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(CF). 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 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 CO 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.

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.

The activation of NH and CO is still an ambitious target for multiply bonded sub-valent silicon compounds. Now, the precise splitting of the N-H bond of ammonia by (Z)-imino(silyl)disilene 1 to give trans-1,2-adduct 2 a at low temperatures (-78 °C) is presented. According to DFT calculations, the stereospecific hydroamination follows a similar mechanism as the recently reported anti-addition of H to the Si=Si bond of 1. The aminosilane 2 b could also be obtained as the formal silylene addition product under thermodynamic reaction control. By applying low temperatures, the activation of CO with 1 selectively afforded the cis-oxadisilacyclobutanone 7-c as [2+2] cycloadduct. By performing the reaction directly at ambient temperatures, a mixture of three different-sized silacycles (4-6) was observed. Their formation was investigated theoretically and their structures were revealed with separate experiments using 1 and the oxygenation agents N O and O .

A comprehensive reactivity study of an acyclic iminosiloxysilylene provides further insights into this relatively unexplored compound class and revealed analogies to both classical transition metal complexes and the lighter silicon congener carbon.