Neue Element-Kohlenstoff-(p-p)μ-Bindungen

Metathesis Reactions of a NHC-Stabilized Phosphaborene

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P-protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N-heterocyclic carbene (NHC)-stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P-terminal, as a room-temperature-stable crystalline solid accessible via facile NHC-induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P-centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X-ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

Geometrically Constrained Cationic Low-Coordinate Tetrylenes: Highly Lewis Acidic σ-Donor Ligands in Catalytic Systems

A novel non-innocent ligand class, namely cationic single-centre ambiphiles, is reported in the phosphine-functionalised cationic tetrylene Ni0 complexes, [PhR DippENi(PPh3 )3 ]+ (4 a/b (Ge) and 5 (Sn); PhR Dipp={[Ph2 PCH2 SiR2 ](Dipp)N}- ; R=Ph, i Pr; Dipp=2,6-i Pr2 C6 H3 ). The inherent electronic nature of low-coordinate tetryliumylidenes, combined with the geometrically constrained [N-E-Ni] bending angle enforced by the chelating phosphine arm in these complexes, leads to strongly electrophilic EII centres which readily bind nucleophiles, reversibly in the case of NH3 . Further, the GeII centre in 4 a/b readily abstracts the fluoride ion from [SbF6 ]- to form the fluoro-germylene complex PhR DippGe(F)Ni(PPh3 )3 9, despite this GeII centre simultaneously being a σ-donating ligand towards Ni0 . Alongside the observed catalytic ability of 4 and 5 in the hydrosilylation of alkynes and alkenes, this forms an exciting introduction to a multi-talented ligand class in cationic single-centre ambiphiles.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

Directed Gas Phase Formation of Silene (H2 SiCH2 )

The silene molecule (H₂ SiCH₂ ; X¹ A₁ ) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH₄ ). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH₂ SiH₃ ; X² A') complex followed by hydrogen migration to the methylsilyl radical (SiH₂ CH₃ ; X² A'). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H₂ SiCH₂ ; X¹ A₁ ). The exploration of the elementary reaction of methylidyne with silane delivers a unique view at the widely uncharted reaction dynamics and isomerization processes of the carbon-silicon system in the gas phase, which are noticeably different from those of the isovalent carbon system thus contributing to our knowledge on carbon silicon bond couplings at the molecular level.

Heterodiatomic Multiple Bonding in Group 13: A Complex with a Boron-Aluminum π Bond Reduces CO2

Heterodiatomic multiple bonds have never been observed within Group 13. Herein, we disclose a method that generates [(CAAC)PhB=AlCp^3t ] (1), a complex featuring π bonding between boron and aluminum through the association of singlet fragments. We present the properties of this multiple bond as well as the reactivity of the complex with carbon dioxide, which yields a boron CO complex via an unusual metathesis reaction.

Limitations of Steric Bulk: Towards Phospha-germynes and Phospha-stannynes

The use of bulky aryl(silyl)amides (R) as substituents for the stabilisation of phospha-germynes and phospha-stannynes (R-Ge≡P and R-Sn≡P, respectively) is described. Such species can be transiently generated by photolysis of the phosphaketene precursors (RE(PCO); E=Ge, Sn). Utilisation of bulky amides R¹ and R² (R¹ =Ar**NSi(OtBu)₃ , where Ar**=2,6-bis[bis(4-tert-butylphenyl)methyl]-4-methylphenyl; R² =Ar***NSi(iPr)₃ , where Ar***=2,6-bis[bis(3,5-di-tert-butylphenyl)methyl]-4-methylphenyl) facilitates the formation of diphosphene-type dimers, [(RGe)P]₂ and [(RSn)P]₂ . In an effort to circumvent dimerisation, the bulkier R³ substituent (R³ =Ar***NSi(4-tert-butylphenyl)₃ ) was employed in an analogous series of experiments. This affords cyclic germylenes and stannylenes due to insertion of the terminal phosphide into Si-C bonds of the R³ substituent, which in case of the stannylene could act as a trap for another R³ -Sn≡P moiety. All attempts to isolate terminal phosphide species were unsuccessful due to the reactivity of such compounds towards the organic periphery of the bulky amides, highlighting the limitations of highly sterically demanding functionalities.

Synthesis of a Metallo-Iminosilane via a Silanone-Metal π-Complex

Facile oxygenation of the acyclic amido-chlorosilylene bis(N-heterocyclic carbene) Ni⁰ complex [{N(Dipp)(SiMe₃ )ClSi:→Ni(NHC)₂ ] (1; Dipp=2,6-ⁱ Pr₂ C₆ H₄ ; N-heterocyclic carbene=C[(ⁱ Pr)NC(Me)]₂ ) with N₂ O 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.

Donor-Functionalized Polydentate Pyrylium Salts and Phosphinines: Synthesis, Structural Characterization, and Photophysical Properties

A series of donor-functionalized pyrylium salts have been prepared by classical condensation reactions which were further converted into the corresponding thienyl- and pyridyl-substituted polydentate lambda(3)-phosphinines by reaction with P(SiMe(3))(3). Further chemical modification of these phosphorus heterocycles with Hg(OAc)(2) in the presence of methanol resulted in the formation of lambda(5)-phosphinines. The photophysical properties of a selected series of thienyl- and pyridyl-functionalized pyrylium salts, lambda(3)- and lambda(5)-phosphinines, were investigated and the results compared and supported by theoretical calculations on the DFT level. Significant fluorescence was observed for the pyrylium salts and lambda(5)-phosphinines. In contrast, the heteroaromatic substituted lambda(3)-phosphinines show very little emission which is consistent with the low oscillator strength predicted by DFT calculations for this pi-->pi* transition. Furthermore, all three classes of compounds show readily observable phosphorescence in solution, which was determined by time-gated detection at low temperature.

Regioselective formation of beta-alkyl-alpha-phenyliridabenzenes via unsymmetrical 3-vinylcyclopropenes: probing steric and electronic influences by varying the alkyl ring substituent

The synthesis of unsymmetrical (Z)-1-alkyl-3-(2-iodovinyl)-2-phenyl-1-cyclopropenes (R=Me (8 a), Et (8 b), iPr (8 c), and tBu (8 d)) and their reactions with Vaska's complex [Ir(CO)Cl(PPh3)2] and its trimethylphosphine analogue [Ir(CO)Cl(PMe3)2] were investigated. Iridabenzvalene (13/20), iridabenzene (14/21), and/or eta(5)-cyclopentadienyliridium complexes (15/22) were obtained in modest yields and were fully characterized by spectroscopic means. X-ray structural data was secured for iridabenzvalene 13 d and iridabenzenes 14 a,b,d. Whereas iridabenzenes 14 a-c were stable at 75 degrees C for 48 h, 14 d, which possesses a bulky tBu group, rearranged cleanly to cyclopentadienyliridium 15 d at 50 degrees C over 15 h and displayed first-order kinetics. The influence of the alkyl substituent on the mechanisms of iridacycle generation, isomerization, and iridabenzene regioselectivity is discussed.

Recent Advances in Metallabenzene Chemistry

Research into aromatic metallacycles, though discussed in the literature over the last quarter century, has undergone a major expansion since 2000. A wide variety of new metallabenzenes, encompassing new synthetic methods and new metal centers, is now available. New aromatic metallacycle topologies (iridanaphthalene, osmabenzynes) have been isolated and characterized. The first metallabenzene valence isomers (iridabenzvalenes, rhodabenzvalenes) and constitutional isomers (isoosmabenzenes) are now known. This review discusses the synthesis, chemistry, and physical properties of these intriguing aromatic compounds.