The Silabenzenes: Structure, Properties, and Aromaticity

Structures and Properties of Planar Ge3O3 Cluster and Its Buckled Honeycomb Two-Dimensional Nanostructure

The discovery of graphene and its excellent properties inspired the search for more two-dimensional (2D) materials. Understanding the structures and properties of the smallest repeating units as well as crystal 2D materials is helpful for designing 2D materials. As germanium tends to form three-dimensional structures, the preparation of germanium-based 2D nanomaterials is still a challenge. Herein, we report a Ge3O3 cluster with the potential to construct a germanium oxide 2D nanostructure. We conduct a combined anion photoelectron spectroscopy and theoretical study on Ge3O3-/0. The structure of Ge3O3- is a Cs symmetric nonplanar six-membered ring, while that of Ge3O3 is a D3h symmetric planar six-membered ring. Chemical bonding analyses reveal that Ge3O3 exhibits π aromaticity. First-principle results suggest that a buckled honeycomb 2D nanostructure with a wide band gap of 3.14 eV may be produced based on Ge3O3, which is promising in optoelectronic applications especially in blue, violet, and ultraviolet regions.

On-surface synthesis of disilabenzene-bridged covalent organic frameworks

Substituting carbon with silicon in organic molecules and materials has long been an attractive way to modify their electronic structure and properties. Silicon-doped graphene-based materials are known to exhibit exotic properties, yet conjugated organic materials with atomically precise Si substitution have remained difficult to prepare. Here we present the on-surface synthesis of one- and two-dimensional covalent organic frameworks whose backbones contain 1,4-disilabenzene (C₄Si₂) linkers. Silicon atoms were first deposited on a Au(111) surface, forming a AuSi_x film on annealing. The subsequent deposition and annealing of a bromo-substituted polyaromatic hydrocarbon precursor (triphenylene or pyrene) on this surface led to the formation of the C₄Si₂-bridged networks, which were characterized by a combination of high-resolution scanning tunnelling microscopy and photoelectron spectroscopy supported by density functional theory calculations. Each Si in a hexagonal C₄Si₂ ring was found to be covalently linked to one terminal Br atom. For the linear structure obtained with the pyrene-based precursor, the C₄Si₂ rings were converted into C₄Si pentagonal siloles by further annealing.

Revival of Hückel Aromatic (Poly)benzenoid Subunits in Triplet State Polycyclic Aromatic Hydrocarbons by Silicon Substitution

By employing density functional theory (DFT) calculations we show that mono- and disilicon substitution in polycyclic aromatic hydrocarbons, having two to four benzene units, quenches their triplet state antiaromaticity by creating Hückel aromatic (poly)benzenoid subunit(s) and weakly antiaromatic, or almost nonaromatic silacycle. Therefore, such systems are predicted to be globally aromatic in both the ground state and the first excited triplet state. Putting the silicon atom(s) into various positions of a hydrocarbon provides an opportunity to tune the singlet-triplet energy gaps. They depend on the global aromaticity degree which, in turn, depends on the type of aromatic carbocyclic subunit(s) and the extent of their aromaticity. On the basis of the set of studied compounds, some preliminary rules on how to regulate the extent of global, semiglobal and local aromaticity are proposed. The results of this work extend the importance of Hückel aromaticity concept to excited triplet states which are usually characterized by the Baird type of (anti)aromaticity.

Fluoromaticity: The Molecular Orbital Contributions of Fluorine Substituents to the π-Systems of Aromatic Rings

The addition of fluorine atoms to an aromatic ring brings about an additional set of π-bonding and antibonding orbitals culminating after the addition of the sixth fluorine with a new set of π-aromatic-like orbitals that affect the molecule in a way that we will refer to hereafter as "fluoromaticity". Depending on the number and position of the fluorine atoms, the contributed π-orbitals can even further stabilize the ring leading to smaller bond lengths within the ring and higher resistance to addition reactions. This added ring stability partially explains the high thermostability and chemical resistance found in polymers containing fluorinated aromatics in their architecture. A similar molecular orbital effect is seen with the addition of other halogen atoms to aromatic rings, though to a much smaller degree and not resulting in the additional ring stability.

