Sandra Künzler

An unusual germole-to-silole transformation is described. As key intermediates hetero-fulvenes are formed which rearrange to more stable bicyclic carbene analogues. The so-formed germylenes undergo a reductive elimination yielding elemental germanium and siloles. In contrast, the analogous silylenes are stable at ambient conditions and were identified by MS spectrometry and NMR spectroscopy supported by the results of quantum mechanical calculations. These bicyclic silylenes are stable derivatives of the global minimum of the C₄Si₂H₆ potential energy surface.

Always expect the unexpected: An unprecedented germole to silole transformation opened the way for the synthesis of novel bicyclic silylenes that are stable derivatives of the global minimum on the C₄Si₂H₆ potential energy surface.

The solution structure of AlMe₂F and its reactivity with a prototypical ansa-metallocene have been investigated by advanced NMR techniques, in an attempt to indirectly shed some light on the structure and working principles of methylalumoxane (MAO) mixtures in olefin polymerization. In solution, AlMe₂F gives rise to a complex equilibrium of oligomeric species, including a heterocubane [(Me₂Al)₄F₄] tetramer, resembling the behavior of MAO. This complex mixture reacts with (ETH)ZrMe₂ (ETH=rac-[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]) to afford [(ETH)ZrMe^δ+(μ-F)(AlMe₂F)_nAlMe₃^δ−] inner-sphere ion pairs through successive insertions/deinsertions of AlMe₂F units into the Zr⋅⋅⋅(μ-F) bond.

One after the other: AlMe₂F mimics the behavior of AlMeO units in methylalumoxane (MAO) mixtures and can thus be used to simulate metallocene/MAO interactions in [metallocenium^δ+⋅⋅⋅MAO^δ−] polymerization catalysts. Mixtures of AlMe₂F and metallocenes contain several inner-sphere ion pairs that equilibrate via successive insertions/deinsertions of AlMe₂F units into the Zr⋅⋅⋅(μ-F) bonds.

An homologous series of divinylchalcogenophene-bridged binuclear ruthenium complexes [{(PMe₃)₃Cl(CO)Ru}₂(µ-CH=CH-C₄H₂E-CH=CH)] (4a–4d, E = O, S, Se, Te) have been synthesised and fully characterised by X-ray crystallography and various spectroscopic techniques. The single-crystal X-ray diffraction results reveal a distinct short/long bond-length alternation along the polyene-like hydrocarbon backbone, with geometric constraints imposed by the chalcogenophene leading to an increasing distance between the two metal centres (d_Ru–Ru) in complexes 4a–4d as the heteroatom in the five-membered ring is changed from oxygen (9.980 Å in 4a) to tellurium (11.063 Å in 4d). The complexes undergo two sequential one-electron oxidation processes, the half-wave potential and separation of which appear to be sensitive to a range of factors, including aromatic stabilisation and re-organisation energies. Analysis of [4a–4d]ⁿ⁺ (n = 0, 1, 2) by UV/Vis/NIR and IR spectroelectrochemical methods, supported by DFT calculations (n = 0, 1), revealed that the redox character of the complexes is dominated by the polyene-like backbone with the chalcogenide playing a subtle but influential, structural rather than electronic, role. In the radical cations [4a–4d]⁺, the charge is rather effectively delocalised over the 10-atom Ru–[4-s-cis-all-trans-(CH=CH)₄]–Ru chain, giving rise to a species with spectroscopic properties not dissimilar to oxidised polyaceylene.

What's in a name? Frontier orbitals of divinylchalcogenophene-bridged binuclear ruthenium complexes and the radical cations derived by one-electron oxidation exhibit considerable octatetraene character, thus making these systems better described as organic redox systems rather than Class III mixed-valence complexes.

Chalcogen bonding is a little explored noncovalent interaction similar to halogen bonding. This manuscript describes the first application of selenium-based chalcogen bond donors as Lewis acids in organic synthesis. To this end, the solvolysis of benzhydryl bromide served as a halide abstraction benchmark reaction. Chalcogen bond donors based on a bis(benzimidazolium) core provided rate accelerations relative to the background reactivity by a factor of 20–30. Several comparative experiments provide clear indications that the observed activation is due to chalcogen bonding. The performance of the chalcogen bond donors is superior to that of a related brominated halogen bond donor.

Activation Se(en) from a different angle: Chalcogen bond donors (chalcogen-based Lewis acids) accelerate a halide abstraction benchmark reaction by a factor of 20–30. Several comparative experiments provide clear indications that the observed activation is due to chalcogen bonding.

