Kai Abersfelder

A Tricyclic Aromatic Isomer of Hexasilabenzene

Aromatic Silicon Benzene has long intrigued chemists on account of the energy stabilization, termed aromaticity, which arises from π-electron delocalization around its ring framework. A persistent question has been how such stabilization would be impacted were the carbons to be replaced by heavier atoms such as silicon. Abersfelder et al. (p. 564) have prepared a benzene analog with Si atoms in place of all six-ring carbons, but a slightly altered bonding framework in which substituents outside the ring are no longer evenly distributed. Instead, the substituents pair up at two Si sites, leaving two other ring sites with no external appendages. The resulting compound no longer has a continuous network of π-electrons, but retains a degree of aromatic stabilization involving sigma and nonbonding electrons. A structural isomer of benzene in which carbon is replaced by silicon exhibits unexpected electronic stabilization. Benzene represents the showcase of Hückel aromaticity. The silicon analog, hexasilabenzene, has consequently been targeted for decades. We now report an intensely green isomer of Si6R6 (R being 2,4,6-triisopropylphenyl) with a tricyclic structure in the solid state featuring silicon atoms with two, one, and no substituents outside the ring framework. The highly dispersed 29Si nuclear magnetic resonance shifts in solution ranging from +125 to −90 parts per million indicate an inhomogeneous electron distribution due to the dismutation of formal oxidation numbers as compared with that of benzene. Theoretical analysis reveals nonetheless the cyclic delocalization of six mobile electrons of the π-, σ- and non-bonding type across the central four-membered ring. For this alternative form of aromaticity, in principle applicable to many Hückel aromatic species, we propose the term dismutational aromaticity.

A Stable Derivative of the Global Minimum on the Si6H6 Potential Energy Surface

The higharomatic stabilization that is conferred by the cyclic delocal-ization of six p electrons distinguishes benzene from itsapproximately 200 theoretically known isomers and accountsfor the ubiquitous occurrence of the benzene motif in manyareas of chemistry. Despite impressive progress regarding thesynthesis of stable compounds with Si Si p bonds,

Reversible Base Coordination to a Disilene

During the last few decades, alkene and alkyne analogues of the heavier Group 14 elements have attracted considerable interest. Their isolation as stable derivatives has become possible by the use of carefully designed bulky substituents that provide kinetic (and to some extent thermodynamic) stabilization. The considerable differences in structure, bonding, and reactivity of such compounds in comparison to the carbon-based species have prompted various experimental and theoretical studies. By and large, the lower electronegativity of heavier elements and the increasing spatial extension of their valence electron shells are responsible for many of these differences. One of several rationalizations for structure and reactivity of such doubly and triply bonded species is based on zwitterionic (Ib and IIb in Scheme 1) and

Syntheses of Trisila Analogues of Allyl Chlorides and Their Transformations to Chlorocyclotrisilanes, Cyclotrisilanides, and a Trisilaindane

The rearrangements of (chlorosilyl)disilenes R2(Cl)Si-(Tip)Si=SiTip2 (5a,b: Tip = 2,4,6-iPr3C6H2, a: R = Me, b: R = Ph) quantitatively yield the isomeric chlorocyclotrisilanes (6a,b). The disilene precursors 5a,b are, in turn, accessible from the reactions of the disilenide Tip2Si=Si(Tip)Li (1), that is, a disila analogue to vinyl anions, with dichlorosilanes R2SiCl2. This novel approach to cyclotrisilanes potentially allows for the facile variation of the substitution pattern and grants access to the first anionic derivatives; while the rearrangement of 5a,b to 6a,b is slow at room temperature and additionally requires the presence of THF or other n-donors, reduction of 5b with lithium instantly yields the corresponding cyclotrisilanide (7b) without detection of any open-chained isomer. Heating of a neat sample of 5b to 150 degrees C provides a completely characterized 1,2,3-trisilaindane derivative (13), strongly supporting the intermediacy of a disilanyl silylene species that inserts into an ortho-CH bond of the phenyl substituents. The X-ray diffraction studies on single crystals of 6a,b and 7b reveal that the Si-Si bond distance in cyclotrisilanes depends significantly on the electronegativity of the opposing silicon atoms substituents, which is rationalized by density functional theory (DFT) calculations on model systems.

An experimental charge density study of two isomers of hexasilabenzene.

