Equilibrium Formation of Stable All-Silicon Versions of 1,3-Cyclobutanediyl

Amidinato silylene-based inorganic aromatic rings

Aromaticity is a key concept that underpins the behavior and applications of a wide range of chemical compounds. Its impact on stability, reactivity, biological functions, material properties, and environmental persistence underscores the importance of understanding and harnessing aromaticity in chemistry and materials sciences. We have been pioneers in the field of silylene chemistry and recently, our silylene molecules have been used to synthesize several inorganic aromatic ring compounds. Aromaticity in inorganic compounds is not commonly observed; hence, inorganic aromatic rings derived from silylene would further enhance our understanding of aromaticity and stability. Herein, we discuss the inorganic aromatic rings which have been synthesized from amidinato silylene.

Preparation of a high-coordinated-silicon-centered spiro-cyclic compound

Silicon compounds containing silicon-silicon bond with a variety of unusual oxidation states are quite important, because their high reactivity leads to the formation of a variety of silicon compounds. The isolation of such compounds with unusual oxidation states requires a resilient synthetic strategy. Herein, we report the synthesis of a silicon based spirocyclic compound containing a hyper-valent silicon atom and a silicon-silicon bond. The computational calculations employing natural bond orbital (NBO) analysis and energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) reveal that the nature of bonding between the silicon atoms is of an electron sharing nature.

Synthesis and Characterization of an All-Germanium Analogue of Cyclobutane-1,3-diyl

A tetragermacyclobutane-1,3-diyl was prepared and structurally characterized via the reduction of chlorogermylene-coordinated germylgermylene with potassium graphite, which represents the first all-germanium analogue of cyclobutane-1,3-diyl. Single-crystal X-ray analysis of the tetragermacyclobutane-1,3-diyl disclosed that it adopts a planar-cis structure, which is different from those reported all-silicon cyclobutane-1,3-diyls. DFT calculations revealed that both the bulky substituents on the germanium atoms and the tethering of the amido groups are important for the planar cis-configuration of 5.

Isolation of Fluorescent 2π-Aromatic 1,3-Disiladiboretenes

Herein, we reported the isolation of 2π-aromatic disiladiboretenes (L2Si2B2Ph2) [L = ArC(NtBu)2, Ar = Ph (1), Mes (2)], which have been synthesized from the straightforward reduction of silylene-borane adducts (LSiX → BX2Ph) [X = Cl, Br] with potassium graphite (KC8). X-ray diffraction analysis of 1 and 2 revealed that the Si2B2 units are completely planar, and DFT calculations suggested delocalization of 2π-electrons over the Si2B2 rings. Moreover, their photophysical properties and reactivity toward sulfur were also investigated in detail.

Heavy Tetrel Clusters Based on the Bicyclobutane Framework: Bicyclo[1.1.0]butanes, [1.1.1]Propellanes, and Tricyclo[2.1.0.02,5 ]pentanes

In this review, the current state of affairs in the novel field of the group 14 element clusters based on the bicyclo[1.1.0]butane framework, namely, bicyclo[1.1.0]butane, [1.1.1]propellane, and tricyclo[2.1.0.02,5 ]pentane derivatives, are overviewed. Structural similarities and differences between these three classes of highly strained polycyclic compounds are considered from the viewpoint of the critically important nature of their bridgehead bonds. Synthetic strategies towards bicyclo[1.1.0]butanes, [1.1.1]propellanes, and tricyclo[2.1.0.02,5 ]pentanes, as well as their peculiar structural and bonding features, and specific reactivity, are also presented and discussed in this review.

An Isolable Radical Anion Featuring a 2-Center-3-Electron π-Bond without a Clearly Defined σ-Bond

Featuring an extra electron in the π* antibonding orbital, species with a 2-center-3-electron (2c3e) π bond without an underlying σ bond are scarcely known. Herein, we report the synthesis, isolation and characterization of a radical anion salt [K(18-C-6)]+ {[(HCNDipp)2 Si]2 P2 }⋅- (i.e. [K(18-C-6)]+ 3⋅- ) (18-C-6=18-crown-6, Dipp=2,6-diisopropylphenyl), in which 3⋅- features a perfectly planar Si2 P2 four-membered ring. This species represents the first example of a Si- and P-containing analog of a bicyclo[1.1.0]butane radical anion. The unusual bonding motif of 3⋅- was thoroughly investigated via X-ray diffraction crystallography, electron paramagnetic resonance spectroscopy (EPR), and calculations by density functional theory (DFT), which collectively unveiled the existence of a 2c3e π bond between the bridgehead P atoms and no clearly defined supporting P-P σ bond.