The reaction of Ph₂P(O)H with B(C₆F₅)₃ provided the Lewis pair complex Ph₂(H)POB(C₆F₅)₃ (5) containing a bipolar ⁺P–O–B^– linkage. The reaction of 5 with [(tht)AuCl] (tht = tetrahydrothiophene) proceeded with elimination of HCl and formation of [{(C₆F₅)₃BOPPh₂}Au(tht)] (6), which can be regarded as push–pull complex between a zwitterionic OPPh₂Au unit and a hard Lewis acid, B(C₆F₅)₃, coordinating at the oxygen atom, and a soft Lewis base, tht, coordinating at the gold atom.

[{(C₆F₅)₃BOPPh₂}Au(tht)] contains a push–pull complex between a zwitterionic OPPh₂Au unit and a hard Lewis acid, B(C₆F₅)₃, coordinating at the oxygen atom, and a soft Lewis base, tht, coordinating at the gold atom (tht = tetrahydrothiophene).

The reaction of naphthalene-1,8-diylbis[(trimethylsilyl)methanide] and dimethyl arylboronates afforded the corresponding 2,3-dihydro-1H-naphtho[1,8-cd]borinine as single diastereomers. Single-electron oxidation of the boron-containing heterocycles provided acenaphthylene through the generation boron-containing cyclic singlet biradicals.

2,3-Dihydro-1H-naphtho[1,8-cd]borinines were synthesized as single diastereomers from naphthalene-1,8-diylbis[(trimethylsilyl)methanide] and dimethyl arylboronates. A single-electron oxidation of the boron-containing heterocycles provided acenaphthylene by generating boron-containing cyclic singlet biradicals.

This review is dedicated to a relatively new, but nevertheless very quickly evolving field of chemistry – closomer chemistry. Closomers are complex organoelement molecules built on a [B₁₂]^2– icosahedral platform that have the combined benefits of the symmetry and stability of polyhedral boranes and the diversity of organic chemistry. On a material level, closomers can be considered as monodisperse molecular nanoparticles because of their size, spherical symmetry, and ease of adjusting the arm length. The ease of twelvefold side-chain functionalization and the possibility of heterosubstitution makes closomers very useful medical tools for targeted and traceable imaging and drug delivery applications.

This review is dedicated to the chemistry of closomers – monodisperse molecular nanoparticles built on a [B₁₂]^2– icosahedral platform, incorporating combined benefits of polyhedral boranes and organic molecules. The diverse side-chain functionalization and heterosubstitution possibilities make closomers very useful medical tools for targeted and traceable imaging and drug delivery applications.

The first 16 valence electron [bis(NHC)](silylene)Ni⁰ complex 1, [(^TMSL)ClSi:Ni(NHC)₂], bearing the acyclic amido-chlorosilylene (^TMSL)ClSi: (^TMSL=N(SiMe₃)Dipp; Dipp=2,6-Prⁱ₂C₆H₄) and two NHC ligands (N-heterocyclic carbene=:C[(Prⁱ)NC(Me)]₂) was synthesized in high yield and structurally characterized. Compound 1 is capable of facile dihydrogen activation under ambient conditions to give the corresponding HSi-NiH complex 2. Most notably, 1 reacts with catechol borane to afford the unprecedented hydroborylene-coordinated (chloro)(silyl)nickel(II) complex 3, {[cat(^TMSL)Si](Cl)Ni:BH(NHC)₂}, via the cleavage of two B−O bonds and simultaneous formation of two Si−O bonds. The mechanism for the formation of 3 was rationalized by means of DFT calculations, which highlight the powerful synergistic effects of the Si:Ni moiety in the breaking of incredibly strong B−O bonds.

Si–Ni cooperativity: The first 16 valence electron amido-chlorosilylene nickel(0) complex 1 readily activates H₂ to give the hydrido(silyl)nickel(II) complex 2. With catechol borane, 1 affords the deep-blue hydroborylene nickel(II) complex 3, the first example of a hydroborylene metal complex and is the result of Si^II−Ni⁰ cooperativity in the B−O bond activation processes.

Herein we report synthesis, structure and properties of a new type of twisted nanographene, which contains an [8]circulene moiety in a polycyclic framework of 96 sp² carbon atoms. The key steps in this synthesis are the Diels–Alder reaction of a macrocyclic diyne and the subsequent Scholl reaction forming the [8]circulene moiety. Two incompletely cyclized products were isolated from the Scholl reaction, providing insight into the cyclization of the strained octagon. This nanographene is twisted along two directions with end-to-end twists of 142.4° and 140.2° as revealed by X-ray crystallography, and is flexible at room temperature as found from the computational and experimental studies.

A new type of twisted nanographene containing an [8]circulene moiety in a polycyclic framework of 96 sp² carbon atoms was synthesized from a macrocyclic diyne (see picture). Its structure was studied with X-ray crystallography and DFT calculations.

