Sandra Künzler

Novel silylated diols and polyols were prepared using a recently developed synthesis route with bifunctionalized silyl triflates. These silyl derivatives include two triflate functions which allow a selective protection of two hydroxyl groups. Moreover, the conformation of the silyl chain in the silane backbone led to exceptional UV properties.

We report the spectroscopic characterization of protonated monosilanol (SiH₃OH₂⁺) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH₃OH₂⁺ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH₃OH₂⁺ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H₃Si⁺-OH₂ bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence p_z orbital of SiH₃⁺ and CH₃⁺.

Detection of protonated monosilanol: The IR spectrum of protonated silanol, SiH₃OH₂⁺, has been derived by resonant IR photodissociation of weakly-bound SiH₃OH₂⁺⋅Ar clusters. Its analysis provides the first spectroscopic characterization of this fundamental silanol cation and is consistent with dative bonding of H₂O to SiH₃⁺.

The highly reactive silicon congeners of cyclopropene, cyclotrisilenes (c-Si₃R₄), typically undergo either π-addition to the Si=Si double bond or σ-insertion into the Si−Si single bond. In contrast, treatment of c-Si₃Tip₄ (Tip=2,4,6-ⁱPr₃C₆H₂) with styrene and benzil results in ring opening of the three-membered ring to formally yield the [1+2]- and [1+4] cycloaddition product of the isomeric disilenyl silylene to the C=C bond and the 1,2-diketone π system, respectively. At elevated temperature, styrene is released from the [1+2]-addition product leading to the thermodynamically favored housane species after [2+2] cycloaddition of styrene and c-Si₃Tip₄.

Speed is of the essence to ring-open the peraryl-substituted cyclotrisilene (R=2,4,6-ⁱPr₃C₆H₂) and coax disilenyl silylene reactivity towards benzil and styrene from it. In case of styrene, the kinetic product with a residual R₂Si=SiR- substituent is transformed into the saturated thermodynamic product via the re-detachment of the styrene reagent at elevated temperature as confirmed by a control experiment under vacuum.

Equating (3,−1) critical points (CPs), derived from the topological analysis of the electron densities, to chemical bonds has triggered a lot of confusion in recent years. Part of this confusion stems from calling these CPs “bond” CPs (BCPs). While the origin of this terminology is traceable to the late seventies and beginning of eighties, when it sounded reasonable, new computational studies conducted on molecular electron densities cast serious doubt on the supposed universal equivalence between the chemical bonds and (3,−1) CPs. Herein, recent computational studies are briefly reviewed to demonstrate why (3,−1) CPs are not indicators of chemical bonds. It is discussed why this confusing terminology needs to be changed and reemphasized that (3,−1) CPs should be called “line” critical points (LCPs). The proposed terminology detaches the topological properties of molecular electron densities from any a priori chemical interpretation. Such detachment, if adopted by other authors, will hopefully prevent further misinterpretation of the data emerging from the quantum theory of atoms in molecules (QTAIM).

When is a bond not a bond? A careful consideration of the results of the topological analysis of electron densities within the context of the quantum theory of atoms in molecules reveals that equating (3,−1) critical points and chemical bonds, and calling them bond critical points lead to serious inconsistencies. It is argued that by calling (3,−1) critical points as line critical points part of this misinterpretation is circumvented.

Organic functional groups attract rare-synthons from main group complexes as black holes do the matter from a star. No escape, no other ways. DFT-calculations reveal that phosphasilenes can act as synthetic equivalents of highly reactive phosphinidene moieties. In the presence of properly tuned bulky groups, the usual reactivity of phosphasilenes towards unsaturated organic reactants is altered and a synthon transfer reaction takes place. For more information see the Full Paper by T. Szilvási et al. on page 17908 ff.

The paper by Gershoni-Poranne and Chen (R. Gershoni-Poranne, P. Chen, Chem. Eur. J. 2017, 23, 4659) gives an incorrect definition of covalent bonding. Furthermore, the assignment of so-called charge shift bonds in ammonium compounds has no physical foundation and is conceptually redundant.

