of these compounds with shorter retention times, while Toxaphene is mostly stable and can therefore be quantitatively determined by gas-chromatography.

An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone

The first isolable molecular silicon dicarbonate complex (bis-NHC)Si(CO3 )2 2 (bis-NHC=H2 C[{NC(H)=C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) was synthesized by facile reaction of the bis-N-heterocyclic carbene stabilized silylone (bis-NHC)Si 1, bearing a zero-valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis-NHC)SiO2 3 supported by the bis-NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero-valent silicon atom in 1 by two molar equivalents of CO2 under liberation of CO; its subsequent Lewis acid-base reaction with CO2 leads to 2 which has been fully characterized including an single-crystal X-ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum-chemically.

Analysis of Tertiary Phosphanes, Arsanes, and Stibanes as Bridging Ligands in Dinuclear Group 9 Complexes

The unusual bridging and semi-bridging binding mode of tertiary phosphanes, arsanes, and stibanes in dinuclear low-valent Groupu20059 complexes have been studied by density functional methods and bonding analyses. The influence of various parameters (bridging and terminal ligands, metal atoms) on the structural preferences and bonding of dinuclear complexes of the general composition [A(1)M(1)(μ-CH(2))(2)(μ-EX(3))M(2)A(2)] (M(1), M(2) = Co, Rh, Ir; A(1), A(2) = F, Cl, Br, I, κ(2)-acac; E = P, As, Sb, X = H, F, CH(3)) has been analyzed. A number of factors have been identified that favor bridging or semi-bridging modes for the phosphane ligands and their homologues. A more symmetrical position of the bridging ligand EX(3) is promoted by more polar E-X bonding, but by less electronegative (softer) terminal anionic ligands. Among the Groupu20059 metal elements Co, Rh, and Ir, the computations clearly show that the 4d element rhodium exhibits the largest preference for a {M(1)(μ-EX(3))M(2)} bridge, in agreement with experimental observation. Iridium complexes should be valid targets, whereas cobalt does not seem to support well a symmetric bridging mode. Analyses of the Electron Localization Function (ELF) indicate a competition between a delocalized three-center bridge bond and direct metal-metal bonding.

Taming Silicon Congeners of CO and CO2: Synthesis of Monomeric SiII and SiIV Chalcogenide Complexes

The synthesis of the unprecedented monomeric SiII selenide complex (bis-NHC)Si=Se→GaCl3 2 (bis-NHC=bis-N heterocyclic carbene, H2 C[{NC(H)=C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ), bearing the elusive SiSe ligand as a heavy CO homologue by the reaction of the silylone-GaCl3 adduct (bis-NHC)Si→GaCl3 1 with elemental selenium in acetonitrile, is reported. The similar conversion of 1 with excess selenium conducted in THF afforded the SiSe2 complex (bis-NHC)Si(=Se)Se→GaCl3 3. Remarkably, the reaction of 1 with Te=P(nBu)3 as a gentle Te transfer reagent led to the isolation of the monomeric SiTe2 complex (bis-NHC)SiTe2 4, the first structurally characterized Lewis acid free heavy CO2 homologue complex. The isolated compounds 2, 3, and 4 have been fully characterized, including single-crystal X-ray diffraction analyses. Their electronic structures and spectroscopic data have also been studied by quantum-chemical calculations.