Tetrasila-1,3-butadiene and its transformation to disilenyl anions

Isolation of a planar 1,2-dilithio-disilene and its conversion to a Si-B hybrid 2π-electron system and a planar tetraboryldisilene

Lithium reagents have long played important roles in synthetic chemistry. However, unsaturated organosilicon lithium reagents are few in number. Herein, we describe the first isolation of a 1,2-dilithiodisilene: [(boryl)SiLi]₂ (2) was prepared in 73% yield by the reduction of (boryl)tribromosilane (1, boryl = (HCArN)₂B, Ar = 2,6-iPr₂C₆H₃) with lithium in Et₂O. The salt elimination reaction of 2 with dihaloboranes RBX₂ afforded disilaborirenes [(boryl)Si]₂BR (3a–c), whereas the reaction with two equivalents of B-bromocatecholborane ((cat)BBr) yielded the first tetraboryldisilene [(boryl)(cat)BSi]₂ (4). X-ray diffraction analysis and density functional theory calculations indicated that the disilene 2 and tetraboryldisilene 4 feature an almost planar geometry and disilaborirenes 3a–c are aromatic with a silicon–boron hybrid 2π-electron delocalized structure. The results indicate that 1,2-dilithiodisilene 2 is a powerful synthetic reagent for the construction of novel silicon multiply bonded species with unique electronic structures and that the boryl substituents have significant electronic effects on the structure of silicon multiple bonding.

Dianionic disilyne: reduction of boryltribromosilane yielded the 1,2-dilithio-disilene 2, which is a powerful transfer reagent for the synthesis of a novel 2π aromatic system and the first tetraboryldisilene.

Isolation of a 1-Magnesium-2,3-disilacyclopropene and a Related Bis(disilenide)

Herein we report the first synthesis of a 1-magnesium-2,3-disilacyclopropene and a related bis(disilenide). Reduction of (boryl)tribromosilane (1, boryl = (HCArN)₂B, Ar = 2,6-iPr₂C₆H₃) with magnesium in THF afforded boryl-substituted magnesium complex [(boryl)Si]₂Mg(THF)₃ (2) in good yield, whereas reduction of (boryl)trichlorosilane (3) with KC₈ in THF led to the isolation of bridged alkoxy alkyl bis(disilenide) (THF)K(boryl)Si═Si(boryl)O(CH₂)₄(boryl)Si═Si(boryl)K(THF) (4) via ring opening of a THF molecule. X-ray diffraction analysis of 2 confirmed the presence of the novel Si₂Mg three-membered ring as well as the Si═Si double bond, which existed in a noticeably twisted B-Si-Si-B array. Complex 2 also represents the first reported example of a stable disilyne dianion.

Functional Disilenes in Synthesis

Functionalized disilenes are valuable auxiliary tools in preparative chemistry. In the following review the various synthetic potential of disilenes as precursors is presented. Upon consumption of the Si=Si unit in disilenes, cyclic, polycyclic and cluster-like arrangements are obtainable.

Reactivity in the periphery of functionalised multiple bonds of heavier group 14 elements

Heavier group 14 multiple bonds have intrigued chemists since more than a century. The synthesis of stable compounds with double and triple bonds with silicon, germanium, tin and lead had considerable impact on modern ideas of chemical bonding. These developments were made possible by the use of bulky substituents that provide kinetic and thermodynamic protection. Since about a decade the compatibility of heavier multiple bonds with various functional groups has moved into focus. This review covers multiply bonded group 14 species with at least one additional reactive site. The vinylic functionalities of groups 1 and 17, resulting in nucleophilic and electrophilic disila vinyl groups, respectively, are the most prevalent and well-studied. They have been employed repeatedly for the transfer of heavier multiple bonds to yield low-valent group 14 compounds with novel structural motifs. Vinylic functionalities of groups 2 to 16 and a few σ-bonded transition metal complexes are experimentally known, but their reactivity has been studied to a lesser extent. Donor-coordinated multiple bonds are a relatively new field of research, but the large degree of unsaturation as isomers of alkynes (as well as residual functionality in some cases) offers considerable possibility for further manipulation, e.g. for the incorporation into more extended systems. Heavier allyl halides constitute the major part of heavier multiple bonds with a functional group in allylic position and some examples of successful transformations are given. At present, remote functionalities are basically limited to para-phenylene functionalised disilenes. The reported use of the latter for further derivatisation might encourage investigations in this direction. In summary, the study of peripherally functionalised multiple bonds with heavier group 14 elements is already well beyond its infancy and may be an instrumental factor in awakening the potential of group 14 chemistry for applications in polymers and other materials.