One- and Two-Electron Transfer Oxidation of 1,4-Disilabenzene with Formation of Stable Radical Cations and Dications

Electron‐transferable oxidants such as B(C₆F₅)₃/nBuLi, B(C₆F₅)₃/LiB(C₆F₅)₄, B(C₆F₅)₃/LiHBEt₃, Al(C₆F₅)₃/(o‐RC₆H₄)AlH₂ (R=N(CMe₂CH₂)₂CH₂), B(C₆F₅)₃/AlEt₃, Al(C₆F₅)₃, Al(C₆F₅)₃/nBuLi, Al(C₆F₅)₃/AlMe₃, (CuC₆F₅)₄, and Ag₂SO₄, respectively were employed for reactions with (L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂ (L=PhC(NtBu)₂, 1). The stable radical cation [1]^+. was formed and paired with the anions [nBuB(C₆F₅)₃]⁻ (in 2), [B(C₆F₅)₄]⁻ (in 3), [HB(C₆F₅)₃]⁻ (in 4), [EtB(C₆F₅)₃]⁻ (in 5), {[(C₆F₅)₃Al]₂(μ‐F)]⁻ (in 6), [nBuAl(C₆F₅)₃]⁻ (in 7), and [Cu(C₆F₅)₂]⁻ (in 8), respectively. The stable dication [1]²⁺ was also generated with the anions [EtB(C₆F₅)₃]⁻ (9) and [MeAl(C₆F₅)₃]⁻ (10), respectively. In addition, the neutral compound [(L)₂Si₂C₄(SiMe₃)₂(C₂SiMe₃)₂][μ‐O₂S(O)₂] (11) was obtained. Compounds 2–11 are characterized by UV‐vis absorption spectroscopy, X‐ray crystallography, and elemental analysis. Compounds 2–8 are analyzed by EPR spectroscopy and compounds 9–11 by NMR spectroscopy. The structure features are discussed on the central Si₂C₄‐rings of 1, [1]^+., [1]²⁺, and 11, respectively.

Comparison of group 14 elements in sp3 and sp2 environment by fragment structure energy analysis

To explore the characteristics of group 14 elements in sp³ and sp² hybrid environments systematically, the concept of fragment structure energy (FSE) was proposed through ab initio molecular orbital method. A fragment in this study means a building block of molecular structure and the energy of each kind of fragments (FSE) is obtained from the total electronic energy of different size of model compounds by just a simple algebraic method. The model compounds are the hydrogen-terminated diamond and graphene analogs as the typical compounds in the ideal sp³ - and sp² -environment, respectively. As a result, the sp³ environment was found to be more stable than the sp² for all the group 14 elements (C-Pb) in the ideal hybrid environment. Furthermore, a mixing effect of carbon and heavier group 14 elements (E = Si, Ge, Sn, and Pb) was investigated by comparing the E/C-alternately mixed- and the pure FSEs.

Disila- and digermabenzenes

Reactions of isolable disilynes and digermynes with alkynes can result in the formation of the corresponding disila- (DSBs) and digermabenzenes (DGBs), wherein two carbon atoms of the benzene ring are replaced by silicon or germanium atoms. Detailed structural and spectroscopic analyses of these DSBs and DGBs have revealed that they exhibit considerable aromaticity, comparable to that of benzene. However, in contrast to the all-carbon system benzene, these DSBs and DGBs are highly reactive toward small molecules such as oxygen, hydrogen, 1,3-dienes, and water. During the investigation of their reactivity, we discovered that a 1,2-DGB works as a catalyst for the cyclotrimerization of arylalkynes, which provides access to the corresponding 1,2,4-triarylbenzenes. In this perspective article, our recent progress in the area of DSB and DGB chemistry is summarized.

Probing the tautomerization of disilenes and disilabenzenes with their isomeric silylenes: significant substituent, aromaticity and base effects

Disilene has attracted considerable interest due to the trans-bending geometry which is significantly different from the planar alkene. However, the equilibrium between disilene and isomeric silylsilylene has not been fully understood. Here, we report a density functional theory (DFT) study on this equilibrium. Calculations reveal significant effects of substituent, aromaticity and base. Specifically, the parent disilene is thermodynamically more stable than the isomeric silylene. When the methoxy substituent is introduced, the corresponding silylene becomes thermodynamically more stable, which could be rationalized by the Bent's rule. Interestingly, disilabenzene becomes thermodynamically more stable than the isomeric silylene when the concept of aromaticity is taken into account. Finally, once the base is introduced, the silylene could become thermodynamically more stable than the isomeric disilabenzene. The kinetic effect of the tautomerization with several typical substituents (F, Me and OMe) has also been investigated. Some species with a bridged form have been found to have a higher thermodynamic stability over the nonbridged ones. All these findings could be particularly useful to develop the chemistry of disilenes and silylenes.

The planarity of heteroatom analogues of benzene: Energy component analysis and the planarization of hexasilabenzene

There are various nonplanar heteroatom analogues of benzene-cyclic 6π electron systems-and among them, hexasilabenzene (Si₆ H₆ ) is well known as a typical example. To determine the factors that control their planarity, quantum chemical calculations and an energy component analysis were performed. The results show that the energy components mainly controlling the planarity of benzene and hexasilabenzene are different. For hexasilabenzene, electron repulsion energy was found to be significantly important for the planarity. The application of the pseudo Jahn-Teller effect and the Carter-Goddard-Malrieu-Trinquier model for the interpretation of the planarity of the benzene analogues was also investigated. Furthermore, based on the quantitative results, it was revealed that the planarization of hexasilabenzene is realized by introducing substituents with π-accepting ability, such as the boryl group, that bring about a reduction of the π-electron repulsion on the silicon skeleton. © 2018 Wiley Periodicals, Inc.

