Shigeyoshi Inoue

Isolation and Structure of Germylene-Germyliumylidenes Stabilized by N-Heterocyclic Imines

The ditopic germanium complex FGe(NIPr)2 Ge[BF4 ] (3[BF4 ]; IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) is prepared by the reaction of the amino(imino)germylene (Me3 Si)2 NGeNIPr (1) with BF3 ⋅OEt2 . This monocation is converted into the germylene-germyliumylidene 3[BAr(F) 4 ] [Ar(F) =3,5-(CF3 )2 -C6 H3 ] by treatment with Na[BAr(F) 4 ]. The tetrafluoroborate salt 3[BF4 ] reacts with 2 equivalents of Me3 SiOTf to give the novel complex (OTf)(GeNIPr)2 [OTf] (4[OTf]), which affords 4[BAr(F) 4 ] and 4[Al(OR(F) )4 ] [R(F) =C(CF3 )3 ] after anion exchange with Na[BAr(F) 4 ] or Ag[Al(OR(F) )4 ], respectively. The computational, as well as crystallographic study, reveals that 4(+) has significant bis(germyliumylidene) dication character.

A Stable Neutral Compound with an Aluminum–Aluminum Double Bond

Homodinuclear multiple-bonded neutral Al compounds, aluminum analogues of alkenes, have been a notoriously difficult synthetic target over the past several decades. Herein, we report the isolation of a stable neutral compound featuring an Al═Al double bond stabilized by N-heterocyclic carbenes. X-ray crystallographic and spectroscopic analyses demonstrate that the dialuminum entity possesses trans-planar geometry and an Al-Al bond length of 2.3943(16) Å, which is the shortest distance reported for a molecular dialuminum species. This new species reacts with ethylene and phenyl acetylene to give the [2+2] cycloaddition products. The structure and bonding were also investigated by detailed density functional theory calculations. These results clearly demonstrate the presence of an Al═Al double bond in this molecule.

From Si(II) to Si(IV) and Back: Reversible Intramolecular Carbon–Carbon Bond Activation by an Acyclic Iminosilylene

Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a C═C bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV-vis studies of this Si(IV) compound indicated a facile transformation back to Si(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene-borane adduct 3 following addition of B(C6F5)3. This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si(IV) dihydride 6, respectively.

A Tin Analogue of Carbenoid: Isolation and Reactivity of a Lithium Bis(imidazolin‐2‐imino)stannylenoid

The lithium bis(imino)stannylenoid (NIPr)2 Sn(Li)Cl (1; NIPr=bis(2,6-diisopropylphenyl)imidazolin-2-imino) was prepared by the reaction of LiNIPr with 0.5 equiv of SnCl2 ⋅diox (diox=1,4-dioxane) and the ambiphilic character of the compound was demonstrated by investigations into its reactivity. Treatment of 1 with I2 or MeI yielded the oxidative addition products (NIPr)2 SnI2 and (NIPr)2 Sn(Me)I, respectively. In contrast, the reaction of 1 with one equivalent of Me3 SiCl resulted in the formation of Me3 SiNIPr and the chlorostannylene dimer [NIPrSnCl]2 . Moreover, the substitution reaction of compound 1 with MeLi led to the formation of the methyl-substituted stannate (NIPr)2 Sn(Li)Me.

Silicon and Oxygen’s Bond of Affection: An Acyclic Three-Coordinate Silanone and its Transformation to an Iminosiloxysilylene

A long-term dream comes true: An acyclic, neutrally charged silanone at last! Here, we report on the first examples of isolable acyclic, neutral, three-coordinate silanones 2 with indefinite stability as solids and lifetimes in solution of up to 2 days. The electronic properties of the Si═O bond were investigated via DFT calculations and revealed the π-donating N-heterocyclic imino (NHI) and σ-donating silyl groups as key factors for their enhanced stability. Besides initial reactivity studies of 2 toward CO2 and methanol, different isomerization pathways depending on the silyl substitution pattern were found. For 2a (R = TMS), a 1,3-silyl shift gave an intermediary disilene, which was trapped as unique NHC-disilene adduct 6. For the more stable silanone 2b (R = t-Bu), a selective transformation to the first reported room temperature stable, acyclic, two-coordinate N,O-silylene 7 exhibiting a fascinating siloxy ligand was observed. Both compounds were fully characterized experimentally and their bonding features were analyzed by theoretical calculations.

