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Heavier alkene analogues possess unique electronic properties and reactivity, encouraging multidisciplinary research groups to utilize them in the rational design of novel classes of compounds and materials. Phosphasilenes are heavier imine analogues, containing highly reactive Si=P double bonds. Recent achievements in this field are closely related to the progress in the chemistry of stable low-coordinate silicon compounds. In this Review, we have attempted to summarize in a comprehensive way the available data on the structures, syntheses, electronic and chemical properties of these compounds, with an emphasis on recent achievements.

Doubly bonded neighbors: phosphasilenes are heavier imine analogues, containing very reactive Si=P double bonds. This Review summarizes structures, syntheses, electronic, and chemical properties of these and related species with an emphasis on recent achievements.

The dimesitylborinium ion (“horse”), which is the most electron-deficient isolable two-coordinate boron cation, was expected to be transformed into a product with an increased coordination number for the boron atom upon a chemical reaction. However, this boron cation undergoes twofold 1,2-carboboration with diphenylacetylene to form another isolable borinium ion (“Pegasus”) with an extended π-conjugated system. This unusual reactivity is described by Y. Shoji and co-workers in their Communication on page 5312 ff.

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.

CAACs—versatile, readily available, and storable: Combined with most of the elements of the periodic table cyclic (alkyl)(amino)carbenes (CAACs), have influenced various fields of chemistry, and a lot remains to be discovered.

Reduction of the stable germabenzene, 1-Tbt-2-tert-butyl-germabenzene (Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), with KC₈ resulted in the formation of 2-tert-butylgermabenzenylpotassium, that is, the germanium analogue of phenylpotassium, under concomitant elimination of the aryl group from the Ge atom. Under an inert atmosphere, this germabenzenylpotassium could be isolated in the form of stable yellow crystals. In the crystalline state, as well as in solution, the germabenzenyl moiety adopts a monomeric form, even though the X-ray diffraction analysis suggests the presence of highly reactive Ge=C double bonds. The spectroscopic and X-ray crystallographic analyses, in combination with theoretical calculations indicate an ambident character for this germabenzenyl anion, with contributions from aromatic and germylene resonance structures.

Ge is not C: Reduction of a stable germabenzene with KC₈ resulted in the formation of the germanium analogue of phenylpotassium, under concomitant elimination of the aryl group from the Ge atom. Its ambident character with contributions from aromatic and germylene resonance structures was supported experimentally and theoretically.

The cyclic alkyl(amino) carbene-anchored silylene–phosphinidene was isolated as L−Si−P(:cAAC−Me) (L=benzamidinate) at room temperature, synthesized from the reduction of L−Si(Cl₂)−P(:cAAC−Me) (1) using two equivalents of KC₈. Compound 1 was prepared by the oxidative addition of a chlorophosphinidene to the benzamidinate substituted silylene center. This is the first molecular example of a silylene–phosphinidene characterized by single-crystal X-ray structural analysis. Moreover, ¹H, ³¹P, and also ²⁹Si NMR spectroscopic data supported the formulation of the products. The theoretical calculations of compound 2 are in good agreement with the experimental results.

Ein Silylen-Phosphiniden mit cyclischer Alkyl(amino)carben-Einheit wurde in Form von L−Si−P(:cAAC−Me) (L=Benzamidinat) bei Raumtemperatur isoliert. Die Charakterisierung erfolgte mit ¹H-, ³¹P- und ²⁹Si-NMR-Spektroskopie sowie, erstmalig für ein Silylen-Phosphiniden, auch durch Röntgen-Strukturanalyse an Einkristallen. Rechnungen geben zusätzliche Einblicke in die Bindungssituation.

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon–boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([2]⁻), which was obtained as an orange crystalline solid. Single-crystal X-ray diffraction analysis and reactivity studies on [2]⁻ confirmed the double-bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl⁻ to the boron center. A thermal reaction of [2]⁻ afforded a bicyclic product by formal intramolecular C−H insertion across the Si=B bond of 1, which was corroborated by a theoretical study.

The potassium salt of a borasilene–chloride adduct was synthesized and isolated. Its reactivity and single-crystal X-ray diffraction analysis revealed the double-bond character of its Si=B bond and reduced Lewis acidity. This adduct provides a cyclic product by formal intramolecular C−H insertion across the Si=B bond, possibly via the corresponding base-free borasilene.