The similarities and differences between carbon and its heavier congener silicon generate challenging synthetic targets, especially when multiply bonded systems are concerned. The first stable compound with a Si=Si bond goes back to West et al. in 1981, and conjugated systems with Si= Si double bonds were pioneered in 1997 by Weidenbruch et al. Whether silicon analogues of benzene can show aromatic character is still a point of constant debate. The aromatic nature of silabenzenes has been predicted theoretically, but the synthesis of a stable silabenzene was not accomplished until Tokitoh et al. reported the sterically encumbered 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl-substituted monosila derivative. At the same time, Ando et al. independently reported the synthesis of 1,4-disila Dewar benzene. Only two years later, Sekiguchi et al. accomplished the synthesis of 1,2-disilabenzene by reacting RSi SiR (R = Si(CH(SiMe3)2iPr) with PhC CH in a formal [2+2+2] cycloaddition reaction. Recently, in a cooperative effort with the Roesky group, we reported the synthesis of a 1,4-disilabenzene by reacting [{PhC(NtBu)2}Si]2 with diphenyl alkyne. [8] With regards to homonuclear systems, the Scheschkewitz group recently made groundbreaking progress with the isolation of ring and cage isomers of hexasilabenzene (Scheme 1), which prompted the present experimental charge-density study. The dark-green-colored six-membered ring system 1 rearranges upon heating or UV irradiation to the red silicon cage compound 2 with a bridged propellane structure. An analogous transformation for fully saturated silicon compounds under irradiative conditions has been described by Kira and co-workers. We propose a transition of 1 to 2 by the reaction pathway in Scheme 1 (bottom). The transformation proceeds by the breaking of the Si1 Si3 and Si2 Si4 bonds in 1, followed by a twist of the four-membered silicon ring and the formation of the new Si1 Si2 and Si3 Si4 bonds (see Scheme 1 and video in the Supporting Information). In the following, we analyze the bonding situation in the ring (1) and cage (2) isomer of hexasilabenzene (TipSi)6 on the basis of experimental charge-density investigations (Figure 1). High-resolution X-ray data with (sinq/l)max =

Transmetallation reactions of a lithium disilenide

The first magnesium, copper and zinc disilenides were prepared via transmetallation reactions of a lithium disilenide and structurally characterised. The copper and zinc derivatives show red-shifted UV/vis absorptions due to admixture of metal d-orbitals to the highest occupied molecular orbital.

Isolation and Versatile Derivatization of an Unsaturated Anionic Silicon Cluster (Siliconoid).

Abstract The characteristic features of bulk silicon surfaces are echoed in the related partially substituted—and thus unsaturated—neutral silicon clusters (siliconoids). The incorporation of siliconoids into more‐extended frameworks is promising owing to their unique electronic features, but further developments in this regard are limited by the notable absence of functionalized siliconoid derivatives until now. Herein we report the isolation and full characterization of the lithium salt of an anionic R5Si6‐siliconoid, thus providing the missing link between silicon‐based Zintl anions and siliconoid clusters. Proof‐of‐principle for the high potential of this species for the efficient transfer of the intact unsaturated R5Si6 moiety is demonstrated by clean reactions with representative electrophiles of Groups 13, 14, and 15.

The Cp*Si+ cation as a stoichiometric source of silicon

The Cp*Si(+) cation acts as a stoichiometric source of silicon in the reaction with the disilenide Tip(2)Si=Si(Tip)Li (Tip = 2,4,6-(i)Pr(3)C(6)H(2)) affording known neutral unsaturated silicon clusters. It thereby provides a conceptually different approach to this novel class of compounds. The proposed mechanism involves a Cp*-substituted cyclotrisilene in which Cp*(-) acts as a leaving group upon single electron reduction or in a nucleophilic substitution step.

Synthesis of homo- and heterocyclic silanes via intermediates with Si=Si bonds

An account is given of our efforts in the synthesis of homo- and heterocyclic silanes via occasionally stable unsymmetrically substituted disilene intermediates of the A2Si=Si(A)B type that are accessible from a disila analog of a vinyl lithium. This approach is particularly powerful in the preparation of three-membered rings such as cyclotrisilanes with a residual functionality that can be either electro- or nucleophilic in nature and can even give rise to ring-expanded products with preserved Si=Si moiety. The intermediacy of transient silylenes in some of these transformations is discussed as well as the structural effects of the electronic properties of the residual functional group. The relevance of these studies for the understanding of Si(100) surface annealed species during the epitaxial growth of elemental silicon is pointed out, and the potential of the methodology for the synthesis of unusual polycyclic silicon clusters is noted.