Wolfgang Saak

A New Synthesis of Triarylsilylium Ions and Their Application in Dihydrogen Activation

Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions.

Silyl Cation Mediated Conversion of CO2 into Benzoic Acid, Formic Acid, and Methanol

The use of CO2 as a renewable and environmentally friendly C1 source for the synthesis of carboxylic acids and fuels such as methanol and methane is a topic of current interest. The high thermodynamic stability of CO2 clearly calls for highly efficient activation which must be combined with a strong thermodynamic driving force to ensure irreversible fixation. While in the past transition-metal chemistry played a dominant role in CO2 conversion chemistry, [5] recent years have seen the emergence of organocatalytic methods for CO2 reduction. For example N-heterocyclic carbenes have been applied for the nucleophilic activation of CO2, and subsequent reduction of the resulting imidazolium carboxylates by silanes yielded methanol. In addition stoichiometric and catalytic reductions of CO2 have been reported which utilize frustrated Lewis pairs (FLPs) for the activation, and dihydrogen, silanes, or ammonia borane as the hydrogen source. In view of the high electrophilic activity of silyl cations and their complexes with solvents and counteranions, we were intrigued by the possibility of exploiting this extreme reactivity in CO2 activation. Silanes then would be the logical hydrogen source for the reduction and would provide the desired thermodynamic driving force through the formation of siloxanes. Here we report on a metal-free reduction of CO2 by trialkylsilanes, R3SiH (R = Et, iPr), using stoichiometric amounts of trityl borate [Ph3C][B(C6F5)4]. The reduction is fast under ambient conditions, but different products— depending on the applied solvent—are formed. In chlorobenzene (PhCl) either disilylated formic acid 1 or the disilylmethyl oxonium ion 2 is formed, depending on the substituent R at the silane. Simple hydrolysis of these compounds yields formic acid and methanol (Scheme 1). In benzene (PhH), the reaction of CO2 with the preformed silylbenzenium salt [Et3Si(C6H6)][B(C6F5)4] (3[B(C6F5)4]) leads to further functionalization of CO2. [8b] In this case the benzylic cation 4 is formed, which can be easily transformed either by hydrolysis into benzoic acid (PhCO2H, 6), or, by careful deprotonation, into the silylester 5 (Scheme 1). Reaction of triethylsilane with one equivalent of the trityl borate [Ph3C][B(C6F5)4] in benzene yields benzenium ion 3, identified by its characteristic Si NMR resonance at d(Si) = 97.4 ppm (Table 1). Also when PhCl is used as the solvent, cationic complexes [R3Si(PhCl)] + (7) are formed, as indicated by the strongly deshielded Si NMR resonances at d(Si) = 99.9 (R = Et, 7a) and 103.3 ppm (R = iPr, 7b ; Table 1). Based on the results of our quantum mechanical calculations, however, we assign to these complexes the chloronium ion structure 7 rather than the chloroarenium ion structure 8. Scheme 1. Reduction of CO2 by [Ph3C][B(C6F5)4]/R3SiH in different solvents (the B(C6F5)4 anion is omitted, 1a : R = Et, 1b : R = iPr). Conditions: a) 0.1013 MPa CO2, 1 equiv Ph3C , 2 equiv R3SiH, PhCl, room temperature; b) 2 equiv Et3SiH, RT; c) H2O; d) 0.1013 MPa CO2,1 equiv Ph3C , 1 equiv Et3SiH, PhH, room temperature; e) symcollidine, PhH, temperature.

Tetrakis(2,4,6-triisopropylphenyl)diplumbene: A Molecule with a Lead–Lead Double Bond

There is good agreement between the length of the lead–lead double bond calculated for H2Pb=PbH2 and that present in the tetraaryldiplumbene 1 (R=2,4,6-iPr3C6H2), which has now been synthesized and structurally characterized. Furthermore, the calculated and experimentally observed trans bent angles Θ are similar. Thus, compounds with homonuclear double bonds between all elements of Group 14 are now known.

A carbene–stannylene adduct with a long tin–carbon double bond?

Bis(2,4,6-triisopropylphenyl)stannylene, formed by thermolysis of the cyclotristannane 4, reacts with 4,5-dimethyl-1,3-diisopropylimidazol-2-ylidene to furnish the adduct 8; an X-ray structural analysis of this compound reveals the presence of a long tin–carbon ‘double bond’ with a length of 237. 9(5) pm and a distinctly pyramidal tin centre.