Energetically More Stable Singlet Cyclopentane-1,3-diyl Diradical with π-Single Bonding Character than the Corresponding σ-Single Bonded Compound

Carbon-carbon σ-single bonds are crucial for constructing molecules like ethane derivatives (R3C-CR3), which are composed of tetrahedral four-coordinate carbons. Molecular functions, such as light absorption or emission, originate from the π-bonds existing in ethylene derivatives (R2C═CR2). In this study, a relatively stable cyclopentane-1,3-diyl species with π-single bonding system (C-π-C) with planar four-coordinate carbons is constructed. This diradicaloid is energetically more stable than the corresponding σ-single bonding system. The π-electron single bonding system provides deeper insights into the chemical bonding and the physical properties derived from the small energy gaps between the bonding and antibonding molecular orbitals.

π-Aromaticity Dominating in a Saturated Ring: Neutral Aromatic Silicon Analogues of Cyclobutane-1,3-diyls

The synthesis, structures, and reactivity of the first neutral 2π-aromatic Si4 rings [LSiSiAr(X)]2 (3: X = Br; 4: X = Cl; L = PhC(NtBu)2, Ar = 2,4,6-Me3C6H2) were described. Compounds 3 and 4 were obtained by 1,3-halogenation of tetrasilacyclobutadiene (LSiSiAr)2 (2), which was prepared by the reductive cross-coupling of trisilane (ArSiCl2)2SiHAr with two equiv of chlorosilylene LSiCl. The reaction of 3 with two equiv of PhLi yielded the corresponding substitution Si4 ring [LSiSiAr(Ph)]2 (5). Single-crystal X-ray diffraction analysis of 3 disclosed that it adopts both puckered (3a) and planar (3b) structures in the solid state, whereas 4 and 5 exhibit only a puckered structure. DFT calculations suggested that the puckered 3a features almost the same electronic structure with fully delocalized 2π planar 3b. The dominant 2π-aromaticity of 3 in a σ-frame has been demonstrated by DFT calculations, providing the first example of aromatics featuring both planar and puckered structures.

Heteroatom-Based Diradical(oid)s

Heteroatom-centered diradical(oid)s have been in the focus of molecular main group chemistry for nearly 30 years. During this time, the diradical concept has evolved and the focus has shifted to the rational design of diradical(oid)s for specific applications. This review article begins with some important theoretical considerations of the diradical and tetraradical concept. Based on these theoretical considerations, the design of diradical(oid)s in terms of ligand choice, steric, symmetry, electronic situation, element choice, and reactivity is highlighted with examples. In particular, heteroatom-centered diradical reactions are discussed and compared with closed-shell reactions such as pericyclic additions. The comparison between closed-shell reactivity, which proceeds in a concerted manner, and open-shell reactivity, which proceeds in a stepwise fashion, along with considerations of diradical(oid) design, provides a rational understanding of this interesting and unusual class of compounds. The application of diradical(oid)s, for example in small molecule activation or as molecular switches, is also highlighted. The final part of this review begins with application-related details of the spectroscopy of diradical(oid)s, followed by an update of the heteroatom-centered diradical(oid)s and tetraradical(oid)s published in the last 10 years since 2013.

Pyramidanes: newcomers to the anti-van't Hoff-Le Bel family

In this feature article, an overview of the chemistry of pyramidanes, as a novel class of main group element clusters, is given. A general introduction sets the scene, briefly presenting the non-classical pyramidal geometry of tetracoordinate carbon, as opposed to the classical tetrahedral configuration. Pyramidanes, as the simplest organic compounds possessing a pyramidal carbon atom, are then discussed from both computational and experimental viewpoints, to show the theoretical predictions on the stability and thus the feasibility of pyramidanes has finally culminated in the isolation of the first stable representatives of the pyramidane family featuring heavy main group elements at the apex of the square pyramid. Synthetic strategies towards pyramidanes, as well as their peculiar structural features, non-classical bonding situations, and specific reactivity, are presented and discussed in this review.

