André Schäfer

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.

Iron-Catalyzed Dehydrocoupling/Dehydrogenation of Amine–Boranes

The readily available iron carbonyl complexes, [CpFe(CO)2]2 (1) and CpFe(CO)2I (2) (Cp = η-C5H5), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine-borane Me2NH·BH3 (3) to afford the cyclodiborazane [Me2N-BH2]2 (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by (11)B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me2N═BH2 (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me2NH-BH2-NMe2-BH3 (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp2Fe2(CO)3(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (≤10 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine-borane ligated [CpFe(CO)](+) as a key intermediate.

Iron‐Catalyzed Dehydropolymerization: A Convenient Route to Poly(phosphinoboranes) with Molecular‐Weight Control

The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.

Dihydrogen Activation by a Silylium Silylene Frustrated Lewis Pair and the Unexpected Isomerization Reaction of a Protonated Silylene

The isolable silylene 1 forms a frustrated Lewis pair with silylium ion 2 that is able to split hydrogen under ambient conditions. The protonated silylene 3 obtained in this reaction isomerizes to yield the hydrogen-bridged disilyl cation 4. Cation 4 is fully characterized by NMR spectroscopy and by XRD analysis. Its formation via the protonated silylene 3 is indicated by independent synthesis of cation 3 by hydride abstraction from the corresponding dihydridosilane 11 and is further supported by the results of density functional calculations.

σ-π conjugated organosilicon hybrid polymers from copolymerization of a tetrasiladiene and 1,4-diethynylbenzene.

A catalyst- and by-product-free protocol for the synthesis of σ-π conjugated organosilicon polymers is reported. The regiospecific [2+2] cycloaddition of C≡C triple bonds to Si=Si double bonds allowed the preparation of air-stable ethynyl-terminated extended monomers from 1,4-bis(ethynyl)benzene and the para-phenylene bridged tetrasiladiene, Tip2 Si=SiTip-pC6H4-SiTip=SiTip2 (Tip = 2,4,6-iPr3C6H2). The polymer obtained from the extended monomer and further tetrasiladiene exhibits pronounced σ-π conjugation, as was evident from the red-shift in the absorption spectrum compared to model systems. We show that the thermal stability of the employed bis(alkyne) co-monomer is translated into this polymer.

Cyclic Silylated Onium Ions of Group 15 Elements

Five- and six-membered cyclic silylated onium ions of group 15 elements I were synthesized by intramolecular cyclization of transient silylium ions II. Silylium ions II were prepared by the hydride transfer reaction from silanes III using trityl cation as hydride acceptor. It was found that smaller ring systems could not be obtained by this approach. In these cases tritylphosphonium ions IV were isolated instead. Cations I and IV were isolated in the form of their tetrakispentafluorphenyl borates and characterized by multinuclear NMR spectroscopy and, in two cases, by X-ray diffraction analysis. Cyclic onium ions I showed no reactivity similar to that of isoelectronic intramolecular borane/phosphane frustrated Lewis pairs (FLPs). The results of DFT computations at the M05-2X level suggest that the strength of the newly formed Si-E linkage is the major reason for inertness of I[B(C6F5)4] versus molecular hydrogen.

Carbene Complexes of Stannocenes

Several stannocene carbene complexes, 3a-3g, were synthesized and examined in solution by NMR spectroscopy and in the solid state by single-crystal X-ray diffraction. In this new class of metallocene carbene complexes, coordination of the carbene to the tin atom was found to be comparably weak and mostly due to attractive dispersion forces, as indicated by density functional theory calculations. Furthermore, coordination of the N-heterocyclic carbenes results in a weakening of the Sn-Cp bonds, making these complexes very reactive and short-lived at room temperature.