The formation of a 1,4-disilabenzene and its isomerization into a disilabenzvalene derivative

A stable 1,4-disilabenzene was generated from the reaction of a stable disilyne with 3-hexyne.

Recent Advances in Metallaaromatic Chemistry

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Group 14 inorganic hydrocarbon analogues

This Review article deals with the synthesis and properties of inorganic hydrocarbon analogues: binary chemical species that contain heavier Group 14 elements (Si, Ge, Sn or Pb) and hydrogen as components. Rapid advances in our general knowledge of these species have enabled the development of industrially relevant processes such as the hydrosilylation of unsaturated substrates and the chemical vapor deposition of semi-conducting films.

Structures and electronic properties of B2Si6−/0/+: anion photoelectron spectroscopy and theoretical calculations

The lowest-energy structures of B₂Si₆^q(q= −1, 0, +1) clusters are a peculiar structure with a silicon atom hanging over a distorted bowl-like B₂Si₅framework. It is characterized with σ or π delocalization in chemical bonding.

Reexamination of the origin of the pseudo Jahn–Teller puckering instability in silicene

The pseudo Jahn–Teller puckering instability in the basic ring unit of silicene is due to the coupling of its ground state with two excited states through the puckering mode.

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.

Structures and chemical properties of silicene: unlike graphene

The discovery of graphene and its remarkable and exotic properties have aroused interest in other elements and molecules that form 2D atomic layers, such as metal chalcogenides, transition metal oxides, boron nitride, silicon, and germanium. Silicene and germanene, the Si and Ge counterparts of graphene, have interesting fundamental physical properties with potential applications in technology. For example, researchers expect that silicene will be relatively easy to incorporate within existing silicon-based electronics. In this Account, we summarize the challenges and progress in the field of silicene research. Theoretical calculations have predicted that silicene possesses graphene-like properties such as massless Dirac fermions that carry charge and the quantum spin Hall effect. Researchers are actively exploring the physical and chemical properties of silicene and tailoring it for wide variety of applications. The symmetric buckling in each of the six-membered rings of silicene differentiates it from graphene and imparts a variety of interesting properties with potential technological applications. The pseudo-Jahn-Teller (PJT) distortion breaks the symmetry and leads to the buckling in silicenes. In graphene, the two sublattice structures are equivalent, which does not allow for the opening of the band gap by an external electric field. However, in silicene where the neighboring Si atoms are displaced alternatively perpendicular to the plane, the intrinsic buckling permits a band gap opening in silicene in the presence of external electric field. Silicene's stronger spin orbit coupling than graphene has far reaching applications in spintronic devices. Because silicon prefers sp(3) hybridization over sp(2), hydrogenation is much easier in silicene. The hydrogenation of silicene to form silicane opens the band gap and increases the puckering angle. Lithiation can suppress the pseudo-Jahn-Teller distortion in silicene and hence can flatten silicene's structure while opening the band gap. So far, chemists have not successfully synthesized and characterized a free-standing silicene. But recently chemists have successfully produced silicene sheets and nanoribbons over various substrates such as silver, diboride thin films, and iridium. The supporting substrate critically controls the electronic properties of silicene, and the match of the appropriate support and its use is critical in applications of silicene.

Electron-Deficiency Aromaticity in Silicon Nanoclusters

Aromaticity in silicon-containing molecules has been a controversy for more than a century. Combining molecular dynamics simulations with ab initio calculations, we show here that it is possible to obtain aromatic-like behavior with pure hydrogenated silicon clusters without the need for multiple bonds. To this end, we exploit the natural tendency of silicon toward overcoordination to construct electron-deficient molecules with ring structures. Even without the incorporation of any protective bulky substituents the resulting structures are more stable than any other known hydrogenated silicon nanoparticles of this size and exhibit aromatic-like properties due to strong electron delocalization.

A theoretical NMR study of ortho and para-substituted benzenes compared with silabenzenes, pyridines and phosphabenzenes

Calculations of absolute shieldings and nuclear-independent chemical shift [NICS(1)] values of 84 compounds (benzenes, silabenzenes, pyridines and phosphabenzenes) allowed to discuss the absolute shieldings in the function of Brown σ(p)(+) substituent constant and to predict 71 new chemical shifts. The NICS(1) analysis of the aromaticity leads to the unexpected result for the silabenzenes that it is almost insensitive to substituent effects. In these set of compounds, it can be concluded that the aromaticity and its sensitivity to substituent effects are orthogonal.