Concerted addition of aldehydes to the singlet biradical [P(μ-NTer)]2

The reaction of the singlet biradical [P(μ-NTer)]₂ with various aldehydes selectively yielded the corresponding [2.1.1]-bicyclic addition products in a very fast reaction. All products were fully characterized, including by NMR and vibrational spectroscopy as well as single-crystal X-ray diffraction. The mechanism of the addition was investigated theoretically using high-level ab initio methods (CCSD(T) with triple- and quadruple-zeta basis sets) and corresponds to a concerted cycloaddition reaction with a very low activation barrier. For comparison, the mechanisms of the literature-known cycloadditions of H₂, alkenes, and alkynes were also studied, indicating a similar reaction profile for all unsaturated reactants.

Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

Radical Reactivity of the Biradical [⋅P(μ-NTer)2 P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2 P-Et]

The activation of C-Br bonds in various bromoalkanes by the biradical [⋅P(μ-NTer)2 P⋅] (1) (Ter=2,6-bis-(2,4,6-trimethylphenyl)-phenyl) is reported, yielding trans-addition products of the type [Br-P(μ-NTer)2 P-R] (2), so-called 1,3-substituted cyclo-1,3-diphospha-2,4-diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo-1,3-diphospha-2,4-diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in-situ detection of the phosphorus-centered monoradical [⋅P(μ-NTer)2 P-R].< To provide further evidence for the radical mechanism, [⋅P(μ-NTer)2 P-Et] (3Et⋅) was synthesized directly by reduction of the bromoethane addition product [Br-P(μ-NTer)2 P-Et] (2 a) with magnesium, resulting in the formation of the persistent phosphorus-centered monoradical [⋅P(μ-NTer)2 P-Et], which could be isolated and fully characterized, including single-crystal X-ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ-NTer)2 P-Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ-NTer)2 P⋅] (1) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ-NTer)2 P⋅] with EtBr. In the field of hetero-cyclobutane-1,3-diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P-centered biradicals and P-centered monoradicals in terms of radical reactivity.

Impacts of Solvent and Alkyl Chain Length on the Lifetime of Singlet Cyclopentane-1,3-diyl Diradicaloids with π-Single Bonding

The singlet 2,2-dialkoxycyclopentane-1,3-diyl diradicaloids are not only the important key intermediates in the process of bond homolysis but are also attracting attention as π-single bonding compounds. In the present study, the effects of solvent viscosity η (0.24-125.4 mPa s) and polarity π* (-0.11 to 1.00 kcal mol^-1) on the reactivity of localized singlet diradicaloids were thoroughly investigated using 18 different solvents including binary mixed solvent systems containing ionic liquids. In low-η solvents (η < 1 mPa s), the lifetimes of singlet diradicaloids, which are determined by the rate constant for the isomerization of π-single-bonded singlet diradicaloids to the σ-bonded isomer, were substantially dependent on π*. Slower isomerization was observed in more polar solvents. In high-η solvents (η > 2 mPa s), the rate of isomerization was largely influenced by η in addition to π*. Slower isomerization was observed in more viscous solvents. Experimental results demonstrated the crucial roles of both solvent polarity and viscosity in the reactivity of singlet diradicaloids and thus clarified the characters of singlet diradicaloids and molecular motions during the chemical transformation. The dynamic solvent effect was further proved by a long alkyl chain introduced at a remote position of the reaction site.

Long-lived localised singlet diradicaloids with carbon-carbon π-single bonding (C-π-C)

Localised singlet cyclopentane-1,3-diyl diradicaloids have been considered promising candidates for constructing carbon-carbon π-single bonds (C-π-C). However, the high reactivity during formation of the σ-bond has limited a deeper investigation of its unique chemical properties. In this feature article, recent progress in kinetic stabilisation based on the "stretch effect" and the "solvent dynamic effect" induced by the macrocyclic system is summarised. Singlet diradicaloids S-DR4a/b and S-DR4d containing macrocyclic rings showed much longer lifetimes at 293 K (14 μs for S-DR4a and 156 μs for S-DR4b in benzene) compared to the parent singlet diradicaloid S-DR2 having no macrocyclic ring (209 ns in benzene). Furthermore, the dynamic solvent effect in viscous solvents was observed for the first time in intramolecular σ-bond formation, the lifetime of S-DR4d increased to 400 μs in the viscous solvent glycerin triacetin at 293 K. The experimental results proved the validity of the "stretch effect" and the "solvent dynamic effect" on the kinetic stabilisation of singlet cyclopentane-1,3-diyl diradicaloids, and provided a strategy for isolating the carbon-carbon π-single bonded species (C-π-C), and towards a deeper understanding of the nature of chemical bonding.