The end of the metal age: Recent developments in small-molecule activation and chemical transformations of main-group species pose the question as to whether metal catalysts could be avoided altogether in the activation of dinitrogen. A ground-breaking study by Stephan and co-workers clearly implies that the metal-free activation of N₂ with frustrated Lewis pairs may be achievable in the not-too-distant future.

Functionalized disilenes are valuable auxiliary tools in preparative chemistry. In the following review the various synthetic potential of disilenes as precursors is presented. Upon consumption of the Si=Si unit in disilenes, cyclic, polycyclic and cluster-like arrangements are obtainable.

Double-bonding silicons: Due to their relatively weak Si=Si double bonds, disilenes are versatile starting materials for a whole variety of silicon containing products. Disilenes act as synthons for transient silylenes, the Si=Si moiety can be transferred to various substrates using functionalized derivatives, unsaturated clusters, oligomers and polymers can be obtained by exploitation of the inherently high reactivity of the Si=Si moiety towards unsaturated functional groups.

Single defluorination of aryl polyfluoromethyl functionalities is achieved by both intra- and intermolecular silylium cation/phosphine Lewis pairs. Phosphine-captured aryl fluoromethyl cations are then treated with Brønsted base to complete the first mono-hydrodefluorinations of PhCF₃, Ph₂CF₂, and PhCF₂.

Reactions of CF₃: Silylium/phosphine- based frustrated Lewis pairs are shown to effect the selective abstraction of fluoride ion from aryl-CF₃ and CF₂ groups, ultimate converting CF₃ to CF₂H to CFH₂ fragments.

A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium bora-ylide substituent, the lifetime at room temperature of the silanone is dramatically extended (t_1/2=4 days) compared to the related (amino)(phosphonium ylide)silanone VI (t_1/2=5 h), allowing easier manipulation and its use as precursor of new valuable silicon compounds. The interaction of silanone with a weak Lewis acid such as MgBr₂ increases further its stability (no degradation after 3 weeks at room temperature).

A silanone substituted by bulky amino and phosphonium bora-ylide substituents has been synthesized and isolated in crystalline form. Thanks to the exceptionally strong electron-donating phosphonium-bora-ylide substituent, the lifetime is dramatically extended (t_1/2=4 days). The interaction with a weak Lewis acid such as MgBr₂ increases further its stability, and no degradation of silanone was observed after 3 weeks at room temperature.

Chalcogen bonding is a noncovalent interaction based on electrophilic chalcogen substituents, which shares many similarities with the more well-known hydrogen and halogen bonding. Herein, the first application of selenium-based chalcogen bond donors in organocatalysis is described. Cationic bifunctionalized organoselenium compounds activate the carbon–chlorine bond of 1-chloroisochroman in a benchmark reaction. While imidazolium-based derivatives showed no noticeable activation, benzimidazolium backbones yielded potent catalysts. In all cases, syn-isomers were markedly more active, presumably due to bidentate coordination, which was confirmed by DFT calculations. Comparison experiments with the corresponding non-selenated as well as the non-cationic reference compounds clearly indicate that the catalytic activity can be ascribed to chalcogen bonding. The rate acceleration by the catalyst—compared to the non-selenated derivative—was about 10 fold.

The first application of selenium-based chalcogen bond donors in organocatalysis is described. Cationic bifunctionalized organoselenium compounds activate the carbon–chlorine bond of 1-chloroisochroman in a benchmark reaction. Several comparison experiments clearly indicate that the catalytic activity can be ascribed to chalcogen bonding.

The incorporation of carbene (R₂C:) and silylene (R₂Si:) functionalities in the same molecule is a challenging task owing to their inherent reactivity. The synthesis of a stable compound (Si−NHC) featuring an unmasked carbene as well as a silylene is accomplished for the first time by installing a silylene functionality on the backbone of a 1,3-imidazolium-derived NHC. The Si−NHC compound offers significant potential as a ligand in the design of new catalysts, and as a building block for the preparation of new molecules and materials. More information can be found in the Full Paper by R. S. Ghadwal et al. (DOI: 10.1002/chem.201703530).