Pentafluorphenylverbindungen des Zinks and Cadmiums: Bildung und Strukturen von (C6F5)2Zn(thf)2 und von tetramerem C6F5CdOH

Abstract Although single crystals of bis(pentafluorphenyl)zinc ( 1 ) and -cadmium ( 2 ) suitable for X-ray structure analyses could not be isolated, crystallization of 1 in the presence of tetrahydrofuran (THF) gives the adduct (C 6 H 5 ) 2 Zn(thf) 2 , which has tetrahedrally coordinated molecules in the solid state. Toluene, containing small amounts of water, reacts with 2 to give tetrameric pentafluorophenyl cadmium hydroxide, which has a cubane-type structure with the cadmium and the oxygen atoms occupying the corner positions.

Protic ionic liquid and ionic melts prepared from methanesulfonic acid and 1H-1,2,4-triazole as high temperature PEMFC electrolytes

Protic ionic liquid and ionic melts were prepared from the combination of methanesulfonic acid (CH3SO3H) and 1H-1,2,4-triazole (C2H3N3) at various molar ratios. The thermal properties, crystal structure, acid–base interactions, ionic conductivity, proton conduction behavior and electrochemical stability of the system were studied. The equimolar composition, 1,2,4-triazolium methanesulfonate (C2H4N3+·CH3SO3− (1)), was a proton transfer salt with a melting point of around 134 °C. Single crystal and powder XRD data, as well as TGA results, revealed that the base-rich region was a mixture of 1 and 1H-1,2,4-triazole. Infrared analysis and single crystal data suggested that the C2H3N3–CH3SO3H system exists in a strongly hydrogen-bonded network. Systematic investigation of the ionic conductivity showed that the ionic conductivity reached local maxima at the compositions of [C2H3N3]/[CH3SO3H] = 10/90 and 80/20, respectively, while it exhibited a local minimum at the equimolar composition. The temperature dependence of the ionic conductivity was found to obey the Vogel–Fulcher–Tamman (VFT) equation. The fitting of the conductivity data to the VFT equation showed that the carrier ion concentration versus the mole fraction of 1H-1,2,4-triazole exhibited a volcano shape. In addition, the C2H3N3–CH3SO3H system showed adequate electrochemical stability under PEMFC conditions as measured by linear sweep voltammetry. The relatively high ionic conductivity, wide electrochemical window and good thermal stability demonstrated that the C2H3N3–CH3SO3H system is a suitable candidate for high temperature PEMFC electrolytes.

Titanium‐Catalyzed Intermolecular Hydroaminoalkylation of Conjugated Dienes

Ti me kangaroo down: Conjugated dienes undergo intermolecular hydroaminoalkylation in the presence of Ti catalyst [Ind2 TiMe2 ] (Ind=η(5) -indenyl). This new reaction offers a highly atom-efficient approach to homoallylic amines from 1,3-butadienes.

Efficient Access to Titanaaziridines by CH Activation of N‐Methylanilines at Ambient Temperature

Titanaaziridines or η(2)-imine titanium complexes are considered key intermediates of the titanium-catalyzed hydroaminoalkylation of alkenes. Herein, we present an efficient synthetic route to this class of compounds, starting from N-methylanilines and a bis(η(5):η(1)-pentafulvene)titanium complex. Consecutive reactions on the η(2)-methyleneaniline complexes, characterized for the first time, prove a high chemical versatility. In particular, hydroaminoalkylation products were found in reactions of the three-membered titanacycles with alkenes. For the first time, all the intermediates of the hydroaminoalkylation of alkenes were isolated and characterized.

A germanium(II) cation with [1+1] coordination.

Breaking the molecular symmetry by protonation of germylene 1 is the key step in the synthesis of the germyliumylidene 2, which is stabilized by an intramolecular interaction with a distant imido group.

A Heteroleptic Diplumbene and a Magnesium Dibromide Stabilized Plumbylene Dimer

Treatment of the diarylplumbylene R2Pb: (R = 2,4,6-iPr3C6H2) with the disilylplumbylene R′2Pb: [R′ = Si(SiMe3)3] furnishes the heteroleptic plumbylene RR′Pb: which exists as the diplumbene RR′Pb=PbRR′ (7) in the solid state. The X-ray structure analysis of 7 reveals a trans-bent angle of 42.7° and a Pb–Pb bond length of 298.99(5) pm, the shortest observed so far for diplumbenes with a lead–lead double bond. Reaction of mesitylmagnesium bromide with PbCl2 yielded black crystals of an MgBr2 stabilized dimesitylplumbylene dimer with a large trans-bent angle of 71°, a lead–lead separation of 335.49(6) pm, and long Pb···Br contacts of 315.71(8) pm.