π-Conjugated species with an unsupported Si-Si π-bond obtained from direct π-extension

1,3-Diethynylbicyclo[1.1.0]tetrasilanes that contain an unsupported bridgehead Si-Si π-bond and ethynyl π-moieties were synthesized by the reaction of a 1,3-dihalobicyclo[1.1.0]tetrasilane with the corresponding lithium acetylide. The substantial bathochromic shift of the longest-wavelength absorption band observed for the phenylethynyl-substituted bicyclo[1.1.0]tetrasilane in the solid state compared to that of the octynyl-substituted derivative suggested the presence of effective π-conjugation between the unsupported Si-Si π and ethynyl π units.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

Construction of Inorganic Crown Ethers by s-Block-Metal-Templated Si-O Bond Activation

We herein report the synthesis, structures, coordination ability, and mechanism of formation of silicon analogs of crown ethers. An oligomerization of 2 D2 (I) (2 Dn ,=(Me4 Si2 O)n ) was achieved by the reaction with GaI3 and MIx (M=Li, Na, Mg, Ca, Sr). In these reactions the metal cations serve as template and the anions (I- /[GaI4 ]- ) are required as nucleophiles. In case of MIx =LiI, [Li(2 D3 )GaI4 ] (1) is formed. In case of MIx =NaI, MgI2 , CaI2 , and SrI2 the compounds [M(2 D4 )(GaI4 )x ] (M=Mg2+ (3), Ca2+ (4), Sr2+ (5) are obtained. Furthermore the proton complex [H(2 D3 )][Ga2 I7 ] (6) was isolated and structurally characterized. All complexes were characterized by means of multinuclear NMR spectroscopy, DOSY experiments and, except for compound 3, also by single crystal X-ray diffraction. Quantum chemical calculations were carried out to compare the affinity of M+ to 2 Dn and other ligands and to shed light on the formation of larger rings from smaller ones.

Unconventional singlet fission materials

Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is per se problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character

Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical–radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of “stretch effect” to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon–carbon coupling reaction rate of 6.4 × 10³ s⁻¹ (155.9 μs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 10⁴ s⁻¹, 14.2 μs) and the parent system lacking the macrocycle (5 × 10⁶ s⁻¹, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical–radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.

An extremely long-lived localised singlet diradical with π-single bonding character is found in a macrocyclic structure that retards the radical–radical coupling reaction by the “stretch and solvent-dynamic effects”.

Silicon-silicon π single bond

A carbon–carbon double bond consists of a σ bond and a π bond. Recently, the concept of a π single bond, where a π bond is not accompanied by a σ bond, has been proposed in diradicals containing carbon and heteroatom radical centers. Here we report a closed-shell compound having a silicon–silicon π single bond. 1,2,2,3,4,4-Hexa-tert-butylbicyclo[1.1.0]tetrasilane has a silicon−silicon π single bond between the bridgehead silicon atoms. The X-ray crystallographic analysis shows that the silicon−silicon π single bond (2.853(1) Å) is far longer than the longest silicon−silicon bond so far reported. In spite of this unusually long bond length, the electrons of the 3p orbitals are paired, which is confirmed by measurement of electron paramagnetic resonance, and magnetic susceptibility and natural bond orbital analysis. The properties of the silicon−silicon π single bond are studied by UV/Vis and ²⁹Si NMR spectroscopy, and theoretical calculations.

Examples of π single bonds in diradicals are rare, as they are typically unstable. Here, the authors describe a room temperature-stable closed-shell bicyclic tetrasilane that features a Si-Si π single bond between two of the non-adjacent Si centers.

Interconversion between a planar 1,3-dichlorobicyclo[1.1.0]tetrasilane and a (chloro)(chlorosilyl)cyclotrisilene

A 1,3-dichlorotetrasilabicyclo[1.1.0]butane with a π-type bridgehead Si–Si bond undergoes a skeletal isomerisation to a 1-chloro-2-(chlorosilyl)cyclotrisilene.

Dynamic solvent effects in radical-radical coupling reactions: an almost bottleable localised singlet diradical

Localised singlet diradicals are key intermediates in bond homolysis, which plays a crucial role in chemical reactions. However, thorough experimental analyses of the reaction dynamics and chemical properties are generally difficult because bond formation is rapid, even under low-temperature matrix conditions. In this study, the effects of solvent and pressure on the lifetimes of long-lived singlet diradicals with bulky substituents were investigated. The solvent dynamic effect was revealed to provide control over the rate constant of radical-radical coupling reactions, and an almost bottleable singlet diradical with a lifetime of ∼2 s at 293 K was obtained.