Advances in the design and application of catalytic metallodrugs are highlighted in the Review by Z. Yu and J. A. Cowan on page 14113 ff., and approaches to the design of catalytic metallodrugs that selectively target a therapeutically relevant biomolecule are discussed. Incorporation of a recognition domain into catalytic metallodrugs improves substrate selectivity and many examples of catalytic metallodrugs that selectively inactivate or degrade nucleic acid, protein, or other biomolecular targets are described.

Boroles are important motifs within functional materials. With the aim to prepare a pinacolboryl-substituted derivative, the metallacycle transfer from corresponding zirconium and tin precursors has been explored. We show that the reaction of 1,1-dimethyl-2,3,4,5-tetrapinacolborylstannole with dichloro(phenyl)borane does not provide the desired borole, but instead a stannyl-substituted 1-chloroboracyclopent-3-ene. Spectroscopic and structural details of this highly functionalized boracycle indicate that intramolecular interactions between the tin and oxygen atoms of the boryl substituents may account for the unexpected outcome of the tin-boron exchange reaction.

Don′t be a stannole! The tin–boron exchange reaction of 1,1-dimethyl-2,3,4,5-tetrapinacolborylstannole with dichloro(phenyl)borane was shown to give a stannyl-substituted 1-chloroboracyclopent-3-ene instead of the desired borole (see graphic). Spectroscopic and structural properties of this highly functionalized boracycle are presented together with initial results of its reactivity.

Silicon is an extremely important technological material, but its current industrial production by the carbothermic reduction of SiO₂ is energy intensive and generates CO₂ emissions. Herein, we developed a more sustainable method to produce silicon nanowires (Si NWs) in bulk quantities through the direct electrochemical reduction of CaSiO₃, an abundant and inexpensive Si source soluble in molten salts, at a low temperature of 650 °C by using low-melting-point ternary molten salts CaCl₂–MgCl₂–NaCl, which still retains high CaSiO₃ solubility, and a supporting electrolyte of CaO, which facilitates the transport of O²⁻ anions, drastically improves the reaction kinetics, and enables the electrolysis at low temperatures. The Si nanowire product can be used as high-capacity Li-ion battery anode materials with excellent cycling performance. This environmentally friendly strategy for the practical production of Si at lower temperatures can be applied to other molten salt systems and is also promising for waste glass and coal ash recycling.

From old glass to batteries: A new and more sustainable method to produce Si nanowires in bulk quantities through the direct electrochemical reduction of CaSiO₃ at a low temperature of 650 °C was developed. The method uses the low-melting-point ternary molten salts of CaCl₂–MgCl₂–NaCl, which retain high CaSiO₃-solubility, and a supporting electrolyte of CaO, which drastically improves the reaction kinetics and enables the electrolysis at low temperatures.

Facile oxygenation of the acyclic amido-chlorosilylene bis(N-heterocyclic carbene) Ni⁰ complex [{N(Dipp)(SiMe₃)ClSi:Ni(NHC)₂] (1; Dipp=2,6-ⁱPr₂C₆H₄; N-heterocyclic carbene=C[(ⁱPr)NC(Me)]₂) with N₂O furnishes the first Si-metalated iminosilane, [DippN=Si(OSiMe₃)Ni(Cl)(NHC)₂] (3), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)–Ni π-complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3, respectively, show remarkable hydroboration reactivity towards H−B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)–Ni π-complex at low temperature.

A piece of π: An acyclic silylene–Ni⁰ complex undergoes facile oxidation with N₂O to give silanone–Ni⁰ π-complex 1 at low temperature. Upon warming, a cascade rearrangement takes place that exclusively yields the Si-metalated iminosilane 3. Remarkably, the Si=O and Si=N bonds in these complexes readily undergo addition reactions with hydroboranes even though such processes typically require a catalyst for the C=O and C=N analogues.