Twist of a Silicon–Silicon Double Bond: Selective Anti-Addition of Hydrogen to an Iminodisilene

Hydrogenation of alkenes with C═C bonds is a ubiquitous reaction in organic chemistry. However, this transformation remains unknown for heavier counterparts, disilenes with Si═Si bonds. Here we report the isolation of (Z)-diiminodisilyldisilene 2 featuring a highly trans-bent and twisted structure and the longest silicon-silicon double bond reported to date. In silico studies suggested that the Si═Si bond in 2 is described as very weak double donor-acceptor bond. We utilized the remarkable electronic and structural features of this product to achieve the first demonstration of hydrogen activation by a multiply bonded silicon compound under ambient conditions. Interestingly, NMR and X-ray analysis gave exclusively racemic (RR/SS)-1,2-disilane 3a, indicating a stereospecific trans-hydrogenation of the Si═Si bond. In-depth calculations revealed that in strong contrast to the reactivity of C═C bonds, a concerted anti-addition pathway was favored due to the twisted structure of 2.

Facile Access to Stable Silylium Ions Stabilized by N-Heterocyclic Imines.

Novel silylium ions with N-heterocyclic imines were successfully synthesized. The reaction of trimethylsilyl imidazolin-2-imine Me3SiNIPr (NIPr = bis(2,6-diisopropylphenyl)-imidazolin-2-imino) with B(C6F5)3 leads to dimeric imino-substituted silylium ions through a methyl group abstraction on the silicon atom. Meanwhile, the intermolecular imino-coordinated silylium ion is formed by using the less sterically crowded imine Me3SiNItBu (NItBu = bis(tert-butyl)-imidazolin-2-imino). Furthermore, the treatment of dimethylchlorosilane Me2(Cl)SiNIPr with AgOTf affords the contact ion pair Me2(OTf)SiNIPr by substitution of the chloride. A novel complex with the formula [Me2(DMAP)SiNIPr][OTf] was prepared by coordination with 4-dimethylamino-pyridine (DMAP). In the solid state, the DMAP adduct [Me2(DMAP)SiNIPr][OTf] contains a distinct [Me2(DMAP)SiNIPr]+ moiety.

Synthesis of a cyclopentadienyl(imino)stannylene and its direct conversion into halo(imino)stannylenes

The reaction of stannocene Cp2Sn with iminolithium LiNIPr (NIPr = bis(2,6-diisopropylphenyl)imidazolin-2-iminato) afforded the dimeric cyclopentadienyl(imino)stannylene [(η1-Cp)SnNIPr]2 (1). Compound 1 exhibits unexpected reactivity towards haloalkanes. The high-yield conversion of 1 into chlorostannylene [ClSnNIPr]2 (2) and bromostannylene [BrSnNIPr]2 (3) were accomplished by treatment with dichloromethane or 1,2-dibromoethane, respectively, through a Cp-substitution reaction.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Synthesis and reactivity of novel amino(imino)metallylenes

GRAPHICAL ABSTRACT ABSTRACT Novel amino(imino)germylene and stannylene were prepared by conversion of HNIPr [NIPr = bis(2,6-diisopropylphenyl)imidazolin-2-imino] with one equiv of Lapperts reagent (E[N(SiMe3)2]2, E = Ge, Sn). The divalent nature of the iminometallylenes was proved by their reactivity toward Lewis acid [Fe2(CO)9] as well as Lewis base [DMAP (para-dimethylaminopyridine)]. Furthermore, azido-substituted stannylene was isolated in its dimeric form [N3SnNIPr]2 from the reaction of the iminostannylene with one equiv of trimethylsilyl azide. Notably, the reactions of the iminometallylenes with B(C6F5)3 furnish the cyclic germanium(II) and tin(II) cations, respectively, though methyl-abstraction and ring-closing reaction.