Preference for a propellane motif in pure silicon nanosheets

Free standing silicene nanosheets remain elusive presumably due to the instability associated with sp(2) hybridized silicon atoms. Here we show that silicon prefers nanosheets based on the non-classical Si5 unit with a [1.1.1]-propellane motif that has two inverted tetrahedral atoms bridged by three tetrahedral atoms. DFT calculations show that nanosheets constructed exclusively from propellane building blocks are consistently more stable than those with sp(2) silicon atoms or their hybrids. These nanosheets also exhibit a narrow but definite band gap, unlike those reported earlier.

Transfer of a disilenyl moiety to aromatic substrates and lateral functional group transformation in aryl disilenes

The reaction of 1 equiv of the disilenide Tip2Si═Si(Tip)Li (5; Tip = 2,4,6-(i)Pr3C6H2) with para-substituted phenyl iodides, 4-X-PhI, transfers the Tip2Si═Si(Tip) moiety with elimination of lithium iodide to yield the laterally functionalized disilenes Tip2Si═Si(Tip)(4-X-Ph) [X = H (6a), F (6b), Cl (6c), Br (6d), I (6e)]. The UV-vis absorptions of 6a-d suggest a linear correlation with electronic Hammett parameters. In addition, X-ray structural analyses of 6a-d verified the theoretically predicted linear dependence of the Si═Si bond length and trans-bent angles. The p-bromophenyl-substituted disilene 6d undergoes a metal-halogen exchange reaction to give 6f (X = Li), which was trapped with Me3SiCl to afford 6g (X = SiMe3). In the case of simple phenyl halides PhX without additional functionality, the reaction with 5 proceeded smoothly for X = Br, but phenyl chlorides and fluorides did not react at room temperature even after one week, hinting at an S(N)2-type aromatic substitution mechanism. Reactions of p- and m-diiodobenzene with 5 afford the corresponding phenylene-bridged tetrasiladienes p-7 and m-7. While red p-7 (λ(max) = 508 nm) exhibits efficient conjugation of the two Si═Si bonds with the phenylene linker, the conjugation in yellow m-7 (λ(max) = 449 nm) is much less effective. Electrochemical studies of m-7 and p-7 as well as density functional theory calculations and electron paramagnetic resonance studies of their respective radical anions provided further support for the notion of conjugation.

Anthryl-substituted trialkyldisilene showing distinct intramolecular charge-transfer transition

1-Naphthyl-, 9-phenanthryl-, and 9-anthryl-substituted trialkyldisilenes 1-3 were synthesized as the first stable disilenes with single polycyclic aromatic substituents, allowing elucidation of the unprecedented intramolecular charge transfer interaction between disilene pi and aromatic pi systems. Anthryl-substituted disilene 3 having a low-lying pi*(aryl) LUMO showed a distinct ICT absorption band due to the charge transfer from a disilene pi donor to an aromatic pi acceptor.

Electrochemical properties of a disilene, a tetrasila-1,3-butadiene, and their germanium analogues

The redox properties of aryl-substituted disilene (1), tetrasila-1,3-butadiene (2), digermene (3), and tetragerma-1,3-butadiene (4) (in which the aryl group is 2,4,6-triisopropylphenyl) have been studied by cyclic voltammetry (CV) in two different solvents. In o-dichlorobenzene, disilene 1 exhibited reversible redox couples in both oxidation and reduction processes, whereas tetrasilabutadiene 2 showed a reversible behavior only in its oxidation and an irreversible wave in its reduction. Tetragermabutadiene 4 afforded a reversible couple in its reduction and an irreversible wave in its oxidation, whereas digermene 3 showed only an irreversible oxidation wave but no reduction one. In comparison, all observed oxidation and reduction waves in THF were irreversible. In both solvents, the same trend of decreasing ease of oxidation has been observed: 2>4>3>1. The calculated vertical ionization energies (E(i)) for the 2,6-dimethylphenyl-substituted model compounds 9-12 afforded a trend of ease of oxidation that decreases in the series tetrasilabutadiene 10>tetragermabutadiene 12>disilene 9>digermene 11. This predicted trend parallels the measured oxidation potentials in the sense that the heavy butadienes are easier to oxidize than the heavy alkenes. The trend in the reduction direction has been found to be different in the two solvents studied. The computed electron affinities (E(ea)) for compounds 9-12 indicated that the ease of reduction decreased in the sequence tetragermabutadiene 12>tetrasilabutadiene 10>digermene 11>disilene 9. However, no straightforward relation between the computed electron affinities and the experimentally measured reduction potentials has been found.