An isolable donor-stabilized silavinylidene phosphorane was synthesized. This molecule, which can also be regarded as a new carbon(0) complex featuring a phosphine and a donor-stabilized silylene ligand, presents a central carbon atom with a remarkably high electron density (−1.82). Furthermore, the experimental electron-density study of this compound demonstrates the delocalization of the σ-lone pair at the central carbon atom toward the silicon center, a feature which is remarkably different from electronic situation of other bent-allene-type molecules. This result clearly demonstrates the powerful electron-donating ability of donor-stabilized silylene ligands, as well as their excellent electron-acceptor properties.

Generous donors: Donor-stabilized silavinylidene phosphoranes were synthesized and isolated as stable molecules. These new carbon(0) complexes, featuring a phosphine and donor-stabilized silylene (DSS) ligand, present a central carbon atom with a remarkably high electron density (−1.82). This result clearly demonstrates the powerful electron-donating ability of DSS ligands toward atomic carbon.

An example of an octaphosphane of type R₂P₈ (R=(DDP)Ga) was isolated by treatment of cage compound (DDP)GaP₄ (2, DDP=(2,6-diisopropylphenyl)(4-((2,6-diisopropylphenyl)imino)pent-2-en-2-yl)amide) with (C₆F₅)₂PBr. The initially formed endo-exo butterfly shaped pentaphosphane 7 rapidly rearranges to the more stable exo–exo isomer 8, which undergoes dimerization to decaphosphane 11. Compound 11 unexpectedly eliminates tetraaryldiphosphane 13 to give tetracyclo[3.3.0.0^2,7.0^4,6]octaphosphane [(DDP)GaBr]₂P₈ (12). The reaction steps were confirmed by crystal structure analysis of the key intermediates and supported by kinetic studies using NMR techniques.

Building up polyphosphanes: A neutral tetracyclic polyphosphane of the composition R₂P₈ (R=DDPGa, DDP=chelating N-donor ligand) was isolated by addition of halophosphanes to main-group-element-functionalized white phosphorus. Initially formed pentaphosphane inverts, dimerizes, and eliminates a diphosphane, yielding R₂P₈ selectively.

Although chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, the catalytic ability of chiral tertiary sulfonium salts has yet to be demonstrated in asymmetric synthesis. Herein, we show that chiral bifunctional trialkylsulfonium salts catalyze highly enantioselective conjugate additions of 3-substituted oxindoles to maleimides under base-free neutral phase-transfer conditions.

Sulfonium catalyst: Whereas chiral quaternary ammonium and phosphonium salts are commonly used for asymmetric organocatalysis, chiral tertiary sulfonium salts have not been employed for such purposes. It is now shown that chiral bifunctional trialkylsulfonium salts catalyze the enantioselective conjugate addition of 3-substituted oxindoles to maleimides under base-free neutral phase-transfer conditions.

The increasing attention devoted to triangulenes and their heteroatom derivatives inspired us to explore a divergent synthesis of heteroatom-centered 4,8,12-triazatriangulenes, which involved the preparation of a nitrogen-containing macrocyclic precursor and subsequent central heteroatom introduction by electrophilic C−Li and C−H substitution. The boron-centered triangulene has a planar structure unlike the bowl-shaped phosphorus- and silicon-centered triangulenes. The described synthetic procedure can be used to fabricate a broad range of attractive functional materials, for example, for organic light-emitting diodes, based on heteroatom-centered triangulenes.

Spoilt for choice: Boron-, phosphorus-, and silicon-centered 4,8,12-triazatriangulenes were synthesized. The key step involved the efficient incorporation of the heteroatom into a nitrogen-containing macrocyclic precursor through electrophilic C−Li and C−H substitution (see scheme).

The nucleophilicity and Lewis basicity of sterically hindered phosphines, widely used in catalysis and in frustrated Lewis pair (FLP) chemistry, have been quantified by determining the rates and equilibrium constants of their associations with reference systems (benzhydrylium and tritylium ions) of calibrated electrophilicities and Lewis acidities. These structure–reactivity investigations allow a rationalization of the Lewis acid–base interactions all along the way from covalent Lewis adducts to FLPs. Comparisons of the association of phosphines of increasing sizes (Ph₃P, (o-tolyl)₃P, and tBu₃P) with the triarylborane B(C₆F₅)₃ and with the isoelectronic tritylium ions Ar₃C⁺ provide detailed insights for the future fine-tuning of the reactivities of FLPs. As a proof of concept, tritylium-ion-derived FLPs were shown to react with alkynes, as reported for the FLPs derived from the benchmark triarylborane B(C₆F₅)₃.

The Lewis basicities and nucleophilicities of sterically hindered phosphines were determined and used for investigating the structure of carbenium-ion-derived frustrated Lewis pairs and their reactivities towards terminal alkynes (see figure).